Atlantic air-sea interaction and model validation

Abstract

An analysis of observations from 1948-1998 suggests that the atmosphere in the North Atlantic region does respond to North Atlantic Sea-Surface Temperatures (SSTs) throughout the annual cycle. In the subtropics, high geopotential heights are seen to be a local response to warm SSTs. In winter, the North Atlantic Oscillation responds to a «tripole» pattern in North Atlantic SSTs. In summer, anticyclonicity over the U.K. is seen downstream of warm SST anomalies off Newfoundland and is possibly also related to warm subtropical SSTs. Such responses imply a degree of seasonal predictability and help quantify the strength of natural ocean-atmosphere coupled modes of variability. The average of an ensemble of 10 simulations of the HadAM3 atmospheric model forced with observed SSTs for the same period produces robust ocean-forced responses which agree well with those identifi ed in the observations and with a previous model. The agreement is encouraging as it confi rms the physical signifi cance of the observational results and suggests that the model responds with the correct patterns to SST forcing. In the subtropics, the magnitude of the ensemble mean response is comparable with the observational response. In the extratropics, the magnitude of the model response is about half that of the observations. Although atmospheric internal variability may have affected the observed atmospheric patterns and there are considerations regarding the lack of two-way air-sea interaction with an atmospheric model, it is suggested that the model’s extratropical response may be too weak. The 10 individual simulations of HadAM3 and 28 50-year periods of the ocean-atmosphere model, HadCM3, display similar results to each other with generally weaker ocean-forced links than observed. Seasonal predictability may, therefore, be too low in HadCM3 and low-frequency coupled modes under-represented. A moderate increase in the extratropics in the sensitivity of surface heat fl uxes to surface temperatures is one possibility for improving these model deficiencies.