Southern Annular Mode Index Research Articles

Unusual Southern Hemisphere tree growth patterns induced by changes in the Southern Annular Mode

Recent changes in the summer climate of the Southern Hemisphere extra-tropics are primarily related to the dominance of the positive phase of the Southern Annular Mode1, 2. This shift in the behaviour of the Southern Annular Mode—essentially a measure of the pressure gradient between Southern Hemisphere mid and high latitudes—has been predominantly induced by polar stratospheric ozone depletion2, 3, 4. The concomitant southward expansion of the dry subtropical belts5, 6 could have consequences for forest growth. Here, we use tree-ring records from over 3,000 trees in South America, Tasmania and New Zealand to identify dominant patterns of tree growth in recent centuries. We show that the foremost patterns of growth between 1950 and 2000 differed significantly from those in the previous 250 years. Specifically, growth was higher than the long-term average in the subalpine forests of Tasmania and New Zealand, but lower in the dry-mesic forests of Patagonia. We further demonstrate that variations in the Southern Annular Mode can explain 12–48% of the tree growth anomalies in the latter half of the twentieth century. Tree-ring-based reconstructions of summer Southern Annular Mode indices suggest that the high frequency of the positive phase since the 1950s is unprecedented in the past 600 years. We propose that changes in the Southern Annular Mode have significantly altered tree growth patterns in the Southern Hemisphere.

Signature of a southern hemisphere extratropical influence on the summer monsoon over India

The weakening relationship of El Nino with Indian summer monsoon reported in recent years is a major issue to be addressed. The altered relationships of Indian monsoon with various parameters excite to search for other dominant modes of variability that can influence the precipitation pattern. Since the Indian summer monsoon circulation originates in the oceanic region of the southern hemisphere, the present study investigates the association of southern extratropical influence on Indian summer monsoon using rainfall and reanalysis parameters. The effect of Southern Annular Mode (SAM) index during the month of June associated with the onset phase of Indian summer monsoon and that during July–August linked with the active phase of the monsoon were analysed separately for a period from 1951 to 2008. The extra-tropical influence over the monsoon is illustrated by using rainfall, specific humidity, vertical velocity, circulation and moisture transport. The June high SAM index enhances the lower level wind flow during the onset phase of monsoon over Indian sub-continent. The area of significant positive correlation between precipitation and SAM in June also shows enhancement in both ascending motion and specific humidity during the strong phase of June SAM. On the other hand, the June high SAM index adversely affects July–August monsoon over Indian subcontinent. The lower level wind flow weakens due to the high SAM. Enhancement of divergence and reduction in moisture transport results in the Indian monsoon region due to the activity of this high southern annular mode. The effect is more pronounced over the southwest region where the precipitation spell has high activity during the period. Significant correlation exists between SAM and ISMR, even after removing the effect of El Nino. It indicates that the signals of Indian summer monsoon characteristics can be envisaged to a certain extend using the June SAM index.

Antarctic Sea Ice Climatology, Variability, and Late Twentieth-Century Change in CCSM4

Abstract A preindustrial control run and an ensemble of twentieth-century integrations of the Community Climate System Model, version 4 (CCSM4), are evaluated for Antarctic sea ice climatology, modes of variability, trends, and covariance with related physical variables such as surface temperature and sea level pressure. Compared to observations, the mean ice cover is too extensive in all months. This is in part related to excessively strong westerly winds over ~50°–60°S, which drive a large equatorward meridional ice transport and enhanced ice growth near the continent and also connected with a cold bias in the Southern Ocean. In spite of these biases in the climatology, the model’s sea ice variability compares well to observations. The leading mode of austral winter sea ice concentration exhibits a dipole structure with anomalies of opposite sign in the Atlantic and Pacific sectors. Both the El Niño–Southern Oscillation and the southern annular mode (SAM) project onto this mode. In twentieth-century integrations, Antarctic sea ice area exhibits significant decreasing annual trends in all six ensemble members from 1950 to 2005, in apparent contrast to observations that suggest a modest ice area increase since 1979. Two ensemble members show insignificant changes when restricted to 1979–2005. The ensemble mean shows a significant increase in the austral summer SAM index over 1960–2005 and 1979–2005 that compares well with the observed SAM trend. However, Antarctic warming and sea ice loss in the model are closely connected to each other and not to the trend in the SAM.

