Multidecadal Fluctuations of SST and Euphotic Zone Temperature off Northwest Africa

Abstract

The Atlantic multidecadal variability (AMV) switched from a cool to a warm phase in 1995 and the mean euphotic zone (EZT) and sea surface temperature (SST) shifted upward by 0.57° and 0.69°C, respectively, between 1982–91 and 2006–15 in the Atlantic region off northwest Africa. This ocean margin has many marine fisheries, and water temperature fluctuations may cause fish there to switch their habitats. Net radiation flux did not significantly change between these two decades. So, we hypothesized that the key driver of the EZT and SST increase is wind, which controls turbulent (sensible and latent) heat exchange with the atmosphere as well as bulk vertical and horizontal heat transport. Using satellite-derived SST and atmospheric and oceanic reanalyses to analyze the ocean top-200-m heat budget, we compared the relative contributions of the heat budget components to the cyclical changes in EZT and SST between these two decades. Results showed that the dominant heat source is horizontal heat flux convergence: weaker northeasterly trades and stronger southerly winds and monsoon enabled the southerly winds to drive warm water northward that subsequently warmed the domain. The dominant heat sink is latent heat loss: onshore–offshore atmospheric pressure gradients caused a complex wind adjustment that enabled the Sahara wind to accelerate evaporation over large subregions. These results highlight the important roles of ocean heat transport and atmosphere–ocean coupling for the tropical branch of the AMV. The regional EZT and SST anomalies associated with this AMV phase switch are mainly a consequence of wind-driven processes occurring at larger spatial scales.