Geostrophic Currents

1. Geostrophic Currents Ekman transport of surface water can lead to it �piling up� or �being removed� from an area This is known as convergence or divergence Even if the winds blew continuously, we do not have a continual pile-up or removal of water Other motions tend to try to restore the sea surface towards a �flat� state. These motions are associated with horizontal pressure gradients

2. Geostrophic Currents Pressure gradient forces move water from high pressure regions under an elevated sea surface to low pressure regions Unlike Ekman motion, pressure gradient transfer occurs at all depths where a pressure gradient may be found This means that the mean flow in deeper layers may oppose the Ekman transport This will also produce vertical motion, such as upwelling or downwelling

4. Geostrophic Currents Since the pressure gradient forces are producing motion, we must consider the Coriolis Effect As the parcel starts to move, it is deflected by the Coriolis effect so that the direction of flow is no longer directly down the pressure gradient Flows are accelerated and deflected until the pressure gradient is balanced by the Coriolis force The resulting Geostrophic Current is thus deflected 90� to the right of the pressure gradient in the Northern Hemisphere

5. Geostrophic Current Geostrophic Current: A current that is produced by the balance between the pressure gradient force and the Coriolis force with the direction of flow 90� to the right of the pressure gradient in the Northern Hemisphere The flow is 90� to the left of the pressure gradient in the southern hemisphere Mathematically: Note that in reality flows are never perfectly geostrophic as friction and other effects come into play

8. Open Ocean Surface Currents Surface currents initiated by Ekman transport Then maintained as Geostrophic Currents Generally reach a few hundred metres deep, although some can reach 2000 m One typical ocean unit of volume transport is the Sverdrup (Sv) 1 Sv = 106 m3 s-1

9. Water Covered Earth The Ekman transport is based on the large scale atmospheric circulation This leads to bands of convergence and divergence Convergence ~ 30� Divergence ~ Eq., 60� This then generates large-scale geostrophic currents that would flow around the Earth

10. Gyres These circulation patterns are modified by the presence of land Only in the south can a current flow all the way around the world Antarctic Circumpolar Current At most latitudes, we get zonal currents within each ocean The currents are deflected north and south by land to form western and eastern boundary currents

11. Gyres Gyres: Closed circulation loops within the ocean Sub-tropical: gyres which flow clockwise (anti-cyclonic) in the Northern Hemisphere Sub-Polar: gyres flowing counterclockwise (cyclonic) in the Northern Hemisphere Gyre direction reversed in southern hemisphere