An Investigation of the Dependence of Severe Weather Occurrence in North Carolina on the Strength and Duration of ENSO

Connections between regional severe weather and EL Nino/Southern Oscillation (ENSO) are investigated. Underlying hypothesis is that ENSO is not directly responsible for the formation of individual thunderstorms however, the formation of thunderstorms is directly related to synoptic flow patterns. Severe thunderstorms develop as a complex interaction between surface thermodynamic properties and upper-level momentum. ENSO is known to manifest height anomalies and resulting flow patterns. Thus a hypothesis of using ENSO as an index for hazard potential is tested taking North Carolina as a case example.

A series of grouping and statistical methods to delineate the connections between ENSO and severe weather in North Carolina are adopted. Monthly occurrences of tornado and wind/hail frequency are compiled from the National Severe Storms Forecast Center (NSSFC) and compared with sea surface temperature (SST) indices and anomalies in the tropical Pacific. Each month from 1950-1989 is assigned an ENSO class (La Nina, Normal, El Nino) based on prior published methodologies. The data are then grouped into 3-month seasons. Hypothesis test is that a significant difference in seasonal means between ENSO classes exists.

The statistical analyses indicate a significant increase in wind/hail events and days during the months of April, May, and June of La Nina years. Despite a series of discriminating analyses, no direct evidence is found between tornado frequency and ENSO classes. Seasonal composite anomalies in upper-level dynamics and near surface thermodynamics during La Nina springs from 1960-1989 are compiled using the NCEP/NCAR Reanalysis data set. Results show that synoptic flow patterns and thermodynamics are more favorable for the formation of convection over North Carolina. Troughing is more likely over the central portions of the U.S. resulting in favorable upper-level dynamics. Vertical wind shear, divergence aloft, and angular momentum are more favorable as a result of this pattern. The increased troughing results in increasing invasion of Canadian air resulting in more frequent and stronger cold fronts affecting North Carolina. Results also indicate anomalous surface ridging over the Gulf of Mexico and western Atlantic resulting in southwesterly flow over the southeastern U.S. The interaction of this warm moist flow with the Canadian air represents near optimal conditions for convective development in North Carolina.

A series of statistical and dynamical approaches are employed to investigate the correlation of North Carolina severe weather and ENSO. Results indicate a significant increase in the occurrence of wind/hail reports and days during La Nina springs due to increasing invasion of Canadian air interacting with warm moist air off the Gulf of Mexico.