Main features of large-scale oceanic-atmospheric circulation related to strongest droughts during rainy season in Brazilian Sao Francisco River Basin

Severe droughts have caused serious impact on water supply and agriculture in Sao Francisco River Basin (SFRB), Brazil, especially during the rainy season. Observational evidence suggests that droughts in this region could be driven by some large-scale ocean-atmospheric patterns. This study provides a general description of linkage between the ocean-atmospheric circulation patterns and the droughts during the rainy season (November-December), in the SFRB's upper sub-basins. The NCEP reanalysis I and monthly gridded precipitation dataset of the Global Precipitation Climatology Centre from 1948 to 2010 were used. SPI was used as drought index. All analyses were performed in a domain that covered South America and southern-tropical portions of the Atlantic and Pacific Oceans. Both the wind speed and vertical velocity fields at 200 hPa and specific humidity at 700 hPa were analysed. Results indicate that more extensive short-duration droughts are linked to: (1) El Nino conditions highly concentrated inside the Nino 3.4 region; (2) ITCZ with a northernmost-than-normal position; (3) weaker-than-normal convective activity over the SFRB coupled to a weaker-than-normal upper-level westerly jet stream. For decadal scale, the atmospheric anomalies patterns are best correlated to long-duration extensive droughts than the oceanic anomalies patterns (r=-0.84 and -0.14, respectively). This suggests that droughts in SFRB are strongly dependent on large-scale moisture transport and upper-level atmospheric circulation.