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Robert A. Tomas
,
Clara Deser
, and
Lantao Sun

Abstract

The purpose of this study is to elucidate the individual and combined roles of thermodynamic and dynamic ocean–atmosphere coupling in the equilibrium global climate response to projected Arctic sea ice loss using a suite of experiments conducted with Community Climate System Model, version 4, at 1° latitude–longitude spatial resolution. The results highlight the contrasting spatial structures and partially compensating effects of thermodynamic and dynamic coupling. In combination, thermodynamic and dynamic coupling produce a response pattern that is largely symmetric about the equator, whereas thermodynamic coupling alone yields an antisymmetric response. The latter is characterized by an interhemispheric sea surface temperature (SST) gradient, with maximum warming at high northern latitudes decreasing toward the equator, which displaces the intertropical convergence zone (ITCZ) and Hadley circulation northward. In contrast, the fully coupled response shows enhanced warming at high latitudes of both hemispheres and along the equator; the equatorial warming is driven by anomalous ocean heat transport convergence and is accompanied by a narrow equatorward intensification of the northern and southern branches of the ITCZ. In both cases, the tropical precipitation response to Arctic sea ice loss feeds back onto the atmospheric circulation at midlatitudes via Rossby wave dynamics, highlighting the global interconnectivity of the coupled climate system. This study demonstrates the importance of ocean dynamics in mediating the equilibrium global climate response to Arctic sea ice loss.

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Clara Deser
,
Susan Wahl
, and
John J. Bates

Abstract

Satellite observations of visible cloudiness and sea surface temperature (SST) are used to test the hypothesis that the configuration of cool low-level winds blowing across a sharp SST front in the equatorial eastern Pacific gives rise to stratiform clouds on the warm (downstream) side of the front. The results show that there is a maximum in low clouds over the equatorial front during the cold season of 1988 when the front and cross-isotherm winds were strong. The low-cloud maximum was reduced in the warm El Niño year of 1987, consistent with the weakening of the front. Instability waves along the equatorial front were pronounced during the summer and autumn of 1988. The results show a strong association between visible cloud and the SST waves, with enhanced (reduced) cloudiness in the warm troughs (cold crests) of the waves.

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David P. Schneider
,
Yuko Okumura
, and
Clara Deser

Abstract

This study reviews the mechanisms associated with Antarctic–tropical climate linkages and presents new analyses of the seasonality and spatial patterns of tropical climate signals in the Antarctic for the late 1950s to the present. Tropical climate signals are primarily communicated to the Antarctic via the Pacific–South America (PSA) pattern and the southern annular mode (SAM). The impacts of these circulation patterns and their tropical linkages are evident in regressions of seasonally stratified Antarctic station temperature data and annually resolved ice core records on global fields of sea surface temperature, sea level pressure, and precipitation. Temperature and ice core anomalies in the Antarctic Peninsula region and adjoining areas of West Antarctica are significantly impacted by the PSA, interpreted as a Rossby wave train driven by anomalous tropical deep convection during ENSO events. This pattern is most evident in the austral spring, consistent with recent studies, suggesting that atmospheric conditions for Rossby wave propagation are most favorable during this season. During austral summer at the peak of the ENSO cycle, temperature anomalies at East Antarctic coastal stations exhibit significant correlations with tropical Pacific anomalies. This linkage reflects the influence of anomalous tropical heating on the position and strength of the subtropical jets and is consistent with changes in eddy momentum fluxes that alter the mean meridional circulation associated with the SAM. Of the ice cores that exhibit tropical linkages, most tend to be associated with the PSA teleconnection. The implications of the study’s findings for understanding Antarctic climate variability and climate change from seasonal to decadal time scales are also discussed.

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Lantao Sun
,
Clara Deser
, and
Robert A. Tomas

Abstract

The impact of projected Arctic sea ice loss on the atmospheric circulation is investigated using the Whole Atmosphere Community Climate Model (WACCM), a model with a well-resolved stratosphere. Two 160-yr simulations are conducted: one with surface boundary conditions fixed at late twentieth-century values and the other with identical conditions except for Arctic sea ice, which is prescribed at late twenty-first-century values. Their difference isolates the impact of future Arctic sea ice loss upon the atmosphere. The tropospheric circulation response to the imposed ice loss resembles the negative phase of the northern annular mode, with the largest amplitude in winter, while the less well-known stratospheric response transitions from a slight weakening of the polar vortex in winter to a strengthening of the vortex in spring. The lack of a significant winter stratospheric circulation response is shown to be a consequence of largely cancelling effects from sea ice loss in the Atlantic and Pacific sectors, which drive opposite-signed changes in upward wave propagation from the troposphere to the stratosphere. Identical experiments conducted with Community Atmosphere Model, version 4, WACCM’s low-top counterpart, show a weaker tropospheric response and a different stratospheric response compared to WACCM. An additional WACCM experiment in which the imposed ice loss is limited to August–November reveals that autumn ice loss weakens the stratospheric polar vortex in January, followed by a small but significant tropospheric response in late winter and early spring that resembles the negative phase of the North Atlantic Oscillation, with attendant surface climate impacts.

