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This paper introduces and applies a new method to consistently estimate internal climate variability for all models within a multimodel ensemble. The method regresses each model's estimate of internal variability from the preindustrial control simulation on the variability derived from a model's ensemble simulations, thus providing practical evidence of the quasi-ergodic assumption. The method allows one to test in a multimodel consensus view how the internal variability of a variable changes for different forcing scenarios. Applying the method to the CMIP5 model ensemble shows that the internal variability of global-mean surface air temperature remains largely unchanged for historical simulations and might decrease for future simulations with a large CO2 forcing. Regionally, the projected changes reveal likely increases in temperature variability in the tropics, subtropics, and polar regions, and extremely likely decreases in midlatitudes. Applying the method to sea ice volume and area shows that their respective internal variability likely or extremely likely decreases proportionally to their mean state, except for Arctic sea ice area, which shows no consistent change across models. For the evaluation of CMIP5 simulations of Arctic and Antarctic sea ice, the method confirms that internal variability can explain most of the models' deviation from observed trends but often not the models' deviation from the observed mean states. The new method benefits from a large number of models and long preindustrial control simulations, but it requires only a small number of ensemble simulations. The method allows for consistent consideration of internal variability in multimodel studies and thus fosters understanding of the role of internal variability in a changing climate. [ABSTRACT FROM AUTHOR]

Changes in internal variability of seasonal and annual mean 2-m temperature in response to anthropogenic forcing are quantified for a global domain using climate models driven by a twenty-first-century high-emissions scenario. While changes in variance have been quantified previously in a univariate sense, the field significance of such changes has remained unclear. This paper proposes a new field significance test for changes in variance that accounts for spatial and temporal relationships within the domain. The test proposed here uses an optimization technique based on discriminant analysis, yielding results that are invariant to linear transformations of the data and therefore independent of normalization procedures. Multiple significance tests are employed because spatial fields can differ in many ways in a multivariate space. All climate models investigated here predict significant changes in internal variability of temperature in response to anthropogenic forcing. The models consistently predict decreases to temperature variance in regions of seasonal sea ice formation and across the Southern Ocean by the end of the twenty-first century. While more than half the models also predict significant changes in variance over ENSO regions and the North Atlantic Ocean, the direction of this change is model dependent. Seasonal mean changes are remarkably similar to annual mean changes, but there are model-dependent exceptions. Some models predict future variability that is more than double their preindustrial control variability, raising questions about the adequacy of doubling uncertainty estimates to test robustness in detection and attribution studies. [ABSTRACT FROM AUTHOR]

Much of the existing theory for the ocean's overturning circulation considers steady-state equilibrium solutions. However, Earth's climate is not in a steady state, and a better understanding of the ocean's nonequilibrium response to changes in the surface climate is urgently needed. Here, the time-dependent response of the deep-ocean overturning circulation to atmospheric warming is examined using a hierarchy of idealized ocean models. The transient response to surface warming is characterized by a shoaling and weakening of the Atlantic meridional overturning circulation (AMOC)—consistent with results from coupled climate simulations. The initial shoaling and weakening of the AMOC occurs on decadal time scales and is attributed to a rapid warming of northern-sourced deep water. The equilibrium response to warming, in contrast, is associated with a deepening and strengthening of the AMOC. The eventual deepening of the AMOC is argued to be associated with abyssal density changes and driven by modified surface fluxes in the Southern Ocean, following a reduction of the Antarctic sea ice cover. Full equilibration of the AMOC requires a diffusive adjustment of the abyss and takes many millennia. The equilibration time scale is much longer than most coupled climate model simulations, highlighting the importance of considering integration time and initial conditions when interpreting the deep-ocean circulation in climate models. The results also show that past climates are unlikely to be an adequate analog for changes in the overturning circulation during the coming decades or centuries. [ABSTRACT FROM AUTHOR]

The Southern Ocean (SO) is the region of the World Ocean bordering on Antarctica over which significant exchanges between the atmosphere, the ocean, and the sea ice take place. Here, the strong and nearly unhindered eastward flow of the Antarctic Circumpolar Current plays an important role in mean global climate as it transmits climate anomalies around the hemisphere. Features of interannual variability have been observed to propagate eastward around the SO with the circumpolar flow in the form of a system of coupled anomalies, known as the Antarctic circumpolar wave (ACW). In the present study, the 142-yr series of the Twentieth Century Reanalysis, version 2, dataset (850-hPa geopotential height, sea level pressure, sea surface temperature, surface meridional wind, and surface air temperature) spanning from 1871 to 2012 is used to investigate the presence and variability of ACWs. This examination shows, for the first time, the presence of the ACW before the mid-1950s and interdecadal changes in its characteristics. Modifications in the strength and speed of the circumpolar wave are shown to be linked with large-scale climate changes. Complex empirical orthogonal function analyses confirm that the ACW becomes apparent when the tropical El Niño-Southern Oscillation (ENSO) signal gives rise to the Pacific-South American (PSA) pattern and is a consequence of the constructive combination of the PSA and the subantarctic zonal wavenumber 3. Correlation analyses are also performed to quantify the role played by ENSO teleconnections for the appearance of the ACW, and the impact on the presence of ACWs of three super-El Niño events is investigated. [ABSTRACT FROM AUTHOR]

