Revisiting Trends in the North Atlantic Subtropical High for the Southeast US Summer Climate and Climate Change

We investigated trends in the summer position of the North Atlantic Subtropical High (NASH) western ridge using two reanalysis datasets: NCEP-NCAR Reanalysis (NNRP; 1948-2014) and 20^th Century Reanalysis (1851-2014) data.  Based on 1948-2007 NNRP data, the climatology of the NASH westward ridge position is at 86°W.  This study revisits the observational record that highlighted a significant observed westward shift in the NASH from 1948 through 2007 (1.22° per decade) and the potential that anthropogenic warming is contributing to the westward shift.  Such a shift in the NASH has important implications for the Southeast US summer climate now and into the future.  By calculating the trend using seven additional years through 2014, the westward trend in the NASH is no longer significant (new westward trend of 0.5° per decade).  We hypothesize that the decrease in the significance of the trend is associated with the phase of the Atlantic Multidecadal Oscillation (AMO).  Our results illustrate that the NASH generally shifts westward with cooler North Atlantic SST anomalies and vice versa.  Presently, the North Atlantic SSTs are in a positive phase of the AMO (warm North Atlantic SSTs) and contributing to the changes in the NASH trend.  However, we have found more exceptions in the AMO (SST)-NASH (westward location) relationship since 1998, with land being more anomalously warmer than the ocean for those years.  Our analysis suggests that the impact of anthropogenic warming on the NASH westward placement is emerging, and the natural relationship of the NASH with the AMO could also be changing.