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Clara Deser
,
Robert A. Tomas
, and
Lantao Sun

Abstract

The role of ocean–atmosphere coupling in the zonal-mean climate response to projected late twenty-first-century Arctic sea ice loss is investigated using Community Climate System Model version 4 (CCSM4) at 1° spatial resolution. Parallel experiments with different ocean model configurations (full-depth, slab, and no interactive ocean) allow the roles of dynamical and thermodynamic ocean feedbacks to be isolated. In the absence of ocean coupling, the atmospheric response to Arctic sea ice loss is confined to north of 30°N, consisting of a weakening and equatorward shift of the westerlies accompanied by lower tropospheric warming and enhanced precipitation at high latitudes. With ocean feedbacks, the response expands to cover the whole globe and exhibits a high degree of equatorial symmetry: the entire troposphere warms, the global hydrological cycle strengthens, and the intertropical convergence zones shift equatorward. Ocean dynamics are fundamental to producing this equatorially symmetric pattern of response to Arctic sea ice loss. Finally, the absence of a poleward shift of the wintertime Northern Hemisphere westerlies in CCSM4’s response to greenhouse gas radiative forcing is shown to result from the competing effects of Arctic sea ice loss and greenhouse warming on the meridional temperature gradient in middle latitudes.

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Jie He
,
Clara Deser
, and
Brian J. Soden

Abstract

The intrinsic atmospheric and ocean-induced tropical precipitation variability is studied using millennial control simulations with various degrees of ocean coupling. A comparison between the coupled simulation and the atmosphere-only simulation with climatological sea surface temperatures (SSTs) shows that a substantial amount of tropical precipitation variability is generated without oceanic influence. This intrinsic atmospheric variability features a red noise spectrum from daily to monthly time scales and a white noise spectrum beyond the monthly time scale. The oceanic impact is inappreciable for submonthly time scales but important at interannual and longer time scales. For time scales longer than a year, it enhances precipitation variability throughout much of the tropical oceans and suppresses it in some subtropical areas, preferentially in the summer hemisphere. The sign of the ocean-induced precipitation variability can be inferred from the local precipitation–SST relationship, which largely reflects the local feedbacks between the two, although nonlocal forcing associated with El Niño–Southern Oscillation also plays a role. The thermodynamic and dynamic nature of the ocean-induced precipitation variability is studied by comparing the fully coupled and slab ocean simulations. For time scales longer than a year, equatorial precipitation variability is almost entirely driven by ocean circulation, except in the Atlantic Ocean. In the rest of the tropics, ocean-induced precipitation variability is dominated by mixed layer thermodynamics. Additional analyses indicate that both dynamic and thermodynamic oceanic processes are important for establishing the leading modes of large-scale tropical precipitation variability. On the other hand, ocean dynamics likely dampens tropical Pacific variability at multidecadal time scales and beyond.

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Clara Deser
,
Laurent Terray
, and
Adam S. Phillips

Abstract

This study elucidates the physical mechanisms underlying internal and forced components of winter surface air temperature (SAT) trends over North America during the past 50 years (1963–2012) using a combined observational and modeling framework. The modeling framework consists of 30 simulations with the Community Earth System Model (CESM) at 1° latitude–longitude resolution, each of which is subject to an identical scenario of historical radiative forcing but starts from a slightly different atmospheric state. Hence, any spread within the ensemble results from unpredictable internal variability superimposed upon the forced climate change signal. Constructed atmospheric circulation analogs are used to estimate the dynamical contribution to forced and internal components of SAT trends: thermodynamic contributions are obtained as a residual. Internal circulation trends are estimated to account for approximately one-third of the observed wintertime warming trend over North America and more than half locally over parts of Canada and the United States. Removing the effects of internal atmospheric circulation variability narrows the spread of SAT trends within the CESM ensemble and brings the observed trends closer to the model’s radiatively forced response. In addition, removing internal dynamics approximately doubles the signal-to-noise ratio of the simulated SAT trends and substantially advances the “time of emergence” of the forced component of SAT anomalies. The methodological framework proposed here provides a general template for improving physical understanding and interpretation of observed and simulated climate trends worldwide and may help to reconcile the diversity of SAT trends across the models from phase 5 of the Coupled Model Intercomparison Project (CMIP5).

