The Roles of Radiative Forcing, Sea Surface Temperatures, and Atmospheric and Land Initial Conditions in U.S. Summer Warming Episodes

Abstract:

This study investigates the roles of radiative forcing, sea surface temperatures (SSTs), and atmospheric and land initial conditions in the summer warming episodes of the United States. The summer warming episodes are defined as the above normal (1983-2012) June-August 2-m temperature anomalies, and are referred to as heat waves in this study. Two contrasting cases, the summers of 2006 and 2012, are explored in detail to illustrate the distinct roles of SSTs, and atmospheric and land initial conditions in driving U.S. summer heat waves. For 2012, simulations with the GFDL atmospheric general circulation model reveal that SSTs play a critical role. Further sensitivity experiments reveal the contributions of global SST warming and SSTs in individual ocean basins to the geographic distribution and magnitudes of warm temperature anomalies. In contrast to 2012, for 2006, the atmospheric and land initial conditions are key drivers. The atmospheric (land) initial conditions play a key (minor) role in the central and northwestern (north and east) of U.S..

Due to changes in radiative forcing, the probability of areal averaged U.S. summer temperature anomalies exceeding the observed 2012 temperature anomaly increases with time over the early 21st century. La Nina (El Nino) events tend to increase (reduce) the occurrence of heat waves. The temperatures over the great central U.S. are mostly influenced by El Nino/La Nina. And, the central/western tropical Pacific plays a more important role than the eastern tropical Pacific. Thus, atmospheric and land initial conditions, SSTs and radiative forcing are all important drivers of, and sources of predictability for U.S. summer heat waves.