Pacific sea surface temperature linkages with Tanzania's multi-season drying trends

摘要

During 1999-2014, Tanzania experienced below average precipitation in two important seasons: December to February (DJF) in the south and during March to June (MAMJ) in the northeast. We explore DJF and MAMJ precipitation in the areas with drying trends and examine their relationships with anomalous sea surface temperatures (SSTs) in the Indo-Pacific and corresponding circulation patterns. At seasonal time scales, precipitation in DJF and MAMJ trend areas appears inversely related to diabatic forcing in the equatorial Pacific. Dominant influence for droughts in DJF is eastern Pacific SST while for droughts in MAMJ it is West Pacific SST. A bivariate regression model with West Pacific and Nino3.4 region SST as predictors is found to recreate multidecadal DJF variability after the 1950s and the extreme drying in MAMJ during the 2000s. The regression model coefficients also indicate differential eastern versus western Pacific forcing for DJF versus MAMJ. Thus, we suggest that recent La Nina-like conditions, characterized by an enhanced Pacific SST gradient due to cooling in the eastern Pacific and warming in the western Pacific, played a substantial role in Tanzania's recent multi-season drying trends. SST change scenarios (difference between 2023-2037 and 2000-2014 means) based on CMIP5 projections and observed trends illustrate the uncertainty of future precipitation outcomes, as well as the potential implications of contrasting linkages to eastern versus western Pacific SSTs. These scenarios are mainly optimistic for the DJF southern Tanzania trend area, because it appears dominated by Nino3.4 cooling at both seasonal and decadal time scales. Conversely, our scenarios are quite pessimistic for the MAMJ northeastern Tanzania trend area, because we find a dominant negative influence of warming West Pacific SST.