Using multiple passive tracers to identify the importance of the North Brazil undercurrent for Atlantic cold tongue variability

Abstract

The use of multiple passive tracers in a dynamical ocean model is presented as a method to study water mass origins. The multiple passive tracer method is successfully applied to determine the origin regions of the Atlantic cold tongue (ACT), using 31 passive tracers. At the time of the ACT minimum in sea surface temperature, 38 ± 3% of the surface water in the central equatorial Atlantic is found to originate in the western side of the basin, with only 11 ± 2% originating in the east. Conversely, for the ACT surface cooling off the west coast of Africa, only 2 ± 1% is from the west of the Atlantic basin, with 71 ± 7% from the east. The region of the North Brazil undercurrent is found to be the most important origin region for the central Atlantic ACT region, for a 6–8 month lead time. Using the Met Office EN4 observational dataset of ocean temperatures, correlations are found between January temperatures in the North Brazil undercurrent region and both the ACT mean temperature and ACT spatial extent in July–August, significant at the 0.95 level. This suggests that the 6‐month lead‐time predictability of the ACT may be increased by improved knowledge of the North Brazil undercurrent temperature. Results from previous studies suggest that such increased predictability of the ACT could potentially help improve predictions of the West African monsoon.