Southern Annular Mode Research Articles

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.

Predictable and Unpredictable Components of Cape Town Winter Rainfall

Abstract In early 2018, due in part to a severe and extended meteorological drought, Cape Town was at risk of being one of the first major metropolitan areas in the world to run out of water. The magnitude of the crisis was exacerbated by the fact that such a prolonged and severe drought was both unanticipated and unpredicted. In this work, we analyze data from both observations and seasonal forecasts made as part of the North American Multimodel Ensemble (NMME) to better understand the predictability of rainfall in the Cape Town (CT) region. We find that there are statistically significant correlations between observed CT rainfall and sea surface temperatures in the tropical Atlantic (∼0.45) as well as a pattern of 200-mb geopotential height (z200) anomalies resembling the Southern Annular Mode (SAM; ∼0.4). Examination of hindcasts from the NMME demonstrates that the models accurately reproduce the observed correlation between CT rainfall and z200 anomalies. However, they fail to reproduce correlations between CT rainfall and the tropical South Atlantic. Decomposition of the correlations into contributions from predictable and unpredictable components indicates that CT rainfall in the models is dominated by unpredicted atmospheric variability (correlation ∼ 0.84) relative to predicted (correlation ∼ 0.14), which may be related to the failure to simulate the connection with the tropical Atlantic. Significance Statement Water crises are occurring with increasing severity and frequency around the globe. The ability to accurately forecast wet season rainfall would be invaluable to water managers and other decision-makers. Here, we explore the reasons behind the failure of a suite of operational seasonal forecast models to accurately predict rainfall in the Cape Town region of South Africa.

Prediction of Coral Sea tropical cyclone power and latitude of maximum intensity using climate indices

Tropical cyclone (TC) wind power, often highly destructive, can be quantified using the power dissipation index (PDI) and in this study, the PDIs for Coral Sea TC tracks, as well as the latitude of maximum intensity (LMI) were investigated for correlation with climatological indices. Multiple linear regression with collinearity removed and an overall Pearson correlation of 0.7 or above was used for this. The results for all tracks showed that three indices dominated for PDI: Niño 4 Sea Surface Temperature (SST), the Dipole Mode Index (DMI) and the Madden Julian Oscillation (MJO). Coral Sea TC tracks clustered by maximum windspeed-weighted locations were then examined: For cluster 1 (located more south-east), the additional influence of the Southern Annular Mode (SAM) was apparent, whilst for cluster 2 (located more north-west), the same indices dominated as for the all-tracks model. For LMI, four indices were identified: the Indian Ocean East SST Anomaly (DMI E), the MJO, the Southern Oscillation Index (SOI) and the SAM. Only TCs clustered in the northeast of Australia had a valid model for LMI, with correlation 0.8, using three indices: DMI E, DMI and the SOI. Overall, raised Niño 4 SST combined with a negative DMI and low MJO amplitude were shown to predict large increases in TC power, whilst a combination of increasing DMI E temperature anomaly with a positive SOI moves LMI equator-wards. The models compiled in this study identified the most significant climatic indices and successfully predicted TC power dissipation and LMI.

Role of the Australian High in Seasonal Phase Locking of the Indian Ocean Dipole

AbstractThis paper analyzes the effect of the Australian High (AH) on the seasonal phase locking of Indian Ocean Dipole (IOD) events. The anomalous strong AH associated with the positive phase of the Antarctic Oscillation can cause significant easterly wind anomalies and northward cross‐equatorial flow over the western Maritime Continent (MC) by strengthening the Australian winter monsoon during May–August. The AH‐associated easterly anomalies and northward cross‐equatorial flow can create thermodynamic air‐sea feedback and contribute to a significant cooling anomaly in the western MC and the tropical eastern Indian Ocean. Without considering the effect of ENSO, these processes contribute to the occurrence of positive IOD events, which begin in early summer, peak in late summer, and decay rapidly thereafter. The effect of ENSO can extend the peak period of IOD into the boreal autumn of that year. An anomalous weak AH corresponds to the occurrence and seasonal phase locking of negative IOD events. Through combined empirical orthogonal function analysis, we find that the effect of the AH can well explain the seasonal phase locking of 34 IOD events (40 in total), which provides an important theoretical basis for the prediction of IOD events.

