Negative Southern Annular Mode Research Articles

Observed Associations between Fire Danger and Climate Modes and Their Representation in ACCESS-S2

Abstract The increasing frequency and severity of wildfires in Australia, driven by climate change, pose a significant threat to ecosystems, lives, and property. This study examines the impact of climate drivers, specifically El Niño–Southern Oscillation (ENSO), Indian Ocean dipole (IOD), Southern Annular Mode (SAM), Madden–Julian oscillation (MJO), and two modes of persistent high pressure in the Australia–Pacific region, on extreme fire danger. By analyzing observed and simulated fire danger in relation to these climate drivers, we aim to enhance our understanding of climate–fire mechanisms and contribute to Australia’s bushfire preparedness. Our findings indicate that all assessed drivers influence extreme fire danger, with key influences related to the drivers’ established relationships with rainfall and temperature. El Niño, positive IOD, and negative SAM events generally increase extreme fire danger across most of Australia and in most seasons. The two modes of Australian blocking exhibit similar effects, varying spatially. Specific phases of the MJO have significant seasonal relationships with fire danger. In some instances, increased fire danger is not directly linked to temperature or precipitation changes but rather driven by remote teleconnections, airflow, or pressure anomalies. Evaluating the accuracy of weather and climate forecasting systems in representing these relationships is crucial for the effective prediction and mitigation of fire hazards. The leading Australian climate simulation model effectively reproduces observed relationships but reveals biases in capturing certain aspects of climate variability. Advancements in this field would enhance fire weather forecasts, including those by the Australian Bureau of Meteorology with lead times of up to 4 months. Significance Statement This assessment of climate–fire relationships and modeling accuracy is pivotal in understanding the factors behind severe fire events and improving future predictions and mitigation. Prior research has already established links between elevated fire weather conditions and these climate drivers, highlighting their potential for predicting severe fire events. This study aims to deepen our comprehension of the factors driving severe fire events and facilitate improved bushfire risk management in Australia.

Supraglacial lake evolution and its drivers in Dronning Maud Land, East Antarctica

Abstract Supraglacial lakes on Antarctic ice shelves can have far-reaching implications for ice-sheet stability, highlighting the need to understand their dynamics, controls and role in the ice-sheet mass budget. We combine a detailed satellite-based record of seasonal lake evolution in Dronning Maud Land with a high-resolution simulation from the regional climate model Modèle Atmosphérique Régional to identify drivers of lake variability between 2014 and 2021. Correlations between summer lake extents and climate parameters reveal complex relationships that vary both in space and time. Shortwave radiation contributes positively to the energy budget during summer melt seasons, but summers with enhanced longwave radiation are more prone to surface melting and ponding, which is further enhanced by advected heat from summer precipitation. In contrast, previous winter precipitation has a negative effect on summer lake extents, presumably by increasing albedo and pore space, delaying the accumulation of meltwater. Downslope katabatic or föhn winds promote ponding around the grounding zones of some ice shelves. At a larger scale, we find that summers during periods of negative southern annular mode are associated with increased ponding in Dronning Maud Land. The high variability in seasonal lake extents indicates that these ice shelves are highly sensitive to future warming or intensified extreme events.

Evidence for large-scale climate forcing of dense shelf water variability in the Ross Sea

Antarctic Bottom Water (AABW), which supplies the lower limb of the thermohaline circulation, originates from dense shelf water (DSW) forming in Antarctic polynyas. Here, combining a long mooring record of DSW measurements with numerical simulations and satellite data, we show that significant correlation exists between interannual variability of DSW production in the Ross Sea polynyas, where DSW contributes between 20–40% of the global AABW production, and the Southern Annular Mode (SAM). The correlation is largest when the Amundsen Sea Low (ASL) is weakened and shifted east of the Ross Sea. During positive SAM phases, enhanced offshore winds and lower air temperatures over the western Ross Sea increase sea ice production and promote DSW formation, with the opposite response during negative SAM phases. These processes ultimately modulate AABW thickness in the open ocean. A projected positive shift of the SAM and eastward displacement of the ASL thus has implications for the future of DSW and AABW formation.

