Southern Annular Mode Index Research Articles

Seasonal-to-Interannual Response of Southern Ocean Mixed Layer Depth to the Southern Annular Mode from a Global 1/10° Ocean Model

Abstract The relationship between the southern annular mode (SAM) and Southern Ocean mixed layer depth (MLD) is investigated using a global 0.1° resolution ocean model. The SAM index is defined as the principal component time series of the leading empirical orthogonal function of extratropical sea level pressure from September to December, when the zonally symmetric SAM feature is most prominent. Following positive phases of the SAM, anomalous deep mixed layers occur in the subsequent fall season, starting in May, particularly in the southeast Pacific. Composite analyses reveal that for positive SAM phases enhanced surface cooling caused by anomalously strong westerlies weakens the stratification of the water column, leading to deeper mixed layers during spring when the SAM signal is at its strongest. During the subsequent summer, the surface warms and the mixed layer shoals. However, beneath the warm surface layer, anomalously weak stratification persists throughout the summer and into fall. When the surface cools again during fall, the mixed layer readily deepens due to this weak interior stratification, a legacy from the previous springtime conditions. Therefore, the spring SAM–fall MLD relationship is interpreted here as a manifestation of reemergence of interior water mass anomalies. The opposite occurs after negative phases of the SAM, with anomalously shallow mixed layers resulting. Additional analyses reveal that for the MLD region in the southeast Pacific, the effects of salinity variations and Ekman heat advection are negligible, although Ekman heat transport may play an important role in other regions where mode water is formed, such as south of Australia and in the Indian Ocean.

Recent land surface temperature patterns in Antarctica using satellite and reanalysis data

Antarctica is among the most important regions when it comes to observing the effects of climate change. One important part of the variability of the land surface temperature (LST) observed on this continent is related to the sea surface temperature of the Tropical Pacific Ocean, which is associated with El Niño-Southern Oscillation (ENSO). This large-scale phenomenon is called tele-connection. In this study, we investigate the recent trends of LST in Antarctica over the last few decades and its relationship with one of the most intense El Niño events in 2015–2016. LST anomalies derived from satellite data (MODIS) and skin temperature from re-analysis (ERA-5 and ERA-Interim) were used, in addition to the Oceanic Niño Index (ONI) and Southern Annular Mode (SAM) time series. A non-parametric time series analysis was carried out to estimate LST trends over different zones of Antarctica. The results show a warming trend obtained from ERA-5 of about 0.9 K/decade in the interior of Antarctica, while MODIS showed the lowest temperature trends of −1 K/decade in East Antarctica. The analysis between LST and the SAM index shows a negative relationship in the extreme periods of ENSO 2015–2016, showing colder autumns and warmer springs than in previous years. These results aide in understanding the effects of extreme phases of ENSO on Antarctica, which are of great importance given the projection of more intense phases over the next century as foreseen in future scenarios of climate change.

Holocene glacier fluctuations in Patagonia are modulated by summer insolation intensity and paced by Southern Annular Mode-like variability

Alpine glaciers are sensitive indicators of changes in climate, and their ubiquity in mountainous regions make them valuable proxies for terrestrial climate reconstructions worldwide. However, the timing and extent of glacier change across the South American mid-latitudes through the Holocene are still poorly constrained relative to their counterparts in the Northern Hemisphere. Here we report a new 10Be surface exposure-based chronology of moraines recording a series of progressively less-extensive glacier advances of Glaciar Torre (Argentina, 49.3°S/73.0°W) since the Last Glacial Maximum, with expansions culminating at 17,600 ± 900, 13,500 ± 500, 9700 ± 400, 6900 ± 200, 6100 ± 300, 4500 ± 200, and 530 ± 60 yr BP. The declining magnitude of Holocene glacier expansions parallels a gradual rise in local summer insolation intensity during the Holocene, while individual advances occurred during inferred negative Southern Annular Mode (SAM)-like states at centennial to millennial timescales. These observations suggest that (i) summer insolation intensity modulated antiphased trends in glacier extent in the polar hemispheres during the Holocene, and that (ii) centennial-scale ‘SAM-like’ temperature and precipitation anomalies paced glacier fluctuations throughout Patagonia. Given the persistence of the inferred ’SAM-like’ anomalies throughout the Holocene, the modern measured trend towards positive SAM index conditions could mark the onset of a fundamental shift in the climate of the Southern Hemisphere midlatitudes that warrants consideration in projections of future climate.