Morphological changes and future projections of the Getz Ice shelf, western Antarctica—A statistical and geospatial approach

The study assessed the extent of the Getz Ice Shelf over a 19-year period using satellite data and divided it into three sectors. The average of rates (AOR), end point rate (EPR), and linear regression (LR) methods were employed to estimate the rate of change, with the LR method showing the strongest correlation. The analysis revealed a shrinkage rate of 42 m/year during February from 2003 to 2019. Over the last 17 years, recession was observed in 60% of transects, while progradation occurred in 40%. The recent decline in Getz ice shelf extent is closely linked to the record low sea ice extent in the Amundsen Sea sector. Predictions for 2024 and 2029 indicated progradation in Sector I and recession in Sectors II and III, as determined by LR analysis. The root mean square error values of transects in Sectors II and III showed good correlation with past positions and satellite-based observations of ice shelves. To validate the predicted ice shelf extent cross-verified with the satellite and predicted data from 2020 to 2022. These findings highlight the complex interactions between climatic forcings, such as wind speed, sea surface temperature (SST), and the Southern Annular Mode (SAM). Positive SAM and increased zonal winds can contribute to warm water upwelling near the Antarctic coast, impacting ice shelf extent. The relationship between ice mass loss and SST provides valuable insights into the dynamic processes of melting and progradation in the Getz Ice Shelf during February. Ocean warming plays a significant role in increased basal melting and changes in sea ice mass. The study emphasizes the significant impact of ocean-atmospheric factors on Antarctic ice shelf dynamics and highlights the necessity for ongoing satellite observations and enhanced comprehension of these processes to accurately predict future changes in Antarctic ice shelves.

Impact of Zonal and Meridional Atmospheric Flow on Surface Climate and Extremes in the Southern Hemisphere

Abstract The Southern Annular Mode (SAM) describes the annular or zonal component of the large-scale atmospheric circulation in the Southern Hemisphere (SH) extratropics and influences surface climate across the SH. Although this annular flow is dominant in austral summer, in other seasons considerable zonal asymmetries are evident, reflecting a zonal wave 3 (ZW3) pattern. We define an index representing asymmetric flow using the first two leading modes of meridional wind variability in the SH. Two orthogonal ZW3 indices are used together to capture longitudinal shifts in the wave train and their connection to tropical convection. We compare the impacts of the SAM and ZW3 on surface climate by examining composites of temperature and precipitation fields during each season. Impacts on mean and extreme surface climates are assessed. We find that the SAM and ZW3 are not clearly separated modes, but rather, ZW3 modulates the impact of the SAM across the midlatitudes. The SAM influence on regional temperature and precipitation is similar for both mean impacts and extremes. The ZW3 influence on extremes is more varied across indices and does not always reflect the ZW3 impact on mean fields. Notably, amplified ZW3 activity has a significant influence on the number of midlatitude fronts and frontal rainfall, highlighting the importance of considering ZW3 when exploring the surface climate impacts of large-scale SH circulation states, particularly for nonsummer seasons. Significance Statement Variations in the strength and position of the midlatitude westerly winds have a strong influence on surface climates. While these winds are predominantly zonally symmetric in the Southern Hemisphere, few studies to date have explored the role of the asymmetric component of this circulation, particularly for seasons outside of summer. By defining two new indices of meridional circulation, this study reveals new important impacts on temperature, rainfall, and the likelihood of extreme climates in regions of southern Australia and South America, and sea ice regions around Antarctica. These findings question the validity of considering only zonal-mean winds for climate studies of the Southern Hemisphere and have important implications for the seasonal forecasting and predictability of extreme climate events in the near future.