The average predictability time (APT) method is used to identify the most predictable components of decadal sea surface temperature (SST) variations over the Southern Ocean (SO) in a 4000-yr unforced control run of the GFDL CM2.1 model. The most predictable component shows significant predictive skill for periods as long as 20 years. The physical pattern of this variability has a uniform sign of SST anomalies over the SO, with maximum values over the Amundsen-Bellingshausen-Weddell Seas. Spectral analysis of the associated APT time series shows a broad peak on time scales of 70-120 years. This most predictable pattern is closely related to the mature phase of a mode of internal variability in the SO that is associated with fluctuations of deep ocean convection. The second most predictable component of SO SST is characterized by a dipole structure, with SST anomalies of one sign over the Weddell Sea and SST anomalies of the opposite sign over the Amundsen-Bellingshausen Seas. This component has significant predictive skill for periods as long as 6 years. This dipole mode is associated with a transition between phases of the dominant pattern of SO internal variability. The long time scales associated with variations in SO deep convection provide the source of the predictive skill of SO SST on decadal scales. These analyses suggest that if the SO deep convection in a numerical forecast model could be adequately initialized, the future evolution of SO SST and its associated climate impacts are potentially predictable. [ABSTRACT FROM AUTHOR]

This study explores the potential predictability of the Southern Ocean (SO) climate on decadal time scales as represented in the GFDL CM2.1 model using prognostic methods. Perfect model predictability experiments are conducted starting from 10 different initial states, showing potentially predictable variations of Antarctic bottom water (AABW) formation rates on time scales as long as 20 years. The associated Weddell Sea (WS) subsurface temperatures and Antarctic sea ice have potential predictability comparable to that of the AABW cell. The predictability of sea surface temperature (SST) variations over the WS and the SO is somewhat smaller, with predictable scales out to a decade. This reduced predictability is likely associated with stronger damping from air-sea interaction. As a complement to this perfect predictability study, the authors also make hindcasts of SO decadal variability using the GFDL CM2.1 decadal prediction system. Significant predictive skill for SO SST on multiyear time scales is found in the hindcast system. The success of the hindcasts, especially in reproducing observed surface cooling trends, is largely due to initializing the state of the AABW cell. A weak state of the AABW cell leads to cooler surface conditions and more extensive sea ice. Although there are considerable uncertainties regarding the observational data used to initialize the hindcasts, the consistency between the perfect model experiments and the decadal hindcasts at least gives some indication as to where and to what extent skillful decadal SO forecasts might be possible. [ABSTRACT FROM AUTHOR]

The relationship between climate modes and Antarctic sea ice is explored by separating the variability into intraseasonal, interannual, and decadal time scales. Cross-spectral analysis shows that geopotential height and Antarctic sea ice extent are most coherent at periods between about 20 and 40 days (the intraseasonal time scale). In this period range, where the atmospheric circulation and the sea ice extent are most tightly coupled, sea ice variability responds strongly to Rossby waves with the structure of the Pacific-South American (PSA) pattern. The PSA pattern in this time scale is not directly related to El Niño-Southern Oscillation (ENSO) or the southern annular mode (SAM), which have received much attention for explaining Antarctic sea ice variability. On the interannual time scale, ENSO and SAM are important, but a large fraction of sea ice variance can also be explained by Rossby wave-like structures in the Drake Passage region. After regressing out the sea ice extent variability associated with ENSO, the observed positive sea ice trends in Ross Sea and Indian Ocean during the satellite era become statistically insignificant. Regressing out SAM makes the sea ice trend in the Indian Ocean insignificant. Thus, the positive trends in sea ice in the Ross Sea and the Indian Ocean sectors may be explained by the variability and decadal trends of known interannual climate modes. [ABSTRACT FROM AUTHOR]

