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Ocean Surface Circulation

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Ocean Surface Circulation Ocean surface circulation Recall from Last Time The three drivers of atmospheric circulation we discussed: • Differential heating • Pressure gradients • Earth’s rotation (Coriolis) Last two show up as direct forcing of ocean surface circulation, the first indirectly (it drives the winds, also transport of heat is an important consequence). Coriolis In northern hemisphere wind or currents deflect to the right. Equator In the Southern hemisphere they deflect to the left. Major surfaceA schematic currents of them anyway Surface salinity A reasonable indicator of the gyres 31.0 30.0 32.0 31.0 31.030.0 33.0 33.0 28.0 28.029.0 29.0 34.0 35.0 33.0 33.0 33.034.035.0 36.0 34.0 35.0 37.0 35.036.0 36.0 34.0 35.0 35.0 35.0 34.0 35.0 37.0 35.0 36.0 36.0 35.0 35.0 35.0 34.0 34.0 34.0 34.0 34.0 34.0 Ocean Gyres Surface currents are shallow (a few hundred meters thick) Driving factors • Wind friction on surface of the ocean • Coriolis effect • Gravity (Pressure gradient force) • Shape of the ocean basins Surface currents Driven by Wind Gyres are beneath and driven by the wind bands . Most of wind energy in Trade wind or Westerlies Again with the rotating Earth: is a major factor in ocean and Coriolisatmospheric circulation. • It is negligible on small scales. • Varies with latitude. Ekman spiral Consider the ocean as a Wind series of thin layers. Friction Direction of Wind friction pushes on motion the top layers. Coriolis deflects this motion to the right (in Northern Hemisphere). Ekman spiral Current from the top layer Wind exerts friction on second Friction Direction layer, causing it to move. of motion This motion is again Friction deflected to the right. Direction motion of and so on ... Ekman transport At some depth, the flow will be in the opposite direction to the wind. Total layer down to this depth is known as the Ekman Layer. Ekman layer deeps decreases with increasing latitude. Net transport in Ekman Layer is 90o to the wind. Why do gyres exist Consider a rectangular ocean in Northern Hemisphere Why do gyres exist Polar Easterlies Mid-latitude Westerlies Easterly trades The three main wind bands. Why do gyres exist Mid-latitude Westerlies Easterly trades The mid-latitude gyre is the largest, as most of the energy is in the trade winds and mid- latitude westerlies. Why do gyres exist Ekman transport moves water at 90o to the wind. Why do gyres exist The resulting convergence piles water up in the middle. Why do gyres exist H L The extra weight under the ‘hill’ causes a pressure gradient. Why do gyres exist Ekman transport (Coriolis) Pressure gradient (Gravity) The Ekman transport (Coriolis) and Pressure Gradient (Gravity) forces balance each other. This is known as a Geostrophic balance. Why do gyres exist Ekman transport (Coriolis) Water motion Pressure gradient (Gravity) Water moves around the ‘hill’. Geostrophic balance Balance of Ekman transport (Coriolis) and Pressure Gradient (Gravity) forces. Importantly, the Geostrophic flows occur over a larger depth range than direct wind forcing. Ekman transport (Coriolis) Water motion Pressure gradient (Gravity) Geostrophic hill is closer to the western side because of Coriolis’ latitude dependence. Currents in a gyre Western Boundary currents are narrow, deep, fast (warm) currents. Eastern Boundary currents are shallow, broad (cold) currents. N Northern Hemisphere Western Boundary Currents Gulf Stream (North Atlantic ocean) Kuroshio Current (North Pacific ocean) Brazil Current (South Atlantic ocean) Agulhas Current (Indian ocean) East Australian Current (South Pacific) Eastern Boundary Currents Canary Current (North Atlantic ocean) California Current (North Pacific ocean) Benguela Current (South Atlantic ocean) West Australian Current (Indian ocean) Peru Current (South Pacific ocean) Never that simple... Talley et al., 2011 et al., Talley and its still a simplification. Ocean is dynamic North Atlantic currents In Sverdrups: 1 million cubic meters / sec Gulf Stream The gulf stream transport about 55 million cubic meters per second (55 Sv) Ansel Adams Ansel 2.5 million cubic meters of concrete in Hoover Dam. Gulf Stream http://eoimages.gsfc.nasa.gov/ve/215/gulf_stream_modis_lrg.gif Heat redistribution Heat out t r o p s n a r Heat in t t a e h t e N Increasing heat The oceans account for about a third of the heat redistribution. Ocean affects climate Anchorage, AK 61oN Faeroe Islands 62oN Ocean affects climate Faeroe Islands 60 o 62 N F] o 50 Mean December 2010 40 o Temperature = 34.5 F 30 20 Anchorage, AK 10 o [ Temperature 61 N 0 Mean December 2010 o Jan Jul Temperature = 10.6 F Mar May Sep Nov 2010 Some key ideas • Ocean gyres are wind driven. • Ekman transport is 90o to the right of the wind in the northern hemisphere. (related ideas: Ekman spirals, Ekman depth). • Geostrophic balance is when Ekman transport (Coriolis) equals pressure gradient force. • Western boundary currents are narrow, deep, and fast. Eastern boundary currents are broad and slow. • Western boundary currents transport heat poleward. • One Sverdrup = 1x106 m3/s..
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