Intraseasonal Kelvin waves and their relation with El Nino development

Previous studies suggest that the unusually rapid development of 1997-98 El Nino could be attributed to at least two Madden-Julian Oscillation (MJO) events in winter 1996-97, which passed eastward across the tropical Indian and western Pacific Oceans prior to the onset. They generated oceanic equatorial Kelvin waves and may have initiated the eastward displacement of the warm pool. However, strong MJO activity is not necessarily associated with the subsequent development of El Nino. One possible reason is that ocean response to the MJO may depend on other factors such as the basic state of the upper ocean.

Present study investigates the influence of upper ocean basic state on intraseasonal Kelvin waves produced by the MJO. The tropical Indo-Pacific basin model was first integrated with daily surface forcing fields during 1981-2002. Then the model was integrated with same intraseasonal forcings with different basic states. These model experiments indicate that generation and propagation of intraseasonal Kelvin waves are sensitive to the upper ocean basic state which varies interannually. Previous observational studies also show the systematic decrease in phase speed of Kelvin waves during the development of some El Nino events. The mechanism by which the phase speed of Kelvin wave changes during these periods is further discussed based on a series of model experiments.