Understanding sudden changes in cloud amount: The Southern Annular Mode and South American weather fluctuations

This work investigates the cause and effects of extreme changes in synoptic‐scale cloud cover operating at daily timescales using a variety of satellite‐based and reanalysis data sets. It is found that the largest sudden increases detected in globally averaged cloud cover over the last ten years of satellite‐based observations occur following positively correlated shifts in the phase of the Southern Annular Mode (SAM) index. The associated pressure anomalies are found to generate frontal cloud formation over large areas of the South American continent, increasing regional cloud cover by up to 20%; these changes are correlated to statistically significant reductions in local temperatures of approximately −2.5°C with a +1 day time lag, indicating the SAM index is associated with large scale weather fluctuations over South America.

An analysis of trends in the boreal winter mean tropospheric circulation during the second half of the 20th century

A relaxation technique applied to the ECMWF model is used to analyse 11, 21 and 31 year trends in the boreal winter mean 500 hPa North Atlantic Oscillation (NAO), Pacific North America pattern (PNA) and Southern Annular Mode (SAM) indices. For the PNA, the results indicate a strong influence from the tropics on all time scales, whereas for the NAO, the stratosphere is important on time scales of 11 and 21 but with an indication of feedback from extratropical sea surface temperature and sea‐ice (SSTSI) anomalies on the 11 year time scale. For the SAM, the tropics emerge as the most important influence. We find an influence from the stratosphere consistent with expectations based on ozone depletion, although no clear role for stratospheric forcing of the SAM is found in these experiments.

Seesaw does not explain Antarctic climate change

The Arctic has warmed rapidly in the past 3 decades, but temperature records show that the Antarctic on average has warmed less. This observation hints at a bipolar seesaw mechanism, in which changes in ocean circulation result in changes in heat transport between the hemispheres, linking the warming Arctic to a lack of warming in the Antarctic. However, a new analysis by Schneider and Noone shows that this mechanism cannot explain Antarctic temperature variability or trends. The authors reviewed the data set used as the basis of the bipolar seesaw hypotheses and note that for the Antarctic region the data come from only one station prior to the mid‐1940s and include only a few stations until the late 1950s. They estimated Antarctic climate variability using a variety of independent data, including the long station time series, a reconstruction of the Southern Annular Mode index (a measure of Southern Hemisphere atmospheric circulation variability), and an ice core–based temperature reconstruction.

The Southern Annular Mode Seen through Weather Regimes

This article investigates the prominent features of the Southern Hemisphere (south of 20°S) atmospheric circulation when extracted using EOF analysis and a k-means clustering algorithm. The focus is on the southern annular mode (SAM), the nature of its recent trend, and the zonal symmetry of associated spatial patterns. The study uses the NCEP–Department of Energy Atmospheric Model Intercomparison Project II Reanalysis (NCEP-2) (period 1979–2009) to obtain robust patterns over the recent years and the Twentieth Century Reanalysis Project (period 1871–2008) to document decadal changes. Also presented is a comparison of these signals against a station-based reconstruction of the SAM index and a gridded interpolated dataset [Hadley Centre Sea Level Pressure dataset version 2 (HadSLP2)]. Over their common period, both reanalyses are in fair agreement, both in terms of spatial patterns and temporal variability. In particular, both datasets show weather regimes that can be interpreted as the opposite phases of the SAM. At the decadal time scale, the study shows that the trend toward the positive SAM phase (as inferred from the usual EOF-based index) is related more to an increase in the frequency of clusters corresponding to the positive phase, with little changes in the frequency of the negative SAM events. Similarly, the long-term tropospheric warming trend already discussed in the literature is shown to be related more to a decrease in the number of abnormally cold days, with little changes in the number of abnormally warm days. The cluster analysis therefore allows for complement descriptions based on simple indexes or EOF decompositions, highlighting the nonlinear nature of the decadal changes in the Southern Hemisphere atmospheric circulation and temperature.