Using optimal fingerprinting techniques, a detection analysis is performed to determine whether observed trends in Southern Ocean sea ice extent since 1979 are outside the expected range of natural variability. Consistent with previous studies, it is found that for the seasons of maximum sea ice cover (i.e., winter and early spring), the observed trends are not outside the range of natural variability and in some West Antarctic sectors they may be partially due to tropical variability. However, when information about the spatial pattern of trends is included in the analysis, the summer and autumn trends fall outside the range of internal variability. The detectable signal is dominated by strong and opposing trends in the Ross Sea and the Amundsen-Bellingshausen Sea regions. In contrast to the observed pattern, an ensemble of 20 CMIP5 coupled climate models shows that a decrease in Ross Sea ice cover would be expected in response to external forcings. The simulated decreases in the Ross, Bellingshausen, and Amundsen Seas for the autumn season are significantly different from unforced internal variability at the 95% confidence level. Unlike earlier work, the authors formally show that the simulated sea ice response to external forcing is different from both the observed trends and simulated internal variability and conclude that in general the CMIP5 models do not adequately represent the forced response of the Antarctic climate system. [ABSTRACT FROM AUTHOR]

The sensitivity of the reflection of shortwave radiation over the Southern Ocean to the cloud properties there is estimated using observations from a suite of passive and active satellite instruments in combination with radiative transfer modeling. A composite cloud property observational data description is constructed that consistently incorporates mean cloud liquid water content, ice water content, liquid and ice particle radius information, vertical structure, vertical overlap, and spatial aggregation of cloud water as measured by optical depth versus cloud-top pressure histograms. The observational datasets used are Moderate Resolution Imaging Spectroradiometer (MODIS) effective radius filtered to mitigate solar zenith angle bias, the Multiangle Imaging Spectroradiometer (MISR) cloud-top height-optical depth (CTH-OD) histogram, the liquid water path from the University of Wisconsin dataset, and ice cloud properties from CloudSat. This cloud database is used to compute reflected shortwave radiation as a function of month and location over the ocean from 40° to 60°S, which compares well with observations of reflected shortwave radiation. This calculation is then used to test the sensitivity of the seasonal variation of shortwave reflection to the observed seasonal variation of cloud properties. Effective radius decreases during the summer season, which results in an increase in reflected solar radiation of 4-8 W m−2 during summer compared to what would be reflected if the effective radius remained constant at its annual-mean value. Summertime increases in low cloud fraction similarly increase the summertime reflection of solar radiation by 9-11 W m−2. In-cloud liquid water path is less in summertime, causing the reflected solar radiation to be 1-4 W m−2 less. [ABSTRACT FROM AUTHOR]

Unlike the rapid sea ice losses reported in the Arctic, satellite observations show an overall increase in Antarctic sea ice concentration over recent decades. However, observations of decadal trends in Antarctic ice thickness, and hence ice volume, do not currently exist. In this study a model of the Southern Ocean and its sea ice, forced by atmospheric reanalyses, is used to assess 1992-2010 trends in ice thickness and volume. The model successfully reproduces observations of mean ice concentration, thickness, and drift, and decadal trends in ice concentration and drift, imparting some confidence in the hindcasted trends in ice thickness. The model suggests that overall Antarctic sea ice volume has increased by approximately 30 km3 yr−1 (0.4% yr−1) as an equal result of areal expansion (20 × 103 km2 yr−1 or 0.2% yr−1) and thickening (1.5 mm yr−1 or 0.2% yr−1). This ice volume increase is an order of magnitude smaller than the Arctic decrease, and about half the size of the increased freshwater supply from the Antarctic Ice Sheet. Similarly to the observed ice concentration trends, the small overall increase in modeled ice volume is actually the residual of much larger opposing regional trends. Thickness changes near the ice edge follow observed concentration changes, with increasing concentration corresponding to increased thickness. Ice thickness increases are also found in the inner pack in the Amundsen and Weddell Seas, where the model suggests that observed ice-drift trends directed toward the coast have caused dynamical thickening in autumn and winter. Modeled changes are predominantly dynamic in origin in the Pacific sector and thermodynamic elsewhere. [ABSTRACT FROM AUTHOR]

