Re-emergence of North Atlantic subsurface ocean temperature anomalies in a seasonal forecast system

A high-resolution coupled ocean atmosphere model is used to study the effects of seasonal re-emergence of North Atlantic subsurface ocean temperature anomalies on northern hemisphere winter climate. A 50-member control ensemble is integrated from 1 September 2007 to 28 February 2008 and compared with a parallel ensemble with perturbed ocean initial conditions. The perturbation consists of a density-compensated subsurface Atlantic temperature anomaly corresponding to the observed subsurface temperature anomaly for September 2010. The experiment is repeated for two atmosphere horizontal resolutions (~ 60 km and ~ 25 km) in order to determine whether the sensitivity of the atmosphere to re-emerging temperature anomalies is dependent on resolution. A wide range of re-emergence behavior is found within the perturbed ensembles. While the observations seem to indicate that most of the re-emergence is occurring in November, most members of the ensemble show re-emergence occurring later in the winter. However, when re-emergence does occur it is preceded by an atmospheric pressure pattern that induces a strong flow of cold, dry air over the mid-latitude Atlantic, and enhances oceanic latent heat loss. In response to re-emergence (negative SST anomalies), there is reduced latent heat loss, less atmospheric convection, a reduction in eddy kinetic energy and positive low-level pressure anomalies downstream. Within the framework of a seasonal forecast system the results highlight the atmospheric conditions required for re-emergence to take place and the physical processes that may lead to a significant effect on the winter atmospheric circulation.