MODELING THE SURFACE HEAT-FLUX RESPONSE TO LONG-LIVED SST ANOMALIES IN THE NORTH-ATLANTIC

An atmospheric general circulation model(AGCM), a simplified atmospher ic model (SAM) of surface heat flux, and various idealized analytic mo dels have been used to investigate the atmospheric response over the N orth Atlantic to SST anomalies, including a general cooling associated with a weakened thermohaline circulation. Latent heating dominates th e surface heat flux response, while sensible heating plays an importan t secondary role. The total heat flux response is weaker than presumed in recent studies using ocean models under highly idealized surface b oundary conditions. This implies that stability of the thermohaline ci rculation to high-latitude freshening in more sophisticated coupled sy stems (that incorporate either AGCMs or models like SAM) will be incre ased. All three kinds of atmospheric models exhibit nonrestorative beh avior away from the anomaly peak that is primarily associated with the advection of cooled air eastward. This simple picture is complicated in the AGCM by the fact that the winds weaken over the SST anomaly, wh ich helps to moderate the response. Analytic models for atmospheric te mperature forced using imposed surface temperature anomalies highlight conditions under which a nonrestorative response can arise. Previous work has shown that the length scale of spatially periodic anomalies p artially determines the magnitude of the response in a diffusive atmos phere. Here the authors show that this scale dependence has much wider applicability by considering more localized anomalies and by the incl usion of advective transport processes. The modification of the respon se by sea ice changes and the absence of any statistically significant change in the basin-averaged hydrological cycle are also discussed.