Quasi‐Decadal Temperature Variability in the Intermediate Layer of Subtropical South Indian Ocean During the Argo Period

AbstractIt has been reported that the subtropical South Indian Ocean (SIO) has been rapidly warming over the past two decades and can therefore be characterized as one of the major heat accumulators among the oceanic basins. However, this strong warming is not uniformly distributed in the vertical direction. In comparison to the decade‐long warming in the upper layer (0–300 m) in 2004–2013, the intermediate layer (300–1,000 m) displays a shorter warming during 2004–2009 and an intense cooling during 2010–2016. By decomposing temperature variations into heaving and spice components, and performing a heat budget analysis, we show that temperature variations in the intermediate layer during these two periods are primarily contributed by isopycnal migrations driven by local wind forcing. Local wind change in the subtropical SIO can be explained by the Indian Ocean Dipole and El Niño–Southern Oscillation during 2004–2016, while Southern Annular Mode (SAM) favors anomalous wind change in mid‐latitudes and the formation of basin‐wide wind change in the SIO. Additionally, wind forcing in the Subantarctic Mode Water (SAMW) formation region, which is closely linked to the SAM, modulates the anomalous spreading of SAMW into the interior of the subtropical SIO. This, therefore, leads to the SAMW intrusion being of secondary importance to the quasi‐decadal temperature variability. Our findings demonstrate the independence of wind‐driven temperature changes on the quasi‐decadal scale in the intermediate layer of the subtropical SIO under the overall warming background of SIO waters.

Long-term trends and wave climate variability in the South Atlantic Ocean: The influence of climate indices

Linking wave climate variability and trends with climate indices is important to better understand and predict large-scale patterns of wave variability down to wave conditions at the coast. This study investigates such links in the South Atlantic Ocean using 72 years of ERA5 wave hindcast. Different wave parameters are computed, including storm wave statistics, and are further analyzed in terms of long-term trends and interannual changes. Our results indicate that, over the last decades, wave height has been significantly increasing across the entire domain, while extreme events statistics are also increasing, although with more complex spatial variability. The variations of these wave properties are primarily correlated, from low to high latitudes, with the Atlantic Multidecadal Oscillation (AMO), Tropical Southern Atlantic Index (TSA) and Southern Annular Mode (SAM), with different preferred timescales. We think that better understanding and predicting the evolution of these climate indices, including under climate change, will be critical to anticipate coastal hazards in this region.

Atmospheric Drivers of Tasman Sea Marine Heatwaves

Abstract Marine heatwaves (MHWs) can severely impact marine biodiversity, fisheries, and aquaculture. Consequently, there is an increasing desire to understand the drivers of these events to inform their predictability so that proactive decisions may be made to reduce potential impacts. In the Tasman Sea (TS), several relatively intense and broad-scale MHWs have caused significant damage to marine fisheries and aquaculture industries. To assess the potential predictability of these events, we first determined the main driver of each MHW event in the TS from 1993 to 2021. We found that those MHWs driven by ocean advection—approximately 45% of all events—are generally longer in duration and less intense and affected a smaller area compared with the remaining 55%, which are driven by air–sea heat fluxes, are shorter in duration, and are more surface intense. As ocean advection–driven events in the TS have been closely studied and reported previously, we focus here on atmospherically driven MHWs. The predictability of these events is assessed by identifying the patterns of atmospheric pressure, winds, and air–sea heat fluxes in the Southern Hemisphere that coincide with MHWs in the Tasman Sea. We found that atmospherically driven MHWs in this region are more likely to occur during the positive phase of the asymmetric Southern Annular Mode (A-SAM)—which presents as an atmospheric zonal wave-3 pattern and is more likely to occur during La Niña years. These A-SAM events are linked to low wind speeds and increased downward solar radiation in the TS, which lead to increased surface ocean temperatures through the reduction of mixing. Significance Statement The purpose of this study is to understand factors of the atmosphere that contribute to an accumulation of heat in the upper ocean in the Tasman Sea to better inform predictability. Higher incidences of ocean extreme thermal events (known as marine heatwaves) in this region are becoming increasingly more common and threatening the important marine industries that support the people of both Australia and New Zealand. We need to know the sources of this extra heat to understand whether such events can be predicted. Previous studies have found the East Australian Current to be responsible for around half of these events, and our results show a connection between a known atmospheric pattern and the other half. As we continue to improve our ability to anticipate this pattern, this suggests that we may also be able to predict these extreme heating events.

