JISAO data

SLP-based Antarctic Oscillation (AAO) time series
1948 - May 2011

Definition and analyses | Data


The Antarctic Oscillation (AAO) is the dominant pattern of non-seasonal tropospheric circulation variations south of 20S, and it is characterized by pressure anomalies of one sign centered in the Antarctic and anomalies of the opposite sign centered about 40-50S. The AAO is also referred to as the Southern Annular Mode (SAM). There is a Northern Hemisphere analog to the AAO, and it is called the
Arctic Oscillation (or Northern Annular Mode).

The AAO is defined as the leading principal component (PC) of 850 hPa geopotential height anomalies south of 20S (Thompson and Wallace 2000) and that index is available through this link. On this page I present an index which is calculated from sea-level pressure (SLP) anomalies south of 20S. This choice is the Southern Hemisphere counterpart of the Arctic Oscillation. The usefulness of the Antarctic analysis in the NCEP - NCAR reanalysis before the satellite era (pre-1979) has been questioned by Marshall (2002). For this reason, the pre-1979 values of the index are included for completeness, but should be used only after reading Marshall.

The time series is calculated as follows:
1) Anomalies are calculated for each calendar month with respect to the 1979-2010.
2) Each grid box is area weighted by the square root of the cosine of the central latitude.
3) The covariance matrix of the area-weighted data is calculated for the grid boxes south of 20S, and the principal component (PC, timeseries ) calculated for that matrix.
4) The PC is correlated with global SLP anomalies, and that correlation map projected onto the data, including area weighting, to produce a time series for SAM for the entire record.
5) The mean and standard deviation are adjusted to be 0 and 1, respectively, for the period 1979-2010, all calendar months combined. This is a standard way to perform this calculation.

see caption
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The AO exhibits large amplitudes in all calendar months and a pronounced upward trend.


South Pole view
PostScript | JPEG | PNG
Regression of sea-level pressure anomalies onto the AAO (units of mb per standard deviation of the index). Anomalies of one sign centered over the pole (several millibars in amplitude), and weaker anomalies (mostly < 1 mb) centered at 40-50S. Netcdf files are provided of the digital values for the map-form analyses on this page (correlation, regression).

Global domain
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SLP anomaly correlations with the AAO. The AAO explains several percent of the variance in the tropics.


References:
Marshall, G.J., 2002: Trends in Antarctic geopotential height and temperature: A comparison between radiosonde and NCEP-NCAR reanalysis data. J. Climate, 15, 659-674.
Thompson, D. W. J., and J. M. Wallace, 2000: Annular modes in the extratropical circulation. Part I: Month-to-month variability. J. Climate, 13, 1000-1016.

Data:
The time series is standardized with respect to 1979-2010, all calendar months combined. The time series is offered in 3 formats
  • 3-column table: year, calendar month, pc1 (SAM,AAO)
  • 1-column table.
  • Table in year-month form.

