Atlantic Seasonal Hurricane Frequency. Part I: El Niño and 30 mb Quasi-Biennial Oscillation Influences

This is the first of two papers on Atlantic seasonal hurricane frequency. In this paper, seasonal hurricane frequency as related to E1 Niño events during 1900–82 and to the equatorial Quasi-Biennial Oscillation (QBO) of stratospheric zonal wind from 1950 to 1982 is discussed. It is shown that a substantial negative correlation is typically present between the seasonal number of hurricanes, hurricane days and tropical storms, and moderate or strong (15 cases) El Niñ off the South American west coast. A similar negative anomaly in hurricane activity occurs when equatorial winds at 30 mb are from an easterly direction and/or are becoming more easterly with time during the hurricane season. This association of Atlantic hurricane activity with El Niño can also be made with the Southern Oscillation Index. By contrast, seasonal hurricane frequency is slightly above normal in non-El Niño years and substantially above normal when equatorial stratospheric winds blow from a westerly direction and/or are becoming more westerly with time during the storm season. El Niño events are shown to be related to an anomalous increase in upper tropospheric westerly winds over the Caribbean basin and the equatorial Atlantic. Such anomalous westerly winds inhibit tropical cyclone activity by increasing tropospheric vertical wind shear and giving rise to a regional upper-level environment which is less anticyclonic and consequently less conductive to cyclone development and maintenance. The seasonal frequency of hurricane activity in storm basis elsewhere is much less affected by El Niño events and the QBO. Seasonal hurricane frequency in the Atlantic and the stratospheric QBO is hypothesized to be associated with the trade-wind nature of Atlantic cyclone formation. Tropical cyclone formation in the other storm basins is primarily associated with monsoon trough conditions which are absent in the Atlantic. Quasi-Biennial Oscillation-induced influences do not positively enhance monsoon trough region vorticity fields as they apparently do with cyclone formations within the trade winds. Part II discusses the utilization of the information in this paper for the development of a forecast scheme for seasonal hurricane activity variations.