Comparative Analysis for Atmospheric Oscillations

In recent decades, studies on atmospheric circulations indicate that those patterns have influences on meteorological variables. This paper investigates the comparative statistical analysis of atmospheric oscillations with climatological elements. Based on analysis of the climate data obtained from observed values of meteorological station in Antalya, it was pointed that atmospheric elements such as meteorological variables were associated with atmospheric oscillations such as North Atlantic Oscillation, Arctic Oscillation, Antarctic Oscillation and Pacific-North American pattern. Spearman’s rho and Kendall’s tau statistics were employed to reveal the relations between atmospheric variables and atmospheric oscillations as statistically significant. Both coefficients were compared in interpreting the direction and strength of the relationships. It was seen that Spearman’s rho coefficients presented more suitable values generally.

Foehn winds at Pine Island Glacier and their role in ice changes

Abstract. Pine Island Glacier (PIG) has recently experienced increased ice loss that has mostly been attributed to basal melt and ocean ice dynamics. However, atmospheric forcing also plays a role in the ice mass budget, as besides lower-latitude warm air intrusions, the steeply sloping terrain that surrounds the glacier promotes frequent Foehn winds. An investigation of 41 years of reanalysis data reveals that Foehn occurs more frequently from June to October, with Foehn episodes typically lasting about 5 to 9 h. An analysis of the surface mass balance indicated that their largest impact is on the surface sublimation, which is increased by about 1.43 mm water equivalent (w.e.) per day with respect to no-Foehn events. Blowing snow makes roughly the same contribution as snowfall, around 0.34–0.36 mm w.e. d−1, but with the opposite sign. The melting rate is 3 orders of magnitude smaller than the surface sublimation rate. The negative phase of the Antarctic oscillation and the positive phase of the Southern Annular Mode promote the occurrence of Foehn at PIG. A particularly strong event took place on 9–11 November 2011, when 10 m winds speeds in excess of 20 m s−1 led to downward sensible heat fluxes higher than 75 W m−2 as they descended the mountainous terrain. Surface sublimation and blowing-snow sublimation dominated the surface mass balance, with magnitudes of up to 0.13 mm w.e. h−1. Satellite data indicated an hourly surface melting area exceeding 100 km2. Our results stress the importance of the atmospheric forcing on the ice mass balance at PIG.

The influence of large-scale climate modes on tropical cyclone tracks in the southwest Pacific

Tropical cyclones (TCs) impact the economy, properties, lives and infrastructure of island nations and territories of the southwest Pacific (SWP), accounting for three in four regional disasters each year. To increase the resilience of the SWP to the destructive impacts of TCs, improved TC track forecasts are needed since a high degree of uncertainty exists around the likely path a TC will take in this region post-formation. This requires better comprehension of the factors contributing to TC track variability occurring at different timescales. Therefore, we examine the modulating impact of key Indo-Pacific climate drivers: the El Niño-Southern Oscillation (ENSO), Interdecadal Pacific Oscillation (IPO), Southern Annular Mode (SAM) and the Indian Ocean Dipole (IOD), on SWP TC track variability. We present new insights into the spatial (i.e. prevailing trajectories) and temporal (i.e. track length, average speed and duration) components of TC tracks, being modulated by both individual and combined climate modes. Overall, TC tracks tend to shift northeast during El Niño, IPO positive, IOD east positive and/or SAM negative phases (with a southwest shift observed during the opposite climate phases). Further, we show that when two of these climate modes are in their positive phase (e.g. El Niño with the positive phases of IPO or eastern pole of IOD and SAM), TC track length and average speed are enhanced. However, for cases where either one (e.g. El Niño/negative phase of IPO and IOD east) or two (La Niña/negative phase of IPO, IOD east and SAM) climate modes were in the negative phase, an increase in TC track duration was observed. The findings of this study may be used to improve TC forecasting and better quantify TC-related risks.

