A Study of the Relationship between Mid-Tropospheric Large-scale Circulation Pattern and US Landfall Hurricane Activities

In the past two decades there have been several attempts (Elsner 2003, Klotzbach and Gray 2004, and Saunders and Lea 2005) to predict seasonal US landfall hurricane activities for upcoming hurricane season in the Atlantic basin. The general approach is to statistically link the landfall hurricane activities of next season to various climate indices such as SOI, NAOI, AO, and QBO, or to wind anomalies in certain regions. The forecast skill of such an approach needs further evaluation. Furthermore, the physical mechanisms behind current statistical forecast models call for more detailed investigation. This current research examines how US landfall hurricane activities vary with mid-tropopheric large-scale circulation pattern over the North American Continent and the Atlantic Basins. The primary datasets used are the NCEP/NCAR reanalysis data and NHC's North Atlantic hurricane database. For the purpose of this study, US landfall hurricanes since 1950 were divided into two groups based on their landfall locations. The first group includes hurricanes making landfall along Gulf coast and Florida (or west of 80oW and south of 31oN), and the second group has hurricanes making landfall from Georgia to Maine (or east of 80oW and north of 31oN). Composites of August-October 500mb geopotential height anomalies on decadal and annual scales were then created using the Climate Diagnostic Center's “Monthly/Seasonal Climate Composites” Web page (http://www.cdc.noaa.gov/cgi-bin/Composites/printpage.pl). The reason for choosing 500mb pressure level is the optimum steering level for hurricane-strength tropical cyclones according to Velden (1993). Examination of the composites shows that hurricanes in the second group happened in years when there was an anomaly height from mid-to-west US Continent and anomaly low along US East Coast. On the other hand hurricanes in the first group happened in years when there was no distinct closed center of either anomaly height or low over US Continent. Analysis of composite plots clearly indicates the persistence of the large-circulation circulation, for example, in the decade of 1951-1960. The finding in this study is consistent with Namias's study back in 1955. Several theories are proposed to explain the persistence of general circulation anomalies.