Southern Annular Mode Variability Research Articles

The Role of the Subtropical and Extratropical Southern Hemisphere Anomalous Sea Surface Temperature in the Trans‐Seasonal Influence of Southern Annular Mode on Rainfall Over Southeastern South America

AbstractThe current study explores the linkage and possible mechanisms of the Southern Annular Mode (SAM) with rainfall variability over southeastern South America (SESA). The findings indicate that the preceding May SAM can modulate the subsequent rainfall over the SESA region in September‐October‐November (SON), independent of the El Niño‐Southern Oscillation (ENSO) signal. Further analysis suggests that the SAM‐rainfall relationship is mainly due to the response of sea surface temperature (SST) in the subtropical and extratropical Southern Hemisphere to SAM variability via air‐sea interactions. Such SST anomalies persist to the subsequent SON and stimulate an upstream wave train extending into the study region from the South Pacific. Such wave patterns form a cyclone anomaly over SESA, indicating strong subsidence and reduced convection over the region. Meanwhile, the low‐level anomalous cyclone and anticyclone located over the subtropical South Atlantic (on the coast of the study region) are associated with easterly winds to the South (North) of the anomalous cyclone (anticyclone), which advect colder and drier air into the study region due to the cold SST over the subtropical South Atlantic and leading to increased moisture scarcity and decreased rainfall over SESA. Besides, model results based on the joint climate indices (i.e., SAM, ENSO and Indian Ocean Dipole (IOD)) revealed a satisfactory estimate of the SON rainfall compared with either single index, indicating that the SAM can be one of the precursors of the SESA rainfall, besides ENSO and IOD.

Technical note: An improved methodology for calculating the Southern Annular Mode index to aid consistency between climate studies

Abstract. The Southern Annular Mode (SAM) strongly influences climate variability in the Southern Hemisphere. The SAM index describes the phase and magnitude of the SAM and can be calculated by measuring the difference in mean sea level pressure (MSLP) between middle and high latitudes. This study investigates the effects of calculation methods and data resolution on the SAM index, and subsequent interpretations of SAM impacts and trends. We show that the normalisation step that is traditionally used in calculating the SAM index leads to substantial differences in the magnitude of the SAM index calculated at different temporal resolutions. Additionally, the equal weighting that the normalisation approach gives to MSLP variability at the middle and high southern latitudes artificially alters temperature and precipitation correlations and the interpretation of climate change trends in the SAM. These issues can be overcome by instead using a natural SAM index based on MSLP anomalies, resulting in consistent scaling and variability in the SAM index calculated at daily, monthly and annual data resolutions. The natural SAM index has improved representation of SAM impacts in the high southern latitudes, including the asymmetric (zonal wave-3) component of MSLP variability, whereas the increased weighting given to mid-latitude MSLP variability in the normalised SAM index incorporates a stronger component of tropical climate variability that is not directly associated with SAM variability. We conclude that an improved approach of calculating the SAM index from MSLP anomalies without normalisation would aid consistency across climate studies and avoid potential ambiguity in the SAM index, including SAM index reconstructions from palaeoclimate data, and thus enable more consistent interpretations of SAM trends and impacts.

Lagged effect of Southern Annular Mode on chlorophyll-a in the mid-latitude South Pacific and Indian Oceans

