A Two-Gyre Ocean Model Based on Similarity Solutions

Abstract

Abstract An ocean model based on similarity solutions derived from the thermocline equations is defined, and its properties are studied in a stationary case. This model only applies to cases that show small departures from zonality. It is continuously stratified, has open boundaries (in particular it extends indefinitely westward), and does not represent boundary currents. The zonal gradient of density is prescribed constant at the northern and southern boundaries, the density along the eastern coast is prescribed at the surface and bottom, and a zonal (null) Ekman pumping is applied at the surface (bottom). The mean state obtained with this simple model shows a quite realistic thermocline pattern. In particular, two gyres are represented, which improves the results obtained from the previous models based on similarity solutions. Waters of nearly constant density are found in the first 600 m in the subpolar gyre, and the northward “tilt of the thermocline” is also reproduced. In this model, the vertical diffusion coefficient governs the detailed structure of the thermocline. However, the role of the horizontal diffusion coefficient is crucial as it allows mass exchanges between the gyres and has a strong impact on the meridional and vertical velocities, which in turn affect the thickness of the thermocline. Eventually, the results suggest that, as soon as the classical integral constraints exerting on the potential vorticity fluxes are taken into account, a (detailed) representation of the boundary currents is not needed to model the gross features of the thermocline in the open ocean.