Polar Cyclones at the Origin of the Reoccurrence of the Maud Rise Polynya in Austral Winter 2017

AbstractThis study examines the role of atmospheric forcings in the occurrence of open‐ocean polynyas by investigating the case of the austral winter 2017's polynya located in the Lazarev Sea sector to the east of the Weddell Sea, known as the Maud Rise polynya or the Weddell Polynya. The ice‐free zone appeared in mid‐September 2017 and grew to as large as 80,000 km2 by the end of October 2017 before merging with the open ocean after the sea ice started to retreat at the beginning of the austral summer. Using a combination of satellite observations and reanalysis data at high spatiotemporal resolution, we found that severe cyclones, occurring over the ice pack, have a deterministic role in creating strong divergence in the sea ice field through strong cyclonic surface winds leading to the opening of the polynya. The occurrence of intense and frequent cyclones over the ice pack during austral winter 2017 was unusual, and it occurred under an enhanced strong positive meridional transport of heat flux and moisture toward Antarctica associated with an amplification of the atmospheric zonal wave 3 and a strong positive Southern Annular Mode index. We found that the opening of the polynya was not primarily due to direct ice melt by thermodynamic effects but rather to strong dynamical forcing by the winds on the sea ice, as in the case of coastal polynyas. Indeed, the meridional transport of heat toward Antarctica occurred over the Weddell Sea sector (i.e., to the east of the Lazarev Sea sector where the polynya is located) whereas the Lazarev Sea sector was under the influence of equatorward transport of cold air masses at that time. Our results show that the supply of warm and moist air coming from the west side of the South Atlantic Ocean into the Weddell Sea significantly increased the potential for cyclone formation as measured by the Eady growth rate leading to intense and frequent cyclogenesis over the ice pack, far south from the ice edge. After cyclogenesis in the Weddell Sea, these cyclones intensified as they moved eastward spinning over the Lazarev Sea with intensity comparable to category 11—violent storms—in the Beaufort scale. The cyclonic winds generated sea ice divergence by pushing the ice away from the cyclone center: To the east, north of it and to the west, south of it, which led to the reoccurrence of the Maud Rise polynya in mid‐September 2017.

Streamflow variability and its relationship with climate indices in western rivers of Argentina

ABSTRACT The objective of this study was to examine the streamflow variability of Argentinean Andean basins (22°–52°S). Trends and step changes of seven hydrological variables were analysed. In addition, relationships between the hydrological variables and Pacific Decadal Oscillation (PDO), Niño 3.4, and Southern Annular Mode (SAM) indices were analysed. Most streamflow variables showed upward trends in the northwest and central-western basins, while downward trends were identified in the Patagonia (southwestern) region. Streamflow of the central-western and Patagonian basins was positively correlated with the Niño 3.4 index. Moreover, an inverse relationship with the SAM was found in watersheds south of 37°S. Positive step changes associated with the PDO phases in the north and central-western basins in the mid-1970s were detected, while negative step changes resulted in Patagonia between 1970 and 2000. This research provides new evidence of the influence of major climate modes on streamflow variability in the western rivers of Argentina.

