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Lecture 24: SURFACE CIRCULATION OF THE OCEANS- In More Detail

Powerpoint Lecture Slides

Major surface currents- Review
Water transport -- Ekman model
Rotary motion of major gyres -- "geostrophic" currents
Intensification of western boundary currents
Upwelling and downwelling
Eddies in surface circulation


MAJOR SURFACE CURRENT SYSTEMS
Initiated by global winds
Deflected by continents and Coriolis effect
Circular systems of currents- Gyres
Identification of some important major currents
Gulf Stream
Equatorial currents and counter-currents
West wind drift - circles antarctica
Very important- connects Atlantic and Pacific
Note that ocean circulation may change as continents drift
e.g., Global climate 100 million years ago?
 
WATER TRANSPORT AT SURFACE AND DEPTH
-- model of V. W. Ekman
Surface currents are at 45° to wind direction (due to Coriolis effect)
Successive deeper layers- angle is greater and greater
This is the "Ekman spiral" of water movement
- to a depth of about 150 m
"Ekman transport" - - average direction of this layer is 90° from wind direction
 
Slight mound of water inside each Gyre
1. Circular currents around a gyre
2. Ekman transport forces water into the gyre, creates a "hill"
of water. For example, the middle of the Sargasso Sea
(tropical N. Atl.) is 1.5 m higher than the edges
3. Gravity tends to push water down and away from the "hill"
This leads to a balance of forces: circular currents, not much inward flow
Western Intensification of currents in the gyres
Because of the details of winds and the Coriolis effect:
-Western currents: strong, narrow, deep (Gulf Stream)
- Eastern currents: slow, broad, shallow (Canary Current)
-This effect not very important in S. Hemisphere (positions of land masses)

 

UPWELLING AND DOWNWELLING INDUCED BY EKMAN TRANSPORT
Ekman transport can....
1) Drive surface waters apart creating zones of upwelling,
2) Force them together creating zones of downwelling
3) Drive surface waters away from coasts (upwelling)
4) or force them onto coasts (downwelling)

Regions of important upwelling and downwelling
1. Equatorial upwelling
2. Upwelling along west side of continents
(especially South America and Africa)
* Upwelling and downwelling along coasts
may change with seasons due to seasonal wind changes
Importance of upwelling: Brings nutrient-rich deep waters
close to surface creating regions of high productivity
 
EDDIES IN SURFACE CIRCULATION
Rotation created at the edges of currents
Rings of circulating water, 10-100 km wide
Commonly develop in the Gulf Stream:
Cold water - Warm water boundary meanders like a river
Eddies become pinched-off and move with prevailing current
Waters in eddies have different properties (T, S, motion)
than the water masses or currents in which they are embedded
Rotary motion in eddies can extend to great depths and
even stir-up bottom sediments; they are analagous to
"storms" in the oceans
 

(Detailed notes start here)
 
The major surface current systems of the world oceans are initiated by global winds (i.e., trades, westerlies, etc.) and deflected by the Coriolis effect and continents. Surface current systems are made up of a series of E-W currents (wind-driven) and N-S "boundary"currents (deflection) that are coupled together in major rotational gyres.

Water transport at the surface and at depth by wind-driven currents and Coriolis deflection. V. W. Ekman addressed this problem from a theoretical perspective. Ekman's model showed that surface currents move a 45 deg. to the direction of prevailing winds; the deflection is due to the Coriolis effect -- to the right in the N.H. and left in the S.H. With greater depth, successive layers are deflected even more, producing what is called the "Ekman spiral" of water movement to a depth of about 150 m. Ekman further showed that the net transport of water from the surface to 150 m is at 90 deg. to the wind direction (again, right in N.H., left in S.H.).

Continuous rotary flow of surface currents around gyres. Ekman's model really helps us understand this phenomenon. First of all, because Ekman transport is at right angles to the direction of prevailing wind, it actually forces water to flow into the center of a gyre, creating a "hill" of water. (The middle of the Sargasso Sea in the tropical North Atlantic is 1.5 meters higher than the edges!) But then gravity tends to force water down and away from that hill. When gravity and Ekman transport balance one another, there is a steady-state flow of currents parallel to the elevation contours of the hill. Currents flow around the hill due to the Coriolis effect -- they are continuously deflected as they travel over long distances. Keep in mind that this situation is analogous to winds circulating around high-pressure zones.

Intensification of western boundary currents. The flow of currents around Northern-Hemisphere gyres is not symmetrical. Currents flowing south to north on the western side of gyres (western boundary currents) are strong, narrow, and deep, such as the Gulf Stream. In contrast, currents flowing on the eastern side (e.g., Canary Current) are slow, broad, and shallow. Several factors probably contribute to this western intensification:

Trade winds displace currents to west side.
Because of the piling up of waters on the west side, there is increased
friction between the currents and continents, and currents must flow faster.
The Coriolis effect increases with latitude. Thus, northward-flowing currents
are given an additional "twist" as they move.

Intensification of western boundary currents is less important in the Southern Hemisphere.
Deflection of West Wind Drift around Antarctica by South America and Africa
create strong currents on eastern side of Pacific and Atlantic.
Deflection of South Equatorial Current in the Pacific to the north by the
islands of Indonesia decreases the flow of water to the south.

Upwelling and downwelling induced by Ekman transport. In the open ocean, Ekman transport can drive surface waters apart creating zones of upwelling, or force them together creating zones of downwelling. In coastal areas, Ekman transport can drive surface waters away from coasts (upwelling)
or force them onto coasts (downwelling). Regions of important upwelling and downwelling include the following:
Equatorial upwelling in the open ocean. Trade winds generate Equatorial
currents, and the net transport of water is away from the Equator.
Upwelling along west side of continents, especially South America and
Africa. Strong northward-flowing currents parallel to these (Southern
Hemisphere) coasts transports water away from the coasts.
Upwelling and downwelling along coasts due to seasonal wind changes
and current flow.
Upwelling is important to the biology of the seas because it brings nutrient-rich deep waters close to surface, creating regions of high productivity.

Eddies in surface circulation. Eddies are rings of circulating water, 10-100 km in diameter. They develop at the boundaries of major surface currents, like the Gulf Stream. Major currents meander much like rivers. When the meanders become extreme, part of the current or adjacent water through which the current moves can become pinched off -- a rotating eddy is formed. Waters in eddies have different properties (T, S, motion) than the water masses or currents in which they are embedded. The rotary motion in eddies can extend to great depths and even stir-up bottom sediments. In that way, eddies are analagous to "storms" in the oceans.


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