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 southcentral 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.

Variability of Convective Activity over the North Indian Ocean and its Associations with Monsoon Rainfall over India

“…Thus, the convective activity increases over the Arabian Sea and the Bay of Bengal in the recent period compared to that of the former period during the monsoon season. A similar increase of convective activity over the equatorial Indian Ocean was also indicated by CHU and WANG (1997) during the monsoon season associated with decreasing trends of OLR.…”

Trends in upper‐level cloud cover and surface divergence over the tropical Indo‐Pacific Ocean between 1952 and 1997

“…Despite the direct link to deep convection, this study does not examine trends in surface‐observed cumulonimbus cloudiness since that cloud type exhibits dubiously large changes in zonal mean cloud amount [ London et al , 1991; Bajuk and Leovy , 1998a]. The reduction in surface‐observed upper‐level cloud cover is consistent with the decreasing trend in satellite‐observed high‐level cloud cover reported by Nitta and Kachi [1994] for 1978–1994, but not with the decreasing trends in OLR documented by Chu and Wang [1997] for 1974–1992 and by Waliser and Zhou [1997] for 1974–1996. This disagreement remains even if upper‐level cloud trends are calculated for the same time period as the OLR trends, implying that either the reported OLR decrease over the tropical western Pacific is due to changes in factors besides cloud cover (e.g., emissivity and temperature) or that trends in one of the data sets are spurious.…”

“…Specifically, the latter study found an overall decline in upper‐level cloud cover between 1952 and 1997 when averaged over the tropical ocean in general and the Indo‐Pacific region in particular. Previous research has documented significant decadal and multidecadal changes in OLR, surface‐observed total cloud cover, precipitation, atmospheric circulation, and sea surface temperature over the tropical Indo‐Pacific during the later decades of the 20th century [e.g., Nitta and Yamada , 1989; Graham , 1994; Nitta and Kachi , 1994; Morrissey and Graham , 1996; Chu and Wang , 1997; Waliser and Zhou , 1997; Zhang et al , 1997; Curtis and Hastenrath , 1999; Deser et al , 2004], and an investigation of the relationships between meteorological and cloud trends would aid attribution of the regional mean decrease in upper‐level cloud cover noted by Norris [2005].…”

Tropical Rainfall Variability on Interannual-to-Interdecadal and Longer Time Scales Derived from the GPCP Monthly Product

“…Long-term rainfall changes represented by the slopes of the regression lines are computed for P LO , P O , and P L , respectively. The degrees of freedom (dof's) are estimated by means of dof ϭ n(1 Ϫ ␥)/(1 ϩ ␥) Ϫ 2 (e.g., Livezey and Chen 1983;Chu and Wang 1997;Bretherton et al 1999), where n is the length of time series and ␥ is the lag-one correlation coefficient of the residual time series (Santer et al 2000;Feidas et al 2004). 2 but are denoted by different units (Fig.…”

“…Longer-term change and/or trend are also widely recognized, and are considered to be a consequence of probably both natural and human-induced effects (e.g., Chu and Wang 1997;Bates and Jackson 2001). The gradually warming climate is a good example (e.g., Yang et al 2003;Neelin et al 2003;Kumar et al 2004).…”