Seasonal Antarctic pressure variability during the twentieth century from spatially complete reconstructions and CAM5 simulations

As most permanent observations in Antarctica started in the 1950s, understanding Antarctic climate variations throughout the twentieth century remains a challenge. To address this issue, the non-summer multi-decadal variability in pressure reconstructions poleward of 60°S is evaluated and assessed in conjunction with climate model simulations throughout the twentieth and early twenty-first centuries to understand historical atmospheric circulation variability over Antarctica. Austral autumn and winter seasons show broadly similar patterns, with negative anomalies in the early twentieth century (1905–1934), positive pressure anomalies in the middle twentieth century (1950–1980), and negative pressure anomalies in the most recent period (1984–2013), consistent with concurrent trends in the SAM index. In autumn, the anomalies are significant in the context of estimates of interannual variability and reconstruction uncertainty across most of the Antarctic continent, and the reconstructed patterns agree best with model-generated patterns when the simulation includes the forced response to tropical sea surface temperatures and external radiative forcing. In winter and spring, the reconstructed anomalies are less significant and are consistent with internal atmospheric variability alone. The specific role of tropical SST variability on pressure trends in these seasons is difficult to assess due to low reconstruction skill in the region of strongest tropical teleconnections, the large internal atmospheric variability, and uncertainty in the SST patterns themselves. Indirect estimates of pressure variability, whether through sea ice reconstructions, proxy records, or improved models and data assimilation schemes, will help to further constrain the magnitude of internal variability relative to the forced responses expected from SST trends and external radiative forcing.

The state of the atmosphere in the 2016 southern Kerguelen Axis campaign region

The near-surface environment of the Southern Ocean is subject to particular biases in weather and climate simulations, particularly during the summer season, and relatively few analyses of cloud and radiation properties have been reported for the region. Here we provide an analysis of ship-based measurements of downwelling radiation, cloud fraction and cloud base height from the RSV Aurora Australis during the Kerguelen Axis marine science campaign which was conducted in the Southern Ocean south-east of the Kerguelen Plateau between January and March 2016. Our study period focussed on a 22-day interval during the first two months of the campaign. We compared estimates of cloud fraction obtained with a cloud imager and ceilometer, and found good agreement between the two measurement types, particularly when the camera images were analysed in a narrow overhead field to account for differences in the measurement techniques. We used the Interim European Centre for Medium-Range Weather Forecasts Reanalysis (ERA-Interim) and the Antarctic Mesoscale Prediction System Polar Weather and Research Forecasting model (Polar WRF) to provide comparison data for our measurements. We found that both comparison data sets generally underestimated cloud cover (observed cloud fraction ~0.96 compared with 0.87 for ERA-Interim and 0.63 for Polar WRF). As a consequence, the comparison data showed biases in both the surface shortwave irradiance (+59 W m−2 for ERA-Interim and +154 W m−2 for Polar WRF) and the longwave irradiance (−23 W m−2 for ERA-Interim and −46 W m−2 for Polar WRF). The observed mean net surface cloud radiative effect (CRE) of −228 W m−2 was significantly more negative than found in previous observations in the Southern Ocean region, and compares with a net surface CRE of − 138 W m−2 for ERA-Interim which also showed relatively strong cloud forcing. The observed net surface CRE bias for ERA-Interim of + 90 W m−2 appears primarily the result of the reanalysis underestimating the cloud fraction, which at least partly relates to a lack of low clouds. Polar WRF was also found to have a deficit of low clouds. We characterised the relationship between the ratio of irradiances by Photosynthetically Active Radiation (PAR) and shortwave radiation and cloud transmittance. As a consequence of cloud, light levels were estimated as being below the level for light-limited photosynthesis during 31% of the available time the sun was above the horizon (69% of each day on average), compared with the expected clear-sky value of 10%. Over the campaign period, the Indian Ocean sector of the Southern Ocean was influenced by the positive phase of the Southern Annular Mode (SAM). Notably, the surface SAM index in January and March was the most positive observed since 1957. This situation generally led to near-surface climatological differences over the southern part of the campaign region over much of the period, which included significant negative anomalies in mean sea level pressure and air temperature, and positive anomalies in zonal wind. Overall, the cloudiness of our study region appeared to be above average for the time of year, but we could not identify a clear cause for this in the prevailing climatic conditions. While the level of shortwave radiation was likely below average for the time of year, this deficit is not likely to have significantly impacted on photosynthesis in the mixed layer of the ocean.

