Recent Summer Rainfall Increase and Surface Cooling over Northern Australia since the Late 1970s: A Response to Warming in the Tropical Western Pacific

Abstract

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 the south Indian Ocean and the rainfall decrease in NEA was associated with SST in the 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 in the post-1979 satellite era. The 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 and northeast Indian Ocean as the controlling factor responsible for the 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.

Historically hottest summers projected to be the norm for more than half of the world’s population within 20 years

“…For the sub-period of 1979-2010, Li et al (2013) even report a cooling trend over the region and suggest that the cooling is due to the increase in North Australian summer rainfall and surface evaporation. The reason for the positive trend in precipitation is still unclear, although Li et al (2013) suggest that it is linked to an anomalous Gill-type cyclone response to increasing sea surface temperatures in the tropical Western Pacific. While such variability is present in observations and thus mask the response to ALL forcing, ensemble averaging would have largely removed the influence of internal variability in the ensemble mean of ALL simulations, leaving essentially only the forced response.…”

Interdecadal change in the lagged relationship between the Pacific–South American pattern and ENSO

“…For the correlation between the monthly data of two variables X and Y, the effective sample size N * was estimated using the modified Chelton method (Pyper and Peterman 1998;Li et al 2012Li et al , 2013. N * can be obtained from the theoretical approximation:…”

Influence of the North Pacific Victoria mode on the Pacific ITCZ summer precipitation

“…We estimate the significance of all correlations between two time series taking into account the effect of their autocorrelations. For two time series of variables X and Y, the effective sample size N* was estimated using the modified Chelton method [Pyper and Peterman, 1998;Li et al, 2013]. The N* can be obtained from the theoretical approximation…”