Journal articles on the topic 'Atmospheric circulation Tropics Mathematical models'
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Igel, Matthew R., and Joseph A. Biello. "The Nontraditional Coriolis Terms and Tropical Convective Clouds." Journal of the Atmospheric Sciences 77, no. 12 (December 2020): 3985–98. http://dx.doi.org/10.1175/jas-d-20-0024.1.
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AbstractThe full, three-dimensional Coriolis force includes the familiar sine-of-latitude terms as well as frequently dropped cosine-of-latitude terms [nontraditional Coriolis terms (NCT)]. The latter are often ignored because they couple the zonal and vertical momentum equations that in the large-scale limit of weak vertical velocity are considered insignificant almost everywhere. Here, we ask whether equatorial mesoscale clouds that fall outside the large-scale limit are affected by the NCT. A simple scaling indicates that a Lagrangian parcel convecting at 10 m s−1 through the depth of the troposphere should be deflected over 2 km to the west. To understand the real impact of NCT, we develop a mathematical framework that describes an azimuthally symmetric convective circulation with an analytical expression for an incompressible poloidal flow. Because the model incorporates the full three-dimensional flow associated with convection, it uniquely predicts not only the westward tilt of clouds but also a meridional diffluence of western cloud outflow. To test these predictions, we perform a set of cloud-resolving simulations whose results show preferential lifting of surface parcels with positive zonal momentum and zonal asymmetry in convective strength. RCE simulations show changes to the organization of coherent precipitation regions and a decrease in mean convective intensity of approximately 2 m s−1 above the freezing level. An additional pair of dry cloud-resolving simulations designed to mimic the steady-state flow of the model show maximum perturbations to the upper-level zonal flow of 8 m s−1. Together, the numerical and analytic results suggest the NCT consequentially alter equatorial mesoscale convective circulations and should be considered in conceptual models.
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Grise, Kevin M., Sean M. Davis, Paul W. Staten, and Ori Adam. "Regional and Seasonal Characteristics of the Recent Expansion of the Tropics." Journal of Climate 31, no. 17 (September 2018): 6839–56. http://dx.doi.org/10.1175/jcli-d-18-0060.1.
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In recent decades, the subtropical edges of Earth’s Hadley circulation have shifted poleward. Some studies have concluded that this observed tropical expansion is occurring more rapidly than predicted by global climate models. However, recent modeling studies have shown that internal variability can account for a large fraction of the observed circulation trends, at least in an annual-mean, zonal-mean framework. This study extends these previous results by examining the seasonal and regional characteristics of the recent poleward expansion of the Hadley circulation using seven reanalysis datasets, sea level pressure observations, and surface wind observations. The circulation has expanded the most poleward during summer and fall in both hemispheres, with more zonally asymmetric circulation trends occurring in the Northern Hemisphere (NH). The seasonal and regional characteristics of these observed trends generally fall within the range of trends predicted by climate models for the late twentieth and early twenty-first centuries, and in most cases, the magnitude of the observed trends does not exceed the range of interdecadal trends in the models’ control runs, which arise exclusively from internal variability. One exception occurs during NH fall when large observed poleward shifts in the atmospheric circulation over the North Atlantic sector exceed nearly all trends projected by models. While most recent NH circulation trends are consistent with a change in phase of the Pacific decadal oscillation (PDO), the observed circulation trends over the North Atlantic instead reflect 1) large natural variability unrelated to the PDO and/or 2) a climate forcing (or the circulation response to that forcing) that is not properly captured by models.
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Long, Shang-Min, Shang-Ping Xie, and Wei Liu. "Uncertainty in Tropical Rainfall Projections: Atmospheric Circulation Effect and the Ocean Coupling." Journal of Climate 29, no. 7 (March 29, 2016): 2671–87. http://dx.doi.org/10.1175/jcli-d-15-0601.1.
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Abstract Uncertainty in tropical rainfall projections under increasing radiative forcing is studied by using 26 models from phase 5 of the Coupled Model Intercomparison Project. Intermodel spread in projected rainfall change generally increases with interactive sea surface temperature (SST) warming in coupled models compared to atmospheric models with a common pattern of prescribed SST increase. Moisture budget analyses reveal that much of the model uncertainty in tropical rainfall projections originates from intermodel discrepancies in the dynamical contribution due to atmospheric circulation change. Intermodel singular value decomposition (SVD) analyses further show a tight coupling between the intermodel variations in SST warming pattern and circulation change in the tropics. In the zonal mean, the first SVD mode features an anomalous interhemispheric Hadley circulation, while the second mode displays an SST peak near the equator. The asymmetric mode is accompanied by a coupled pattern of wind–evaporation–SST feedback in the tropics and is further tied to interhemispheric asymmetric change in extratropical shortwave radiative flux at the top of the atmosphere. Intermodel variability in the tropical circulation change exerts a strong control on the spread in tropical cloud cover change and cloud radiative effects among models. The results indicate that understanding the coupling between the anthropogenic changes in SST pattern and atmospheric circulation holds the key to reducing uncertainties in projections of future changes in tropical rainfall and clouds.
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Kiladis, George N., and Steven B. Feldstein. "Rossby wave propagation into the tropics in two GFDL general circulation models." Climate Dynamics 9, no. 4-5 (January 1, 1994): 245–52. http://dx.doi.org/10.1007/s003820050023.
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Kiladis, George N., and Steven B. Feldstein. "Rossby wave propagation into the tropics in two GFDL general circulation models." Climate Dynamics 9, no. 4-5 (January 1994): 245–52. http://dx.doi.org/10.1007/bf00208256.
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Byrne, Michael P., and Tapio Schneider. "Atmospheric Dynamics Feedback: Concept, Simulations, and Climate Implications." Journal of Climate 31, no. 8 (March 26, 2018): 3249–64. http://dx.doi.org/10.1175/jcli-d-17-0470.1.
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AbstractThe regional climate response to radiative forcing is largely controlled by changes in the atmospheric circulation. It has been suggested that global climate sensitivity also depends on the circulation response, an effect called the “atmospheric dynamics feedback.” Using a technique to isolate the influence of changes in atmospheric circulation on top-of-the-atmosphere radiation, the authors calculate the atmospheric dynamics feedback in coupled climate models. Large-scale circulation changes contribute substantially to all-sky and cloud feedbacks in the tropics but are relatively less important at higher latitudes. Globally averaged, the atmospheric dynamics feedback is positive and amplifies the near-surface temperature response to climate change by an average of 8% in simulations with coupled models. A constraint related to the atmospheric mass budget results in the dynamics feedback being small on large scales relative to feedbacks associated with thermodynamic processes. Idealized-forcing simulations suggest that circulation changes at high latitudes are potentially more effective at influencing global temperature than circulation changes at low latitudes, and the implications for past and future climate change are discussed.
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Tomassini, Lorenzo. "The Interaction between Moist Convection and the Atmospheric Circulation in the Tropics." Bulletin of the American Meteorological Society 101, no. 8 (August 1, 2020): E1378—E1396. http://dx.doi.org/10.1175/bams-d-19-0180.1.
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Abstract Theories of the interaction between moist convection and the atmospheric circulation in the tropics are reviewed. Two main schools of thought are highlighted: (i) one that emphasizes the lower-level control of convection through moisture convergence and variations in convective inhibition, and (ii) one that sees convection as an adjustment process in reaction to larger-scale instabilities, referred to as convective quasi-equilibrium theory. Conceptually the two views consider moist convection to have fundamentally different roles in the tropical circulation. In one case the presence of low-level inhibition and the conditional nature of the atmospheric instability allows for convective vertical motion and latent heating to drive and reinforce synoptic-scale disturbances and overturning circulations; in the other case, because low-level inhibition is not acknowledged to be a widespread controlling barrier, convection is believed to balance and dampen vertical instabilities at the rate they are created by larger-scale processes over the vertical extent of the atmosphere. More recently, investigations of the moisture dynamics surrounding organized convective structures have led to an emerging consensus on the theory of convection–circulation coupling in the tropics that acknowledges the important role of lower- to midtropospheric moisture variations, and the significance of moist convection and convective clouds for initiating and establishing circulations. However, the implementation of these new insights in numerical models lags behind. This is exemplified by the apparent inadequacy of climate models to correctly represent decadal variability in the tropical Pacific, a fact that potentially has implications for the confidence in climate change projections based on such models.
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Boos, William R. "Thermodynamic Scaling of the Hydrological Cycle of the Last Glacial Maximum." Journal of Climate 25, no. 3 (February 1, 2012): 992–1006. http://dx.doi.org/10.1175/jcli-d-11-00010.1.
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Abstract In climate models subject to greenhouse gas–induced warming, vertically integrated water vapor increases at nearly the same rate as its saturation value. Previous studies showed that this increase dominates circulation changes in climate models, so that precipitation minus evaporation (P − E) decreases in the subtropics and increases in the tropics and high latitudes at a rate consistent with a Clausius–Clapeyron scaling. This study examines whether the same thermodynamic scaling describes differences in the hydrological cycle between modern times and the last glacial maximum (LGM), as simulated by a suite of coupled ocean–atmosphere models. In these models, changes in water vapor between modern and LGM climates do scale with temperature according to Clausius–Clapeyron, but this thermodynamic scaling provides a poorer description of the changes in P − E. While the scaling is qualitatively consistent with simulations in the zonal mean, predicting higher P − E in the subtropics and lower P − E in the tropics and high latitudes, it fails to account for high-amplitude zonal asymmetries. Large horizontal gradients of temperature change, which are often neglected when applying the scaling to next-century warming, are shown to be important in large parts of the extratropics. However, even with this correction the thermodynamic scaling provides a poor quantitative fit to the simulations. This suggests that circulation changes play a dominant role in regional hydrological change between modern and LGM climates. Changes in transient eddy moisture transports are shown to be particularly important, even in the deep tropics. Implications for the selection and interpretation of climate proxies are discussed.
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Xu, Kang, Chi-Yung Tam, Congwen Zhu, Boqi Liu, and Weiqiang Wang. "CMIP5 Projections of Two Types of El Niño and Their Related Tropical Precipitation in the Twenty-First Century." Journal of Climate 30, no. 3 (January 12, 2017): 849–64. http://dx.doi.org/10.1175/jcli-d-16-0413.1.
