Oceanic Circulation •

Current

= a moving mass of water • OCEAN WATER MOVES IN CURRENTS CAUSED BY WIND AND DIFFERENCE IN WATER DENSITY (beneath surface zone)

Oceanic Circulation •

Surface Currents

– horizontally flowing water in the uppermost 400m of the ocean – 10% of water in Oceans moves this way – driven by thermal expansion & contraction and WIND friction

Oceanic Circulation •

Surface Currents

– horizontally flowing water in the uppermost 400m of the ocean – driven by thermal expansion & contraction and wind friction •

Thermohaline Circulation

– slower, deeper circulation

Oceanic Circulation • •

Surface Currents

– horizontally flowing water in the uppermost 400m of the ocean (above pycnocline) – driven by thermal expansion & contraction and wind friction

Thermohaline Circulation

– slower, deeper circulation (below pycnocline) – due to the action of gravity on water masses of different densities

Refresher From Ch. 6

• “Thermocline” (refresher) • Tropical and subtropical oceans are permanently layered with warm, less dense surface water separated from cold, dense deep water by a

thermocline

.

– The thermocline is a layer in which water temperature and density change rapidly.

• Temperate regions have a seasonal thermocline and polar regions have none.

Thermocline, Halocline, and Pycnocline

Surface Currents • Solar heating – water expands at equator and contracts near poles

Surface Currents • Solar heating – water expands at equator and contracts near poles – water moves toward poles due to gravity

Surface Currents • Solar heating – water expands at equator and contracts near poles – water moves toward poles due to gravity – water lags behind earth’s rotation & piles up on west sides of oceans

Surface Currents • Wind – primary force responsible for surface currents • Friction drags water along • Coriolis effect The “piled up” water will move in direction the wind is blowing it UNTIL the coreolis determines final direction (right of wind direction in N. Hemisphere)

Surface Currents • Continents prevent continuos flow and deflect water…

Surface Currents • Continents prevent continuous flow and deflect water… • Gyre – the circular flow around the periphery of an ocean basin

Trade Winds = Easterlies Winds are Driven by Uneven Solar Heating and the earths spin

Fig 8-1, g

Surface Winds, Sun’s heat, Coreolis Effect and Gravity = surface current = gyres

Fig 8-2, g

5 major ocean gyres

Six great current circuits • North Atlantic Gyre • South Atlantic Gyre • North Pacific Gyre • South Pacific Gyre • West Wind drift or Antarctic Circumpolar Current

Sea Surface Temperatures • Insolation and ocean-surface water temperature vary with the season.

• Ocean temperature is highest in the tropics (25 o C) and decreases poleward.

Figure 5-9a Sea-Surface Temperature in August Using Thermocline Principles

Northern Atlantic Gyre

Fig 8-3, g

Flow within Gyres • Western Boundary Currents (ex: Gulf Stream) – narrow, fast, deep currents that carry warm water toward poles

Flow within Gyres • Western Boundary Currents (ex: Gulf Stream) – narrow, fast, deep currents that carry warm water toward poles – westward intensification - more concentrated due to water piling up due to eastward rotation of earth

Fig 8-13b, g

Flow within Gyres • Eastern Boundary Currents (ex: Canary Current) – broad, slow, shallow currents that carry cold water toward equator

Flow within Gyres • Transverse Currents (ex: North Atlantic Current, North Equatorial current) – currents that flow from east to west or west to east

Flow within Gyres • Currents affect climate: – North Atlantic current warms England – California current cools San Francisco in the summer

Fig 8-8, g

Fig. 8-9, g

…Gyres…a final word • Gyres consist of currents that blend into 1 • Flow is continuous • Caused by combo of: wind energy, friction, the Coreolis effect and the pressure gradient

EKMAN SPIRAL • Sum of water direction in (multi) layered Ocean • Net motion of water (down to 100 meters) w/ ekman spiral included = ekman transport • Each layer in “spiral” acts differently

Fig 8-5, g

Fig 8-5a, g

Fig 8-5b, g

Fig 8-5c, g

fnft

fnft Figure 15.32

Convergence & Divergence of Water Currents in the Northern Hemisphere

Eddy Formation • Western boundary of Gulf Stream has distinct Temperature, Speed and Direction • Meanders (EDDIES) form here • Eddies pinch off and become isolated cells of either warm or cold water

Eddy

Gulf stream Viewed from Space 1=east coast off Fl.

