Assessment of climatological tropical cyclone activity over the north Indian Ocean in the CORDEX-South Asia regional climate models

Abstract

This study evaluates the ability of the regional climate models (RCMs) in the Coordinated Regional Climate Downscaling Experiment for South Asia (CORDEX-SA) to simulate the tropical cyclone activity in the north Indian Ocean. The RCMs used in the CORDEX-SA are Rossby Centre regional atmospheric model version 4 (RCA4) and Regional Climatic Model version 4 (RegCM4) forced by the ECMWF Interim Reanalysis (ERA-Interim) from 1989 to 2008. Both models have reasonable skill in capturing the observed climatological seasonal and spatial distribution of the genesis and track of the tropical cyclones. However, the two issues faced are (i) in RegCM4 model, higher number of tropical cyclones throughout the year compared to observations in addition to the unrealistic simulation of tropical cyclones during winter and (ii) in the RCA4 model, relatively higher tropical cyclone activity over the Arabian Sea compared to the Bay of Bengal. In the RCA4, the anomalous large-scale lower tropospheric cyclonic circulation observed over the southern Arabian Sea might be responsible for the overestimation in the number of tropical cyclones over there. In both models, the relationship between minimum pressure and the maximum wind is well simulated even though there is an underestimation in the intensity of the tropical cyclones. Quantitative analysis of large-scale environmental parameters that influence the tropical cyclone activity using an index called genesis potential index showed that in the RegCM4 model, the model simulates seasonal variation (enhancement/suppression) of environmental conditions fairly good as seen in the observation. However, the model has higher number of tropical cyclones in all the seasons, which is due to the enhanced large scale dynamical and thermodynamical parameters. The cold and wet bias in the middle troposphere causes the reduction in the mid-layer saturation deficit throughout the year, which in turn favours an unrealistic high number of tropical cyclone genesis. Therefore, the changes in the large scale dynamics and relatively saturated middle troposphere in the model might be responsible for the higher number of tropical cyclones.