Water mass pathways between the subtropical and tropical ocean in a climatological simulation of the North Atlantic ocean circulation

Abstract

Abstract A primitive equation, hydrostatic, terrain-following coordinate ocean general circulation model (OGCM) is used to investigate the mean water mass pathways from the subtropics to the tropics in the Atlantic Ocean. The OGCM is used in a fully realistic configuration of the Atlantic, from 30°S to 65°N, with realistic bathymetry. Surface forcings are provided by the COADS climatology. A non-eddy-resolving numerical simulation is analyzed with 3/4° horizontal resolution and 20 terrain-following vertical levels. The primary objective of this study is to assess the theoretical framework extending the ventilated thermocline theory to the equator in the context of the numerical calculation, and to establish whether the predictions of a steady-state theory can be verified in a time-dependent simulation, in which rectified seasonal effects on the time mean yearly circulation may be important. The Bernoulli function is evaluated on isopycnal surfaces outcropping in the subtropics in both hemispheres and floats are injected at different northern and southern latitudes. In both hemispheres, the interior flow velocities are parallel to the Bernoulli streamlines that are significantly modified by inertia only very near the equator and on the Equatorial UnderCurrent (EUC). In the Northern Atlantic, pathways from the subtropics to the tropics exist for the isopycnal surfaces outcropping at 20–22°N. The injected floats reach the EUC following a zigzag pattern determined by the tropical current system. It is impossible to distinguish between the western boundary and the interior exchange windows as they are merged together forming a broad exchange pathway east of the northwestward flowing North Brazil Current (NBC). This exchange window disappears for the floats injected north of ∼30°N, and corresponding outcropping isopycnals σ θ >25.5 kg/m 3 , where only the recirculating window of the subtropical gyre remains. In the Southern Atlantic, all the floats injected between 6° and 15°S migrate to the western boundary where they are entrained in the NBC. There is no interior exchange window. At the equator, some are directly entrained into the EUC, some overshoot and retroflect at ∼8°N, then join the EUC. As the numerical simulation is carried out under surface forcings that include the seasonal cycle, we can assess the impact of the seasonal cycle on the steady-state analysis. The most important effect is due to the Atlantic Intertropical Convergence Zone (ITCZ), which in summer is strong, and produces an “island” of Ekman upwelling between 10° and 20°N, which is reflected in the yearly mean properties. The ICTZ-induced upwelling and interior stratification support a corresponding “island” of high potential vorticity that penetrates in depth to all the isopycnals outcropping between 20° and 25°N. This high potential vorticity island creates a barrier that constrains the floats injected at and north of 20°N to flow around it to reach the Equator and the EUC.