An integrative analysis of drought in the Southeastern United States

Droughts over the SE US have been poorly understood due to its humid coastal environment through the year and its complicate weather and climate systems. We diagnosed the SE US droughts and pluvials and investigated their driving mechanisms using a hierarchical approach based on land-atmosphere and atmosphere-ocean interactions. Data from the North American Regional Reanalysis (NARR) are used as observational estimates. The six driest and wettest years during the study period (1979-2007) were selected based on summertime (JJA) and wintertime (DJF) precipitation and composited to understand land-atmosphere-ocean conditions during drought and pluvial years for both seasons. Typical conditions for the SE US droughts (pluvials) are identified as follows: 1) weaker (stronger) southerly meridional fluxes and weaker (stronger) westerly zonal fluxes, 2) strong moisture flux divergence (convergence) by transient eddies and 3) strong (weak) coupling between the land surface and atmosphere. Our results demonstrated the summertime SE droughts are driven from the shift and persistence of the North Atlantic Sub-tropical High (NASH) westwards and are associated with the gradient in SSTs between the North Atlantic and Northwest Pacific, with the wintertime droughts associated with the cold phase over the Northwest Pacific. Finally, our study evaluated the development of conditions for the recent multi-year drought (2005-2009) over the SE US. The analysis showed that the ongoing drought conditions are led by a combination of wintertime La-Niña conditions, a persistent warm season SST gradient between the North Atlantic and Pacific during the period, dormant TC activity and a strong NASH during summers. Also, strong coupling between land surface and atmosphere during summers prolongs the onset of the drought.