Interannual and interdecadal variability of atmospheric synoptic‐scale activity in the Southern Hemisphere

Abstract

A rotated empirical orthogonal function analysis was applied to anomalies of July atmospheric short time scale variability (or storm tracks) over the Southern Hemisphere (SH) using NCEP reanalyses. The study reveals the existence of two leading modes of storm‐track variations, the first one describing storm‐track changes along the subtropical Pacific and the second one associated with variations over southeast Pacific Ocean. The principal components of both modes exhibit significant variability on interannual (2–7 years) and interdecadal (longer than 7 years) time scales. The interannual variation of the first mode is closely linked to ENSO variability. The corresponding signal on SST anomalies shows a conspicuous maximum over central equatorial Pacific, while the signature on geopotential‐height anomalies shows the typical Pacific South America pattern characterized by a Rossby wave train propagating poleward from the tropics. On interdecadal time scales, the first mode is related to SST anomalies over the subtropical portions of the Pacific, Atlantic, and Indian Oceans and to a Rossby wave train with strongest signals over the southeast Pacific and the Antarctic Peninsula. The second mode that represents storm‐track changes west of the Antarctic Peninsula is related on interannual time scales to SST anomalies over western Pacific between 40°S and 20°S, while on interdecadal time scales, it does not seem to be significantly related to SST changes in any of the basins. In particular, the spatial signature of the REOF2 interdecadal variability on the SH circulation corresponds to the well‐known high latitude mode characterized by a monopole over the Antarctica and a three‐wave pattern at middle latitudes.