On gyre interactions

The principal meeting point of the subtropical and subpolar gyres of the North Atlantic is at the Tail of the Grand Banks where the two western boundary currents, the Gulf Stream and Labrador Current, join forces as the North Atlantic Current, which flows northeast almost 10 degrees in latitude before turning east as the Subpolar Front, ultimately feeding the Labrador and Nordic Seas and the thermohaline overturning. After the Gulf Stream turns into the North Atlantic Current at the Grand Banks, its role shifts from a wind-driven current to a link in the large-scale thermohaline circulation. The processes governing this transition, in particular the continued transport north of mass and heat, are questions of considerable climatic importance. The North Atlantic Current is a very unusual western boundary current in that its mass transport decreases in the downstream direction. The mean path and annual shifting of the eastward flowing Gulf Stream is conjectured to result from a time-varying shelf-Slope Water overflow of waters from the Labrador shelf. As the volume transport increases in fall and deepens the Slope Water pycnocline, it forces the Gulf Stream south and deepens the Sargasso Sea thermocline as well. The timing of these steps governs the June maximum in baroclinic transport. There is some evidence that this 'back-door' gyre interaction may operate on interannual time scales as well. The question then arises whether the shelf-to-Slope Water Sea transport also plays a role in governing the separation of the Gulf Stream. The widely observed robustness of the width of the Gulf Stream appears to result from a tight balance between the release of available potential energy and the kinetic energy of the current. A broader current would release more energy than can be 'disposed of', while a narrower current requires more kinetic energy than is available to sustain it. It is shown that for plausible dissipation rates in the recirculation gyres, the amount of energy that needs to be expelled from the Gulf Stream is such a small fraction of that advected through as to be vitually undetectable, hence the stiffness of the current. (C) 1999 Elsevier Science Ltd. All rights reserved.