Interannual Variability of Barrier Layer in the Tropical Atlantic and Its Relationship with the Tropical Atlantic Modes

Abstract

Abstract This study reveals the role of the tropical Atlantic variability in modulating barrier layer thickness (BLT) in peak seasons. Based on reanalysis data during 1980–2016, statistical and dynamical analyses are performed to investigate the mechanism of BLT variability associated with the tropical Atlantic modes. The regions with significant correlation between BLT and tropical Atlantic modes are located on the northwest and southeast coasts of the tropical Atlantic, which are consistent with BLT maximum variability regions. In boreal spring, BLT decreases in the northwest because less latent heat release affected by weak trade wind related to the Atlantic meridional mode (AMM) shoals the isothermal layer depth (ITLD). In the south equatorial Atlantic, deepened mixed layer depth (MLD) is controlled by the decreasing freshwater input brought by a northward shift of the intertropical convergence zone (ITCZ) and further leads to a thinner barrier layer (BL). However, a shoaling MLD appears in the north equatorial Atlantic, which results from excessive freshwater input, causing a thick BL there. In boreal summer, positive runoff anomaly caused by the Atlantic equatorial mode (AEM) leads to upper warming of the tropical northwest Atlantic and a shallowing ITLD, favoring a thinner BL there. However, a southward shift of ITCZ brings more freshwater into the south equatorial Atlantic, inducing a shallowing MLD as well as a thicker BL. AEM-driven horizontal heat advection of the south equatorial current contributes to a thick ITLD in the central southern tropical Atlantic and thus increases BLT. Significance Statement This research aims to reveal how the tropical Atlantic meridional and equatorial interannual climatic modes affect barrier layer thickness (BLT). These two climate modes can affect the wind field, ocean current, and precipitation through air–sea interaction processes, and further affect mixing, heat–salt transport, and stratification in the upper ocean and thus BLT. This finding is important because the barrier layer restricts the exchange of heat, momentum, mass, and nutrients between the mixed layer and the thermocline, thereby impacting local and remote weather events, the ecological environment, and the climate. Our results provide guidance for interpreting the interannual variability of BLT in the tropical Atlantic.