The sustained North American warming of 1997 and 1998

North America experienced sustained and strong surface warming during 1997 and 1998. This period coincided with a dramatic swing of the El Nino-Southe rn 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 w armth of the world oceans as a whole for the entire period, with unpreceden ted amplitudes within the recent instrumental record. Using a suite of dyna mical and empirical model simulations, this study examines the causes for t he North American warming, focusing on the role of the sea surface boundary conditions. Two sets of atmospheric general circulation model experiments, one forced w ith the observed global SSTs and the other with the tropical east Pacific p ortion only, produce similar North American-wide warming during fall and wi nter of 1997. The GCM results match empirical estimates of the canonical te mperature response related to a strong El Nino and confirm that east equato rial Pacific SST forcing was a major factor in the continental warming of 1 997. Perpetuation of that warming from spring through fall of 1998 is shown to b e unrelated to equatorial east Pacific SSTs and thus cannot be attributed t o the ENSO cycle directly. Yet, simulations using the observed global SSTs are shown to reproduce realistically the continuation of North American war ming throughout 1998. The continental warmth occurs in tandem with a warmin g of the troposphere that, initially confined to tropical latitudes during El Nino's peak in 1997, spreads poleward and covers the entire globe in 199 8. This evolutionary aspect of the global circulation anomalies during 1997 and 1998 is found to be a response to global SSTs and not linked directly to ENSO's evolution. Results presented here demonstrate that a significant fraction of the North American warming in 1997 and 1998 is explainable as the forced response to sea surface boundary conditions. The hand-over in the impact of those SSTs , with a classic ENSO driven signal in 1997 but an outwardly independent si gnal in 1998 related to the disposition of global SSTs outside the ENSO reg ion is emphasized. The high potential predictability of North American clim ate during this 2-yr period raises new questions on the role of global SSTs in climate variability and the ability to predict them skillfully.