Rainfall variability in the rainy season of semiarid zone of Northeast Brazil (NEB) and its relation to wind regime

摘要

The rainfall and the 850 hPa wind characteristic of the semiarid interior zone of the northeast Brazil (NEB) are analyzed. The database used are the daily mean u, v, z, T, q and theta, at 850 hPa, 500 hPa and 200 hPa, displayed on a 2.5 degrees x 2.5 degrees grid, from the NCEP/NCAR reanalysis; the daily mean OLR data from the Climate Diagnostics Center, from June 1986 to December 1997; and the gridded daily precipitation (P) analysis data in Brazil (10 degrees S-40 degrees S and 30 degrees W-80 degrees W), produced by the Climate Prediction Center, from 1977 to 1997. Daily time series of the normalized rainfall index (RI) and the normalized 850 hPa wind indexes (i.e. zonal index (ZI), the meridional index (MI) and the u minus v index (UVI)) were calculated for northern (Al: 7 degrees S-10 degrees S and 37 degrees W-44 degrees W) and southern (A2: 10 degrees S-15 degrees S and 40 degrees W-45 degrees W) domains. The RI, ZI, MI and UVI were defined by area-average of P, u, v, and u minus v, divided by the number of grid points corresponding to each area, respectively. As for regional features this region presents a well-defined annual cycle of rainfall with rainy seasons from December to April in A1 and from November to March in A2, and a few extreme rainfall events (during a 20-year period, 26 cases occurred in A1 and 39 cases in A2). Several synoptic scale systems acting alone or simultaneously in the NEB could explain the occurrence of heavy rainfall in this region. The RI is correlated positively with both the ZI and the UVI, and negatively with the MI, which might indicate that when easterly and southerly winds intensify (weaken), the rainfall decreases (increases). From the statistical perspective the UVI appears to be more effective to describe the daily rainfall variability in the NEB semiarid interior region, compared to the ZI or MI. Copyright (c) 2006 Royal Meteorological Society.