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Abstract Future projections of the eastern-Pacific (EP) and central-Pacific (CP) types of El Niño in the twenty-first century, as well as their associated tropical circulation and precipitation variability, are investigated using historical runs and representative concentration pathway 8.5 (RCP8.5) simulations from 31 coupled models in phase 5 of the Coupled Model Intercomparison Project (CMIP5). As inferred from CMIP5 models that best capture both El Niño flavors, EP El Niño sea surface temperature (SST) variability will become weaker in the future climate, while no robust change of CP El Niño SST is found. Models also reach no consensus on the future change of relative frequency from CP to EP El Niño. However, there are robust changes in the tropical overturning circulation and precipitation associated with both types of El Niño. Under a warmer climate, magnitudes of precipitation anomalies during EP El Niño are projected to increase, presenting significant enhancement of the dry (wet) signal over the western (central–eastern) Pacific. This is consistent with an accelerated hydrological cycle in the deep tropics; hence, a “wet get wetter” picture appears under global warming, accompanied by a weakened anomalous Walker circulation. For CP El Niño, drier-than-normal conditions will be intensified over the tropical central–eastern Pacific in the future climate, with stronger anomalous sinking related to the strengthened North Pacific local Hadley cell. These results suggest that, besides the enhanced basic-state hydrological cycle over the tropics, other elements, such as the anomalous overturning circulation, might also play a role in determining the ENSO precipitation response to a warmer background climate.
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Mitovski, Toni, Ian Folkins, Knut von Salzen, and Michael Sigmond. "Temperature, Relative Humidity, and Divergence Response to High Rainfall Events in the Tropics: Observations and Models." Journal of Climate 23, no. 13 (July 1, 2010): 3613–25. http://dx.doi.org/10.1175/2010jcli3436.1.
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Abstract Radiosonde measurements and Tropical Rainfall Measuring Mission (TRMM) 3B42 rainfall are used to construct composite anomaly patterns of temperature, relative humidity, and divergence about high rainfall events in the western Pacific. The observed anomaly patterns are compared with anomaly patterns from four general circulation models [Third and Fourth Generation Atmospheric General Circulation Model (AGCM3 and AGCM4), Geophysical Fluid Dynamics Laboratory Climate Model version 2.1 (GFDL CM2.1), and European Center Hamburg Model version 5 (ECHAM5)] and two reanalysis products [40-yr ECMWF Re-Analysis (ERA-40) and ERA-Interim]. In general, the models and reanalyses do not fully represent the timing, strength, or altitude of the midlevel congestus divergence that precedes peak rainfall or the midlevel stratiform convergence that occurs after peak rainfall. The surface cold pools that develop in response to high rainfall events are also either not present or somewhat weaker than observations. Surface cold pools originate from the downward transport within mesoscale downdrafts of midtropospheric air with low moist static energy into the boundary layer. Differences between the modeled and observed response to high rainfall events suggest that the convective parameterizations used by the models and reanalyses discussed here may underrepresent the strength of the mesoscale downdraft circulation.
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Punge, H. J., and M. A. Giorgetta. "Net effect of the QBO in a chemistry climate model." Atmospheric Chemistry and Physics 8, no. 21 (November 13, 2008): 6505–25. http://dx.doi.org/10.5194/acp-8-6505-2008.
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Abstract. The quasi-biennial oscillation (QBO) of zonal wind is a prominent mode of variability in the tropical stratosphere. It affects not only the meridional circulation and temperature over a wide latitude range but also the transport and chemistry of trace gases such as ozone. Compared to a QBO less circulation, the long-term climatological means of these quantities are also different. These climatological net effects of the QBO can be studied in general circulation models that extend into the middle atmosphere and have a chemistry and transport component, so-called Chemistry Climate Models (CCMs). In this work we show that the CCM MAECHAM4-CHEM can reproduce the observed QBO variations in temperature and ozone mole fractions when nudged towards observed winds. In particular, it is shown that the QBO signal in transport of nitrogen oxides NOx plays an important role in reproducing the observed ozone QBO, which features a phase reversal slightly below the level of maximum of the ozone mole fraction in the tropics. We then compare two 20-year experiments with the MAECHAM4-CHEM model that differ by including or not including the QBO. The mean wind fields differ between the two model runs, especially during summer and fall seasons in both hemispheres. The differences in the wind field lead to differences in the meridional circulation, by the same mechanism that causes the QBO's secondary meridional circulation, and thereby affect mean temperatures and the mean transport of tracers. In the tropics, the net effect on ozone is mostly due to net differences in upwelling and, higher up, the associated temperature change. We show that a net surplus of up to 15% in NOx in the tropics above 10 hPa in the experiment that includes the QBO does not lead to significantly different volume mixing ratios of ozone. We also note a slight increase in the southern vortex strength as well as earlier vortex formation in northern winter. Polar temperatures differ accordingly. Differences in the strength of the Brewer-Dobson circulation and in further trace gas concentrations are analysed. Our findings underline the importance of a representation of the QBO in CCMs.
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Chakraborty, Arindam, and T. N. Krishnamurti. "Improved Forecasts of the Diurnal Cycle in the Tropics Using Multiple Global Models. Part II: Asian Summer Monsoon." Journal of Climate 21, no. 16 (August 15, 2008): 4045–67. http://dx.doi.org/10.1175/2008jcli2107.1.
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Abstract The diurnal mode of the Asian summer monsoon during active and break periods is studied using four versions of the Florida State University (FSU) global spectral model (GSM). These versions differ in the formulation of cloud parameterization schemes in the model. Observational-based estimates show that there exists a divergent circulation at 200 hPa over the Asian monsoon region in the diurnal time scale that peaks at 1200 local solar time (LST) during break monsoon and at 1800 LST during active monsoon. A circulation in the opposite direction is seen near the surface. This circulation loop is completed by vertical ascending/descending motion over the monsoon domain and its surroundings. This study shows that global models have large phase and amplitude errors for the 200-hPa velocity potential and vertical pressure velocity over the monsoon region and its surroundings. Construction of a multimodel superensemble could reduce these errors substantially out to five days in advance. This was on account of assigning differential weights to the member models based on their past performance. This study also uses a unified cloud parameterization scheme that inherits the idea of a multimodel superensemble for combining member model forecasts. The advantage of this model is that it is an integrated part of the GSM and thus can improve the forecasts of other parameters as well through improved cloud cover. It was seen that this scheme had a larger impact on forecasting the diurnal cycle of cloud cover and precipitation of the Asian summer monsoon compared to circulation. The authors show that the diurnal circulation contributes to about 10% of the rate of change of total kinetic energy of the monsoon. Therefore, forecasting this pronounced diurnal mode has important implications for the energetics of the Asian summer monsoon.
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Hagos, Samson, and L. Ruby Leung. "On the Relationship between Uncertainties in Tropical Divergence and the Hydrological Cycle in Global Models." Journal of Climate 25, no. 1 (January 1, 2012): 381–91. http://dx.doi.org/10.1175/jcli-d-11-00058.1.
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Abstract A survey of tropical divergence from three GCMs, three global reanalyses, and four in situ soundings from field campaigns shows the existence of large uncertainties in the ubiquity of shallow divergent circulation as well as the depth and strength of the deep divergent circulation. More specifically, only two out of the three GCMs and three global reanalyses show significant shallow divergent circulation, which is present in all in situ soundings, and of the three GCMs and three global reanalyses, only two global reanalyses have deep divergence profiles that lie within the range of uncertainty of the soundings. The relationships of uncertainties in the shallow and deep divergent circulation to uncertainties in present-day and projected strength of the hydrological cycle from the GCMs are assessed. In the tropics and subtropics, deep divergent circulation is the largest contributor to moisture convergence that balances the net precipitation (precipitation minus evaporation), and intermodel differences in the present-day simulations carry over onto the future projections. In comparison to the soundings and reanalyses, the GCMs are found to have deeper and stronger divergent circulation. While these two characteristics of GCM divergence affect the strength of the hydrological cycle, they tend to compensate for each other so that their combined effect is relatively modest.
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Hsia, Chun-Hsiung, Chang-Shou Lin, Tian Ma, and Shouhong Wang. "Tropical atmospheric circulations with humidity effects." Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 471, no. 2173 (January 2015): 20140353. http://dx.doi.org/10.1098/rspa.2014.0353.
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The main objective of this article is to study the effect of the moisture on the planetary scale atmospheric circulation over the tropics. The modelling we adopt is the Boussinesq equations coupled with a diffusive equation of humidity, and the humidity-dependent heat source is modelled by a linear approximation of the humidity. The rigorous mathematical analysis is carried out using the dynamic transition theory. In particular, we obtain mixed transitions, also known as random transitions, as described in Ma & Wang (2010 Discrete Contin. Dyn. Syst. 26 , 1399–1417. ( doi:10.3934/dcds.2010.26.1399 ); 2011 Adv. Atmos. Sci. 28 , 612–622. ( doi:10.1007/s00376-010-9089-0 )). The analysis also indicates the need to include turbulent friction terms in the model to obtain correct convection scales for the large-scale tropical atmospheric circulations, leading in particular to the right critical temperature gradient and the length scale for the Walker circulation. In short, the analysis shows that the effect of moisture lowers the magnitude of the critical thermal Rayleigh number and does not change the essential characteristics of dynamical behaviour of the system.
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Grewe, V. "The origin of ozone." Atmospheric Chemistry and Physics 6, no. 6 (May 10, 2006): 1495–511. http://dx.doi.org/10.5194/acp-6-1495-2006.
