JISAO data

SLP-based Antarctic Oscillation (AO) time series, 1948 - June 2005

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 was calculated in the following steps:
1) Anomalies were calculated for each calendar month with respect to the 1979-2004 period.
2) Each grid box was appropriately area weighted (square root of the cosine of the central latitude).
3) The covariance matrix was calculated for the grid boxes south of 20S, and the principal component (PC, timeseries ) calculated for that matrix.
4) The PC was correlated with global SLP anomalies, and that correlation map was projected onto the data, including area weighting, to produce a time series for SAM for the entire record 1948 - June 2005.
5) The mean and standard deviation were adjusted to be 0 and 1, respectively, for the period 1979-2004, 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
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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. A netcdf file is provided of the digital values for the map-form analyses on this page. (There are 6 global maps: EOF1 correlation, EOF1 regression, EOF2 (not discussed) ..., EOF3 (not discussed) ... read the history variable).

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 1979-2004, 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
           jan  feb  mar  apr  may  jun  jul  aug  sep  oct  nov  dec
     1948  -96 -172 -182 -124    0 -150 -349 -363 -153 -259 -225 -147
     1949 -318 -442 -239  -54   19   46 -193  -92 -104  -97 -108 -286
     1950 -290 -214 -251 -215 -142 -169 -340 -287  -97 -117 -185  -86
     1951  -45 -132 -158 -247 -276 -258 -231 -272 -281 -242 -170 -232
     1952 -281 -129 -184 -262 -221 -174 -166 -194 -197 -150 -217 -257
     1953 -223 -155 -159 -278 -214 -169 -261 -121 -124 -230 -118 -220
     1954  -69 -124 -323 -193  -84 -172 -199 -410  -62  -94   11 -185
     1955  -57  -52  -21  -82 -160 -173  -51 -342 -141 -277 -205 -126
     1956 -157 -165 -127 -136 -122   39 -197 -241 -257 -148 -121 -215
     1957  -94  -85  -28 -142 -155 -286 -157 -174  -21 -219 -189 -186
     1958   13  -49 -106 -182 -161  -65 -241  -87   62  -42    1   79
     1959   52  -35  -19 -191 -254 -185 -208 -248  -42 -107   61  -38
     1960   -8   -1   72 -186 -220  -60 -124 -253 -106  -34   -1  -24
     1961  -33 -111  -78  -97 -136 -168 -130  -67 -158  -55   -2   97
     1962   78  -39  -32   12  -62 -221  -22  -81  -21 -155 -127   39
     1963   68   34  -21 -160 -138 -161 -198 -446 -147  -67  -43  -37
     1964 -155 -117  -70  -96  -75 -337 -398 -302  -43   -9 -138 -109
     1965  -69 -298  -84 -181 -305  -82 -206 -192 -176 -217   -5 -207
     1966  -38 -112 -275 -140 -267  -28 -152  -80  -85   11 -108  -75
     1967 -161 -102  -32  -86 -111 -115  -87  -82  -92  -60 -107  -39
     1968  -80 -122  -74  -99 -239  -41  -10 -113 -214 -172 -165  -82
     1969 -245 -131  -60 -158 -110 -139  -17  -27    3 -113   25  -44
     1970   22    8  -72  -61  -18   -1 -106   72  -32 -124   -2 -140
     1971  -94  -26   17  -68 -177    1 -154  -52  -24  -43  -25 -131
     1972 -179  -91 -110 -200 -100 -185 -174 -204  -58  -26   38 -113
     1973 -119  -21   75  -53 -178  -43   10   51  -31    2  -24   97
     1974  -58   97   36  -39  -49  -83  -66 -282   83  -74 -170 -142
     1975 -111   -7    0  -10 -141  -77 -182   30  102   79  -24  -42
     1976  -77   45  -18   59    0  -32  -78  -55  -22  -82 -207 -366
     1977 -213  -36  -37 -118 -113 -106 -128 -136   12   -1   12  -71
     1978   46   -7  -45  -26  -75  -27 -246  105   94   -3  -41   37
     1979   43   33   93   69   85  143  170  -31   32  -10  -58 -110
     1980  -43 -119 -144 -203  -47   49 -180  -95 -149   52 -185  -74
     1981    9   13 -100 -179  -52   40 -117 -228 -143  -45   74  130
     1982  -33   35  132  124    4   61  -90   -8  105 -100  -64 -203
     1983 -117  -96   -6   -1  -52  -22   31   36  -41  107   74   38
     1984  -87  -63   25   -2  -83   22   28  -91   -3    1   20 -203
     1985  -80   10  -23   10  -39  -36  180   39  144   26  145  109
     1986   23 -161  -85   -4 -219  -99    4  -53  108   42   11   91
     1987 -110  -70  -31  -49  -26  -55 -173  109  -84   37  114   17
     1988  -52   54   -3   16  -70  -96   79  -62  -66 -258   74  112
     1989   44  108   57  -70  262  201  134  -47  -20   41   57  -34
     1990  -16  138   33 -200 -189   -5 -102   19  168   21    6  -18
     1991   97  -71   36  -51  -36  -80 -105   31  -69  -55  -61 -219
     1992   -2 -137  -87  -57 -208 -227  -39  -12   14  -48   20   -4
     1993 -196   40  -43  -18   91  131  174  105  106   81   32  100
     1994   67  105   77    5  -42 -178  -53  162  -78  -45  -55   88
     1995  119   62  -18   44  145  -66 -286  -71  138  -26   27  137
     1996   -6  -65   39   25   72  -27  -19 -115 -158   94 -141  -21
     1997   16  -33   90  -56   90  -51   47   46  -13  -67 -185 -105
     1998   38   85   60  209   54  188  198  134   15   40  114  152
     1999   96   59    2  124  181 -116   49   34   20  224  137  202
     2000  132   92   14   39  136   50  -17  -55 -174   38 -124 -167
     2001  -17  -58    7   86    4   86  -69   74  121  122  110  100
     2002   79  157 -145   50 -205  -30  -53   -3  -49 -260  -90  112
     2003  -94  -37  -39   51   67  -74   91  100    2  -19  -37 -142
     2004   88  -84   58   39   79  193  116  -18   73    6  -16  -91
     2005    8  118   27   84    6  -82                              
    
    
    

    August 2005
    Todd Mitchell ( mitchell@atmos.washington.edu )
    JISAO data