7.6 How do deep ocean currents form?

• deep currents occur in deep zone below pycnocline, so they influence 90% of all ocean water

• density differences create deep currents

• density differences usually small, but large enough to cause denser waters to sink

• deep-water currents move larger volumes of water and are much slower than surface currents (10-20 km/year)

  • takes a deep current an entire year to travel same distance that a western intensified surface current can move in one hour

• deep ocean circulation = thermohaline circulation

Origin of Thermohaline Circulation

• temp has the greater influence on density than salinity

  • density changes due to salinity only important in very high lats, where water temp remains low and relatively constant

• most water involved in deep-ocean currents originates in high lats at surface: surface water becomes cold and salinity increases as sea ice forms —> waterm becomes dense enough —> sinks, initiating deep-ocean currents

  • once water sinks, it’s removed from physical processes that increased its density —> temp and salinity remain largely unchanged for the duration it spends in the ocean

• temp-salinity diagram (T—S) can be used to identify deep-water masses based on their charactertistic temp, salinity, and resulting density

• as surface masses sink, deep-water masses rise

• water temp in high lat regions is same at surface as it is down bwlo, water column is isothermal —> no thermocline or pycnocline associated = easy upwelling and downwelling

Sources of Deep Water

• Antarctic Bottom Water (AABW): huge masses of deep water form beneath sea ice along margins of Antarctica formed by rapid winter freezing

  • densest water in open ocean

  • sinks slowly beneath surface and spreads into all world’s ocean basins, eventually returning to surface approx. 1K years later

• North Atlantic Deep Water (NADW): large masses of deep water form in Norwegian sea, flows as a subsurface current into N Atlantic

  • also comes from margins of Irminger Sea off of Greenland, Labrador Sea, Mediterranean Sea

  • spreads throughout ocean basins, but less dense than AABW, so layers on top of AABW

• surface water masses converge w/in subtropical gyres in arctic and antarctic

  • subtropical convergence does not produce deep currents bc density of warm surface water is too low

• arctic & antarctic convergence: deep-water formed = Antarctic Intermediate Water

• highest density water found along ocean bottom

• prominent thermocline in warm surface water at low lats = no vertical mixing

• pacific and indian oceans have layered density too but no source of northern hemisphere deep water

• N Pacific, low salinity of surface waters prevents them from sinking into deep ocean

  • oceanic common water: created from mixture of AABW and NADW = bottom of pacific ocean and indian ocean

World-wide Deep Water Circulation

• difficult to identify specifically where this upward flow to the surface is occurring

• believed that it occurs as a gradual, uniform upwelling throughout the ocean basins and that it may be somewhat greater in low-lat regions where surface temps are higher

Conveyor-Belt Circulation

• overall circulation pattern resembles a large conveyor belt —> model is called conveyor-belt circulation

• begins in N Atlantic, surface water carries heat to high lats via the Gulf Stream; during cold winter months, this heat is transferred to the overlying atmosphere, warming northern Europe

• source areas for deep water exist in high-lat regions where surface water cools, becomes high density, and sinks

  • source areas feed flow of deep, high-density waters, which slowly drifts into all oceans

  • deep water returns to the surface in localized areas of upwelling and as gradual, uniform upwelling throughout ocean basins

Dissolved Oxygen in Deep Water

• deep-water circulation brings dense, cold, oxygen-enriched water from surface in high-lats to deep ocean —> deep water has higher dissolved oxygen content than water closer to the surface

  • in its time in deep ocean, deep water becomes enriched in nutrients bc of decomposition of dead organisms and lack of living organisms using nutrients there

CONCEPT CHECK 7.6

1. discuss origin of thermohaline vertical circulation. why do deep currents form only in high-lat regions?

originates from thermo and haloclines, but mostly thermocline, which doesn’t really exist at high lats and at the poles. temp has the highest impact on density, so downwelling happens super often here.

2. what are the two major deep-water masses? where do they form at the ocean’s surface?

NADW: north atlantic, weddell’s sea, norwegian waters

AABW: off coast of antarctica, south pole, densest

3. explain why no matter where you are in the ocean, if you go deep enough, you’ll encounter oceanic common water.

As you go deeper in ocean below the surface currents, the deep currents dominate and move about the world’s oceans very slowly through the global conveyor system.

4. describe how the distribution of life in the ocean would be different if there were very little dissolved oxygen in deep-water currents.

oxygen content in deep ocean decreases —> life in ocean decreases