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1
Yang, Qianzi, Yingying Zhao, Qin Wen, Jie Yao, and Haijun Yang. "Understanding Bjerknes Compensation in Meridional Heat Transports and the Role of Freshwater in a Warming Climate." Journal of Climate 31, no. 12 (June 2018): 4791–806. http://dx.doi.org/10.1175/jcli-d-17-0587.1.
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The Bjerknes compensation (BJC) under global warming is studied using a simple box model and a coupled Earth system model. The BJC states the out-of-phase changes in the meridional atmosphere and ocean heat transports. Results suggest that the BJC can occur during the transient period of global warming. During the transient period, the sea ice melting in the high latitudes can cause a significant weakening of the Atlantic meridional overturning circulation (AMOC), resulting in a cooling in the North Atlantic. The meridional contrast of sea surface temperature would be enhanced, and this can eventually enhance the Hadley cell and storm-track activities in the Northern Hemisphere. Accompanied by changes in both ocean and atmosphere circulations, the northward ocean heat transport in the Atlantic is decreased while the northward atmosphere heat transport is increased, and the BJC occurs in the Northern Hemisphere. Once the freshwater influx into the North Atlantic Ocean stops, or the ocean even loses freshwater because of strong heating in the high latitudes, the AMOC would recover. Both the atmosphere and ocean heat transports would be enhanced, and they can eventually recover to the state of the control run, leading to the BJC to become invalid. The above processes are clearly demonstrated in the coupled model CO2 experiment. Since it is difficult to separate the freshwater effect from the heating effect in the coupled model, a simple box model is used to understand the BJC mechanism and freshwater’s role under global warming. In a warming climate, the freshwater flux into the ocean can cool the global surface temperature, mitigating the temperature rise. Box model experiments indicate clearly that it is the freshwater flux into the North Atlantic that causes out-of-phase changes in the atmosphere and ocean heat transports, which eventually plays a stabilizing role in global climate change.
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Xu, Xiaobiao, Peter B. Rhines, and Eric P. Chassignet. "Temperature–Salinity Structure of the North Atlantic Circulation and Associated Heat and Freshwater Transports." Journal of Climate 29, no. 21 (October 6, 2016): 7723–42. http://dx.doi.org/10.1175/jcli-d-15-0798.1.
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Abstract This study investigates the circulation structure and relative contribution of circulation components to the time-mean meridional heat and freshwater transports in the North Atlantic, using numerical results of a high-resolution ocean model that are shown to be in excellent agreement with the observations. The North Atlantic circulation can be separated into the large-scale Atlantic meridional overturning circulation (AMOC) that is diapycnal and the subtropical and subpolar gyres that largely flow along isopycnal surfaces but also include prominent gyre-scale diapycnal overturning in the Subtropical Mode Water and Labrador Sea Water. Integrals of the meridional volume transport as a function of potential temperature θ and salinity S yield streamfunctions with respect to θ and to S, and heat functions. These argue for a significant contribution to the heat transport by the southward circulation of North Atlantic Deep Water. At 26.5°N, the isopycnic component of the subtropical gyre is colder and fresher in the northward-flowing western boundary currents than the southward return flows, and it carries heat southward and freshwater northward, opposite of that of the diapycnal component. When combined, the subtropical gyre contributes virtually zero to the heat transport and the AMOC is responsible for all the heat transport across this latitude. The subtropical gyre however significantly contributes to the freshwater transport, reducing the 0.5-Sv (1 Sv ≡ 106 m3 s–1) southward AMOC freshwater transport by 0.13 Sv. In the subpolar North Atlantic near 58°N, the diapycnal component of the circulation, or the transformation of warm saline upper Atlantic water into colder fresher deep waters, is responsible for essentially all of the heat and freshwater transports.
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Hall, Stephen A. "Atmospheric transport of freshwater algaePediastrumin the American Southwest." Grana 37, no. 6 (January 1998): 374–75. http://dx.doi.org/10.1080/00173139809362693.
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Wang, Xiaoli, Peter H. Stone, and Jochem Marotzke. "Global Thermohaline Circulation. Part II: Sensitivity with Interactive Atmospheric Transports." Journal of Climate 12, no. 1 (January 1, 1999): 83–91. http://dx.doi.org/10.1175/1520-0442-12.1.83.
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Abstract A hybrid coupled ocean–atmosphere model is used to investigate the stability of the thermohaline circulation (THC) to an increase in the surface freshwater forcing in the presence of interactive meridional transports in the atmosphere. The ocean component is the idealized global general circulation model used in Part I. The atmospheric model assumes fixed latitudinal structure of the heat and moisture transports, and the amplitudes are calculated separately for each hemisphere from the large-scale sea surface temperature (SST) and SST gradient, using parameterizations based on baroclinic stability theory. The ocean–atmosphere heat and freshwater exchanges are calculated as residuals of the steady-state atmospheric budgets. Owing to the ocean component’s weak heat transport, the model has too strong a meridional SST gradient when driven with observed atmospheric meridional transports. When the latter are made interactive, the conveyor belt circulation collapses. A flux adjustment is introduced in which the efficiency of the atmospheric transports is lowered to match the too low efficiency of the ocean component. The feedbacks between the THC and both the atmospheric heat and moisture transports are positive, whether atmospheric transports are interactive in the Northern Hemisphere, the Southern Hemisphere, or both. However, the feedbacks operate differently in the Northern and Southern Hemispheres, because the Pacific THC dominates in the Southern Hemisphere, and deep water formation in the two hemispheres is negatively correlated. The feedbacks in the two hemispheres do not necessarily reinforce each other because they have opposite effects on low-latitude temperatures. The model is qualitatively similar in stability to one with conventional “additive” flux adjustment, but quantitatively more stable.
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Holfort, Jürgen, and Jens Meincke. "Time series of freshwater-transport on the East Greenland Shelf at 74N." Meteorologische Zeitschrift 14, no. 6 (December 19, 2005): 703–10. http://dx.doi.org/10.1127/0941-2948/2005/0079.
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Nilsson, Johan, and Heiner Körnich. "A Conceptual Model of the Surface Salinity Distribution in the Oceanic Hadley Cell." Journal of Climate 21, no. 24 (December 15, 2008): 6586–98. http://dx.doi.org/10.1175/2008jcli2284.1.
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Abstract A conceptual model of the salinity distribution in the oceanic Hadley cell is presented. The model pertains to the region of tropical easterly surface winds, where the surface salinity increases poleward from a local salinity minimum near the equator to a subtropical salinity maximum. A fundamental constraint is that the meridional freshwater transports in the atmosphere and the ocean have the same magnitude but opposite directions. A key assumption is that the strength of the meridional overturning cells in the atmosphere and the ocean is proportional and set by the surface layer Ekman transport. It is further assumed that, to the lowest order of approximation, the zonal-mean Ekman transports accomplish the meridional freshwater transports, that is, eddy fluxes and gyre-induced transports are ignored. The model predicts that the salinity variation in the oceanic cell is directly proportional to the specific humidity of the near-surface air, but independent of the meridional mass transport (as long as the atmospheric and oceanic mass transports remain proportional). If the relative humidity of the near-surface air is constant, the salinity variation in the oceanic Hadley cell varies essentially with the surface temperature according to the Clausius–Clapeyron expression for the saturation vapor pressure. Further, the model is compared to observations and a global warming simulation and found to give a leading-order description of the tropical surface salinity range.
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Wang, Xiaoli, Peter H. Stone, and Jochem Marotzke. "Global Thermohaline Circulation. Part I: Sensitivity to AtmosphericMoisture Transport." Journal of Climate 12, no. 1 (January 1, 1999): 71–82. http://dx.doi.org/10.1175/1520-0442-12.1.71.
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Abstract A global ocean general circulation model of idealized geometry, combined with an atmospheric model based on observed transports of heat, momentum, and moisture, is used to explore the sensitivity of the global conveyor belt circulation to the surface freshwater fluxes, in particular the effects of meridional atmospheric moisture transports. The numerical results indicate that the equilibrium strength of the North Atlantic Deep Water (NADW) formation increases as the global freshwater transports increase. However, the global deep water formation—that is, the sum of the NADW and the Southern Ocean Deep Water formation rates—is relatively insensitive to changes of the freshwater flux. Perturbations to the meridional moisture transports of each hemisphere identify equatorially asymmetric effects of the freshwater fluxes. The results are consistent with box model results that the equilibrium NADW formation is primarily controlled by the magnitude of the Southern Hemisphere freshwater flux. However, the results show that the Northern Hemisphere freshwater flux has a strong impact on the transient behavior of the North Atlantic overturning. Increasing this flux leads to a collapse of the conveyor belt circulation, but the collapse is delayed if the Southern Hemisphere flux also increases. The perturbation experiments also illustrate that the rapidity of collapse is affected by random fluctuations in the wind stress field.
