Where’s the heat? Insights from GCM experiments into the lack of Eastern US warming

During the past half century the Eastern United States (EUS) has cooled slightly, while the earth as a whole has warmed. Here we address possible mechanisms for this regional lack of warming using ensembles of retrospective simulations with an atmospheric general circulation model. The model (GISS SI99 GCM) exhibits EUS cooling in all simulations in which the observed time varying tropical ocean temperatures are imposed, and fails to exhibit this cooling in all other simulations. This result holds regardless of what time-varying radiative forcings, such as increasing greenhouse gases or sulfate aerosols, are applied.

The EUS cooling is investigated in a 5-member ensemble of runs that use observed time-varying sea surface temperatures (SST) and sea ice, but include fixed greenhouse and aerosol forcing. The EUS cooling in these runs is robust - it is present in every ensemble member - and it is broadly consistent with observations. The modeled EUS cooling is radiative, driven by increased cloud cover. EUS cloud cover is highly correlated with SST averaged over the Tropical Pacific Ocean, which, therefore, correlate negatively with EUS temperatures. The role of tropical SST is confirmed by an ensemble of runs in which observed SST are imposed only in the tropics - these runs yield EUS cooling nearly identical to that obtained with global SST. Further diagnostic analyses of the model output show how Tropical Pacific SST controls EUS cloudiness.

To project future regional climatic change, it is necessary to determine whether the observed lack of warming in the Eastern United States is a manifestation of global warming or a result of natural climate variability. Our results suggest that the latter explanation is more likely correct.