Journal articles on the topic 'Atmospheric circulation Southern Hemisphere Mathematical models'
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Braesicke, P., J. Keeble, X. Yang, G. Stiller, S. Kellmann, N. L. Abraham, A. Archibald, P. Telford, and J. A. Pyle. "Circulation anomalies in the Southern Hemisphere and ozone changes." Atmospheric Chemistry and Physics 13, no. 21 (November 4, 2013): 10677–88. http://dx.doi.org/10.5194/acp-13-10677-2013.
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Abstract. We report results from two pairs of chemistry-climate model simulations using the same climate model but different chemical perturbations. In each pair of experiments an ozone change was triggered by a simple change in the chemistry. One pair of model experiments looked at the impact of polar stratospheric clouds (PSCs) and the other pair at the impact of short-lived halogenated species on composition and circulation. The model response is complex with both positive and negative changes in ozone concentration, depending on location. These changes result from coupling between composition, temperature and circulation. Even though the causes of the modelled ozone changes are different, the high latitude Southern Hemisphere response in the lower stratosphere is similar. In both pairs of experiments the high-latitude circulation changes, as evidenced by N2O differences, are suggesting a slightly longer-lasting/stronger stratospheric descent in runs with higher ozone destruction (a manifestation of a seasonal shift in the circulation). We contrast the idealised model behaviour with interannual variability in ozone and N2O as observed by the MIPAS instrument on ENVISAT, highlighting similarities of the modelled climate equilibrium changes to the year 2006–2007 in observations. We conclude that the climate system can respond quite sensitively in its seasonal evolution to small chemical perturbations, that circulation adjustments seen in the model can occur in reality, and that coupled chemistry-climate models allow a better assessment of future ozone and climate change than recent CMIP-type models with prescribed ozone fields.
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Grainger, S., C. S. Frederiksen, and X. Zheng. "Interannual modes of variability of Southern Hemisphere atmospheric circulation in CMIP3 models." IOP Conference Series: Earth and Environmental Science 11 (August 1, 2010): 012027. http://dx.doi.org/10.1088/1755-1315/11/1/012027.
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Barnes, Elizabeth A., Susan Solomon, and Lorenzo M. Polvani. "Robust Wind and Precipitation Responses to the Mount Pinatubo Eruption, as Simulated in the CMIP5 Models." Journal of Climate 29, no. 13 (June 14, 2016): 4763–78. http://dx.doi.org/10.1175/jcli-d-15-0658.1.
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Abstract The volcanic eruption of Mount Pinatubo in June 1991 is the largest terrestrial eruption since the beginning of the satellite era. Here, the monthly evolution of atmospheric temperature, zonal winds, and precipitation following the eruption in 14 CMIP5 models is analyzed and strong and robust stratospheric and tropospheric circulation responses are demonstrated in both hemispheres, with tropospheric anomalies maximizing in November 1991. The simulated Southern Hemisphere circulation response projects strongly onto the positive phase of the southern annular mode (SAM), while the Northern Hemisphere exhibits robust North Atlantic and North Pacific responses that differ significantly from that of the typical northern annular mode (NAM) pattern. In contrast, observations show a negative SAM following the eruption, and internal variability must be considered along with forced responses. Indeed, evidence is presented that the observed El Niño climate state during and after this eruption may oppose the eruption-forced positive SAM response, based on the El Niño–Southern Oscillation (ENSO) state and SAM response across the models. The results demonstrate that Pinatubo-like eruptions should be expected to force circulation anomalies across the globe and highlight that great care must be taken in diagnosing the forced response as it may not fall into typical seasonal averages or be guaranteed to project onto typical climate modes.
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Grainger, Simon, Carsten S. Frederiksen, and Xiaogu Zheng. "Assessment of Modes of Interannual Variability of Southern Hemisphere Atmospheric Circulation in CMIP5 Models." Journal of Climate 27, no. 21 (October 24, 2014): 8107–25. http://dx.doi.org/10.1175/jcli-d-14-00251.1.
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Abstract An assessment is made of the modes of interannual variability in the seasonal mean summer and winter Southern Hemisphere (SH) 500-hPa geopotential height in the twentieth century in models from the Coupled Model Intercomparison Project (CMIP) phase 5 (CMIP5) dataset. Modes of variability of both the slow (signal) and intraseasonal (noise) components in the CMIP5 models are evaluated against those estimated from reanalysis data. There is general improvement in the leading modes of the slow (signal) component in CMIP5 models compared with the CMIP phase 3 (CMIP3) dataset. The largest improvement is in the spatial structures of the modes related to El Niño–Southern Oscillation variability in SH summer. An overall score metric is significantly higher for CMIP5 over CMIP3 in both seasons. The leading modes in the intraseasonal noise component are generally well reproduced in CMIP5 models, and there are few differences from CMIP3. A new total overall score metric is used to rank the CMIP5 models over both seasons. Weighting the seasons by the relative spread of overall scores is shown to be suitable for generating multimodel ensembles for further analysis of interannual variability. In multimodel ensembles, it is found that an ensemble of size 5 or 6 is sufficient in SH summer to reproduce well the dominant modes. In contrast, about 13 models are typically are required in SH winter. It is shown that it is necessary that the selected models individually reproduce well the leading modes of the slow component.
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Totz, Sonja, Alexey V. Eliseev, Stefan Petri, Michael Flechsig, Levke Caesar, Vladimir Petoukhov, and Dim Coumou. "The dynamical core of the Aeolus 1.0 statistical–dynamical atmosphere model: validation and parameter optimization." Geoscientific Model Development 11, no. 2 (February 22, 2018): 665–79. http://dx.doi.org/10.5194/gmd-11-665-2018.
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Abstract. We present and validate a set of equations for representing the atmosphere's large-scale general circulation in an Earth system model of intermediate complexity (EMIC). These dynamical equations have been implemented in Aeolus 1.0, which is a statistical–dynamical atmosphere model (SDAM) and includes radiative transfer and cloud modules (Coumou et al., 2011; Eliseev et al., 2013). The statistical dynamical approach is computationally efficient and thus enables us to perform climate simulations at multimillennia timescales, which is a prime aim of our model development. Further, this computational efficiency enables us to scan large and high-dimensional parameter space to tune the model parameters, e.g., for sensitivity studies. Here, we present novel equations for the large-scale zonal-mean wind as well as those for planetary waves. Together with synoptic parameterization (as presented by Coumou et al., 2011), these form the mathematical description of the dynamical core of Aeolus 1.0. We optimize the dynamical core parameter values by tuning all relevant dynamical fields to ERA-Interim reanalysis data (1983–2009) forcing the dynamical core with prescribed surface temperature, surface humidity and cumulus cloud fraction. We test the model's performance in reproducing the seasonal cycle and the influence of the El Niño–Southern Oscillation (ENSO). We use a simulated annealing optimization algorithm, which approximates the global minimum of a high-dimensional function. With non-tuned parameter values, the model performs reasonably in terms of its representation of zonal-mean circulation, planetary waves and storm tracks. The simulated annealing optimization improves in particular the model's representation of the Northern Hemisphere jet stream and storm tracks as well as the Hadley circulation. The regions of high azonal wind velocities (planetary waves) are accurately captured for all validation experiments. The zonal-mean zonal wind and the integrated lower troposphere mass flux show good results in particular in the Northern Hemisphere. In the Southern Hemisphere, the model tends to produce too-weak zonal-mean zonal winds and a too-narrow Hadley circulation. We discuss possible reasons for these model biases as well as planned future model improvements and applications.
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Lin, Pu, Qiang Fu, and Dennis L. Hartmann. "Impact of Tropical SST on Stratospheric Planetary Waves in the Southern Hemisphere." Journal of Climate 25, no. 14 (July 15, 2012): 5030–46. http://dx.doi.org/10.1175/jcli-d-11-00378.1.
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Abstract The impact of tropical sea surface temperature (SST) on stratospheric planetary waves in the Southern Hemisphere (SH) is investigated in austral spring using observed SST and reanalysis data for the past three decades. Maximum covariance analysis indicates that the tropical SST and the SH stratospheric planetary wave activity are primarily coupled through two modes. The leading two modes show the La Niña–like and the central-Pacific El Niño–like SST anomalies in their positive polarities, respectively, which each are related to enhanced stratospheric planetary wave activity. These two modes also introduce phase shifts to the stratospheric stationary planetary waves: a westward shift is seen for La Niña and an eastward shift for warm SST anomalies is seen in the central Pacific. The Eliassen–Palm fluxes associated with the two modes indicate that the anomalous stratospheric wave activity originates in the troposphere and propagates upward over the mid–high latitudes, so that the linkages between tropical SST and extratropical tropospheric circulation appear to play a key role. Furthermore, the observed circulation anomaly patterns for the two modes change rapidly from spring to summer, consistent with a sharp seasonal transition in the SH basic state. Similar SST and circulation anomaly patterns associated with the two modes are simulated in chemistry–climate models.