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Murakami, Shigenori, Rumi Ohgaito, Ayako Abe-Ouchi, Michel Crucifix, and Bette L. Otto-Bliesner. "Global-Scale Energy and Freshwater Balance in Glacial Climate: A Comparison of Three PMIP2 LGM Simulations." Journal of Climate 21, no. 19 (October 1, 2008): 5008–33. http://dx.doi.org/10.1175/2008jcli2104.1.
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Abstract Three coupled atmosphere–ocean general circulation model (AOGCM) simulations of the Last Glacial Maximum (LGM: about 21 000 yr before present), conducted under the protocol of the second phase of the Paleoclimate Modelling Intercomparison Project (PMIP2), have been analyzed from a viewpoint of large-scale energy and freshwater balance. Atmospheric latent heat (LH) transport decreases at most latitudes due to reduced water vapor content in the lower troposphere, and dry static energy (DSE) transport in northern midlatitudes increases and changes the intensity contrast between the Pacific and Atlantic regions due to enhanced stationary waves over the North American ice sheets. In low latitudes, even with an intensified Hadley circulation in the Northern Hemisphere (NH), reduced DSE transport by the mean zonal circulation as well as a reduced equatorward LH transport is observed. The oceanic heat transport at NH midlatitudes increases owing to intensified subpolar gyres, and the Atlantic heat transport at low latitudes increases in all models whether or not meridional overturning circulation (MOC) intensifies. As a result, total poleward energy transport at the LGM increases in NH mid- and low latitudes in all models. Oceanic freshwater transport decreases, compensating for the response of the atmospheric water vapor transport. These responses in the atmosphere and ocean make the northern North Atlantic Ocean cold and relatively fresh, and the Southern Ocean relatively warm and saline. This is a common and robust feature in all models. The resultant ocean densities and ocean MOC response, however, show model dependency.
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Cvijanovic, I., P. L. Langen, and E. Kaas. "Weakened atmospheric energy transport feedback in cold glacial climates." Climate of the Past Discussions 7, no. 2 (April 13, 2011): 1235–59. http://dx.doi.org/10.5194/cpd-7-1235-2011.
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Abstract. The response of atmospheric energy transport during Northern Hemisphere cooling and warming from present day (PD) and Last Glacial Maximum (LGM) conditions is investigated using sea surface temperature anomalies derived from a freshwater hosing experiment. The present day climate shows enhanced sensitivity of the atmospheric energy transport compared to that of the LGM suggesting an ability of the PD atmosphere to reorganize more easily and thereby dampen temperature anomalies that may arise from changes in the oceanic transport. The increased PD sensitivity relative to that of the LGM is due mainly to a stronger dry static energy transport response which, in turn, is driven chiefly by larger changes in the transient eddy heat flux. In comparison, changes in latent heat transport play a minor role in the overall transport sensitivity.
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Liu, Wei, and Zhengyu Liu. "A Note on the Stability Indicator of the Atlantic Meridional Overturning Circulation." Journal of Climate 27, no. 2 (January 15, 2014): 969–75. http://dx.doi.org/10.1175/jcli-d-13-00181.1.
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Abstract This study examines the validity of the net freshwater transport ΔMov as a stability indicator of the Atlantic meridional overturning circulation (AMOC) in a low-resolution version of the NCAR Community Climate System Model, version 3 (CCSM3). It is shown that the sign of ΔMov indicates the monostability or bistability of the AMOC, which is based on a hypothesis that a collapsed AMOC induces a zero net freshwater transport. In CCSM3, this hypothesis is satisfied in that the collapsed AMOC, with a nonzero strength, induces a zero net freshwater transport ΔMov across the Atlantic basin by generating equivalent freshwater export MovS and freshwater import MovN at the southern and northern boundaries, respectively. Because of the satisfaction of the hypothesis, ΔMov is consistent with a generalized indicator L for a slowly evolving AMOC, both of which correctly monitor the AMOC stability.
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Florindo-López, Cristian, Sheldon Bacon, Yevgeny Aksenov, Léon Chafik, Eugene Colbourne, and N. Penny Holliday. "Arctic Ocean and Hudson Bay Freshwater Exports: New Estimates from Seven Decades of Hydrographic Surveys on the Labrador Shelf." Journal of Climate 33, no. 20 (October 15, 2020): 8849–68. http://dx.doi.org/10.1175/jcli-d-19-0083.1.
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AbstractWhile reasonable knowledge of multidecadal Arctic freshwater storage variability exists, we have little knowledge of Arctic freshwater exports on similar time scales. A hydrographic time series from the Labrador Shelf, spanning seven decades at annual resolution, is here used to quantify Arctic Ocean freshwater export variability west of Greenland. Output from a high-resolution coupled ice–ocean model is used to establish the representativeness of those hydrographic sections. Clear annual to decadal variability emerges, with high freshwater transports during the 1950s and 1970s–80s, and low transports in the 1960s and from the mid-1990s to 2016, with typical amplitudes of 30 mSv (1 Sv = 106 m3 s−1). The variability in both the transports and cumulative volumes correlates well both with Arctic and North Atlantic freshwater storage changes on the same time scale. We refer to the “inshore branch” of the Labrador Current as the Labrador Coastal Current, because it is a dynamically and geographically distinct feature. It originates as the Hudson Bay outflow, and preserves variability from river runoff into the Hudson Bay catchment. We find a need for parallel, long-term freshwater transport measurements from Fram and Davis Straits to better understand Arctic freshwater export control mechanisms and partitioning of variability between routes west and east of Greenland, and a need for better knowledge and understanding of year-round (solid and liquid) freshwater fluxes on the Labrador shelf. Our results have implications for wider, coherent atmospheric control on freshwater fluxes and content across the Arctic Ocean and northern North Atlantic Ocean.
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Zhang, Liping, Lixin Wu, and Jiaxu Zhang. "Simulated Response to Recent Freshwater Flux Change over the Gulf Stream and Its Extension: Coupled Ocean–Atmosphere Adjustment and Atlantic–Pacific Teleconnection." Journal of Climate 24, no. 15 (August 1, 2011): 3971–88. http://dx.doi.org/10.1175/2011jcli4020.1.
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Abstract Recent observation has shown that the dominant mode of the net freshwater flux variations over the North Atlantic Ocean is the significant trend of freshwater loss over the Gulf Stream region and its extension. In this paper, the coupled ocean–atmosphere response to this freshwater flux change is investigated based on a series of the Fast Ocean–Atmosphere Model coupled-model experiments. The model demonstrates that the freshwater loss over the Gulf Stream and its extension region directly forces an anomalous cyclonic gyre and triggers a SST dipole with cooling in the western subtropical and warming in the eastern subpolar North Atlantic. The freshwater loss also forces a significant response in the atmosphere with a negative NAO-like response in early winter and a basin-scale ridge resembling the eastern Atlantic mode (EAM) in late winter. The salinification also strengthens the Atlantic meridional overturning circulation and thus the poleward heat transport, leading to tropical cooling. The freshwater loss over the Gulf Stream and its extension also leads to an El Niño–like warming in the tropical Pacific and cooling in the North Pacific, similar to the responses in previous water-hosing experiments with an input of freshwater in the subpolar North Atlantic. The tropical Pacific responses subsequently strengthen the Northern Hemispheric atmospheric anomalies in early winter, but reverse them in late winter through an emanation of Rossby wave trains. Overall, the tropical Pacific air–sea coupling plays a damping role, while local air–sea coupling tends to enhance the ocean and atmospheric responses over the North Atlantic.
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Mertz, G., S. Narayanan, and J. Helbig. "The freshwater transport of the labrador current." Atmosphere-Ocean 31, no. 2 (June 1993): 281–95. http://dx.doi.org/10.1080/07055900.1993.9649472.
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Köhl, Armin, and Nuno Serra. "Causes of Decadal Changes of the Freshwater Content in the Arctic Ocean." Journal of Climate 27, no. 9 (April 23, 2014): 3461–75. http://dx.doi.org/10.1175/jcli-d-13-00389.1.