This study investigates the influence of the Southern Annular Mode (SAM) on chlorophyll-a (Chl-a) concentrations and the underlying mechanisms governing their associated environmental variations in the mid-latitude (35–50° S) ocean from 1998 to 2021. The intensification of westerly winds during positive SAM phases influences meridional water transport and mixed layer depth (MLD), which are both critical factors that affect surface nutrient availability. A marked contrast in the relationship between the meridional current anomaly and the SAM was observed, with reduced northward transport of nutrient-rich water in regions north of 50° S during positive SAM phases. This reduction could be attributed to the poleward migration of the westerly winds, which impeded the meridional current from reaching the mid-latitudes. The relationship between SAM and MLD south of 50° S was positive whereas that in the mid-latitude eastern (60–110° E) South Indian Ocean and eastern (90–140° W) South Pacific Ocean was negative or weak. The immediate effect of a more positive SAM on Chl-a in the mid-latitude ocean was reduced productivity caused by enhanced nutrient depletion. However, in the mid-latitude eastern South Pacific Ocean, the northward migration of the zonal mean meridional current anomaly closely aligned with the lagged correlation pattern between SAM variability and Chl-a over time, suggesting that the delayed northward transport of nutrient-rich waters may partially counterbalance the immediate effects of the SAM on ocean productivity. This mechanism was not present in the mid-latitude eastern South Indian Ocean, implying that future climate change may variably affect these regions. Our findings emphasize the importance of considering regional differences and temporal lags when evaluating the influence of SAM variability on ocean productivity and nutrient dynamics in the context of climate change.

Southern Hemisphere Storm Tracks and Large-Scale Variability: What Do the Latest Reanalyses Say?

Abstract In this study, we explore linkages between the monthly mean 500 hPa height field (Z500) and its high-frequency variability over 2–8-day periods, a proxy for Southern Hemisphere storm track activity. We apply maximum covariance analysis to identify leading modes of covariability between the Z500 and high-frequency variance anomalies on monthly and submonthly time scales, using two reanalysis products. We also calculate covariance with indices of large-scale variability [Southern Annular Mode (SAM), El Niño–Southern Oscillation (ENSO), and zonal wave 3 (ZW3)]. We find large-scale circulation patterns emerge as prominent modes of covariability, particularly SAM and ENSO, accounting for 11.1% and 7.8% of covariability on the monthly scale, respectively. The seasonal cycle plays a prominent role in explaining variability in both SAM and ENSO interactions with the storm track. We find that despite a broadly linear response, both SAM and ENSO teleconnections present additional complexities and nonlinearities. Despite strong ZW3 signals in the mean height field, its influence on the high-frequency variance field remains unclear. We conclude the mean height field is most likely strongly linked with high-frequency variance, but interference from other influences may lead to an inconsistent response. We note both fields have an apparent hemispheric response to the SAM, but ENSO has a more regional response and ZW3 a relatively incoherent response. This suggests ENSO and ZW3 patterns depend less on feedbacks between the storm track and the mean height field than the SAM.

The Intrinsic 150‐Day Periodicity of the Southern Hemisphere Extratropical Large‐Scale Atmospheric Circulation

AbstractThe variability of the Southern Hemisphere (SH) extratropical large‐scale circulation is dominated by the Southern Annular Mode (SAM), whose timescale is extensively used as a key metric in evaluating state‐of‐the‐art climate models. Past observational and theoretical studies suggest that the SAM lacks any internally generated (intrinsic) periodicity. Here, we show, using observations and a climate model hierarchy, that the SAM has an intrinsic 150‐day periodicity. This periodicity is robustly detectable in the power spectra and principal oscillation patterns (aka dynamical mode decomposition) of the zonal‐mean circulation, and in hemispheric‐scale precipitation and ocean surface wind stress. The 150‐day period is consistent with the predictions of a new reduced‐order model for the SAM, which suggests that this periodicity is associated with a complex interaction of turbulent eddies and zonal wind anomalies, as the latter propagate from low to high latitudes. These findings present a rare example of periodic oscillations arising from the internal dynamics of the extratropical turbulent circulations. Based on these findings, we further propose a new metric for evaluating climate models, and show that some of the previously reported shortcomings and improvements in simulating SAM's variability connect to the models' ability in reproducing this periodicity. We argue that this periodicity should be considered in evaluating climate models and understanding the past, current, and projected Southern Hemisphere climate variability.

