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Vorticity Balance of Outcropping Isopycnals

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  • 1 Center for Coastal Physical Oceanography, Old Dominion University, Norfolk, Virginia
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Abstract

The authors extend Marshall and Nurser's analysis of potential vorticity (PV) flux into outcropping isopycnic layers of the oceanic thermocline to the nonstationary case, allowing for the seasonal migration of isopycnal surfaces under surface heating and cooling. The most important new result is that the bulk of the surface PV flux arising from seasonal heating is used up in creating stratification as an isopycnal outcrop moves northward, extending the stratified layers of the thermocline. Residual PV transport (flux times the separation distance between adjacent isopycnals) reaching the interior thermocline is small in quiescent regions where only mean advection (connecting to subduction or upwelling at the outcrop) operates, and is given by Marshall and Nurser's formula, unaffected by the migration of the isopycnals. Where geostrophic turbulence is vigorous, it supports another pathway of PV transport, via Reynolds flux of vorticity. Larger PV transports are then possible within the range of action of the geostrophic turbulence in locations where Ekman transport only partly balances wind stress.

Abstract

The authors extend Marshall and Nurser's analysis of potential vorticity (PV) flux into outcropping isopycnic layers of the oceanic thermocline to the nonstationary case, allowing for the seasonal migration of isopycnal surfaces under surface heating and cooling. The most important new result is that the bulk of the surface PV flux arising from seasonal heating is used up in creating stratification as an isopycnal outcrop moves northward, extending the stratified layers of the thermocline. Residual PV transport (flux times the separation distance between adjacent isopycnals) reaching the interior thermocline is small in quiescent regions where only mean advection (connecting to subduction or upwelling at the outcrop) operates, and is given by Marshall and Nurser's formula, unaffected by the migration of the isopycnals. Where geostrophic turbulence is vigorous, it supports another pathway of PV transport, via Reynolds flux of vorticity. Larger PV transports are then possible within the range of action of the geostrophic turbulence in locations where Ekman transport only partly balances wind stress.

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