Dines Compensation in the Ocean

Divergence can occur at the ocean surface when two ocean currents split. Upwelling will occur underneath that location.

Divergence often occurs when geostrophy is disturbed. In other words, when new accelerations are imposed on an ocean current or ocean parcels already in geostrophic balance. (Remember, in both the atmosphere and ocean, geostrophic flow is non-divergent).

Upwelling on the West Coast of Continents

In the eastern portion of the oceans, boundary currents, such as the California Current, are already in geostrophic balance. Flow will be parallel to the altimeter contours, as discussed in Metr 201.

On summer mornings, atmospheric pressure gradients and winds are often light over the region in which the California Current is present. By mid-afternoon, a strong onshore pressure gradient develops, causing northwest winds to strengthen, typically to 20 to 30 mph sustained by 4PM Local Time. Thus, additional wind stress develops on the ocean surface, so that ocean parcels at the surface are accelerated by frictional drag, initially in the direction of the wind.

Schematic of California coastal upwelling

The surface water parcels then are deflected to the right, because their motion is no longer in geostrophic balance. Coriolis acceleration is now greater than the pressure (altimetry) gradient acceleration, and the surface water is deflected about 45 degrees to the right of the wind. This alone would produce divergence in the surface layers, which must be compensated for by upwelling.

 

Upwelling calculation

 

However, frictional drag causes the next layer of ocean downward to also drift in the direction of the surface flow but with lesser speed, originally, but in turn is deflected by Coriolis acceleration. This second layer then impacts the next layer down, creating the well known Ekman spiral, which also would finally contribute to bottom flow towards the coast, and upwelling as well.

The offshore deflection of surface waters is not uniform along the coast, with some areas topographically favored. This creates the well known upwelling plumes visible on infrared imagery.

Upwelling Plumes

Upwelling Generated by Persistant Atmospheric Disturbances

When closed atmospheric surface cyclones become quasistationary in a region, an ocean disturbance can be imposed on current systems initially in geostrophic equilibrium. As the ocean surface begins to turn counterclockwise, each fluid parcel will begin to be deflected to its right by Coriolis acceleration. Until the ocean surface reachieves geostrophy again, the ocean surface will be accelerated outward with respect to the cyclonic circulation center. Water will upwell underneath that center of divergence. Thus, the center of the eddy will be cold, and a cold ring will have formed.

Closed atmospheric surface antityclones will impose a convergent circulation in the ocean. Such centers will be associated with downwelling and the development of a warm ring.

Eddy downwelling

 

 

Upwelling Generated by Topographic Interference With The Wind

When uniform winds encounter islands, it is common for winds to be deflected and slowed in such a manner, that differential motion is imparted to the ocean. Downwind of a major island, such as Hawaii, wind on either side of the island will be unimpeded by the island, whereas a back eddy in the wind will develop immediately downwind of the island. If the island is large enough, a cyclonic and anticyclonic couplet will occur in the wind, with a similar flow imparted to the ocean surface.

As in the last example, the cyclonic ocean eddy will have upwelled waters at its center, and the anticyclonic ocean eddy will have downwelled waters at its center. And, as in the last example, this leads to the production of ocean rings.

Hawaii