Transcript Ocean Circulation II Ekman transport & Geostrophic currents

Ocean Circulation II
Ekman transport & Geostrophic currents
Ocean Circulation
1. Surface Circulation  Wind Driven
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Ekman Transport and Geostrophic Currents
Surface layer and Picnocline zone
0 – 50,100m / 50,100m - ~1000m
Affects ~25% of total water mass
Fast (1-2 m/s)
2. Deep Circulation  Density Driven
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Thermohaline Circulation
Below Picnocline zone (>~1000m)
Affects ~75% of total water mass
Slow (~ m/day)
Surface Ocean Circulation
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Ekman Spiral
Geostrophic Currents
Open Ocean Currents
Global Surface Circulation
Ekman Motion
Ekman Motion
Ekman Spiral
Ekman Spiral
• 45o surface transport offset
• 90o net water transport
• Stronger winds cause deeper water
movement
– Wind 10 m/s  50 – 100 m watercolumn
– Wind 20 m/s  100 – 200 m watercolumn
Thermo, halo & Picnoclines
Coastal Ekman Transport
Consequences of Ekman Transport
• Ekman affects picnocline layer
• Coastal Upwelling
– Cold surface water, high nutrients
– High phytoplankton productivity
– Great fisheries (Sahara bank, Peru)
• Coastal Downwelling
– Warm surface water, low nutrients
– Low phytoplankton
– Poor fisheries (Northern Brazil)
Pressure Gradient and Coriolis
equilibrium: Geostrophic currents
Geostrophic currents
Geostrophic currents
follow lines of equal
pressure / height
Open Ocean Surface Currents
Ekman surface transport
Open Ocean Geostrophic currents
Geostrophic currents and
continents
Geostrophic currents and
continents
Upwelling and Downwelling
Convergence and Divergence
• Downwelling – Convergence
– Subtropical Gyres
– Sargasso Sea example of Geostrophic
current
• Upwelling – Divergence
Global Surface Currents
Oceanic Gyres
Western Boundary Currents
• Gulf Stream, Kuroshio,
Agulas currents
Eastern Boundary Currents
• California, Canary, Peru
currents
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Narrow (100 km)
Deep (<2000 m)
Swift (1.5 m/s)
Warm
High Volume
(50-75 Sv)
Broad (1000 km)
Shallow (<500 m)
Sluggish (0.3 m/s)
Cool
Low Volume
(10-15 Sv)
1 Sverdrup = 100000 m3/s
Western Boundary Current
Intensification
Stronger Coriolis force at higher latitudes
Western Boundary Current
Intensification
Subtropical Gyres displaced to the West
Gulf Stream
150 Sv
30 Sv
Gulf Stream
Cold and Warm Water Eddies
SST – Gulf Stream
SST – Gulf Stream
California Seasonal Reversal
The End