Oceanic Circulation • Current = a moving mass of water • OCEAN WATER MOVES IN CURRENTS CAUSED BY WIND AND DIFFERENCE IN WATER DENSITY.
Download ReportTranscript Oceanic Circulation • Current = a moving mass of water • OCEAN WATER MOVES IN CURRENTS CAUSED BY WIND AND DIFFERENCE IN WATER DENSITY.
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