Trends and variability in the Southern Annular Mode over the Common Era

The Southern Annular Mode (SAM) is the leading mode of atmospheric variability in the extratropical Southern Hemisphere and has wide ranging effects on ecosystems and societies. Despite the SAM’s importance, paleoclimate reconstructions disagree on its variability and trends over the Common Era, which may be linked to variability in SAM teleconnections and the influence of specific proxies. Here, we use data assimilation with a multi-model prior to reconstruct the SAM over the last 2000 years using temperature and drought-sensitive climate proxies. Our method does not assume a stationary relationship between the SAM and the proxy records and allows us to identify critical paleoclimate records and quantify reconstruction uncertainty through time. We find no evidence for a forced response in SAM variability prior to the 20th century. We do find the modern positive trend falls outside the 2σ range of the prior 2000 years at multidecadal time scales, supporting the inference that the SAM’s positive trend over the last several decades is a response to anthropogenic climate change.

Evaluation of seasonal teleconnections to remote drivers of Australian rainfall in CMIP5 and CMIP6 models

This study describes how coupled climate models participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6) simulate the primary climate drivers that affect Australian climate, and their seasonal relationship to Australian rainfall, namely the El Niño–Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD) and the Southern Annular Mode (SAM). As results from the earlier generation of models (CMIP5) are still in use, the CMIP6 multi-model mean teleconnections between climate drivers and seasonal Australian rainfall are compared to CMIP5. Collectively, an improvement is found in CMIP6 relative to CMIP5 in the representation of the relationship between ENSO and IOD events and Australia’s springtime rainfall. Overall, CMIP6 models are also able to reproduce the asymmetric relationship between ENSO and eastern Australian rainfall, which exhibits a more robust signal during La Niña than during El Niño years. Both CMIP5 and CMIP6 models are also generally able to capture the stronger relationship between Central Pacific La Niñas, compared to Eastern Pacific La Niñas. However, the large spread in model-to-model behaviour, and among ensemble members, remains a source of uncertainty. Although CMIP6 models have improved in their representation of SAM variability, the simulated relationship between SAM and Australian rainfall has not materially improved. Additionally, this study is accompanied by an extensive Appendix in which each model’s ENSO, IOD and SAM seasonal teleconnection patterns to rainfall are presented and ranked.

A global analysis of austral summer ocean wave variability during SAM–ENSO phase combinations

Anticipating and mitigating wave-related hazards rely heavily on understanding wave variability drivers. Here, we describe wave conditions related to concurrent Southern Annular Mode (SAM) and El Nino–Southern Oscillation (ENSO) phases during the austral summer. To identify such conditions, significant wave height (Hs) and peak wave period (Tp) daily anomalies were composited during different SAM–ENSO phase combinations over the last four decades (1979–2018). Surface wind anomalies were also composited to assist in the interpretation of wave conditions. The composites show significant wave variability across all ocean basins and in several semi-enclosed seas throughout the different SAM–ENSO phase combinations. The Southern, Indian, and Pacific Oceans generally experience the strongest Tp anomalies during combinations of SAM phases with El Nino, and the weakest Tp anomalies during combinations of SAM phases with La Nina. The anomalously large waves observed in the south-western Pacific, Tasman Sea, and the Southern Ocean, previously ascribed to ENSO conditions, seem to be instead associated with the SAM variability. SAM-related atmospheric conditions are found to be able to modulate the intensity of ENSO-related winds over the South China Sea, which, in turn, alter the magnitude of waves in that region. These and other wave anomaly structures described here, especially those contrasting the behaviour expected for a given ENSO phase, such as the one found along the California coast, stress the importance of understanding relationships between wave parameters and climate patterns interactions.

Southern Annular Mode Influence on Wintertime Ventilation of the Southern Ocean Detected in Atmospheric O2 and CO2 Measurements

AbstractThe Southern Annular Mode (SAM) is the dominant mode of climate variability in the Southern Ocean, but only a few observational studies have linked variability in SAM to changes in ocean circulation. Atmospheric potential oxygen (APO) combines atmospheric O2/N2 and CO2 data to mask the influence of terrestrial exchanges, yielding a tracer that is sensitive mainly to ocean circulation and biogeochemistry. We show that observed wintertime anomalies of APO are significantly correlated to SAM in 25‐ to 30‐year time series at three Southern Hemisphere sites, while CO2 anomalies are also weakly correlated. We find additional correlations between SAM and O2 air‐sea fluxes in austral winter inferred from both an atmospheric inversion of observed APO and a forced ocean biogeochemistry model simulation. The model results indicate that the correlation with SAM is mechanistically linked to stronger wind speeds and upwelling, which brings oxygen‐depleted deep waters to the surface.