The authors compare Community Earth System Model results to marine observations for the 1990s and examine climate change impacts on biogeochemistry at the end of the twenty-first century under two future scenarios (Representative Concentration Pathways RCP4.5 and RCP8.5). Late-twentieth-century seasonally varying mixed layer depths are generally within 10 m of observations, with a Southern Ocean shallow bias. Surface nutrient and chlorophyll concentrations exhibit positive biases at low latitudes and negative biases at high latitudes. The volume of the oxygen minimum zones is overestimated. The impacts of climate change on biogeochemistry have similar spatial patterns under RCP4.5 and RCP8.5, but perturbation magnitudes are larger under RCP8.5. Increasing stratification leads to weaker nutrient entrainment and decreased primary and export production (>30% over large areas). The global-scale decreases in primary and export production scale linearly with the increases in mean sea surface temperature. There are production increases in the high nitrate, low chlorophyll (HNLC) regions, driven by lateral iron inputs from adjacent areas. The increased HNLC export partially compensates for the reductions in non-HNLC waters (~25% offset). Stabilizing greenhouse gas emissions and climate by the end of this century (as in RCP4.5) will minimize the changes to nutrient cycling and primary production in the oceans. In contrast, continued increasing emission of CO2 (as in RCP8.5) will lead to reduced productivity and significant modifications to ocean circulation and biogeochemistry by the end of this century, with more drastic changes beyond the year 2100 as the climate continues to rapidly warm. [ABSTRACT FROM AUTHOR]

The new Community Climate System Model, version 4 (CCSM4), provides a powerful tool to understand and predict the earth's climate system. Several aspects of the Southern Ocean in the CCSM4 are explored, including the surface climatology and interannual variability, simulation of key climate water masses (Antarctic Bottom Water, Subantarctic Mode Water, and Antarctic Intermediate Water), the transport and structure of the Antarctic Circumpolar Current, and interbasin exchange via the Agulhas and Tasman leakages and at the Brazil-Malvinas Confluence. It is found that the CCSM4 has varying degrees of accuracy in the simulation of the climate of the Southern Ocean when compared with observations. This study has identified aspects of the model that warrant further analysis that will result in a more comprehensive understanding of ocean-atmosphere-ice dynamics and interactions that control the earth's climate and its variability. [ABSTRACT FROM AUTHOR]

The sensitivity of the overturning circulation in the Southern Ocean to the recent decadal strengthening of the overlying winds is being discussed intensely, with some works attributing an inferred saturation of the Southern Ocean CO2 sink to an intensification of the overturning circulation, while others have argued that this circulation is insensitive to changes in winds. Fundamental to reconciling these diverse views is to understand properly the role of eddies in counteracting the directly wind-forced changes in overturning. Here, the authors use novel theoretical considerations and fine-resolution ocean models to develop a new scaling for the sensitivity of eddy-induced mixing to changes in winds, and they demonstrate that changes in Southern Ocean overturning in response to recent and future changes in wind stress forcing are likely to be substantial, even in the presence of a decadally varying eddy field. This result has significant implications for the ocean's role in the carbon cycle, and hence global climate. [ABSTRACT FROM AUTHOR]

The atmospheric moisture budget from the Modern Era Retrospective-Analysis for Research and Applications (MERRA) is evaluated in polar regions for the period 1979--2005 and compared with previous estimates, accumulation syntheses over polar ice sheets, and in situ Arctic precipitation observations. The system is based on a nonspectral background model and utilizes the incremental analysis update scheme. The annual moisture convergence from MERRA for the north polar cap is comparable to previous estimates using 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40) and earlier reanalyses but it is more than 50%% larger than MERRA precipitation minus evaporation ( P −− E) computed from physics output fields. This imbalance is comparable to earlier reanalyses for the Arctic. For the south polar cap, the imbalance is 20%%. The MERRA physics output fields are also found to be overly sensitive to changes in the satellite observing system, particularly over data-sparse regions of the Southern Ocean. Comparisons between MERRA and prognostic fields from two contemporary reanalyses yield a spread of values from 6%% of the mean over the Antarctic Ice Sheet to 61%% over a domain of the Arctic Ocean. These issues highlight continued problems associated with the representation of cold-climate physical processes in global data assimilation models. The distribution of MERRA surface fluxes over the major polar ice sheets emphasizes larger values along the coastal escarpments, which agrees more closely with recent assessments of ice sheet accumulation using regional models. Differences between these results and earlier assessments illustrate a continued ambiguity in the surface moisture flux distribution over Greenland and Antarctica. The higher spatial and temporal resolution as well as the availability of all budget components, including analysis increments in MERRA, offer prospects for an improved representation of the high-latitude water cycle in reanalyses. [ABSTRACT FROM AUTHOR]