The Antarctic Amplification Based on MODIS Land Surface Temperature and ERA5

With global warming accelerating, polar amplification is one of the hot issues in climate research. However, most studies focus on Arctic amplification, and little attention has been paid to Antarctic amplification (AnA), and there is no relevant research based on MODIS (Moderate Resolution Imaging Spectroradiometer) land surface temperature observations. Compared with 128 stations’ observations, MODIS can capture the variations in temperature over Antarctica. In addition, the temperature changes in Antarctica, East Antarctica, West Antarctica and the Antarctic Peninsula during the period 2001–2018 reflected by the MODIS and ERA5 are basically consistent, and the temperature changes in Antarctica are negatively correlated with the Southern Annular Mode. AnA occurs under all annual and seasonal scales, with an AnA index greater than 1.27 (1.31) from the MODIS (ERA5), and is strongest in the austral winter and weakest in summer. AnA displays regional differences, and the signal from the MODIS is similar to that from ERA5. The strongest amplification occurs in East Antarctica, with an AnA index greater than 1.45 (1.48) from the MODIS (ERA5), followed by West Antarctica, whereas the amplified signal is absent at the Antarctic Peninsula. In addition, seasonal differences can be observed in the sub regions of Antarctica. For West Antarctica, the greatest amplification appears in austral winter, and in austral spring for East Antarctica. The AnA signal also can be captured in daytime and nighttime observations, and the AnA in nighttime observations is stronger than that in daytime. Generally, the MODIS illustrates the appearance of AnA for the period 2001–2018, and the Antarctic climate undergoes drastic changes, and the potential impact should arouse attention.

Environmental effects on movement and breeding of Australasian Gannets: insights from banding records

ABSTRACT Banding has historically been the most efficient method of marking individuals to gather large datasets on bird movement, especially in seabirds which spend a large proportion of their life-cycle at sea. The Australian Bird and Bat Banding Scheme has gathered such long-term records, including for the Australasian Gannet (Morus serrator). Between 1954 and 2022, a total of 12,583 bands were deployed at seven breeding sites in Australia, of which 522 were recovered dead. An additional 325 individuals from four banding sites in New Zealand were recovered along the Australian coastline. The effects of broadscale environmental indices on movement (inferred from banding recoveries) during the post-breeding period were investigated at two colonies with the most banding effort (Pope’s Eye, Lawrence Rocks). Individuals >1 year old and those from Pope’s Eye were recovered closer to their banding site. There were negative effects of current-year Southern Oscillation Index (SOI) and 1-year lagged Southern Annular Mode (SAM) on recovery distances for Lawrence Rocks individuals, while 2-year lagged SAM positively and negatively affected recovery distance and direction, respectively, for Pope’s Eye individuals. The number of chicks banded (index of chick production) at Pope’s Eye was negatively influenced by 1-year lagged SAM and 2-year lagged SOI, while the likelihood of recoveries of first-year individuals was positively influenced by 1-year lagged SAM. The results of the present study revealed a potential effect of environmental conditions on post-breeding movement with seemingly contrasting effects between colonies, and on reproduction, highlighting the usefulness of long-term banding and recovery efforts.