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Ivanciu, Ioana, Katja Matthes, Sebastian Wahl, Jan Harlaß, and Arne Biastoch. "Effects of prescribed CMIP6 ozone on simulating the Southern Hemisphere atmospheric circulation response to ozone depletion." Atmospheric Chemistry and Physics 21, no. 8 (April 19, 2021): 5777–806. http://dx.doi.org/10.5194/acp-21-5777-2021.
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Abstract. The Antarctic ozone hole has led to substantial changes in the Southern Hemisphere atmospheric circulation, such as the strengthening and poleward shift of the midlatitude westerly jet. Ozone recovery during the twenty-first century is expected to continue to affect the jet's strength and position, leading to changes in the opposite direction compared to the twentieth century and competing with the effect of increasing greenhouse gases. Simulations of the Earth's past and future climate, such as those performed for the Coupled Model Intercomparison Project Phase 6 (CMIP6), require an accurate representation of these ozone effects. Climate models that use prescribed ozone fields lack the important feedbacks between ozone chemistry, radiative heating, dynamics, and transport. In addition, when the prescribed ozone field was not generated by the same model to which it is prescribed, the imposed ozone hole is inconsistent with the simulated dynamics. These limitations ultimately affect the climate response to ozone depletion. This study investigates the impact of prescribing the ozone field recommended for CMIP6 on the simulated effects of ozone depletion in the Southern Hemisphere. We employ a new state-of-the-art coupled climate model, Flexible Ocean Climate Infrastructure (FOCI), to compare simulations in which the CMIP6 ozone is prescribed with simulations in which the ozone chemistry is calculated interactively. At the same time, we compare the roles played by ozone depletion and by increasing concentrations of greenhouse gases in driving changes in the Southern Hemisphere atmospheric circulation using a series of historical sensitivity simulations. FOCI captures the known effects of ozone depletion, simulating an austral spring and summer intensification of the midlatitude westerly winds and of the Brewer–Dobson circulation in the Southern Hemisphere. Ozone depletion is the primary driver of these historical circulation changes in FOCI. The austral spring cooling of the polar cap in the lower stratosphere in response to ozone depletion is weaker in the simulations that prescribe the CMIP6 ozone field. We attribute this weaker response to a prescribed ozone hole that is different to the model dynamics and is not collocated with the simulated polar vortex, altering the strength and position of the planetary wavenumber one. As a result, the dynamical contribution to the ozone-induced austral spring lower-stratospheric cooling is suppressed, leading to a weaker cooling trend. Consequently, the intensification of the polar night jet is also weaker in the simulations with prescribed CMIP6 ozone. In contrast, the differences in the tropospheric westerly jet response to ozone depletion fall within the internal variability present in the model. The persistence of the Southern Annular Mode is shorter in the prescribed ozone chemistry simulations. The results obtained with the FOCI model suggest that climate models that prescribe the CMIP6 ozone field still simulate a weaker Southern Hemisphere stratospheric response to ozone depletion compared to models that calculate the ozone chemistry interactively.
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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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Cai, Wenju, and Tim Cowan. "Trends in Southern Hemisphere Circulation in IPCC AR4 Models over 1950–99: Ozone Depletion versus Greenhouse Forcing." Journal of Climate 20, no. 4 (February 15, 2007): 681–93. http://dx.doi.org/10.1175/jcli4028.1.
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Abstract Simulations by the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) models on the Southern Hemisphere (SH) circulation are assessed over the period 1950–99, focusing on the seasonality of the trend and the level of its congruency with the southern annular mode (SAM) in terms of surface zonal wind stress. It is found that, as a group, the models realistically produce the seasonality of the trend, which is strongest in the SH summer season, December–February (DJF). The modeled DJF trend is principally congruent with the modeled SAM trend, as in observations. The majority of models produce a statistically significant positive trend, with decreasing westerlies in the midlatitudes and increasing westerlies in the high latitudes. The trend pattern from an all-experiment mean achieves highest correlation with that from the National Centers for Environmental Prediction (NCEP) data. A total of 48 out of the 71 experiments were run with ozone-depletion forcing, which offers an opportunity to assess the importance of ozone depletion in driving the late-twentieth-century trends. The AR4 model ensemble that contains an ozone-depletion forcing produces an averaged trend that is comparable to the trend from the NCEP outputs corrected by station-based observations. The trend is largely generated after the mid-1970s. Without ozone depletion the trend is less than half of that in the corrected NCEP, although the errors in the observed trend are large. The impact on oceanic circulation is inferred from wind stress curl in the group with ozone-depletion forcing. The result shows an intensification of the southern midlatitude supergyre circulation, including a strengthening East Australian Current flowing through the Tasman Sea. Thus, ozone depletion also plays an important role in the subtropical gyre circulation change over the past decades.
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Sen Gupta, Alexander, Agus Santoso, Andréa S. Taschetto, Caroline C. Ummenhofer, Jessica Trevena, and Matthew H. England. "Projected Changes to the Southern Hemisphere Ocean and Sea Ice in the IPCC AR4 Climate Models." Journal of Climate 22, no. 11 (June 1, 2009): 3047–78. http://dx.doi.org/10.1175/2008jcli2827.1.
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Abstract Fidelity and projected changes in the climate models, used for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), are assessed with regard to the Southern Hemisphere extratropical ocean and sea ice systems. While individual models span different physical parameterizations and resolutions, a major component of intermodel variability results from surface wind differences. Projected changes to the surface wind field are also central in modifying future extratropical circulation and internal properties. A robust southward shift of the circumpolar current and subtropical gyres is projected, with a strong spinup of the Atlantic gyre. An associated increase in the core strength of the circumpolar circulation is evident; however, this does not translate into robust increases in Drake Passage transport. While an overarching oceanic warming is projected, the circulation-driven poleward shift of the temperature field explains much of the midlatitude warming pattern. The effect of this shift is less clear for salinity, where, instead, surface freshwater forcing dominates. Surface warming and high-latitude freshwater increases drive intensified stratification, and a shoaling and southward shift of the deep mixed layers. Despite large intermodel differences, there is also a robust weakening in bottom water formation and its northward outflow. At the same time the wind intensification invigorates the upwelling of deep water, transporting warm, salty water southward and upward, with major implications for sequestration and outgassing of CO2. A robust decrease is projected for both the sea ice concentration and the seasonal cycling of ice volume, potentially altering the salt and heat budget at high latitudes.
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Naud, Catherine M., Derek J. Posselt, and Susan C. van den Heever. "Observational Analysis of Cloud and Precipitation in Midlatitude Cyclones: Northern versus Southern Hemisphere Warm Fronts." Journal of Climate 25, no. 14 (July 15, 2012): 5135–51. http://dx.doi.org/10.1175/jcli-d-11-00569.1.
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Abstract Extratropical cyclones are responsible for most of the precipitation and wind damage in the midlatitudes during the cold season, but there are still uncertainties on how they will change in a warming climate. A ubiquitous problem among general circulation models (GCMs) is a lack of cloudiness over the southern oceans that may be in part caused by a lack of clouds in cyclones. This study analyzes CloudSat, Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), and Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) observations for three austral and boreal cold seasons, and composites cloud frequency of occurrence and precipitation at the warm fronts for Northern and Southern Hemisphere oceanic cyclones. The authors find that cloud frequency of occurrence and precipitation rate are similar in the early stage of the cyclone life cycle in both the Northern and Southern Hemispheres. As cyclones evolve and reach their mature stage, cloudiness and precipitation at the warm front increase in the Northern Hemisphere but decrease in the Southern Hemisphere. This is partly caused by lower amounts of precipitable water being available to Southern Hemisphere cyclones, and smaller increases in wind speed as the cyclones evolve. Southern Hemisphere cloud occurrence at the warm front is found to be more sensitive to the amount of moisture in the warm sector than to wind speeds. This suggests that cloudiness in Southern Hemisphere storms may be more susceptible to changes in atmospheric water vapor content, and thus to changes in surface temperature than their Northern Hemisphere counterparts. These differences between Northern and Southern Hemisphere cyclones are statistically robust, indicating A-Train-based analyses as useful tools for the evaluation of GCMs in the next Intergovernmental Panel on Climate Change (IPCC) report.
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Ackerley, D., and J. A. Renwick. "The Southern Hemisphere semiannual oscillation and circulation variability during the Mid-Holocene." Climate of the Past 6, no. 4 (July 5, 2010): 415–30. http://dx.doi.org/10.5194/cp-6-415-2010.
