Thursday, 18 January 2007: 1:30 PM
Tropical cyclones, drought, and the seasonal reversal of the ENSO rainfall signal in the Philippines
214B (Henry B. Gonzalez Convention Center)
We utilize over a half a century of precipitation data from observing stations across the Philippines to show that the ENSO rainfall signal reverses sign between boreal summer and the subsequent fall. During summer, it is shown that El Nino conditions bring a statistically significant occurrence of above average rainfall to several locations before unusually dry conditions, more typical of El Nino events, develop during the fall. Conversely, La Nina conditions are shown to often accompanied by unusually dry conditions in summer prior to the development of excessive rainfall by boreal fall. These results are shown to relate to similar behavior of tropical cyclones affecting the Philippines, with a greater number of cyclones occuring in boreal summer under El Nino conditions compared with La Nina. The relative number of tropical cyclones then reverses by fall, with far fewer cyclones occurring in El Nino than La Nina. Using Reanalysis data, these results are shown to not simply be a consequence of ENSO phase transitions during boreal spring, but are instead related to a temporally varying influence of ENSO on the large scale monsoon system during the life cycles of ENSO warm and cold events. Key characteristics of the observed large and regional scale flow associated with this signal reversal are presented, including the physical factors which are associated with the time-varying behavior of tropical cyclones in the vicinity of the Philippines as modulated by the ENSO phenomenon.