Potential Predictability of Geopotential Heights over the Southern Hemisphere

Kevin E. Trenberth National Center for Atmospheric Research, Boulder, CO 80307

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Abstract

An analysis of variance approach is used to estimate the potential predictability of interannual fluctuations of the seasonal mean flow over the Southern Hemisphere. The potential predictability of the 1000 and 500 mb geopotential height fields for both summer and winter is assessed. Daily variances at 500 mb are roughly double those at 1000 mb, but otherwise the patterns are quite similar in both seasons. The level of climatic noise is estimated from the daily variances and compared with the actual interannual variances under a null hypothesis that no signal exists. Results reveal that interannual or longer-term variability clearly exceeds the noise level at both levels and in both seasons over Antarctica and in the tropics No clear statement can be made one way or the other in midlatitudes. The methodology appears to fail over the Australia-New Zealand region where clear interannual signals associated with the Southern Oscillation and a quasi-biennial oscillation have previously been found. However, the noise in this region appears to include the 40–50 day oscillation which may be organized by or systematically contribute to the interannual variability. The implied correlation between signal and noise voids a basic assumption of the methodology.

Abstract

An analysis of variance approach is used to estimate the potential predictability of interannual fluctuations of the seasonal mean flow over the Southern Hemisphere. The potential predictability of the 1000 and 500 mb geopotential height fields for both summer and winter is assessed. Daily variances at 500 mb are roughly double those at 1000 mb, but otherwise the patterns are quite similar in both seasons. The level of climatic noise is estimated from the daily variances and compared with the actual interannual variances under a null hypothesis that no signal exists. Results reveal that interannual or longer-term variability clearly exceeds the noise level at both levels and in both seasons over Antarctica and in the tropics No clear statement can be made one way or the other in midlatitudes. The methodology appears to fail over the Australia-New Zealand region where clear interannual signals associated with the Southern Oscillation and a quasi-biennial oscillation have previously been found. However, the noise in this region appears to include the 40–50 day oscillation which may be organized by or systematically contribute to the interannual variability. The implied correlation between signal and noise voids a basic assumption of the methodology.

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