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Lecture 14 (Honors Section)
Deep Ocean Circulation

Ocean Currents

Wind-driven
Set in motion by moving air masses
Horizontal currents
Primarily near the surface

Winds-->Ekman T.-->Mounds-->Geostrophic Flow

Geostrophic flows are the major currents

a balance between a pressure gradient and Coriolis deflection

Western boundary currents faster

Ekman Transport (Figure 6-6)

Mounds of Water (Figure 6-9)

Ocean Currents

Thermohaline
Initiated at the surface by temp/salinity
Vertical circulation
Primarily at depth

Deep-Ocean Circulation

Powered by thermohaline circulation due to density differences between water masses. The differences reflect:
Temperature - thermal
Salinity - haline

"You are my DENSITY!"
-- George McFly

Density of water determined primarily by temperature and salinity

pressure effects are negligible.

Density near surface:
Evaporation
Precipitation
Runoff, river drainage

Water Masses at Depth

When water masses sink they are isolated from the major processes that affect density.

Hence they retain the temperature and salinity signature established at the sea surface.

Mixing is slow because flow rates are very slow.

Water Mass Types

Central Waters
e.g., NPCW = North Pacific Central Water

Intermediate Waters
e.g., NPIW = North Pacific Intermediate Water

Deep and Bottom Waters
e.g., CoW = Common Water
NADW = North Atlantic Deep Water
AABW = Antarctic Bottom Water

"T-S" Diagrams


Image courtesy of Lamont-Doherty Earth Observatory

Water Masses on T-S Diagram


Image courtesy of Lamont-Doherty Earth Observatory

Thermohaline "Conveyer Belt" (Figure 6-16b)

At the poles:
cold, salty dense water sinks and spreads into the ocean basins along the bottom.

At the equator:
warm low density water floats as a lens slowly moving poleward.

Water that sinks at the poles takes about 1000 years to rise back to the surface.

Location of Water Profiles (Figure 6-14)

Salinity Profile of the W. Atlantic (Figure 6-14)

Temperature Profile of the W. Atlantic (Figure 6-14)

Water Mass Structure of the Atlantic (Figure 6-15)

Water Mass Structure of the Indian Ocean (Figure 6-15)

Salinity Profile of the East Pacific (Figure 6-14)

Temperature Profile of the East Pacific (Figure 6-14)

Water Mass Structure of the Pacific (Figure 6-15)

Why is the Atlantic saltier?
Why is deep water formed in Atlantic?
Look again at Figures 6-16b and 6-4 for a clue.

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Last update: April 27, 2000
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