Warming trends of southwestern Atlantic SST and the summer's warmest SST's impact on South American climate

AbstractSeveral atmospheric variables are directly influenced by oceanic conditions, mainly the sea surface temperature (SST). SST trends and variability over the southwestern Atlantic (SWA) have affected the South American climate, principally the monsoonal period (austral summer). This study analysed the SST trends over the SWA for all seasons. In addition, possible large‐scale triggers of the austral summer's warmest SST over the SWA and the straight impacts of these warmest SSTs on the South American climate were investigated. The results showed a significant positive SST trend of approximately 0.02°C·year−1 over the SWA for all seasons. Since 2000, positive SST anomalies have increased in frequency in all seasons, with the seven warmest summers occurring during the 2010s. The highest SST summers over the SWA have been related to four leading causes: (i) planetary waves triggered by warm anomalies over the subtropical South Pacific; (ii) an omega‐type blocking configuration over the South Atlantic; (iii) a Southern Annular Mode positive phase; and (iv) a strengthening of the Hadley Cell, responding to warm SST over tropical North Atlantic. They all reflected on intensification of the South Atlantic Subtropical High and the western boundary current, heating the SWA. The summer's warmest SST led to positive air temperature anomalies extending in almost all of eastern South America, with significantly highest temperatures over southeastern Brazil, northern Argentina and western Paraguay. Clouds decrease strengthen the incident shortwave in southeastern Brazil, warming the region. Significant reduction of cloudiness and precipitation indicate a low performance of the South Atlantic Convergence Zone. The comparison of the warmest summers in 1980s–1990s in the SWA ratified the role of SST intensification. In conclusion, the positive SST trends over the SWA are related to the summer's warmest SST after 2000s, causing heating and dryness in eastern South America, mainly over southeastern Brazil.

Recent Decline in Antarctic Sea Ice Cover From 2016 to 2022: Insights From Satellite Observations, Argo Floats, and Model Reanalysis

Ever since the abrupt drop in Antarctic sea ice extent (SIE) began in spring of 2016, as opposed to its consistent growth (1.95% decade–1 from 1979 to 2015), the SIE in the satellite era has reached record lows in 2017 and 2022. From spring 2016, the satellite-based SIE remained consistently lower than the long-term mean, with the trend dropping to 0.11% decade–1 from 1979 to 2022. The top record lowest SIE years were observed from 2016 to 2022, corresponding to the warmest years dating back to 1979. With this background, the rare features of Antarctic polynyas reoccurred frequently and the west Antarctic Peninsula remained ice-free throughout 2022. Recently, the SIE dropped to a record low in June 2022, July 2022, August 2022, January 2023, and February 2023, which were 13.67%, 9.91%, 6.79%, 39.29%, 39.56% below the long-term mean value, respectively for months described above. We find that the observed decline in SIE during 2016–2022 occurred due to the combined influences from the intensification of atmospheric zonal waves with enhanced poleward transport of warm-moist air and anomalous warming in the Southern Ocean mixed layer (>1°C). Although the sudden sea ice decline in spring of 2016 occurred corresponding to the transitional climate shift from IPO– (Interdecadal Pacific Oscillation, 2000–2014) to IPO+ (2014–2016), the recent decline after 2016 occurred in a dominant IPO– and Southern Annular Mode (SAM+). CMIP6 models showed a consistent decrease in ensemble-mean SIE from 1979 to 2022. The model trend exhibits similarities to the recent declining trend in SIE from satellite observations since 2016, suggesting a possible shift towards a warmer climatic regime.

Influence of the Antarctic oscillation on summer precipitation over East Asia

As the strongest atmospheric mode in the extratropical Southern Hemisphere, the Antarctic oscillation (AAO) can affect regional and even global climate through zonal asymmetric and symmetric dynamic mechanisms. Observational analysis indicates that AAO can affect the zonal-mean vertical velocity of the Northern Hemisphere through eddy-induced meridional circulation in boreal summer, corresponding to the out-of-phase zonal precipitation anomaly at approximately 20°N and the in-phase zonal precipitation anomalies at approximately 30°N and 50°N. In particular, the precipitation anomalies related to AAO in East Asia are much stronger than those in the other regions at the same latitude, and the AAO-related precipitation anomaly agrees well with the precipitation leading mode over East Asia. The strong meridional circulation anomaly in the East Asia sector associated with the asymmetry of AAO results in strong subsidence over the Philippine Sea, and the tropical cooling triggers a Pacific-Japan-like (PJ-like) pattern anomaly, which is conducive to moisture transport over East Asia. In addition, the transient eddy forcing related to AAO causes abnormal circulation over East Asia, which further strengthens the PJ-like pattern. The zonal precipitation anomalies in the Northern Hemisphere are superimposed on the moisture transport and precipitation anomalies related to the PJ-like pattern, hence promoting positive precipitation anomalies in the Yangtze River valley, northeastern China, South Korea and southern Japan. In contrast, negative precipitation anomalies occur in southern China and North China. Our results indicate that AAO is a potential indicator for the East Asian summer precipitation anomaly.