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Abstract. The Paleoclimate Modelling Intercomparison Project (PMIP) was undertaken to assess the climatic effects of the presence of large ice-sheets and changes in the Earth's orbital parameters in fully coupled Atmosphere-Ocean General Circulation Models (AOGCMs). Much of the previous literature has focussed on the tropics and the Northern Hemisphere during the last glacial maximum and Mid-Holocene whereas this study focuses only on the Southern Hemisphere. This study addresses the representation of the Semiannual Oscillation (SAO) in the PMIP2 models and how it may have changed during the Mid-Holocene. The output from the five models suggest a weakening of the (austral) autumn circumpolar trough (CPT) and (in all but one model) a strengthening of the spring CPT. The effects of changing the orbital parameters are to cause warming and drying during spring over New Zealand and a cooling and moistening during autumn. The amount of spring warming/drying and autumn cooling/moistening is variable between the models and depends on the climatological locations of surface pressure anomalies associated with changes in the SAO. This study also undertakes an Empirical Orthogonal Function (EOF) analysis of the leading modes of atmospheric variability during the control and Mid-Holocene phases for each model. Despite the seasonal changes, the overall month by month and interannual variability was simulated to have changed little from the Mid-Holocene to present.
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Ackerley, D., and J. A. Renwick. "The Southern Hemisphere semiannual oscillation and circulation variability during the Mid-Holocene." Climate of the Past Discussions 6, no. 1 (February 22, 2010): 185–224. http://dx.doi.org/10.5194/cpd-6-185-2010.
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Abstract. The Paleoclimate Modelling Intercomparison Project (PMIP) was undertaken to assess the climatic effects of the presence of large ice-sheets and changes in the Earth's orbital parameters in fully coupled Atmosphere-Ocean General Circulation Models (AOGCMs). Much of the previous literature has focussed on the tropics and the Northern Hemisphere during the last glacial maximum and Mid-Holocene whereas this study focuses only on the Southern Hemisphere. This study addresses the representation of the Semiannual Oscillation (SAO) in the PMIP2 models and how it may have changed during the Mid-Holocene. The output from the models suggest a weakening of the (austral) autumn circumpolar trough (CPT) and (in all but one model) a strengthening of the spring CPT. The effects of changing the orbital parameters are to cause warming and drying during spring over New Zealand and a cooling and moistening during autumn. The amount of spring warming/drying and autumn cooling/moistening is variable between the models and depends on the climatological locations of surface pressure anomalies associated with changes in the SAO. This study also undertakes an Empirical Orthogonal Function (EOF) analysis of the leading modes of atmospheric variability during the control and Mid-Holocene phases for each model. Despite the seasonal changes, the overall month by month and interannual variability was simulated to have changed little from the Mid-Holocene to present.
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Goyal, Rishav, Martin Jucker, Alex Sen Gupta, and Matthew H. England. "A New Zonal Wave-3 Index for the Southern Hemisphere." Journal of Climate 35, no. 15 (August 1, 2022): 5137–49. http://dx.doi.org/10.1175/jcli-d-21-0927.1.
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Abstract Zonal wave 3 (ZW3) is an important feature of the Southern Hemisphere extratropical atmospheric circulation and has strong impacts on meridional heat and momentum transport, regional Antarctic sea ice extent, and Southern Hemisphere blocking events. Attempts have been made in the past to define an index that quantifies the variability in the ZW3 pattern; however, existing methods are based on fixed geographical locations and fail to capture certain ZW3 events because of strong variability in phase. In addition, a fixed spatial index poorly characterizes ZW3 in CMIP models, which can exhibit biases in the mean phase of the ZW3 pattern. In this study, we introduce a new way to characterize ZW3 variability by incorporating two indices, one each for magnitude and phase, based on the combination of the first two empirical orthogonal functions (EOFs) of the 500-hPa meridional wind anomalies. We show that the new ZW3 index provides a clear advantage over past indices because it captures a substantially higher proportion of variance (∼40% compared to ∼16%), and it can be used for both reanalysis datasets and coupled climate models regardless of model biases. A composite analysis associated with the new index reveals a strong relationship between the ZW3 defined by our index and sea ice fraction around Antarctica, with significant regional sea ice anomalies during strong ZW3 events with different phases.
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Karpechko, A. Yu, N. P. Gillett, B. Hassler, K. H. Rosenlof, and E. Rozanov. "Quantitative assessment of Southern Hemisphere ozone in chemistry-climate model simulations." Atmospheric Chemistry and Physics 10, no. 3 (February 8, 2010): 1385–400. http://dx.doi.org/10.5194/acp-10-1385-2010.
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Abstract. Stratospheric ozone recovery in the Southern Hemisphere is expected to drive pronounced trends in atmospheric temperature and circulation from the stratosphere to the troposphere in the 21st century; therefore ozone changes need to be accounted for in future climate simulations. Many climate models do not have interactive ozone chemistry and rely on prescribed ozone fields, which may be obtained from coupled chemistry-climate model (CCM) simulations. However CCMs vary widely in their predictions of ozone evolution, complicating the selection of ozone boundary conditions for future climate simulations. In order to assess which models might be expected to better simulate future ozone evolution, and thus provide more realistic ozone boundary conditions, we assess the ability of twelve CCMs to simulate observed ozone climatology and trends and rank the models according to their errors averaged across the individual diagnostics chosen. According to our analysis no one model performs better than the others in all the diagnostics; however, combining errors in individual diagnostics into one metric of model performance allows us to objectively rank the models. The multi-model average shows better overall agreement with the observations than any individual model. Based on this analysis we conclude that the multi-model average ozone projection presents the best estimate of future ozone evolution and recommend it for use as a boundary condition in future climate simulations. Our results also demonstrate a sensitivity of the analysis to the choice of reference data set for vertical ozone distribution over the Antarctic, highlighting the constraints that large observational uncertainty imposes on such model verification.
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Grainger, Simon, Carsten S. Frederiksen, and Xiaogu Zheng. "Modes of interannual variability of Southern Hemisphere atmospheric circulation in CMIP3 models: assessment and projections." Climate Dynamics 41, no. 2 (January 23, 2013): 479–500. http://dx.doi.org/10.1007/s00382-012-1659-7.
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Wood, T., A. C. Maycock, P. M. Forster, T. B. Richardson, T. Andrews, O. Boucher, G. Myhre, et al. "The Southern Hemisphere Midlatitude Circulation Response to Rapid Adjustments and Sea Surface Temperature Driven Feedbacks." Journal of Climate 33, no. 22 (November 15, 2020): 9673–90. http://dx.doi.org/10.1175/jcli-d-19-1015.1.
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AbstractRapid adjustments—the response of meteorology to external forcing while sea surface temperatures (SST) and sea ice are held fixed—can affect the midlatitude circulation and contribute to long-term forced circulation responses in climate simulations. This study examines rapid adjustments in the Southern Hemisphere (SH) circulation using nine models from the Precipitation Driver and Response Model Intercomparison Project (PDRMIP), which perform fixed SST and coupled ocean experiments for five perturbations: a doubling of carbon dioxide (2xCO2), a tripling of methane (3xCH4), a fivefold increase in sulfate aerosol (5xSO4), a tenfold increase in black carbon aerosol (10xBC), and a 2% increase in solar constant (2%Sol). In the coupled experiments, the SH eddy-driven jet shifts poleward and strengthens for forcings that produce global warming (and vice versa for 5xSO4), with the strongest response found in austral summer. In austral winter, the responses project more strongly onto a change in jet strength. For 10xBC, which induces strong shortwave absorption, the multimodel mean (MMM) rapid adjustment in DJF jet latitude is ~75% of the change in the coupled simulations. For the other forcings, which induce larger SST changes, the effect of SST-mediated feedbacks on the SH circulation is larger than the rapid adjustment. Nevertheless, for these perturbations the magnitude of the MMM jet shift due to the rapid adjustment is still around 20%–30% of that in the coupled experiments. The results demonstrate the need to understand the mechanisms for rapid adjustments in the midlatitude circulation, in addition to the effect of changing SSTs.
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Lin, Pu, Qiang Fu, Susan Solomon, and John M. Wallace. "Temperature Trend Patterns in Southern Hemisphere High Latitudes: Novel Indicators of Stratospheric Change." Journal of Climate 22, no. 23 (December 1, 2009): 6325–41. http://dx.doi.org/10.1175/2009jcli2971.1.
