Decadal deep‐water variability in the subtropical Atlantic and convection in the Weddell Sea

Abstract

This study suggests a link between convection in the Weddell Sea and deepwater flows in the (sub)tropical Atlantic. Employing a global sea ice‐ocean general circulation model, events of enhanced convection are found to create enhanced outflow of Antarctic Bottom Water (AABW), the associated anomaly of which propagates rapidly along the model's deep western boundary. The propagation timescale of the order of a few years to reach the tropical Atlantic matches baroclinic Kelvin waves with phase speed corrected for the model's grid resolution. This feature is consistent with earlier findings on the baroclinic adjustment to perturbations of the rate of North Atlantic Deep Water (NADW) formation. The mean deep ocean transports, as well as the timescales and amplitudes of the deep ocean anomalies, are in good agreement with observation‐based estimates. Upon arrival in the deep western tropical Atlantic, the model AABW anomalies induce strong anomalies in NADW outflow. This behavior suggests that the actual magnitude of NADW outflow across 30°S is highly susceptible to external perturbations on the baroclinic adjustment timescale. The implied higher susceptibility is supported by recent observations that NADW turns mostly eastward between 20°S and 30°S. The NADW anomalies eventually propagate southward and join the Antarctic Circumpolar Current. There is weak indication that the associated anomalies enter the eastern Weddell Gyre to eventually feed back on the critical convection site in the Weddell Sea, possibly sustaining the oscillation. However, there is stronger evidence that the anomalies initially created by the convective events are advected with the model's Weddell Gyre, the advective timescale of which (10–12 years) is consistent with the periodicity of the convective events.