IMS 12 - Deep Current

Deep Currents

·         Below pycnocline; 90% of seawater

·         Due to density difference (caused by temp & sal diff)

·         Deep Ocean Circulation = thermohaline circulation

·         Surface density 1020 – 1029 kg/m³

Thermohaline Circulation

·         Begin @ high latitude

·         Cold saline dense surface water sinks – deep current spread laterally

·         Thin subsurface flow horizontally

Dense surface water

·         Downwelling & upwelling easier @ high latitude coz isothermal, x thermocline

Deep current

·         Move larger water volume slowly

·         10 – 20 km/year

·         Resurface after ~ 1000 yrs

Sources

·         Antarctic Bottom Water

ü  34.65 ppt, – 0.5 °C, 1.0279 g/m³

ü  Formed @ Weddell Sea surface of continental shelf

ü  Huge deep water beneath ice; rapid winter freezing – dense water sink

Conveyer-belt Circulation

·         Combining thermohaline circulation & surface current

·         North Atlantic Deep Water (NADW)

ü  From South-eastern Greenland, Labrador Sea, Mediterranean Sea

ü  Like Antarctic Bottom water; spread to Indian & Pacific ocean

ü  Less dense, above Antarctic Bottom water

·         Antarctic Bottom Water (AABW)

·         Northern Pacific Ocean

ü  No source of N Hemisphere deep water

ü  Low density of surface prevent sinking

·         Northern Indian

ü  Warm water x sink

Deep Water DO

·         Cold water more DO

·         Deep water circulation cause upwelling

·         Without resurface, x oxygen, x deep life, reduced surface life

Normal Condition

·         Cold Peru-Chile current – richest fishing ground

·         High press & sinking air @ coastal S America (dry)

·         Low press & rising air @ Indon, New Guinea, N Australia (rainy)

·         Great press diff – strong SE trade wind across equatorial S Pacific

·         Results in atm circulation cell (Walker cell)

·         Warm western water; 100 m thermocline

·         Cool western water; 30 m thermocline

El Nino

·         Unusually cold temperatures @ Equatorial Pacific

·         High press @ S America; press diff reduced

·         Diminished reversed SE trade winds

·         Western warm  water flow back to S America

·         Aided by Equatorial Countercurrent (band of warm water stretched over equatorial Pacific Ocean)

·         Temperature rise to 10 °C

·         Avr sea lvl rise to 20 cm

·         Increase temperature – coral bleaching @ Tahiti & Pacific Island

·         Causes Peru rain, eastern hurricanes, flatten thermocline, disrupt upwelling

·         Western stacking cause downwelling; productivity low; western droughts

La Nina

·         Unusually warm temperatures @ Equatorial Pacific (intensified)

·         Stronger Walker Circulation & trade winds

·         More upwelling; shallower thermocline < 30 m

·         Cooler