Some potential forcing mechanisms of the year-to-year variability of the tropical convection and its intraseasonal (25–70-day) variability

Abstract

ABSTRACTThree potential forcing mechanisms for the convective signal of the Madden–Julian Oscillation (MJO) are investigated.Outgoing longwave radiation (OLR) is used to indicate convection, and data from 1974 to 1994 are subjected to a 25–70-daybandpass filter to represent the MJO. The focus of the paper is on interannual variability and the three mechanisms diagnosedare: tropical sea-surface temperature (SST); the phase of El Nin˜o–Southern Oscillation (ENSO); and atmospheric precipitablewater (W). The respective roles of these potential forcings are also considered for the year-to-year variability of the seasonalanomalies of mean convection.Observed seasonal anomalies of both mean convection and MJO activity over the equatorial Pacific from 160  E to thecoast of South America are highly correlated to collocated anomalies of SST and the phase of ENSO. During El Nin˜o wintersthe eastward-propagating convective MJO events penetrate farther than normal into the central Pacific before curvingpoleward into the Southern Hemisphere. Over the maritime continent, moist convection is significantly suppressed during ElNin˜o. A corresponding signal in the MJO activity is confined to the Philippine Sea and North Australia during boreal winter,and to the Bay of Bengal in autumn. In contrast, there is no uniform MJO response over the eastern Indian Ocean when thelatest five ENSO warm events (1976–1977, 1982–1983, 1986–1987, 1991–1992 and 1993–1994) are considered.On a seasonal time-scale, W over the Indian and Pacific Oceans is closely related to convection anomalies, as well as tothe variance of the high-frequency convective perturbations. In contrast, convective activity of the MJO shows no relation tovariations in the amount of W. It is concluded that the interannual variability of the convective MJO signal over the IndianOcean and western Pacific Warm Pool is largely determined by the frequency and duration of ‘weak-signal’ episodes ratherthan by modulations of its amplitude. # 1997 Royal Meteorological Society. Int. J. Climatol., Vol. 17, 1513–1534 (1997)(No. of Figures: 12. No. of Tables: 2. No. of