Moderate Resolution Imaging Spectroradiometer (MODIS) Level 2 observations from the Terra satellite are used to create a 3-yr climatology of cloud-top phase over a section of the Southern Ocean (south of Australia) and the North Pacific Ocean. The intent is to highlight the extensive presence of supercooled liquid water over the Southern Ocean region, particularly during summer. The phase of such clouds directly affects the absorbed shortwave radiation, which has recently been found to be ''poorly simulated in both state-of-the-art reanalysis and coupled global climate models'' (Trenberth and Fasullo). The climatology finds that supercooled liquid water is present year-round in the low-altitude clouds across this section of the Southern Ocean. Further, the MODIS cloud phase algorithm identifies very few glaciated cloud tops at temperatures above −−20°°C, rather inferring a large portion of ''uncertain'' cloud tops. Between 50°° and 60°°S during the summer, the albedo effect is compounded by a seasonal reduction in high-level cirrus. This is in direct contrast to the Bering Sea and Gulf of Alaska. Here MODIS finds a higher likelihood of observing warm liquid water clouds during summer and a reduction in the relative frequency of cloud tops within the 0°° to −−20°°C temperature range. As the MODIS cloud phase product has limited ability to confidently identify cloud-top phase between −−5°° and −−25°°C, future research should include observations from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and other space-based sensors to help with the classification within this temperature range. Further, multiregion in situ verification of any remotely sensed observations is vital to further understanding the cloud phase processes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The increase in atmospheric CO2 over this century depends on the evolution of the oceanic air--sea CO2 uptake, which will be driven by the combined response to rising atmospheric CO2 itself and climate change. Here, the future oceanic CO2 uptake is simulated using an ensemble of coupled climate--carbon cycle models. The models are driven by CO2 emissions from historical data and the Special Report on Emissions Scenarios (SRES) A2 high-emission scenario. A linear feedback analysis successfully separates the regional future (2010--2100) oceanic CO2 uptake into a CO2-induced component, due to rising atmospheric CO2 concentrations, and a climate-induced component, due to global warming. The models capture the observation-based magnitude and distribution of anthropogenic CO2 uptake. The distributions of the climate-induced component are broadly consistent between the models, with reduced CO2 uptake in the subpolar Southern Ocean and the equatorial regions, owing to decreased CO2 solubility; and reduced CO2 uptake in the midlatitudes, owing to decreased CO2 solubility and increased vertical stratification. The magnitude of the climate-induced component is sensitive to local warming in the southern extratropics, to large freshwater fluxes in the extratropical North Atlantic Ocean, and to small changes in the CO2 solubility in the equatorial regions. In key anthropogenic CO2 uptake regions, the climate-induced component offsets the CO2-induced component at a constant proportion up until the end of this century. This amounts to approximately 50%% in the northern extratropics and 25%% in the southern extratropics and equatorial regions. Consequently, the detection of climate change impacts on anthropogenic CO2 uptake may be difficult without monitoring additional tracers, such as oxygen. [ABSTRACT FROM AUTHOR]

The role of tectonic Southern Ocean gateway changes in driving Antarctic climate change at the Eocene–Oligocene boundary remains a topic of debate. One approach taken in previous idealized modeling studies of gateway effects has been to alter modern boundary conditions, whereby the Drake Passage becomes closed. Here, the authors follow this approach but vary atmospheric pCO2 over a range of values when comparing gateway configurations. They find a significantly greater sensitivity of Antarctic temperatures to Southern Ocean gateway changes when atmospheric pCO2 is high than when concentrations are low and the ambient climate is cool. In particular, the closure of the Drake Passage (DP) gap is a necessary condition for the existence of ice-free Antarctic conditions at high CO2 concentrations in this coupled climate model. The absence of the Antarctic Circumpolar Current (ACC) is particularly conducive to warm Antarctic conditions at higher CO2 concentrations, which is markedly different from previous simulations conducted under present-day CO2 conditions. The reason for this is the reduction of sea ice associated with higher CO2. Antarctic sea surface temperature and surface air temperature warming due to a closed DP gap reach values around ∼5° and ∼7°C, respectively, for high concentrations of CO2 (above 1250 ppm). In other words, the authors find a significantly greater sensitivity of Antarctic temperatures to atmospheric CO2 concentration when the DP is closed compared to when it is open. The presence of a DP gap inhibits a return to warmer and more Eocene-like Antarctic and deep ocean conditions, even under enhanced atmospheric greenhouse gas concentrations. [ABSTRACT FROM AUTHOR]

