A Diagnostic Comparison of the 1980 and 1988 U.S. Summer Heat Wave-Droughts

Abstract

Observational analyses are performed to examine the roles of remote and local forcing in the evolutions of the extreme U.S. summer heat wave-drought cases of 1980 and 1988. At early stages, both events are associated with anomalous stationary wave patterns. Wave activity flux analyses suggest that in the 1980 case anomalous wave activity propagates southeastward from an apparent source region to the south of the Aleutians. The flux pattern is more complex in the 1988 case but suggests two possible source regions, one over the central North Pacific to the north of the Hawaiian Islands and a second located over the far western Pacific. The 1988 analyses show no anomalous wave propagation out of the eastern tropical Pacific, although this result does not necessarily preclude a role for tropical forcing in generating the anomalous wave train. In both cases the anomalous wave trains and associated wave activity fluxes become very weak by early July, indicating that remotely forced anomalous stationary waves are unlikely to account for the later stages of the heat wave-droughts. This leads us to examine whether these events were enhanced or prolonged by changes in the local surface energy budget associated with reductions in evapotranspiration (ET) over the drought regions. Water vapor budgets show a systematic decrease in monthly mean ET from June to August during both events. Comparisons with nondrought summers support the idea that by late summer ET rates in both events are anomalously low. Estimated reductions in surface latent heat fluxes relative to the control years are approximately 50 W m?2 in 1980 and 20 W m?2 in 1988, with implied increases in sensible heating of similar magnitudes. Overall, the results indicate the importance of both dynamical forcing from remote sources and anomalous local boundary conditions in accounting for the two extreme heat wave-drought events. The relative importance of these factors varies significantly during the evolution of the events, with remote forcing playing a predominant role at early stages and anomalous local boundary conditions assuming increasing importance at later stages.