Impacts of the MJO in the Indian Ocean and on the Western Australian coast

The large sea surface temperature variations induced by the Madden-Julian Oscillation (MJO) on the northwest shelf of Australia and the remote influence of the MJO on the subtropical Western Australian coast are explored using the POAMA Ensemble Ocean Data Assimilation System reanalyses (PEODAS) for the period 1980–2010. The focus here is during the November–April extended summer season when the impacts of the MJO on and along the west coast of Australia are greatest. The MJO is well known to force equatorial Kelvin and Rossby waves in the Indian Ocean, and these are well depicted in the PEODAS reanalyses. When the downwelling Kelvin waves (forced by the westerly-convective phase of the MJO) reach the Indonesian region at the eastern boundary of the Indian Ocean, a coastally trapped Kelvin wave appears to propagate southeast along the Indonesian coastline. At the same time, the suppressed convection/easterly phase of the MJO arrives in the eastern Indian Ocean, with increased heat flux into the ocean due to reduced latent heat flux and increased insolation. The coastally trapped Kelvin waves do not appear to get onto the Western Australian coast. Rather, the increased heat flux and Ekman-induced downwelling onto the northwest (NW) coast in the suppressed/easterly phase of the MJO drive an increase in sea surface temperature on the NW Australian shelf. The piling up of warm water and associated sea level rise on the NW shelf is then communicated down the Western Australian coast as a coastally trapped wave, resulting in an increase in the Leeuwin current. Thus we conclude that the MJO signal in sea level along the west coast of Australia does not result from transmission of equatorial waves onto the Western Australian coast, but rather a southward-propagating coastal trapped wave that is directly forced on the NW shelf through Ekman-induced vertical advection and surface heat fluxes in the easterly phase of the MJO. Additionally, subtropical coastal sea level variability is reinforced locally via a teleconnection of the MJO to the local meridional wind off the southwest Australian coast. Considering the capability to predict the MJO to about 4 weeks lead time plus the 2 weeks taken for the MJO signal on the NW shelf to influence sea level at Fremantle, the use of MJO forecasts in management of the Western Australian marine environment should be considered for future application.