Influence of the Tropics on the Southern Annular Mode

Abstract Perturbations in the southern annular mode (SAM) are shown to be significantly correlated with SST anomalies in the central tropical Pacific during austral winter and SST anomalies in the eastern tropical Pacific during austral summer. The SAM signature in the Pacific sector resembles a tropically forced Rossby wave train, the so-called Pacific–South American pattern, while the signature in the Indian Ocean sector is a zonally elongated meridional dipole. Thus, the SAM contains strong zonally asymmetric variability and tends to behave differently in the Eastern and Western Hemispheres, with internal dynamics prevailing in the Indian Ocean sector and the forced response to tropical SST anomalies exerting a strong influence in the Pacific sector. The tropically forced component of the SAM in the Pacific sector is related to a geographically fixed active Rossby wave source to the east of Australia within the core of the subtropical jet. In addition to the well-documented positive trend in summer, the SAM also exhibits a negative wintertime trend since 1979, characterized by prominent geopotential height increases over the high latitudes. In both seasons, SAM trends are closely linked to long-term trends in tropical Pacific SST that are independent of the canonical eastern Pacific ENSO variability. Although the SAM is an intrinsic pattern of high-latitude variability, the SAM index reflects the superposition of both high-latitude and tropically forced variability.

The Southern Annular Mode: a comparison of indices

Abstract. The Southern Annular Mode (SAM) has been identified as a climate mechanism with potentially significant impacts on the Australian hydroclimate. However, despite the identification of relationships between SAM and Australia's hydroclimate using certain data sets, and focussed on certain time periods, the association has not been extensively explored and significant uncertainties remain. One reason for this is the existence of numerous indices, methods and data sets by which SAM has been approximated. In this paper, the various SAM definitions and indices are reviewed and the similarities and discrepancies are discussed, along with the strengths and weaknesses of each index development approach. Further, the sensitivity of the relationship between SAM and Australian rainfall to choice of SAM index is quantified and recommendations are given as to the most appropriate index to use when assessing the impacts of the SAM on Australia's hydroclimate. Importantly this study highlights the need to consider the impact that the choice of SAM index, and data set used to calculate the index, has on the outcomes of any SAM attribution study.

Is a bipolar seesaw consistent with observed Antarctic climate variability and trends?

A bipolar seesaw of Arctic and Antarctic temperature anomalies has been reported to be evident in instrumental data on decadal timescales during the last century. This finding hinges upon a global temperature data set that for the area poleward of ∼60°S is derived from only one sub‐Antarctic station prior to the mid‐1940s, and does not include a substantial number of Antarctic stations until the late 1950s. The timeseries of the single‐station record for the early period spliced to the data based on broader coverage for the latter period is an artificial estimate of the Antarctic climate trend and its variability. We estimate the real variability using the original timeseries from the sub‐Antarctic station, a reconstruction of the Southern Annular Mode index, and an ice‐core based reconstruction of Antarctic temperature. None of these Antarctic timeseries are significantly correlated with Arctic or North Atlantic climate records, nor with the index of the Atlantic Multidecadal Oscillation, which was proposed as the driving mechanism of the seesaw. Instead, each of these records is consistently correlated with tropical Pacific sea surface temperatures. However, neither the seesaw nor the tropics alone can fully capture the complexity of Antarctic climate variability and climate change.

Climatology and recent increase of westerly winds over the Amundsen Sea derived from six reanalyses

AbstractThe observed acceleration of glaciers from West Antarctica into the Amundsen Sea is estimated to be contributing 6% to current sea‐level rise with the estimated potential to add 0.24 m to global sea level. Stronger westerly winds over the Amundsen Sea can increase the flow of relatively warm ocean water to the base of ice shelves that flow from glaciers into the Amundsen Sea. Thinning of the glaciers caused by this warming is a potentially important factor in driving the observed acceleration of glaciers. However, the climatology of winds in the region has not been extensively studied due to a lack of in situ observational long‐term records. Here six different reanalysis datasets are assessed (CFSR, ERA‐40, ERA‐Interim, JRA‐25, MERRA and NNR1) to determine a best estimate of variability and change since 1979 when the widespread monitoring of the atmosphere from satellites was introduced. A comparison with independent mean sea‐level pressure data from ice drifting buoys shows that ERA‐Interim is clearly the most accurate at capturing the details of individual weather systems over the neighbouring Bellingshausen Sea, implying that it is also accurate over the Amundsen Sea. In terms of climatological means, the five recently‐produced (after ∼2000) reanalysis datasets show only small differences. Decadal variations of westerly winds congruent with the observed increases in the southern annular mode (SAM) index are a consistent feature across the reanalysis datasets. In particular, the strong seasonal dependence of observed trends in the SAM (i.e. significant positive trends in the summer and autumn in recent decades) is also seen in the strength of westerly winds over the Amundsen Sea. In terms of year‐to‐year variability, the annual mean westerly winds over the Amundsen Sea were found to be significantly correlated with the SAM in summer (r = 0.35; p≤0.05) and El Nino‐Southern Oscillation in spring (September to November) (r = 0.41; p≤0.05). Copyright © 2012 Royal Meteorological Society

