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 Niño-Southern Oscillation (ENSO), with El Niño in 1997 rapidly replaced by La Niña 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 Niño 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 of North American warming throughout 1998. The continental warmth occurs in tandem with a warming of the troposphere that, initially confined to tropical latitudes during El Niño's peak in 1997, spreads poleward and covers the entire globe in 1998. 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 signal in 1998 related to the disposition of global SSTs outside the ENSO region is emphasized. The high potential predictability of North American climate during this 2-yr period raises new questions on the role of global SSTs in climate variability and the ability to predict them skillfully.