The Burning Boy and the Goose Girl: On the Economies of Poetry and Nature

The Burning Boy and the Goose GirlOn the Economies of Poetry and Nature Maurice Manning (bio) Among the piles on my wholly disordered bookshelves I have a cheap, yellowed Macmillan paperback edition of the Selected Poems of Thomas Hardy. This volume was selected by John Crowe Ransom and comes with his high-minded introduction. It is courtesy of Ransom, I suspect, that we think of Thomas Hardy as a modern poet. This edition of Hardy, published in 1961, has become one of the subtle jewels in my home library. The poems are essential; Ransom's introduction is instructive and can apply to many other poets as well, even to the vast province of poetry itself. But more recently I've found another shimmering facet in this volume, namely, a browned index card left in the book by a previous owner. On the card, in careful blue ink, is a list: ticketsclothes bag [End Page 89] tux, suspenders, garterslaundryMary's addressshoe laces — blackpoetry — Thomas, Yeats Without intending to, the previous owner of this book has left his own poem behind. The card contains a list, a basic version of poetic form, yet the order of the list is revealing, something we appreciate in actual poems. Does this poem move from the most important element to the least significant? Or is bringing along the poetry of Thomas and Yeats the full blossom of this poem's passion? And it's tantalizing that wedged into the middle of the list is "Mary's address." Is Mary only casually a member of this list, or is she at its very center? We can't answer such questions, and that ambiguity — never to be resolved — gives this "poem" its most poignant dimension. We have a lyric version of a larger narrative. Let's say the poet in this case is a young man, perhaps a college student named Sam. Earnest, studious Sam has a hot date with the sensitive and deep Mary. They're probably going to a formal dance. Beforehand, however, they'll have supper at a fancy restaurant, a place far beyond Sam's usual budget. After the dance, or perhaps while they are cheek-to-cheek, Sam will mention these fellows he's been reading lately, Thomas — certainly in 1961 this is Dylan Thomas — and the high-water mark for modern poetry, W. B. Yeats. "There's such power in their words," he tells her, "and the metrical complexity is like listening to Stravinsky or way out-there jazz. Yeats has these things he calls gyres, circular, recurrent cycles of history he ties, symbolically, to phases of the moon. It's all a real mind-bender!" That's about as far as we can go with the story of Sam and Mary. I expect he [End Page 90] read "Fern Hill" out loud to her, or "Sailing to Byzantium." And I hope for Sam's sake that reading poems to Mary achieved the envisaged effect. Sam's list, interestingly, has not a shred of irony in it; the list believes that the world is ordered and that it is a just order. But it is ironic to the point of absolute wonder that Sam's unironic list appears in a volume of poems by a poet whose chief fuel is irony, one edited by another poet who is also a paid subscriber to irony. And I don't mean cheap, affected irony; I mean incisive, drama-forming irony, which flows through poems like a bloodstream. Sam's innocent little poem is floating unknowingly in the Black Sea of irony. How did Sam's index card that mentions the poetry of Thomas and Yeats become lodged in a volume of poems by Thomas Hardy? And why, fifty years later, did I, a poet with an interest in each of these poets, stumble upon Sam's index card and marvel at it, as if it's proof that poetry is a naturally occurring force in the universe? Whereas Nature provides the seed that begets the tree that begets an infinite number of seeds over its lifetime, that beget forests of trees, that beget infinities of seeds, the metaphor...