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Abstract Decadal changes of the liquid freshwater content in the Arctic Ocean are studied with a suite of forward and adjoint model simulations. Adjoint sensitivities show that freshwater volume changes in the Norwegian Atlantic Current north of the Lofoten basin and a salinity maximum in the Fram Strait and in the Canadian Archipelago lead to an enhanced northward transport of freshwater. The dynamical sensitivities indicate that stronger freshwater export from the Arctic is related to an enhanced cyclonic circulation around Greenland, with an enhanced export through the Canadian Archipelago and a stronger circulation within the Fram Strait. Associated with this circulation around Greenland is a large-scale cyclonic circulation in the Arctic. Cyclonic wind stress anomalies in the Arctic Ocean as well as over the Nordic seas and parts of the subpolar Atlantic are optimal to force the freshwater transport changes. Results from a simulation over the period 1948–2010 corroborate the result that Arctic freshwater content changes are mainly related to the strength of the circulation around Greenland. Volume transport changes are more important than salinity changes. Freshwater content changes can be explained by wind stress–driven transport variability, with larger export for cyclonic atmospheric forcing. By redistributing freshwater within the Arctic, cyclonic wind stress leads to high sea level in the periphery of the Arctic, and the stronger gradient from the Arctic to the North Atlantic enhances the export through the passages. A second mechanism is the wind-driven Sverdrup circulation, which can be described by Godfrey’s (1989) “island rule” including friction. For this, wind stress in the Arctic is not important.
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St. Pierre, Kyra A., Vincent L. St. Louis, Sherry L. Schiff, Igor Lehnherr, Paul G. Dainard, Alex S. Gardner, Pieter J. K. Aukes, and Martin J. Sharp. "Proglacial freshwaters are significant and previously unrecognized sinks of atmospheric CO2." Proceedings of the National Academy of Sciences 116, no. 36 (August 19, 2019): 17690–95. http://dx.doi.org/10.1073/pnas.1904241116.
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Carbon dioxide (CO2) emissions from freshwater ecosystems are almost universally predicted to increase with climate warming. Glacier-fed rivers and lakes, however, differ critically from those in nonglacierized catchments in that they receive little terrestrial input of organic matter for decomposition and CO2 production, and transport large quantities of easily mobilized comminuted sediments available for carbonate and silicate weathering reactions that can consume atmospheric CO2. We used a whole-watershed approach, integrating concepts from glaciology and limnology, to conclusively show that certain glacier-fed freshwater ecosystems are important and previously overlooked annual CO2 sinks due to the overwhelming influence of these weathering reactions. Using the glacierized Lake Hazen watershed (Nunavut, Canada, 82°N) as a model system, we found that weathering reactions in the glacial rivers actively consumed CO2 up to 42 km downstream of glaciers, and cumulatively transformed the High Arctic’s most voluminous lake into an important CO2 sink. In conjunction with data collected at other proglacial freshwater sites in Greenland and the Canadian Rockies, we suggest that CO2 consumption in proglacial freshwaters due to glacial melt-enhanced weathering is likely a globally relevant phenomenon, with potentially important implications for regional annual carbon budgets in glacierized watersheds.
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LOHMANN, GERRIT. "Atmospheric and oceanic freshwater transport during weak Atlantic overturning circulation." Tellus A 55, no. 5 (October 2003): 438–49. http://dx.doi.org/10.1034/j.1600-0870.2003.00028.x.
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Lohmann, Gerrit. "Atmospheric and oceanic freshwater transport during weak Atlantic overturning circulation." Tellus A: Dynamic Meteorology and Oceanography 55, no. 5 (January 2003): 438–49. http://dx.doi.org/10.3402/tellusa.v55i5.12108.
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Solomon, Amy, Céline Heuzé, Benjamin Rabe, Sheldon Bacon, Laurent Bertino, Patrick Heimbach, Jun Inoue, et al. "Freshwater in the Arctic Ocean 2010–2019." Ocean Science 17, no. 4 (August 17, 2021): 1081–102. http://dx.doi.org/10.5194/os-17-1081-2021.
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Abstract. The Arctic climate system is rapidly transitioning into a new regime with a reduction in the extent of sea ice, enhanced mixing in the ocean and atmosphere, and thus enhanced coupling within the ocean–ice–atmosphere system; these physical changes are leading to ecosystem changes in the Arctic Ocean. In this review paper, we assess one of the critically important aspects of this new regime, the variability of Arctic freshwater, which plays a fundamental role in the Arctic climate system by impacting ocean stratification and sea ice formation or melt. Liquid and solid freshwater exports also affect the global climate system, notably by impacting the global ocean overturning circulation. We assess how freshwater budgets have changed relative to the 2000–2010 period. We include discussions of processes such as poleward atmospheric moisture transport, runoff from the Greenland Ice Sheet and Arctic glaciers, the role of snow on sea ice, and vertical redistribution. Notably, sea ice cover has become more seasonal and more mobile; the mass loss of the Greenland Ice Sheet increased in the 2010s (particularly in the western, northern, and southern regions) and imported warm, salty Atlantic waters have shoaled. During 2000–2010, the Arctic Oscillation and moisture transport into the Arctic are in-phase and have a positive trend. This cyclonic atmospheric circulation pattern forces reduced freshwater content on the Atlantic–Eurasian side of the Arctic Ocean and freshwater gains in the Beaufort Gyre. We show that the trend in Arctic freshwater content in the 2010s has stabilized relative to the 2000s, potentially due to an increased compensation between a freshening of the Beaufort Gyre and a reduction in freshwater in the rest of the Arctic Ocean. However, large inter-model spread across the ocean reanalyses and uncertainty in the observations used in this study prevent a definitive conclusion about the degree of this compensation.
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Cheng, Wei, Wilbert Weijer, Who M. Kim, Gokhan Danabasoglu, Steve G. Yeager, Peter R. Gent, Dongxiao Zhang, John C. H. Chiang, and Jiaxu Zhang. "Can the Salt-Advection Feedback Be Detected in Internal Variability of the Atlantic Meridional Overturning Circulation?" Journal of Climate 31, no. 16 (August 2018): 6649–67. http://dx.doi.org/10.1175/jcli-d-17-0825.1.
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Evidence for the assumptions of the salt-advection feedback in box models is sought by studying the Atlantic meridional overturning circulation (AMOC) internal variability in the long preindustrial control runs of two Earth system models. The first assumption is that AMOC strength is proportional to the meridional density difference between the North Atlantic and the Southern Oceans. The model simulations support this assumption, with the caveat that nearly all the long time-scale variability occurs in the North Atlantic density. The second assumption is that the freshwater transport variability by the overturning at the Atlantic southern boundary is controlled by the strength of AMOC. Only one of the models shows some evidence that AMOC variability at 45°N leads variability in the overturning freshwater transport at the southern boundary by about 30 years, but the other model shows no such coherence. In contrast, in both models this freshwater transport variability is dominated by local salinity variations. The third assumption is that changes in the overturning freshwater transport at the Atlantic southern boundary perturb the north–south density difference, and thus feed back on AMOC strength in the north. No evidence for this assumption is found in either model at any time scale, although this does not rule out that the salt-advection feedback may be excited by a strong enough freshwater perturbation.
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Wolfe, Christopher L., and Paola Cessi. "Salt Feedback in the Adiabatic Overturning Circulation." Journal of Physical Oceanography 44, no. 4 (April 1, 2014): 1175–94. http://dx.doi.org/10.1175/jpo-d-13-0154.1.
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Abstract The adiabatic overturning circulation is the part of the meridional overturning circulation that persists in the limit of vanishing diffusivity. Two conditions are required for the existence of the adiabatic overturning circulation: a high-latitude zonally reentrant channel subject to surface westerlies and a set of outcropping isopycnals shared between the channel and the opposite hemisphere. This paper examines how different buoyancy forcing regimes, particularly freshwater flux, affect the surface buoyancy distribution and the strength of the adiabatic overturning circulation. Without freshwater forcing, salinity is uniform and buoyancy is determined by temperature only. In this case, the size of the shared isopycnal window is effectively fixed by the coupling between atmospheric and sea surface temperatures. With freshwater forcing (applied as a surface flux), the salinity, and thus the sea surface buoyancy and the size of the shared isopycnal window, is not specified by the atmospheric state alone. It is found that a salt–advection feedback leads to surface buoyancy distributions that increase the size of the isopycnal window and strengthen the adiabatic overturning circulation. The strength of the feedback is controlled by processes in high latitudes—the southern channel, where the surface salinity is determined by a balance between freshwater input from the atmosphere, salt input from upwelling deep water, and freshwater export by Ekman transport; and the Northern Hemisphere, where the overturning and wind-driven transport in the thermocline advect salty water from the subtropics, mitigating the freshening effect of the surface freshwater flux. The freshwater budget in the channel region provides an estimate of the size of the isopycnal window.
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Justino, Flávio Barbosa, and Jeferson Prietsch Machado. "Climate feedbacks induced by the North Atlantic freshwater forcing in a coupled model of intermediate complexity." Revista Brasileira de Meteorologia 25, no. 1 (March 2010): 103–13. http://dx.doi.org/10.1590/s0102-77862010000100009.
