Mechanisms for El Nino and La Nina induced anomalies in tropical cyclone formation, intensity, and motion in the northwest Pacific

We have used atmospheric and oceanic reanalysis fields to analyze the impacts of El Nino and La Nina events on tropical cyclone (TC) formation, intensity, and motion in the northwest Pacific during the northern summer and fall. Our study period is June-December of 1970-2003. El Nino and La Nina periods were identified using the Multivariate ENSO Index. Our focus was on June-December periods at the beginning of El Nino and La Nina events. A variety of statistical methods, dynamical diagnostic tools, and atmospheric model experiments were used to characterize the atmospheric and oceanic anomalies during El Nino and La Nina events. The dominant feature of the El Nino and La Nina composite anomaly patterns is an equatorial Rossby-Kelvin wave in the upper and lower troposphere that develops in response to the El Nino and La Nina atmospheric heating anomalies. This response develops early in the northern summer and intensifies into the northern winter.

The Rossby-Kelvin wave response plays a large role in inducing major anomalies in the large scale environmental factors that affect tropical cyclone formation, intensity, and motion, including upper ocean thermal energy, tropospheric humidity, instability, vorticity, vertical wind shear, middle and upper tropospheric steering flow, and extratropical long waves. The impacts of these factor anomalies on TCs are indicated by pronounced El Nino - La Nina differences in TC formation, intensity, and motion, especially in the late summer and fall as the Rossby-Kelvin wave response intensifies, and when tropical cyclone activity is still relatively strong. The major formation and motion differences can be directly related to the sign, intensity, and position of the Rossby-Kelvin wave response. Our results indicate that intraseasonal to seasonal forecasts of TC activity may be improved by increased attention to the monitoring and forecasting of the Rossby-Kelvin response, especially the upper tropospheric component of this response.