Response of west pacific subtropical high to northern hemispheric warming: insights from paleo climate models

The present study investigates the response of west Pacific subtropical high (WPSH) to northern hemispheric warming due to natural and anthropogenic forcings. We carried out the analysis using a suite of climate model simulations for the mid-Holocene conditions based on the Paleoclimate Modeling Inter-comparison Project-3 (PMIP3) and quadrupled CO₂ simulations based on the Climate Model Intercomparison Project-5 (CMIP5), representatives of natural and anthropogenic forcings. Both sets of simulations depict northern hemispheric warming and westward shift of WPSH during boreal summer. We found that the northern hemispheric warming increases the seasonal precipitation over Africa, central India and central-eastern Himalayas in the mid-Holocene and quadrupled CO₂ climate, by causing northward shift of the ITCZ and enhanced moisture flux convergence. Enhanced condensational heating associated with precipitation increase over central India and eastern Himalaya, in both the climates, not only reinforces the South Asian High (SAH) in the upper-troposphere but also strengthens the WPSH and promotes its westward extension. Additionally, mid-tropospheric condensational heating over parts of northern Africa modulates the near-equatorial east–west circulation, induces anti-cyclonic circulation over equatorial Indian Ocean and Maritime continent and fosters westward extension of WPSH during boreal summer in both the climate. In short, precipitation changes induced by northern hemispheric warming modulates equatorial and subtropical east–west circulations and play a vital role in the longitudinal variations of WPSH under northern hemispheric warming climate. In a climate with strong northern hemispheric surface warming, we also noted a minimal role of warm pool SST in the westward extension of WPSH.