Spring Soil Temperature Anomalies in the Western U.S. and Summer Droughts in the Southern Plains

The Southern Plains (SP) are one of the most drought-prone regions in the United States. In the summer of 2011, Texas and parts of several surrounding states suffered a drought nearly as severe as some of the world's previous worst. A number of observational and modeling studies have shown strong sensitivity of rainfall deficits in the Southern Plains to strong La Niņa conditions consisting of a cold eastern Pacific Ocean. This study presents the results showing the close relationship between land surface temperature anomaly in Western U.S. and precipitation deficit in SP, such as 2011 drought. Investigations based on observational data have established a close relationship between spring land surface temperature (LST) and snowfall/snow accumulation in the Western U.S. We further study the observed LST and precipitation relationship and found that the conditions with heavy snow and cold LST in the Western U.S. during the spring have a high probability of being associated with drier conditions in the Southern U.S. In particular, the correlation between May LST in the Western U.S. and June precipitation in the Southern Plains for the 19 years when absolute anomalies over the Western U.S. are larger than 0.5°C since 1980 is 0.53, with statistical significant at 0.95 level with two tailed-test.

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.