Author(s): Li, X; Cai, W; Meehl, GA; Chen, D; Yuan, X; Raphael, M; Holland, DM; Ding, Q; Fogt, RL; Markle, BR; Wang, G; Bromwich, DH; Turner, J; Xie, SP; Steig, EJ; Gille, ST; Xiao, C; Wu, B; Lazzara, MA; Chen, X; Stammerjohn, S; Holland, PR; Holland, MM; Cheng, X; Price, SF; Wang, Z; Bitz, CM; Shi, J; Gerber, EP; Liang, X; Goosse, H; Yoo, C; Ding, M; Geng, L; Xin, M; Li, C; Dou, T; Liu, C; Sun, W; Wang, X; Song, C | Abstract: Over the modern satellite era, substantial climatic changes have been observed in the Antarctic, including atmospheric and oceanic warming, ice sheet thinning and a general Antarctic-wide expansion of sea ice, followed by a more recent rapid loss. Although these changes, featuring strong zonal asymmetry, are partially influenced by increasing greenhouse gas emissions and stratospheric ozone depletion, tropical–polar teleconnections are believed to have a role through Rossby wave dynamics. In this Review, we synthesize understanding of tropical teleconnections to the Southern Hemisphere extratropics arising from the El Nino–Southern Oscillation, Interdecadal Pacific Oscillation and Atlantic Multidecadal Oscillation, focusing on the mechanisms and long-term climatic impacts. These teleconnections have contributed to observed Antarctic and Southern Ocean changes, including regional rapid surface warming, pre-2015 sea ice expansion and its sudden reduction thereafter, changes in ocean heat content and accelerated thinning of most of the Antarctic ice sheet. However, due to limited observations and inherent model biases, uncertainties remain in understanding and assessing the importance of these teleconnections versus those arising from greenhouse gases, ozone recovery and internal variability. Sustained pan-Antarctic efforts towards long-term observations, and more realistic dynamics and parameterizations in high-resolution climate models, offer opportunities to reduce these uncertainties.

Tropical teleconnection impacts on Antarctic climate changes

The southern boundary (SB) of the Antarctic Circumpolar Current, the southernmost extent of the upper overturning circulation, regulates the Antarctic thermal conditions. The SB's behavior remains unconstrained because it does not have a clear surface signature. Revisited hydrographic data from off East Antarctica indicate full-depth warming from 1996 to 2019, concurrent with an extensive poleward shift of the SB subsurface isotherms (>50 km), which is most prominent at 120°E off the Sabrina Coast. The SB shift is attributable to enhanced upper overturning circulation and a depth-independent frontal shift, generally accounting for 30 and 70%, respectively. Thirty years of oceanographic data corroborate the overall and localized poleward shifts that are likely controlled by continental slope topography. Numerical experiments successfully reproduce this locality and demonstrate its sensitivity to mesoscale processes and wind forcing. The poleward SB shift under intensified westerlies potentially induces multidecadal warming of Antarctic shelf water.

Multidecadal poleward shift of the southern boundary of the Antarctic Circumpolar Current off East Antarctica

The effective climate sensitivity estimates the equilibrium response of near-surface temperature to doubling atmospheric carbon dioxide concentration and is a widely used metric to characterize potential global warming. Earth system models participating in phase 6 of the Coupled Model Intercomparison Project (CMIP6) exhibit considerable spread in effective climate sensitivity estimates. Cloud feedbacks are thought to be the cause of this, with marine boundary layer clouds over the Southern Ocean playing an important role. Here, we show that Southern Ocean deep convection is a major contributor to the CMIP6 intermodel spread in effective climate sensitivity. By comparing two Earth system models with very different sensitivities, we find that greater storage of heat at depth can delay the Southern Ocean surface warming and associated cloud response, thereby delaying global surface warming by centuries. The link between Southern Ocean convection and effective climate sensitivity is seen across 41 CMIP6 models, with low-sensitivity models exhibiting substantial deep ocean warming. Our results reveal the influence of Southern Ocean convection on potential long-term climate warming. Southern Ocean deep convection governs the magnitude of long-term warming in response to greenhouse gases, according to an analysis of Earth system models.

Shutdown of Southern Ocean convection controls long-term greenhouse gas-induced warming

The open ocean nitrogen cycle is being altered by increases in anthropogenic atmospheric nitrogen deposition and climate change. How the nitrogen cycle responds will determine long-term trends in net primary production (NPP) in the nitrogen-limited low latitude ocean, but is poorly constrained by uncertainty in how the source-sink balance will evolve. Here we show that intensifying nitrogen limitation of phytoplankton, associated with near-term reductions in NPP, causes detectable declines in nitrogen isotopes (δ15N) and constitutes the primary perturbation of the 21st century nitrogen cycle. Model experiments show that~75% of the low latitude twilight zone develops anomalously low δ15N by 2060, predominantly due to the effects of climate change that alter ocean circulation, with implications for the nitrogen source-sink balance. Our results highlight that δ 15 N changes in the low latitude twilight zone may provide a useful constraint on emerging changes to nitrogen limitation and NPP over the 21st century.

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Impact of intensifying nitrogen limitation on ocean net primary production is fingerprinted by nitrogen isotopes.

