The 1998 Oklahoma-Texas drought: Mechanistic experiments with NCEP global and regional models

This study presents results from mechanistic experiments to clarify the origin and maintenance of the Oklahoma-Texas (OK-TX) drought of the 1998 summer, using the National Centers for Environmental Prediction (NCEP) global and regional models. In association with this unprecedented drought, three major mechanisms that can produce extended atmospheric anomalies have been identified: (i) sea surface temperature (SST) anomalies, (ii) soil moisture anomalies, and (iii) atmospheric initial conditions favorable to such a climate extreme even in the absence of surface forcing (i.e., internal forcing). The authors found that the SST anomalies during April-May 1998 established the large-scale conditions for the drought. However, the warm El Nino-Southern Oscillation (ENSO) SST anomalies over the central and eastern tropical Pacific alone did not play a major role in initiating the drought. The internal structure of atmospheric conditions played as significant a role as the SST anomalies over the globe. In June 1998, soil moisture anomalies started to play an important role in maintaining the drought, and the regional positive feedback associated with lower evaporation/lower precipitation explained most of the water deficit in July. After July, synoptic-scale disturbances overwhelmed the impact of dry soil moisture near the Gulf of Mexico states where above-normal precipitation occurred, but the regional feedback was still prominent over the OK-TX region, where the drought persisted until early October.