The Patagonian shelf circulation: Drivers and variability

A high-resolution ocean model is used to characterize the local and remote driving mechanisms of the variability of the Patagonian shelf circulation. Local forcing includes the effects of tides, buoyancy fluxes and wind, while remote forcing represents the impact of the adjacent deep-ocean currents. There is an abrupt change of the dynamical characteristics of the shelf circulation at 40°S. South of 40°S, the seasonal variations of the shelf circulation are out of phase with the local wind stress and are driven by deep ocean inflows originated in the Drake Passage. The inter-annual variability of the shelf circulation is principally driven by the wind and shows a significant correlation with the time variations of the Southern Annular Mode index. The variability of the circulation and upwelling at the shelfbreak region are modulated by the variability of the Malvinas Current transport at low frequency (periods higher than two years), and by the local wind stress at higher frequencies. North of 40°S, the local wind forcing drives the seasonal variations of the shelf transport. The inter-annual variability of the flow is driven by the combined action of the Rio de la Plata discharge (significantly correlated with the El Niño Southern Oscillation), local wind stress and the Brazil-Malvinas Confluence in the outer shelf. In agreement with previous studies, we show that while the position of the confluence marks the location of the largest offshelf transports, it does not determine their magnitude. The offshelf transport variability is controlled by the local wind at high frequency (periods less than a year) and by the equatorward inflow of southern waters at longer periods. Our simulation indicates that the variability of the Subtropical Shelf Front is modulated by the local wind stress forcing, position of the Brazil/Malvinas Confluence and the equatorward inflow of Subantarctic waters.

Assessment of the Carbonate Chemistry Seasonal Cycles in the Southern Ocean From Persistent Observational Platforms

AbstractObservations from Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) biogeochemical profiling Argo floats are used to characterize the climatological seasonal cycles and drivers of dissolved inorganic carbon, total alkalinity, pH, the partial pressure of carbon dioxide (CO2), and the saturation state of aragonite at the surface and at 200 m across five Southern Ocean frontal regimes, including under sea ice. The Southern Ocean ranges from a temperature‐dominated system in the northernmost Subtropical Zone to a biologically dominated system in the most poleward Seasonal Sea Ice Zone. In all zones, the ingassing or outgassing of CO2 must be balanced by geostrophic and Ekman transport, mixing from below, and particle transport of carbon into or out of the euphotic zone. The climatological seasonal cycles spanning the period from 2014 to 2017 compare favorably with existing climatologies in spring and summer and less so during winter months, at higher latitudes, and in ice‐covered regions due, in part, to limited wintertime observations before SOCCOM. We observe increases in the carbon and nutrient content of surface waters south of the Subantarctic Front between climatological data products and the SOCCOM float climatologies, even after adjusting for anthropogenic change, suggesting a large‐scale increase in the amount of upwelled carbon‐ and nutrient‐rich deep waters. This increased upwelling corresponds to a positive Southern Annular Mode Index over 2014–2017 and likely acts to decrease the magnitude of the Southern Ocean sink of total carbon by increasing outgassing of natural CO2, especially during winter months.

High-latitude Southern Hemisphere fire history during the mid- to late Holocene (6000–750 BP)

Abstract. We determined the specific biomass burning biomarker levoglucosan in an ice core from the TALos Dome Ice CorE drilling project (TALDICE) during the mid- to late Holocene (6000–750 BP). The levoglucosan record is characterized by a long-term increase with higher rates starting at ∼ 4000 BP and peaks between 2500 and 1500 BP. The anomalous increase in levoglucosan centered at ∼ 2000 BP is consistent with other Antarctic biomass burning records. Multiple atmospheric phenomena affect the coastal Antarctic Talos Dome drilling site, where the Southern Annular Mode (SAM) is the most prominent as the Southern Annular Mode Index (SAMA) correlates with stable isotopes in precipitation throughout the most recent 1000 years of the ice core. If this connection remains throughout the mid- to late Holocene, then our results demonstrate that changes in biomass burning, rather than changes in atmospheric transport, are the major influence on the TALDICE levoglucosan record. Comparisons with charcoal syntheses help evaluate fire sources, showing a greater contribution from southern South American fires than from Australian biomass burning. The levoglucosan peak centered at ∼ 2000 BP occurs during a cool period throughout the Southern Hemisphere, yet during a time of increased fire activity in both northern and southern Patagonia. This peak in biomass burning is influenced by increased vegetation in southern South America from a preceding humid period, in which the vegetation desiccated during the following cool, dry period. The Talos Dome ice core record from 6000 to ∼ 750 BP currently does not provide clear evidence that the fire record may be strongly affected by anthropogenic activities during the mid- to late Holocene, although we cannot exclude at least a partial influence.