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Abstract Robust stratospheric temperature trend patterns are suggested in the winter and spring seasons in the Southern Hemisphere high latitudes from the satellite-borne Microwave Sounding Unit (MSU) measurement for 1979–2007. These patterns serve as indicators of key processes governing temperature and ozone changes in the Antarctic. The observed patterns are characterized by cooling and warming regions of comparable magnitudes, with the strongest local trends occurring in September and October. In September, ozone depletion induces radiative cooling, and strengthening of the Brewer–Dobson circulation (BDC) induces dynamical warming. Because the trends induced by these two processes are centered in different locations in September, they do not cancel each other, but rather produce a wavelike structure. In contrast, during October, the ozone-induced radiative cooling and the BDC-induced warming exhibit a more zonally symmetric structure than in September, and hence largely cancel each other. However, the October quasi-stationary planetary wavenumber 1 has shifted eastward from 1979 to 2007, producing a zonal wavenumber-1 trend structure, which dominates the observed temperature trend pattern. Simulated temperature changes for 1979–2007 from coupled atmosphere–ocean general circulation model (AOGCM) experiments run for the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) are compared with the observations. In general, the simulated temperature changes are dominated by zonally symmetric ozone-induced radiative cooling. The models fail to simulate the warming in the southern polar stratosphere, implying a lack of the BDC strengthening in these models. They also fail to simulate the quasi-stationary planetary wave changes observed in October and November.
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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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Karpechko, Alexey Yu, Nathan P. Gillett, Mauro Dall'Amico, and Lesley J. Gray. "Southern Hemisphere atmospheric circulation response to the El Chichón and Pinatubo eruptions in coupled climate models." Quarterly Journal of the Royal Meteorological Society 136, no. 652 (October 2010): 1813–22. http://dx.doi.org/10.1002/qj.683.
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Garfinkel, Chaim I., Ian White, Edwin P. Gerber, and Martin Jucker. "The Impact of SST Biases in the Tropical East Pacific and Agulhas Current Region on Atmospheric Stationary Waves in the Southern Hemisphere." Journal of Climate 33, no. 21 (November 1, 2020): 9351–74. http://dx.doi.org/10.1175/jcli-d-20-0195.1.
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AbstractClimate models in phase 5 of the Coupled Model Intercomparison Project (CMIP5) vary significantly in their ability to simulate the phase and amplitude of atmospheric stationary waves in the midlatitude Southern Hemisphere. These models also suffer from a double intertropical convergence zone (ITCZ), with excessive precipitation in the tropical eastern South Pacific, and many also suffer from a biased simulation of the dynamics of the Agulhas Current around the tip of South Africa. The intermodel spread in the strength and phasing of SH midlatitude stationary waves in the CMIP archive is shown to be significantly correlated with the double-ITCZ bias and biases in the Agulhas Return Current. An idealized general circulation model (GCM) is used to demonstrate the causality of these links by prescribing an oceanic heat flux out of the tropical east Pacific and near the Agulhas Current. A warm bias in tropical east Pacific SSTs associated with an erroneous double ITCZ leads to a biased representation of midlatitude stationary waves in the austral hemisphere, capturing the response evident in CMIP models. Similarly, an overly diffuse sea surface temperature gradient associated with a weak Agulhas Return Current leads to an equatorward shift of the Southern Hemisphere jet by more than 3° and weak stationary wave activity in the austral hemisphere. Hence, rectification of the double-ITCZ bias and a better representation of the Agulhas Current should be expected to lead to an improved model representation of the austral hemisphere.
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Lyu, Kewei, Xuebin Zhang, John A. Church, and Quran Wu. "Processes Responsible for the Southern Hemisphere Ocean Heat Uptake and Redistribution under Anthropogenic Warming." Journal of Climate 33, no. 9 (May 1, 2020): 3787–807. http://dx.doi.org/10.1175/jcli-d-19-0478.1.
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AbstractThe Southern Hemisphere oceans absorb most of the excess heat stored in the climate system due to anthropogenic warming. By analyzing future climate projections from a large ensemble of the CMIP5 models under a high emission scenario (RCP8.5), we investigate how the atmospheric forcing and ocean circulation determine heat uptake and redistribution in the Southern Hemisphere oceans. About two-thirds of the net surface heat gain in the high-latitude Southern Ocean is redistributed northward, leading to enhanced and deep-reaching warming at middle latitudes near the boundary between the subtropical gyres and the Antarctic Circumpolar Current. The projected magnitudes of the ocean warming are closely related to the magnitudes of the wind and gyre boundary poleward shifts across the models. For those models with the simulated gyre boundary biased equatorward, the latitude where the projected ocean warming peaks is also located farther equatorward and a larger poleward shift of the gyre boundary is projected. In a theoretical framework, the subsurface ocean changes are explored using three distinctive processes on the temperature–salinity diagram: pure heave, pure warming, and pure freshening. The enhanced middle-latitude warming and the deepening of isopycnals are attributed to the pure heave and pure warming processes, likely related to the wind-driven heat convergence and the accumulation of extra surface heat uptake by the background ocean circulation, respectively. The equatorward and downward subductions of the surface heat and freshwater input at high latitudes (i.e., pure warming and pure freshening processes) result in cooling and freshening spiciness changes on density surfaces within the Subantarctic Mode Water and Antarctic Intermediate Water.
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Liu, Xiaojuan, David S. Battisti, and Aaron Donohoe. "Tropical Precipitation and Cross-Equatorial Ocean Heat Transport during the Mid-Holocene." Journal of Climate 30, no. 10 (April 24, 2017): 3529–47. http://dx.doi.org/10.1175/jcli-d-16-0502.1.
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Abstract Summertime insolation intensified in the Northern Hemisphere during the mid-Holocene, resulting in enhanced monsoonal precipitation. In this study, the authors examine the changes in the annual-mean tropical precipitation as well as changes in atmospheric circulation and upper-ocean circulation in the mid-Holocene compared to the preindustrial climate, as simulated by 12 coupled climate models from PMIP3. In addition to the predominant zonally asymmetric changes in tropical precipitation, there is a small northward shift in the location of intense zonal-mean precipitation (mean ITCZ) in the mid-Holocene in the majority (9 out of 12) of the coupled climate models. In contrast, the shift is southward in simulations using an atmospheric model coupled to a slab ocean. The northward mean ITCZ shift in the coupled simulations is due to enhanced northward ocean heat transport across the equator [OHT(EQ)], which demands a compensating southward atmospheric energy transport across the equator, accomplished by shifting the Hadley cell and hence the mean ITCZ northward. The increased northward OHT(EQ) is primarily accomplished by changes in the upper-ocean gyre circulation in the tropical Pacific acting on the zonally asymmetric climatological temperature distribution. The gyre intensification results from the intensification of the monsoonal winds in the Northern Hemisphere and the weakening of the winds in the Southern Hemisphere, both of which are forced directly by the insolation changes.
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Hua, Quan, and Mike Barbetti. "Review of Tropospheric Bomb 14C Data for Carbon Cycle Modeling and Age Calibration Purposes." Radiocarbon 46, no. 3 (2004): 1273–98. http://dx.doi.org/10.1017/s0033822200033142.
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Comprehensive published radiocarbon data from selected atmospheric records, tree rings, and recent organic matter were analyzed and grouped into 4 different zones (three for the Northern Hemisphere and one for the whole Southern Hemisphere). These 14C data for the summer season of each hemisphere were employed to construct zonal, hemispheric, and global data sets for use in regional and global carbon model calculations including calibrating and comparing carbon cycle models. In addition, extended monthly atmospheric 14C data sets for 4 different zones were compiled for age calibration purposes. This is the first time these data sets were constructed to facilitate the dating of recent organic material using the bomb 14C curves. The distribution of bomb 14C reflects the major zones of atmospheric circulation.
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Lipat, Bernard R., Aiko Voigt, George Tselioudis, and Lorenzo M. Polvani. "Model Uncertainty in Cloud–Circulation Coupling, and Cloud-Radiative Response to Increasing CO2, Linked to Biases in Climatological Circulation." Journal of Climate 31, no. 24 (December 2018): 10013–20. http://dx.doi.org/10.1175/jcli-d-17-0665.1.
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Recent analyses of global climate models suggest that uncertainty in the coupling between midlatitude clouds and the atmospheric circulation contributes to uncertainty in climate sensitivity. However, the reasons behind model differences in the cloud–circulation coupling have remained unclear. Here, we use a global climate model in an idealized aquaplanet setup to show that the Southern Hemisphere climatological circulation, which in many models is biased equatorward, contributes to the model differences in the cloud–circulation coupling. For the same poleward shift of the Hadley cell (HC) edge, models with narrower climatological HCs exhibit stronger midlatitude cloud-induced shortwave warming than models with wider climatological HCs. This cloud-induced radiative warming results predominantly from a subsidence warming that decreases cloud fraction and is stronger for narrower HCs because of a larger meridional gradient in the vertical velocity. A comparison of our aquaplanet results with comprehensive climate models suggests that about half of the model uncertainty in the midlatitude cloud–circulation coupling stems from this impact of the circulation on the large-scale temperature structure of the atmosphere, and thus could be removed by improving the climatological circulation in models. This illustrates how understanding of large-scale dynamics can help reduce uncertainty in clouds and their response to climate change.