ACCESS-S2 seasonal forecasts of rainfall and the SAM–rainfall relationship during the grain growing season in south-west Western Australia

South-west Western Australia (SWWA) is home to a world class grains industry that is significantly affected by periods of drought. Previous research has shown a link between the Southern Annular Mode (SAM) and rainfall in SWWA, especially during winter months. Hence, the predictability of the SAM and its relationship to SWWA rainfall can potentially improve forecasts of SWWA drought, which would provide valuable information for farmers. In this paper, focusing on the 0-month lead time forecast, we assess the bias and skill of ACCESS-S2, the Australian Bureau of Meteorology’s current operational sub-seasonal to seasonal forecasting system, in simulating seasonal rainfall for SWWA during the growing season (May–October). We then analyse the relationship between the SAM and SWWA precipitation and how well this is captured in ACCESS-S2 as well as how well ACCESS-S2 forecasts the monthly SAM index. Finally, ACCESS-S2 rainfall forecasts and the simulation of SAM are assessed for a case study of extreme drought in 2010. Our results show that forecasts tend to have greater skill in the earlier part of the season (May–July). ACCESS-S2 captures the significant inverse SAM–rainfall relationship but underestimates its strength. The model also shows overall skill in forecasting the monthly SAM index and simulating the MSLP and 850-hPa wind anomaly patterns associated with positive and negative SAM phases. However, for the 2010 drought case study, ACCESS-S2 does not indicate strong likelihoods of the upcoming dry conditions, particularly for later in the growing season, despite predicting a positive (although weaker than observed) SAM index. Although ACCESS-S2 is shown to skillfully depict the SAM–SWWA rainfall relationship and generally forecast the SAM index well, the seasonal rainfall forecasts still show limited skill. Hence it is likely that model errors unrelated to the SAM are contributing to limited skill in seasonal rainfall forecasts for SWWA, as well as the generally low seasonal-timescale predictability for the region.

Local and Remote Atmosphere‐Ocean Coupling During Extreme Warming Events Impacting Subsurface Ocean Temperature in an Antarctic Embayment

AbstractCoastal ocean temperatures can respond to different atmospheric and oceanic processes at local spatial scales or through remote teleconnections. This study focused on subsurface ocean temperatures (subT) at 10 m depth in Maxwell Bay, northern Antarctic Peninsula from February 2017 and January 2022. It investigated extreme warming events during austral summers and their interaction with atmospheric and oceanic conditions regionally and locally. The analysis identified active and extreme Marine Heat Waves (MHWs) in March 2017 and January‐February 2020 associated with a significantly negative Southern Annular Mode index observed 3–4 months before the temperature increase. In March 2017, temperatures exceeded the climatological mean by over 1°C. This anomaly was linked to a strengthened Amundsen Sea Low and a blocking anticyclone moving between the Scotia Sea and the South‐West Atlantic Ocean that deflected westerly winds and facilitated the anomalous transport of warmer northern air masses to the AP. In January‐February 2020, the highest recorded subT was observed (2.97°C), although air‐sea heat fluxes did not show a similar pattern. In February 2020, one of the most intense atmospheric heatwaves ever recorded in West Antarctica was observed. This heatwave corresponded with maximum subT and positive sea surface temperature anomalies extending throughout the western region of the Southern Ocean, related to an extremely negative Southern Annular Mode. This study provides valuable insights into the impact of strong MHWs, a phenomenon that has been less documented in Antarctic coastal regions.

Interannual Influence of Antarctic Sea Ice on Southern Hemisphere Stratosphere‐Troposphere Coupling

AbstractWhile weakening of the boreal polar vortex may be caused by autumnal Arctic sea ice loss, less is known about the interannual influence of Antarctic sea ice on stratosphere‐troposphere coupling in the Southern Hemisphere. Identifying any relationship over the short satellite period is difficult due to sampling variability and anthropogenic modification of the austral polar vortex. To circumvent these issues, we use large ensembles of fixed boundary condition simulations from the Community Atmosphere Model (CAM) and Whole Atmosphere Community Climate Model (WACCM) to assess if and how interannual fluctuations in winter Antarctic sea ice influence spring planetary‐scale waves and the coupled stratosphere‐troposphere circulation. Low Antarctic sea ice conditions are found to modulate tropospheric stationary waves to project constructively onto the climatological stationary wave, enhancing upward planetary wave propagation into the austral polar stratosphere. In WACCM, the resulting vortex weakening coincides with development of negative Southern Annular Mode conditions during September–November.

