An interannual dipole mode of midsummer persistent extreme high-temperature days between South China and Southwest China: effects of Indo-Pacific sea surface temperature

One prominent mode of interannual variability in the number of persistent extreme high-temperature days (PEHDs) in midsummer (July-August) over southern China exhibits a dipole mode between South China (SC) and Southwest China (SWC), whose underlying mechanism remains unknown. Utilizing observations, reanalysis datasets, and numerical experiments, this study shows that this dipole mode is primarily contributed by local anomalous diabatic processes and is further related to the anomalies of the western North Pacific subtropical high (WNPSH), South Asian high (SAH), and East Asian westerly jet (EAJ). Meanwhile, midsummer sea surface temperature (SST) anomalies in the North Indian Ocean (NIO) and the central Pacific-type El Ni & ntilde;o-Southern Oscillation play crucial roles, with the former making a relatively larger contribution and the latter being influenced by the Pacific meridional mode in the preceding April via wind-evaporation-SST feedback. Warmer NIO SST leads to the WNPSH extending southwestward via an eastward-propagating Kelvin wave and further causes a southward-displacing EAJ through the associated East Asia-Pacific/Pacific-Japan wave train. Additionally, it also favors an eastward-extending SAH by heating the troposphere over the NIO and its surrounding areas. A midsummer central Pacific-type La Ni & ntilde;a contributes to a southwestward-extending WNPSH through a Gill response and influences the vertical velocity anomalies over SC and SWC by modulating the Walker circulation and associated local meridional circulations. Consequently, warmer NIO SST and a central Pacific-type La Ni & ntilde;a lead to an increase (decrease) in PEHDs over SC (SWC) during midsummer, and vice versa.