Role of sea surface temperature and soil-moisture feedback in the 1998 Oklahoma-Texas drought

The drought that affected the US states of Oklahoma and Texas in the summer of 1998 was strong and persistent, with soil moisture reaching levels comparable to those of the 1930s 'dust bowl'(1,2). Although other effects of the record-strength 1997-98 El Nino were successfully predicted over much of the United States, the Oklahoma-Texas drought was not(3). Whereas the response of the tropical atmosphere to strong anomalies in sea surface temperature is quite predictable, the response of the extratropical atmosphere is more variable(4,5). Here we present results from mechanistic experiments to clarify the origin and maintenance of this extratropical climate extreme. In addition to global tmospheric models(6-11), we use a regional model(12,13) to isolate regional climate feedbacks. We conclude that during April and May 1998, sea surface temperature anomalies combined with a favourable atmospheric circulation to establish the drought. In June-August, the regional positive feedback associated with lower evaporation and precipitation contributed substantially to the maintenance of the drought. The drought ended in the autumn, when stronger large-scale weather systems were able to penetrate the region and overwhelm the soil-moisture feedback. Our results show the potential for numerical models including appropriate physical processes to make skilful predictions of regional climate.