Combined Impacts of Southern Annular Mode and Zonal Wave 3 on Antarctic Sea Ice Variability

Abstract Large-scale modes of atmospheric variability in the southern midlatitudes can influence Antarctic sea ice concentrations (SIC) via diverse processes. For instance, variability in both the Southern Annular Mode (SAM) and zonal wave 3 (ZW3) have been linked to the abrupt 2015/16 sea ice decline. While SIC responses to each of SAM and ZW3 have been examined previously, their interaction and synchronous impact on Antarctic sea ice has not. Here, we investigate SAM/ZW3 interactions and their associated combined impacts on Antarctic sea ice using a 1200-yr simulation from a state-of-the-art climate model. Our results suggest that zonal wind anomalies associated with SAM drive SIC anomalies in the marginal ice-zone via advection of ice normal to the ice edge and Ekman drift. In contrast, meridional wind anomalies associated with ZW3 can have opposing dynamic and thermodynamic effects on SIC. Both SAM- and ZW3-related SIC anomalies propagate eastward, likely by the Antarctic Circumpolar Current. The interaction of SAM and ZW3 leads to interesting regional SIC responses. During negative SAM, ZW3-associated meridional wind anomalies across western Antarctica are closer to the ice edge and have a stronger impact on sea ice overall. ZW3 phase affects meridional wind anomalies across the whole ice edge, whereas it affects SIC anomalies mainly over western Antarctica. In parts of eastern Antarctica, SIC anomalies are less sensitive to ZW3 phase, but are sensitive to SAM, particularly in locations where the ice edge has a prominent angle relative to the SAM-related zonal wind anomalies. Significance Statement The Southern Annular Mode (SAM) and zonal wave 3 (ZW3) are large-scale atmospheric circulation patterns affecting midlatitude east–west and north–south winds, respectively, over the Southern Ocean. Variations in winds can affect sea ice formation, which can feed back to influence Southern Hemisphere climate. We examine how variations in SAM and ZW3 affect Antarctic sea ice due to a combination of wind- and ocean-driven ice movement and sea ice growth or melting. Regional variations in ice concentrations are due both to alternating north–south ZW3 winds and to the interaction of SAM-related east–west winds with the ice edge. SAM and ZW3 can also interact, leading to stronger north–south wind and sea ice responses over western Antarctica when SAM-related midlatitude winds weaken.

Evaporative controls on Antarctic precipitation: an ECHAM6 model study using innovative water tracer diagnostics

Abstract. Improving our understanding of the controls on Antarctic precipitation is critical for gaining insights into past and future polar and global environmental changes. Here we develop innovative water tracing diagnostics in the atmospheric general circulation model ECHAM6. These tracers provide new detailed information on moisture source locations and properties of Antarctic precipitation. In the preindustrial simulation, annual mean Antarctic precipitation originating from the open ocean has a source latitude range of 49–35∘ S, a source sea surface temperature range of 9.8–16.3 ∘C, a source 2 m relative humidity range of 75.6 %–83.3 %, and a source 10 m wind velocity (vel10) range of 10.1 to 11.3 m s−1. These results are consistent with estimates from existing literature. Central Antarctic precipitation is sourced from more equatorward (distant) sources via elevated transport pathways compared to coastal Antarctic precipitation. This has been attributed to a moist isentropic framework; i.e. poleward vapour transport tends to follow constant equivalent potential temperature. However, we find notable deviations from this tendency especially in the lower troposphere, likely due to radiative cooling. Heavy precipitation is sourced by longer-range moisture transport: it comes from 2.9∘ (300 km, averaged over Antarctica) more equatorward (distant) sources compared to the rest of precipitation. Precipitation during negative phases of the Southern Annular Mode (SAM) also comes from more equatorward moisture sources (by 2.4∘, averaged over Antarctica) compared to precipitation during positive SAM phases, likely due to amplified planetary waves during negative SAM phases. Moreover, source vel10 of annual mean precipitation is on average 2.1 m s−1 higher than annual mean vel10 at moisture source locations from which the precipitation originates. This shows that the evaporation of moisture driving Antarctic precipitation occurs under windier conditions than average. We quantified this dynamic control of Southern Ocean surface wind on moisture availability for Antarctic precipitation. Overall, the innovative water tracing diagnostics enhance our understanding of the controlling factors of Antarctic precipitation.