2=warm eddy 3=cold eddy 4=mix of surface and surrounding waters

Wind- Induced Vertical Circulation • Upwelling – upward movement of water • Downwelling – downward movement of water

Fig 8-16, g

Principal regions of coastal upwelling and down-current areas of increased primary productivity

Wind- Induced Vertical Circulation • Coastal Upwelling – cold, deeper water upwells to replace the surface water – leads to increased nutrients & productivity and cooler climates

Coastal upwelling in the Northern Hemisphere

Wind- Induced Vertical Circulation • Equatorial Upwelling – westward flowing equatorial currents are deflected poleward – deeper water comes up to replace the surface water

Fig 8-14a, g

Wind- Induced Vertical Circulation • Downwelling – water driven toward the coast will be forced down – Brings down dissolved gases

Deep Circulation Thermohaline Circulation • Driven by density differences • water masses do not mix easily but flow above or beneath each other

Classic thermohaline circulation

Fnft

Cross-section of the South Atlantic Ocean

5 Common Water Masses • Surface water – to 200m • Central water – to bottom of thermocline • Intermediate Water – to 1500m • Deep water – below intermediate but not in contact with bottom • Bottom water – in contact with bottom

Some Water Masses in the Deep Atlantic • Antarctic Bottom Water • North Atlantic Deep water • Mediterranean Intermediate Water • Antarctic Intermediate Water

Water layers and deep circulation of Atlantic

• The water sinks to a density-appropriate level and then slowly flows equatorward across the basin.

• Deep water gradually mixes with other water masses and eventually rises to the surface.

Fig. 8-23, p. 190

Fig. 8-24, g

Thermohaline Circulation • Sinking of water masses is offset by slow, gradual rising across warmer temperate and tropical zones

Thermohaline Circulation • Much slower than surface circulation – hundreds of years (1500?) vs 1 year (North Atlantic Gyre)

Remember… • From lecture #1 of the course – those 1 st “facts” are extremely important • Let’s review…

Important Facts • 81% of the Southern Hemisphere is covered by Ocean (remember that! It’ll become really important later…); while only 61% of the Northern Hemisphere is covered – WHY?

• The Oceans are 4X as deep as the Continents are high (average depth = 2.5 miles).

• The Pacific (Ocean) is so huge that it covers almost ½ of the Earth’s surface; it is also the Earth’s largest collection of water.

• We have 100X more “aquatic” habitats available on earth than terrestrial habitats (1.4 billion cubic kilometers).

© digitalife/ShutterStock, Inc.

A synthetic view of our ocean planet

• (if time permits) El Nino

ENSO events • Surface winds generally move from East to West in Tropical (equator) Pacific but every 3-8 yrs. these pressure areas (typically high to low) change and you get a reversal of wind direction/atmospheric pressure (low to high) = southern oscillation • El nino = water flow name (+ southern osc. = ENSO event) • Still no one really knows why/how this occurs

El Nino • “Current of the Christ Child” because Peru had an (unexpected) abundance of fishing 1 X-Mas.

Why 2 names?

• Southern oscillation (ENSO): Weather related name (pressure changes in wind patterns are clearly associated; these drive water change) • El Nino: oceanographic name (associated w/ water and temp. patterns that change biological species’ productivity in the Pacific)

A non el nino year Thermocline rises Upwelling of cold water

Fig 8-16a, g

What happens (non El Nino)?

• Warmest part of the Worlds’ ocean = western Pacific b/c warm water moves East to West and builds up there • As a result you get a decreased thermocline (lower in water column, less upwelling)

Fig. 8-21, g

Fig 8-16d, g

What happens (El Nino)?

• Warm water (that usually moves East to West) shifts to West –> East movement • Slows trade winds (southern oscillation) • Pressure system shifts • Warm water on other side, less where expected • As a result you get an increased thermocline (higher in water column, more upwelling)

Fig. 8-22a, g

Fig 8-16b, g

El nino year Eastward Movement of Water, no upwelling

Fig 8-16c, g

The opposite – La Nina • “The Girl” • When we “return to normal,” it is fast w/ a huge change and increased currents, increased upwelling (thus increased cold water upwards) result.

• Trade winds renewed

Fig. 8-22b, g

Fig 8-21b, g

North Pole 60 °N Latitude Equator South Pole 30 °N 0 ° 30 °S North Pole Longitude 60 °W 0 ° Prime meridien North Pole 60 °N 30 °N 60 °W 30 °W 0 ° 0 ° 30 °E 30 °S South Pole South Pole Stepped Art