The role of eastern North Pacific tropical storms in the rainfall climatology of western Mexico

This paper explores the role that tropical storms (TS) of the eastern North Pacific play in the rainfall climatology of western Mexico. It uses an 18-station rainfall grid, along with a data base of storm tracks (1949-1997). TS rainfall is defined by a distance threshold, so that for each station in the grid, daily rainfall recorded when a TS is located within 550 km of the station is considered as TS-derived; rainfall recorded in the absence of a TS or when a storm is beyond 550 km from a station is by default 'non-TS' rainfall. A variety of statistics are presented to demonstrate the static associations that exist between TS activity and regional rainfall patterns. In sum, they clearly suggest that improved seasonal forecasts of TS activity in the eastern North Pacific will be critical to advancing seasonal climate prediction for Mexico. This study also addresses the interannual variability in TS and non-TS rainfall. Principal component analysis is used to define the region's primary modes of rainfall variation. Considering the temporal behaviour of the rainfall modes, analyses reveal no statistically significant trends in TS rainfall. In contrast, non-TS rainfall modes exhibit significant trends. This study provides support for the idea that these trends are partly tied to long-period fluctuations in the ocean-atmosphere system. The results also indicate that while TS rainfall is not subject to trend behaviour, its year-to-year fluctuations, apparently, are linked to large-scale processes like El Nino-Southern Oscillation (ENSO) and the state of North Pacific sea-surface temperatures (SSTs). Copyright (C) 2001 Royal Meteorological Society.