Influence of geomagnetic activity on the southern atmosphere and its cross‐seasonal association with SAM

AbstractThis study investigates the interannual cross‐seasonal association between the winter energetic particle precipitation, which is related to the geomagnetic activity (GA), and the southern annular mode (SAM) in the following summer by employing the ERA5 reanalysis dataset. The results reveal a marked negative correlation between these variables on an interannual timescale. Therefore, we analyse the GA signatures in the southern atmosphere and find that the evolution of SAM to the negative phase driven by GA may be caused by the following two mechanisms: First, for high GA levels, the middle and lower stratospheric polar vortex becomes weaker from October to November, with considerable changes in the stratospheric thermal structure and more anomalous planetary wave upwelling, accompanied by weakening of the circumpolar westerly. Over time, the anomalous signal from the upper layer can be transmitted downward and is conducive to inducing and strengthening the negative SAM in the following summer, which may serve as the stratospheric pathway for the geomagnetic impact on the SAM and Southern Hemisphere (SH) climate. Second, another possible way is that the preceding GA also appreciably regulates negative anomalies in the Ferrel circulation in SH, implying poleward anomalies in the direction of the pressure gradient, which promotes the development and enhancement of the negative phase of the SAM. In addition, easterly quasi‐biennial oscillation conditioning plays a positive role in the response of the southern atmosphere to geomagnetic forcing, thereby strengthening the connection between them.

Impact of southern annular mode on the Indian Ocean surface waves

AbstractInterannual climate modes, especially the southern annular mode (SAM), significantly influence the wave climate modulation of the Indian Ocean (IO). The present study, aligned with previous research, identifies two crucial swell generation regions in the IO: the extratropical southern Indian Ocean (ETSI) and the tropical southern Indian Ocean (TSIO). The SAM, governing Southern Ocean surface winds, significantly shapes wave generation in these zones, thereby dominantly regulating IO wave conditions. Positive SAM phases shifts the westerlies poleward creating significant negative anomalies in the northern ETSI, reducing wave generation and swell propagation into the northern IO (NIO) basins, while the positive wind anomalies in the western TSIO creates a new swell generation area that directs swells towards the Arabian Sea, elevating wave heights there during monsoons. Conversely, negative SAM phases enhance TSIO easterlies, making it the primary IO swell source, increasing swell activity in the Bay of Bengal, notably during premonsoon and monsoon seasons. SAM impact expands beyond swells, influencing NIO wave climate by altering wind seas through Hadley cell (HC) circulation shifts. Positive SAM phases trigger NIO and midlatitude anomalous warming, intensifying HC and NIO surface winds in the next season, thereby affecting convection and subsequent sea surface temperature (SST) anomaly changes. The “SAM positive anomaly wind‐SST oscillations (SPAWSO)” pattern emerges, where warm SST anomalies in DJF and JJA precede increased surface winds in MAM and SON, thus increasing the wind‐sea conditions in the NIO. SPAWSO, hence, acts as a delayed mode, season‐dependent positive air–sea interaction cycle linked to positive SAM phases, significantly impacting NIO's wind‐sea dynamics. Thus the present study provides better insights into the long‐term wave prediction accuracy in the IO by considering the direct swell influence and SPAWSO‐driven wind‐sea changes, aiding preparations for changing wave dynamics and their impacts.