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Garny, H., G. E. Bodeker, D. Smale, M. Dameris, and V. Grewe. "Drivers of hemispheric differences in return dates of mid-latitude stratospheric ozone to historical levels." Atmospheric Chemistry and Physics 13, no. 15 (August 1, 2013): 7279–300. http://dx.doi.org/10.5194/acp-13-7279-2013.
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Abstract. Chemistry-climate models (CCMs) project an earlier return of northern mid-latitude total column ozone to 1980 values compared to the southern mid-latitudes. The chemical and dynamical drivers of this hemispheric difference are investigated in this study. The hemispheric asymmetry in return dates is a robust result across different CCMs and is qualitatively independent of the method used to estimate return dates. However, the differences in dates of return to 1980 levels between the southern and northern mid-latitudes can vary between 0 and 30 yr across the range of CCM projections analyzed. Positive linear trends in ozone lead to an earlier return of ozone than expected from the return of Cly to 1980 levels. This forward shift is stronger in the Northern than in the Southern Hemisphere because (i) trends have a larger effect on return dates if the sensitivity of ozone to Cly is lower and (ii) the trends in the Northern Hemisphere are stronger than in the Southern Hemisphere. An attribution analysis performed with two CCMs shows that chemically-induced changes in ozone are the major driver of the earlier return of ozone to 1980 levels in northern mid-latitudes; therefore transport changes are of minor importance. This conclusion is supported by the fact that the spread in the simulated hemispheric difference in return dates across an ensemble of twelve models is only weakly related to the spread in the simulated hemispheric asymmetry of trends in the strength of the Brewer–Dobson circulation. The causes for chemically-induced asymmetric ozone trends relevant for the total column ozone return date differences are found to be (i) stronger increases in ozone production due to enhanced NOx concentrations in the Northern Hemisphere lowermost stratosphere and troposphere, (ii) stronger decreases in the destruction rates of ozone by the NOx cycle in the Northern Hemisphere lower stratosphere linked to effects of dynamics and temperature on NOx concentrations, and (iii) an increasing efficiency of heterogeneous ozone destruction by Cly in the Southern Hemisphere mid-latitudes as a~result of decreasing lower stratospheric temperatures.
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Grise, Kevin M., and Sean M. Davis. "Hadley cell expansion in CMIP6 models." Atmospheric Chemistry and Physics 20, no. 9 (May 6, 2020): 5249–68. http://dx.doi.org/10.5194/acp-20-5249-2020.
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Abstract. In response to increasing greenhouse gases, the subtropical edges of Earth's Hadley circulation shift poleward in global climate models. Recent studies have found that reanalysis trends in the Hadley cell edge over the past 30–40 years are within the range of trends simulated by Coupled Model Intercomparison Project Phase 5 (CMIP5) models and have documented seasonal and hemispheric asymmetries in these trends. In this study, we evaluate whether these conclusions hold for the newest generation of models (CMIP6). Overall, we find similar characteristics of Hadley cell expansion in CMIP5 and CMIP6 models. In both CMIP5 and CMIP6 models, the poleward shift of the Hadley cell edge in response to increasing greenhouse gases is 2–3 times larger in the Southern Hemisphere (SH), except during September–November. The trends from CMIP5 and CMIP6 models agree well with reanalyses, although prescribing observed coupled atmosphere–ocean variability allows the models to better capture reanalysis trends in the Northern Hemisphere (NH). We find two notable differences between CMIP5 and CMIP6 models. First, while both CMIP5 and CMIP6 models contract the NH summertime Hadley circulation equatorward (particularly over the Pacific sector), this contraction is larger in CMIP6 models due to their higher average climate sensitivity. Second, in recent decades, the poleward shift of the NH annual-mean Hadley cell edge is slightly larger in CMIP6 models. Increasing greenhouse gases drive similar trends in CMIP5 and CMIP6 models, so the larger recent NH trends in CMIP6 models point to the role of other forcings, such as aerosols.
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Li, Shuanglin, Judith Perlwitz, Martin P. Hoerling, and Xiaoting Chen. "Opposite Annular Responses of the Northern and Southern Hemispheres to Indian Ocean Warming." Journal of Climate 23, no. 13 (July 1, 2010): 3720–38. http://dx.doi.org/10.1175/2010jcli3410.1.
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Abstract Atmospheric circulation changes during boreal winter of the second half of the twentieth century exhibit a trend toward the positive polarity of both the Northern Hemisphere annular mode (NAM) and the Southern Hemisphere annular mode (SAM). This has occurred in concert with other trends in the climate system, most notably a warming of the Indian Ocean. This study explores whether the tropical Indian Ocean warming played a role in forcing these annular trends. Five different atmospheric general circulation models (AGCMs) are forced with an idealized, transient warming of Indian Ocean sea surface temperature anomalies (SSTA); the results of this indicate that the warming contributed to the annular trend in the NH but offset the annular trend in SH. The latter result implies that the Indian Ocean warming may have partly cancelled the influence of the stratospheric ozone depletion over the southern polar area, which itself forced a trend toward the positive phase of the SAM. Diagnosis of the physical mechanisms for the annular responses indicates that the direct impact of the diabatic heating induced by the Indian Ocean warming does not account for the annular response in the extratropics. Instead, interactions between the forced stationary wave anomalies and transient eddies is key for the formation of annular structures.
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Grise, Kevin M., and Lorenzo M. Polvani. "Southern Hemisphere Cloud–Dynamics Biases in CMIP5 Models and Their Implications for Climate Projections." Journal of Climate 27, no. 15 (July 29, 2014): 6074–92. http://dx.doi.org/10.1175/jcli-d-14-00113.1.
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Abstract This study quantifies cloud–radiative anomalies associated with interannual variability in the latitude of the Southern Hemisphere (SH) midlatitude eddy-driven jet, in 20 global climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). Two distinct model types are found. In the first class of models (type I models), total cloud fraction is reduced at SH midlatitudes as the jet moves poleward, contributing to enhanced shortwave radiative warming. In the second class of models (type II models), this dynamically induced cloud radiative warming effect is largely absent. Type I and type II models have distinct deficiencies in their representation of observed Southern Ocean clouds, but comparison with two independent satellite datasets indicates that the cloud–dynamics behavior of type II models is more realistic. Because the SH midlatitude jet shifts poleward in response to CO2 forcing, the cloud–dynamics biases uncovered from interannual variability are directly relevant for climate change projections. In CMIP5 model experiments with abruptly quadrupled atmospheric CO2 concentrations, the global-mean surface temperature initially warms more in type I models, even though their equilibrium climate sensitivity is not significantly larger. In type I models, this larger initial warming is linked to the rapid adjustment of the circulation and clouds to CO2 forcing in the SH, where a nearly instantaneous poleward shift of the midlatitude jet is accompanied by a reduction in the reflection of solar radiation by clouds. In type II models, the SH jet also shifts rapidly poleward with CO2 quadrupling, but it is not accompanied by cloud radiative warming anomalies, resulting in a smaller initial global-mean surface temperature warming.
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Hansen, Anthony R., Alfonso Sutera, and Joseph J. Tribbia. "The Relation of Multiple Flow Regimes to the Climatic Error in General Circulation Models: Southern Hemisphere Winter." Journal of the Atmospheric Sciences 48, no. 11 (June 1991): 1329–35. http://dx.doi.org/10.1175/1520-0469(1991)048<1329:tromfr>2.0.co;2.
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Raphael, Marilyn N., and Marika M. Holland. "Twentieth century simulation of the southern hemisphere climate in coupled models. Part 1: large scale circulation variability." Climate Dynamics 26, no. 2-3 (November 23, 2005): 217–28. http://dx.doi.org/10.1007/s00382-005-0082-8.
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Kurzke, H., M. V. Kurgansky, K. Dethloff, D. Handorf, D. Olbers, C. Eden, and M. Sempf. "Simulating Southern Hemisphere extra-tropical climate variability with an idealized coupled atmosphere-ocean model." Geoscientific Model Development Discussions 4, no. 3 (August 16, 2011): 1907–40. http://dx.doi.org/10.5194/gmdd-4-1907-2011.
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Abstract. The design and implementation of a simplified coupled atmosphere-ocean model over mid and high Southern Hemisphere latitudes are described. The development of the model is motivated by the clear indications of important low-frequency variability of extratropical origin in atmosphere-only models and the crucial role of atmosphere-ocean interaction in altering and shaping the climate variability on decadal and multidecadal time-scales. The basic model consists of an idealized quasi-geostrophic model of Southern Hemisphere's wintertime atmospheric circulation coupled to a general ocean circulation model with simplified physics. Model spin-up is described, some basic descriptors of the model climatology are discussed, and it is argued that the model exhibits skill in reproducing essential features of decadal and multi-decadal climate variability in the extratropical Southern Hemisphere. Notably, 1000 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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Osman, Marisol, and Carolina S. Vera. "Predictability of Extratropical Upper-Tropospheric Circulation in the Southern Hemisphere by Its Main Modes of Variability." Journal of Climate 33, no. 4 (February 15, 2020): 1405–21. http://dx.doi.org/10.1175/jcli-d-19-0122.1.
