Buoyancy-forced circulation in an idealized Red Sea

The MIT General Circulation Model (MITgcm) is used to study the buoyancy-driven circulation in the idealized Red Sea on a β-plane, where Coriolis parameter f=f₀+ βy. The surface buoyancy loss, due to both cooling and a fresh water flux, in the Red Sea has a strong meridional gradient with smaller buoyancy loss in the southern Red Sea and larger buoyancy loss in the northern Red Sea. Light water flows into the Red Sea and dense water flows out in order to balance the buoyancy loss over the Red Sea surface. The northward surface flow in the Red Sea crosses from the western boundary to the eastern boundary on its journey north. This western boundary current in the southern Red Sea disappears if horizontally uniform buoyancy forcing is applied in the numerical model. Analysis of the potential vorticity (PV) budget indicates that the balance between the advection terms and friction is consistent with the circulation patterns in both cases. The potential vorticity is the sum of planetary PV, relative PV and stretching PV due to stratification. Planetary PV increases with latitude and stretching PV decreases with latitude due to surface buoyancy loss. Contribution of stretching PV to the PV budget must be balanced with a negative PV production due to friction, which requires a cyclonic boundary circulation. Contribution of planetary PV to the PV budget must be balanced with a positive PV production due to friction, which requires an anticyclonic circulation. In the experiment which is driven by the surface buoyancy loss with a meridional gradient, the competition between planetary PV and stretching PV forms an anticyclonic circulation in the southern Red Sea and a cyclonic circulation in the northern Red Sea. The contribution of stretching PV to the total PV increases with larger f₀, while the contribution of planetary PV increases with larger β. Numerical experiments with different values of β or f₀ support that the crossover latitude of the western boundary current in the southern Red Sea moves north with larger β or smaller f₀.