Mechanisms for Abrupt Summertime Circumpolar Surface Warming in the Southern Ocean

Abstract In recent years, the Southern Ocean has experienced unprecedented surface warming and sea ice loss—a stark reversal of the sea ice expansion and surface cooling that prevailed over the preceding decades. Here, we examine the mechanisms that led to the abrupt circumpolar surface warming events that occurred in late 2016 and 2019 and assess the role of internal climate variability. A mixed layer heat budget analysis reveals that these recent circumpolar surface warming events were triggered by a weakening of the circumpolar westerlies, which decreased northward Ekman transport and accelerated the seasonal shoaling of the mixed layer. We emphasize the underappreciated effect of the latter mechanism, which played a dominant role and amplified the warming effect of air–sea heat fluxes during months of peak solar insolation. An examination of the CESM1 large ensemble demonstrates that these recent circumpolar warming events are consistent with the internal variability associated with the Southern Annular Mode (SAM), whereby negative SAM in austral spring favors shallower mixed layers and anomalously high summertime SST. A key insight from this analysis is that the seasonal phasing of springtime mixed layer depth shoaling is an important contributor to summertime SST variability in the Southern Ocean. Thus, future Southern Ocean summertime SST extremes will depend on the coevolution of mixed layer depth and surface wind variability. Significance Statement This study examines how reductions in the strength of the circumpolar westerlies can produce abrupt and extreme surface warming across the Southern Ocean. A key insight is that the mixed layer temperature is most sensitive to surface wind perturbations in late austral spring, when the regional mixed layer depth and solar insolation approach their respective seasonal minimum and maximum. This heightened surface temperature response to surface wind variability was realized during the austral spring of 2016 and 2019, when a dramatic weakening of the circumpolar westerlies triggered unprecedented warming across the Southern Ocean. In both cases, the anomalously weak circumpolar winds reduced the northward Ekman transport of cool subpolar waters and caused the mixed layer to shoal more rapidly in the spring, with the latter mechanism being more dominant. Using results from an ensemble of coupled climate simulations, we demonstrate that the 2016 and 2019 Southern Ocean warming events are consistent with the internal variability associated with the Southern Annular Mode (SAM). These results suggest that future Southern Ocean surface warming extremes will depend on both the evolution of regional mixed layer depths and interannual wind variability.

The circulation and rainfall response in the southern hemisphere extra-tropics to climate stabilisation

Changes from transient to equilibrium climate states following stabilisation of anthropogenic forcings will determine the long-term future of our climate, including the future of rainfall and drought. However, there is a lack of research and very long climate simulations to characterise the future equilibrium climate. As forcing and global temperature stabilise, some regions will likely continue to warm, including much of the Southern Ocean. Here we explore the change in surface warming, atmospheric circulation, and rainfall in two case studies of high CO2-only and high all forcings from LongRunMIP. We find an initial delay in Southern Ocean warming, enhancing the meridional temperature gradient between the tropics and the Southern Ocean. The enhanced gradient peaks sooner and with lower magnitude over the Indian Ocean relative to the gradient over the Pacific Ocean. Coincident with the peak meridional temperature gradient there is clear stabilisation and then reversal of mid-latitude annual drying. The response in atmospheric circulation in one case study has the character of a shift from positive to negative Southern Annular Mode (SAM). Some regions of the mid-latitudes, such as southern Australia, become drier and do not rebound to wetter conditions during the transition to an equilibrium climate state, whereas other regions such as eastern Australia may see transient drying that then rebounds to a wetter climate as a new equilibrium is reached. This study adds to growing evidence of a time-dependent response in rainfall and water availability to greenhouse gas emissions across key regions, including Australia, and further understanding is needed about short-term and long-term changes.

Characteristics of Surface “Melt Potential” over Antarctic Ice Shelves based on Regional Atmospheric Model Simulations of Summer Air Temperature Extremes from 1979/80 to 2018/19

