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The Southern Annular Mode: Variability, trends, and climate impacts across the Southern Hemisphere

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Abstract The Southern Annular Mode (SAM) is the leading mode of extratropical Southern Hemisphere climate variability, associated with changes in the strength and position of the polar jet around Antarctica. This variability in the polar jet drives large fluctuations in the Southern Hemisphere climate, from the lower stratosphere into the troposphere, and stretching from the midlatitudes across the Southern Ocean to Antarctica. Notably, the SAM index has displayed marked positive trends in the austral summer season (stronger and poleward shifted westerlies), associated with stratospheric ozone loss. Historical reconstructions demonstrate that these recent positive SAM index values are unprecedented in the last millennia, and fall outside the range of natural climate variability. Despite these advances in the understanding of the SAM behavior, several areas of active research are identified that highlight gaps in our present knowledge. This article is categorized under: Paleoclimates and Current Trends > Earth System Behavior
Seasonal mean EOFs calculated as in Figure 1 and the corresponding standardized seasonal mean PC and other SAM index time series. The percentage of variance explained for each EOF is given in the upper right of each EOF spatial plot. Each time series was standardized over its full length (1957–2018 for Marshall (2003) index, 1979–2018 for all others, and regression lines over the full period are also shown (thin lines). The correlation between the Marshall (2003) SAM index with the other indices is provided at the top of each panel as well as the trends and 95% confidence intervals during 1979–2018 (units of standardized SAM index per decade) at the bottom of each panel. EOF, empirical orthogonal function; SAM, Southern Annular Mode
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Leading EOF of ERA5 (the fifth global reanalysis produced by the European Centre for Medium‐Range Weather Forecasts, ECMWF) (Hersbach et al., 2019) 500 hPa geopotential height based on (a) monthly and (b) annual mean data during 1979–2018. The EOFs are based on detrended weighted (by cosine of latitude) 500 hPa geopotential height anomaly data from 20° to 90°S and are shown here as regression maps of the leading PC time series on the time‐varying height anomalies (contour interval 10 m/std dev of PC timeseries). The percentage of explained variance and corresponding standardized PC time series/SAM index is also given in each panel. EOF, empirical orthogonal function; SAM, Southern Annular Mode
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Annual mean SAM indices, smoothed with an 11‐year Hamming filter. The Fogt reconstruction (J. M. Jones et al., 2009) is an example of a station‐based SAM reconstruction, while the Abram reconstruction uses multiple paleo‐proxies (Abram et al., 2014). Note that the annual mean Fogt reconstruction was calculated by averaging the four seasonal SAM reconstructions. The shading about each reconstruction represents the 95% confidence interval based on comparison with the Marshall (2003) index, and all data have been standardized over 1957–2005 prior to smoothing. SAM, Southern Annular Mode
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(a) Annual mean Marshall (2003) SAM index (red line, right axis) and 30‐year running trend and 95% confidence interval (black line with shading, left axis). The running trend is plotted on the x‐axis by the middle year of the trend. (b) Seasonal time‐varying trends, with the start of the time interval for the trend indicated by the y‐axis and the ending year by the x‐axis. The trends on the diagonal line are exactly 30 years (trends are only shown if longer than 30 years). Cross‐hatching and stippling in (b) indicate trends statistically different from zero at p < .10 and p < .05, respectively. SAM, Southern Annular Mode
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As in Figure 3, but for ERA5 monthly composites for (a) MSLP and (b) 2 m temperature. Composites of ERA5 seasonal mean total precipitation anomalies for (c) JJA and (d) DJF. Contour interval is 1 hPa in (a), 0.3°C in (b), and 0.3 mm in (c)–(d). In all composites, anomalies statistically different from zero at p < .05 are stippled. MSLP, mean sea level pressure
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(a) ERA5 Zonal mean zonal wind seasonal climatology for austral summer (left column) and winter (right column), contour interval is 5 m/s, with approximate locations of the polar front jet (PFJ) and the subtropical jet (STJ) indicated. b) ERA5 Zonal mean monthly anomaly composites for SAM + (left column) and SAM – (right column) for zonal wind (contoured) and temperature (shaded). The stippling indicates zonal wind anomalies statistically different from zero at p < .05 while only temperature anomalies significantly different from zero at p < .05 are shown. Composites are based on months that are outside of 0.75 standard deviations from the 1979 to 2018 mean, and the distribution of the months for each composite is given in the histograms in (c). The pronounced zonal wind anomalies at 60°S show the stronger SAM impacts on the PFJ than on the STJ. SAM, Southern Annular Mode
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