Abrupt ice-age shifts in southern westerly winds and Antarctic climate forced from the north.

The mid-latitude westerly winds of the Southern Hemisphere play a central role in the global climate system via Southern Ocean upwelling1, carbon exchange with the deep ocean2, Agulhas leakage (transport of Indian Ocean waters into the Atlantic)3 and possibly Antarctic ice-sheet stability4. Meridional shifts of the Southern Hemisphere westerly winds have been hypothesized to occur5,6 in parallel with the well-documented shifts of the intertropical convergence zone7 in response to Dansgaard-Oeschger (DO) events- abrupt North Atlantic climate change events of the last ice age. Shifting moisture pathways to West Antarctica8 are consistent with this view but may represent a Pacific teleconnection pattern forced from the tropics9. The full response of the Southern Hemisphere atmospheric circulation to the DO cycle and its impact on Antarctic temperature remain unclear10. Here we use five ice cores synchronized via volcanic markers to show that the Antarctic temperature response to the DO cycle can be understood as the superposition of two modes: a spatially homogeneous oceanic 'bipolar seesaw' mode that lags behind Northern Hemisphere climate by about 200 years, and a spatially heterogeneous atmospheric mode that is synchronous with abrupt events in the Northern Hemisphere. Temperature anomalies of the atmospheric mode are similar to those associated with present-day Southern Annular Mode variability, rather than the Pacific-South American pattern. Moreover, deuterium-excess records suggest a zonally coherent migration of the Southern Hemisphere westerly winds over all ocean basins in phase with Northern Hemisphere climate. Our work provides a simple conceptual framework for understanding circum-Antarctic temperature variations forced by abrupt Northern Hemisphere climate change. We provide observational evidence of abrupt shifts in the Southern Hemisphere westerly winds, which have previously documented1-3 ramifications for global ocean circulation and atmospheric carbon dioxide. These coupled changes highlight the necessity of a global, rather than a purely North Atlantic, perspective on the DO cycle.

Variability of the Southern Hemisphere Subtropical Jet Stream in the Second Half of the 20th Century and Early 21st Century

Latitudinal position and wind speed of the Southern Hemisphere subtropical jet stream have been investigated on the basis of ERA-Interim, JRA-55, and NCEP–NCAR reanalysis data for 1948–2013. The analysis covers different time intervals in summer and winter seasons, as well as different spatial domains. It has been shown that the variability of the southern jet stream parameters in both winter and summer seasons is predominantly characterized by wind-speed weakening on the jet-stream axis and its poleward shift. The winter seasons of 2000–2013 identified a shift in the jet-stream axis toward the equator in the Atlantic (60°–0° W) and African (0°–60° E) sectors; the wind-speed increase in the Atlantic sector was statistically significant. The wind speed on the jet-stream axis in both winter and summer is closely related to the temperature difference in the upper tropospheric layer of 200–400 hPa between the latitudinal zones of 0°–30° S and 30°–60° S. A significant negative correlation (r = −0.78) between wind speed and temperature difference has been revealed for the winter season in the upper tropospheric layer between the latitudinal zones of 30°–60° S and 60°–90° S, which can be explained by the Southern Annular Mode variability in this season. No such relationship has been found for the summer season.