Abstract We calculate a regional surface “melt potential” index (MPI) over Antarctic ice shelves that describes the frequency (MPI-freq; %) and intensity (MPI-int; K) of daily maximum summer temperatures exceeding a melt threshold of 273.15 K. This is used to determine which ice shelves are vulnerable to melt-induced hydrofracture and is calculated using near-surface temperature output for each summer from 1979/80 to 2018/19 from two high-resolution regional atmospheric model hindcasts (using the MetUM and HIRHAM5). MPI is highest for Antarctic Peninsula ice shelves (MPI-freq 23%–35%, MPI-int 1.2–2.1 K), lowest (2%–3%, <0 K) for the Ronne–Filchner and Ross ice shelves, and around 10%–24% and 0.6–1.7 K for the other West and East Antarctic ice shelves. Hotspots of MPI are apparent over many ice shelves, and they also show a decreasing trend in MPI-freq. The regional circulation patterns associated with high MPI values over West and East Antarctic ice shelves are remarkably consistent for their respective region but tied to different large-scale climate forcings. The West Antarctic circulation resembles the central Pacific El Niño pattern with a stationary Rossby wave and a strong anticyclone over the high-latitude South Pacific. By contrast, the East Antarctic circulation comprises a zonally symmetric negative Southern Annular Mode pattern with a strong regional anticyclone on the plateau and enhanced coastal easterlies/weakened Southern Ocean westerlies. Values of MPI are 3–4 times larger for a lower temperature/melt threshold of 271.15 K used in a sensitivity test, as melting can occur at temperatures lower than 273.15 K depending on snowpack properties.

How well do forecast models represent observed long‐lived Rossby wave packets during southern hemisphere summer?

AbstractRossby wave packets (RWPs), are atmospheric perturbations linked to the occurrence of extreme weather events such as heatwaves, extratropical cyclone development and other equally destructive phenomena. Under certain circumstances, these packets can last from several days to 2–3 weeks in the atmosphere. Therefore, forecast models should be able to correctly predict their formation and development to enhance extreme weather events prediction from 10 to 30 days in advance. In this study, we assess whether the NCEP and IAP‐CAS sub‐seasonal forecast models can predict the evolution of observed RWPs that last more than 8 days (long‐lived RWPs or LLRWPs) during southern hemisphere summer. Results show that the NCEP (IAP‐CAS) model forecasts LLRWPs that appear eastward (westward) from the observed LLRWPs. Both models forecasted LLRWPs that rapidly lose energy after the 6th–7th lead day of simulation, which could limit LLRWPs prediction to the synoptic time scale. Additionally, both models better forecast LLRWPs when the packets manifest in the eastern Pacific. Southern Annular mode (SAM) and El Niño Southern‐Oscillation (ENSO) do not seem to exert a large influence in the representation of LLRWPs. Nevertheless, during the best LLRWPs forecasts, the observed circulation anomalies signal the manifestation of negative SAM events. In contrast, both forecast models struggle at forecasting LLRWPs when a blocking situation develops to the South of Australia. Lastly, an inactive Madden Julian Oscillation (MJO) seems to favor the development of accurate LLRWPs forecasts, whereas during phases 3, 5 in the NCEP model and 3, 8 for IAP‐CAS, the models struggle at forecasting LLRWPs.

Southern Hemisphere Response to the Quasi-Biennial Oscillation in the CMIP5/6 Models

Abstract Using 25 quasi-biennial oscillation (QBO)-resolving models from phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5/6), this study systematically explores the Southern Hemisphere (SH) extratropical response to the QBO. The observed maximum stratospheric polar vortex–weakening response to the 20-hPa easterly QBO appears in the austral spring, which is reproduced qualitatively by seven models. The Eliassen–Palm (EP) flux convergence associated with enhanced upward propagation of waves from the SH midlatitude troposphere is also reasonably simulated following easterly QBO in these seven models. The Brewer–Dobson (BD) circulation response, especially its deep branch, has a large bias among models. The anomalous upwelling of the deep branch of the BD circulation and the cold anomalies are confined to midlatitudes above 20 hPa in reanalyses, while this deep branch is simulated to expand to the Antarctic stratosphere with unrealistically cold anomalies for some models. Models with a decent stratospheric pathway tend to simulate the observed negative Southern Annular Mode (SAM)-like response in the troposphere. Further, the largest enhanced convection center is observed over the tropical Indo-Pacific Ocean and simulated by 12 models, most of which also simulate a more reasonable height anomaly pattern in the troposphere. The near-surface response to the QBO also resembles the SAM pattern with warm anomalies over eastern Antarctic, which are simulated by only five models. Seven models simulate the observed wet signals in the midlatitude southern Indo-Pacific Ocean, where an anomalous low develops. Overall, models with a better reproduction of the SH stratospheric polar vortex response exhibit a more reasonable near-surface response, though for all models the response is too weak. Significance Statement The equatorial stratospheric quasi-biennial oscillation (QBO) has been widely reported to influence the Northern Hemisphere. This paper focuses on the possible impact of the QBO on the Southern Hemisphere (SH) using 25 models. The maximum QBO signal in the Antarctic stratosphere forms in austral spring when the climatological vortex intensity has weakened relative to austral winter. The timing and magnitude of the QBO signal is simulated with different degrees of success by the 25 models, and those with a reasonable polar vortex response tend to better simulate the near-surface response. The QBO has another pathway for the extratropical circulation change via modifying tropical convection, and models also simulate a large spread in the tropical convection change associated with the QBO.

