High- and Low-Frequency Intraseasonal Variance of OLR on Annual and ENSO Timescales

Abstract

Using 20 yr of outgoing longwave radiation observations, the complex behavior of the higher- (6?25-day) and lower- (25?70-day) frequency bands of tropical intraseasonal convective oscillations is investigated. Emphasis is given to the mean annual cycle and interannual variability of both bands and to the interaction between the two bands. The focus with regard to the interannual variability within each band is on the warm and cold events associated with the El Niño?Southern Oscillation (ENSO) cycle. The study encompasses the tropical and subtropical Indian and Pacific Oceans (including Australasia). The strongest intraseasonal signals are, for the most part, aligned with the intertropical convergence zone (ITCZ) and South Pacific convergence zone. In some cases, the 6?25-day signal is not collocated with the Madden?Julian oscillation (MJO) signal and/or occurs remotely from the ITCZ. In these cases, the higher-frequency intraseasonal convective perturbations are associated with phenomena independent from the MJO, such as easterly waves, monsoon depressions, typhoons, or circulations involved in tropical?extratropical interactions. Over the equatorial eastern Indian Ocean, strong activity in both bands persists throughout the year, but the bands are found to be anticorrelated, regardless of the ENSO phase. The effect of ENSO timescales is further examined by looking at December?February anomalies for five El Niño and two La Niña events during this 20-yr sample. A well-defined response of the two bands is restricted to the northwestern and central Pacific. Over the northwestern Pacific Ocean, the two bands complement one another with suppressed (enhanced) convection occurring during El Niño (La Niña) events. Both bands also complement each other over the equatorial central Pacific but are out-of-phase with those in the western Pacific on ENSO timescales. In contrast, over the Australian monsoon region and the eastern Indian Ocean, neither band shows a uniform response in terms of anomalous activity when the latest five ENSO warm events, 1977?78, 1982?83, 1986?87, 1991?92, and 1992?93, are considered.