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Abstract. Highest atmospheric ozone production rates can be found at around 30 km in the tropical stratosphere, leading to ozone mixing ratios of about 10 ppmv. Those stratospheric air masses are then transported to extra-tropical latitudes via the Brewer-Dobson circulation. This is considered the main mechanism to generate mid- and high latitude ozone. By applying the climate-chemistry models E39/C and MAECHAM4/CHEM, this view is investigated in more detail. The origin of ozone in the troposphere and stratosphere is analysed, by incorporating a diagnostics ("marked ozone origin tracers") into the models, which allows to identify the origin of ozone. In most regions the simulated local ozone concentration is dominated by local ozone production, i.e. less than 50% of the ozone at higher latitudes of the stratosphere is produced in the tropics, which conflicts with the idea that the tropics are the global source for stratospheric ozone. Although episodic stratospheric intrusions occur basically everywhere, the main ozone stratosphere-to-troposphere exchange is connected to exchange processes at the sub-tropical jet-stream. The simulated tropospheric influx of ozone amounts to 420 Tg per year, and originates in the Northern Hemisphere from the extra-tropical stratosphere, whereas in the Southern Hemisphere a re-circulation of tropical tropospheric ozone contributes most to the influx of ozone into the troposphere. In the model E39/C, the upper troposphere of both hemispheres is clearly dominated by tropical tropospheric ozone (40%–50%) except for northern summer hemisphere, where the tropospheric contribution (from the tropics as well as from the Northern Hemisphere) does not exceed 20%.
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Li, Ying, David W. J. Thompson, and Sandrine Bony. "The Influence of Atmospheric Cloud Radiative Effects on the Large-Scale Atmospheric Circulation." Journal of Climate 28, no. 18 (September 11, 2015): 7263–78. http://dx.doi.org/10.1175/jcli-d-14-00825.1.
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Abstract The influence of clouds on the large-scale atmospheric circulation is examined in numerical simulations from an atmospheric general circulation model run with and without atmospheric cloud radiative effects (ACRE). In the extratropics of both hemispheres, the primary impacts of ACRE on the circulation include 1) increases in the meridional temperature gradient and decreases in static stability in the midlatitude upper troposphere, 2) strengthening of the midlatitude jet, 3) increases in extratropical eddy kinetic energy by up to 30%, and 4) increases in precipitation at middle latitudes but decreases at subtropical latitudes. In the tropics, the primary impacts of ACRE include 1) eastward wind anomalies in the tropical upper troposphere–lower stratosphere (UTLS) and 2) reductions in tropical precipitation. The impacts of ACRE on the atmospheric circulation are interpreted in the context of a series of dynamical and physical processes. The changes in the extratropical circulation and precipitation are consistent with the influence of ACRE on the baroclinicity and eddy fluxes of momentum in the extratropical upper troposphere, the changes in the zonal wind in the UTLS with the influence of ACRE on the amplitude of the equatorial planetary waves, and the changes in the tropical precipitation with the energetic constraints on the tropical troposphere. The results make clear that ACRE have a pronounced influence on the atmospheric circulation not only at tropical latitudes, but at extratropical latitudes as well. They highlight the critical importance of correctly simulating ACRE in global climate models.
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JOSHI, P. C., B. SIMON, and P. K. THAPLIYAL. "Satellite estimated water vapour in relation to Asian monsoon Circulation." MAUSAM 52, no. 1 (December 29, 2021): 109–16. http://dx.doi.org/10.54302/mausam.v52i1.1681.
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Water vapour plays a crucial role in various exchange and transport processes in the atmosphere and its knowledge in the tropics is extremely important for input to various global circulation models. The vast oceans of earth's surface provide a large source of moisture and continuously modify the thermodynamics of the atmosphere through latent heat flux and condensational heating. In the tropics, especially in the Indian ocean the water vapour is highly heterogeneousin nature, and is one of the parameters which is responsible for cloud formation, associated with tropical systems like monsoon flows, depression, cyclones etc. The present paper reviews the various information’s available from deferent geostationary and polar orbiting satellites about water vapour affecting the southwest monsoon region around the Indian Ocean and Indian subcontinent.
The temperature and moisture data from TIROS operational vertical sounder (TOVS) and INSAT-2E water vapour channel are examined to study water vapour distribution. Their usefulness in characterizing the Asian south-west (SW) monsoon circulation is focused. The Western Indian ocean showed an increase in mid-tropospheric moisture (700-500hPa) over about 8 to 10 days prior to the onset over Kerala coast. NOAA/TOVS layer tmperature and humidity is used to extrapolate the humidity profile at standard pressure levels. It is also used to compute latent and sensible heat flux. Total integrated water vapour from SSM/I is also used for estimating latent heat fluxes and for the diagnostics of NWP models. Recently, INSAT-2E water vapour channel was used to monitor the monsoon circiulation features. The new WV channel brought out clearly the feeding of various air masses, especially water vapour associated with monsoon onset and monsoon lows.
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Norris, Jesse, Alex Hall, J. David Neelin, Chad W. Thackeray, and Di Chen. "Evaluation of the Tail of the Probability Distribution of Daily and Subdaily Precipitation in CMIP6 Models." Journal of Climate 34, no. 7 (April 2021): 2701–21. http://dx.doi.org/10.1175/jcli-d-20-0182.1.
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AbstractDaily and subdaily precipitation extremes in historical phase 6 of the Coupled Model Intercomparison Project (CMIP6) simulations are evaluated against satellite-based observational estimates. Extremes are defined as the precipitation amount exceeded every x years, ranging from 0.01 to 10, encompassing the rarest events that are detectable in the observational record without noisy results. With increasing temporal resolution there is an increased discrepancy between models and observations: for daily extremes, the multimodel median underestimates the highest percentiles by about a third, and for 3-hourly extremes by about 75% in the tropics. The novelty of the current study is that, to understand the model spread, we evaluate the 3D structure of the atmosphere when extremes occur. In midlatitudes, where extremes are simulated predominantly explicitly, the intuitive relationship exists whereby higher-resolution models produce larger extremes (r = −0.49), via greater vertical velocity. In the tropics, the convective fraction (the fraction of precipitation simulated directly from the convective scheme) is more relevant. For models below 60% convective fraction, precipitation amount decreases with convective fraction (r = −0.63), but above 75% convective fraction, this relationship breaks down. In the lower-convective-fraction models, there is more moisture in the lower troposphere, closer to saturation. In the higher-convective-fraction models, there is deeper convection and higher cloud tops, which appears to be more physical. Thus, the low-convective models are mostly closer to the observations of extreme precipitation in the tropics, but likely for the wrong reasons. These intermodel differences in the environment in which extremes are simulated hold clues into how parameterizations could be modified in general circulation models to produce more credible twenty-first-century projections.
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Zhang, Lei, and Tim Li. "A Simple Analytical Model for Understanding the Formation of Sea Surface Temperature Patterns under Global Warming*." Journal of Climate 27, no. 22 (November 4, 2014): 8413–21. http://dx.doi.org/10.1175/jcli-d-14-00346.1.
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Abstract How sea surface temperature (SST) changes under global warming is critical for future climate projection because SST change affects atmospheric circulation and rainfall. Robust features derived from 17 models of phase 5 of the Coupled Model Intercomparison Project (CMIP5) include a much greater warming in high latitudes than in the tropics, an El Niño–like warming over the tropical Pacific and Atlantic, and a dipole pattern in the Indian Ocean. However, the physical mechanism responsible for formation of such warming patterns remains open. A simple theoretical model is constructed to reveal the cause of the future warming patterns. The result shows that a much greater polar, rather than tropical, warming depends primarily on present-day mean SST and surface latent heat flux fields, and atmospheric longwave radiation feedback associated with cloud change further enhances this warming contrast. In the tropics, an El Niño–like warming over the Pacific and Atlantic arises from a similar process, while cloud feedback resulting from different cloud regimes between east and west ocean basins also plays a role. A dipole warming over the equatorial Indian Ocean is a response to weakened Walker circulation in the tropical Pacific.
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Fueglistaler, S., P. H. Haynes, and P. M. Forster. "The annual cycle in lower stratospheric temperatures revisited." Atmospheric Chemistry and Physics 11, no. 8 (April 21, 2011): 3701–11. http://dx.doi.org/10.5194/acp-11-3701-2011.
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Abstract. Observed lower stratospheric temperatures show a prominent annual cycle. The cycles in the tropics and Northern Hemisphere are in phase and the cycle in the Southern Hemisphere has the opposite phase. In an elegant and influential paper, Yulaeva, Holton and Wallace (1994) explained the observed pattern as a direct consequence of hemispheric asymmetries in the dynamical forcing of the stratospheric circulation. They showed that in Microwave Sounding Unit channel 4 (weighting centered in the lower stratosphere) data the combined extratropical and the tropical temperature cycle nearly compensate and interpreted the out-of-phase temperature variations between tropics and extratropics as the temperature response to an annual cycle in the wave driven residual circulation. We show that the near-compensation of temperature variations observed by Yulaeva et al. (1994) is artefact of the weighting function of the MSU-4 channel and does not hold on individual pressure levels. We discuss in detail the conditions required that temperature variations compensate, and what insights can be obtained from analysis of tropical, extratropical and global mean temperature variations. Dynamically induced seasonal variations of lower stratospheric ozone lead to an amplification of the seasonal temperature cycle particularly in the tropics. The latitudinal structure of static stability also induces a significant deviation from compensation of tropical and combined extratropical temperature variations. In line with Yulaeva et al. (1994) we affirm that the see-saw pattern in the annual cycles of tropical and combined extratropical temperatures provides an important pointer to mechanistic models for interannual variability and trends, but additionally conclude that the feedback of dynamically induced ozone variations on temperatures and the latitudinal structure of static stability should be included as leading order processes in such models.
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Takahashi, H. G., M. Hara, M. Fujita, and T. Yoshikane. "A discrepancy in precipitable water among reanalyses and the impact of forcing dataset on downscaling in the tropics." Atmospheric Chemistry and Physics Discussions 12, no. 9 (September 12, 2012): 23759–91. http://dx.doi.org/10.5194/acpd-12-23759-2012.
