Mechanisms of Java Rainfall Anomalies

Abstract

The large-scale circulation departure patterns associated with the interannual variability of (July?June) rainfall in Java are studied on the basis of ship observations (1911?73) in the Indian Ocean and surface station records. Circulation mechanisms of interannual variability can, in part, be understood as modulations of the average annual cycle. Abundant rainfall years are characterized by an anomalously strong Northwest monsoon, and drought years by approximately inverse circulation characteristics. In December?January of the wet years, anomalously high pressure near Southeast Asia along with anomalously low pressure over Indonesia entail an enhanced meridional pressure gradient, stronger northeasterly flow over the South China Sea and Bay of Bengal, and enhanced northwesterlies over the Indonesian waters. The increased northerly wind component to the north and intensified westerlies over the equatorial Indian Ocean result in enhanced convergence and cloudiness over Indonesia, while surface waters are cold. In May?October, anomalously low pressure, abundant cloudiness, and anomalous northwesterlies consistent with the enhanced pressure gradient are associated with a positive sea surface temperature anomaly in the Indonesian waters. By contrast, in November?April, abundant cloudiness and anomalous northwesterlies, again associated with anomalously low pressure but now resulting in higher wind speed, accompany a negative sea surface temperature anomaly. Anomalously low pressure and warm surface waters in May?October are followed by low pressure and low sea temperature in November?April, which in turn are succeeded by high May?October pressure. This seasonally varying relationship among cloudiness, wind, and sea surface temperature appears instrumental for the functioning of the Southern Oscillation. inasmuch as a warm sea surface is conducive to an inflation of the atmospheric column and thus a rise of upper-tropospheric constant pressure topographies and a drop of surface pressure, with the inverse effects ensuing from a negative sea surface temperature anomaly. This chain of causalities must be regarded as an essential part of the Southern Oscillation pressure seesaw between the Indonesian and eastern South Pacific dipoles. The memory of the combined atmosphere?ocean system presumably ensures the strong pressure persistence from the drier to the rainy half-year, which in turn provides a basis for climate prediction.