Heatwave Variability in the Southeast United States

This study was motivated to assess the variability of the heat wave season in the Southeast United States from a 103 year (1903-2005) long downscaled atmospheric reanalysis of 10 km grid resolution. A heat wave day was defined as a day in which the maximum hourly heat index measurement for a particular location reached a value of 90°F or more. The heatwaves were classified based on their duration as 1-day, 2-day, 3-day, 4-day, and a 5-day event.

A heat wave season is characterized by a few parameters in this study: the total number of heat wave events per year (season events), the first day a heat wave occurs in a year (season onset), the last day a heat wave occurs in a year (season demise), and the season length in a year (season demise minus season onset gives season length). We determine how the heat wave season varies year to year by examining changes in the frequency of the heatwave events in the season, seasonal onset, seasonal demise, and seasonal length. The variations of the heatwave season are compared separately with March-April-May and June-July-August seasonal anomalies of El Nino Index, the Bermuda High Index, the gradient of sea surface temperature (SST) between the Atlantic and East Pacific Ocean (Atlantic SST minus East Pacific SST).

Our results suggests that the (1 to 3-day) heatwaves are widely prevalent across southeastern US with 4-day and 5-day heatwave events limited to eastern Texas, Louisiana, southern Mississippi, southern Alabama, and southern Georgia. The earliest heatwave events occur in the southern latitudes of south Florida and southeastern Texas gradually progressing northwards along the Gulf coastline and moving northward. Similarly, the last (1 and 2-day) heatwave of the season occurs progressively earlier in the year from northern latitudes to the southern latitudes of the domain. The other (3 to 5 day) heatwave events terminate progressively from east to west in the domain.

The heatwaves in the southeastern US display significant interannual variability. The large-scale drivers of ENSO variations, Bermuda high variations, and the SST gradient between the tropical Atlantic and Pacific oceans show a varied and spatially discontinuous influence on heat waves. This suggests that in the absence of strong large-scale drivers, the heatwave seasonal variability is dictated largely by internal variations.