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David P. Schneider
,
Clara Deser
, and
Tingting Fan

Abstract

Westerly wind trends at 850 hPa over the Southern Ocean during 1979–2011 exhibit strong regional and seasonal asymmetries. On an annual basis, trends in the Pacific sector (40°–60°S, 70°–160°W) are 3 times larger than zonal-mean trends related to the increase in the southern annular mode (SAM). Seasonally, the SAM-related trend is largest in austral summer, and many studies have linked this trend with stratospheric ozone depletion. In contrast, the Pacific sector trends are largest in austral autumn. It is proposed that these asymmetries can be explained by a combination of tropical teleconnections and polar ozone depletion. Six ensembles of transient atmospheric model experiments, each forced with different combinations of time-dependent radiative forcings and SSTs, support this idea. In summer, the model simulates a positive SAM-like pattern, to which ozone depletion and tropical SSTs (which contain signatures of internal variability and warming from greenhouse gasses) contribute. In autumn, the ensemble-mean response consists of stronger westerlies over the Pacific sector, explained by a Rossby wave originating from the central equatorial Pacific. While these responses resemble observations, attribution is complicated by intrinsic atmospheric variability. In the experiments forced only with tropical SSTs, individual ensemble members exhibit wind trend patterns that mimic the forced response to ozone. When the analysis presented herein is applied to 1960–2000, the primary period of ozone loss, ozone depletion largely explains the model’s SAM-like zonal wind trend. The time-varying importance of these different drivers has implications for relating the historical experiments of free-running, coupled models to observations.

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Clara Deser
,
Robert Tomas
,
Michael Alexander
, and
David Lawrence

Abstract

The authors investigate the atmospheric response to projected Arctic sea ice loss at the end of the twenty-first century using an atmospheric general circulation model (GCM) coupled to a land surface model. The response was obtained from two 60-yr integrations: one with a repeating seasonal cycle of specified sea ice conditions for the late twentieth century (1980–99) and one with that of sea ice conditions for the late twenty-first century (2080–99). In both integrations, a repeating seasonal cycle of SSTs for 1980–99 was prescribed to isolate the impact of projected future sea ice loss. Note that greenhouse gas concentrations remained fixed at 1980–99 levels in both sets of experiments. The twentieth- and twenty-first-century sea ice (and SST) conditions were obtained from ensemble mean integrations of a coupled GCM under historical forcing and Special Report on Emissions Scenarios (SRES) A1B scenario forcing, respectively.

The loss of Arctic sea ice is greatest in summer and fall, yet the response of the net surface energy budget over the Arctic Ocean is largest in winter. Air temperature and precipitation responses also maximize in winter, both over the Arctic Ocean and over the adjacent high-latitude continents. Snow depths increase over Siberia and northern Canada because of the enhanced winter precipitation. Atmospheric warming over the high-latitude continents is mainly confined to the boundary layer (below ∼850 hPa) and to regions with a strong low-level temperature inversion. Enhanced warm air advection by submonthly transient motions is the primary mechanism for the terrestrial warming. A significant large-scale atmospheric circulation response is found during winter, with a baroclinic (equivalent barotropic) vertical structure over the Arctic in November–December (January–March). This response resembles the negative phase of the North Atlantic Oscillation in February only. Comparison with the fully coupled model reveals that Arctic sea ice loss accounts for most of the seasonal, spatial, and vertical structure of the high-latitude warming response to greenhouse gas forcing at the end of the twenty-first century.

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Clara Deser
,
Gudrun Magnusdottir
,
R. Saravanan
, and
Adam Phillips

Abstract

The wintertime atmospheric circulation responses to observed patterns of North Atlantic sea surface temperature and sea ice cover trends in recent decades are studied by means of experiments with an atmospheric general circulation model. Here the relationship between the forced responses and the dominant pattern of internally generated atmospheric variability is focused on. The total response is partioned into a portion that projects onto the leading mode of internal variability (the indirect response) and a portion that is the residual from that projection (the direct response). This empirical decomposition yields physically meaningful patterns whose distinctive horizontal and vertical structures imply different governing mechanisms. The indirect response, which dominates the total geopotential height response, is hemispheric in scale with resemblance to the North Atlantic Oscillation or Northern Hemisphere annular mode, and equivalent barotropic in the vertical from the surface to the tropopause. In contrast, the direct response is localized to the vicinity of the surface thermal anomaly (SST or sea ice) and exhibits a baroclinic structure in the vertical, with a surface trough and upper-level ridge in the case of a positive heating anomaly, consistent with theoretical models of the linear baroclinic response to extratropical thermal forcing. Both components of the response scale linearly with respect to the amplitude of the forcing but nonlinearly with respect to the polarity of the forcing. The deeper vertical penetration of anomalous heating compared to cooling is suggested to play a role in the nonlinearity of the response to SST forcing.

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