http://svs.gsfc.nasa.gov/goto?10841 Ocean Surface Currents AdOc 4060 Spring 2013 Chris Jenkins 21 Feb 2013 Recommended reading, Benson Library: Ocean Circulation: A. Colling: Butterworth-Heinemann, Oxford UK, 1999, 2nd Edition, 286 pp. See Chs 3, 4, 5 Currents: Velocity (m/s) or Flow (Sv = 10^6 m3/s) Mechanical current meters • instrument fixed in place (moored) • anchored to seafloor or suspended from an anchored buoy • usually have impeller and directional vane 1992 North Pacific: 29,000 rubber duckies Accidental floats 1990 North Pacific: 21 containers of Nikes (40,000 pairs) http://www.google.com/url?sa=t&rct=j&q=acoustic%20release%20ocean&source= video&cd=5&ved=0CFEQtwIwBA&url=http%3A%2F%2Fwww.youtube.com%2F watch%3Fv%3DBClBozY7Ysk&ei=AdpHT- TVDsv3sQLd4ujqCA&usg=AFQjCNGRlld2DaHGH1359e2UwJwhHGxbgQ B. Doppler (acoustic) current meters • attached to ship or seafloor • sound waves bouncing off suspended particles give depth • Doppler shift of sound frequency gives speed of particles • Doppler comparison of different beams gives flow direction ADCP – Acoustic Doppler Current Profiler Turbulence sensors D. Models Debris: http://www.bbc.co.uk/news/science-environment- 17121086 GFDL: http://www.youtube.com/watch?v=JMLy4jV7Xgo Eulerian / Lagrangian Methods http://www.youtube.com/watch?v=05crV-RidF4 Ekman spiral • force balance & net motion (90° to R) distributed over ~100 m • friction causes reduced velocity with depth • surface water moves ~45° to right of wind direction (NH) • water beneath that “feels” surface water, moves farther to R Ekman transport • Balance of wind stress (τ) and Coriolis effect is in the direction of the wind • Coriolis increases with water velocity (and latitude): 2Ωsinφ×V • Coriolis vector is 90° to right of flow in Northern Hemisphere • Net flow is 90° to right of wind in Northern Hemisphere Wind-driven gyre circulation • surface currents: mostly <400 m, ~10% of ocean volume • dominated by the subtropical gyres (may extend to ~1000 m) • dynamics: 3 main factors • Ekman transport • Geostrophic balance • Westward intensification Ekman convergence due to trades and westerlies creates a “hill” of water (high pressure) sea surface height (North Atlantic Subtropical Gyre) Upwelling and downwelling Vertical movement of water (½) Upwelling = movement of deep water to surface Hoists cold, nutrient-rich water to surface Produces high productivities and abundant marine life Downwelling = movement of surface water down Moves warm, nutrient-depleted surface water down Not associated with high productivities or abundant marine life Coastal upwelling and downwelling Ekman transport moves surface water away from shore, producing upwelling Ekman transport moves surface water towards shore, producing downwelling Other types of upwelling Offshore wind Sea floor obstruction Sharp bend in coastal geometry Question: Flow ? wind C Pressure ? Recall that geostrophy is the balance between a pressure gradient (P) and Coriolis effect (C), with the force vectors indicated above for the Northern Hemisphere. If Ekman transport is the balance between wind stress (τ) and Coriolis effect, which vectors below describe this balance in the Northern Hemisphere? τττ τ A. B. C. D. flow C flow C flow flow C C Antarctic surface circulation 1. Wind stress patterns 2. Eckman drift 3. Density water masses 4. Fronts, (convergences, divergences) Antarctic Circumpolar Wave http://www.antarctica.gov.au/science/climate-processes-and-change Also: http://oceanworld.tamu.edu/resources/ocng_textbook/chapter13/chapter13_04.htm Arctic circulation Also: http://nsidc.org/cryosphere/se aice/processes/circulation.ht ml.