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AbstractThe predictability and forecast skill of the models participating in the Climate Historical Forecast Project (CHFP) database is assessed through evaluating the representation of the upper-tropospheric extratropical circulation in the Southern Hemisphere (SH) in winter and summer and its main modes of variability. In summer, the predictability of 200-hPa geopotential height anomalies mainly comes from the ability of the multimodel ensemble mean (MMEM) to forecast the first three modes of interannual variability with high fidelity. The MMEM can reproduce not only the spatial patterns of these modes but also their temporal evolution. On the other hand, in JJA only the second and fourth modes of variability are predictable by the MMEM. These seasonal differences in the performance of the MMEM seem to be related to the role that the sea surface temperature (SST) anomalies have in influencing the variability of each mode. Accordingly, modes that are strongly linked to tropical SST anomalies are better forecast by the MMEM and show less spread among models. The analysis of both 2-m temperature and precipitation anomalies in the SH associated with the predictable modes reveals that DJF predictable modes are accompanied by significant temperature anomalies. In particular, temperatures at polar (tropical) latitudes are significantly correlated with the first (second) mode. Furthermore, these links obtained with observations are also well forecast by the MMEM and can help to improve seasonal forecast of climate anomalies in those regions with low skill.
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Rashid, Harun A., and Ian Simmonds. "Southern Hemisphere Annular Mode Variability and the Role of Optimal Nonmodal Growth." Journal of the Atmospheric Sciences 62, no. 6 (June 1, 2005): 1947–61. http://dx.doi.org/10.1175/jas3444.1.
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Abstract The southern annular mode is the leading mode of Southern Hemisphere circulation variability, the temporal evolution of which is characterized by large amplitudes and significant persistence. Previous investigators have suggested a positive feedback mechanism that explains some of this low-frequency variance. Here, a mechanism is proposed, involving transient nonmodal growths of the anomalies, that is at least as effective as the positive feedback mechanism in increasing the low-frequency variance of the southern annular mode. Using the vector autoregressive modeling technique, a number of linear inverse models of southern annular mode variability from National Centers for Environmental Prediction–Department of Energy (NCEP–DOE) Reanalysis 2 is derived. These models are then analyzed applying the ideas of the generalized stability theory. It is found that, as a consequence of the nonnormality of the system matrices, a significant increase in the low-frequency variance of the southern annular mode occurs through optimal nonmodal growth of the zonal wind anomalies. The nonnormality arises mainly from the relative dominance of the eddy forcing, while the nonmodal growth is caused by the interference of the nonorthogonal eigenvectors of the nonnormal system matrix. These results are demonstrated first in a simple model that retains only the two leading modes of the zonally averaged zonal wind and eddy-forcing variability, and then in a more general model that includes all the important modes. Using the more general model the authors have determined, among other things, the optimal initial perturbation and the time scale over which it experiences the maximum nonmodal growth to evolve into the pattern associated with the southern annular mode.
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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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Parsons, Simon, James A. Renwick, and Adrian J. McDonald. "An Assessment of Future Southern Hemisphere Blocking Using CMIP5 Projections from Four GCMs." Journal of Climate 29, no. 21 (October 5, 2016): 7599–611. http://dx.doi.org/10.1175/jcli-d-15-0754.1.
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AbstractThis study is concerned with blocking events (BEs) in the Southern Hemisphere (SH), their past variability, and future projections. ERA-Interim (ERA-I) is used to compare the historical output from four general circulation models (GCMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5); the output of the representative concentration pathway 4.5 and 8.5 (RCP4.5 and RCP8.5) projections are also examined. ERA-I shows that the higher latitudes of the South Pacific Ocean (SPO) are the main blocking region, with blocking occurring predominantly in winter. The CMIP5 historical simulations also agree well with ERA-I for annual and seasonal BE locations and frequencies. A reduction in BEs is observed in the SPO in the 2071–2100 period in the RCP4.5 projections, and this is more pronounced for the RCP8.5 projections and occurs predominantly during the spring and summer seasons. Preliminary investigations imply that the southern annular mode (SAM) is negatively correlated with blocking activity in the SPO in all seasons in the reanalysis. This negative correlation is also observed in the GCM historical output. However, in the RCP projections this correlation is reduced in three of the four models during summer, suggesting that SAM may be less influential in summertime blocking in the future.
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Simpson, Isla R., Peter Hitchcock, Richard Seager, Yutian Wu, and Patrick Callaghan. "The Downward Influence of Uncertainty in the Northern Hemisphere Stratospheric Polar Vortex Response to Climate Change." Journal of Climate 31, no. 16 (July 13, 2018): 6371–91. http://dx.doi.org/10.1175/jcli-d-18-0041.1.
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Abstract General circulation models display a wide range of future predicted changes in the Northern Hemisphere winter stratospheric polar vortex. The downward influence of this stratospheric uncertainty on the troposphere has previously been inferred from regression analyses across models and is thought to contribute to model spread in tropospheric circulation change. Here we complement such regression analyses with idealized experiments using one model where different changes in the zonal-mean stratospheric polar vortex are artificially imposed to mimic the extreme ends of polar vortex change simulated by models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). The influence of the stratospheric vortex change on the tropospheric circulation in these experiments is quantitatively in agreement with the inferred downward influence from across-model regressions, indicating that such regressions depict a true downward influence of stratospheric vortex change on the troposphere below. With a relative weakening of the polar vortex comes a relative increase in Arctic sea level pressure (SLP), a decrease in zonal wind over the North Atlantic, drying over northern Europe, and wetting over southern Europe. The contribution of stratospheric vortex change to intermodel spread in these quantities is assessed in the CMIP5 models. The spread, as given by 4 times the across-model standard deviation, is reduced by roughly 10% on regressing out the contribution from stratospheric vortex change, while the difference between models on extreme ends of the distribution in terms of their stratospheric vortex change can reach up to 50% of the overall model spread for Arctic SLP and 20% of the overall spread in European precipitation.
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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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Thompson, Andrew F., Sophia K. Hines, and Jess F. Adkins. "A Southern Ocean Mechanism for the Interhemispheric Coupling and Phasing of the Bipolar Seesaw." Journal of Climate 32, no. 14 (June 21, 2019): 4347–65. http://dx.doi.org/10.1175/jcli-d-18-0621.1.
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Abstract The last glacial period is punctuated by abrupt changes in Northern Hemisphere temperatures that are known as Dansgaard–Oeschger (DO) events. A striking and largely unexplained feature of DO events is an interhemispheric asymmetry characterized by cooling in Antarctica during periods of warming in Greenland and vice versa—the bipolar seesaw. Methane-synchronized ice core records indicate that the Southern Hemisphere lags the Northern Hemisphere by approximately 200 years. Here, we propose a mechanism that produces observed features of both the bipolar seesaw and the phasing of DO events. The spatial pattern of sea ice formation and melt in the Southern Ocean imposes a rigid constraint on where water masses are modified: waters are made denser near the coast where ice forms and waters are made lighter farther north where ice melts. This pattern, coupled to the tilt of density surfaces across the Southern Ocean and the stratification of the ocean basins, produces two modes of overturning corresponding to different bipolar seesaw states. We present evolution equations for a simplified ocean model that describes the transient adjustment of the basin stratification, the Southern Ocean surface density distribution, and the overturning strength as the ocean moves between these states in response to perturbations in North Atlantic Deep Water formation, which we take as a proxy for Greenland temperatures. Transitions between different overturning states occur over a multicentennial time scale, which is qualitatively consistent with the observed Southern Hemisphere lag. The volume of deep density layers varies inversely with the overturning strength, leading to significant changes in residence times. Evidence of these dynamics in more realistic circulation models is discussed.
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Byrne, Nicholas J., Theodore G. Shepherd, Tim Woollings, and R. Alan Plumb. "Nonstationarity in Southern Hemisphere Climate Variability Associated with the Seasonal Breakdown of the Stratospheric Polar Vortex." Journal of Climate 30, no. 18 (August 8, 2017): 7125–39. http://dx.doi.org/10.1175/jcli-d-17-0097.1.
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Abstract Statistical models of climate generally regard climate variability as anomalies about a climatological seasonal cycle, which are treated as a stationary stochastic process plus a long-term seasonally dependent trend. However, the climate system has deterministic aspects apart from the climatological seasonal cycle and long-term trends, and the assumption of stationary statistics is only an approximation. The variability of the Southern Hemisphere zonal-mean circulation in the period encompassing late spring and summer is an important climate phenomenon and has been the subject of numerous studies. It is shown here, using reanalysis data, that this variability is rendered highly nonstationary by the organizing influence of the seasonal breakdown of the stratospheric polar vortex, which breaks time symmetry. It is argued that the zonal-mean tropospheric circulation variability during this period is best viewed as interannual variability in the transition between the springtime and summertime regimes induced by variability in the vortex breakdown. In particular, the apparent long-term poleward jet shift during the early-summer season can be more simply understood as a delay in the equatorward shift associated with this regime transition. The implications of such a perspective for various open questions are discussed.
