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

Ø Troposphere (dense, weather layer)

Ø Stratosphere (ozone layer)

Ø Mesosphere (middle layer)

Ø Thermosphere (ionized layer)

· The Ocean

Ø Surface zone (mixed layer)

Ø Pycnocline (middle layer with rapid density change)

Ø 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 Atmosphere–Ocean 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)

Ø Terrigenous materials from land (wind-carried)

Ø 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 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

ü The Gulf Stream: Northern Atlantic

ü The Brazil Current: Southern Atlantic

ü The Japan or Kuroshio: North Pacific

ü The East Australian Current: South Pacific

ü The Agulhas Current: Indian Ocean

· The Eastern Boundary Currents

ü The Canary Current: North Atlantic

ü The Benguela Current: South Atlantic

ü The California Current: North Pacific

ü The Peru or Humboldt Current: South Pacific

ü The West Australian Current: Indian Ocean

· The Transverse Currents

ü North Equatorial Currents: North Atlantic and Pacific

ü South Equatorial Currents: South Atlantic and Pacific

3. The Western Boundary Currents

· The fastest and deepest of the three current types

Ø Up to 10 km/hr

Ø 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

Ø Up to 50 million cubic meters per second

· Maintains its identity for very long distances

Ø 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

Ø Up to 2 km/hr

Ø Typically reaches down to less than 500 m deep in places

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

Ø 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

Ø 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

Ø 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

Ø No continental interference

· Carries more water than any other current in the world ocean

Ø 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

Ø 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. Don’t 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 Ñino / 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 Ñino 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 (7°C) 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 Ñino: 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: 

Ø Depth distribution of temperature and salinity are distinctive

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

Ø Depth plots are T-S curves

· T-S Curves and Water Masses: 

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

Ø Define water masses by depth and location

Ø 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 water masses of equal density but different

temperatures and salinities converge can mix readily; 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 - 4°C, 34.9‰)

· Sinks, moves southward

2. In the South Atlantic: 

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

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

· Surface waters: 25°C, 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, 13°C, 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