Ekman Balance and Wind-Driven Circulation

In the last lecture you saw geostrophic balance – the flow that follows sea-level slopes. Now Gael Alory turns to the other great driver of ocean circulation: the wind. This lecture follows the wind’s influence from the sea surface all the way down to the deep circulation of entire ocean basins.

It builds up in stages:

• Wind stress – how the wind grips the sea surface and passes its momentum to the water, creating the surface mixed layer
• The Ekman spiral – the surprising result that wind-driven surface water flows at an angle to the wind, and that the Ekman transport as a whole moves at right angles to the wind – to its right in the northern hemisphere, to its left in the southern
• Upwelling – how this sideways transport lifts cool, nutrient-rich water to the surface along coasts and at the equator, sustaining some of the world’s richest fisheries
• Ekman pumping – how patterns in the wind push water down or draw it up across the open ocean
• Wind-driven gyres – how, through these mechanisms, the wind ultimately drives the great basin-scale circulations, including the western boundary currents like the Gulf Stream

Watch for the recurring theme: the wind rarely pushes the ocean in the direction you would expect. Because of the Earth’s rotation, its effects appear at right angles, and those right-angle turns explain upwelling, the gyres, and much of the circulation you will meet for the rest of the course. As always, the aim is physical intuition, not the mathematics.