Atmospheric influences on the anomalous 2016 Antarctic sea ice decay

Abstract. In contrast to the Arctic, where total sea ice extent (SIE) has been decreasing for the last three decades, Antarctic SIE has shown a small, but significant, increase during the same time period. However, in 2016, an unusually early onset of the melt season was observed; the maximum Antarctic SIE was already reached as early as August rather than the end of September, and was followed by a rapid decrease. The decay was particularly strong in November, when Antarctic SIE exhibited a negative anomaly (compared to the 1979–2015 average) of approximately 2 million km2. ECMWF Interim reanalysis data showed that the early onset of the melt and the rapid decrease in sea ice area (SIA) and SIE were associated with atmospheric flow patterns related to a positive zonal wave number three (ZW3) index, i.e., synoptic situations leading to strong meridional flow and anomalously strong southward heat advection in the regions of strongest sea ice decline. A persistently positive ZW3 index from May to August suggests that SIE decrease was preconditioned by SIA decrease. In particular, in the first third of November northerly flow conditions in the Weddell Sea and the Western Pacific triggered accelerated sea ice decay, which was continued in the following weeks due to positive feedback effects, leading to the unusually low November SIE. In 2016, the monthly mean Southern Annular Mode (SAM) index reached its second lowest November value since the beginning of the satellite observations. A better spatial and temporal coverage of reliable ice thickness data is needed to assess the change in ice mass rather than ice area.

Tritium Records to Trace Stratospheric Moisture Inputs in Antarctica

AbstractBetter assessing the dynamic of stratosphere‐troposphere exchange is a key point to improve our understanding of the climate dynamic in the East Antarctica Plateau, a region where stratospheric inputs are expected to be important. Although tritium (3H or T), a nuclide naturally produced mainly in the stratosphere and rapidly entering the water cycle as HTO, seems a first‐rate tracer to study these processes, tritium data are very sparse in this region. We present the first high‐resolution measurements of tritium concentration over the last 50 years in three snow pits drilled at the Vostok station. Natural variability of the tritium records reveals two prominent frequencies, one at about 10 years (to be related to the solar Schwabe cycles) and the other one at a shorter periodicity: despite dating uncertainty at this short scale, a good correlation is observed between 3H and Na+ and an anticorrelation between 3H and δ18O measured on an individual pit. The outputs from the LMDZ Atmospheric General Circulation Model including stable water isotopes and tritium show the same 3H‐δ18O anticorrelation and allow further investigation on the associated mechanism. At the interannual scale, the modeled 3H variability matches well with the Southern Annular Mode index. At the seasonal scale, we show that modeled stratospheric tritium inputs in the troposphere are favored in winter cold and dry conditions.

Historical SAM index time series: linear and nonlinear analysis

ABSTRACTA statistical analysis of 100‐year historic Southern Annular Mode (SAM) time series is carried out, for a set of indices calculated by different methods, in view to understanding their value as simple indicators of climate variability and of the physical processes involved, particularly for the early part of the 20th century. Historic SAM time series available in the literature are analysed together with ones calculated using the 20th‐century reanalysis. A preliminary analysis and comparison is carried out using standard time and space correlations. Distinct differences are observed in the linear relations between the SAM index time series before and after 1950s. A detailed study is carried out using wavelet transform (WT) analysis, in order to better determine the spectral nature and non‐stationarity of the timeseries. The WT spectra reveal an aperiodic, non‐stationary evolution in all cases, with differences in the spectral signature of the various SAM indices during the 20th century. Preferred oscillation periods mainly appear between 2 and 20 years. Important differences between the SAM indices arise from different behaviour on the interannual time scale even in more recent years, as well as non‐stationarity and phase differences on longer time scales. A wavelet coherency (WTC) analysis between the 20th‐century‐reanalysis‐derived SAM time series shows that differences may also arise for indices calculated from the same data source, particularly during the earlier part of the sample. WTC differences appear at times of perturbed El Niño‐Southern Oscillation (ENSO) events. Results using linear as well as a mutual information analysis suggest links between SAM, ENSO, and Pacific‐South American patterns of variability, which may depend on the SAM time series definition. Such behaviour could be due to a combination of factors including the geographic coverage of the SAM calculation methods and data quality, as well as ENSO‐SAM relationships.

