IMS 11 - Surface Current

Ocean Surface Currents

Wind Driven Currents

·         Current depth depend on strength & persistence

·         Current speed depend on strength

·         3 /4 days wind of 20 knots, current of 0.4 knot

Nautical miles & Knot

·         Earth has 360 degrees; 21600 minutes

·         1 minute = 1 nautical mile = 1.852 km

·         1 knot = 1 nautical mile per hour (speed)

Coriolis Force

·         Wind-driven current deflected by earth’s rotation / Coriolis force

·         Coriolis greater in high latitude & deep water

Arrows = Trade winds

ITCZ = Equatorial Current (travel westward)

Ekman Spiral

·         Describes speed & direction water flow at depths (due to wind & Coriolis)

·         Layer below deflected by Coriolis deflect to the right (N hemisphere)

·         Deeper layer move slower due to energy lost between transfer

·         Angle increase, depth increase

·         Wind usually not constant – Ekman spiral incomplete

Gyre

·         Equatorial Counter – equatorial current reflected back by continental barrier

·         Warm current / western boundary currents (red arrow) – due to Coriolis; eg: Gulf Stream

·         Cold current (blue arrow) – due to Coriolis & continental barrier; eg: Canary

·         Warm cold currents combine – circular flow @ ocean basin called Gyre

·         6 gyre:

ü  N / S Atlantic Gyre

ü  N / S Pacific Gyre

ü  Indian Gyre

ü  Antarctic Circumpolar current (west wind drift)

Equatorial Countercurrents (ECC)

·         Coriolis effect minimal @ equator; water unturned

·         Piles at west; western sea lvl 2 m higher than east

·         Western water flow downhill by gravity – narrow ECC eastward

Upwelling

·         Vertical motion of cold, deep, nutrient rich water to surface – high productivity

Downwelling

·         Low surface productivity – DO carried to benthos

Divergence surface water – upwelling

·         Surface water current move away

·         Common @ equator; esp Pacific (equatorial upwelling)

Convergence surface water – downwelling

·         Surface water current join; water stack up & goes down

·         Gulf Stream, Labrador & E. Greenland