Dynamic impact of oscillating sources on the North Atlantic deep circulation: A process study

Abstract

In this process study the dynamic effects of fluctuating deep water sources on the deep circulation of the North Atlantic are examined by means of a 1½‐layer model. The model is used with a realistic topography of the Atlantic Ocean between 65°N and 25°S and has an open southern boundary. According to observations, the northern inflow of deep water is subject to variability on timescales between days and decades. The same holds for the Deep Western Boundary Current (DWBC), which acts as a link between the deep water inflow and the interior circulation. The timescales of the observed variability motivate the forcing functions used in the model. They determine the nature of the models response. The flow response relative to the local mean flow can be considerably larger than the original perturbation at the source. Normalizing the perturbed flow with the local steady state flow leads to the identification of sensitive areas. Their location and shape are determined by the difference between the characteristic travel paths of the perturbation and the streamlines of the steady state circulation. In the eastern equatorial Atlantic a forcing with periods between 1 and 3 months gives rise to larger flow perturbations than a forcing with periods between half a year and 1 year. Forcing with the latter periods leads to a stronger response in the western equatorial Atlantic and in the interior ocean. Changes in the direction of the deep equatorial flow are driven by low‐frequency oscillations of the DWBC in the model. This is consistent with observations in the area. Perturbations radiate into the interior away from the eastern boundary as long Rossby waves. As this propagation is along geostrophic contours, consequently large parts of the abyssal interior are inaccessible. The results support the notion that the area where the DWBC encounters the equator is not only a key region for the “slow” interhemispheric exchange of heat and tracers but also for the transport of information by “fast” internal waves along the equator.