On the Origin of the Solar Cycle Modulation of the Southern Annular Mode

AbstractThe Southern Annular Mode (SAM) is the dominant hemisphere‐scale mode of variability in the Southern Hemisphere. Analysis of the European Centre for Medium‐range Weather Forecasts Reanalysis data set (ERA‐Interim) for 1979–2015 shows that stratospheric variability associated with the October–November mean SAM index is modulated in synchrony with solar activity. The sequence of stratospheric variability after July, similar to that associated with the Polar‐night Jet Oscillation in the Southern Hemisphere, is well connected to the SAM during years of stronger solar activity, but the connection is weak during years of weaker solar activity. The source of this solar cycle modulation of the SAM was examined by performing momentum and energy budget (wave energy) analyses. The momentum analysis shows that the modulation of the zonal wind comes directly from the acceleration of zonal wind produced by waves. The wave‐energy analysis suggests that the modulation is produced through energy conversion from zonal‐mean fields, mainly by the baroclinic mechanism, although diabatic processes are also important. These analyses also suggest that a key factor regarding the effect of solar activity on the structural modulation of the SAM is the intensification of the sensitivity of the wave‐mean flow interaction in the stratosphere.

Mean, Variability, and Trend of Southern Ocean Wind Stress: Role of Wind Fluctuations

The Southern Ocean (SO) surface westerly wind stress plays a fundamental role in driving the Antarctic Circumpolar Current and the global meridional overturning circulation. Here, the authors investigate the contributions of atmospheric wind fluctuations to the mean, variability, and trend of SO wind stress over the last four decades using NCEP reanalysis and ERA-Interim products. Including wind variability at synoptic frequencies (2–8 days) and higher in the stress calculation is found to increase the strength of the mean SO wind stress by almost 40% in both reanalysis products. The southern annular mode index is found to be a good indicator for the strength of the mean wind and mean wind stress, but not as good an indicator for wind fluctuations, at least for the chosen study period. Large discrepancies between reanalysis products emerge regarding the contributions of wind fluctuations to the strengthening trend of SO wind stress. Between one-third and one-half of the stress trend in NCEP can be explained by the increase in the intensity of wind fluctuations, while the stress trend in ERA-Interim is due entirely to the increasing strength of the mean westerly wind. This trend discrepancy may have important climatic implications since the sensitivity of SO circulation to wind stress changes depends strongly on how these stress changes are brought about. Given the important role of wind fluctuations in shaping the SO wind stress, studies of the SO response to wind stress changes need to account for changes of wind fluctuations in the past and future.

Teleconnection stationarity, variability and trends of the Southern Annular Mode (SAM) during the last millennium

The Southern Annular Mode (SAM) is the leading mode of atmospheric interannual variability in the Southern Hemisphere (SH) extra-tropics. Here, we assess the stationarity of SAM spatial correlations with instrumental and paleoclimate proxy data for the past millennium. The instrumental period shows that temporal non-stationarities in SAM teleconnections are not consistent across the SH land areas. This suggests that the influence of the SAM index is modulated by regional effects. However, within key-regions with good proxy data coverage (South America, Tasmania, New Zealand), teleconnections are mostly stationary over the instrumental period. Using different stationarity criteria for proxy record selection, we provide new austral summer and annual mean SAM index reconstructions over the last millennium. Our summer SAM reconstructions are very robust to changes in proxy record selection and the selection of the calibration period, particularly on the multi-decadal timescale. In contrast, the weaker performance and lower agreement in the annual mean SAM reconstructions point towards changing teleconnection patterns that may be particularly important outside the summer months. Our results clearly portend that the temporal stationarity of the proxy-climate relationships should be taken into account in the design of comprehensive regional and hemispherical climate reconstructions. The summer SAM reconstructions show no significant relationship to solar, greenhouse gas and volcanic forcing, with the exception of an extremely strong negative anomaly following the AD 1257 Samalas eruption. Furthermore, reconstructed pre-industrial summer SAM trends are very similar to trends obtained by model control simulations. We find that recent trends in the summer SAM lie outside the 5–95% range of pre-industrial natural variability.

