The Sustained North American Warming of 1997 and 1998.

ABSTRACT North America experienced sustained and strong surface warming during 1997 and 1998. This period coincided with a dramatic swing of the El Nino-Southern Oscillation (ENSO), with El Nino in 1997 rapidly replaced by La Nina in 1998. An additional aspect of the sea surface temperatures (SSTs) was the warmth of the world oceans as a whole for the entire period, with unprecedented amplitudes within the recent instrumental record. Using a suite of dynamical and empirical model simulations, this study examines the causes for the North American warming, focusing on the role of the sea surface boundary conditions. Two sets of atmospheric general circulation model experiments, one forced with the observed global SSTs and the other with the tropical east Pacific portion only, produce similar North American-wide warming during fall and winter of 1997. The GCM results match empirical estimates of the canonical temperature response related to a strong El Nino and confirm that east equatorial Pacific SST forcing was a major factor in the continental warming of 1997. Perpetuation of that warming from spring through fall of 1998 is shown to be unrelated to equatorial east Pacific SSTs and thus cannot be attributed to the ENSO cycle directly. Yet, simulations using the observed global SSTs are shown to reproduce realistically the continuation... [ABSTRACT FROM AUTHOR]