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England, Matthew H., David K. Hutchinson, Agus Santoso, and Willem P. Sijp. "Ice–Atmosphere Feedbacks Dominate the Response of the Climate System to Drake Passage Closure." Journal of Climate 30, no. 15 (August 2017): 5775–90. http://dx.doi.org/10.1175/jcli-d-15-0554.1.
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The response of the global climate system to Drake Passage (DP) closure is examined using a fully coupled ocean–atmosphere–ice model. Unlike most previous studies, a full three-dimensional atmospheric general circulation model is included with a complete hydrological cycle and a freely evolving wind field, as well as a coupled dynamic–thermodynamic sea ice module. Upon DP closure the initial response is found to be consistent with previous ocean-only and intermediate-complexity climate model studies, with an expansion and invigoration of the Antarctic meridional overturning, along with a slowdown in North Atlantic Deep Water (NADW) production. This results in a dominance of Southern Ocean poleward geostrophic flow and Antarctic sinking when DP is closed. However, within just a decade of DP closure, the increased southward heat transport has melted back a substantial fraction of Antarctic sea ice. At the same time the polar oceans warm by 4°–6°C on the zonal mean, and the maximum strength of the Southern Hemisphere westerlies weakens by ≃10%. These effects, not captured in models without ice and atmosphere feedbacks, combine to force Antarctic Bottom Water (AABW) to warm and freshen, to the point that this water mass becomes less dense than NADW. This leads to a marked contraction of the Antarctic overturning, allowing NADW to ventilate the abyssal ocean once more. Poleward heat transport settles back to very similar values as seen in the unperturbed DP open case. Yet remarkably, the equilibrium climate in the closed DP configuration retains a strong Southern Hemisphere warming, similar to past studies with no dynamic atmosphere. However, here it is ocean–atmosphere–ice feedbacks, primarily the ice-albedo feedback and partly the weakened midlatitude jet, not a vigorous southern sinking, which maintain the warm polar oceans. This demonstrates that DP closure can drive a hemisphere-scale warming with polar amplification, without the presence of any vigorous Southern Hemisphere overturning circulation. Indeed, DP closure leads to warming that is sufficient over the West Antarctic Ice Sheet region to inhibit ice-sheet growth. This highlights the importance of the DP gap, Antarctic sea ice, and the associated ice-albedo feedback in maintaining the present-day glacial state over Antarctica.
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Simpson, Isla R., Theodore G. Shepherd, Peter Hitchcock, and John F. Scinocca. "Southern Annular Mode Dynamics in Observations and Models. Part II: Eddy Feedbacks." Journal of Climate 26, no. 14 (July 12, 2013): 5220–41. http://dx.doi.org/10.1175/jcli-d-12-00495.1.
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Abstract Many global climate models (GCMs) have trouble simulating southern annular mode (SAM) variability correctly, particularly in the Southern Hemisphere summer season where it tends to be too persistent. In this two-part study, a suite of experiments with the Canadian Middle Atmosphere Model (CMAM) is analyzed to improve the understanding of the dynamics of SAM variability and its deficiencies in GCMs. Here, an examination of the eddy–mean flow feedbacks is presented by quantification of the feedback strength as a function of zonal scale and season using a new methodology that accounts for intraseasonal forcing of the SAM. In the observed atmosphere, in the summer season, a strong negative feedback by planetary-scale waves, in particular zonal wavenumber 3, is found in a localized region in the southwest Pacific. It cancels a large proportion of the positive feedback by synoptic- and smaller-scale eddies in the zonal mean, resulting in a very weak overall eddy feedback on the SAM. CMAM is deficient in this negative feedback by planetary-scale waves, making a substantial contribution to its bias in summertime SAM persistence. Furthermore, this bias is not alleviated by artificially improving the climatological circulation, suggesting that climatological circulation biases are not the cause of the planetary wave feedback deficiency in the model. Analysis of the summertime eddy feedbacks in the models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) confirms that this is indeed a common problem among GCMs, suggesting that understanding this planetary wave feedback and the reason for its deficiency in GCMs is key to improving the fidelity of simulated SAM variability in the summer season.
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Barnes, Elizabeth A., Nicholas W. Barnes, and Lorenzo M. Polvani. "Delayed Southern Hemisphere Climate Change Induced by Stratospheric Ozone Recovery, as Projected by the CMIP5 Models." Journal of Climate 27, no. 2 (January 15, 2014): 852–67. http://dx.doi.org/10.1175/jcli-d-13-00246.1.
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Abstract Stratospheric ozone is expected to recover by the end of this century because of the regulation of ozone-depleting substances by the Montreal Protocol. Targeted modeling studies have suggested that the climate response to ozone recovery will greatly oppose the climate response to rising greenhouse gas (GHG) emissions. However, the extent of this cancellation remains unclear since only a few such studies are available. Here, a much larger set of simulations performed for phase 5 of the Coupled Model Intercomparison Project is analyzed, which includes ozone recovery. It is shown that the closing of the ozone hole will cause a delay in summertime [December–February (DJF)] Southern Hemisphere climate change between now and 2045. Specifically, it is found that the position of the jet stream, the width of the subtropical dry zones, the seasonality of surface temperatures, and sea ice concentrations all exhibit significantly reduced summertime trends over the first half of the twenty-first century as a consequence of ozone recovery. After 2045, forcing from GHG emissions begins to dominate the climate response. Finally, comparing the relative influences of future GHG emissions and historic ozone depletion, it is found that the simulated DJF tropospheric circulation changes between 1965 and 2005 (driven primarily by ozone depletion) are larger than the projected changes in any future scenario over the entire twenty-first century.
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Cariolle, D., and H. Teyssèdre. "A revised linear ozone photochemistry parameterization for use in transport and general circulation models: multi-annual simulations." Atmospheric Chemistry and Physics 7, no. 9 (May 2, 2007): 2183–96. http://dx.doi.org/10.5194/acp-7-2183-2007.
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Abstract. This article describes the validation of a linear parameterization of the ozone photochemistry for use in upper tropospheric and stratospheric studies. The present work extends a previously developed scheme by improving the 2-D model used to derive the coefficients of the parameterization. The chemical reaction rates are updated from a compilation that includes recent laboratory work. Furthermore, the polar ozone destruction due to heterogeneous reactions at the surface of the polar stratospheric clouds is taken into account as a function of the stratospheric temperature and the total chlorine content. Two versions of the parameterization are tested. The first one only requires the solution of a continuity equation for the time evolution of the ozone mixing ratio, the second one uses one additional equation for a cold tracer. The parameterization has been introduced into the chemical transport model MOCAGE. The model is integrated with wind and temperature fields from the ECMWF operational analyses over the period 2000–2004. Overall, the results from the two versions show a very good agreement between the modelled ozone distribution and the Total Ozone Mapping Spectrometer (TOMS) satellite data and the "in-situ" vertical soundings. During the course of the integration the model does not show any drift and the biases are generally small, of the order of 10%. The model also reproduces fairly well the polar ozone variability, notably the formation of "ozone holes" in the Southern Hemisphere with amplitudes and a seasonal evolution that follow the dynamics and time evolution of the polar vortex. The introduction of the cold tracer further improves the model simulation by allowing additional ozone destruction inside air masses exported from the high to the mid-latitudes, and by maintaining low ozone content inside the polar vortex of the Southern Hemisphere over longer periods in spring time. It is concluded that for the study of climate scenarios or the assimilation of ozone data, the present parameterization gives a valuable alternative to the introduction of detailed and computationally costly chemical schemes into general circulation models.
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Kim, Seo-Yeon, and Seok-Woo Son. "Breakdown of the Linear Relationship between the Southern Hemisphere Hadley Cell Edge and Jet Latitude Changes in the Last Glacial Maximum." Journal of Climate 33, no. 13 (July 1, 2020): 5713–25. http://dx.doi.org/10.1175/jcli-d-19-0531.1.
