Climatology of rainfall and wind for the Philippine-South Sea region: 1. Monthly variations

A diagnostic study of the rainfall and the wind climatology of the Philippine–South China Sea region has been conducted using data obtained from the ECMWF reanalysis. The study shows unusually complex rainfall patterns even over the sea where no complications can occur due to mountainous terrain. The most interesting of these patterns is a zone of high rainfall which is found during the southwest monsoon season over the central areas of the South China Sea. Surprisingly, the high rainfall is located over the zone of strongest southwesterlies. This is named the Central South China Sea Rainfall System (CSCSRS). During the northeast monsoon season, a comparable maximum can be found in relatively the same area. It also occurs where the northeasterlies are strong, but is not as defined and does not last as long as the CSCSRS. Other rainfall maxima that develop according to season are those caused by orographic effects. For example, when the prevailing flow is southwesterly, a maximum develops near southwestern Luzon, a Philippine island. When the northeast monsoon season arrives, rainfall maxima over eastern Luzon and eastern Mindanao develop.

Another rainfall system worth noting is one which affects the Philippine-Indonesian region. Called the Philippine-Indonesian Rainfall System (PIRS), it is present throughout the year and appears to be related to the ITCZ.

The study also shows that over the Western Pacific (roughly from 127°E to 140°E), there is usually a northward advance of a rainfall maximum during the first half of the year, followed by a retreat of the maximum to the equatorial regions during the second half. These movements are generally associated with that of the ITCZ. In contrast, over the South China Sea – around 105°E to 120°E – the northward-advancing rainfall maximum is absent. Only the southward
retreat is found.