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Based on coupled model simulations (ECBilt-Clio), we investigate the atmospheric and oceanic response to sustained freshwater input into the North Atlantic under the glacial maximum background state. The results demonstrate that a weakening of the thermohaline circulation triggered by weaker density flux leads to rapid changes in global sea-ice volume and reduced poleward heat transport in the Northern Hemisphere (NH). In the Southern Hemisphere (SH), however, the oceanic heat transport increases substantially. This in turn leads to strong cooling over the North Atlantic whereas the SH extratropical region warms up. The suppression of the NADW also drastically changes the atmospheric circulation. The associated northward wind anomalies over the North Atlantic increase the warm air advection from the tropics and induce the transport of tropical saltier water to mid-latitudes. This negative atmospheric-oceanic feedback should play an important role to resume the NADW, after the freshwater forcing ends up.
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Liu, Wei, and Zhengyu Liu. "A Diagnostic Indicator of the Stability of the Atlantic Meridional Overturning Circulation in CCSM3." Journal of Climate 26, no. 6 (March 15, 2013): 1926–38. http://dx.doi.org/10.1175/jcli-d-11-00681.1.
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Abstract A diagnostic indicator ΔMov is proposed in this paper to monitor the stability of the Atlantic meridional overturning circulation (AMOC). The ΔMov is a diagnostic for a basinwide salt-advection feedback and defined as the difference between the freshwater transport induced by the AMOC across the southern border of the Atlantic Ocean and the overturning liquid freshwater transport from the Arctic Ocean to the North Atlantic. As validated in the Community Climate System Model, version 3 (CCSM3), for an AMOC in the conveyor state, a positive ΔMov (freshwater convergence) in the Atlantic basin indicates a monostable AMOC and a negative ΔMov (freshwater divergence) indicates a bistable AMOC. Based on ΔMov, the authors investigate the AMOC stability in the Last Glacial Maximum (LGM) and analyze the modulation of the AMOC stability by an open/closed Bering Strait. Moreover, the authors estimate that the real AMOC is likely to be bistable in the present day, since some observations suggest a negative ΔMov (freshwater divergence) is currently in the Atlantic basin. However, this estimation is very sensitive to the choice of the observational data.
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Cvijanovic, I., P. L. Langen, and E. Kaas. "Weakened atmospheric energy transport feedback in cold glacial climates." Climate of the Past 7, no. 4 (October 6, 2011): 1061–73. http://dx.doi.org/10.5194/cp-7-1061-2011.
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Abstract. The response of atmospheric energy transport during Northern Hemisphere cooling and warming from present day (PD) and Last Glacial Maximum (LGM) conditions is investigated using sea surface temperature anomalies derived from a freshwater hosing experiment. The present day climate shows enhanced sensitivity of the atmospheric mid-latitude energy transport compared to that of the LGM, suggesting its ability to reorganize more easily and thereby dampen high latitude temperature anomalies that may arise from changes in the oceanic transport. This effect is found to be a result of both the atmospheric and surface flux response. The increased PD transport sensitivity relative to that of the LGM is linked to a stronger dry static energy transport response which, in turn, is mainly driven by larger changes in the transient eddy heat flux. In comparison, changes in mid-latitude latent heat transport play a minor role in the overall transport sensitivity.
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McGeehan, Timothy, and Wieslaw Maslowski. "Impact of Shelf–Basin Freshwater Transport on Deep Convection in the Western Labrador Sea." Journal of Physical Oceanography 41, no. 11 (November 1, 2011): 2187–210. http://dx.doi.org/10.1175/jpo-d-11-01.1.
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Abstract Freshwater exiting the Arctic Ocean through the Canadian Arctic Archipelago (CAA) has been shown to affect meridional overturning circulation and thereby the global climate system. However, because of constraints of spatial resolution in most global ocean models, neither the flow of low salinity water through the CAA to the Labrador Sea nor the eddy activity that may transport freshwater from the shelf to areas of open ocean convection can be directly simulated. To address these issues, this study uses a high-resolution ice–ocean model of the pan-Arctic region with a realistic CAA and forced with realistic atmospheric data. This model resolves conditions in the Arctic Ocean upstream of the Labrador Sea and is coupled to a thermodynamic–dynamic sea ice model that responds to the atmospheric forcing. The major shelf–basin exchange of liquid freshwater occurs south of Hamilton Bank, whereas the largest ice flux occurs in the northwest of the basin. Freshwater flux anomalies entering the Labrador Sea through Davis Strait do not immediately affect deep convection. Instead, eddies acting on shorter time scales can move freshwater to locations of active convection and halt the process. Convection is modulated by the position of the ice edge, highlighting the critical need for a coupled ice–ocean model. Finally, the size of eddies and the short duration of events demonstrate the need for high resolution, both spatial and temporal.
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Pardaens, A. K., H. T. Banks, J. M. Gregory, and P. R. Rowntree. "Freshwater transports in HadCM3." Climate Dynamics 21, no. 2 (August 1, 2003): 177–95. http://dx.doi.org/10.1007/s00382-003-0324-6.
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Fuentes-Franco, Ramón, and Torben Koenigk. "Sensitivity of the Arctic freshwater content and transport to model resolution." Climate Dynamics 53, no. 3-4 (March 25, 2019): 1765–81. http://dx.doi.org/10.1007/s00382-019-04735-y.
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Rahmstorf, S. "On the freshwater forcing and transport of the Atlantic thermohaline circulation." Climate Dynamics 12, no. 12 (November 1996): 799–811. http://dx.doi.org/10.1007/s003820050144.
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Wen, Qin, Jie Yao, Kristofer Döös, and Haijun Yang. "Decoding Hosing and Heating Effects on Global Temperature and Meridional Circulations in a Warming Climate." Journal of Climate 31, no. 23 (December 2018): 9605–23. http://dx.doi.org/10.1175/jcli-d-18-0297.1.
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The global temperature changes under global warming result from two effects: one is the pure radiative heating effect caused by a change in greenhouse gases, and the other is the freshwater effect related to changes in precipitation, evaporation, and sea ice. The two effects are separated in a coupled climate model through sensitivity experiments in this study. It is indicated that freshwater change has a significant cooling effect that can mitigate the global surface warming by as much as ~30%. Two significant regional cooling centers occur: one in the subpolar Atlantic and one in the Southern Ocean. The subpolar Atlantic cooling, also known as the “warming hole,” is triggered by sea ice melting and the southward cold-water advection from the Arctic Ocean, and is sustained by the weakened Atlantic meridional overturning circulation. The Southern Ocean surface cooling is triggered by sea ice melting along the Antarctic and is maintained by the enhanced northward Ekman flow. In these two regions, the effect of freshwater flux change dominates over that of radiation flux change, controlling the sea surface temperature change in the warming climate. The freshwater flux change also results in the Bjerknes compensation, with the atmosphere heat transport change compensating the ocean heat transport change by about 80% during the transient stage of global warming. In terms of global temperature and Earth’s energy balance, the freshwater change plays a stabilizing role in a warming climate.
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Kirkman, Clark H., and Cecilia M. Bitz. "The Effect of the Sea Ice Freshwater Flux on Southern Ocean Temperatures in CCSM3: Deep-Ocean Warming and Delayed Surface Warming." Journal of Climate 24, no. 9 (May 1, 2011): 2224–37. http://dx.doi.org/10.1175/2010jcli3625.1.
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Abstract This study explores the role of sea ice freshwater and salt fluxes in modulating twenty-first-century surface warming in the Southern Ocean via analysis of sensitivity experiments in the Community Climate System Model, version 3 (CCSM3). In particular, the role of a change in these fluxes in causing surface cooling, expanding sea ice, and increasing deep oceanic storage of heat in the Southern Ocean is investigated. The results indicate that in response to the doubling of CO2 concentrations in the atmosphere in CCSM3, net freshwater input from sea ice to the ocean increases south of 58°S (owing to less growth) and decreases from 48° to 58°S (owing to less melt). The freshwater source from changing precipitation in the model is considerably less than from sea ice south of 58°S, but it serves to compensate for the reduction in sea ice melt near the ice edge, leaving almost no net freshwater flux change between about 48° and 58°S. As a result, freshwater input principally from sea ice reduces ocean convection, which in turns reduces the entrainment of heat into the mixed layer and reduces the upward heat transport along isopycnals below about 1000 m. The reduced upward heat transport (from all sources) causes deep-ocean heating south of 60°S and below 500-m depth, with a corresponding surface cooling in large parts of the Southern Ocean in the model. These results indicate that changing sea ice freshwater and salt fluxes are a major component of the twenty-first-century delay in surface warming of the Southern Ocean and weak reduction in Antarctic sea ice in model projections.
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Dinniman, Michael S., John M. Klinck, Eileen E. Hofmann, and Walker O. Smith. "Effects of Projected Changes in Wind, Atmospheric Temperature, and Freshwater Inflow on the Ross Sea." Journal of Climate 31, no. 4 (February 2018): 1619–35. http://dx.doi.org/10.1175/jcli-d-17-0351.1.
