title: An objective tropical Atlantic sea surface temperature gradient index for studies of south Amazon dry-season climate variability and change.
abstract: Future changes in meridional sea surface temperature (SST) gradients in the tropical Atlantic could influence Amazon dry-season precipitation by shifting the patterns of moisture convergence and vertical motion. Unlike for the El Niño-Southern Oscillation, there are no standard indices for quantifying this gradient. Here we describe a method for identifying the SST gradient that is most closely associated with June-August precipitation over the south Amazon. We use an ensemble of atmospheric general circulation model (AGCM) integrations forced by observed SST from 1949 to 2005. A large number of tropical Atlantic SST gradient indices are generated randomly and temporal correlations are examined between these indices and June-August precipitation averaged over the Amazon Basin south of the equator. The indices correlating most strongly with June-August southern Amazon precipitation form a cluster of near-meridional orientation centred near the equator. The location of the southern component of the gradient is particularly well defined in a region off the Brazilian tropical coast, consistent with known physical mechanisms. The chosen index appears to capture much of the Atlantic SST influence on simulated southern Amazon dry-season precipitation, and is significantly correlated with observed southern Amazon precipitation. We examine the index in 36 different coupled atmosphere-ocean model projections of climate change under a simple compound 1% increase in CO2. Within the large spread of responses, we find a relationship between the projected trend in the index and the Amazon dry-season precipitation trends. Furthermore, the magnitude of the trend relationship is consistent with the inter-annual variability relationship found in the AGCM simulations. This suggests that the index would be of use in quantifying uncertainties in climate change in the region.
authors.affiliation: Met Office Hadley Centre, FitzRoy Road, Exeter, Devon EX1 3PB, UK. peter.good@metoffice.gov.uk
authors.forename: Peter
authors.initials: P
authors.lastname: Good
authors.forename: Jason A
authors.initials: JA
authors.lastname: Lowe
authors.forename: Mat
authors.initials: M
authors.lastname: Collins
authors.forename: Wilfran
authors.initials: W
authors.lastname: Moufouma-Okia
mesh_terms: D001267:Atlantic Ocean; D003247:Conservation of Natural Resources; D017752:Greenhouse Effect; D008962:Models, Theoretical; D011891:Rain; D012621:Seasons; D013696:Temperature; D014329:Tropical Climate