The Impacts of Combined SAM and ENSO on Seasonal Antarctic Sea Ice Changes

Abstract Both the Southern Annular Mode (SAM) and El Niño–Southern Oscillation (ENSO) are critical factors contributing to Antarctic sea ice variability on interannual time scales. However, their joint effects on sea ice are complex and remain unclear for each austral season. In this study, satellite sea ice concentration (SIC) observations and atmospheric reanalysis data are utilized to assess the impacts of combined SAM and ENSO on seasonal Antarctic sea ice changes. The joint SAM–ENSO impacts on southern high latitudes are principally controlled by the strength and position of the wave activity and associated atmospheric circulation anomalies affected by their interactions. In-phase events (La Niña/positive SAM and El Niño/negative SAM) are characterized with an SIC dipole located in the Weddell/Bellingshausen Seas and Amundsen/Ross Seas, while out-of-phase events (El Niño/positive SAM and La Niña/negative SAM) experience significant SIC anomalies in the Indian Ocean and western Pacific Ocean. Sea ice budget analyses are conducted to separate the dynamic and thermodynamic contributions inducing the sea ice intensification anomalies. The results show that in-phase intensification anomalies also display a pattern similar to the SIC dipole and are mainly driven by the direct thermodynamic forcing at the ice edge and thermodynamic responses to meridional sea ice drift in the inner pack, especially in autumn and winter. Dynamic processes caused by zonal sea ice drift also play an important role during out-of-phase conditions in addition to the same mechanisms during in-phase conditions.

Interdecadal Changes in the Dominant Modes of Spring Snow Cover over the Tibetan Plateau around the Early 1990s

Abstract The current work investigated the interdecadal changes in the leading empirical orthogonal function (EOF) pattern of the interannual variation in spring [March–May (MAM)] snow-cover extent (SCE) over the Tibetan Plateau (TP) (SSC_TP). The leading EOF pattern of the SSC_TP is transformed from an east to west dipole pattern during the period 1970–89 (P1) to a monopole structure during the period 1991–2020 (P2). Observational analysis shows that during P1, the negative Antarctic Oscillation (AAO) (−AAO) is associated with low-level cross-equator southeasterly anomalies across the Bay of Bengal and transports more water vapor to the eastern TP. Moreover, at a high level, anomalous northerly winds accompanied by an anomalous sinking motion dominate the western TP, favoring an east-wet–west-dry dipole pattern of SSC_TP. Further analysis shows that the −AAO induces anomalous divergence over the Antarctic, which contributes to the formation of a Rossby wave source (RWS). This RWS is related to a northeastward-propagating atmospheric wave train that crosses the equator and contributes to the SSC_TP variation during P1. In contrast, in P2, the Arctic Oscillation (AO) is associated with a barotropic atmospheric wave train originating from southern Greenland, moving across the North Atlantic Ocean and North Africa and reaching the TP. This wave train results in significant positive vorticity and ascending airflow above the TP and favors a monopole pattern of the SSC_TP. Further analysis shows that the AO can induce divergence anomalies over southeastern Greenland and RWS anomalies there. This RWS induces an atmospheric wave train that propagates eastward and reaches the TP during P2. The above mechanisms have been supported by the results of numerical experiments performed using the linear baroclinic model.

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.