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A 5-km horizontal resolution regional ocean–sea ice–ice shelf model of the Ross Sea is used to examine the effects of changes in wind strength, air temperature, and increased meltwater input on the formation of high-salinity shelf water (HSSW), on-shelf transport and vertical mixing of Circumpolar Deep Water (CDW) and its transformation into modified CDW (MCDW), and basal melt of the Ross Ice Shelf (RIS). A 20% increase in wind speed, with no other atmospheric changes, reduced summer sea ice minimum area by 20%, opposite the observed trend of the past three decades. Increased winds with spatially uniform, reduced atmospheric temperatures increased summer sea ice concentrations, on-shelf transport of CDW, vertical mixing of MCDW, HSSW volume, and (albeit small) RIS basal melt. Winds and atmospheric temperatures from the SRES A1B scenario forcing of the MPI ECHAM5 model decreased on-shelf transport of CDW and vertical mixing of MCDW for 2046–61 and 2085–2100 relative to the end of the twentieth century. The RIS basal melt increased slightly by 2046–61 (9%) and 2085–2100 (13%). Advection of lower-salinity water onto the continental shelf did not significantly affect sea ice extent for the 2046–61 or 2085–2100 simulations. However, freshening reduces on-shelf transport of CDW, vertical mixing of MCDW, and the volume of HSSW produced. The reduced vertical mixing of MCDW, while partially balanced by the reduced on-shelf transport of CDW, enhances the RIS basal melt rate relative to the twentieth-century simulation for 2046–61 (13%) and 2085–2100 (17%).
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Sandal, Cathrine, and Doron Nof. "A New Analytical Model for Heinrich Events and Climate Instability." Journal of Physical Oceanography 38, no. 2 (February 1, 2008): 451–66. http://dx.doi.org/10.1175/2007jpo3722.1.
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Abstract The authors focus on Heinrich events and the question of whether the arrest and restart of convection can explain the associated sudden changes in oceanic and atmospheric temperature. For this purpose, a new (mixed) dynamical-box model is developed in which the ocean and atmosphere communicate via both Ekman layers and convection. The conservation of heat, salt, volume flux, and a “convection condition” yields a system of algebraic equations that are solved analytically. As expected, it is found that as the freshwater flux increases, the convective ocean temperature decreases. The heat flux from the ocean to the atmosphere, the transport of the oceanic meridional overturning cell (MOC), and the corresponding atmospheric flow generated by the heat flux from the ocean all decrease. However, the outgoing air temperature increases with increasing freshwater flux. This counterintuitive increase is because a decreased latent and sensible heat flux (to a humid atmosphere) means a reduced temperature difference between the warmer ocean and the cooler atmosphere, implying a cooler ocean and warmer atmosphere. For each wind speed, there is a critical freshwater flux beyond which the convection collapses and the temperatures of both the ocean and the air plunge because equatorial water is no longer flowing northward to replace the frigid northern waters. The above points to a potentially new instability process that was probably active during glaciation periods—when ice and snow are abundant, even the smallest amount of freshwater flux will cause local warming which, in turn, will cause increased melting, resulting in an ever-increased freshwater flux until the critical flux is reached and the MOC collapses. The model suggests that switching convection on and off changed the glacial ocean temperature by 4°C and the glacial air temperature by 12.5°C, both consistent with the Greenland Ice Sheet Project (GISP II) ice core record and the Centre Européen de Recherche et d’Enseignement des Géosciences de l’Environnement (CEREGE) alkenone record.
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Mayer, Johannes, Michael Mayer, and Leopold Haimberger. "Consistency and Homogeneity of Atmospheric Energy, Moisture, and Mass Budgets in ERA5." Journal of Climate 34, no. 10 (May 2021): 3955–74. http://dx.doi.org/10.1175/jcli-d-20-0676.1.
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AbstractThis study uses advanced numerical and diagnostic methods to evaluate the atmospheric energy budget with the fifth major global reanalysis produced by ECMWF (ERA5) in combination with observed and reconstructed top of the atmosphere (TOA) energy fluxes for the period 1985–2018. We assess the meridional as well as ocean–land energy transport and perform internal consistency checks using mass-balanced data. Furthermore, the moisture and mass budgets in ERA5 are examined and compared with previous budget evaluations using ERA-Interim as well as observation-based estimates. Results show that peak annual mean meridional atmospheric energy transports in ERA5 (4.58 ± 0.07 PW in the Northern Hemisphere) are weaker compared to ERA-Interim (4.74 ± 0.09 PW), where the higher spatial and temporal resolution of ERA5 can be excluded as a possible reason. The ocean–land energy transport in ERA5 is reliable at least from 2000 onward (~2.5 PW) such that the imbalance between net TOA fluxes and lateral energy fluxes over land are on the order of ~1 W m−2. Spinup and spindown effects as revealed from inconsistencies between analyses and forecasts are generally smaller and temporally less variable in ERA5 compared to ERA-Interim. Evaluation of the moisture budget shows that the ocean–land moisture transport and parameterized freshwater fluxes agree well in ERA5, while there are large inconsistencies in ERA-Interim. Overall, the quality of the budgets derived from ERA5 is demonstrably better than estimates from ERA-Interim. Still some particularly sensitive budget quantities (e.g., precipitation, evaporation, and ocean–land energy transport) show apparent inhomogeneities, especially in the late 1990s, which warrant further investigation and need to be considered in studies of interannual variability and trends.
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Wang, Chunzai, Liping Zhang, and Sang-Ki Lee. "Response of Freshwater Flux and Sea Surface Salinity to Variability of the Atlantic Warm Pool." Journal of Climate 26, no. 4 (February 15, 2013): 1249–67. http://dx.doi.org/10.1175/jcli-d-12-00284.1.
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Abstract The response of freshwater flux and sea surface salinity (SSS) to the Atlantic warm pool (AWP) variations from seasonal to multidecadal time scales is investigated by using various reanalysis products and observations. All of the datasets show a consistent response for all time scales: A large (small) AWP is associated with a local freshwater gain (loss) to the ocean, less (more) moisture transport across Central America, and a local low (high) SSS. The moisture budget analysis demonstrates that the freshwater change is dominated by the atmospheric mean circulation dynamics, while the effect of thermodynamics is of secondary importance. Further decomposition points out that the contribution of the mean circulation dynamics primarily arises from its divergent part, which mainly reflects the wind divergent change in the low level as a result of SST change. In association with a large (small) AWP, warmer (colder) than normal SST over the tropical North Atlantic can induce anomalous low-level convergence (divergence), which favors anomalous ascent (decent) and thus generates more (less) precipitation. On the other hand, a large (small) AWP weakens (strengthens) the trade wind and its associated westward moisture transport to the eastern North Pacific across Central America, which also favors more (less) moisture residing in the Atlantic and hence more (less) precipitation. The results imply that variability of freshwater flux and ocean salinity in the North Atlantic associated with the AWP may have the potential to affect the Atlantic meridional overturning circulation.
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Condron, Alan, Peter Winsor, Chris Hill, and Dimitris Menemenlis. "Simulated Response of the Arctic Freshwater Budget to Extreme NAO Wind Forcing." Journal of Climate 22, no. 9 (May 1, 2009): 2422–37. http://dx.doi.org/10.1175/2008jcli2626.1.
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Abstract The authors investigate the response of the Arctic Ocean freshwater budget to changes in the North Atlantic Oscillation (NAO) using a regional-ocean configuration of the Massachusetts Institute of Technology GCM (MITgcm) and carry out several different 10-yr and 30-yr integrations. At 1/6° (∼18 km) resolution the model resolves the major Arctic transport pathways, including Bering Strait and the Canadian Archipelago. Two main calculations are performed by repeating the wind fields of two contrasting NAO years in each run for the extreme negative and positive NAO phases of 1969 and 1989, respectively. These calculations are compared both with a control run and the compiled observationally based freshwater budget estimate of Serreze et al. The results show a clear response in the Arctic freshwater budget to NAO forcing, that is, repeat NAO negative wind forcing results in virtually all freshwater being retained in the Arctic, with the bulk of the freshwater content being pooled in the Beaufort gyre. In contrast, repeat NAO positive forcing accelerates the export of freshwater out of the Arctic to the North Atlantic, primarily via Fram Strait (∼900 km3 yr−1) and the Canadian Archipelago (∼500 km3 yr−1), with a total loss in freshwater storage of ∼13 000 km3 (15%) after 10 yr. The large increase in freshwater export through the Canadian Archipelago highlights the important role that this gateway plays in redistributing the freshwater of the Arctic to subpolar seas, by providing a direct pathway from the Arctic basin to the Labrador Sea, Gulf Stream system, and Atlantic Ocean. The authors discuss the sensitivity of the Arctic Ocean to long-term fixed extreme NAO states and show that the freshwater content of the Arctic is able to be restored to initial values from a depleted freshwater state after ∼20 yr.