Does It Pay or Does Firm Pay? The Relation between CSR Performance and the Cost of Debt

This paper investigates the link between corporate social performance and cost of debt financing. The literature on the determinants of the cost of debt generally documents a negative association between measures of the risk of the firm and its cost of debt. The literature on Corporate Social Responsibility, instead, presents risk reduction as one of the potential benefits related to these investments. Thanks to that effect, therefore, an efficient market should recognize a ‘ethical financial premium’ to socially responsible firms, corresponding to a less cost of debt financing.In order to test this hypothesis, the developed model investigates the relation between firms’ CSR performance (measured through the SAM index*) and their cost of debt. Potentially confounding factors such as industry, size and time lag effects were also analyzed. Employing a unique data set of 332 firms over a time period of five years (1641 observations) we find some evidence that there is no ‘ethical financial premium’ for improved corporate social responsibility in term of cost of debt applied by banks and financial institutions to the company. Instead, the results document a positive relation between the CSR performance proxy and the cost of debt, demonstrating that CSR is not considered a value driver with an impact on the firm’s risk profile, but a sort of waste of resources that can negatively affect the performance of the firm, independently from the country in which the firm operates.

Multidecadal Warming and Shoaling of Antarctic Intermediate Water*

Abstract Antarctic Intermediate Water (AAIW) is a dominant Southern Hemisphere water mass that spreads from its formation regions just north of the Antarctic Circumpolar Current (ACC) to at least 20°S in all oceans. This study uses an isopycnal climatology constructed from Argo conductivity–temperature–depth (CTD) profile data to define the current state of the AAIW salinity minimum (its core) and thence compute anomalies of AAIW core pressure, potential temperature, salinity, and potential density since the mid-1970s from ship-based CTD profiles. The results are used to calculate maps of temporal property trends at the AAIW core, where statistically significant strong circumpolar shoaling (30–50 dbar decade−1), warming (0.05°–0.15°C decade−1), and density reductions [up to −0.03 (kg m−3) decade−1] are found. These trends are strongest just north of the ACC in the southeast Pacific and Atlantic Oceans and decrease equatorward. Salinity trends are generally small, with their sign varying regionally. Bottle data are used to extend the AAIW core potential temperature anomaly analysis back to 1925 in the Atlantic and to ~1960 elsewhere. The modern warm AAIW core conditions appear largely unprecedented in the historical record: biennially and zonally binned median AAIW core potential temperatures within each ocean basin are, with the notable exception of the subtropical South Atlantic in the 1950s–70s, 0.2–1°C colder than modern values. Zonally averaged sea surface temperature anomalies around the AAIW formation latitudes in each ocean and sectoral southern annular mode indices are used to put the AAIW core property trends and variations into context.

