Oceanography Lecture Notes Outline

Ocean circulation

I. Contents -  Topics Covered

The Atmosphere Ocean Interface

Wind-Driven Surface Currents

Geostrophic Gyres

Countercurrents and Undercurrents

Other Important Currents

Upwelling and Downwelling

Surface Currents Affect on Climate

El Nino and the Southern Oscillation

Thermohaline Circulation

 

II. The ATmosphere Ocean Interface

A. The Atmosphere and Ocean Are Dynamic Fluid Layers

1. Both are dynamic, density-stratified, multi-layered, fluid

spheres

        The Atmosphere

      Troposphere (dense, weather layer)

      Stratosphere (ozone layer)

      Mesosphere (middle layer)

      Thermosphere (ionized layer)

 

        The Ocean

      Surface zone (mixed layer)

      Pycnocline (middle layer with rapid density change)

      Deep zone (cold stable layer)

 

2. Convection in the atmosphere is driven by latitudinal

variations in solar input (uneven heating of the planet),

which in turn powers the wind-driven ocean surface

currents

        Convection is the transfer of energy via mass transfer

        Equatorial regions have a heat surplus

        Polar regions have a heat deficit

        Atmosphere and ocean act in concert in an

attempt to redistribute the excess heat from low to

high latitudes

 

3. The more fluid atmosphere convects (moves) much more

rapidly than the underlying ocean

        Air currents (wind) flow rates up to 200 kilometers per hour

        Ocean currents flow rates up to 10 kilometers per hour

 

B. The Atmosphere and Ocean are in a Never-Ending

Dynamic State of Heat Energy Exchange

1. This exchange is powered by solar energy

 

2. Exchange of solar-derived heat between the ocean and

atmosphere is the heart of the hydrologic cycle

        Evaporation

        Condensation

        Precipitation

 

C. The AtmosphereOcean Interface is a Very Dynamic

Interface

1. The great density difference between bottom of

atmosphere and the ocean surface

 

2. Large difference in flow regimes between the two (see A3

above)

 

3. Friction coupling between moving air (wind) and water

 

4. Exchange of heat and gasses

 

5. Significant changes in surface area as a function of wind

speed

        Calm conditions smooth seas; minimum surface area

        Stormy conditions - rough seas; much higher surface area

 

III. Wind-Driven surface currents

A. Surface Currents Mainly Confined to the Surface Zone

1. Involve about 10% (by volume) of the world ocean

 

2. Flow horizontally

 

3. Typically extend down to about 400 meters (top of the

pycnocline)

 

4. Driven by wind-driven friction (ocean-air coupling)

      Terrigenous materials from land (wind-carried)

      Sea salt from ocean surface

 

B. Wind is the Primary Agent Responsible for Surface

Currents

1. Friction coupling between wind and ocean surface causes

surface water to get piled up perpendicular to direction of

wind

 

2. Higher pressure on upwind side of piling up water

 

3. Piled-up water flows downhill toward low pressure side

of pile

        Net water current flow is in the downwind direction

 

4. A persistent wind can generate an ocean surface current

beneath it.

 

5. Factors involved in the initial generation of an ocean

surface current:

        Wind persistence

        Wind strength

        Length of continuous stretch of ocean surface under a

a persistent wind current (termed a fetch)

 

6. The prime global-scale winds responsible for surface

current generation are the powerful Westerlies and the

persistent Trades (Easterlies)

 

7. Once generated, the direction of a surface current will

become affected by the Coriolis effect

        Surface currents deflected to the right in the Northern Hemisphere

        Surface currents deflected to the left in the Southern Hemisphere

 

        Ocean surface currents not found along the equator tend to follow

curved paths

 

8. Continents and ocean basin topography will block

surface current flow and further deflect the surface flow

into a circular pattern

 

9. The combination of the Coriolis effect and ocean basin

margins produce circular surface current flow around

the periphery of ocean basins

        These circular-flowing surface currents are called gyres.

        See Figures 9.2 to 9.4 (page 210)

 

C. The Different Ways Currents Flow

1. Upwelling: ascending water masses

 

2. Downwelling: sinking water masses

        Maintain continuity of flow, vertical movement (0.1 - 1.5m/day) 

        Sinking waters may take 1000 years to reach great depths. 

 

3. Horizontal water movement:

        Convergence (meeting) and divergence (spreading out)

 

D. Influence of Ekman Spiral and Ekman Transport: 

1. Coriolis effect acts on surface current water 

        Deflects it from the wind direction

 

2. Deflected by Earth's rotation

        Right in N. hemisphere

        Left in S. hemisphere

 

3. Transfer through the water column of wind-driven motion

with depth to about 100 -150m down

        Top layer of current (directly powered by wind)

transfers some of its kinetic energy to the layer

beneath it.