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Snow, Kate, Andrew McC Hogg, Bernadette M. Sloyan, and Stephanie M. Downes. "Sensitivity of Antarctic Bottom Water to Changes in Surface Buoyancy Fluxes." Journal of Climate 29, no. 1 (December 31, 2015): 313–30. http://dx.doi.org/10.1175/jcli-d-15-0467.1.
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Abstract The influence of freshwater and heat flux changes on Antarctic Bottom Water (AABW) properties are investigated within a realistic bathymetry coupled ocean–ice sector model of the Atlantic Ocean. The model simulations are conducted at eddy-permitting resolution where dense shelf water production dominates over open ocean convection in forming AABW. Freshwater and heat flux perturbations are applied independently and have contradictory surface responses, with increased upper-ocean temperature and reduced ice formation under heating and the opposite under increased freshwater fluxes. AABW transport into the abyssal ocean reduces under both flux changes, with the reduction in transport being proportional to the net buoyancy flux anomaly south of 60°S. Through inclusion of shelf-sourced AABW, a process absent from most current generation climate models, cooling and freshening of dense source water is facilitated via reduced on-shelf/off-shelf exchange flow. Such cooling is propagated to the abyssal ocean, while compensating warming in the deep ocean under heating introduces a decadal-scale variability of the abyssal water masses. This study emphasizes the fundamental role buoyancy plays in controlling AABW, as well as the importance of the inclusion of shelf-sourced AABW within climate models in order to attain the complete spectrum of possible climate change responses.
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Hu, Aixue, Bette L. Otto-Bliesner, Gerald A. Meehl, Weiqing Han, Carrie Morrill, Esther C. Brady, and Bruce Briegleb. "Response of Thermohaline Circulation to Freshwater Forcing under Present-Day and LGM Conditions." Journal of Climate 21, no. 10 (May 15, 2008): 2239–58. http://dx.doi.org/10.1175/2007jcli1985.1.
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Abstract Responses of the thermohaline circulation (THC) to freshwater forcing (hosing) in the subpolar North Atlantic Ocean under present-day and the last glacial maximum (LGM) conditions are investigated using the National Center for Atmospheric Research Community Climate System Model versions 2 and 3. Three sets of simulations are analyzed, with each set including a control run and a freshwater hosing run. The first two sets are under present-day conditions with an open and closed Bering Strait. The third one is under LGM conditions, which has a closed Bering Strait. Results show that the THC nearly collapses in all three hosing runs when the freshwater forcing is turned on. The full recovery of the THC, however, is at least a century earlier in the open Bering Strait run than the closed Bering Strait and LGM runs. This is because the excessive freshwater is diverged almost equally toward north and south from the subpolar North Atlantic when the Bering Strait is open. A significant portion of the freshwater flowing northward into the Arctic exits into the North Pacific via a reversed Bering Strait Throughflow, which accelerates the THC recovery. When the Bering Strait is closed, this Arctic to Pacific transport is absent and freshwater can only be removed through the southern end of the North Atlantic. Together with the surface freshwater excess due to precipitation, evaporation, river runoff, and melting ice in the closed Bering Strait experiments after the hosing, the removal of the excessive freshwater takes longer, and this slows the recovery of the THC. Although the background conditions are quite different between the present-day closed Bering Strait run and the LGM run, the THC responds to the freshwater forcing added in the North Atlantic in a very similar manner.
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Nihashi, Sohey, Kay I. Ohshima, and Noriaki Kimura. "Creation of a Heat and Salt Flux Dataset Associated with Sea Ice Production and Melting in the Sea of Okhotsk." Journal of Climate 25, no. 7 (March 28, 2012): 2261–78. http://dx.doi.org/10.1175/jcli-d-11-00022.1.
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Abstract Sea ice formation, its transport, and its melting cause the redistribution of heat and salt, which plays an important role in the climate and biogeochemical systems. In the Sea of Okhotsk, a heat and salt flux dataset is created in which such sea ice processes are included, with a spatial resolution of ~12.5 km. The dataset is based on a heat budget analysis using ice concentration, thickness, and drift speed from satellite observations and the ECMWF Interim Re-Analysis (ERA-Interim) data. The salt flux calculation considers both salt supplied to the ocean from sea ice production and freshwater supplied when the ice melts. This dataset will be useful for the validation and boundary conditions of modeling studies. The spatial distribution of the annual fluxes shows a distinct contrast between north and south: significant ocean cooling with salt supply is shown in the northern coastal polynya region, while ocean heating with freshwater supply is shown in the south. This contrast suggests a transport of freshwater and negative heat by ice advection. The annual fluxes also show ocean cooling with freshwater supply in the Kashevarov Bank (KB) region and the central and eastern Sea of Okhotsk, suggesting the effect of warm water advection. In the ice melt season, relatively prominent ice melting is shown in the coastal polynya region, probably due to large solar heating of the upper ocean. This indicates that the polynya works as a “meltwater factory” in spring, contrasting with its role as an “ice factory” in winter. In the coastal polynya region, the spatial distribution of phytoplankton bloom roughly corresponds with the ice melt region.
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Levang, Samuel J., and Raymond W. Schmitt. "Intergyre Salt Transport in the Climate Warming Response." Journal of Physical Oceanography 50, no. 1 (January 2020): 255–68. http://dx.doi.org/10.1175/jpo-d-19-0166.1.
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ABSTRACTRegional connectivity is important to the global climate salinity response, particularly because salinity anomalies do not have a damping feedback with atmospheric freshwater fluxes and may therefore be advected over long distances by ocean circulation, resulting in nonlocal influences. Climate model intercomparison experiments such as CMIP5 exhibit large uncertainty in some aspects of the salinity response, hypothesized here to be a result of ocean dynamics. We use two types of Lagrangian particle tracking experiments to investigate pathways of exchange for salinity anomalies. The first uses forward trajectories to estimate average transport time scales between water cycle regimes. The second uses reverse trajectories and a freshwater accumulation method to quantitatively identify remote influences in the salinity response. Additionally, we compare velocity fields with both resolved and parameterized eddies to understand the impact of eddy stirring on intergyre exchange. These experiments show that surface anomalies are readily exchanged within the ocean gyres by the mean circulation, but intergyre exchange is slower and largely eddy driven. These dynamics are used to analyze the North Atlantic salinity response to climate warming and water cycle intensification, where the system is broadly forced with fresh surface anomalies in the subpolar gyre and salty surface anomalies in the subtropical gyres. Under these competing forcings, strong intergyre eddy fluxes carry anomalously salty subtropical water into the subpolar gyre which balances out much of the local freshwater input.
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Obata, Atsushi. "Climate–Carbon Cycle Model Response to Freshwater Discharge into the North Atlantic." Journal of Climate 20, no. 24 (December 15, 2007): 5962–76. http://dx.doi.org/10.1175/2007jcli1808.1.
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Abstract The response of a coupled climate–carbon cycle model to discharge of freshwater into the North Atlantic is investigated with regard to cold reversals caused by meltwater from northern continental ice sheets such as the Younger Dryas during the last deglaciation. The extreme case in which the North Atlantic thermohaline circulation ceases in several decades is discussed. In the preindustrial case, northern severe cooling is reproduced by the collapse of the Atlantic northward heat transport, and land carbon decreases because of a decrease in net primary production (NPP) by the cooling or precipitation decrease, resulting in a decrease in global air temperature and an increase in atmospheric carbon dioxide (CO2) concentration. The atmospheric CO2 increase by the land carbon decrease is consistent with a previous terrestrial vegetation model study and a minimum in ice core δ13CO2 during the Younger Dryas. The atmospheric CO2 increase in this model is less than 10 ppmv, consistent with the paleoclimatic records at the beginning of the Younger Dryas. This small increase results from the compensation between changes in CO2 sources and sinks of the land and ocean carbon cycles, such as the decrease in both NPP and soil respiration by the cooling. In the postindustrial case with fossil fuel CO2 emission, the same anomalies as the preindustrial case are found in the climate and the land carbon cycle in comparison with the control experiment without the freshwater discharge. After the termination of the freshwater discharge, the cold anomaly is globally reduced by the intense greenhouse effect of the increasing atmospheric CO2 on the order of 2000 ppmv, despite the weak thermohaline circulation in the North Atlantic. In contrast to the preindustrial case, the atmospheric CO2 concentration about 90 ppmv higher than the control experiment is caused mostly by the decrease in the ocean carbon uptake, which results from the decrease in the transport of anthropogenic carbon into the deep ocean by the collapse of the North Atlantic thermohaline circulation. The model results show the fact that there can be a case in the earth system where a decoupling can be seen between two environmental variables, air temperature and atmospheric CO2, that normally correlate with each other.