Characterization of the inter-annual, seasonal, and diurnal variations of condensation particle concentrations at Neumayer, Antarctica

Abstract. Continuous condensation particle (CP) observations were conducted from 1984 through 2009 at Neumayer Station under stringent contamination control. During this period, the CP concentration (median 258 cm−3) showed no significant long term trend but exhibited a pronounced seasonality characterized by a stepwise increase starting in September and reaching its annual maximum of around 103 cm−3 in March. Minimum values below 102 cm–3 were observed during June/July. Dedicated time series analyses in the time and frequency domain revealed no significant correlations between inter-annual CP concentration variations and atmospheric circulation indices like Southern Annular Mode (SAM) or Southern Ocean Index (SOI). The impact of the Pinatubo volcanic eruption and strong El Niño events did not affect CP concentrations. From thermodenuder experiments we deduced that the portion of volatile (at 125 °C) and semi-volatile (at 250 °C) particles which could be both associated with biogenic sulfur aerosol, was maximum during austral summer, while during winter non-volatile sea salt particles dominated. During September through April we could frequently observe enhanced concentrations of ultrafine particles within the nucleation mode (between 3 nm and 7 nm particle diameter), preferentially in the afternoon.

Differences in coastal and oceanic SST trends due to the strengthening of coastal upwelling along the Benguela current system

Coastal and oceanic sea surface temperature (SST) trends were analyzed in the Benguela upwelling ecosystem from the seventies on. Monthly SST data were obtained from the UK Meteorological Office, Hadley Center at two transects in front of the Namibia coast and the western coast of South Africa. A positive SST trend (0.06 °C dec −1) is observed at open sea locations and a negative one (−0.13 °C dec −1) near shore. The observed negative ΔSST (SST coast−SST ocean) trend (−0.18 °C dec −1) is linked to the strengthening of upwelling (87 m 3 s −1 km −1 dec −1) during the same period. For this purpose, Ekman transport was directly obtained from the Pacific Fisheries Environmental Laboratory in the area under study and calculated from reanalysis wind data from NCEP/NCAR at six locations in front of the Namibia coast and the western coast of South Africa. This coastal upwelling enhancement is in good agreement with changes in the intensity and location of the South Atlantic High, which are also reflected in the Southern Annular Mode (SAM) index.

Role of medium-scale waves on the Southern Annular Mode

[1] We examined the role of short-period waves on the Southern Annular Mode (SAM) through analysis of 6-hourly reanalysis data of the European Centre for Medium-Range Weather Forecasts (ECMWF ERA-Interim). We focused on medium-scale waves, those with periods shorter than about 6 days. Although medium-scale waves generally have much smaller amplitudes than synoptic waves, they have a much higher correlation with the SAM index. As a result, about one third of wave-driven meridional circulation and zonal wind acceleration associated with the SAM is generated by medium-scale waves. Their contribution is large in spite of their climatologically smaller amplitude. Moreover, two thirds of SAM-related meridional circulation and zonal wind acceleration comes from waves of periods shorter than about 3 days. Our analysis demonstrates a positive feedback between westerly winds around 60°S and amplification of synoptic and medium-scale waves, which sustains hemispheric SAM variability. For the feedback process between zonal wind and waves, baroclinic conversion is a key process to both synoptic waves and medium-scale waves. Our analysis also shows that wave forcings calculated from daily data explain only half of the total forcings and that the contribution from short-period waves is very significant around 50°S.

Decadal variability in the Southern Hemisphere

[1] This study reveals that a quasi‐decadal variability exists in the climate system of southern high latitudes, particularly in the Southern Annular Mode (SAM) and subpolar to mid latitudes sea surface temperature (SST), based on in situ observations, reanalysis data, and the 20th Century runs of IPCC AR4 coupled climate models. Spectral analysis reveals that a statistically significant variability with periods of 8–16 years appears in the SAM indices based on about 50 years of reanalysis data and observed SST. Observations of air temperature and sea level pressure from weather stations confirm that the decadal variability is more evident in the midlatitudes than over Antarctica. Cross‐spectral analysis indicates that the SAM index is related to the SST in the subpolar seas of Antarctica and SST gradient at mid‐high latitudes at this decadal frequency band. The SAM indices from 20th century runs (longer than 100 years) of eighteen IPCC coupled climate models are also examined for the decadal variability. Sixteen out of the eighteen models exhibit decadal variability in SAM that is significant at least at the 90% of confidence level while eight are significant at the 95% confidence level. Seven models produce significant co‐variability between SAM and subpolar SST at this quasi‐decadal frequency.