Stable Southern Hemisphere westerly winds throughout the Holocene until intensification in the last two millennia

The Southern Hemisphere westerly winds sustain the Southern Ocean’s role as one of Earth’s main carbon sinks, and have helped sequester nearly half the anthropogenic CO2 stored in the ocean. Observations show shifts in the vigor of this climate regulator, but models disagree how future change impacts carbon storage due to scarce baseline data. Here, we use the hydrogen isotope ratios of sedimentary lipids to resolve Holocene changes in Southern Hemisphere westerly wind strength. Our reconstruction reveals stable values until ~2150 years ago when aquatic compounds became more 2H-enriched. We attribute this isotope excursion to wind-driven lake water evaporation, and regional paleoclimate evidence shows it marks a trend towards a negative Southern Annular Mode – the Southern Ocean’s main mode of atmospheric variability. Because this shift is unmatched in the past 7000 years, our findings suggest that previously published millennium-long Southern Annular Mode indices used to benchmark future change may not capture the full range of natural variability.

Subantarctic Mode Water Variations in the Three Southern Hemisphere Ocean Basins During 2004–2019

AbstractSubantarctic mode water (SAMW) variations and their forcing mechanisms in the three Southern Hemisphere ocean basins are studied using Argo data and reanalysis products during the 2004–2019 period. In the Indian and Pacific Oceans, SAMW is characterized by potential vorticity (PV) minimums lower than 0.5 × 10−10 m−1 s−1. In the Atlantic Ocean, the PV minimum is generally lower than 1 × 10−10 m−1 s−1. The variations in SAMW thickness are significantly correlated with changes in the mixed layer depth (MLD) and show large differences among different ocean basins. The winter MLD in the Indian Ocean SAMW formation region is significantly positively correlated with the local wind stress curl (correlation coefficient = 0.61; 95% confidence level). In the Pacific and Atlantic SAMW formation regions, the variations in the winter MLD have significant negative correlations with the buoyancy flux; the correlation coefficients reach −0.57 and −0.67 (95% confidence level), respectively. These different mechanisms of the MLD variations in SAMW in the three ocean basins are related to the Southern Annular Mode (SAM) variation in winter. During positive (negative) SAM index periods, a strong positive (negative) wind stress curl anomaly occurs in association with a meridionally asymmetric westerly wind anomaly distribution. This phenomenon results in a corresponding MLD anomaly over the Indian Ocean SAMW formation region. In the Pacific and Atlantic SAMW formation regions, a negative (positive) westerly wind anomaly induces buoyancy gain (loss), leading to mixed layer shallowing (deepening) during positive (negative) SAM periods.