Precipitation anomalies in the Brazilian southern coast related to the SAM and ENSO climate variability modes

ABSTRACT The precipitation pattern of the State of Rio Grande do Sul (RS) is changing, indicating an increase, although there are long periods of drought. Several studies indicate the influence of climate variability modes on RS precipitation. This work analyzes the influence of the Southern Annular Mode (SAM) and the El Niño - Southern Oscillation (ENSO) on precipitation anomalies (PP) of the Rio Grande do Sul Coastal Plain (RGSCP), dividing it into three regions: south, central and north. Contingency tables were used to correlate the indices, classifying them as neutral, below ou above the mean. To statistical significance we used percentage correctly classified with which the Student’s t was aplied for each region. The PP of the RGSCP and the south coast have correlation with the ENSO and SAM; the central only with SAM. The PP of the north is not correlated to either index. Periods with more than 5 months of PP, SAM and ENSO anomalies were identified. Below average events were majority. The results indicate greater influence of SAM + and La Niña. The trend towards SAM+ and intensification of ENSO, could increase the frequency of droughts in RGSCP. In addition, it was possible to interpret that the geographical differences of the RGSCP can influence the results of precipitation totals. This work contributes to the understanding of the effects of the new trends of climatic variability under regional and geographical aspects.

Different Behaviours of the Ross and Weddell Seas Surface Heat Fluxes in the Period 1972–2015

Operational analyses and re-analyses, provided by ECMWF for the period 1972–2015, were used to investigate the behaviour of the surface heat fluxes between ocean and atmosphere, estimated via empirical formulae, over the Ross and Weddell Seas. The presence and thickness of sea ice cover, which strongly affects ocean-atmosphere interactions, was estimated through Special Sensor Microwave Imager and Special Sensor Microwave Imager Sounder brightness temperatures. Because of the lack of ice information before 1992, daily averaged ice and snow thickness obtained from the 1992–2012 dataset has been used as a ‘climatological year’ for the 1972–2015 period. The heat loss in the Ross Sea reached its maximum in 2008 (−98 W∙m−2) and its minimum (−58 W∙m−2) in 1980, while in the Weddell Sea, it ranged between −65 W∙m−2 (1999) and −99 W∙m−2 (2015). Results showed that the surface heat fluxes behaviour in the two seas moved from opposite to synchronous during the study period. The wavelet analysis was applied to evaluate if this result might be linked to the signature of global climate variability expressed by El Niño Southern Oscillation (ENSO) and Southern Annular Mode (SAM). The synchronous behaviour of the surface heat fluxes in the Ross and Weddell seas, observed since 2001, coincides with a change in the energy peak associated to the time scale of the SAM variability, which moved from 32 to 64 months during 1990s. This change generates a common energy peak for the SAM and ENSO with a lagged in phase relationship between the signals, possibly influencing the behaviour of the surface heat fluxes.

Western Antarctic ocean surface variability during the 20th century in the NCAR-CCSM4

Variability in the Southern Ocean is strongly connected with climate modes, in particular with the Southern Annular Mode (SAM). SAM variability shows strong seasonal asymmetries in the Antarctic Peninsula which could impact ocean-atmosphere dynamics. Thus, the goal of this study is to understand the atmosphere-ocean response to seasonality in the Southern Ocean during the 20th century with a specific focus on the western Antarctic Peninsula. We analyze the seasonal and long-term aspects of the ocean-atmosphere properties in the NCAR-CCSM4 model. Given the high correlation between the CCSM4-SAM index and the significant (at the 95% level) regional long-term trend, both the zonal wind stress (~0.75 and 0.04Nm−2 century−1) and barotropic transport (~0.78 and 12Sv century−1) in the western domain (155–130W), are associated with the SAM. This is in contrast to the barotropic transport in the north-central domain (130–80W and 55–60S), which shows great seasonality: The changes from the austral winter (JJA) to summer (DJF) are approximately 10Sv, while its long-term trend is small (7Sv century−1). On the other hand, the decrease in the surface salinity in the end of the century (~0.06), mainly near Antarctic Peninsula, and the long-term trend of 0.2 century−1 are not seasonal and were not attributed directly to SAM variability. Thus, an increase in precipitation of 8mm month−1, mostly during the austral summer (DJF) in the late 20th century over the northern Antarctic Peninsula, together with the positive long-term trend of 8mm month−1 century−1 over the Amundsen-Bellingshausen Seas, contributed, among other factors, to surface desalinization in the late 20th century. SST warmed throughout the 20th century, independent of the season displaying changes that were not directly related with the SAM. Our study shows how different sea-air properties interact with each other and, also, when the SAM plays an important role in controlling long-term variability.

