Notes on Chapter 9 (10/27) (10/29) (11/3)

Ocean Circulation

Ocean currents (surface & deep)

• Surface (driven by wind) (Fig 9-1)

• Influenced by global wind patterns (Figs 8-8)

• Deep (driven by density differences)

• Deep currents affect water below pycnocline (90% of ocean water)

Ocean gyres

• Subtropical gyres (five total)

• One in each ocean basin

• Generally composed of 4 currents

• Centered ~ 30°N and S

• Subpolar gyres

• Generally composed of 2 currents

• Centered ~ 60°N and S

• Rotate opposite their adjoining subtr. gyres

Ekman spiral (Fig 9-2) describes water movement with respect to wind direction

• Influenced by Coriolis and wind

• Explains motions of gyres

• Geostrophic flow (Fig 9-3)

• “Hills” affected by gravity and Coriolis (Fig 9.4)

• Western intensification (subtr. gyres)

• Western vs. eastern boundary currents are: Warmer, faster, narrower, and deeper.

• Warm and cold core rings (Gulf Stream; p 245)

• Last for months to years

• 100-300 km diameter

Upwelling and downwelling (areas of high vs. low biological productivity) influenced by Ekman spiral and major wind belts, NOT regional/local winds!

• Coastal and equatorial (Figs 9-5 and 9-6)

• Upwelling – Brings cold, nutrient-rich water to the surface

• Downwelling – Brings warm, nutrient-poor water to the deep

Chapter 9 continued (10/27) (10/29) (11/3)

ENSO – El Niño Southern Oscillation is a combined oceanic-atmospheric disturbance

• El Niño is a warm phase (warm water piles against eastern ocean boundary)

• La Niña is a cooling phase (an extreme of normal conditions)

• Recurrence interval (2-12 years) (Fig 9-22)

• Both have worldwide effects

• Can be predicted; SST measured across equatorial Pacific by TOGA array

Deep ocean currents (a.k.a. thermohaline circulation)

• Created in subpolar regions by density differences surface water (Figs 9-8 thru 9-12, and see handout)

Some major deep and intermediate water masses include:

• North Atlantic Deep Water (NADW)
• Antarctic Bottom Water (ABW)
• Mediterranean Intermediate Water (MIW)
• Antarctic Intermediate Water (AIW)
• North Atlantic Central Surface Water (NACSW)

• See Fig 9-8 and handout

Different water masses are distinguished on T-S diagrams (Fig 7-5). Each mass is a straight line, and the curves represent mixing between water masses. The masses are distinguished by T, S, and subsequently, density.

Sverdrup (Sv) is the unit for ocean current volume. One Sv is one million cubic meters per second.

The Antarctic Circumpolar Current carries the largest amount of ocean water (Fig 9-11)

Notes on Ch 9 continued  (10/27) (10/29) (11/3)

In terms of density, NACSW<AIW<MIW<NADW<ABW

The NADW upwells at Antarctica and comprises most of the Circumpolar Current water (Fig 9-14)

Oceanic Common Water (OCW) is created by mixing between ABW and NADW (Fig 9-14)

Conveyor Belt Circulation (Fig 9-10) is a representation of thermohaline circulation. The mean time for flow of a body of water from NADW, through the conveyor, and back is 1000 years. Importance of thermohaline circulation:

• Heat transport from high to low latitudes

• Ventilation of deep water with oxygen-rich water

• Transport of nutrients

• Relation with large-scale climate variability (e.g., cooling events during interglacial periods; dryas)

Dryas (intense global warming, right before global cooling)

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