INTRODUCTORY
OCEANOGRAPHYINTRODUCTORY
OCEANOGRAPHY
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In the last part of the lesson on circulation, we will look a the intensification of currents in the western boundaries of ocean basins, discuss undersea currents driven by density, and how Eddies are formed from a meandering Gulf Stream.
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Westward Intensification of ocean currents |
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] as shown in Fig. 9.9). 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 (more properly called Vorticity Tendancies) are:
The fact that the vorticity due to CE is opposite on the two sides of the basin makes balancing the three vorticities difficult. While the explanation is beyond the scope of this class, 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). To see the details of the argument for this balance, go to Balance of Vorticies (I will not hold you responsible for these details on the exam).
As
implied in Fig. 9.7 (a&b), 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, as shown in Fig. 9.12 on the right, 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.
We already have discussed one type of vertical circulation (upwelling), but of far greater significance is the subsurface circulation that occurs at intermediate, deep and bottom depths in the ocean. These density-driven currents are called thermo-haline currents because differences in temperature and/or salinity at the ocean surface cause density differences that drive these subsurface water masses.
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Thermo-haline circulation and conservative properties of a fluid |
Before
we begin our discussion, however, we need to define some terms and
lay the conceptual groundwork for tracing thermo-haline currents. At
depths more than a couple of hundred meters below the surface of the
ocean, the temperature and salinity of a water mass normally changes
very slowly (unlike the rapid loss of gain of heat or water across
the air-sea boundary for the surface waters). When it does change
rapidly it is only because a 'new' water
mass (with new salinity and temperature values) advects (flows) into,
and displaces the original water from the location. Therefore, we can
trace this new water mass by its temperature and salinity. We call
temperature and salinity 'conservatve properties of a fluid' because
they are changed only by advection.
Why are dissolved oxygen or carbon dioxide considered 'non-conservative properties of a fluid'? Oxygen and carbon dioxide may be changed by chemical or biological processes and, therefore, cannot reliably be used to trace the advection of a water mass (remember that hydrogenous calcium carbonate can be precipitated several kilometers below the surface, and when that happens, the amount of carbon dioxide dissolved in the seawater will change.
The classical model for thermo-haline circulation is shown in Fig. 9.22 and the places in the world's oceans where water sinks and upwells is shown in Fig. 9.20.
We also use a relative depth to characterize water masses, and there are four general depths beginning at the surface and extending to the bottom: Surface, Intermediate, Deep & Bottom, respectively.
Due to the circumstances surrounding the formation of a water mass, each water mass usually has a fixed temperature and salinity by which it can be identified (see NADW below). Sometimes a fixed temperature (18 degree Water in the Pacific) or a characteristic salinity value (see AAIW below) are used to trace a water mass.
We actually trace water masses using a plot of temperature and salinity (called a T-S Diagram - as shown above in Fig. 9.19, where I have added the labels for the three water masses discussed below: AAIW, NADW & AABW). These water masses have 'characteristic' features that identify them visually on the T-S Diagram and make them easy to recognize. For instance, AAIW, when present in the ocean, always shows up on the T-S Diagram with the shape shown above and with the minimum salinity in the water colum (i.e., nowhere else from the surface to the seafloor is there any seawater with less salinity). Also note on Fig. 9.19 that the curve extends from the top (the ocean surface) to the bottom (the seafloor) and depths are shown, but that the actual depth is NOT preserved on the graph.
To summarize: because the temperature and salinity of subsurface water masses changes only as a result of advection, we can use temperature and salinity to trace that advection. Now we will discuss the three thermo-haline masses in the Atlantic Ocean identified earlier.
There are three important water subsurface water masses in the Atlantic Ocean as shown in Fig. 9.23. Each is identified by the its point of origin and its relative depth:
Under special winter conditions, very cold air off the southern coast of Greenland lowers the temperature of the water and evaporation increases its salinity, and this very dense and highly oxygenated water sinks rapidly below the surface in a near vertical direction. As it gets deeper in the ocean it begins to encounter water of equal density and eventually it moves in a more horizontal direction toward the south at a deep depth. This NADW "hugs" the western boundary of the basin and flows as far south as the Antarctic Convergence Zone (at about 60 S latitude) before it begins rising back toward the surface. NADW is characterized by a coupled-pair of fixed salinity and temperature values and by its relatively high oxygen content.
