Australian Monsoon Variability Driven by a Gill-Matsuno-Type Response to Central West Pacific Warming

The objective of this study is to investigate the mechanisms that cause the anomalous intensification of tropical Australian rainfall at the height of the monsoon during El Nino Modoki events. In such events, northwestern Australia tends to be wetter in January and February when the SST warming is displaced to the central west Pacific, the opposite response to that associated with a traditional El Nino. In addition, during the bounding months, that is, December and March, there is below-average rainfall induced by an anomalous Walker circulation. This behavior tends to narrow and intensify the annual rainfall cycle over northwestern Australia relative to the climatology, causing a delayed monsoonal onset and an earlier retreat over the region. Observational datasets and numerical experiments with a general circulation model are used to examine the atmospheric response to the central west Pacific SST warming. It is shown here that the increase of precipitation, particularly in February, is phased locked to the seasonal cycle when the intertropical convergence zone is displaced southward and the South Pacific convergence zone is strengthened. An interaction between the interannual SST variability associated with El Nino Modoki events and the evolution of the seasonal cycle intensifies deep convection in the central west Pacific, driving a Gill-Matsuno-type response to the diabatic heating. The westward-propagating disturbance associated with the Gill-Matsuno mechanism generates an anomalous cyclonic circulation over northwestern Australia, leading to convergence of moisture and increased precipitation. The Gill-Matsuno-type response overwhelms the subsidence of the anomalous Walker circulation associated with Modoki events over Australia during the peak of the monsoon.