<-previous | Geol 117 Home | Lectures | Review | next ->

Lecture 23: OCEAN CIRCULATION -- BASIC CONCEPTS

Wind-driven surface currents
Density-driven deep circulation

Powerpoint Lecture Slides


OCEAN CIRCULATION -- INTRODUCTION

Surface Circulation

* Horizontal currents in upper few 100 meters; speed = about 1 m/s
* Driving force: prevailing winds initiate currents
* Modifying factors: deflection by ...

Deep Circulation

* Driving force: creation of dense water masses at the surface of high-latitudes oceans (particularly the Atlantic).
1) Coriolis effect
2) Continents and mid-ocean ridges
* General pattern -- sinking, spreading, eventual upwelling


Surface Current Details: Winds drive surface currents, BUT...

Deflection of surface currents by Earth's rotation and continental positions

Atlantic -- rotary, or circular motion- the gyres (concentrate on N. Hemisph.)
Equatorial currents
Gulf Stream
North Atlantic Drift --climate effects
Canary Current
 
Currents in the North Pacific Ocean
- - California Current
- - Kuroshio Current
- - North Pacific Drift
- - polar currents (Alaska, Oyashio)
- - Equatorial Counter Current (deflected by Asia)

General model of wind-driven currents and gyres

Trade winds and westerlies by themsleves would create circular motion
Additional help from Caoiolis Effect and Continents

Circulation around Antarctica
No continental barriers
Continuous current (surface-to-deep water)
Deep Circulation- Convection in the Oceans

Review: Controls on the density of seawater -- T, S, pressure (depth)

Temperature
- - The most important factor controlling surface density
- - Warm waters are at surface, especially at low and mid latitudes
- - Cold waters formed during seasonal cooling at high latitudes tend to sink

Salinity
- - Important at high latitudes in the open ocean where waters are uniformly cold
- - - - Seasonal cooling and sea-ice formation
--> Increases S and density and thus leads to sinking
- - Important in coastal areas and marginal, semi-enclosed seas where evaporation is high
- - - - High surface S
--> Sinking and outflow at depth to adjacent oceans

Pressure (depth)
- - Sea water is compressible (density increases with pressure)
- - But pressure effects do not cause seawater to sink (or rise)
- - So we can ignore the effects of depth in our analysis

Review: Temperature and salinity control the density of surface sea water and thus its tendency to sink.

Density-driven circulation is "thermohaline" circulation.
T and S are determined by processes occuring at the surface:
- - Exchange of heat with atmosphere (T)
- - Exchange of water with atmosphere (S)

Surface and deep circulation are coupled in the "Global Conveyor Belt"

- - Transport of surface waters to high latitudes
- - Sinking and flow at depth
- - Upwelling -- return to surface


Video: "Ocean Currents and Wind"
First part -- Surface Currents

Introduction:
Kon-Tiki demonstrated currents in equ. Pac.
Ponce de Leon used Gulf Stream to return
Colonial sea captains charted position of GS by temp.
B. Franklin mapped GS
Deep circulation and upwelling also occur

Relationship between solar heating, winds, wind stress and surface currents

Defelction by "rotation" and continental position
-- North Atlantic as an example

Deflection of equatorial currents by South America
---> (G. of Mexico, Florida Current) ---> Gulf Stream

Gulf Stream (- North Atlantic Drift):
> warms Britain (compare to Labrador)
> splits into two currents (continental deflection)
-- Norwegian Current --> Norway, Arctic Russia
-- S-flowing (cool) Canary Current

North Atlantic "gyre" [clockwise circulation]
Sargasso Sea (warm, saline waters) in the center

Mixing (upwelling) of Labrador Current and Gulf Stream off of
New England and maritime Canada
---> nutrient rich waters
phytoplankton production
major fisheries

Pacific Ocean surface currents

Equatorial Counter Current (equatorial deflection by Asia)

General model of wind-driven currents and gyres

Circulation around Antarctica
Prevailing westerly winds; no continental barriers
Antarctic Circumpolar Current -- surface-to-deep current
encircling Antarctica.



Additional Notes Start Here

Ocean Circulation -- an introduction. We usually describe ocean circulation as two separate systems (although, as we shall see, they are coupled).

Surface circulation describes the large-scale horizontal-flowing currents in the upper few hundred meters of the oceans. We think of these currents as largely wind-driven; that is, prevailing winds initiate surface currents. They are modified (deflected) by the Coriolis effect and by continental barriers. The general pattern of surface currents are large, rotating "gyres." Major gyres are centered in subtropical oceans.

Deep circulation is driven by the creation of dense masses of water at the surface of high-latitude oceans, particularly in the Atlantic. Cooling, sea-ice formation, and evaporation produce dense water-masses at the surface that sink and spread laterally. Deep current of the ocean are deflected as they flow by the Coriolis effect and by continental and mid-ocean ridge barriers.

Video: "Ocean Currents and Wind" -- First part on Surface Currents
Historical demonstration of surface currents
Relationship between solar heating, winds, wind stress and surface currents
Deflection of surface currents by Earth's rotation and continental positions in the North Atlantic -- rotary, or "gyric" motion
Equatorial currents
Gulf Stream
North Atlantic Drift --climate effects
Canary Current
Upwelling and nutrient supply
Currents in the North Pacific Ocean
California Current
Kuroshio Current
North Pacific Drift
polar currents (Alaska, Oyashio)
Equatorial Counter Current (deflected by Asia)
General model of wind-driven currents and gyres
Circulation around Antarctica
No continental barriers
Continuous current (surface-to-depth)


<-previous | Geol 117 Home | Lectures | Review | next ->