Changes in the temperature, salinity, and subduction of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) between the 1950s and 2090s are diagnosed using the CSIRO Mark version 3.5 (Mk3.5) climate system model Caps under a CO2 forcing that reaches 860 ppm by the year 2100. These Southern Ocean upper-limb water masses ventilate the ocean interior, and changes in their properties have been related to climate change in numerous studies. Over time, the authors follow the low potential vorticity and salinity minimum layers describing SAMW and AAIW and find that the water column in the 2090s shifts to lighter densities by approximately 0.2 kg m-3. The model projects a reduction in the SAMW and AAIW annual mean subduction rates as a result of a combination of a shallower mixed layer, increased potential vorticity at the base of the mixed layer, and a net buoyancy gain. There is little change in the projected total volume of SAMW transported into the ocean interior via the subduction process; however, the authors find a significant decrease in the subduction of AAIW. The authors find overall that increases in the air–sea surface heat and freshwater fluxes mainly control the reduction in the mean loss of the SAMW and AAIW surface buoyancy flux when compared with the effect of changes supplied by Ekman transport because of increased zonal wind stress. In the A2 scenario, there are cooling and freshening on neutral density surfaces less than 27.3 kg m-3 in response to the warming and freshening observed at the ocean’s surface. The model projects deepening of density surfaces due to southward shifts in the outcrop regions and the downward displacement of these surfaces north of 45°S. The volume transport across 32°S is predicted to decrease in all three basins, with southward transport of SAMW and AAIW decreasing by up to 1.2 and 2.0 Sv (1 Sv ≡ 106 m3 s-1), respectively, in the Indian Ocean. These projected reductions in the subduction and transport of mode and intermediate water masses in the CSIRO Mk3.5 model could potentially decrease the absorption and storage of CO2 in the Southern Ocean. [ABSTRACT FROM AUTHOR]

Climate change over the last several decades is suggested to cause a decrease in the magnitude of the uptake of CO2 by the Southern Ocean (Le Quere et al.). In this study, the atmospheric fields from NCEP R1 for the years 1948–2003 are used to drive an ocean biogeochemical model to probe how changes in the heat and freshwater fluxes and in the winds affect the Southern Ocean’s uptake of carbon. Over this period, the model simulations herein show that the increases in heat and freshwater fluxes drive a net increase in Southern Ocean uptake (south of 40°S) while the increases in wind stresses drive a net decrease in uptake. The total Southern Ocean response is nearly identical with the simulation without climate change because the heat and freshwater flux response is approximately both equal and opposite to the wind stress response. It is also shown that any change in the Southern Ocean anthropogenic carbon uptake is always opposed by a much larger change in the natural carbon air–sea exchange. For the 1948–2003 period, the changes in the natural carbon cycle dominate the Southern Ocean carbon uptake response to climate change. However, it is shown with a simple box model that when atmospheric CO2 levels exceed the partial pressure of carbon dioxide (pCO2) of the upwelled Circumpolar Deep Water (≈450 μatm) the Southern Ocean uptake response will be dominated by the changes in anthropogenic carbon uptake. Therefore, the suggestion that the Southern Ocean carbon uptake is a positive feedback to global warming is only a transient response that will change to a negative feedback in the near future if the present climate trend continues. Associated with the increased outgassing of carbon from the natural carbon cycle was a reduction in the aragonite saturation state of the high-latitude Southern Ocean (south of 60°S). In the simulation with just wind stress changes, the reduction in the high-latitude Southern Ocean aragonite saturation state (≈0.2) was comparable to the magnitude of the decline in the aragonite saturation state over the last 4 decades because of rising atmospheric CO2 levels (≈0.2). The simulation showed that climate change could significantly impact aragonite saturation state in the Southern Ocean. [ABSTRACT FROM AUTHOR]

The role played by Southern Hemisphere sea ice in the global climate system is explored using an earth system climate model of intermediate complexity. An ensemble of experiments is analyzed in which freshwater forcing equivalent to a complete 100-yr meltback of Southern Hemisphere sea ice is applied to a model run that simulates the present climate. This freshwater forcing acts to mildy subdue Southern Ocean deep overturning, reducing mean Antarctic Bottom Water (AABW) export by 0.5 Sv (1 Sv ≡ 106 m3 s-1) in the ensemble average. The decreased convective overturning cools the surface waters, thereby increasing sea ice volume and thus forming a negative feedback that stabilizes Antarctic sea ice. In contrast, the reduced convective overturn warms subsurface waters in the Southern Ocean, which, combined with the imposed freshening, results in a reduction in the meridional steric height gradient and hence a slowdown of the Antarctic Circumpolar Current (ACC). The reduction in ACC strength is, however, only modest at 1.5 Sv. These responses are thus of only weak magnitude, and the system recovers to its original state over time scales of decades. An extreme scenario experiment with essentially instantaneous addition of this meltwater load shows similar results, indicating the limited response of the climate system to the freshening implied by Antarctic sea ice melt. An additional experiment in which a much larger freshwater forcing of approximately 0.4 Sv is applied over 100 yr confirms the relatively weak response of the model’s climate state to such forcing, relative to the well-documented climatic effects of freshwater forcing added to the North Atlantic. [ABSTRACT FROM AUTHOR]