The Influence of Recurrent Modes of Climate Variability on the Occurrence of Monthly Temperature Extremes Over South America

AbstractThe associations between extreme temperature months and four prominent modes of recurrent climate variability are examined over South America. Associations are computed as the percent of extreme temperature months concurrent with the upper and lower quartiles of the El Niño–Southern Oscillation (ENSO), the Atlantic Niño, the Pacific Decadal Oscillation (PDO), and the Southern Annular Mode (SAM) index distributions, stratified by season. The relationship is strongest for ENSO, with nearly every extreme temperature month concurrent with the upper or lower quartiles of its distribution in portions of northwestern South America during some seasons. The likelihood of extreme warm temperatures is enhanced over parts of northern South America when the Atlantic Niño index is in the upper quartile, while cold extremes are often association with the lowest quartile. Concurrent precipitation anomalies may contribute to these relations. The PDO shows weak associations during December, January, and February, while in June, July, and August its relationship with extreme warm temperatures closely matches that of ENSO. This may be due to the positive relationship between the PDO and ENSO, rather than the PDO acting as an independent physical mechanism. Over Patagonia, the SAM is highly influential during spring and fall, with warm and cold extremes being associated with positive and negative phases of the SAM, respectively. Composites of sea level pressure anomalies for extreme temperature months over Patagonia suggest an important role of local synoptic scale weather variability in addition to a favorable SAM for the occurrence of these extremes.

The evolution and volcanic forcing of the southern annular mode during the past 300 years

ABSTRACTA positive change in the southern annular mode (SAM), which is the primary pattern of climate variability in the Southern Hemisphere, has been induced predominantly by polar stratospheric ozone depletion. However, the lack of long‐term observational records limits our understanding of the long‐term SAM behaviour. In this study, we found that the geochemical record of the LGB69 ice core from the eastern coast of Antarctica was significantly correlated with the winter SAM index (SAMI). In addition, we developed an annual mean SAMI beginning in 1701 based on 15 annually resolved ice cores and relevant proxy–climate relationships. Our reconstruction accounted for 54.8% of the total variance from 1957 to 2000 (the calibration period). We demonstrate that the recent positive phase shift in the annual mean SAMI since the 1970s is unprecedented, with the estimate for the latest regime in the 1990s reaching values 2.5 times the standard deviation above the baseline (1701–2000). This peak value also coincides with the largest 30‐ and 50‐year trends, which have occurred at the end of the 20th century. From the reconstructed SAMI, we also found that the response to large volcanic events was likely positive in the 3 years after the eruption, but this positive response can be masked by internal climate variability when a strong El Niño event occurs in the eruption year.

Variation of Antarctic circumpolar current and its intensification in relation to the southern annular mode detected in the time-variable gravity signals by GRACE satellite

The southern annular mode (SAM) in the atmosphere and the Antarctic circumpolar current (ACC) in the ocean play decisive roles in the climatic system of the mid- to high-latitude southern hemisphere. Using the time-variable gravity data from the GRACE satellite mission, we find the link between the space–time variabilities of the ACC and the SAM. We calculate the empirical orthogonal functions (EOF) of the non-seasonal ocean bottom pressure (OBP) field in the circum-Antarctic seas from the GRACE data for the period from 2003 to 2015. We find that the leading EOF mode of the non-seasonal OBP represents a unison OBP oscillation around Antarctica with time history closely in pace with that of the SAM Index with a high correlation of 0.77. This OBP variation gives rise to a variation in the geostrophic flow field; the result for the same EOF mode shows heightened variations in the zonal velocity that resides primarily in the eastern hemispheric portion of the ACC and coincided geographically with the southernmost boundary of the ACC’s main stream. Confirming previous oceanographic studies, these geodetic satellite results provide independent information toward better understanding of the ACC–SAM process.Graphical .