The Arctic and Subarctic Ocean Flux of Potential Vorticity and the Arctic Ocean Circulation

摘要

According to observations, the Arctic Ocean circulation beneath a shallow thermocline can be schematized by cyclonic rim currents along shelves and over ridges. In each deep basin, the circulation is also believed to be cyclonic. This circulation pattern has been used as an important benchmark for validating Arctic Ocean models. However, modeling this grand circulation pattern with some of the most sophisticated ocean-ice models has been often difficult. The most puzzling and thus perhaps the most interesting finding from the Arctic Ocean Model Intercomparison Project (AOMIP), an international consortium that runs 14 Arctic Ocean models by using the identical forcing fields, is that its model results can be grouped into two nearly exact opposite patterns. While some models produce cyclonic circulation patterns similar to observations, others do the opposite. This study examines what could be possibly responsible for such strange inconsistency. It is found here that the flux of potential vorticity (PV) from the subarctic oceans strongly controls the circulation directions. For a semienclosed basin like the Arctic, the PV integral over the whole basin yields a balance between the net lateral PV inflow and the PV dissipation along the boundary. When an isopycnal layer receives a net positive PV through inflow/outflow, the circulation becomes cyclonic so that friction can generate a flux of negative PV to satisfy the integral balance. For simplicity, a barotropic ocean model is used in this paper but its application to the 3D models will be discussed. In the first set of experiments, the model with a realistic Arctic bathymetry is forced by observed inflows and outflows. In this case, there is a net positive PV inflow to the basin, due to the fact that inflow layer is thinner than that of outflow. The model produces a circulation field that is remarkably similar to the one from observations. In the second experiment, the model bathymetry at Fram Strait is modified so that the same inflows and outflows of water masses lead to a net negative PV flux into the Arctic. The circulation is reversed and becomes nearly the opposite of the first experiment. In the third experiment, the net PV flux is made to be zero by modifying again the sill depth at Fram Strait. The circulation becomes two gyres, a cyclonic one in the Eurasian Basin and an anticyclonic one in the Canada Basin. To elucidate the control of the PV integral, a second set of model experiments is conducted by using an idealized Arctic bathymetry so that the PV dynamics can be better explained without the complication of rough topography. The results from five additional experiments that used the idealized topography will be discussed. While the model used in this study is one layer, the same PV-integral constraint can be applied to any isopycnal layer in a three-dimensional model. Variables that affect the PV fluxes to this density layer at any inflow/outflow channel, such as layer thickness and water volume flux, can affect the circulation pattern. The relevance to 3D models is discussed in this paper.