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Abstract. Seven major reanalyses of precipitable water (PW) are compared in this paper. In addition, using a regional climate model, we also investigated the impact of the boundary conditions on downscaling simulations in the tropics with a particular focus on the differences in the absolute value of PW among reanalyses. Results showed that the absolute amounts of PW in some reanalyses were very small compared to the observation, although most spatial patterns of PW in the reanalyses agreed closely with the observation. Particularly over the tropics, most of reanalyses tended to have dry biases throughout the annual cycle. The range of inter-reanalysis dispersion in the tropical mean PW is very large compared with their seasonal variations of the tropical mean PW. In addition, the discrepancies of the 12-yr mean PW in July over the Southeast Asian monsoon region among the reanalyses exceeded their inter-annual standard deviation of the PW. Therefore, the inter-reanalyses dispersion in the tropical PW is significantly large. We also conducted the downscaling experiments, which were forced by the different four reanalyses. The spatial and temporal variations of atmospheric circulation, including monsoon westerlies and various disturbances, were very similar among the reanalyses. However, the simulated precipitation was 40% less than the observed precipitation amounts, although the dry bias in the boundary conditions was only 6%, and the simulated atmospheric circulation was also basically the same. This result indicates that the dry bias has large effects on precipitation in downscaling experiments over the tropics even if atmospheric circulation is well simulated. Downscaled models can provide realistic simulations of regional tropical climates only if the boundary conditions include realistic absolute amounts of PW. Use of boundary conditions that include realistic absolute amounts of PW in downscaling in the tropics is imperative at the present time.
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DelSole, Timothy, Mei Zhao, and Paul Dirmeyer. "A New Method for Exploring Coupled Land–Atmosphere Dynamics." Journal of Hydrometeorology 10, no. 4 (August 1, 2009): 1040–50. http://dx.doi.org/10.1175/2009jhm1071.1.
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Abstract This paper proposes a new method for investigating coupled land–atmosphere interactions. The method is to apply an empirical correction technique to distinct components of a model and then examine differences between forecasts of the empirically corrected models. The correction technique is based on adding a time-dependent term to the tendency equations that subtracts the estimated tendency error at every time step. This methodology can be interpreted more generally as a series of data assimilation experiments in which only certain components of a coupled model are assimilated at a time. The correction is applied to a state-of-the-art coupled land–atmosphere model in three different ways, namely, to the atmosphere only, to the land only, and to the land and atmosphere simultaneously. The land–atmosphere interactions are inferred from monthly-mean differences between experiments. The results suggest that the land–atmosphere coupling in midlatitudes can be understood from straightforward water balance considerations, whereas the coupling in the deep tropics involves a more complicated change in regional circulation. Specifically, in midlatitudes, moisture injected into the soil is transferred to the atmosphere directly above, which in turn advects downstream and subsequently moistens the atmosphere in the downwind regions to produce positive precipitation anomalies. In the deep tropics, the regional circulation, including precipitation, is sensitive to perturbations and has no obvious relation to corrections in the atmosphere or land. The similarity of biases among different models suggests that the conclusions and methodology may be relevant to other models.
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Sherwood, S. C., and C. L. Meyer. "The General Circulation and Robust Relative Humidity." Journal of Climate 19, no. 24 (December 15, 2006): 6278–90. http://dx.doi.org/10.1175/jcli3979.1.
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Abstract The sensitivity of free-tropospheric relative humidity to cloud microphysics and dynamics is explored using a simple 2D humidity model and various configurations of the National Center for Atmospheric Research (NCAR) Community Atmosphere Model version 3 (CAM3) atmospheric general circulation model (AGCM). In one configuration the imposed surface temperatures and radiative perturbations effectively eliminated the Hadley and Walker circulations and the main westerly jet, creating instead a homogeneous “boiling kettle” world in low and midlatitudes. A similarly homogeneous state was created in the 2D model by rapid horizontal mixing. Relative humidity ℛ simulated by the AGCM was insensitive to surface warming. Doubling a parameter governing cloud water reevaporation increased tropical mean ℛ near the midtroposphere by about 4% with a realistic circulation, but by more than 10% in the horizontally homogeneous states. This was consistent in both models. AGCM microphysical sensitivity decreased in the upper troposphere, and vanished outside the Tropics. Convective organization by the general circulation evidently makes relative humidity much more robust to microphysical details by concentrating the rainfall in moist environments. Models that fail to capture this will overestimate the microphysical sensitivity of humidity. Based on these results, the uncertainty in the strength of the water vapor feedback associated with cloud microphysical processes seems unlikely to exceed a few percent. This does not include uncertainties associated with large-scale dynamics or cloud radiative effects, which cannot be quantified, although radical CAM3 circulation changes reported here had surprisingly little impact on simulated relative humidity.
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Ling, Jian, and Chidong Zhang. "Diabatic Heating Profiles in Recent Global Reanalyses." Journal of Climate 26, no. 10 (May 8, 2013): 3307–25. http://dx.doi.org/10.1175/jcli-d-12-00384.1.
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Abstract Diabatic heating profiles are extremely important to the atmospheric circulation in the tropics and therefore to the earth’s energy and hydrological cycles. However, their global structures are poorly known because of limited information from in situ observations. Some modern global reanalyses provide the temperature tendency from the physical processes. Their proper applications require an assessment of their accuracy and uncertainties. In this study, diabatic heating profiles from three recent global reanalyses [ECMWF Interim Re-Analysis (ERA-Interim), Climate Forecast System Reanalysis (CFSR), and Modern Era Retrospective Analysis for Research and Applications (MERRA)] are compared to those derived from currently available sounding observations in the tropics and to each other in the absence of the observations. Diabatic heating profiles produced by the reanalyses match well with those based on sounding observations only at some locations. The three reanalyses agree with each other better in the extratropics, where large-scale condensation dominates the precipitation process in data assimilation models, than in the tropics, where cumulus parameterization dominates. In the tropics, they only agree with each other in gross features, such as the contrast between the ITCZs over different oceans. Their largest disagreement is the number and level of heating peaks in the tropics. They may produce a single, double, or triple heating peak at a given location. It is argued that cumulus parameterization cannot be the sole source of the disagreement. Implications of such disagreement are discussed.
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Xiang, Baoqiang, Ming Zhao, Yi Ming, Weidong Yu, and Sarah M. Kang. "Contrasting Impacts of Radiative Forcing in the Southern Ocean versus Southern Tropics on ITCZ Position and Energy Transport in One GFDL Climate Model." Journal of Climate 31, no. 14 (June 19, 2018): 5609–28. http://dx.doi.org/10.1175/jcli-d-17-0566.1.
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Abstract Most current climate models suffer from pronounced cloud and radiation biases in the Southern Ocean (SO) and in the tropics. Using one GFDL climate model, this study investigates the migration of the intertropical convergence zone (ITCZ) with prescribed top-of-the-atmosphere (TOA) shortwave radiative heating in the SO (50°–80°S) versus the southern tropics (ST; 0°–20°S). Results demonstrate that the ITCZ position response to the ST forcing is twice as strong as the SO forcing, which is primarily driven by the contrasting sea surface temperature (SST) gradient over the tropics; however, the mechanism for the formation of the SST pattern remains elusive. Energy budget analysis reveals that the conventional energetic constraint framework is inadequate in explaining the ITCZ shift in these two perturbed experiments. For both cases, the anomalous Hadley circulation does not contribute to transport the imposed energy from the Southern Hemisphere to the Northern Hemisphere, given a positive mean gross moist stability in the equatorial region. Changes in the cross-equatorial atmospheric energy are primarily transported by atmospheric transient eddies when the anomalous ITCZ shift is most pronounced during December–May. The partitioning of energy transport between the atmosphere and ocean shows latitudinal dependence: the atmosphere and ocean play an overall equivalent role in transporting the imposed energy for the extratropical SO forcing, while for the ST forcing, the imposed energy is nearly completely transported by the atmosphere. This contrast originates from the different ocean heat uptake and also the different meridional scale of the anomalous ocean circulation.
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Bretherton, Christopher S. "Insights into low-latitude cloud feedbacks from high-resolution models." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 373, no. 2054 (November 13, 2015): 20140415. http://dx.doi.org/10.1098/rsta.2014.0415.
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Cloud feedbacks are a leading source of uncertainty in the climate sensitivity simulated by global climate models (GCMs). Low-latitude boundary-layer and cumulus cloud regimes are particularly problematic, because they are sustained by tight interactions between clouds and unresolved turbulent circulations. Turbulence-resolving models better simulate such cloud regimes and support the GCM consensus that they contribute to positive global cloud feedbacks. Large-eddy simulations using sub-100 m grid spacings over small computational domains elucidate marine boundary-layer cloud response to greenhouse warming. Four observationally supported mechanisms contribute: ‘thermodynamic’ cloudiness reduction from warming of the atmosphere–ocean column, ‘radiative’ cloudiness reduction from CO 2 - and H 2 O-induced increase in atmospheric emissivity aloft, ‘stability-induced’ cloud increase from increased lower tropospheric stratification, and ‘dynamical’ cloudiness increase from reduced subsidence. The cloudiness reduction mechanisms typically dominate, giving positive shortwave cloud feedback. Cloud-resolving models with horizontal grid spacings of a few kilometres illuminate how cumulonimbus cloud systems affect climate feedbacks. Limited-area simulations and superparameterized GCMs show upward shift and slight reduction of cloud cover in a warmer climate, implying positive cloud feedbacks. A global cloud-resolving model suggests tropical cirrus increases in a warmer climate, producing positive longwave cloud feedback, but results are sensitive to subgrid turbulence and ice microphysics schemes.
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Zhao, Ping, Song Yang, Maoqiu Jian, and Junming Chen. "Relative Controls of Asian–Pacific Summer Climate by Asian Land and Tropical–North Pacific Sea Surface Temperature." Journal of Climate 24, no. 15 (August 1, 2011): 4165–88. http://dx.doi.org/10.1175/2011jcli3915.1.
