The Mann Eddy: formation and interaction with the North Atlantic Current

The Mann eddy (ME) is a mesoscale anticyclone, adjacent to the path of the North Atlantic Current (NAC) in the Newfoundland basin. The eddy has been detected consistently in synoptic surveys since first discovered [1], and manifests in Argo- and altimetry-based climatologies. The interaction of the Mann eddy (ME) and NAC is of interest since their poleward transport signals are difficult to distinguish in observations [2]. Furthermore, decadal ME strength variability is correlated with AMOC variability in climate models [3]. Yet no detailed dynamical investigation of the ME has been conducted yet, to our knowledge.

In this study we investigate the phenomenology of the ME, its dynamics, and its interactions with the NAC. Our primary tool is a high-resolution (2.5 km) realistic regional numerical simulation, complemented by existing hydrographic data and satellite altimetry. The eddy is consistently identified in instantaneous model output, surface-intensified but bottom reaching, as in observations. It migrates significant distances relative to its ~100 km diameter, within a ~500 km wide topographic depression. We apply eddy tracking to define an eddy-centered ME climatology, since in Eulerian climatology it is several times weaker due to its non-stationarity. The climatology, as well as particle tracking, are used to deduce the source water of the eddy and fraction originating in the NAC. In satellite altimetry and model output we quantify the frequency of mergers with other anticyclones that contribute to sustaining the ME, and track their origins.

To study its formation process, we conduct idealized numerical experiments with a layered hydrostatic model in a parameter regime relevant to the ME. A ME-like anticyclone is produced within the topographic depression, as controlled by an identified nonlinearity parameter. Vertical structure is consistent with Taylor-cone height scaling of surface-intensified forcing. Eddy-eddy and topographic interactions play significant roles in the aggregation of anticyclonic vorticity. Finally, we evaluate the extent to which topographic turbulence theories are consistent with the sustained presence and characteristics of the eddy.

[1] Mann, C. R., Deep Sea Res. 14.3, 1967

[2] Meinen, C. S., Deep Sea Res. I, 48.7, 2001.

[3] Tulloch, R. & J. Marshall, J. Climate, 25.12, 2012