EMPIRICAL ORTHOGONAL FUNCTION ANALYSIS FOR CLIMATE VARIABILITY OVER THE INDONESIA-PACIFIC REGION

Abstract An Empirical Orthogonal Function analysis has been performed on monthly mean Outgoing Longwave Radiation anomaly for the greater part of Indonesia-Pacific region (30°S – 30°N; 80°E – 80°W) during the period 1982 – 2003. In this paper, the first three Empirical Orthogonal Function modes are presented. A mode is defined spatially in terms of an empirical orthogonal function, which describes the degree of coherence of variation. The principal component’s corresponding coefficients depict the evolution of the mode in time. The results show the most important non-seasonal convective variation over the Indonesia-Pacific region (the first mode) is governed by the west-east overturning Walker circulation. The spatial and temporal patterns of this mode are identified by the El Nino/Southern Oscillation phenomenon. Other two modes are discussed. 1. INTRODUCTION The climate of Indonesia is determined by the temporal and spatial distribution of the Inter-Tropical Convergence Zone (ITCZ) and South Pacific Convergence Zone (SPCZ). ITCZ is a zone of low-pressure near the equator where two easterly trade winds originating from the Northern and Southern hemispheres converge. This zone of enhanced convection, cloudiness, and rainfall constitutes the rising branch of the meridional Hadley circulation. SPCZ is a band of abundant cloudiness extends southeastward the Indonesian convection center. One of the most striking characteristics of the ITCZ and SPCZ is its variability on a wide range of temporal scales. On the interannual time scales, the ITCZ and SPCZ over Indonesia is dominated by the El Nino/Southern Oscillation (ENSO). The El Nino is associated with the anomalous warm sea surface temperature (SST) in the eastern-central tropical Pacific Ocean. The Southern Oscillation is referred to the strength of the Walker circulation over the Pacific region, i. e., to the difference in surface pressure between the southeastern tropical Pacific and the Indonesian-Australian regions. Ropelewski and Halpert (1987) demonstrated that 80% of the ENSO events from 1879 to 1982 were accompanied by below normal rainfall in Indonesia between June and November. Moreover, Ropelewski and Halpert (1989) found that 90% of La Nina events were associated with abnormally wet season over Indonesia between July and December. Table 1 shows the El Nino and La Nina (the anomalous cold SST in the eastern-central tropical Pacific Ocean) events from 1951 to 2000 identified using SST anomalies in the Nino 3.4 region (5°N-5°S; 120°-170°W) and 0.4°C threshold (Trenberth 1997).