        This is repeated for numerous horizontal sheets of

water in the the ocean column down to about 100

meters.

 

        The Coriolis effect affects each of the moving horizontal layers

 

        The key point is that each layer responds only to

the layer above it, and since there is a time lag

involved, each horizontal layer in the current will

have a unique direction.

 

                    The overall effect is to produce a vertically-

oriented helix pattern of current directions

Called the Ekman Spiral

 

                    See Figure 9.5 on page 211

 

4. Current speed in the Ekman spiral decreases with depth

5. Net result: 

        Overall water movement is at 90 to wind direction

        Net current motion is called the Ekman Transport

        Dependent on wind persistence.

 

6. In nature we find that the overall water movement is

around 45 - not the theorized 90

        Another factor is working against the Coriolis effect

 

        Attributed to a current-induced pressure gradient (pile-up)

 

        See Figures 9.6 and 9.7 (pages 211 and 212)

 

7. A deflecting surface current converges, creating a hill of

water piling up on one side of the in the direction of the

deflection

 

        Current tends to want to turn towards the downhill

 

        direction from the hill opposite to Coriolis effect

 

        Overall effect is a path between wind direction and

90 to the wind direction

 

        See Figures 9.6 and 9.7 (pages 211 and 212)

 

IV. Geostrophic Gyres

A. Geostrophic Gyres Defined

1. Circular, basin-peripheral surface currents that are in

balance between the pressure gradient and the Coriolis

effect.

 

2. Geostrophic gyres of the Northern Hemisphere are

independent to the ones in the Southern Hemisphere

 

B. Major Geostrophic Gyres of the World Ocean

1. There are five great Geostrophic gyres in the world ocean

        Northern Atlantic

        South Atlantic

        North Pacific

        South Pacific

        Indian Ocean

 

2. There is another major surface current that is technically

not a gyre:

        The West Wind Drift or Antarctic Circumpolar Current

        Not confined to the periphery of a single ocean basin

 

3. The convergence between Northern and Southern

Hemispheric gyres does not coincide with the geographic

equator

        Coincides with the meteorological equator

        Displaced about 5 to 8 north of geographic equator

 

4. Pattern of driving winds and positions of continents shape

the gyres

 

C. The Major Surface Currents Within Geostrophic Gyres

1. The major currents within a single Geostrophic gyre have

different characteristics

        Each current reflects differences in the factors that shape them

 

        Each gyre has a similar set of unique currents

        Each current within a gyre blends into one another

 

2. Currents are classified by geographic position within the gyre

        Western boundary currents

      The Gulf Stream: Northern Atlantic

      The Brazil Current: Southern Atlantic

      The Japan or Kuroshio: North Pacific

      The East Australian Current: South Pacific

      The Agulhas Current: Indian Ocean

 

        The Eastern Boundary Currents

      The Canary Current: North Atlantic

      The Benguela Current: South Atlantic

      The California Current: North Pacific

      The Peru or Humboldt Current: South Pacific

      The West Australian Current: Indian Ocean

 

        The Transverse Currents

      North Equatorial Currents: North Atlantic and Pacific

      South Equatorial Currents: South Atlantic and Pacific

 

3. The Western Boundary Currents

        The fastest and deepest of the three current types

      Up to 10 km/hr

      Can reach down to 1500 m deep in places

 

        Form narrow, deep currents along the eastern margins of ocean basins

 

        Move warm water poleward

 

        Each individual current moves massive amounts of water

      Up to 50 million cubic meters per second

 

        Maintains its identity for very long distances

          Sharp boundaries with coastal circulation system

 

        Prone to form warm-and cold-water eddies

 

        Coastal upwelling uncommon

 

        Waters derived from trade wind belts

 

        Waters tend to be very clear and nutrient poor

 

        Likely responsible for unusual abyssal ocean bottom storms

 

        See Figures 9.8 to 9.12 for illustrations (pages 213 to 218)

 

3. The Eastern Boundary Currents

        Have virtually opposite characteristics compared to the western

boundary currents

 

        Slower and more shallow of the western boundary currents

      Up to 2 km/hr

      Typically reaches down to less than 500 m deep in places

 

        Form broad, shallow currents along the eastern margins of ocean basins

      Up to 1000 kilometers wide

 

        Move cold water towards the equator

 

        Each individual current moves relatively small

amounts of water compared to its western

counterpart

      Up to 15 million cubic meters per second

 

        Has diffuse boundaries separating from coastal currents

 

        Coastal upwelling common

 

        Waters derived from mid-latitudes

 

        See Figures 9.8, 9.9 and 9.12 for illustrations (pages 213, 214, and 218)

 

4. The Transverse Currents

        Directly derived from the trade winds and mid-latitude Westerlies

 