The Antarctic Slope Front (ASF) is a fundamental feature of the subpolar Southern Ocean that is still poorly observed. In this study we build a statistical climatology of the temperature and salinity fields of the upper 380 m of the Antarctic margin. We use a comprehensive compilation of observational datasets including the profiles gathered by instrumented marine mammals. The mapping method consists first of a decomposition in vertical modes of the combined temperature and salinity profiles. Then the resulting principal components are optimally interpolated on a regular grid and the monthly climatological profiles are reconstructed, providing a physically plausible representation of the ocean. The ASF is located with a contour method and a gradient method applied on the temperature field, two complementary approaches that provide a complete view of the ASF structure. The front extends from the Amundsen Sea to the eastern Weddell Sea and closely tracks the continental shelf break. It is associated with a sharp temperature gradient that is stronger in winter and weaker in summer. The emergence of the front in the Amundsen and Bellingshausen sectors appears to be seasonally variable (slightly more westward in winter than in summer). Investigation of the density gradients across the shelf break indicates a winter slowdown of the baroclinic component of the Antarctic Slope Current at the near surface, in contrast with the seasonal variability of the temperature gradient.

/pdf/seasonal-variation-of-the-antarctic-slope-front-occurrence-4xq5lrjrnm.pdf

Seasonal Variation of the Antarctic Slope Front Occurrence and Position Estimated from an Interpolated Hydrographic Climatology

Despite playing a major role in global ocean heat storage, the Southern Ocean remains the most sparsely measured region of the global ocean Here, a unique 25-year temperature time-series of the upper 800 m, repeated several times a year across the Southern Ocean, allows us to document the long-term change within water-masses and how it compares to the interannual variability Three regions stand out as having strong trends that dominate over interannual variability: warming of the subantarctic waters (029 ± 009 °C per decade); cooling of the near-surface subpolar waters (−007 ± 004 °C per decade); and warming of the subsurface subpolar deep waters (004 ± 001 °C per decade) Although this subsurface warming of subpolar deep waters is small, it is the most robust long-term trend of our section, being in a region with weak interannual variability This robust warming is associated with a large shoaling of the maximum temperature core in the subpolar deep water (39 ± 09 m per decade), which has been significantly underestimated by a factor of 3 to 10 in past studies We find temperature changes of comparable magnitude to those reported in Amundsen–Bellingshausen Seas, which calls for a reconsideration of current ocean changes with important consequences for our understanding of future Antarctic ice-sheet mass loss

/pdf/southern-ocean-in-situ-temperature-trends-over-25-years-2qxgtmpxdt.pdf

Southern Ocean in-situ temperature trends over 25 years emerge from interannual variability.

The Southern Ocean greatly contributes to the regulation of the global climate by controlling important heat and carbon exchanges between the atmosphere and the ocean. Rates of climate change on decadal timescales are therefore impacted by oceanic processes taking place in the Southern Ocean, yet too little is known about these processes. Limitations come both from the lack of observations in this extreme environment and its inherent sensitivity to intermittent processes at scales that are not well captured in current Earth system models. The Southern Ocean Carbon and Heat Impact on Climate programme was launched to address this knowledge gap, with the overall objective to understand and quantify variability of heat and carbon budgets in the Southern Ocean through an investigation of the key physical processes controlling exchanges between the atmosphere, ocean and sea ice using a combination of observational and modelling approaches. Here, we provide a brief overview of the programme, as well as a summary of some of the scientific progress achieved during its first half. Advances range from new evidence of the importance of specific processes in Southern Ocean ventilation rate (e.g. storm-induced turbulence, sea–ice meltwater fronts, wind-induced gyre circulation, dense shelf water formation and abyssal mixing) to refined descriptions of the physical changes currently ongoing in the Southern Ocean and of their link with global climate. This article is part of a discussion meeting issue ‘Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities’.

Southern ocean carbon and heat impact on climate

https://www.nature.com/articles/s41597-022-01166-z.pdf

Southern ocean sea level anomaly in the sea ice-covered sector from multimission satellite observations

A novel multi-satellite product is used to shed light on the sea surface height seasonal cycle and associated geostrophic circulation in the subpolar Southern Ocean. We find three main modes of variability governing the SLA seasonal cycle, all of them primarily governed by wind forcing. The main mode of seasonal variability is associated with the seasonality of the main subpolar gyres governed by large-scale wind stress curl, qualitatively consistent with Sverdrup dynamics. The second seasonal mode is related to the Antarctic Slope Current (ASC), governed by the coastal easterlies, with a rapid circumpolar propagation of anomalous sea-level along the continental slope that is dynamically consistent with the so-called Southern Mode. The first two modes induce an acceleration of the gyre and the ASC in winter. The third seasonal mode appears to be driven by sea ice-modulated surface stress, and induces an offshore extension of the ASC from autumn to winter. 

/pdf/subpolar-southern-ocean-seasonal-variability-of-the-1p1sly8x.pdf

Matthis Auger

Papers

Southern Ocean in-situ temperature trends over 25 years emerge from interannual variability.

Southern ocean carbon and heat impact on climate

Southern ocean sea level anomaly in the sea ice-covered sector from multimission satellite observations

Southern ocean sea level anomaly in the sea ice-covered sector from multimission satellite observations

Subpolar Southern Ocean Seasonal Variability of the Geostrophic Circulation from Multi‐mission Satellite Altimetry