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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 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.
References:
Data:
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