        Tropical trade winds drive the east to west transverse currents

          The convergent effect of the trades cause

the east to west current to be stronger

than its west to east counterpart

 

        Mid-latitude Westerlies drive the west to east transverse currents

 

        Moderately shallow and broad

 

        Links the western and eastern boundary currents

 

5. The West Wind Drift Current

        Generated by the unimpeded Southern

Hemisphere mid-latitude Westerlies

      No continental interference

 

        Carries more water than any other current in the world ocean

      100 million cubic meters per second

 

        Technically a transverse current

 

V. Countercurrents and undercurrents

A. Equatorial Countercurrents

1. Lack of persistent equatorial winds allows a west-to-east

backward flow of water between the North and South

Equatorial Currents

 

2. Form very narrow surface currents along the intertropical

convergence zone

 

3. Helps balance mass transfer flow of equatorial waters

 

B. Countercurrents Can Exist Beneath Surface Currents

1. Subsurface countercurrents are termed undercurrents

        Flow beneath surface currents but in the opposite direction

        Flow velocities of averaging up to 5 kilometers per hour

 

2. Undercurrents found beneath most of the major surface

currents

 

3.      These currents can be very large in volume

        Volume can equal the opposite-flowing overlying surface current

 

        Best studied undercurrent is the Pacific Equatorial Undercurrent

      Also called the Cromwell Current

 

4.      Undercurrents probably help to balance the mass transfer

flow of ocean circulating ocean waters

 

VI. Other important Surface Currents

A. Monsoon Currents:

1. Reversal of normal surface current circulation of the

Equatorial Current

2. Caused by a northward shift in the position of the

intertropical convergence zone (ITCZ) during the

summer months

 

3. Reversal of regional high and low pressure cells

 

4. Characterized by a summer rainy season

 

5. A temporary seasonal current

 

6. Best developed is the Southwest Monsoon Current in the

Indian Ocean

 

B. High Latitude Cold Currents:

1. Non-geostrophic currents originating in polar regions

 

2. These smaller sized currents move from high latitude to

low latitude

        Powered by polar easterlies

        Modified and shaped by geographic obstacles

 

3. Dont appear to be controlled by the Coriolis effect,

gravity, or friction

 

4. The Greenland and Labrador Currents are good examples

 

VII. Wind-Induced Upwelling and downwelling

A. Wind-driven Horizontal Currents Can Induce Vertical

Water Motion

1. Upwelling Ascending water movement

        Brings up cold, nutrient-rich waters

 

2. Downwelling Descending water movement

        Caused by water driven against edge of a continent

 

        Important for global-scale mixing of ocean

 

3. Equatorial Upwelling

        Generated by divergence of the opposing Equatorial Currents

        Direct effect on global climate and the marine life found along the equator

4.      Coastal Upwelling

        Caused by winds blowing either parallel or offshore along a coastline

 

        Effect of the Ekman transport

 

        Brings up cold nutrient-rich waters

 

        Affects regional climate

 

VIII. Surface Currents Affect World CLimate

A. Causes of Seasonal Changes:

1. Caused by differential solar heating of ocean and land

2. Product of high heat capacity of water

 

B. Weather Characteristics of Summer

1. Low pressure areas over land caused by warm rising air

2. High pressure over ocean

 

C. Weather Characteristics of Winter

1. Winter produces the opposite effect

        High pressure areas over land caused by cold sinking air

        Low pressure over ocean

 

IX. El ino / SOuthern Oscillation (ENSO)

A. Causes Large Climatic Fluctuation

1. Breakdown in the normal atmospheric circulation

patterns in the Pacific

2. Irregular cycle, occurs every 2 - 10 yrs.

 

3. The 1997-1998 weather season was last large El Nino

 

4. The 1982-1983 season was another major episode

 

B. Obvious Signs That an El ino is Underway

1. Diminishment of the Equatorial Trade winds

 

2. The appearance of unusually warm water off the coast of

Ecuador and Peru.

 

C. The Sequence of Events -

1. Southern Oscillation - Prevailing Trades Weaken -

        Sub-tropical high in the eastern Pacicfic

        Low pressure cell over Indonesia

 

2. Weak westerlies develop and the Indonesia low moves

eastward

 

3. East to west sea slope collapses (sea level rises in the

east by up to 20 cm)

 

4. East-Pacific surface waters warm (7C) warm layer

suppresses upwelling of cooler water

 

5. See Figures 9.17 and 9.18 in the text (pages 223-224)

 

D. Some Global Environmental Effects of El ino: Vary

from event to event

1. Marine productivity declines - Upwelling ceases off Peru

 

2. Storm frequency increases- greater precipitation in the

western Americas

 