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Kolpakova, E. S. "Chlorophenol compounds in freshwater lakes of subarctic regions." Arctic and Antarctic Research 64, no. 4 (December 26, 2018): 380–90. http://dx.doi.org/10.30758/0555-2648-2018-64-4-380-390.
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Chlorophenol compounds, representing a group of chlorinated phenols and their derivatives, are ionic compounds with different degree of hydrophobicity, lipophilicity and acidity. As potential anthropogenic sources of pollution of ecosystems by chlorophenol compounds is the regional atmospheric transport of these compounds from the territories of border regions. At the same time, the sources of the diversity of chlorophenol compounds are the natural processes of enzymatic biosynthesis in the components of the environment. These compounds are especially dangerous for the Arctic and subarctic ecosystems, since the conditions of the cold climate contribute to their longterm preservation, which increases their negative impact on living organisms.The research area includes subarctic small thermokarst lakes located in the central part of Vaigach Island and in Bolshezemelskaya tundra.The isolation of chlorophenol compounds from bottom sediment samples was carried out by accelerated liquid flow-through extraction with a hot mixture of organic solvents with extraction of easily and hardly extractable chlorophenol compounds and subsequent analysis on a gas chromatograph with an electron-capture detector. The total content of chlorophenol compounds was determined by summing their concentrations in the easily and hardly extractable fractions.The bottom sediments of the investigated lakes were characterized by a relatively high content of chlorophenol compounds. Toxic pentachlorophenol was found only in the sediments of lake Tundra Bolshezemelskaya. The revealed presence of chlorophenol compounds in the bottom sediments of the investigated lakes suggests their spread in the environment by atmospheric transport from abiogenic sources, as well as natural enzymatic and biochemical processes in these Arctic waters.
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Pietsch, Renee B., Hinrich Grothe, Regina Hanlon, Craig W. Powers, Sunghwan Jung, Shane D. Ross, and David G. Schmale III. "Wind-driven spume droplet production and the transport ofPseudomonas syringaefrom aquatic environments." PeerJ 6 (September 26, 2018): e5663. http://dx.doi.org/10.7717/peerj.5663.
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Natural aquatic environments such as oceans, lakes, and rivers are home to a tremendous diversity of microorganisms. Some may cross the air-water interface within droplets and become airborne, with the potential to impact the Earth’s radiation budget, precipitation processes, and spread of disease. Larger droplets are likely to return to the water or adjacent land, but smaller droplets may be suspended in the atmosphere for transport over long distances. Here, we report on a series of controlled laboratory experiments to quantify wind-driven droplet production from a freshwater source for low wind speeds. The rate of droplet production increased quadratically with wind speed above a critical value (10-m equivalent 5.7 m/s) where droplet production initiated. Droplet diameter and ejection speeds were fit by a gamma distribution. The droplet mass flux and momentum flux increased with wind speed. Two mechanisms of droplet production, bubble bursting and fragmentation, yielded different distributions for diameter, speed, and angle. At a wind speed of about 3.5 m/s, aqueous suspensions of the ice-nucleating bacteriumPseudomonas syringaewere collected at rates of 283 cells m−2 s−1at 5 cm above the water surface, and at 14 cells m−2 s−1at 10 cm above the water surface. At a wind speed of about 4.0 m/s, aqueous suspensions ofP. syringaewere collected at rates of 509 cells m−2 s−1at 5 cm above the water surface, and at 81 cells m−2 s−1at 10 cm above the water surface. The potential for microbial flux into the atmosphere from aquatic environments was calculated using known concentrations of bacteria in natural freshwater systems. Up to 3.1 × 104 cells m−2 s−1of water surface were estimated to leave the water in potentially suspended droplets (diameters <100 µm). Understanding the sources and mechanisms for bacteria to aerosolize from freshwater aquatic sources may aid in designing management strategies for pathogenic bacteria, and could shed light on how bacteria are involved in mesoscale atmospheric processes.
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Mengel, M., A. Levermann, C. F. Schleussner, and A. Born. "Enhanced Atlantic subpolar gyre variability through baroclinic threshold in a Coarse Resolution Model." Earth System Dynamics Discussions 3, no. 1 (April 13, 2012): 259–78. http://dx.doi.org/10.5194/esdd-3-259-2012.
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Abstract. Direct observations, satellite measurements and paleorecords reveal strong variability in the Atlantic subpolar gyre on various time scales. Here we show that variations of comparable amplitude can only be simulated in a coupled climate model in the proximity of a dynamical threshold. The threshold and the associated dynamic response is due to a positive feedback involving increased salt transport in the subpolar gyre and enhanced deep convection in its center. A series of sensitivity experiments is performed with a coarse resolution ocean general circulation model coupled to a statistical-dynamical atmosphere model which in itself does not produce atmospheric variability. To simulate the impact of atmospheric variability, the model system is perturbed with freshwater forcing of varying but small amplitude and multidecadal to centennial periodicity, and observational variations in wind stress. While both freshwater and wind-stress-forcing have a small direct effect on the strength of the subpolar gyre, the magnitude of the gyre's response is strongly increased in the vicinity of the threshold. Our results thus indicate that baroclinic self-amplification in the North Atlantic ocean can play an important role in presently observed SPG variability and thereby North Atlantic climate variability on multidecadal scales.
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Yang, Haijun, Qin Wen, Jie Yao, and Yuxing Wang. "Bjerknes Compensation in Meridional Heat Transport under Freshwater Forcing and the Role of Climate Feedback." Journal of Climate 30, no. 14 (July 2017): 5167–85. http://dx.doi.org/10.1175/jcli-d-16-0824.1.
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Using a coupled Earth climate model, freshwater forcing experiments are performed to study the Bjerknes compensation (BJC) between meridional atmosphere heat transport (AHT) and meridional ocean heat transport (OHT). Freshwater hosing in the North Atlantic weakens the Atlantic meridional overturning circulation (AMOC) and thus reduces the northward OHT in the Atlantic significantly, leading to a cooling (warming) in the surface layer in the Northern (Southern) Hemisphere. This results in an enhanced Hadley cell and northward AHT. Meanwhile, the OHT in the Indo-Pacific is increased in response to the Hadley cell change, partially offsetting the reduced OHT in the Atlantic. Two compensations occur here: compensation between the AHT and the Atlantic OHT, and that between the Indo-Pacific OHT and the Atlantic OHT. The AHT change undercompensates the OHT change by about 60% in the extratropics, while the former overcompensates the latter by about 30% in the tropics due to the Indo-Pacific change. The BJC can be understood from the viewpoint of large-scale circulation change. However, the intrinsic mechanism of BJC is related to the climate feedback of the Earth system. The authors’ coupled model experiments confirm that the occurrence of BJC is an intrinsic requirement of local energy balance, and local climate feedback determines the extent of BJC, consistent with previous theoretical results. Even during the transient period of climate change, the BJC is well established when the ocean heat storage is slowly varying and its change is much weaker than the net local heat flux change at the ocean surface. The BJC can be deduced from the local climate feedback. Under the freshwater forcing, the overcompensation in the tropics is mainly caused by the positive longwave feedback related to clouds, and the undercompensation in the extratropics is due to the negative longwave feedback related to surface temperature change. Different dominant feedbacks determine different BJC scenarios in different regions, which are in essence constrained by local energy balance.
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Haines, K., M. Valdivieso, H. Zuo, and V. N. Stepanov. "Transports and budgets in a 1/4 ° global ocean reanalysis 1989–2010." Ocean Science 8, no. 3 (June 7, 2012): 333–44. http://dx.doi.org/10.5194/os-8-333-2012.
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Abstract. Large-scale ocean transports of heat and freshwater have not been well monitored, and yet the regional budgets of these quantities are important to understanding the role of the oceans in climate and climate change. In contrast, atmospheric heat and freshwater transports are commonly assessed from atmospheric reanalysis products, despite the presence of non-conserving data assimilation based on the wealth of distributed atmospheric observations as constraints. The ability to carry out ocean reanalyses globally at eddy-permitting resolutions of 1/4 ° or better, along with new global ocean observation programs, now makes a similar approach viable for the ocean. In this paper we examine the budgets and transports within a global high resolution ocean model constrained by ocean data assimilation, and compare them with independent oceanic and atmospheric estimates.
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Haines, K., M. Valdivieso, H. Zuo, and V. N. Stepanov. "Transports and budgets in a 1/4° global ocean reanalysis 1989–2010." Ocean Science Discussions 9, no. 1 (January 23, 2012): 261–90. http://dx.doi.org/10.5194/osd-9-261-2012.
