Numerical Experiments Related to the Summer 1980 U.S. Heat Wave

Abstract

A series of numerical forecast experiments has been conducted with the Goddard Laboratory for Atmospheres (GLA) fourth-order general circulation model in order to study the role of (i) sea-surface temperature (SST) anomalies in the North Pacific, (ii) soil moisture anomalies over the continental United States, as derived from observations of precipitation and surface temperature, and (iii) solar radiational forcing in the maintenance and breakdown of the severe heat wave over the United States in the summer of 1980. Results from these experiments show opposite effects of the SST and soil moisture anomalies on the model's medium-range (ten-day) simulations of the maintenance of the heat wave. The lower than normal soil moisture over the United States during the summer of 1980 resulted in reduced surface evaporation, higher ground temperature, increased sensible heat flux from ground to air, higher surface temperature, lower sea-level pressure and higher 500 mb height over much of the Great Plains in the model forecasts. In contrast, the SST anomalies in the North Pacific during the same period resulted in an enhanced northerly flow of cooler and dryer air, increased evaporation, decreased ground and air temperature and reduced 500-mb heights over the Great Plains. These results suggest that once established, soil moisture deficits contribute to maintaining warm, dry conditions. Although long-term effects of North Pacific SST anomalies may be to create or enhance the heat wave, ten-day simulations showed that such anomalies contributed to lower temperatures over shorter time scales. A limited number of experiments, with modified solar radiational forcing, showed a dramatic weakening of the heat wave pattern in the model forecasts and indicated that the interaction of changing solar declination with the prevailing synoptic situation was probably responsible for the breakdown of the heat wave in September 1980.