The Holocene hypsithermal in the Australian region

Close examination of key and well-dated Holocene sites, both on land and at sea in the Australian region indicates that at the very beginning of the Holocene, as a result of strong westerlies, there must have been a continuous positive Southern Annular Mode (SAM). Following from that, the entire region switched to a negative SAM scenario and, during that time, the westerlies must have retreated further south. Afterwards, a period of time spaning ∼8200 to ∼5500 years ago temperatures were higher than today. We refer to it as the Holocene Hyspithermal. Coincident to this period, lake levels and postulated rainfall were extraordinarily high and vegetation spectra in places very different compared to today. The extent of this period varies by a few centuries between sites, but this may result from the level of resolution and also appears to be controlled by latitude. There is also clear indication that the influence of the westerlies was reduced over Australia during those two and a half millennia.Nevertheless, air temperatures recognised in Antarctic ice cores are at the opposite to those recognised in Australia. In addition, during the Australian Holocene Hypsithermal, CO2 levels were at their lowest in Antarctic ice cores.Climatic conditions then progressively deteriorated everywhere a bit after ∼6000 years BP until recent times as ENSO signals with alternating El Niño and La Niña conditions across the entire Pacific region as already described by Perner et al. (2018) based on the same cores studied here.Brief mention is also made to the presence of humans in SE Australia during the Holocene. It seems that human activities changed well after the period of high temperatures and rainfall, with more sedentary activities along the major rivers, with an enhancement of food production in organized settings suggestive of villages.

Distinct patterns of monthly Southern Annular Mode events

The Southern Annular Mode (SAM) is the leading mode of atmospheric variability in the mid–high latitudes of the Southern Hemisphere, representing large-scale variations in pressure and the polar front jet (PFJ). In SAM events, the combination of the SAM and other modes may result in different atmospheric patterns. In this study, a neural-network-based cluster technique, the self-organizing map, was applied to extract the distinct patterns of SAM events on the monthly time scale based on geopotential height anomalies at 500 hPa. Four pairs of distinguishable patterns of positive and negative SAM events were identified, representing the diversity in spatial distribution, especially the zonal symmetry of the center of action at high latitudes—that is, symmetric patterns, split-center patterns, West Antarctica patterns, and a tripole pattern. Although the SAM is well known to be belt-shaped, within the selected SAM events, the occurrence frequency of symmetric patterns is only 23.8%—less than that of West Antarctica patterns. Diverse PFJ variations were found in the symmetric and asymmetric patterns of SAM events. The more asymmetric the spatial distribution of the pressure anomaly, the more localized the adjusted zonal wind anomaly. The adjusted PFJ varied in meridional displacement and strength in different patterns of SAM events. In addition, the entrance and exit of the jet changed in most of the patterns, especially in the asymmetric patterns, which might result in different climate impacts of the SAM.摘要南半球环状模 (SAM) 是南半球中–高纬度地区大气变化的主导模态, 表现为气压和极锋急流 (PFJ) 的大尺度变动, 形成强烈的气候影响. 当SAM事件发生时, 气压场异常可呈现出不同的空间结构. 本文利用自组织映射网络方法对月尺度的SAM事件进行分类, 可识别出四对具有显著差异的正, 负SAM事件类型, 包括对称型, 中心分裂型, 西南极洲型和一种三极型分布. 气压异常的空间分布越不对称, 调整后的纬向风异常越局地化. PFJ的经向位移和强度变化入口和出口的变化, 可能导致了SAM的不同气候影响.

Variability in High Wave Energy Events Around the Southern African Coast

AbstractThe wave energy flux along the southern African coastline often reaches extreme levels, seriously impacting coastal communities, infrastructure, as well as near‐coast and offshore marine operations. Understanding the drivers behind past high wave energy events and their frequency is key to forecasting future events. Using in‐situ wave buoy data recorded by the Council for Scientific and Industrial Research (CSIR), the ERA5 and CACWR global wave model products are evaluated. The better‐performing ERA5 product is used to assess long‐term variability and trends around the coastline between 1979 and 2020. There are significant increasing trends in offshore flux for all seasons, with spring having the strongest coastal trends. Substantial interannual variability exists in wave energy flux and direction. Possible drivers, the Southern Annular Mode (SAM), El Niño Southern Oscillation (ENSO), and the semi‐annual oscillation (SAO), were assessed to find that SAM appears to show the strongest relationship overall. Negative (positive) SAM is associated with above (below) average and westerly (easterly) anomalies in both flux and direction. ENSO directly impacts the summer wave climate while the SAO indirectly impacts the wave energy flux over spring and winter. During many seasons, more than one climate mode is active. It is found the strongest significant near‐coast positive westerly anomalies occurred under negative SAM and negative SAO, with more intense offshore anomalies under El Niño, whereas the strongest significant negative east‐northeasterly anomalies occurred under a combination of La Niña with positive SAM and SAO phases.