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Abstract The dominant pattern of summertime tropical and North Pacific sea surface temperature (SST) is characterized by an out-of-phase relationship between the tropics and the extratropics. This pattern, defined as the tropical–North Pacific mode (TNPM) in this study, is closely correlated with the variability of climate over Asia and the Pacific Ocean. A high TNPM index, with positive (negative) SST anomalies over the extratropics (tropics) of the Pacific, is linked to deep negative anomalies of tropospheric temperature over the extratropical Pacific, with shallow positive anomalies in the lower troposphere, and is also linked to deep positive temperature over Asia. It is also found that these anomalies of tropospheric temperature and SST are significantly related to the Asian–Pacific Oscillation (APO), an extratropical zonal–vertical atmospheric pattern connecting Asia and the Pacific. Indeed, when the variability of APO is removed, the above-described climate anomalies weaken significantly. Although the above relationships observed between atmospheric circulation and SST can be captured by general circulation models, sensitivity experiments show that the variations of summertime Asian–Pacific atmospheric circulation may not be mainly forced by the Pacific SST. Instead, the Asian land elevated heating seems to play a more important role in generating the climate anomalies, as shown by model-sensitivity experiments in which changes in topographic height are included. Moreover, the relative importance of Asian land and Pacific SST for the variations of Asian–Pacific climate in summer and winter is compared in this study. In winter the most dominant mode of Pacific SST exerts a stronger impact on the Asian–Pacific climate.
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McCrystall, Michelle R., J. Scott Hosking, Ian P. White, and Amanda C. Maycock. "The Impact of Changes in Tropical Sea Surface Temperatures over 1979–2012 on Northern Hemisphere High-Latitude Climate." Journal of Climate 33, no. 12 (June 15, 2020): 5103–21. http://dx.doi.org/10.1175/jcli-d-19-0456.1.
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AbstractWhile rapid changes in Arctic climate over recent decades are widely documented, the importance of different driving mechanisms is still debated. A previous study proposed a causal connection between recent tropical Pacific sea surface temperature (SST) trends and circulation changes over northern Canada and Greenland (NCG). Here, using the HadGEM3-A model, we perform a suite of sensitivity experiments to investigate the influence of tropical SSTs on winter atmospheric circulation over NCG. The experiments are forced with observed SST changes between an “early” (1979–88) and “late” period (2003–12) and applied across the entire tropics (TropSST), the tropical Pacific (PacSST), and the tropical Atlantic (AtlSST). In contrast to the previous study, all three experiments show a negative 200-hPa eddy geopotential height (Z200) anomaly over NCG in winter, which is similar to the response in AMIP experiments from four other climate models. The positive Z200 NCG anomaly in ERA-Interim between the two periods is inside the bounds of internal variability estimated from bootstrap sampling. The NCG circulation anomaly in the TropSST experiment is associated with a Rossby wave train originating from the tropical Pacific, with an important contribution coming from the tropical Atlantic SSTs connected via an atmospheric bridge through the tropical Pacific. This generates anomalous upper-level convergence and a positive Rossby wave source anomaly near the North Pacific jet exit region. Hence, while a tropics–Arctic teleconnection is evident, its influence on recent Arctic regional climate differs from observed changes and warrants further research.
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Chadwick, Robin, Ian Boutle, and Gill Martin. "Spatial Patterns of Precipitation Change in CMIP5: Why the Rich Do Not Get Richer in the Tropics." Journal of Climate 26, no. 11 (May 31, 2013): 3803–22. http://dx.doi.org/10.1175/jcli-d-12-00543.1.
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Abstract Changes in the patterns of tropical precipitation (P) and circulation are analyzed in Coupled Model Intercomparison Project phase 5 (CMIP5) GCMs under the representative concentration pathway 8.5 (RCP8.5) scenario. A robust weakening of the tropical circulation is seen across models, associated with a divergence feedback that acts to reduce convection most in areas of largest climatological ascent. This is in contrast to the convergence feedback seen in interannual variability of tropical precipitation patterns. The residual pattern of convective mass-flux change is associated with shifts in convergence zones due to mechanisms such as SST gradient change, and this is often locally larger than the weakening due to the divergence feedback. A simple framework is constructed to separate precipitation change into components based on different mechanisms and to relate it directly to circulation change. While the tropical mean increase in precipitation is due to the residual between the positive thermodynamic change due to increased specific humidity and the decreased convective mass flux due to the weakening of the circulation, the spatial patterns of these two components largely cancel each other out. The rich-get-richer mechanism of greatest precipitation increases in ascent regions is almost negated by this cancellation, explaining why the spatial correlation between climatological P and the climate change anomaly ΔP is only 0.2 over the tropics for the CMIP5 multimodel mean. This leaves the spatial pattern of precipitation change to be dominated by the component associated with shifts in convergence zones, both in the multimodel mean and intermodel uncertainty, with the component due to relative humidity change also becoming important over land.
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Lan, Chia-Wei, Yen-Ting Hwang, Rong-You Chien, Agnès Ducharne, and Min-Hui Lo. "Responses of Global Atmospheric Energy Transport to Idealized Groundwater Conditions in a General Circulation Model." Journal of Climate 35, no. 21 (November 2022): 3291–303. http://dx.doi.org/10.1175/jcli-d-20-0753.1.
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Abstract The representation of groundwater dynamics in land surface models and their roles in global precipitation variations has received attention in recent years. Studies have revealed the overall higher soil moisture but rather diverse precipitation changes after incorporating the groundwater component in climate models. However, groundwater effects on large-scale atmospheric energy transport, the fundamental atmospheric variable regulating Earth’s climate, have not been explored thoroughly. In this study, a pair of idealized experiments corresponding to contrast globally fixed water table depths by AMIP-type simulations in the Community Earth System Model was conducted. In the wet (shallow water table) experiments, an increased meridional surface temperature gradient makes the mean meridional energy transports and Hadley circulation stronger than dry (deep water table) experiments over the tropics. Such energy transport changes are primarily attributed to the dynamic contribution (intensified Hadley circulation). The wet experiments make the simulated world be like an aquaplanet simulation with less land–sea temperature contrast and the enhancement (reduction) of mean meridional circulation (stationary eddies) energy transports. Furthermore, the South Asian monsoon circulation in the wet experiment shows a southward shift in the premonsoon season (April–June) and slight weakening in the mature phase (July and August). This study explores the impacts of the soil conditions caused by various water table depths on global energy transport and has further implications for climate model developments and experiment designs.
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Tandon, Neil F., Edwin P. Gerber, Adam H. Sobel, and Lorenzo M. Polvani. "Understanding Hadley Cell Expansion versus Contraction: Insights from Simplified Models and Implications for Recent Observations." Journal of Climate 26, no. 12 (June 15, 2013): 4304–21. http://dx.doi.org/10.1175/jcli-d-12-00598.1.
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Abstract This study seeks a deeper understanding of the causes of Hadley Cell (HC) expansion, as projected under global warming, and HC contraction, as observed under El Niño. Using an idealized general circulation model, the authors show that a thermal forcing applied to a narrow region around the equator produces “El Niño–like” HC contraction, while a forcing with wider meridional extent produces “global warming–like” HC expansion. These circulation responses are sensitive primarily to the thermal forcing’s meridional structure and are less sensitive to its vertical structure. If the thermal forcing is confined to the midlatitudes, the amount of HC expansion is more than three times that of a forcing of comparable amplitude that is spread over the tropics. This finding may be relevant to recently observed trends of rapid tropical widening. The shift of the HC edge is explained using a very simple model in which the transformed Eulerian mean (TEM) circulation acts to diffuse heat meridionally. In this context, the HC edge is defined as the downward maximum of residual vertical velocity in the upper troposphere ; this corresponds well with the conventional Eulerian definition of the HC edge. In response to a positive thermal forcing, there is anomalous diabatic cooling, and hence anomalous TEM descent, on the poleward flank of the thermal forcing. This causes the HC edge () to shift toward the descending anomaly, so that a narrow forcing causes HC contraction and a wide forcing causes HC expansion.
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Han, Zixuan, Qiong Zhang, Qiang Li, Ran Feng, Alan M. Haywood, Julia C. Tindall, Stephen J. Hunter, et al. "Evaluating the large-scale hydrological cycle response within the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) ensemble." Climate of the Past 17, no. 6 (December 8, 2021): 2537–58. http://dx.doi.org/10.5194/cp-17-2537-2021.
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Abstract. The mid-Pliocene (∼3 Ma) is one of the most recent warm periods with high CO2 concentrations in the atmosphere and resulting high temperatures, and it is often cited as an analog for near-term future climate change. Here, we apply a moisture budget analysis to investigate the response of the large-scale hydrological cycle at low latitudes within a 13-model ensemble from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2). The results show that increased atmospheric moisture content within the mid-Pliocene ensemble (due to the thermodynamic effect) results in wetter conditions over the deep tropics, i.e., the Pacific intertropical convergence zone (ITCZ) and the Maritime Continent, and drier conditions over the subtropics. Note that the dynamic effect plays a more important role than the thermodynamic effect in regional precipitation minus evaporation (PmE) changes (i.e., northward ITCZ shift and wetter northern Indian Ocean). The thermodynamic effect is offset to some extent by a dynamic effect involving a northward shift of the Hadley circulation that dries the deep tropics and moistens the subtropics in the Northern Hemisphere (i.e., the subtropical Pacific). From the perspective of Earth's energy budget, the enhanced southward cross-equatorial atmospheric transport (0.22 PW), induced by the hemispheric asymmetries of the atmospheric energy, favors an approximately 1∘ northward shift of the ITCZ. The shift of the ITCZ reorganizes atmospheric circulation, favoring a northward shift of the Hadley circulation. In addition, the Walker circulation consistently shifts westward within PlioMIP2 models, leading to wetter conditions over the northern Indian Ocean. The PlioMIP2 ensemble highlights that an imbalance of interhemispheric atmospheric energy during the mid-Pliocene could have led to changes in the dynamic effect, offsetting the thermodynamic effect and, hence, altering mid-Pliocene hydroclimate.
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Lee, June-Yi, Bin Wang, Kyong-Hwan Seo, Jong-Seong Kug, Yong-Sang Choi, Yu Kosaka, and Kyung-Ja Ha. "Future Change of Northern Hemisphere Summer Tropical–Extratropical Teleconnection in CMIP5 Models*." Journal of Climate 27, no. 10 (May 9, 2014): 3643–64. http://dx.doi.org/10.1175/jcli-d-13-00261.1.
