A pilot study examining US winter cyclone frequency patterns associated with three ENSO parameters

Teleconnections were used to link three El Nino-Southern Oscillation (ENSO) parameters to winter (December-February) cyclone frequencies over the United States during the 1949-96 period. Since each ENSO event is not exactly the same, small subsets of ENSO events were examined in addition to the more common composite ENSO event. Mean winter cyclone frequencies, derived by counting cyclones passing through 30, 5 degrees latitude equal-area circles located in a grid from 70 degrees to 120 degrees W and 30 degrees to 50 degrees N were determined for classes of El Ninos and La Ninas based on 1) the intensity of the equatorial Pacific sea surface temperature anomaly, 2) the intensity of the Tahiti-Darwin sea level pressure anomaly, and 3) the location of the 28 degrees C isotherm. The average cyclone count for each class of El Nino and La Nina was compared to the average count for winters when no ENSO event occurred. Expected differences in cyclone frequency patterns when comparing an average of all El Nino winters to all La Nina winters were found; however, large pattern differences were also determined when comparing winters with strong El Ninos to moderate-weak El Ninos and similarly for La Ninas. Significant differences in number of cyclones were found in 8 of 30 circles located in the Pacific Northwest, the Great Lakes, New England, and the Southeast. The differences found in the cyclone frequency patterns for El Ninos and La Ninas of different intensities and locations indicated that using a composite of air El Ninos or La Ninas may provide misleading information while examination of each of these parameters independently may assist in the preparation of long-range climate predictions.