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Abstract. Large scale ocean transports of heat and freshwater have not been well monitored, and yet the regional budgets of these quantities are vital to understanding the role of the oceans in climate and climate change. In contrast atmospheric heat and freshwater transports are commonly assessed from atmospheric reanalysis products, despite the presence of non-conserving data assimilation based on the wealth of distributed atmospheric observations as constraints. The ability to carry out ocean reanalyses globally at eddy permitting resolutions of 1/4° or better, along with new global ocean observation programs, now make a similar approach viable for the ocean. In this paper we examine the budgets and transports within a global high resolution ocean model constrained by ocean data assimilation, and compare them with independent ocean and atmospheric estimates.
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Spence, J. Paul, Michael Eby, and Andrew J. Weaver. "The Sensitivity of the Atlantic Meridional Overturning Circulation to Freshwater Forcing at Eddy-Permitting Resolutions." Journal of Climate 21, no. 11 (June 1, 2008): 2697–710. http://dx.doi.org/10.1175/2007jcli2103.1.
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Abstract The effect of increasing horizontal resolution is examined to assess the response of the Atlantic meridional overturning circulation (AMOC) to freshwater perturbations. Versions of a global climate model with horizontal resolutions ranging from 1.8° (latitude) × 3.6° (longitude) to 0.2° × 0.4° are used to determine if the AMOC response to freshwater forcing is robust to increasing resolution. In the preindustrial equilibrium climate, the representation of western boundary currents and meridional heat transport are improved with resolution. Freshwater forcings similar to the final drainage of proglacial Lakes Agassiz and Ojibway are applied evenly over the Labrador Sea and exclusively along the western boundary. The duration and maximum amplitude of model responses to freshwater forcing showed little sensitivity to increasing resolution. An evaluation with tracers of the forcing impact on different regions of North Atlantic Deep Water formation revealed the possibility that increases in Labrador Sea deep convection at higher resolution mitigate the effect of stronger boundary currents and enhanced mixing. With increasing resolution, there is less cooling in the subpolar west Atlantic, more cooling in the subpolar east Atlantic, and greater variability in the deep ocean response to the boundary forcing. While differences exist, the coarse-resolution model response remains robust at finer horizontal resolutions.
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Wills, Robert C., and Tapio Schneider. "Stationary Eddies and the Zonal Asymmetry of Net Precipitation and Ocean Freshwater Forcing." Journal of Climate 28, no. 13 (July 1, 2015): 5115–33. http://dx.doi.org/10.1175/jcli-d-14-00573.1.
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Abstract Transport of water vapor in the atmosphere generates substantial spatial variability of net precipitation (precipitation minus evaporation). Over half of the total spatial variability in annual-mean net precipitation is accounted for by deviations from the zonal mean. Over land, these regional differences determine differences in surface water availability. Over oceans, they account, for example, for the Pacific–Atlantic difference in sea surface salinity, with implications for the deep overturning circulation. This study analyzes the atmospheric water budget in reanalyses from ERA-Interim and MERRA, to investigate which physical balances lead to zonal variation in net precipitation. It is found that the leading-order contribution is zonal variation in stationary-eddy vertical motion. Transient eddies modify the pattern of zonally anomalous net precipitation by moving moisture from the subtropical and tropical oceans onto land and poleward across the Northern Hemisphere storm tracks. Zonal variation in specific humidity and stationary-eddy horizontal advection play a secondary role. The dynamics leading to net precipitation via vertical motion in stationary eddies can be understood from a lower-tropospheric vorticity budget. The large-scale variations of vertical motion are primarily described by Sverdrup balance and Ekman pumping, with some modification by transient eddies. These results suggest that it is important to understand changes in stationary eddies and their influence on the zonal variation of transient eddy fluxes, in order to understand regional changes in net precipitation. They highlight the relative importance of different atmospheric mechanisms for the freshwater forcing of the North Pacific and North Atlantic.
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Holland, Marika M., Joel Finnis, and Mark C. Serreze. "Simulated Arctic Ocean Freshwater Budgets in the Twentieth and Twenty-First Centuries." Journal of Climate 19, no. 23 (December 1, 2006): 6221–42. http://dx.doi.org/10.1175/jcli3967.1.
Full textAbstract:
Abstract The Arctic Ocean freshwater budgets in climate model integrations of the twentieth and twenty-first century are examined. An ensemble of six members of the Community Climate System Model version 3 (CCSM3) is used for the analysis, allowing the anthropogenically forced trends over the integration length to be assessed. Mechanisms driving trends in the budgets are diagnosed, and the implications of changes in the Arctic–North Atlantic exchange on the Labrador Sea and Greenland–Iceland–Norwegian (GIN) Seas properties are discussed. Over the twentieth and the twenty-first centuries, the Arctic freshens as a result of increased river runoff, net precipitation, and decreased ice growth. For many of the budget terms, the maximum 50-yr trends in the time series occur from approximately 1975 to 2025, suggesting that we are currently in the midst of large Arctic change. The total freshwater exchange between the Arctic and North Atlantic increases over the twentieth and twenty-first centuries with decreases in ice export more than compensated for by an increase in the liquid freshwater export. Changes in both the liquid and solid (ice) Fram Strait freshwater fluxes are transported southward by the East Greenland Current and partially removed from the GIN Seas. Nevertheless, reductions in GIN sea ice melt do result from the reduced Fram Strait transport and account for the largest term in the changing ocean surface freshwater fluxes in this region. This counteracts the increased ocean stability due to the warming climate and helps to maintain GIN sea deep-water formation.
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Clarke, Garry K. C., Andrew B. G. Bush, and John W. M. Bush. "Freshwater Discharge, Sediment Transport, and Modeled Climate Impacts of the Final Drainage of Glacial Lake Agassiz." Journal of Climate 22, no. 8 (April 15, 2009): 2161–80. http://dx.doi.org/10.1175/2008jcli2439.1.
Full textAbstract:
Abstract A cold event at around 8200 calendar years BP and the release, at around that time, of a huge freshwater outburst from ice-dammed glacial Lake Agassiz have lent support to the idea that the flood triggered the cold event. Some suggest that the freshwater addition caused a weakening of the North Atlantic meridional overturning circulation (MOC) thereby reducing the ocean transport of heat to high northern latitudes. Although several modeling efforts lend strength to this claim, the paleoceanographic record is equivocal. The authors’ aim is to use a coupled ocean–atmosphere model to examine the possibility that the two events are causally linked but that MOC reduction was not the main agent of change. It is found that the outburst flood and associated redirection of postflood meltwater drainage to the Labrador Sea, via Hudson Strait, can freshen the North Atlantic, leading to reduced salinity and sea surface temperature, and thus to increased sea ice production at high latitudes. The results point to the possibility that the preflood outflow to the St. Lawrence was extremely turbid and sufficiently dense to become hyperpycnal, whereas the postflood outflow through Hudson Strait had a lower load of suspended sediment and was buoyant.
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Rathore, Saurabh, Nathaniel L. Bindoff, Caroline C. Ummenhofer, Helen E. Phillips, and Ming Feng. "Near-Surface Salinity Reveals the Oceanic Sources of Moisture for Australian Precipitation through Atmospheric Moisture Transport." Journal of Climate 33, no. 15 (August 1, 2020): 6707–30. http://dx.doi.org/10.1175/jcli-d-19-0579.1.
Full textAbstract:
AbstractThe long-term trend of sea surface salinity (SSS) reveals an intensification of the global hydrological cycle due to human-induced climate change. This study demonstrates that SSS variability can also be used as a measure of terrestrial precipitation on interseasonal to interannual time scales, and to locate the source of moisture. Seasonal composites during El Niño–Southern Oscillation/Indian Ocean dipole (ENSO/IOD) events are used to understand the variations of moisture transport and precipitation over Australia, and their association with SSS variability. As ENSO/IOD events evolve, patterns of positive or negative SSS anomaly emerge in the Indo-Pacific warm pool region and are accompanied by atmospheric moisture transport anomalies toward Australia. During co-occurring La Niña and negative IOD events, salty anomalies around the Maritime Continent (north of Australia) indicate freshwater export and are associated with a significant moisture transport that converges over Australia to create anomalous wet conditions. In contrast, during co-occurring El Niño and positive IOD events, a moisture transport divergence anomaly over Australia results in anomalous dry conditions. The relationship between SSS and atmospheric moisture transport also holds for pure ENSO/IOD events but varies in magnitude and spatial pattern. The significant pattern correlation between the moisture flux divergence and SSS anomaly during the ENSO/IOD events highlights the associated ocean–atmosphere coupling. A case study of the extreme hydroclimatic events of Australia (e.g., the 2010/11 Brisbane flood) demonstrates that the changes in SSS occur before the peak of ENSO/IOD events. This raises the prospect that tracking of SSS variability could aid the prediction of Australian rainfall.