Atmospheric Response Patterns Associated with Tropical Forcing

Abstract

Atmospheric response patterns associated with tropical forcing are examined with general circulation models driven by global sea surface temperature (SST) variations during 1950?99. Specifically the sensitivity of midlatitude responses to the magnitude and position of tropical SST anomalies is explored. This controversial problem, spanning more than a quarter century now, centers on whether response patterns over the Pacific?North American region are affected or changed by inter?El Niño variability in tropical forcing. Ensemble methods are used in this study to reliably identify the signals related to various tropical SST forcings, and the sensitivity is determined from analysis of four different climate models. First, the fraction of Pacific?North American (PNA) wintertime 500-hPa height variability that is potentially predictable and is linked to interannual variations in the global SSTs is identified. This SST-forced component accounts for as much as 20%?30% of the total seasonal mean height variability over portions of the PNA region, and the most important boundary forcing originates from the tropical Pacific Ocean. The spatial expression of the teleconnections that are linked to this potentially predictable SST-forced fraction of height variability is next identified. The leading model pattern is similar to the classic observed teleconnection associated with the linear ENSO signal, and explains 80% of the SST-forced height variance over parts of the North Pacific and North America. Two additional wavelike patterns are identified that are also associated with tropical forcing. One is related to the pattern of tropical SST variations often seen during the transition of the tropical ocean that marks the interlude between ENSO extremes, and the pattern of forcing related to it is distinctly non-ENSO in character. The other is related to the nonlinear component of the atmospheric response to ENSO's extreme opposite phases. Response patterns having annular-like structures over the Northern Hemisphere that are related to multidecadal variations in tropical Indo-Pacific and Atlantic SSTs are also highlighted. Subtle modifications in upper-level responses to different tropical SST forcings are shown to yield disproportionate sensitivity in North American surface climate. Particularly pronounced is the reversal in sign of the precipitation anomalies over the region spanning the Canadian border to southern California in response to equatorial Pacific convection anomalies shifting from 170°E to 140°W. The behavior is reproduced in experiments using both realistic and idealized SST anomalies, and this behavior is found to emerge particularly when the far eastern equatorial Pacific Ocean is strongly warmed as occurred during the 1982/83 and 1997/98 El Niños. Despite the existence of different response patterns to tropical SST forcings, it is shown that the seasonal hindcast skill of PNA 500-hPa heights for 1950?99 originates mainly from the single, leading teleconnection structure. The conclusion drawn from this result is that the atmospheric sensitivity to different tropical SST forcings, though real, is weak and easily masked by the year-to-year climate variations due to internal atmospheric processes.