Developing a Climatology of the South's 'Other' Storm Season: ENSO Impacts on Winter Extratropical Cyclogenesis

Article excerpt

An investigation of extratropical storms in the southeastern U.S. and adjacent waters of the Gulf of Mexico and Atlantic Ocean over the winter half-year (November to April) from 1961 to 1998 reveals a March peak in observations. During El Nino events the peak in population shifts to February, and a large number of intense storms are observed. Also, during El Nino events there are three hot spots of equally favored extratropical cyclogenesis east of the Rockies--near the Oklahoma panhandle, the western coast of the Gulf of Mexico, and off the coast of North Carolina. During La Nina and Neutral winters, cyclogenesis primarily occurs in just the first geographical region. No significant trend in storm observations was found in the 38-yr record.

KEY WORDS: Extratropical storm, sea level pressure, cyclogenesis, ENSO

INTRODUCTION

One only need look at the 2004 and 2005 hurricane seasons to realize the dramatic impacts weather related disasters have on the southeastern U.S. (Bossak 2004; Weunsch, Ast, and Curtis 2004). Tropical storms and hurricanes made up 32% of the billion-dollar weather disasters in the U.S. from 1980 to 2004 (NCDC 2005). Not surprisingly, the climatology of Atlantic tropical storms has been studied extensively (e.g., Landsea 1993; Maloney and Hartmann 2000; Muller and Stone 2001; Elsner 2003; Kimball and Mulekar 2004). Furthermore, the NASA satellite, Tropical Rainfall Measuring Mission (TRMM, Kummerow et al. 2000) has offered exciting new insights over the past eight years into the intensification of storms, like Hurricane Isabel in 2003, while they were well off-shore.

To a lesser extent, the southeastern U.S. is impacted by extratropical cyclones in winter. Extratropical storms can be severe along the eastern seaboard where they are sometimes referred to as nor'easters because of the prevailing wind direction (Northeast) on the western edge of the storm. The result can be snow, ice, or occasionally blizzard conditions with winds gusting to hurricane force. Since 1980, only one extratropical storm, a nor'easter referred to as the "Storm of the Century" or "March 1993 Superstorm," is listed as a billion-dollar weather disaster by the National Climatic Data Center (NCDC). This cyclone tracked from Texas, across the Gulf of Mexico, where it began taking on tropical characteristics, and then up the East Coast (Kocin et al. 1995; Huo et al. 1995). The minimum sea level pressure dropped from 998.5 hPa to 968.3 hPa (a change of 30.2 hPa) in 24 hr, making it what is referred to as a "bomb cyclone," or a rapidly intensifying storm (Sanders and Gyakum 1980). Overall, $56 billion in damage and 270 deaths are attributed to the Storm of the Century (Lott 1993).

There is some debate as to whether storms like these will be more frequent in a potentially warmer future world. Zhang, Douglas, and Leatherman (2000) did not find long term trends in storm severity on the East Coast during the past century. However, the latest report of the Intergovernmental Panel on Climate Change (Houghton 2001) stated that extratropical cyclone activity in the Northern Hemisphere has increased since the mid-twentieth century, but that more studies are needed.

In light of this call for research, recent advances have been made in constructing climatologies of extratropical storms and nor'easters (e.g., Hirsch, DeGaetano, and Colucci 2001; Bradbury, Keim, and Wake 2003), but much of this work has focused on the eastern and northeastern U.S. However, the Gulf of Mexico and Gulf Stream (Whittaker and Horn 1981) can be thought of as a breeding ground for extratropical cyclones, which in turn affect the southeastern U.S. Davis, Dolan, and Demme (1993) found that the most dangerous nor'easters formed either over Florida or north of Cuba between October and April.

Businger, Knapp, and Watson (1990) compiled one of the first climatologies (1960-1983) of cyclones tracking across the Gulf of Mexico and their related precipitation totals. …