A General Circulation Model Study of the Atmospheric Response to Extratropical SST Anomalies Observed in 1950–79

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Abstract
A 30-year experiment with an atmospheric general simulation model has been performed. The lower boundary condition at all oceanic grid points between 38°S and 60°N has been prescribed to follow the observed month-to-month variation of the sea surface temperature (SST) field during the 1950–79 period. Much of the model diagnosis presented here pertains to the midlatitude atmospheric response to recurrent SST patterns in the North Pacific and North Atlantic in winter. The principal modes of variability of the seasonally averaged 515 mb height and SST fields have been identified using rotated principal component (RPC) analysis. The extrema of the first atmospheric mode reside over the North Atlantic and Eurasia, whereas the second mode is associated with height anomalies in the North Pacific/North American sector. Cross-correlation analysis reveals that these two atmospheric modes am linked to leading patterns of the SST field in the North Atlantic and North Pacific, respectively. It is also demonstrated that the extrema in leading RPC patterns of the SST field in the northern oceans are almost coincident with the sites of maximal covariability between the SST and 515 mb height fields. Regression charts of selected model parameters versus the SST variations off the Newfoundland coast and northwest of Hawaii have been constructed. These two reference maritime sites have been identified by the RPC and cross-correlation analyses as being correlated with the strongest atmospheric signals. The model fields examined in this manner include the geopotential height at various pressure levels, precipitation, heat flux across the air-sea interface, as well as temporal variance and covariance statistics. These regression maps indicate that the atmospheric response to midlatitude SST anomalies has an equivalent barotropic structure. The presence of SST perturbations in the extratropics are associated with displacements of the storm track axes, and with relocations of the midlatitude rainbelts and preferred sites of heat transfer from the underlying ocean. The changes in the locality of synoptic scale eddy activity are accompanied by alterations in the transient eddy forcing of the quasi-stationary flow. The geopotential height tendencies associated with these anomalous eddy effects exhibit a positive spatial correlation with the seasonally averaged, downstream atmospheric response. The time scale for the eddy induced tendencies to produce such seasonal height anomalies is on the order of several days. These findings suggest that the transient disturbances act as an essential intermediary between the extratropical SST forcing and the time-mean atmospheric response. The principal atmospheric anomaly pattern in the North Atlantic/Eurasian sector exhibits substantial correlations with SST fluctuations in the tropical South Atlantic; whereas oceanic anomalies in the equatorial Pacific are only weakly associated with atmospheric circulation changes in the North Pacific/North American region. The temporal lead/lag relationships between the simulated atmospheric anomalies and the prescribed SST changes have been explored.

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