Issues in Establishing Causes of the 1988 Drought over North America

Progress toward understanding the causes of and physical mechanisms involved in the 1988 North American drought is reported. An earlier study demonstrated that major sea surface temperature (SST) anomalies in the tropical Pacific Ocean, in association with the 1988 La Niña, may have disrupted atmospheric heating patterns by changing the location and intensity of the intertropical convergence zone and that such heating anomalies could have initiated the circulation anomalies across North America responsible for the drought. A key issue of when the drought circulation anomalies developed and their relation to changes in tropical Pacific SSTs is examined. Although unusually dry soil moisture and heat waves persisted into August, the anomalous atmospheric conditions that brought on the drought occurred in April, May, and June of 1988. The evolution of the Pacific SSTs and tropical convection, as revealed by outgoing longwave radiation, is shown to be consistent with the development of the conditions favorable for initiating the drought circulation pattern in April through June of 1988. On the equator at 110°W, SST anomalies exceeded −2.75°C in only April, May, and June and were largest (−4.1°C) in May 1988. The issues of how the 1988 La Niña differed from those in the past and the importance of the whole SST field in determining the anomalous diabatic heating are also discussed. Diagnostic calculations of atmospheric diabatic heating confirm that atmospheric heating anomalies existed in the tropical Pacific in association with the major SST anomalies during this time. The link between the anomalous heating and the tropical SSTs supports the view that influences external to the atmosphere were important and that the drought was not generated solely by mechanisms internal to the atmosphere. The distribution of diagnosed heating anomalies over North America, together with a planetary wave model response to idealized forcing, is described to clarify the possible role of soil moisture anomalies in perpetuating the drought. It is argued that feedback-caused soil moisture anomalies may have been secondary sources for the drought circulation but could not have been the primary instigator. For the most part, other diagnosed heating anomalies during the drought are found to have little influence on the North American region. Criteria to help judge the ability of general circulation models to simulate the drought are discussed.