Increased Frequency of Summer Extreme Heat Waves over Texas Area Tied to the Amplification of Pacific Zonal SST Gradient

Summer extreme heat waves (EHWs) over the Texas area and their trend are investigated using observations and atmospheric general circulation model (AGCM) output. There is a positive linear trend in Texas EHW days for the period 1979-2015. While the interannual variability of the Texas EHWs is linked to ENSO conditions, the upward trend in Texas EHWs is found to be significantly associated with the tropical Pacific zonal SST gradient (PZSSTG). The amplification of PZSSTG leads to both enhanced convection in the western Pacific and suppressed convection in the central-eastern Pacific (i.e., La Nina-like pattern), both of which can induce anomalous anticyclones over the Texas area through two distinct planetary wave trains in the antecedent spring. As a result, anomalously sinking motions and divergent water vapor flux appear over the Texas area, which reduce precipitation and increase downward solar radiation, leading to dry and hot soil that favors the occurrence of Texas summer EHWs. In addition, all AGCMs using observed SSTs as boundary conditions were able to simulate the observed decreasing trend in Texas summer precipitation and the observed increasing trend in Texas summer surface air temperature. The observed relationships between winter PZSSTG and the following spring-summer Texas precipitation/temperature were also reproduced by these models, where the intensified PZSSTG tended to reduce the Texas precipitation while increasing the surface air temperature.