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Deep currents
Abstract:
A suite of high-resolution models of the Atlantic Ocean circulation is used to study the deep seasonal current variability in the equatorial regime, with a particular emphasis on its manifestation in the variability of the interhemispheric transports near the western boundary. The basic experiment has a resolution of 1 / 3 ∘ horizontally and 45 vertical levels, and is subject to a monthly mean atmospheric forcing based on \ECMWF\} flux fields. Sensitivity experiments explore the effects of higher horizontal resolution ( 1 / 12 ∘ ), and alternative mixing parameterizations. The model behavior near the equator confirms previous suggestions based on solutions of the \{WOCE\} Community Modelling Effort (“CME”) and the “DYNAMO” model intercomparison project, of the presence of a system of vigorous seasonal current oscillations, spanning the whole water column and nearly the whole zonal extent of the basin. The patterns of the primarily zonal current anomalies are fairly robust across the range of model cases investigated, i.e., show relatively little sensitivity to horizontal resolution/mixing, or to the different choices of vertical discretization and vertical mixing as in the \{DYNAMO\} cases. The amplitude of the seasonal variation exceeds 10 cm/s in the surface layer, and decreases to about 5 cm/s near 1000 m and 2–3 cm/s in the deep ocean in both the basic 1 / 3 ∘ - and the 1 / 12 ∘ -cases, thereby leading to seasonally reversing current signatures at all depths below the EUC. A particular aspect of the seasonal current variability concerns its manifestation in the southward transport of North Atlantic Deep Water (NADW) by the Deep Western Boundary Current (DWBC). The temporal characteristics of the \{DWBC\} variability are in agreement with moored current meter observations at 44 ∘ W , with simulated \{DWBC\} transports varying between a maximum of more than 30 Sv in January/February, and almost vanishing transport in September. However, in contrast to the annual-mean deep water transport which is confined to the \{DWBC\ and tight, O(100) km-recirculation cells, the seasonal cycle of transport is not trapped near the boundary: the simulations show that the zonal current variations of the equatorial wave guide, near the western boundary give rise to a broad system of seasonal recirculation cells of the DWBC. Calculations of the amplitude of the seasonal variability in the deep water transport near the equator are therefore strongly dependent of the spatial extent of the cross-section considered; in particular, for being approximately representative of low-frequency variations in the net, zonally-integrated meridional transport of deep water in the equatorial regime, transport sections would need to extend over nearly the whole western basin.