    Digital values * 100
           jan  feb  mar  apr  may  jun  jul  aug  sep  oct  nov  dec
     1948  -97 -175 -180 -115   -8 -156 -342 -351 -154 -262 -220 -141
     1949 -316 -445 -242  -56    6   41 -184  -86 -106 -103 -104 -289
     1950 -285 -219 -254 -212 -147 -173 -337 -281 -102 -123 -180  -89
     1951  -48 -137 -159 -241 -283 -262 -222 -259 -293 -247 -166 -229
     1952 -283 -135 -189 -260 -223 -181 -162 -183 -199 -161 -210 -268
     1953 -231 -162 -165 -279 -216 -167 -256 -117 -128 -236 -120 -216
     1954  -73 -127 -317 -188  -88 -175 -189 -399  -67  -97   16 -182
     1955  -54  -54  -26  -81 -164 -177  -46 -319 -137 -266 -194 -128
     1956 -157 -168 -123 -129 -116   42 -188 -226 -250 -144 -119 -222
     1957  -89 -102  -39 -139 -159 -302 -153 -174  -26 -225 -183 -184
     1958    4  -69 -110 -175 -166  -70 -236  -77   63  -44    7   67
     1959   42  -39  -18 -189 -253 -191 -207 -239  -44 -118   57  -41
     1960  -18   -3   68 -185 -222  -61 -120 -239 -104  -39    1  -29
     1961  -41 -116  -82  -87 -139 -174 -128  -63 -154  -59    3   94
     1962   81  -42  -30   25  -54 -213  -12  -72  -17 -151 -117   37
     1963   60   31  -24 -151 -141 -168 -189 -439 -151  -87  -43  -39
     1964 -151 -119  -66  -91  -79 -335 -382 -292  -44  -13 -141 -110
     1965  -72 -296  -89 -179 -310  -93 -211 -189 -186 -227   -7 -215
     1966  -48 -121 -281 -144 -267  -37 -145  -71  -95    0 -102  -80
     1967 -160  -99  -33  -78 -121 -121  -86  -80  -97  -75 -101  -44
     1968  -86 -133  -70  -89 -240  -47   -6 -105 -223 -178 -164  -82
     1969 -246 -137  -65 -150 -122 -146  -18  -24    0 -129   23  -46
     1970   12   -3  -84  -59  -20   -6  -99   83  -25 -126    5 -138
     1971  -93  -32   12  -62 -177   -3 -149  -40  -23  -48  -24 -135
     1972 -176  -94 -120 -192 -112 -203 -175 -200  -71  -38   24 -109
     1973 -121  -32   73  -56 -180  -47   13   60  -30    4  -20   93
     1974  -64   94   33  -36  -61  -82  -64 -274   76  -82 -172 -144
     1975 -113  -13   -2   -5 -143  -86 -174   38  101   79  -20  -40
     1976  -83   49  -22   59   -9  -46  -83  -57  -43  -92 -203 -373
     1977 -225  -52  -49 -111 -116 -113 -130 -138   -1  -20    9  -77
     1978   33  -22  -55  -26  -78  -36 -243  106   86  -13  -48   35
     1979   40   32   80   63   68  128  159  -30   22  -27  -61 -112
     1980  -53 -128 -156 -210  -54   37 -187  -94 -157   37 -182  -86
     1981   -3    1 -110 -171  -60   29 -109 -226 -153  -64   70  121
     1982  -32   28  126  123   -5   39 -102  -16   85 -120  -77 -206
     1983 -134 -114  -24   -5  -58  -37   32   41  -49   90   69   29
     1984  -94  -66   14   -1  -91    7   26  -87  -10  -20   13 -200
     1985  -82    3  -29    9  -49  -52  171   42  134   11  134   91
     1986    7 -168  -87    0 -229 -102   -7  -60   91   23    1   73
     1987 -114  -81  -40  -55  -36  -73 -184  106 -101   22  110   13
     1988  -59   44   -9   14  -81 -102   81  -59  -67 -255   72  112
     1989   46  104   54  -75  254  190  139  -39  -21   32   54  -39
     1990  -28  130   23 -203 -186  -13 -102   13  153    9    5  -19
     1991   92  -81   31  -47  -50  -95 -110   34  -81  -70  -65 -226
     1992   -9 -150 -101  -66 -217 -237  -45  -14    1  -62   10   -3
     1993 -199   28  -55  -27   86  115  167   98   87   62   40   86
     1994   60   94   63    1  -53 -184  -60  166  -92  -62  -45   73
     1995  104   49  -28   49  135  -76 -281  -67  124  -31   25  123
     1996   -3  -64   41   22   65  -37  -22 -108 -160   84 -137  -26
     1997   17  -45   79  -57   66  -63   36   39  -40  -84 -182 -110
     1998   21   69   50  213   46  177  199  136   12   32  108  147
     1999   90   52    0  126  173 -119   52   31   12  210  140  197
     2000  130   84   15   43  130   42  -14  -61 -176   22 -128 -177
     2001  -18  -65   -2   81    4   72  -78   76  112  113  111   96
     2002   78  147 -149   50 -221  -46  -53   -8  -53 -277 -100  102
     2003 -104  -41  -46   47   53  -82   89  103    0  -29  -37 -143
     2004   83  -93   52   35   67  175  113  -23   62   -4  -11  -91
     2005   -1  109   21   80   -1  -91    1   39   53   20  -35 -213
     2006   34    0   77    1  187   89   84 -167  -18   90   46   46
     2007    7   18  -52  -75    2  -72 -238   19  -10  -43  -76  177
     2008  115  118   67  -44  -39  155   47   21  152  134  128  114
     2009  108   72   64   -2  -43  -26  -61  -79   -1    5 -172   37
     2010 -100  -88   29   82  136  255  258  176   90  152  171   12
     2011  -11  114  -17  -28  161                                   
    
    
    
    

    July 2011
    Todd Mitchell ( mitchell@atmos.washington.edu )
    JISAO data