Geoscience Reference
In-Depth Information
very large since the difference in volumic mass at the interface is
weak compared to the average volumic mass: the motions of the
interface multiply the motions of the free surface a hundredfold, in the
opposite direction. Consequently oceanographers adhered for a long
time to describe the topography of the thermocline as a picture of the
geostrophic circulation in the surface layer.
Cyclonic
Anticyclonic
Horizontal plan
Horizontal plan
Low
pressure
High
pressure
Downwelling
Upwelling
Vertical section
Vertical section
Ekman transport
Wind
Figure 2.10.
Diagram illustrating Ekman pumping in the northern
hemisphere (from [OUC 89])
We will look at what is produced when an atmospheric low pressure
passes over the ocean: the low pressure is marked by cyclonic winds
curling around the center of low pressure in an anticlockwise direction
(it is said that the rotation of these winds is positive). These winds drive
an Ekman transport to their right, thus diverging: the waters of
the surface layer are pulled toward the exterior of the low pressure, the
surface level of the ocean sinks (the surface pressure diminishes), while
the thermocline, between the surface layer and the interior ocean, rises
(Figure 2.10). In the case of a low pressure, this rising of water from the
