Recent summer rainfall increase and surface cooling over Northern Australia since the late 1970s: A response to warming in the tropical Western Pacific

Rainfall over northern Australia (NA) in austral summer is the largest water source of Australia. Previous studies have suggested a strong zonal-dipole trend pattern in austral summer rainfall since 1950, with rainfall increasing in northwest Australia (NWA), but decreasing in northeast Australia (NEA). The dynamics of rainfall increase in NWA was linked to sea surface temperature (SST) in South Indian Ocean and the rainfall decrease in NEA was associated with SST in the Wharton Basin, located in northeast Indian Ocean. This study reports that, in contrast to a zonal-dipole trend pattern, a dominant wetting pattern over NA has recently been observed since the post-1979 satellite era. Recent NA rainfall increase also manifests as the first leading mode of summer rainfall variability over the Australian continent. Further investigation reveals that SST in the Tropical Western Pacific (TWP) has replaced the SST in the South Indian Ocean and Wharton Basin as the controlling factor responsible for recent NA rainfall increase. Direct thermal forcing by increasing TWP SST gives rise to an anomalous Gill-type cyclone centered around NA, leading to anomalously high rainfall. As such, the increasing SST in the TWP induces over 50% of the observed rainfall wetting trend over NA. The increased rainfall in turn induces land surface cooling in NA. This mechanism can be confirmed with results obtained from sensitivity experiments of a numerical spectral atmospheric general circulation model. Thus, increasing SST in the TWP has contributed much of the recent summer rainfall increase, and consequently, the surface cooling over NA.