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Abstract Two dominant global-scale teleconnections in the Northern Hemisphere (NH) extratropics during boreal summer season (June–August) have been identified: the western North Pacific–North America (WPNA) and circumglobal teleconnection (CGT) patterns. These teleconnection patterns are of critical importance for the NH summer seasonal climate prediction. Here, how these teleconnections will change under anthropogenic global warming is investigated using representative concentration pathway 4.5 (RCP4.5) experiments by 20 coupled models that participated in phase 5 of the Coupled Model Intercomparison Project (CMIP5). The six best models are selected based on their performance in simulation of the two teleconnection patterns and climatological means and variances of atmospheric circulation, precipitation, and sea surface temperature. The selected models capture the CGT and its relationship with the Indian summer monsoon (ISM) reasonably well. The models can also capture the WPNA circulation pattern but with striking deficiencies in reproducing its associated rainfall anomalies due to poor simulation of the western North Pacific summer monsoon rainfall. The following changes are anticipated in the latter half of twenty-first century under the RCP4.5 scenario: 1) significant weakening of year-to-year variability of the upper-level circulation due to increased atmospheric stability, although the moderate increase in convective heating over the tropics may act to strengthen the variability; 2) intensification of the WPNA pattern and major spectral peaks, particularly over the eastern Pacific–North America and North Atlantic–Europe sectors, which is attributed to the strengthening of its relationship with the preceding mature phase of El Niño–Southern Oscillation (ENSO); and 3) weakening of the CGT due to atmospheric stabilization and decreasing relationship with ISM as well as weakening of the ISM–ENSO relationship.
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Geil, Kerrie L., Yolande L. Serra, and Xubin Zeng. "Assessment of CMIP5 Model Simulations of the North American Monsoon System." Journal of Climate 26, no. 22 (October 29, 2013): 8787–801. http://dx.doi.org/10.1175/jcli-d-13-00044.1.
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Abstract Precipitation, geopotential height, and wind fields from 21 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are examined to determine how well this generation of general circulation models represents the North American monsoon system (NAMS). Results show no improvement since CMIP3 in the magnitude (root-mean-square error and bias) of the mean annual cycle of monthly precipitation over a core monsoon domain, but improvement in the phasing of the seasonal cycle in precipitation is notable. Monsoon onset is early for most models but is clearly visible in daily climatological precipitation, whereas monsoon retreat is highly variable and unclear in daily climatological precipitation. Models that best capture large-scale circulation patterns at a low level usually have realistic representations of the NAMS, but even the best models poorly represent monsoon retreat. Difficulty in reproducing monsoon retreat results from an inaccurate representation of gradients in low-level geopotential height across the larger region, which causes an unrealistic flux of low-level moisture from the tropics into the NAMS region that extends well into the postmonsoon season. Composites of the models with the best and worst representations of the NAMS indicate that adequate representation of the monsoon during the early to midseason can be achieved even with a large-scale circulation pattern bias, as long as the bias is spatially consistent over the larger region influencing monsoon development; in other words, as with monsoon retreat, it is the inaccuracy of the spatial gradients in geopotential height across the larger region that prevents some models from realistic representation of the early and midseason monsoon system.
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Huang, Ping, Dong Chen, and Jun Ying. "Weakening of the Tropical Atmospheric Circulation Response to Local Sea Surface Temperature Anomalies under Global Warming." Journal of Climate 30, no. 20 (September 11, 2017): 8149–58. http://dx.doi.org/10.1175/jcli-d-17-0171.1.
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Abstract In the tropics, the atmospheric circulation response to sea surface temperature (SST) anomalies is a crucial part of the tropical air–sea interaction—the primary process of tropical climate. How it will change under global warming is of great importance to tropical climate change. Here, it is shown that the atmospheric vertical circulation response to local SST anomalies will likely be weakened under global warming using 28 selected models from phase 5 of the Coupled Model Intercomparison Project. The weakening of the circulation response to SST anomalies is closely tied to the increased atmospheric stability under global warming, which increases at the same rate as the circulation response decreases—around 8% for 1 K of tropical-mean SST warming. The spatial pattern of background warming can modify—especially in the equatorial central-eastern Pacific—the spatial distribution of the changes in the circulation response. The atmospheric response to SST anomalies may increase where the local background warming is pronouncedly greater than the tropical mean. The general weakening of the atmospheric circulation response to SST anomalies leads to a decreased circulation response to the structured variability of tropical SST anomalies, such as the El Niño–Southern Oscillation and the Indian Ocean dipole. The decreased circulation response will offset some of the enhancement of the tropical rainfall response to these SST modes as a result of global-warming-induced moisture increase and also implies a decreased amplitude of the tropical air–sea interaction modes.
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Ern, Manfred, Peter Preusse, and Martin Riese. "Intermittency of gravity wave potential energies and absolute momentum fluxes derived from infrared limb sounding satellite observations." Atmospheric Chemistry and Physics 22, no. 22 (November 28, 2022): 15093–133. http://dx.doi.org/10.5194/acp-22-15093-2022.
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Abstract. Atmospheric gravity waves contribute significantly to the driving of the global atmospheric circulation. Because of their small spatial scales, their effect on the circulation is usually parameterized in general circulation models. These parameterizations, however, are strongly simplified. One important but often neglected characteristic of the gravity wave distribution is the fact that gravity wave sources and, thus, the global distribution of gravity waves are both very intermittent. Therefore, time series of global observations of gravity waves are needed to study the distribution, seasonal variation, and strength of this effect. For gravity wave absolute momentum fluxes and potential energies observed by the High-Resolution Dynamics Limb Sounder (HIRDLS) and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) limb sounding satellite instruments, we investigate the global distribution of gravity wave intermittency by deriving probability density functions (PDFs) in different regions as well as global distributions of Gini coefficients. In the stratosphere, we find that intermittency is strongest in mountain wave regions, followed by the polar night jets and by regions of deep convection in the summertime subtropics. Intermittency is weakest in the tropics. A better comparability of intermittency in different years and regions is achieved by normalizing observations by their spatially and temporally varying monthly median distributions. Our results are qualitatively in agreement with previous findings from satellite observations and quantitatively in good agreement with previous findings from superpressure balloons and high-resolution models. Generally, momentum fluxes exhibit stronger intermittency than potential energies, and lognormal distributions are often a reasonable approximation of the PDFs. In the tropics, we find that, for monthly averages, intermittency increases with altitude, which might be a consequence of variations in the atmospheric background and, thus, varying gravity wave propagation conditions. Different from this, in regions of stronger intermittency, particularly in mountain wave regions, we find that intermittency decreases with altitude, which is likely related to the dissipation of large-amplitude gravity waves during their upward propagation.
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Reichler, Thomas, and John O. Roads. "Long-Range Predictability in the Tropics. Part II: 30–60-Day Variability." Journal of Climate 18, no. 5 (March 1, 2005): 634–50. http://dx.doi.org/10.1175/jcli-3295.1.
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Abstract It is suggested that the slow evolution of the tropical Madden–Julian oscillation (MJO) has the potential to improve the predictability of tropical and extratropical circulation systems at lead times beyond 2 weeks. In practice, however, the MJO phenomenon is extremely difficult to predict because of the lack of good observations, problems with ocean forecasts, and well-known model deficiencies. In this study, the potential skill in forecasting tropical intraseasonal variability is investigated by eliminating all those errors. This is accomplished by conducting five ensemble predictability experiments with a complex general circulation model and by verifying them under the perfect model assumption. The experiments are forced with different combinations of initial and boundary conditions to explore their sensitivity to uncertainties in those conditions. When “perfect” initial and boundary conditions are provided, the model produces a realistic climatology and variability as compared to reanalysis, although the spectral peak of the simulated MJO is too broad. The effect of initial conditions is noticeable out to about 40 days. The quality of the boundary conditions is crucial at all lead times. The small but positive correlations at very long lead times are related to intraseasonal variability of tropical sea surface temperatures (SSTs). When model, initial, and boundary conditions are all perfect, the useful forecast skill of intraseasonal variability is about 4 weeks. Predictability is insensitive to the El Niño–Southern Oscillation (ENSO) phenomenon, but it is enhanced during years when the intraseasonal oscillation is more active. The results provide evidence that the MJO must be understood as a coupled system. As a consequence, it is concluded that further progress in the long-range predictability effort may require the use of fully interactive ocean models.
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Zhang, He, Minghua Zhang, and Qing-cun Zeng. "Sensitivity of Simulated Climate to Two Atmospheric Models: Interpretation of Differences between Dry Models and Moist Models." Monthly Weather Review 141, no. 5 (May 1, 2013): 1558–76. http://dx.doi.org/10.1175/mwr-d-11-00367.1.
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Abstract The dynamical core of the Institute of Atmospheric Physics of the Chinese Academy of Sciences Atmospheric General Circulation Model (IAP AGCM) and the Eulerian spectral transform dynamical core of the Community Atmosphere Model, version 3.1 (CAM3.1), developed at the National Center for Atmospheric Research (NCAR) are used to study the sensitivity of simulated climate. The authors report that when the dynamical cores are used with the same CAM3.1 physical parameterizations of comparable resolutions, the model with the IAP dynamical core simulated a colder troposphere than that from the CAM3.1 core, reducing the CAM3.1 warm bias in the tropical and midlatitude troposphere. However, when the two dynamical cores are used in the idealized Held–Suarez tests without moisture physics, the IAP AGCM core simulated a warmer troposphere than that in CAM3.1. The causes of the differences in the full models and in the dry models are then investigated. The authors show that the IAP dynamical core simulated weaker eddies in both the full physics and the dry models than those in the CAM due to different numerical approximations. In the dry IAP model, the weaker eddies cause smaller heat loss from poleward dynamical transport and thus warmer troposphere in the tropics and midlatitudes. When moist physics is included, however, weaker eddies also lead to weaker transport of water vapor and reduction of high clouds in the IAP model, which then causes a colder troposphere due to reduced greenhouse warming of these clouds. These results show how interactive physical processes can change the effect of a dynamical core on climate simulations between two models.
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Pegion, Kathy, and Prashant D. Sardeshmukh. "Prospects for Improving Subseasonal Predictions." Monthly Weather Review 139, no. 11 (November 1, 2011): 3648–66. http://dx.doi.org/10.1175/mwr-d-11-00004.1.
