Enhanced activity of West North Pacific typhoons in response to anthropogenically-forced weakening of the Indian summer monsoon

The West North Pacific (WNP) is a breeding ground for tropical cyclones (TC) all through the year with peak activity during August-September, in contrast to the North Indian Ocean where tropical cyclones (TC) are observed mainly during April-June and October-December but suppressed in July-August due to the strong vertical wind shear of the Indian summer monsoon (ISM). Interannual variations in the WNP tropical cyclones (typhoons) are known to be closely linked to El Nino - Southern Oscillation (ENSO) with higher number of typhoons forming during El Nino phases as compared to La Nina; however the response of WNP typhoon to human-induced climate change is not well understood. Here, we compare two sets of high-resolution climate model simulations for the historical period 1951-2005 viz., HIST [includes both natural (solar irradiance, volcanic aerosols) and anthropogenic forcing (greenhouse gasses (GHG), aerosols and land use land cover changes)] and HISTNAT (natural-only forcing) to understand the influence of climate change on the ISM and WNP typhoon activity. It is found that an anthropogenically-forced weakening of the ISM circulation, largely due to the influence of Northern Hemispheric aerosols which oppose the GHG forcing, tends to significantly enhance the genesis potential of WNP typhoons. Our results suggest that a forced weakening of the ISM circulation reorients the large-scale flow over the tropical Indo-Pacific in a manner that is conducive for enhanced WNP typhoon genesis through enrichment of low-level relative vorticity, while the intensified mid-latitude westerlies over the Far East and North Pacific region steer the WNP typhoons to move northward with recurving tracks. Additionally the area of extremely low sea-level pressure (SLP) around Taiwan and mainland China is substantially larger in the anthropogenically- forced simulation as compared to the natural run.