Meteorological factors associated with drought on Guam

Lander, M

The rainfall on the island of Guam, at 13.5°N, 144.8°E, has a pronounced temporal asymmetry: roughly one-third of the annual rainfall accumulates during the dry-season months of January through June while two-thirds of the annual rainfall accumulates during the wet-season months of July through Dcccmber. The me:an annual total rainfall on Guam varies from about 85 inches (2170 mm) at drier spots to a little over 100 inches (2540 mm) in wetter area. Despite the relatively high annual rainfall amounts, Guam suffers deleterious effects at drought almost every dry season: dessication of grasslands. desiccation and defoliation of some species of trees, significant reduction of streamflow and significant reduction of the water level in many of Guam's wells. Wildfires burn thousands of acres during many dry seasons. Every three or four years, the dry scacon is especially dry and prolonged. Wildfires and stress to local crops thereby aggravated and prolonged. The seasonal asymmetry of the rainfall on Guam is governed primarily by the scasonal shift of the monsoon trough and its associated zone of monsoonal cloudiness. During January through June, northeasterly tradewinds blow on Guam. In a tradewind regime the air is subsiding, clouds lack vertical development and rainfall comes in the form of sporadic tradewind showers. During the rainy season, the monsoon trough becomes active in the western North Pacific, allowing deep connective clouds, meso-scale convective cloud systems, and tropical cyclones (with their torrential rains) to affect Guam.