What caused the spring intensification and winter demise of the 2011 drought over Texas?

The 2011 Texas drought, the worst 1-year drought on record, was characterized by spring intensification of rainfall deficit and surface dryness. Such spring intensification was led by an unusually strong increase of convective inhibition (CIN), which suppressed convection at the time critical for the onset of the April-June rainfall season. The CIN increase appeared to be caused by strong sub-seasonal anomalously westerly winds at 850 hPa (U850) in April, in addition to surface dryness due to cumulative rainfall deficit since fall of 2010. The anomalous U850 advected warm dry air from the Mexican Plateau to Texas, enhancing cap inversion, and exacerbating static stability initially elevated by an anomalously high surface Bowen ratio due to rainfall deficits from winter through spring over Texas. Strengthened westerly U850 in April, in addition to the persistent rainfall deficits from winter through spring, are common characteristics in other strong drought events experienced over Texas. Atmospheric Model Intercomparison Project-type simulations with prescribed La Nia SSTAs in the tropical Pacific do not show a strengthening of westerly U850 in April, suggesting that internal atmospheric variability at intraseasonal scale, instead of La Nia, may initiate the spring drought intensification over Texas. Soil moisture deficits in late spring are significantly correlated with positive 500 hPa geopotential height anomalies over the south central U.S. 2-3 weeks later, suggesting that intensified surface dryness in late-spring could reinforce the drought-inducing anomalous mid-tropospheric high. The drought diminished in the winter of 2011/2012 despite a second La Nia event. Our analysis suggests an important role for strong westerly wind anomalies, the resultant increase of CIN in spring, and subsequent positive feedback between dry surface anomalies and the anomalous large-scale circulation pattern in drought intensification. Clarification of the mechanisms behind the strong increase of CIN and land-atmosphere feedbacks may provide a key for improving our understanding of drought predictability in spring and summer, and a scientific basis for the early warning of strong summer drought. The demise of the 2011 drought appears to have resulted from internal atmospheric circulation variability, thus intrinsically unpredictable.