Contribution of equatorial Pacific winds to southern tropical Indian Ocean Rossby waves

Abstract

Westward propagating features, identified as Rossby waves, have been observed and modeled in the southern tropical Indian Ocean (STIO) between 10° and 30°S. These STIO Rossby waves, which have broad zonal and meridional extents, could interact with the westward flowing South Equatorial Current (SEC) as well as coastal currents on the south shore of Java (the South Java Current) and along the western shore of Australia (the Leeuwin Current). Previous work has attributed these waves to variations in wind stress along the west coast of Australia, Ekman pumping in the STIO interior, and a combination of both. This study investigates the importance of a third factor: remotely forced coastal Kelvin waves. Observations show that changes in wind stress curl in the eastern equatorial Pacific create annual upwelling and downwelling Rossby waves. Numerical model results confirm previous studies that demonstrate these waves, upon reaching the western boundary of the Pacific, create poleward propagating coastal Kelvin waves along the western shore of the Irian Jaya/Australia land mass. Direct observations of annual sea level variations along the northwest coast of Australia show a phase lag from the northernmost station to the southernmost that is not explained by direct wind forcing, suggesting that this signal is propagating from the Indonesian seas. It is shown in this work that when these waves reach the Indian Ocean, they are in phase with the local Ekman forcing and enhance the STIO Rossby waves. In the model the signal from the equatorial Pacific accounts for almost 80% of the energy of the STIO Rossby wave near the coast of Australia and 10% of the energy offshore.