A GCM Simulation of the Relationship between Tropical-Storm Formation and ENSO

Abstract

A low-resolution Geophysical Fluid Dynamics Laboratory (GFDL) general circulation model has been integrated for 15 years. In the course of this experiment, the observed month-to-month sea surface temperature (SST) variations in the tropical Pacific Ocean were incorporated in the lower boundary condition. The output from this model run was used to investigate the influence of El Niñ-Southern Oscillation (ENSO) events on the variability of tropical-storm formation. Criteria for detecting tropical cyclogenesis and tropical-storm formation were developed for the model. Tropical storms appearing in the model atmosphere exhibit many typhoonlike characteristics: strong cyclonic vorticity and convergence in the lower troposphere, strong anticyclonic vorticity and divergence near the tropopause, and intense precipitation. It is demonstrated that, despite its coarse resolution, the model is capable of reproducing the observed geographical distribution and seasonal variation of tropical-storm formation. The relationship between simulated tropical-storm formation and ENSO was explored using correlation statistics, composite fields for the warm and cold phases of ENSO, and individual case studies. Significant correlations were found between eastern equatorial Pacific SST anomalies and tropical-storm formation over the western North Pacific, western South Pacific, and western North Atlantic. In these areas, below-normal frequency of tropical-storm formation was simulated in warm El Niño years, whereas more tropical storms occurred in La Niña years. The correlation between tropical-storm formation and equatorial SST changes is particularly high for fluctuations on time scales of less than 3?4 years. During the boreal summer months (June?October), there exists a seesaw in the frequency of tropical-storm formation between western and central North Pacific: while more tropical storms were generated over western North Pacific during La Niña years, less tropical storms were detected over central North Pacific. The reverse situation prevails in El Niño years. Over the Indian Ocean, the relationship between storm formation and ENSO exhibits a seasonal dependence.