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The AO exhibits large amplitudes in all calendar months and a pronounced upward trend.
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.
References:
Data: The time series is standardized with respect to 1979-2004, all calendar months combined. The time series is offered in 3 formats
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