INTRODUCTORY
OCEANOGRAPHYINTRODUCTORY
OCEANOGRAPHY
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In this first part of the lesson on ocean circulation, we will introduce the first theory of wind-driven circulation and one of its very important consequences, the upwelling of subsurface water.
As
you can see from Fig. 9.8, wind-driven surface currents have many of
the same characteristics in all three oceans. All have currents that
are intensified in the western boundary of the ocean basins and all
have major current gyres that flow clockwise (in the Northern
Hemisphere or counterclockwise (in the Southern Hemisphere) -- we
will discuss the reasons for this later in this lesson. Note that the
winds that power these major gyres are the Trade Winds and the
Prevailing Westerlies.
Also, note that not all western boundary currents are as large nor as well defined as the Gulf Stream in the Atlantic Ocean. In particular, look again at Fig. 9.8 and note the deflection of part of the S. Equatorial Current into the N. Hemisphere. You can also see some of that in Fig. 9.9 to the right. The numbers in this figure are measured in sverdrups (sv = 1 million cubic m/sec).
Given the circulation shown Fig. 9.8, how do you think the volume transport of the Brazil Current compares with the volume transport of the Gulf Stream?
When wind blows over the surface of the water, energy in the form of momentum is transferred from the air to the water. In Chapter 10, we will see that this transfer creates ocean surface waves, but in the this lesson we will focus on the net movement of water that results from this momentum transfer, the wind-drift current.
The first model describing wind-driven (drift) currents was formulated in the early 20th century by Walfrid Ekman, a Swedish physicist/meteorologist/ oceanographer. He formulated this model after Fridtjof Nansen (a Norwegian oceanographer) described the behavior of ice flows being driven by the wind in the Arctic Ocean. Nansen reported that the ice flows appeared to move between 20-40 deg. to the right of the direction of the wind blowing across the surface. Two of the most important results of that model for the North Hemisphere are discussed below:
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Ekman's contribution to ocean circulation modeling |
Ekman
assumed that a homogenous water column was set in motion by wind
blowing across its surface and, because of the CE, the top-most layer
motion was 45 degrees to the right of the surface wind direction. The
surface layer, moving as a thin lamina, sets the layer beneath it in
motion, which also is subject to deflection to the right by the
Coriolis force. In this manner the wind energy is passed through the
water column from the surface down, with each successive layer being
deflected more to the right and having a lower velocity than the
layer above (as shown in Fig. 9.5 b to the right). This continues
until all the momentum imparted by the wind at the surface has been
distributed to all the lamina, and there is no longer any motion. The
depth at which motion ceases is called the
Depth of Frictional Influence
(usually about 100 - 150 m).
Together, all these motions produce what has been called the Ekman Spiral (shown in Figure 9.5c above). To visualize this Ekman spiral, I want you to imagine a spiral staircase down which you are attempting to flee from danger in a nightmare. Much to your distress, as you descend, you find that the width of the steps (the velocity vectors of each lamina) gets progressively narrower and you are trapped when you reach the point where the steps disappear (the depth of frictional influence in the Ekman Spiral).
Of
much greater interest to physical and biological oceanographers,
however, is the net motion of the water
being moved by the water in the Ekman spiral. This net
motion would include the average velocity (direction and speed) of
all the lamina in the spiral. To illustrate, imagine a spar float (a
long, narrow float that remains vertical in the water) with a length
exactly that of the depth of frictional influence, that is made to
float with NONE of its length being out of the water (so that it will
not be moved by the wind). If this float were put into the ocean in
an active wind field (of sufficient strength and duration to produce
the Ekman spiral) it would act as an integrating (averaging) tool
that would move in the direction of the net movement of all the water
mass between the surface and the depth of frictional influence.
That direction, according the theory, is 90
deg to the right of the wind direction (as shown in the figure at the
right).
If you were in the Southern Hemisphere, in what direction relative to the wind would you find the movement of the surface lamina and the Ekman transport?
A very practical and important consequence of the Ekman transport is the vertical upward movement (upwelling) of water. It is important to understand that this upwelled water will come from below the surface (possibly even as deep as the thermocline), and that this upwelling is one of the few ways in which water is exchanged upward across this substantial barrier. In later lessons we will learn details about the important biological consequences of such upwelling (where nutrients are returned to the euphotic zone), but here we will focus on the physical mechanisms that cause upwelling. There are two primary locations in which upwelling occurs:
Do you see that when the winds blow from the south parallel with the coast in the Northern Hemisphere (Fig 9.15b, above), the Ekman transport moves toward the coast and results in downwelling
What is the relationship of the Ekman Transport to the wind for the Southern Hemisphere - contrast the upwelling and downwelling for the Southern Hemisphere with that from Northern Hemisphere for the coastline and winds shown above?
Recall that the ITCZ migrates semi-annually north and south and, because there is more land mass north of the equator in the Atlantic Ocean, the 'thermal equator' (the book calls it the meteorological equator) is centered around 5 deg. N. Note that even the SE Trade Winds (remember that winds are reconned by the direction FROM WHICH they blow) will create Ekman transport away from the geographical equator north of the equator, ensuring that upwelling will result.
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