Past, present and future of Tropical Cyclone climatology over CORDEX South-Asia domain: A coupled regional climate model study

Abstract:

Tropical cyclones (TC) in the Indian Ocean arise from cyclonic disturbances that naturally form in the inter-tropical convergence zone between 5N and 20N latitudes when sea surface temperatures (SST) are at 27C or above. This relationship between SST and TC formation and intensification implies that rising SST in the climate change era will increase the probability of tropical cyclone formation and that therefore as global temperatures rise so will the frequency and intensity of tropical cyclones. However horizontal resolution of the global general circulation models used in IPCC simulations are usually too coarse to reproduce many smaller scale processes like TC.

We present a novel approach to downscale climate change scenarios, which could be also used to investigate the generation and propagation of tropical cyclones over the northern Indian Ocean. The REgional atmosphere MOdel REMO is coupled to the global ocean - sea ice model MPIOM with increased resolution over the Indian Ocean (~20km). The resulting coupled system called ROM. The coupled domain is standard CORDEX south Asia domain (0.44 deg). The models are coupled via the OASIS coupler. Exchange between ocean and atmosphere was made every three hours. Lateral atmospheric and upper oceanic boundary conditions outside the coupled domain were prescribed using ERA40, MPIESM-LR historical and MPIESM-LR RCP85 data.

Here we present the results of the dynamical downscaling approach both coupled (ROM) and uncoupled (REMO) simulations for the same period. The validation period (ERA40 and Historical) is 1958-2001. While comparing with observations mean (1958-2001) annual frequency of 8 TC/year, ROM simulated with ERA40 (Historical) TCs are 12 (10), whereas with REMO they are 21 (18). Therefore ROM simulated TC frequency is much more realistic than REMO. REMO has simulated two times more TCs than ROM, hence leading to strong overestimation of TC tracks densities over the region. Scenario simulations (RCP85) showed that compared to 20th century frequency of tropical cyclones will be increased at the end of the 21st century both in coupled and uncoupled simulation. There is quite pronounced change in the TCs tracks. The frequency of landfalls increases.