ENSO variations and drought occurrence in Indonesia and the Philippines

Abstract

The “El Nino-Southern Oscillation” (ENSO) consists of a sympathetic movement involving the Pacific ocean and associated atmosphere in an essentially chaotic manner along the equator. The system oscillates between extremes of the so-called “warm events” usually lasting 1 or 2 yr and involving movement of warm sea water from the western Pacific along the equator to impact on the west coast of the American continent and “cold events” associated with easterly trade-wind-induced flows of colder water from the eastern Pacific towards the west. Information drawn from meteorological records in southeast Asia clearly indicates that each event is unique in terms of the signature which it imposes on the rainfall and temperature from location to location. Nevertheless, a strong underlying pattern within the context of each event, itself apparently initiated or molded by the character of the preceding years, can be detected. This pattern permits relatively circumscribed predictions of forward conditions (drought intensity) for 2–3 yr, to be made once the event “locks in” for the duration of the warm event and at least 1 yr beyond. The character of the intervening non-ENSO years can also be projected but in a more tenuous, though fairly regular manner. When the non-ENSO years leading up to a warm event are scored in terms of the extent to which they depart from the secular warming trend for the warmest month using data from Jakarta and Semarang on the north coast of Java, the cumulative temperature deviations signal the character of the upcoming ENSO event. This signal does not, however, allow an exact determination to be made with respect to whether or not an ENSO event will occur in the next year. For the available historical instrumental data, all markedly upward-moving traces eventually delivered a hot dry season in east Indonesia. This sort of tendency within non-ENSO blocks can thus serve as a caution in the sense that a very hot ENSO event is likely in the offing. The background data can also be used to predict actually the probable intensity of an ENSO in the upcoming year in terms of its drought potential, should such an event take place in reality. In this respect the correlation between the cumulative temperature deviation of the inter-ENSO blocks in relation to the temperature deviation of the first ENSO year is 0.43. In the region of southeast Asia represented by Indonesia and the Philippines, relatively secure predictions concerning likely upcoming droughts can be made in specific instances once an ENSO event “locks in” for successive years (2–4) until the warm event set terminates. In the case of Java and within succeeding inter-ENSO years, further predictions can be made with reference to successive years in terms of the character of preceding years. This system, in conjuction with predictions generated by models could form the basis of a crop advisory service for prediction of drought or rainfall within dry seasons from one year to the next. It is anticipated that a broad description of temperature and rainfall patterns associated with space and time with ENSO events will lead to better food security for the region as long as sudden changes do not occur.