Recent Climate Change in the Tropical Western Pacific and Indian Ocean Regions as Detected by Outgoing Longwave Radiation Records

Abstract

Recent climate change in tropical convection in the western Pacific and Indian Ocean regions is inferred from the outgoing longwave radiation (OLR) records. The systematic bias in the OLR series is first corrected and results of the rotated empirical orthogonal function analysis indicate that the bias, to a first approximation, has been corrected. Linear regression analysis and nonparametric Mann?Kendall rank statistics are employed to detect trends. From 1974 to 1992, trend analyses based on the entire consecutive monthly records suggest a significant decrease in OLR over the tropical central?western Pacific and a large portion of the Indian Ocean. In contrast, northern Australia shows the largest increase in OLR over time. The significance of the local linear trend pattern has been determined via a Monte Carlo simulation technique that scrambles OLR time series at each grid point ?simultaneously? and results show the field significance. An increase in convection shows a preference to occur in the summer hemisphere. During the boreal summer half-year, this is seen in a region extending from the Arabian Sea across southeast Asia eastward to the northwest Pacific, with the largest value over the Bay of Bengal. More summer monsoon rainfall is likely to have occurred in these regions. For the austral summer half-year, enhanced convection is found over the equatorial south-central Pacific and the south-central Indian Ocean. Time series of tropical cyclone counts in the northwest Pacific, the Bay of Bengal, and the south-central Indian Ocean also reveal a general level of increase. Regardless of seasonality, a positive trend in OLR is always observed over a large portion of tropical Australia. A sensitivity test is conducted to investigate the change in linear trend patterns by removing the years during which the El Niño?Southern Oscillation phenomenon occurred. Although the enhanced convection over the Bay of Bengal, the south Indian Ocean, and the northwest Pacific are still noticeable, it is much weaker over the equatorial south-central Pacific than when the complete duration series were used. Other sensitivity tests are also conducted to examine the change in linear trend patterns by varying data lengths and by skipping the missing 10-month observation in the OLR time series; results are basically similar to those when complete data are used. The authors speculate that monsoon convection over the tropical western Pacific and the Indian Ocean has undergone a change in the climate mean state, probably on a decadal timescale.