Simulation and prediction of the Southern Annular Mode and its influence on Australian intra-seasonal climate in POAMA

We assess the ability of the Predictive Ocean Atmosphere Model for Australia (POAMA) to simulate and predict the Southern Annular Mode (SAM) and its influence on Australian intra-seasonal climate using a 27-year hindcast dataset. The analysis consists of three stages: (1) prediction of the SAM, (2) simulation of SAM climate anomalies over Australia, and (3) prediction of Australian climate anomalies in association with the SAM. POAMA achieves skilful prediction of the SAM index for lead times out to about 2 weeks with little skill seen beyond 3 weeks when calculated over all hindcast start months; the inherent strong persistence of the SAM appears to be a key factor for its extended-range predictability in a dynamical forecast model. POAMA also simulates SAM climate anomalies over Australia reasonably well despite notable biases in its representation of the SAM to the south and east of the continent. The model reproduces Australian rainfall anomalies most effectively throughout June–November, and least effectively throughout March–May. Skilful prediction of the SAM index, together with realistic simulation of SAM climate anomalies over Australia, translates into more skilful forecasts of rainfall and maximum temperature at intra-seasonal timescales during austral winter and spring. When the SAM is strong in the initial conditions, there is higher skill in forecasting rainfall anomalies over eastern Australia and maximum temperature anomalies over most of the continent during June–November at lead times of 2–3 weeks, compared with when the SAM is weak. The SAM thus contributes to intra-seasonal prediction skill in the Australian region in POAMA.

Sensitivity of the southern annular mode to greenhouse gas emission scenarios

The leading mode of southern hemisphere (SH) climatic variability, the southern annular mode (SAM), has recently seen a shift towards its positive phase due to stratospheric ozone depletion and increasing greenhouse gas (GHG) concentrations. Here we examine how sensitive the SAM (defined as the leading empirical orthogonal function of SH sea level pressure anomalies) is to future GHG concentrations. We determine its likely evolution for three intergovernmental panel on climate change (IPCC) special report on emission scenarios (SRES) for austral summer and winter, using a multi-model ensemble of IPCC fourth assessment report models which resolve stratospheric ozone recovery. During the period of summer ozone recovery (2000–2050), the SAM index exhibits weakly negative, statistically insignificant trends due to stratospheric ozone recovery which offsets the positive forcing imposed by increasing GHG concentrations. Thereafter, positive SAM index trends occur with magnitudes that show sensitivity to the SRES scenario utilised, and thus future GHG emissions. Trends are determined to be strongest for SRES A2, followed by A1B and B1, respectively. The winter SAM maintains a similar dependency upon GHG as summer, but over the entire twenty-first century and to a greater extent. We also examine the influence of ozone recovery by comparing results to models that exclude stratospheric ozone recovery. Projections are shown to be statistically different from the aforementioned results, highlighting the importance of ozone recovery in governing SAM-evolution. We therefore demonstrate that the future SAM will depend both upon GHG emissions and stratospheric ozone recovery.

Climate impact on interannual variability of Weddell Sea Bottom Water

[1] Bottom water formed in the Weddell Sea plays a significant role in ventilating the global abyssal ocean, forming a central component of the global overturning circulation. To place Weddell Sea Bottom Water in the context of larger scale climate fluctuations, we analyze the temporal variability of an 8‐year (April 1999 through January 2007) time series of bottom water temperature relative to the El Nino/Southern Oscillation (ENSO), Southern Annular Mode (SAM), and Antarctic Dipole (ADP). In addition to a pronounced seasonal cycle, the temperature record reveals clear interannual variability with anomalously cold pulses in 1999 and 2002 and no cold event in 2000. Correlations of the time series with ENSO, SAM, and ADP indices peak with the indices leading by 14–20 months. Secondary weaker correlations with the SAM index exist at 1–6 month lead time. A multivariate EOF analysis of surface variables shows that the leading mode represents characteristic traits of out‐of‐phase SAM and ENSO impact patterns and is well separated from other modes in terms of variance explained. The leading principal component correlates with the bottom water temperature at similar time scales as did the climate indices, implying impact from large‐scale climate. Two physical mechanisms could link the climate forcing to the bottom water variability. First, anomalous winds may alter production of dense shelf water by modulating open‐water area over the shelf. Second, surface winds may alter the volume of dense water exported from the shelf by governing the Weddell Gyre’s cyclonic vigor.