Investigating Variability of Outgoing Longwave Radiation over Peninsular Malaysia Using Wavelet Transform

The present study analyzes and quantifies the spatial–temporal variability of outgoing longwave radiation (OLR) over peninsular Malaysia using the continuous wavelet transform (CWT) from 2003 to 2010. The goal is to understand the long-term variability of OLR over Malaysia in terms of time–frequency variations in relation to the monsoon period and other weather phenomena. The study regions selected were the west coast, east coast, and southern part of peninsular Malaysia. The OLR variation characteristics in time and space derived from wavelet transform were found to be distinctly different in these three regions. In these three regions, OLR showed significant periodicities dominated by the annual cycle, followed by a semiannual cycle. The west coast of peninsular Malaysia has a lower annual component compared to the other regions because of the rain-sheltering effect by the mountain range that blocked the heavy rainfall from northeast monsoon winds. Besides that, the results show that the wet and dry spells coincide with local monsoon and intermonsoon periods. Meanwhile, the results also revealed that the semiannual variation is statistically significant during 2004–06. The strong semiannual variation is coincident with several droughts that resulted from the strong El Niño events in 2004–06. In addition, the phase plot of wavelet coefficients shows that the variations at various scales are in phase, which coincided with the sudden variations of OLR, indicating heavy flood occurrences in the southern part of peninsular Malaysia. The results show that CWT is a powerful tool for analysis of phenomena involving multiscale interactions that exhibit localization in both time and frequency.