Antarctic Oscillation (AAO)

Antarctic Oscillation (AAO) time series, 1948 - 2002

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). More information is available on annular modes through this link.

The AAO is defined as the leading principal component (PC) of 850 hPa geopotential height anomalies south of 20S (Thompson and Wallace 2000). (An AAO index calculated from SLP anomalies is available through this link.) I have calculated the PC (time series) of this pattern from the NCEP / NCAR reanalysis, and all calendar months are used. There are concerns about the usefulness of the reanalysis in the Antarctic (see articles in J. Climate). The AAO time series for 1968-98 can be compared with the index calculated by Thompson and Solomon (2002) from 500 hPa geopotential height from radiosondes.

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Plots of PC1 of 850 hPa Z and a 500 hPa index. Both series are standardized with respect to 1968-98, and are correlated at 0.81 . The trend in the reanalysis-based index (the 850 hPa time series) is primarily before 1970. Qualitatively, the 500 radiosonde-based index exhibits less of a trend than the reanalysis-based index.

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850 hPa geopotential height anomalies regressed onto the (normalized) AAO time series. Typical fluctuations in the AAO are associated with anomalies in excess of 30m over the pole and height anomalies of opposite sign and 5-10 m in magnitude at the latitude of New Zealand.

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EOF1 plotted as a correlation map. Fluctuations in the AAO explain several percent of tropical variability.

The AAO is calculated with PC analysis, and this analysis yields a large set of modes, of which the first 3 explain 33, 11, and 9% of the variance, respectively. The spatial patterns and time series of each of these modes are shown in the next figure. The spatial maps are plotted as correlation maps, with the value at each grid point given as the correlation coefficient of the time series at that grid point with the PC for that mode.

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The second EOF is characterized by height anomalies of one sign centered at 50S,120W and weaker anomalies of opposite sign centered over the pole. The third EOF is characterized by zonal wave number 3 with centers of action over each of the oceans at 50S. A netcdf file is provided of the digital values for the above figure. (There are 6 maps: EOF1 correlation, EOF1 regression, ... read the history variable).

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The above plots are of the first 3 principal components. Only the AAO exhibits a strong trend.

References:

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.

Thompson, D. W. J., and S. Solomon, 2002: Interpretation of Recent Southern Hemisphere Climate Change. Science, 296, 895-899.

Data: All of the time series are standardized with respect to 1948-2002.

The PC 1-3 time series as a 5-column table: year, calendar month, pc1 (AAO), pc2, and pc3.

PC1 (AAO) as a one-column table.

PC1 (AAO) as a table in year-month form.

Digital values

       jan  feb  mar  apr  may  jun  jul  aug  sep  oct  nov  dec
 1948  -70 -155 -157  -81   47  -84 -231 -237 -110 -198 -216 -130
 1949 -271 -378 -163    2   79   79  -97    0  -63  -41  -75 -212
 1950 -236 -162 -193 -129  -48  -58 -229 -165  -53  -60 -132  -31
 1951  -43 -108 -104 -169 -203 -153 -124 -171 -223 -148 -105 -176
 1952 -232 -106 -148 -204 -137  -99  -87 -103 -137  -84 -174 -236
 1953 -207 -139 -116 -197 -121  -97 -200  -42  -62 -172  -91 -187
 1954  -34  -84 -242 -140   10  -79  -81 -286  -20  -46   28 -145
 1955  -67  -25   11  -12  -71  -94   14 -207  -98 -193 -155  -76
 1956  -91  -85  -63  -63  -42   92  -75 -152 -171  -94  -66 -149
 1957  -58  -12    4  -42  -60 -172  -22  -32   43 -139 -108 -107
 1958   64   -4  -24  -84  -53   32 -102   15  102   26   66  133
 1959  109    6   25  -94 -156  -76  -84  -97   38  -30  112   27
 1960   40   35  113  -98  -98   35   -8 -104  -52   22   45   25
 1961   42  -44  -14  -15  -62  -74  -28   40  -77   16   42  134
 1962  114   21   14   73   26 -109   71    3   48  -67  -44   85
 1963  119   80   12  -76  -35  -76  -78 -263  -70    6   -4   22
 1964  -89  -59  -10  -14  -13 -190 -225 -182   17   38  -75  -46
 1965  -15 -204  -45  -88 -208  -23  -96  -62  -78 -142   17 -139
 1966   26  -54 -202  -70 -156   32  -24   22  -41   71  -42  -11
 1967  -88  -39    6   -6   -6  -19   13   23  -43    9  -44   13
 1968  -14  -53  -24  -16 -121   32   94  -33 -151 -114  -98   -7
 1969 -122  -51   -1  -66  -22  -39   85   92   41  -30   78   24
 1970   79   58   -5    8   62   63  -33  143   16  -55   50  -56
 1971  -37   27   63    3  -86   50  -41   28   32   21   18  -57
 1972  -95  -25  -30 -104  -13 -100  -53  -79    4   46   74  -37
 1973  -43   42  103   15 -106   15   93  124   16   63    0  112
 1974   -2  120   78   42   58  -14   34 -154   91    5 -105  -85
 1975  -58   24   38   63  -37    5  -63  121  144  113   24   12
 1976  -32   73   37  126   72   33   15   30   16  -34 -135 -245
 1977 -128    7   17  -57  -56  -48  -20  -50    6   28   22  -13
 1978   85   28   -2   28   -1    6 -129  157  109   41    6   77
 1979   90   79  120  130  143  208  250   47   68   45    6  -43
 1980   24  -51  -72 -100   42   65  -77    4 -102   92 -113    3
 1981   67   61  -51  -82   10   70  -34 -138  -93   24   89  138
 1982   24   80  144  158   79  122   32  101  144  -27   -5 -117
 1983  -54  -24   51   73   43   36  120  121  -16  129   74   86
 1984  -21    3   65   51  -12   87  115  -17   11   44   57 -103
 1985  -20   63   39   66   38   18  219  122  155   78  166  154
 1986   77  -81  -27   60 -102  -55   76   22  110   75   54  130
 1987  -25   -7   27   43   67    7  -62  165  -29   95  149   73
 1988   21   95   27   65   26  -13  135   12  -16 -152   92  131
 1989   93  132   87   21  279  228  188   23    7   60   89   18
 1990   39  158   72  -95  -91   37  -20   89  172   63   43   36
 1991  132   -4   66   20   37  -25  -12   78  -16   -4   -2 -136
 1992   44  -67  -18   29 -101 -137   50   48   43    1   46   49
 1993 -110   90   12   69  160  165  234  171  143  123   94  141
 1994  121  151  108   80   51  -84   46  218  -38    5   -1  141
 1995  176   98   37   93  188   -8 -174   17  142   17   82  158
 1996   62   -2   84   87  151   35   68  -39  -98  117  -76   48
 1997   67   21  101   10  142   21  129  109   44    0  -93  -36
 1998   96  107  106  220   98  193  215  174   38   83  143  182
 1999  134   88   46  146  205  -39  100   91   52  226  152  203
 2000  160  106   46   91  165   84   62   30 -114   78  -51  -70
 2001   53    6   35  129   53  112   15  111  133  128  121  145
 2002  107  162  -85  104 -116    1   37   62  -15 -158  -26  151

February 2003
Todd Mitchell ( mitchell@atmos.washington.edu )

JISAO data