Spring Soil Temperature Anomalies in the Western U.S. and Summer Droughts in the Southern Plains
Based on these observed based associations, this study explores the impact of spring sub-surface and surface temperature (SUBT) anomalies in the Western U.S. on southern U.S. summer precipitation, especially the Texas Drought 2011, and possible mechanisms using two regional climate models (RCM) and a general circulation model (GCM). The GCMs produces the lateral boundary condition (LBC) for the RCMs. The study has found that the snow effect is greatly enhanced though the SUBT anomaly memory. In the first experiment, two initial SUBT conditions (one cold and another warm) on May 1st were assigned for the GCM runs and the corresponding RCM runs, to explore the SUBT effect. The results suggest that antecedent May 1st warm (cold) initial SUBT in the Western U.S. contributes positive (negative) June precipitation over the southern U.S. and less (more) precipitation to the north, consistent with the observed anomalies between a year with a warm spring and a year with a cold spring in the Western U.S. The anomalous cyclone induced by the surface heating due to SUBT anomaly propagated eastward through Rossby waves in westerly mean flow. In addition, the steering flow also contributed to the dissipation of perturbation in the northeastern U.S. and its enhancement in southeastern U.S.
In the second experiment, the effects of both snow and SUBT anomalies on the Texas Drought 2011 are investigated. The winter and spring of 2010-2011 had very cold SUBT during the spring. Two initial SUBT and snow cover anomaly conditions (one cold SUBT with heavy snow cover based on 2011 observation/reanalysis and another warm SUBT) on May 1st, 2011 were assigned for the GCM runs and the corresponding RCM runs to explore their effect on the 2011 Texas drought. The anomaly SST effect has also been tested to compare with the SUBT effect. The preliminary results suggest that both SST and SUBT contribute to the 2011 drought. The SUBT effect is consistent with the discoveries in the first study.