Westward intensification in the subtropical gyres using your understanding of relative vorticity supplied by the wind and changes in vorticity due to meridonal transport of rotating parcels of water?

Answer 1

Currents flowing on the western side of ocean basins are intensified when compared with the currents flowing on the eastern side of ocean basins. Intensified currents are those that are narrow, extend to great depth and are fast. The largest and most prominent of such currents is the Gulf Stream in the North Atlantic Ocean, but similar currents also flow in the North Pacific Ocean (the Kuroshio Current), the South Atlantic Ocean (the Brazil Current), the Indian ocean (the Agulhas Current) and the South Pacific (the East Australian Current). The volume transport of the largest of these currents - the Gulf Stream - is about 55 million cubic meters per sec (or 55 sverdrups [sv] ). The reasons for the westward intensification are complex, but can be explained if we balance the three vorticities (induced rotational factors that change the direction of the flow of water) on each side of the basin. Those vorticities and their direction of rotation are:

1) Wind Stress - Trade Winds and Prevailing Westerlies create a clockwise (CW) vorticity that is equal on both sides of the basin.

2) Frictional - The eastern & western boundaries of the basin both induce a weak counter-clockwise (CCW) frictional vorticity (set up as a resistance to the flow of water) that is also equal on both sides of the basin.

3) Coriolis effect (CE) - The Coriolis effect, because it increases with latitude, creates a different vorticity on the two sides of the basin. On the western side, the northward flowing current is subject to an increasing CE deflection, which induces a CW vorticity, while on the eastern side, the southward flowing current is subject to a decreasing CE deflection, which induces a CCW vorticity. Note, therefore, that the CE rotational directions are opposite on the two sides of the basin.

The fact that the vorticity due to CE is opposite on the two sides of the basin makes balancing the three vorticities difficult, but it can be shown that the only way to achieve this balance is to intensify the current in the western side of the ocean basin (i.e., a Westward Intensification of the current).

This imbalance of CE results in a stronger eastward flow of water at the top of the subtropical gyre and a broad equatorward flow over most of the eastern side of the ocean basin.

This eastern boundary flow is consistent with observations in the North Atlantic, where the Canary Current off Africa is broad, slow and shallow, just the opposite of the characteristics of the Gulf Stream, which is very narrow, swift, and extends very deep.