3. Drought in Indonesia, Australia, and Africa (Sahel)

 

4. Winters storms grow or decrease in intensity

 

5. Increased precipitation in the southeastern US

 

X. thermohaline circulation

A. Ocean Water Masses Possess Distinct Characteristics

1. Characteristics include

        Temperature

        Salinity

        Density

 

2. Characteristics determined by:

        Heating

        Cooling

        Evaporation

        Dilution

        Concentration

 

3. Five common water masses

        Surface water

        Central water

        Intermediate water

        Deep water

        Bottom water

 

B. Controlled by Temperature and Salinity

1. Temperature and Salinity Relationships:

        Many combinations of temperature and salinity can yield the same density

        Density of water increases with depth

 

2. The Temperature Salinity Diagram

        Study Figure 9.19 in the text (p226)

        T-S Curves: 

      Depth distribution of temperature and salinity are distinctive

      Plot of temperature vs. salinity forms a T-S diagram

      Depth plots are T-S curves

 

        T-S Curves and Water Masses: 

      T-S curves for large areas of the ocean are vertically similar

      Define water masses by depth and location

      Water masses are related by density.

 

C. Formation of and Downwelling of Deep Water

1. Form mainly in Polar oceans

2. Antarctic Bottom Water (AABW)

        Generation of icy-cold brines due to sea ice formation

        Cold salty water sinks

        Forms a very slow northward-traveling bottom current

 

3. North Atlantic Deep Water (NADW)

        Similar to ABW but far less extensive

        Sits over the top of the ABW

 

4. Mediterranean Intermediate Water (MIW)

        Excess evaporation exceeding freshwater input

        Saltier, but warmer than the AABW and NADW

        Intermediate density to bottom/deep waters and surface waters

 

D. Seasonal Temperature Changes Create Seasonal

Thermocline

1. Affect surface density

 

2. Can form sinking water masses, or freshwater lid.

 

E. Thermohaline Circulation Patterns

1. Thermohaline circulation driven by density differences

between water masses, i.e. gravity driven

 

2. Starts as large volumes of very cold/dense water sinking

rapidly (downwelling) in small areas within polar

regions

 

3. Moves equatorward (horizontally) as very slow bottom

and deep currents

 

4. Eventually slowly rises as diffuse upwelling into broad

regions of ocean within the temperate and tropical zones

        Rises on average at 1 centimeter per day

 

5. These upwelled water masses eventually move back to

to the polar regions as surface currents to start the

cycle over again

            The thermohaline cycle takes about 1000 years

 

6. Illustrations of thermohaline circulation are shown in

Figures 9.22, 9.23, and 9.25

 

7. Upwelling "holds up" the thermocline

 

8. Regions in the ocean where two unique water masses of equal density, but

different temperatures and salinities, converge can mix readily; a new hybrid

water mass with an intermediate temperature and salinity profile results, but

typically with a greater density than the parent water masses; this is termed

caballing

 

9. The thermohaline and surface currents work together

in a continuous, connected global circulation circuit

        Acts as a global heat-transporting conveyor belt

 

        Helps distribute solids, gases and nutrients

 

        Mixes the water masses

 

        Helps move pelagic organisms worldwide

 

XI. Structure of Oceanic Waters:

A. Atlantic and Arctic Oceans:

1. Cooling at high N. latitudes produces North Atlantic Deep

Water 

        NADW (2 - 4C, 34.9)

        Sinks, moves southward

 

2. In the South Atlantic: 

        Antarctic Intermediate W ater (AAIW; 5C, 34.4%o)

        Antarctic Bottom Water (AABW; 0.5C, 34.8%o).  

        Surface waters: 25C, 36.5%o .  

 

4. Arctic Ocean controlled by salinity. 

        Surface low salinity waters

        Affected by seasonal ice formation. 

        At intermediate depths: Norwegian and Greenland

currents

 

B. Pacific Ocean: 

1. No counterpart of NADW, isolated from Arctic

 

2.      No source of deep water, sluggish deep water circulation

 

3.      Subtropical lens of warm, salty water.

 

C. Indian Ocean: 

1. Isolated from Arctic, no source of deep water

 

2. Sluggish deep water circulation

 

D. Mediterranean:

1. Mediterranean Intermediate Water (MIW, 13C, 37.3%o)

 

2. Outflows at depth, mixes in Atlantic

 

E. Red Sea: 

1. Outflow at 40 - 41%o.

 

 

XII. Means of Studying Ocean Currents

A. Two Primary Methods to Measure Currents

  1. Float method

        Movement of a drift bottle or free-floating object

        Example is the rubber duck

        Floats can be on surface or submerged to whatever depth

 

2. Flow method

        Current is measured as it flows past a fixed object

         

 

XIII. Vocabulary Terms