Key Signatures in the Barotropic Vorticity Budget within Ocean Western Boundary Currents : Understanding the Gulf Stream Pathway

Abstract:

A suite of ocean simulations are conducted at resolutions of 1 degree down to 1/36 degree using the MIT General Circulation Model, Regional Ocean Modeling System, and the Community Earth Systems Model. The 1 degree simulations in the CESM exhibit warm sea surface temperature biases (upwards of 8 C) from the Mid-Atlantic Bight through the Gulf of Maine. In a coupled climate simulation the bias can lead to misplaced heat fluxes which drastically reduce the fidelity of the climate simulation. These observed errors are typical of coarsely resolved ocean simulations and are directly linked to the improper location of the Gulf Stream. Thus, determing a the cause of this error requires a detailed study of a more well-behaved numerical Gulf Stream. 1/12 degree and 1/36 degree simulations show an improvement in the modelled Gulf Stream when compared with observations.

All of the simulations show an agreement in the leading balance in the barotropic vorticity budget within the Gulf Stream, dominantly between planetary vorticity advection and bottom pressure torques. This suggests that this inviscid balance is not the only requirement for proper Gulf Stream pathway, since this signature appears in the poorly separating scenario. However, it is shown that the free surface and baroclinic contributions to the bottom pressure torque largely cancel where the modelled Gulf Stream separates. This compensation is necessary for the transport and the upper layer Gulf Stream to cross the edge of the continental shelf. Scaling arguments owe this cancellation to the relative strength between the Gulf Stream and Deep Western Boundary Current. Hence, weak shear may not allow for the appropriate compensation to allow the transport to cross at Cape Hatteras where the bathymetric gradients are upwards of 20%. It is supposed that the sea surface temperature bias is directly caused by the weak vertical shear between the Gulf Stream and Deep Western Boundary Current which disallows decoupling from bathymetry at Cape Hatteras.