Interactions between the Boreal Summer Intraseasonal Oscillation and Higher-Frequency Tropical Wave Activity

Abstract

Interactions between the convection and circulation fields of the boreal summer intraseasonal oscillation (ISO) and two types of higher-frequency tropical wave activity are examined through a statistical analysis of 22 yr of data. During the convectively active phase of the ISO, westward-propagating mixed Rossby?gravity (MRG)?tropical depression (TD)-type wave activity is enhanced within the low-frequency ISO convective envelope, and is strongly correlated with low-frequency 850-hPa westerly anomalies. At the same time, eastward-propagating convectively coupled Kelvin wave activity is enhanced well to the east of the active ISO convection, in the central Pacific. A case study of an ISO event during July?September 1987 illustrates these statistically derived relationships. The enhanced phase of the ISO is shown to consist primarily of westward-propagating higher-frequency variability, including seven named tropical cyclones in the western Pacific, two of which project onto MRG?TD-type modes as they propagate westward across Southeast Asia into the Bay of Bengal. Successive eastward development of three tropical storms is suggested to be associated with an eastward dispersion of energy in the MRG?TD mode. Several Kelvin waves propagate across the Pacific to the east of the active ISO convective envelope. Based on the statistical results and the 1987 case study, it is suggested that the high-frequency, westward-propagating MRG?TD disturbances and tropical cyclones may compose a significant portion of the low-frequency ISO signal. Eastward-propagating Kelvin wave variability, on the other hand, is more active outside the ISO convective envelope, to its east.