Reconstructions of the southern annular mode (SAM) during the last millennium

The leading mode of atmospheric variability in the Southern Hemisphere is the Southern Annular Mode (SAM), which affects the atmosphere and ocean from the mid-latitudes to the Antarctic. However, the short instrumental record of the SAM does not adequately represent its multi-decadal to centennial-scale variability. Long palaeoclimatic reconstructions of the SAM would improve our understanding of its low frequency behavior and its effects on regional temperature, rainfall, sea ice, and ecosystem processes. In this progress report, we review three published palaeoclimatic reconstructions available for understanding multi-decadal to centennial-scale variability of the SAM. Reconstructions reviewed here show similar patterns of decadal SAM variability during the last two centuries, but earlier centuries are less coherent. Reconstructions clearly maintain similar trends towards more positive SAM states since the onset of significant anthropogenic climate forcing from rising greenhouse gas (GHG) concentrations and ozone depletion and these excursions appear unprecedented over at least the last 500 years. We describe how new multi-proxy reconstructions of the SAM could further improve our understanding of its long-term variability and effects across all geographic sectors of the Southern Hemisphere. Here, we recommend careful selection and development of proxies in SAM-sensitive regions and seasons. In particular, proxies related to cool-season conditions and from the poorly-sampled Indian Ocean sector would allow for a true circumpolar and year-round reconstruction of past SAM variability.

Southern Annular Mode drives multicentury wildfire activity in southern South America

The Southern Annular Mode (SAM) is the main driver of climate variability at mid to high latitudes in the Southern Hemisphere, affecting wildfire activity, which in turn pollutes the air and contributes to human health problems and mortality, and potentially provides strong feedback to the climate system through emissions and land cover changes. Here we report the largest Southern Hemisphere network of annually resolved tree ring fire histories, consisting of 1,767 fire-scarred trees from 97 sites (from 22 °S to 54 °S) in southern South America (SAS), to quantify the coupling of SAM and regional wildfire variability using recently created multicentury proxy indices of SAM for the years 1531-2010 AD. We show that at interannual time scales, as well as at multidecadal time scales across 37-54 °S, latitudinal gradient elevated wildfire activity is synchronous with positive phases of the SAM over the years 1665-1995. Positive phases of the SAM are associated primarily with warm conditions in these biomass-rich forests, in which widespread fire activity depends on fuel desiccation. Climate modeling studies indicate that greenhouse gases will force SAM into its positive phase even if stratospheric ozone returns to normal levels, so that climate conditions conducive to widespread fire activity in SAS will continue throughout the 21st century.

Sensitivity of Antarctic sea ice to the Southern Annular Mode in coupled climate models

We assess the sea ice response to Southern Annular Mode (SAM) anomalies for pre-industrial control simulations from the Coupled Model Intercomparison Project (CMIP5). Consistent with work by Ferreira et al. (J Clim 28:1206–1226, 2015. doi:10.1175/JCLI-D-14-00313.1), the models generally simulate a two-timescale response to positive SAM anomalies, with an initial increase in ice followed by an eventual sea ice decline. However, the models differ in the cross-over time at which the change in ice response occurs, in the overall magnitude of the response, and in the spatial distribution of the response. Late twentieth century Antarctic sea ice trends in CMIP5 simulations are related in part to different modeled responses to SAM variability acting on different time-varying transient SAM conditions. This explains a significant fraction of the spread in simulated late twentieth century southern hemisphere sea ice extent trends across the model simulations. Applying the modeled sea ice response to SAM variability but driven by the observed record of SAM suggests that variations in the austral summer SAM, which has exhibited a significant positive trend, have driven a modest sea ice decrease. However, additional work is needed to narrow the considerable model uncertainty in the climate response to SAM variability and its implications for 20th–21st century trends.