The stability of Antarctic Bottom Water (AABW) to freshwater (FW) perturbations is investigated in a coupled climate model of intermediate complexity. It is found that AABW is stable to surface freshwater fluxes greater in volume and rate to those that permanently “shut down” North Atlantic Deep Water (NADW). Although AABW weakens during FW forcing, it fully recovers within 50 yr of termination of FW input. This is due in part to a concurrent deep warming during AABW suppression that acts to eventually destabilize the water column. In addition, the prevailing upwelling of Circumpolar Deep Water and northward Ekman transport across the Antarctic Circumpolar Current, regulated by the subpolar westerly winds, limits the accumulation of FW at high latitudes and provides a mechanism for resalinizing the surface after the FW forcing has ceased. Enhanced sea ice production in the cooler AABW suppressed state also aids in the resalinization of the surface after FW forcing is stopped. Convection then restarts with AABW properties only slightly colder and fresher compared to the unperturbed control climate state. Further experiments with larger FW perturbations and very slow application rates (0.2 Sv/1000 yr) (1 Sv ≡ 106 m3 s-1) confirm the lack of multiple steady states of AABW in the model. This contrasts with the North Atlantic, wherein classical hysteresis behavior is obtained with similar forcing. The climate response to reduced AABW production is also investigated. During peak FW forcing, Antarctic surface sea and air temperatures decrease by a maximum of 2.5° and 2.2°C, respectively. This is of a similar magnitude to the corresponding response in the North Atlantic. Although in the final steady state, the AABW experiment returns to the original control climate, whereas the North Atlantic case transitions to a different steady state characterized by substantial regional cooling (up to 6.0°C surface air temperature). [ABSTRACT FROM AUTHOR]

The authors assess the role of time-dependent eddy variability in the Antarctic Circumpolar Current (ACC) in influencing warming of the Southern Ocean. For this, an eddy-resolving quasigeostrophic model of the wind-driven circulation is used, and the response of circumpolar transport, eddy kinetic energy, and eddy heat transport to changes in winds is quantified. On interannual time scales, the model exhibits the behavior of an “eddy saturated” ocean state, where increases in wind stress do not significantly change the circumpolar transport, but instead enhance the eddy field. This is in accord with previous dynamical arguments, and a recent observational study. The instantaneous response to increased wind stress is to cool temperatures through increased northward Ekman transport of cool water. But, in the longer term, the enhanced eddy state is more efficient at transporting heat, leading to a warming of the ocean. The total eddy heat flux response is greater than the Ekman transport heat flux in this model by a factor of 2, indicating that coarse (non eddy resolving) models may fail to adequately capture the key processes. The authors also test the model response to long-term changes in wind forcing, including steadily increasing circumpolar wind strength over a 30-yr period. The model shows a response in eddy heat flux, and a change in ocean temperature not dissimilar from observed Southern Ocean warming. These findings suggest that eddy heat flux, energized by increasing wind stress, may be a significant contributor to the observed warming of the Southern Ocean. [ABSTRACT FROM AUTHOR]

The effects of clouds and sea ice on the solar radiation budget are determined for the Southern Ocean around Antarctica between latitudes 50° and 80°S. Distributions of cloud optical depth are used, together with distributions of surface albedo, to estimate the geographical and seasonal variations of shortwave irradiance and cloud radiative forcing at the surface, both for the present climate and for altered surface and cloud conditions. Poleward of 68°S in spring, ice causes a greater reduction of solar energy input to the surface than does cloud. However, in summer the clouds are more important than ice at all latitudes in the Southern Ocean. In the present climate the clouds are optically thicker over open water than over sea ice, suggesting a possible negative feedback if the sea ice area shrinks with climatic warming. Compared to the present climate in spring, removing sea ice results in an increase in irradiance reaching the ocean surface, regardless of the type of cloud remaining. However, in summer the removal of ice results in higher irradiance at the surface only if clouds remain unchanged. If clouds become as thick as those presently over the ocean at 55°–60°S, irradiance reaching the ocean surface in summer decreases poleward of 65°S. [ABSTRACT FROM AUTHOR]