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Abstract Extending atmospheric prediction skill beyond the predictability limit of about 10 days for daily weather rests on the hope that some time-averaged aspects of anomalous circulations remain predictable at longer forecast lead times, both because of the existence of natural low-frequency modes of atmospheric variability and coupling to the ocean with larger thermal inertia. In this paper the week-2 and week-3 forecast skill of two global coupled atmosphere–ocean models recently developed at NASA and NOAA is compared with that of much simpler linear inverse models (LIMs) based on the observed time-lag correlations of atmospheric circulation anomalies in the Northern Hemisphere and outgoing longwave radiation (OLR) anomalies in the tropics. The coupled models are found to beat the LIMs only slightly, and only if an ensemble prediction methodology is employed. To assess the potential for further skill improvement, a predictability analysis based on the relative magnitudes of forecast signal and forecast noise in the LIM framework is conducted. Estimating potential skill by such a method is argued to be superior to using the ensemble-mean and ensemble-spread information in the coupled model ensemble prediction system. The LIM-based predictability analysis yields relatively conservative estimates of the potential skill, and suggests that outside the tropics the average coupled model skill may already be close to the potential skill, although there may still be room for improvement in the tropical forecast skill.
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Kurzke, H., M. V. Kurgansky, K. Dethloff, D. Handorf, S. Erxleben, D. Olbers, C. Eden, and M. Sempf. "Simulating Southern Hemisphere extra-tropical climate variability with an idealised coupled atmosphere-ocean model." Geoscientific Model Development 5, no. 5 (September 19, 2012): 1161–75. http://dx.doi.org/10.5194/gmd-5-1161-2012.
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Abstract. A quasi-geostrophic model of Southern Hemisphere's wintertime atmospheric circulation with horizontal resolution T21 has been coupled to a global ocean circulation model with a resolution of 2° × 2° and simplified physics. This simplified coupled model reproduces qualitatively some features of the first and the second EOF of atmospheric 833 hPa geopotential height in accordance with NCEP data. The variability patterns of the simplified coupled model have been compared with variability patterns simulated by four complex state-of-the-art coupled CMIP5 models. The first EOF of the simplified model is too zonal and does not reproduce the right position of the centre of action over the Pacific Ocean and its extension to the tropics. The agreement in the second EOF between the simplified and the CMIP5 models is better. The total variance of the simplified model is weaker than the observational variance and those of the CMIP5 models. The transport properties of the Southern Ocean circulation are in qualitative accord with observations. The simplified model exhibits skill in reproducing essential features of decadal and multi-decadal climate variability in the extratropical Southern Hemisphere. Notably, 800 yr long coupled model simulations reveal sea surface temperature fluctuations on the timescale of several decades in the Antarctic Circumpolar Current region.
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Ball, William T., Gabriel Chiodo, Marta Abalos, Justin Alsing, and Andrea Stenke. "Inconsistencies between chemistry–climate models and observed lower stratospheric ozone trends since 1998." Atmospheric Chemistry and Physics 20, no. 16 (August 20, 2020): 9737–52. http://dx.doi.org/10.5194/acp-20-9737-2020.
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Abstract. The stratospheric ozone layer shields surface life from harmful ultraviolet radiation. Following the Montreal Protocol ban on long-lived ozone-depleting substances (ODSs), rapid depletion of total column ozone (TCO) ceased in the late 1990s, and ozone above 32 km is now clearly recovering. However, there is still no confirmation of TCO recovery, and evidence has emerged that ongoing quasi-global (60∘ S–60∘ N) lower stratospheric ozone decreases may be responsible, dominated by low latitudes (30∘ S–30∘ N). Chemistry–climate models (CCMs) used to project future changes predict that lower stratospheric ozone will decrease in the tropics by 2100 but not at mid-latitudes (30–60∘). Here, we show that CCMs display an ozone decline similar to that observed in the tropics over 1998–2016, likely driven by an increase in tropical upwelling. On the other hand, mid-latitude lower stratospheric ozone is observed to decrease, while CCMs that specify real-world historical meteorological fields instead show an increase up to present day. However, these cannot be used to simulate future changes; we demonstrate here that free-running CCMs used for projections also show increases. Despite opposing lower stratospheric ozone changes, which should induce opposite temperature trends, CCMs and observed temperature trends agree; we demonstrate that opposing model–observation stratospheric water vapour (SWV) trends, and their associated radiative effects, explain why temperature changes agree in spite of opposing ozone trends. We provide new evidence that the observed mid-latitude trends can be explained by enhanced mixing between the tropics and extratropics. We further show that the temperature trends are consistent with the observed mid-latitude ozone decrease. Together, our results suggest that large-scale circulation changes expected in the future from increased greenhouse gases (GHGs) may now already be underway but that most CCMs do not simulate mid-latitude ozone layer changes well. However, it is important to emphasise that the periods considered here are short, and internal variability that is both intrinsic to each CCM and different to observed historical variability is not well-characterised and can influence trend estimates. Nevertheless, the reason CCMs do not exhibit the observed changes needs to be identified to allow models to be improved in order to build confidence in future projections of the ozone layer.
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Exarchou, Eleftheria, Till Kuhlbrodt, Jonathan M. Gregory, and Robin S. Smith. "Ocean Heat Uptake Processes: A Model Intercomparison." Journal of Climate 28, no. 2 (January 15, 2015): 887–908. http://dx.doi.org/10.1175/jcli-d-14-00235.1.
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Abstract The quasi-equilibrium heat balances, as well as the responses to 4 × CO2 perturbation, are compared among three global climate models with the aim to identify and explain intermodel differences in ocean heat uptake (OHU) processes. It is found that, in quasi equilibrium, convective and mixed layer processes, as well as eddy-related processes, cause cooling of the subsurface ocean. The cooling is balanced by warming caused by advective and diapycnally diffusive processes. It is also found that in the CO2-perturbed climates the largest contribution to OHU comes from changes in vertical mixing processes and the mean circulation, particularly in the extratropics, caused both by changes in wind forcing and by changes in high-latitude buoyancy forcing. There is a substantial warming in the tropics: a significant part of which occurs because of changes in horizontal advection in extratropics. Diapycnal diffusion makes only a weak contribution to the OHU, mainly in the tropics, because of increased stratification. There are important qualitative differences in the contribution of eddy-induced advection and isopycnal diffusion to the OHU among the models. The former is related to the different values of the coefficients used in the corresponding scheme. The latter is related to the different tapering formulations of the isopycnal diffusion scheme. These differences affect the OHU in the deep ocean, which is substantial in two of the models, with the dominant region of deep warming being the Southern Ocean. However, most of the OHU takes place above 2000 m, and the three models are quantitatively similar in their global OHU efficiency and its breakdown among processes and as a function of latitude.
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Dietmüller, Simone, Hella Garny, Roland Eichinger, and William T. Ball. "Analysis of recent lower-stratospheric ozone trends in chemistry climate models." Atmospheric Chemistry and Physics 21, no. 9 (May 5, 2021): 6811–37. http://dx.doi.org/10.5194/acp-21-6811-2021.
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Abstract. Recent observations show a significant decrease in lower-stratospheric (LS) ozone concentrations in tropical and mid-latitude regions since 1998. By analysing 31 chemistry climate model (CCM) simulations performed for the Chemistry Climate Model Initiative (CCMI; Morgenstern et al., 2017), we find a large spread in the 1998–2018 trend patterns between different CCMs and between different realizations performed with the same CCM. The latter in particular indicates that natural variability strongly influences LS ozone trends. However none of the model simulations reproduce the observed ozone trend structure of coherent negative trends in the LS. In contrast to the observations, most models show an LS trend pattern with negative trends in the tropics (20∘ S–20∘ N) and positive trends in the northern mid-latitudes (30–50∘ N) or vice versa. To investigate the influence of natural variability on recent LS ozone trends, we analyse the sensitivity of observational trends and the models' trend probability distributions for varying periods with start dates from 1995 to 2001 and end dates from 2013 to 2019. Generally, modelled and observed LS trends remain robust for these different periods; however observational data show a change towards weaker mid-latitude trends for certain periods, likely forced by natural variability. Moreover we show that in the tropics the observed trends agree well with the models' trend distribution, whereas in the mid-latitudes the observational trend is typically an extreme value of the models' distribution. We further investigate the LS ozone trends for extended periods reaching into the future and find that all models develop a positive ozone trend at mid-latitudes, and the trends converge to constant values by the period that spans 1998–2060. Inter-model correlations between ozone trends and transport-circulation trends confirm the dominant role of greenhouse gas (GHG)-driven tropical upwelling enhancement on the tropical LS ozone decrease. Mid-latitude ozone, on the other hand, appears to be influenced by multiple competing factors: an enhancement in the shallow branch decreases ozone, while an enhancement in the deep branch increases ozone, and, furthermore, mixing plays a role here too. Sensitivity simulations with fixed forcing of GHGs or ozone-depleting substances (ODSs) reveal that the GHG-driven increase in circulation strength does not lead to a net trend in LS mid-latitude column ozone. Rather, the positive ozone trends simulated consistently in the models in this region emerge from the decline in ODSs, i.e. the ozone recovery. Therefore, we hypothesize that next to the influence of natural variability, the disagreement of modelled and observed LS mid-latitude ozone trends could indicate a mismatch in the relative role of the response of ozone to ODS versus GHG forcing in the models.
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Cash, Benjamin A., Xavier Rodó, and James L. Kinter. "Links between Tropical Pacific SST and Cholera Incidence in Bangladesh: Role of the Eastern and Central Tropical Pacific." Journal of Climate 21, no. 18 (September 15, 2008): 4647–63. http://dx.doi.org/10.1175/2007jcli2001.1.