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AbstractA poleward displacement of the Hadley cell (HC) edge and the eddy-driven jet latitude has been observed in the Southern Hemisphere (SH) during the last few decades. This change is further projected to continue in the future, indicating coherent tropical and extratropical zonal-mean circulation changes from the present climate to a warm climate. Here we show that such a systematic change in the zonal-mean circulation change does not hold in a cold climate. By examining the Last Glacial Maximum (LGM), preindustrial (PI), and extended concentration pathway 4.5 (ECP4.5) scenarios archived for phase 3 of the Paleoclimate Modeling Intercomparison Project (PMIP3) and phase 5 of the Coupled Model Intercomparison Project (CMIP5), it is shown that while the annual-mean SH HC edge systematically shifts poleward from the LGM scenario to the PI scenario and then to the ECP4.5 scenario the annual-mean SH eddy-driven jet latitude does not. All models show a poleward jet shift from the PI scenario to the ECP4.5 scenario, but over one-half of the models exhibit no trend or even an equatorward jet shift from the LGM scenario to the PI scenario. This decoupling between the HC edge and jet latitude changes is most pronounced in SH winter when the Antarctic surface cooling in the LGM scenario is comparable to or larger than the tropical upper-tropospheric cooling. This result indicates that polar amplification could play a crucial role in driving the decoupling of the tropical and midlatitude zonal-mean circulation in the SH in a cold climate.
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Grise, Kevin M., Sean M. Davis, Isla R. Simpson, Darryn W. Waugh, Qiang Fu, Robert J. Allen, Karen H. Rosenlof, et al. "Recent Tropical Expansion: Natural Variability or Forced Response?" Journal of Climate 32, no. 5 (February 6, 2019): 1551–71. http://dx.doi.org/10.1175/jcli-d-18-0444.1.
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AbstractPrevious studies have documented a poleward shift in the subsiding branches of Earth’s Hadley circulation since 1979 but have disagreed on the causes of these observed changes and the ability of global climate models to capture them. This synthesis paper reexamines a number of contradictory claims in the past literature and finds that the tropical expansion indicated by modern reanalyses is within the bounds of models’ historical simulations for the period 1979–2005. Earlier conclusions that models were underestimating the observed trends relied on defining the Hadley circulation using the mass streamfunction from older reanalyses. The recent observed tropical expansion has similar magnitudes in the annual mean in the Northern Hemisphere (NH) and Southern Hemisphere (SH), but models suggest that the factors driving the expansion differ between the hemispheres. In the SH, increasing greenhouse gases (GHGs) and stratospheric ozone depletion contributed to tropical expansion over the late twentieth century, and if GHGs continue increasing, the SH tropical edge is projected to shift further poleward over the twenty-first century, even as stratospheric ozone concentrations recover. In the NH, the contribution of GHGs to tropical expansion is much smaller and will remain difficult to detect in a background of large natural variability, even by the end of the twenty-first century. To explain similar recent tropical expansion rates in the two hemispheres, natural variability must be taken into account. Recent coupled atmosphere–ocean variability, including the Pacific decadal oscillation, has contributed to tropical expansion. However, in models forced with observed sea surface temperatures, tropical expansion rates still vary widely because of internal atmospheric variability.
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Pinheiro, Henri R., Tercio Ambrizzi, Kevin I. Hodges, and Manoel A. Gan. "Understanding the El Niño Southern Oscillation Effect on Cut-Off Lows as Simulated in Forced SST and Fully Coupled Experiments." Atmosphere 13, no. 8 (July 23, 2022): 1167. http://dx.doi.org/10.3390/atmos13081167.
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In this study, we show that changes in the 250 hPa vorticity cut-off low (COL) activity may possibly be driven by sea surface temperature (SST) variations in the tropical Pacific. Using ERA5 reanalysis, the existence of different large-scale circulation patterns is identified that work to enhance the COL activity with a weakened jet stream, while COLs are suppressed with strengthened westerlies. The present-day simulations of AMIP-CMIP6 models reproduce realistic features of the El Niño Southern Oscillation (ENSO)–COL teleconnection, but biases exist, especially in coupled models. The differences are a priori due to the inability of the models to accurately predict the time-mean zonal flow, which may be in part due to systematic biases in the predicted SST. The underestimation of warm SST anomalies over the eastern Pacific is a common problem in CMIP3 and CMIP5 models and remains a major uncertainty in CMIP6. We find that a reduced bias in the predicted SST by coupled models is most likely to produce more skillful simulations in the Southern Hemisphere, but the same evidence does not hold for the Northern Hemisphere. The study suggests the potential for seasonal prediction of COLs and the benefits that would result using accurate initialization and consistent model coupling.
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Naud, Catherine M., Anthony D. Del Genio, Mike Bauer, and William Kovari. "Cloud Vertical Distribution across Warm and Cold Fronts in CloudSat–CALIPSO Data and a General Circulation Model." Journal of Climate 23, no. 12 (June 15, 2010): 3397–415. http://dx.doi.org/10.1175/2010jcli3282.1.
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Abstract Cloud vertical distributions across extratropical warm and cold fronts are obtained using two consecutive winters of CloudSat–Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) observations and National Centers for Environmental Prediction reanalysis atmospheric state parameters over the Northern and Southern Hemisphere oceans (30°–70°N/S) between November 2006 and September 2008. These distributions generally resemble those from the original model introduced by the Bergen School in the 1920s, with the following exceptions: 1) substantial low cloudiness, which is present behind and ahead of the warm and cold fronts; 2) ubiquitous high cloudiness, some of it very thin, throughout the warm-frontal region; and 3) upright convective cloudiness near and behind some warm fronts. One winter of GISS general circulation model simulations of Northern and Southern Hemisphere warm and cold fronts at 2° × 2.5° × 32 levels resolution gives similar cloud distributions but with much lower cloud fraction, a shallower depth of cloudiness, and a shorter extent of tilted warm-frontal cloud cover on the cold air side of the surface frontal position. A close examination of the relationship between the cloudiness and relative humidity fields indicates that water vapor is not lifted enough in modeled midlatitude cyclones and this is related to weak vertical velocities in the model. The model also produces too little cloudiness for a given value of vertical velocity or relative humidity. For global climate models run at scales coarser than tens of kilometers, the authors suggest that the current underestimate of modeled cloud cover in the storm track regions, and in particular the 50°–60°S band of the Southern Oceans, could be reduced with the implementation of a slantwise convection parameterization.
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Simpson, Isla R., Tiffany A. Shaw, and Richard Seager. "A Diagnosis of the Seasonally and Longitudinally Varying Midlatitude Circulation Response to Global Warming*." Journal of the Atmospheric Sciences 71, no. 7 (June 20, 2014): 2489–515. http://dx.doi.org/10.1175/jas-d-13-0325.1.
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Abstract Zonal-mean or basin-mean analyses often conclude that the midlatitude circulation will undergo a poleward shift with global warming. In this study, the models from phase 5 of the Coupled Model Intercomparison Project are used to provide a detailed examination of midlatitude circulation change as a function of longitude and season. The two-dimensional vertically integrated momentum budget is used to identify the dominant terms that maintain the anomalous surface wind stress, thereby allowing a distinction between features that are maintained by high-frequency eddies and those that involve changes in the lower-frequency or stationary flow. In the zonal mean, in each season and hemisphere there is a poleward shifting of the midlatitude surface wind stress, primarily maintained by high-frequency transient eddies. This is not necessarily the case locally. In the Southern Hemisphere, for the most part, the interpretation of the response as being a high-frequency eddy-driven poleward shifting of the midlatitude westerlies holds true. The Northern Hemisphere is considerably more complex with only the fall months showing a robust poleward shift of both the Atlantic and Pacific jets. During the winter months the jet in the east Pacific actually shifts equatorward and the Atlantic jet strengthens over Europe. An important role for altered climatological stationary waves in these responses is found. This motivates future work that should focus on zonal asymmetries and stationary wave changes, as well as the changes in high-frequency transients that bring about the poleward shifting of the westerlies in the zonal mean.
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Rollings, Michael, and Timothy M. Merlis. "The Observed Relationship between Pacific SST Variability and Hadley Cell Extent Trends in Reanalyses." Journal of Climate 34, no. 7 (April 2021): 2511–27. http://dx.doi.org/10.1175/jcli-d-20-0410.1.
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AbstractReanalysis and other observationally based estimates suggest that the tropics have expanded more than simulated by coupled climate models with historical radiative forcing. Previous research has attempted to reconcile this discrepancy by using climate model simulations with constrained tropical Pacific sea surface temperatures (SSTs) to account for the role of internal variability. Here the relationships between Hadley cell extent and internal SST variability and long-term warming are analyzed using purely observational techniques. Using linearly independent components of SST variability with reanalysis datasets, the statistical relationship between Pacific variability and Hadley cell extent is quantified by time scale. There is a strong correlation between North Pacific decadal SST variability and Southern Hemisphere Hadley cell extent. Conversely, there is a weaker observed relation between El Niño–Southern Oscillation (ENSO) and Hadley cell extent when low-frequency variability is filtered out of the ENSO signal. The observed linear sensitivity of Hadley cell width to long-term warming agrees with coupled general circulation model experiments when accounting for uncertainties, and there is a statistically significant relationship between Northern Hemisphere Hadley cell extent and long-term warming during boreal autumn.