While transient climate change experiments with coupled atmosphere–ocean general circulation models undoubtedly represent the most comprehensive tool for studying the climate response to increasing concentrations of greenhouse gases (GHGs), less computationally expensive time-slice experiments with atmospheric GCMs are still useful to test the robustness of the projected climate change. In the present study, three sets of time-slice experiments with prescribed sea surface temperature (SST) are compared to a reference climate scenario obtained with the Centre National de Recherches Météorologiques Coupled Climate Model (CCM). The main objective is to assess the sensitivity of the monsoon response to the magnitude or pattern of SST anomalies in two regions where such anomalies are highly model dependent, namely, the circumpolar Southern Ocean and the tropical Pacific Ocean. On the one hand, it is shown that the regional climate anomalies predicted by the CCM can be reproduced at least qualitatively by a pair of time-slice experiments in which the present-day SST biases of the CCM are removed. On the other hand, the results indicate that the Indian monsoon response to increasing amounts of GHG is sensitive to regional uncertainties in the prescribed SST warming. Increasing the sea surface warming in the southern high latitudes to compensate for the weak sea ice feedback simulated by the CCM around the Antarctic has a significant influence on the regional climate change simulated over India, through a perturbation of the regional Hadley circulation. Prescribing zonal mean rather than El Niño–like SST anomalies in the tropical Pacific has an even stronger impact on the monsoon response, through a modification of the Walker circulation. These results suggest that both deficiencies in simulating present-day climate (even at high latitudes) and uncertainties in the SST patterns caused by enhanced GHG concentrations (especially in the tropical Pacific) are major obstacles for predicting climate change at the regional scale. [ABSTRACT FROM AUTHOR]

The ensemble rainfall forecasts over the Sahel for July–September (JAS) from the NCEP Coupled Forecast System (CFS) were evaluated for the period 1981–2002. The comparison with the gauge-based precipitation analysis indicates that the predicted Sahel rainfall is light and exhibits little interannual variability. The rain belt is shifted about 4° southward. One major source of rainfall errors comes from the erroneous sea surface temperature (SST) forecasts. The systematic SST error pattern has positive errors in the North Pacific and the North Atlantic and negative errors in the tropical Pacific and the southern oceans. It resembles the decadal SST mode, which has a significant influence on rainfall over the Sahel. Because the systematic SST errors were not corrected during the forecasts, persistent errors serve as an additional forcing to the atmosphere. The second source of error is from the soil moisture feedback, which contributes to the southward shift of rainfall and dryness over West Africa. This was demonstrated by the comparison between simulations (SIMs) and the Atmospheric Model Intercomparison Project (AMIP) run. Both are forced with observed SSTs. The SIMs initialized at the end of June have realistic soil moisture and do not show the southward shift of rainfall. The AMIP, which predicts soil moisture, maintains the dryness through the summer over the Sahel. For AMIP, the decreased rainfall is contributed by the decreased evaporation (E) due to the dry soil and the shift of the large temperature gradients southward. In response, the African easterly jet (AEJ) shifts southward. Since this jet is the primary source of energy for the African waves and their associated mesoscale convective systems, these too shift southward. This negative feedback contributes to increased dryness over the Sahel. [ABSTRACT FROM AUTHOR]

In concert with a poleward shift in baroclinicity, the synoptic environment south of 40°S appears to have changed significantly over recent decades. South of 40°S and north of the Antarctic Ocean the number of cyclones has dramatically decreased, while over the Antarctic Ocean a modest increase has occurred. A global climate model with anthropogenic forcing produces similar historical changes, and under a “business-as-usual” emissions scenario predicts that the number of sub-Antarctic Ocean cyclones will drop by over 30% between now and century's end. [ABSTRACT FROM AUTHOR]

Coupled ocean-atmospheric general circulation models indicate that warming of up to 3°C may occur over the next century in waters immediately to the north of the Amery Ice Shelf. The impact of this warming on the ocean cavity under the Amery Ice Shelf and the mass exchange at the interface between the ocean cavity and the ice shelf is investigated using a three-dimensional ocean model. Warming of between 0.25° and 3.0°C is applied along the ice front in a series of model runs, rather than in a single transient run. Changes in salinity are also considered for larger amounts of warming. The model results show that the circulation in the ocean cavity changes as warming increases, particularly in the gyres that dominate the horizontal circulation. The changes in the heat flux from the warming increase the melt rates from the base of the Amery Ice Shelf, from the present-day mean melt rate and net mass loss estimates of 0.28 m yr[sup -1] and 14.2 Gt yr[sup -1], respectively, by approximately 0.55 m yr[sup -1°]C[sups -1] and 28.4 Gt yr[sup -1°]C[sup -1]. The maximum melt rates increase much more strongly, by around 10 m yr[sup -1°]C[sup -1]. These increased rates of melting suggest substantial modification of the ice shelf would occur in a warmer climate, particularly near the grounding line, and thus indicate that warming of the oceans around Antarctica has the potential for significant impact on the Antarctic ice sheet. [ABSTRACT FROM AUTHOR]