Interpreting the nature of Northern and Southern Annular Mode variability in CMIP5 Models

AbstractCharacteristic timescales for the Northern Annular Mode (NAM) and Southern Annular Mode (SAM) variability are diagnosed in historical simulations submitted to the Coupled Model Intercomparison Project Phase 5 (CMIP5) and are compared to the European Centre for Medium‐Range Weather Forecasts ERA‐Interim data. These timescales are calculated from geopotential height anomaly spectra using a recently developed method, where spectra are divided into low‐frequency (Lorentzian) and high‐frequency (exponential) parts to account for stochastic and chaotic behaviors, respectively. As found for reanalysis data, model spectra at high frequencies are consistent with low‐order chaotic behavior, in contrast to an AR1 process at low frequencies. This places the characterization of the annular mode timescales in a more dynamical rather than purely stochastic context. The characteristic high‐frequency timescales for the NAM and SAM derived from the model spectra at high frequencies are ∼5 days, independent of season, which is consistent with the timescales of ERA‐Interim. In the low‐frequency domain, however, models are slightly biased toward too long timescales, but within the error bars, a finding which is consistent with previous studies of CMIP3 models. For the SAM, low‐frequency timescales in November, December, January, and February are overestimated in the models compared to ERA‐Interim. In some models, the overestimation in the SAM austral summer timescale is partly due to interannual variability, which can inflate these timescales by up to ∼40% in the models but only accounts for about 5% in the ERA‐Interim reanalysis.

Quantifying barotropic and baroclinic eddy feedbacks in the persistence of the Southern Annular Mode

AbstractUnderstanding the persistence of the Southern Annular Mode (SAM) is important for the intraseasonal and decadal predictability of SAM. Using the ERA‐40 and ERA‐Interim reanalysis data, this study introduces a new method to quantify the relative roles of barotropic and baroclinic eddy feedbacks in the SAM persistence. Through a hybrid Eulerian‐Lagrangian Finite Amplitude Wave Activity diagnostic, it is found that (i) transient wave activity is important in driving the SAM, but it provides a negative feedback to the SAM persistence. (ii) Irreversible potential vorticity mixing, through barotropic processes in the upper troposphere, plays an important role in driving and sustaining the SAM variability. Particularly, following the poleward shift of the eddy‐driven jet, the reduction/enhancement in effective diffusivity on the jet's poleward/equatorward flank can be understood by a stronger/weaker zonal jet acting as a robust/leaky mixing barrier. (iii) Baroclinic eddy generation and vertical wave propagation mainly act to sustain the SAM variability.

An examination of the relationship between the Southern Annular Mode and Antarctic surface air temperatures in the CMIP5 historical runs

Strong relationships exist between the Southern Annular Mode (SAM) and surface air temperature (SAT) across much of Antarctica. Changes in the SAM will have a profound influence on future Antarctic climate so it is important that the models used to predict climate change can accurately reproduce current SAM–SAT relationships. We analyse data from 50 Climate Model Intercomparison Project (CMIP) 5 models to assess how well they reproduce the observed mean and variability of annual and seasonal SAM–SAT relationships at six Antarctic stations. Overall, the models do better at reproducing these relationships when meridional flow has its largest influence on SAT, doing best (worst) in winter (autumn and summer). They are generally unable to replicate existing seasonal cycles in the strength of the SAM–SAT relationship and show much less spatial and especially temporal variability in the strength of these relationships than is observed. Using an estimate of intrinsic variability to quantify the skill of the CMIP5 models, their average ability to successfully replicate a seasonal SAM–SAT relationship at the six locations studied ranges from 16 % in autumn to 32 % in winter. The mean success rate of a single model across all four seasons is 24 %, ranging from 8 to 38 % (compared to a ‘perfect model’ with 46 %). Analysing the different atmospheric circulation patterns associated with extreme SAM–SAT correlations in the models demonstrates the importance of correctly reproducing both the climatological mean and variability of the planetary longwaves at Southern Hemisphere high-latitudes (particularly wave-number 3), in order to accurately reproduce observed SAM–SAT relationships across Antarctica.