ENSO and Western North Pacific tropical cyclone activity simulated in a CGCM

Abstract

A high-resolution (T213) coupled ocean–atmosphere general circulation model (CGCM) has been used to examine the relationship between El Nino/Southern Oscillation (ENSO) and tropical cyclone (TC) activity over the western North Pacific (WNP). The model simulates ENSO-like events similar to those observed, though the amplitude of the simulated Nino34 sea surface temperature (SST) anomaly is twice as large as observed. In El Nino (La Nina) years, the annual number of model TCs in the southeast quadrant of the WNP increases (decreases), while it decreases (increases) in the northwest quadrant. In spite of the significant difference in the mean genesis location of model TCs between El Nino and La Nina years, however, there is no significant simultaneous correlation between the annual number of model TCs over the entire WNP and model Nino34 SST anomalies. The annual number of model TCs, however, tends to decrease in the years following El Nino, relating to the development of anticyclonic circulation around the Philippine Sea in response to the SST anomalies in the central and eastern equatorial Pacific. Furthermore, it seems that the number of model TCs tends to increase in the years before El Nino. It is also shown that the number of TCs moving into the East Asia is fewer in October of El Nino years than La Nina years, related to the anomalous southward shift of mid-latitude westerlies, though no impact of ENSO on TC tracks is found in other months. It is found that model TCs have longer lifetimes due to the southeastward shift of mean TC genesis location in El Nino years than in La Nina years. As the result of longer fetch of TCs over warm SST, model TCs appear to be more intense in El Nino years. These relationships between ENSO and TC activity in the WNP are in good agreement with observational evidence, suggesting that a finer-resolution CGCM may become a powerful tool for understanding interannual variability of TC activity.