Interannual Variability of the Australian Summer Monsoon System Internally Sustained Through Wind‐Evaporation Feedback

Interannual variability of Australian summer monsoon (AUSM) activity is hardly forced locally or remotely by tropical sea surface temperature (SST) variability. Despite this lack of SST forcing, convection variability in northern Australia is so strong that it emerges as a distinct peak in climatological variance of convection in austral summer. The present study shows that an internal variability unforced by tropical SST anomaly is dominant in seasonal mean strength of the AUSM system. A mechanism that sustains convection anomaly without SST forcing is also examined. Analysis of latent heat flux reveals that under the climatological monsoon westerlies, the wind‐evaporation feedback in the tropical southeastern Indian Ocean sustains anomalous convection despite a counteracting effect of SST anomalies. The wind anomalies induced by the anomalous AUSM change the subsurface southeastern Indian Ocean, which can contribute to the maintenance of the anomalous convection through weakening the damping effect by SST anomalies. Plain Language Summary Over most of the tropical oceans, deviations of sea surface temperature from its mean seasonal cycle drive anomalous states of atmospheric convection and rainfall. Due to slow nature of ocean variations, this relationship provides an important origin of weather predictions beyond a few weeks. However, this relationship does not always hold depending on regions and seasons. We find that year‐to‐year variation of the Australian summer monsoon is the most pronounced of those unforced by tropical ocean variability in austral summer. We also investigate why the summertime mean anomalous strength of the monsoon can be sustained without ocean forcing. Instead of ocean forcing, an anomalous state of the Australian summer monsoon is sustained through a positive feedback between anomalous surface wind and evaporation over the tropical southeastern Indian Ocean. Besides changes in ocean downwelling in response to the surface wind anomalies suppress sea surface temperature changes, which would otherwise spoil the wind‐evaporation feedback. Key Points Interannual convection variability in summer northern Australia is unforced by tropical ocean variability but forms a distinct variance peak This internal variability of the Australian summer monsoon is maintained through a wind‐evaporation feedback over surrounding oceans Sea surface temperature anomalies act to damp the convection anomalies, but dynamical air‐sea interaction weakens