During the Southern Hemisphere winter, the rapid freezing of ice in the Weddell Sea (a relatively shallow basin sea adjacent to the continent of Antarctica) creates very cold and saline water that flows off the shelf and, because of its high density (the highest in any ocean), it flows north along the bottom into the South Atlantic and under the NADW to as far north as about 8 S latitude. It too is characterized by a coupled-pair of fixed temperatures and salinity values.
AAIW
is formed at the Antarctic Convergence Zone primarily during the
Southern Hemisphere summer by mixing very low salinity surface water
from the melting of Antarctic ice with NADW and warmer surface water
flowing from the north. These three water masses combine to form AAIW
that flows north at an intermediate depth. Sandwiched between very
saline and warm surface water above, and cold and saline NADW below,
AAIW is slowly modified as it flows as far north as about 20 N
latitude, so its original temperature and salinity values are
changed. We trace AAIW, therefore not by a
coupled-pair of temperature and salinity values, but by its minimum
salinity in the water column (i.e., the water masses
everywhere else in the ocean above and below AAIW have higher
salinity values), as can seen in the latitudinal plot of salinity
versus depth above and in Fig. 9.23 below:
As shown in the color-enhanced image in Fig. 9.11, the Gulf Stream, like all western boundary currents, does not flow in a well-defined path. The eastern boundary of the Gulf Stream is quite diverse and its exact boundary is not easily found, but the western boundary has a quite well-defined break between the cool and fairly turbid shelf and slope water to its left, and the warm clear topical water of the current.
The Gulf Stream can move laterally at a rapid pace and, partly as a result of its contact with the edge of the continental shelf and slope, the current may meander. Usually, these meanders intensify so much that they break off, creating a rotating ring of what was very warm Gulf Stream water around a stationary core of cool or warm water.
Have you ever looked down from an aircraft flying over the Midwestern plains of America and noticed the horseshoe (or oxbow) lakes near the sides of large, slow moving and meandering rivers (such as the Missouri or Mississippi)? Did you know that they are formed as these meanders 'intensified' (turned more sharply) and the river bank between two 'loops' of a meander erodes and 'breaks-off', leaving the lake isolated from the river?Now, think about the progression of the meanders in the Gulf Stream and how they may produce eddies on either side of the current, then look at the explaination below.
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Gulf Stream meanders and all those eddies. |
The temperature of the core-water in warm-core and cool-core eddies depends upon the source of the water inside the ring.
The figure shown below shows the progressive development of eddies from meanders in a western boundary current such as the Gulf Stream (GS). Three panels are shown; the panel on the left shows moderately strong meanders in the current and the location of the cool shelf water (shaded blue) to the left of the GS and the warm Sargasso Sea water (shaded red) to the right of the GS; the middle panel shows the further development of the meanders (it is important that you note the warm and cool water 'inside' the meanders as they continue to develop).
It is important that you note the direction of the flow in the meandering current. As shown below, if cool shelf water is trapped within a CCW flowing meander when it 'pinches off', a cool-core eddy (CCE) is formed; if warm Sargasso Sea water is trapped within a CW flowing meander when it 'pinches off', a warm-core eddy (WCE) is formed. The direction of rotation of the rings around these eddies is the same as that of the meanders just before it breaks off.
Can you complete the picture, showing the resulting eddies - cool core or warm core - and the direction of flow of the rings [the remnants of the shear current] around the eddies?
This is shown in a more colorful way in Fig. 9.10, with the progression of the eddies shown in parts a, b, c & d.
Do you see that creating an eddy is the only way that water from one side of the current can be carried to the other side?
What do you think I mean when a say an eddy "spins-down"? What spins-down?
After breaking off, neither of these rings will have a source of energy to keep it rotating and eventually will "spin down" due the internal friction of the water. Then the core will be absorbed into the larger mass of the surrounding ocean. Many warm core eddies are quickly entrained back into the Gulf Stream as its path moves laterally east and west, but those north of Cape Hatteras may last for many months. The longest lasting eddies are cool-core, because they are all in very deep ocean water and generally do not spin down as quickly as warm-core eddies to the left of the current. In fact, many last for several years.
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