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Abstract Recent studies arising from both statistical analysis and dynamical disease models indicate that there is a link between incidence of cholera, a paradigmatic waterborne bacterial disease (WBD) endemic to Bangladesh, and the El Niño–Southern Oscillation (ENSO). However, a physical mechanism explaining this relationship has not yet been established. A regionally coupled, or “pacemaker,” configuration of the Center for Ocean–Land–Atmosphere Studies atmospheric general circulation model is used to investigate links between sea surface temperature in the central and eastern tropical Pacific and the regional climate of Bangladesh. It is found that enhanced precipitation tends to follow winter El Niño events in both the model and observations, providing a plausible physical mechanism by which ENSO could influence cholera in Bangladesh. The enhanced precipitation in the model arises from a modification of the summer monsoon circulation over India and Bangladesh. Westerly wind anomalies over land to the west of Bangladesh lead to increased convergence in the zonal wind field and hence increased moisture convergence and rainfall. This change in circulation results from the tropics-wide warming in the model following a winter El Niño event. These results suggest that improved forecasting of cholera incidence may be possible through the use of climate predictions.
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Weber, Nicholas J., and Clifford F. Mass. "Subseasonal Weather Prediction in a Global Convection-Permitting Model." Bulletin of the American Meteorological Society 100, no. 6 (June 2019): 1079–89. http://dx.doi.org/10.1175/bams-d-18-0210.1.
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AbstractAlthough accurate weather and climate prediction beyond one to two weeks is of great value to society, the skill of such extended prediction is limited in current operational global numerical models, whose coarse horizontal grid spacing necessitates the parameterization of atmospheric processes. Of particular concern is the parameterization of convection and specifically convection in the tropics, which impacts global weather at all time scales through atmospheric teleconnections. Convection-permitting models, which forego convective parameterization by explicitly resolving cumulus-scale motions using fine (1–4 km) horizontal grid spacing, can improve global prediction at extended time scales by more faithfully simulating tropical convection and associated teleconnections. This study demonstrates that convection-permitting resolution in a global numerical model can improve both the statistical features of tropical precipitation and extended predictive skill in the tropics and midlatitudes. Comparing four monthlong global simulations with 3-km grid spacing to coarser-resolution simulations that parameterize convection reveals that convection-permitting simulations improve tropical precipitation rates and the diurnal cycle of tropical convection. The propagation of the Madden–Julian oscillation was better predicted in three of the four 3-km simulations; these three runs also featured more skillful prediction of weekly extratropical circulation anomalies, particularly during week 3 of each forecast. These results, though based on a small sample of four cases, demonstrate that convection-permitting global modeling can benefit extended atmospheric prediction and offers the potential for improved operational subseasonal forecast skill.
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Philbin, R., and M. Jun. "Bivariate spatial analysis of temperature and precipitation from general circulation models and observation proxies." Advances in Statistical Climatology, Meteorology and Oceanography 1, no. 1 (May 22, 2015): 29–44. http://dx.doi.org/10.5194/ascmo-1-29-2015.
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Abstract. This study validates the near-surface temperature and precipitation output from decadal runs of eight atmospheric ocean general circulation models (AOGCMs) against observational proxy data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis temperatures and Global Precipitation Climatology Project (GPCP) precipitation data. We model the joint distribution of these two fields with a parsimonious bivariate Matérn spatial covariance model, accounting for the two fields' spatial cross-correlation as well as their own smoothnesses. We fit output from each AOGCM (30-year seasonal averages from 1981 to 2010) to a statistical model on each of 21 land regions. Both variance and smoothness values agree for both fields over all latitude bands except southern mid-latitudes. Our results imply that temperature fields have smaller smoothness coefficients than precipitation fields, while both have decreasing smoothness coefficients with increasing latitude. Models predict fields with smaller smoothness coefficients than observational proxy data for the tropics. The estimated spatial cross-correlations of these two fields, however, are quite different for most GCMs in mid-latitudes. Model correlation estimates agree well with those for observational proxy data for Australia, at high northern latitudes across North America, Europe and Asia, as well as across the Sahara, India, and Southeast Asia, but elsewhere, little consistent agreement exists.
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Ji, Xuan, J. David Neelin, and C. Roberto Mechoso. "El Niño–Southern Oscillation Sea Level Pressure Anomalies in the Western Pacific: Why Are They There?*." Journal of Climate 28, no. 22 (November 15, 2015): 8860–72. http://dx.doi.org/10.1175/jcli-d-14-00716.1.
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Abstract Although sea level pressure (SLP) anomalies in the western Pacific have long been recognized as an integral part of the classic Southern Oscillation pattern associated with El Niño–Southern Oscillation (ENSO), there is an unresolved question regarding the dynamics that maintain these anomalies. Traditional studies of the ENSO response in the tropics assume a single deep baroclinic mode associated with the tropospheric temperature anomalies. However, the SLP anomalies in the western Pacific are spatially separated from the baroclinic signal in the NCEP–NCAR reanalysis, CMIP5 models, and an intermediate complexity model [a quasi-equilibrium tropical circulation model (QTCM)]. Separation of ENSO SLP anomalies in the tropical Pacific into baroclinic and barotropic components indicates that the barotropic component contributes throughout the tropics and constitutes the primary contribution in the western Pacific. To demonstrate the roles of baroclinic and barotropic modes in ENSO teleconnections within the tropics, a series of QTCM experiments is performed, where anomalies in the interactions between baroclinic and barotropic modes are suppressed over increasingly wider latitudinal bands in the tropical Pacific. If this suppression is done in the 15°N–15°S band, the pressure signals in the western Pacific are only partly removed, whereas if it is done in the 30°N–30°S band, the anomalies in the western Pacific are almost entirely removed. This suggests the following pathway: interactions with SST anomalies create the baroclinic response in the central and eastern Pacific, but baroclinic–barotropic interactions, arising substantially in the subtropical Pacific, generate a barotropic response that yields the SLP anomalies in the western Pacific.
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Popp, Max, and Levi G. Silvers. "Double and Single ITCZs with and without Clouds." Journal of Climate 30, no. 22 (November 2017): 9147–66. http://dx.doi.org/10.1175/jcli-d-17-0062.1.
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A major bias in tropical precipitation over the Pacific in climate simulations stems from the models’ tendency to produce two strong distinct intertropical convergence zones (ITCZs) too often. Several mechanisms have been proposed that may contribute to the emergence of two ITCZs, but current theories cannot fully explain the bias. This problem is tackled by investigating how the interaction between atmospheric cloud-radiative effects (ACREs) and the large-scale circulation influences the ITCZ position in an atmospheric general circulation model. Simulations are performed in an idealized aquaplanet setup and the longwave and shortwave ACREs are turned off individually or jointly. The low-level moist static energy (MSE) is shown to be a good predictor of the ITCZ position. Therefore, a mechanism is proposed that explains the changes in MSE and thus ITCZ position due to ACREs consistently across simulations. The mechanism implies that the ITCZ moves equatorward if the Hadley circulation strengthens because of the increased upgradient advection of low-level MSE off the equator. The longwave ACRE increases the meridional heating gradient in the tropics and as a response the Hadley circulation strengthens and the ITCZ moves equatorward. The shortwave ACRE has the opposite effect. The total ACRE pulls the ITCZ equatorward. This mechanism is discussed in other frameworks involving convective available potential energy, gross moist stability, and the energy flux equator. It is thus shown that the response of the large-scale circulation to the shortwave and longwave ACREs is a fundamental driver of changes in the ITCZ position.
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Ma, Jian, Shang-Ping Xie, and Yu Kosaka. "Mechanisms for Tropical Tropospheric Circulation Change in Response to Global Warming*." Journal of Climate 25, no. 8 (April 10, 2012): 2979–94. http://dx.doi.org/10.1175/jcli-d-11-00048.1.
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Abstract The annual-mean tropospheric circulation change in global warming is studied by comparing the response of an atmospheric general circulation model (GCM) to a spatial-uniform sea surface temperature (SST) increase (SUSI) with the response of a coupled ocean–atmosphere GCM to increased greenhouse gas concentrations following the A1B scenario. In both simulations, tropospheric warming follows the moist adiabat in the tropics, and static stability increases globally in response to SST warming. A diagnostic framework is developed based on a linear baroclinic model (LBM) of the atmosphere. The mean advection of stratification change (MASC) by climatological vertical motion, often neglected in interannual variability, is an important thermodynamic term for global warming. Once MASC effect is included, LBM shows skills in reproducing GCM results by prescribing latent heating diagnosed from the GCMs. MASC acts to slow down the tropical circulation. This is most clear in the SUSI run where the Walker circulation slows down over the Pacific without any change in SST gradient. MASC is used to decelerate the Hadley circulation, but spatial patterns of SST warming play an important role. Specifically, the SST warming is greater in the Northern than Southern Hemisphere, an interhemispheric asymmetry that decelerates the Hadley cell north, but accelerates it south of the equator. The MASC and SST-pattern effects are on the same order of magnitude in our LBM simulations. The former is presumably comparable across GCMs, while SST warming patterns show variations among models in both shape and magnitude. Uncertainties in SST patterns account for intermodel variability in Hadley circulation response to global warming (especially on and south of the equator).
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Kang, Sarah M., Isaac M. Held, Dargan M. W. Frierson, and Ming Zhao. "The Response of the ITCZ to Extratropical Thermal Forcing: Idealized Slab-Ocean Experiments with a GCM." Journal of Climate 21, no. 14 (July 15, 2008): 3521–32. http://dx.doi.org/10.1175/2007jcli2146.1.
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Abstract Using a comprehensive atmospheric GCM coupled to a slab mixed layer ocean, experiments are performed to study the mechanism by which displacements of the intertropical convergence zone (ITCZ) are forced from the extratropics. The northern extratropics are cooled and the southern extratropics are warmed by an imposed cross-equatorial flux beneath the mixed layer, forcing a southward shift in the ITCZ. The ITCZ displacement can be understood in terms of the degree of compensation between the imposed oceanic flux and the resulting response in the atmospheric energy transport in the tropics. The magnitude of the ITCZ displacement is very sensitive to a parameter in the convection scheme that limits the entrainment into convective plumes. The change in the convection scheme affects the extratropical–tropical interactions in the model primarily by modifying the cloud response. The results raise the possibility that the response of tropical precipitation to extratropical thermal forcing, important for a variety of problems in climate dynamics (such as the response of the tropics to the Northern Hemisphere ice sheets during glacial maxima or to variations in the Atlantic meridional overturning circulation), may be strongly dependent on cloud feedback. The model configuration described here is suggested as a useful benchmark helping to quantify extratropical–tropical interactions in atmospheric models.