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. 7.5 and on the right, 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, as shown in Fig. 7.4.
Do you recall in Chapter 6 (Atmospheric Circulation and Air Sea Interaction) that we showed a picture of the Loop Current in the Eastern Gulf of Mexico that provided a continuous source of warm water into that region.
As you can see in the figure above, this Loop Current is part of the North Atlantic Gyre and includes water from the S. Equatorial Current that circulates through the Caribbean Sea and enters the Gulf of Mexico through the Yucatan Straits. From there is goes through the Florida Straits and turns north, joining with water from N. Equatorial Current to form the Gulf Stream.
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. 7.5 and the figure above, and note the deflection of part of the S. Equatorial Current into the N. Hemisphere.
You can also see some of that in Fig. 7.15 & 7.16 and the figure to the left. The numbers in this figure represent volume flow of the currents, and are measured in sverdrups (sv = 1 million cubic m/sec).
Given the circulation shown in Fig. 7.5 (and what we just said in the last paragraph), why do you think the volume transport of the Brazil Current is considerably less than 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 (the transfer to the water is not very efficient, being just a few percent of the wind energy). In the Waves Chapter, we will see that this transfer creates ocean surface waves, but in this lesson we will focus on the net movement of water that results from this momentum transfer; the wind driven current is called a wind-drift current.
The first model describing wind-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 (Fig. 7.6). 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. See Fig. 7.7 for a pictoral representation of that motion.
Two of the most important results of that model for the North Hemisphere are discussed below:
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 moved 45 degrees to the right of the surface wind direction. The surface layer, moving as a thin lamina, then sets the layer beneath it in motion, which also is subject to deflection to the right by the CE. 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 in the figure to the right) as energy is dissipated. 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 Fig. 7.7 below). 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 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 to the theory, is 90 deg to the right of the wind direction (as shown in Fig. 7.7a & b below) in the Northern Hemisphere and is called ET. Note the part of the figure on the right, where the arrows show the spiral of the arrows of each lamina.
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 ET is the vertical upward movement (upwelling) of water that results from the transport. This upwelled water comes from below the surface (usually deeper than the thermocline - a very substantial barrier to the vertical movement of water), and is one of the few ways in which water is exchanged upward across the thermocline. 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 we can also get upwelling along an EASTERN COASTLINE in the Northern Hemisphere, when winds blow parallel with coastline from the south (draw a picture)?
It is important that you understand that to get upwelling as a result of ET along either an Eastern or Western coastline, you must have ET moving offshore (regardless of hemisphere).
Note, in the bottom figure on the right, and Fig. 7.12b, that when the winds blow from the south parallel with this same western coastline in the Northern Hemisphere, the ET moves toward the coast and results in downwelling.
What is the relationship of the ET to the wind for the SOUTHERN HEMISPHERE?
Use this ET direction to contrast especially the conditions that support upwelling and downwelling for the Southern Hemisphere with that for the Northern Hemisphere for the coastline and winds shown above.
Do you recall that in the Sedimentation Chapter we discussed the intense band of Radiolarian production (that produced the Radiolarian ooze) just north of the equator in the Pacific? This was caused by the upwelling of nutrients from below the permanent thermocline because of the Equatorial Upwelling just described.
Recall that the ITCZ migrates semi-annually (twice a year) 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 NAMED 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.