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Equatorial Circulation

Equatorial Circulation. Subtle changes in winds give rise to complicated surface current patterns Equatorial Undercurrent Focus on Pacific circulation, Atlantic patterns are similar. Indian ocean ones are not... Transient behavior - El Nino/Southern Oscillation. Equatorial Currents.

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Equatorial Circulation

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  1. Equatorial Circulation • Subtle changes in winds give rise to complicated surface current patterns • Equatorial Undercurrent • Focus on Pacific circulation, Atlantic patterns are similar. Indian ocean ones are not... • Transient behavior - El Nino/Southern Oscillation

  2. Equatorial Currents

  3. Munk’s Solution

  4. Wind-Driven Equatorial Circulation

  5. Wind-Driven Equatorial Circulation

  6. Equatorial Surface Currents

  7. Wind-Driven Equatorial Circulation

  8. Wind-Driven Equatorial Circulation

  9. Equatorial Convergence Photographs of the convergent front from R/V Thompson, taken by Edward Peltzer .

  10. Equatorial Convergence Photograph from the Space Shuttle Atlantis, at an altitude of 230 km on 7 August, 1992. Distance scale is ~ 100 km.

  11. Equatorial Surface Currents • Subtle changes in wind stress give rise to a complicated set of currents • South Equatorial Current (SEC) • North Equatorial Counter Current (NECC) • North Equatorial Current (NEC) • Vertical circulation cells are also observed

  12. Equatorial Divergence • Westward trades drive a divergence of Ekman transports from equator • Results in upwelling of cool water • Strongest in the eastern portion of Pacific • The cool tongue

  13. Equatorial Divergence

  14. Equatorial Divergence

  15. Equatorial Divergence

  16. Pelagic Ecosystems Ocean Biogeochemistry in a Nutshell • Light energy drives the net fixation of carbon • Within the euphotic zone, nutrients & CO2 produce CO2 & fixed carbon • Below the euphotic zone, the rxn’s reverse hn NUTS Fixed Carbon CO2 O2

  17. Equatorial Divergence

  18. Equatorial Divergence

  19. Equatorial Divergence

  20. The Equatorial Undercurrent • West wind drift piles up water in western equatorial Pacific • Creates a return subsurface flow that is locked to the equator • Rises as a goes across the Pacific • Also found in Atlantic

  21. Equatorial Undercurrents • West wind drift piles up water in western equatorial Pacific • Creates a subsurface return flow locked to the equator • EUC flow rates are huge ~100 cm/s!! • EUC rises due to equatorial divergence • A transient feature

  22. Equatorial Undercurrent

  23. Equatorial Undercurrent

  24. EUC @ 150W

  25. EUC @ 155W

  26. ADCP Observations

  27. ADCP Observations Frequency Range Resolution 38 kHz 800-1000m 24m 75 kHz 560-700m 16m 150 kHz 380-425m 8m

  28. ADCP Principles • Acoustic Doppler Current Profiler (ADCP) • Transmits a succession of acoustic pulses • Segments backscatter echoes into depth cells • Doppler shift in carrier frequency determines along beam velocities • Reconciling four beams - provides estimate of horizontal current

  29. Direct Observations Johnson et al. [2002]

  30. Direct Observations ADCP velocity sections across Pacific SEC NECC EUC EUC

  31. Direct Observations Temperature sections across Pacific

  32. EUC Transport NECC Mid-Basin Transport EUC - 30 Sv NECC - 20 Sv SEC - 50 Sv Peak month EUC - March NECC - December SEC(N) - April SEC(S) - February SEC(N) SEC(S)

  33. Current Location EUC NECC Mid-Basin Transport EUC - On equator NECC - 4 to 6oN SEC(N) SEC(S)

  34. Review • Subtle changes in wind stress give rise to • South Equatorial Current (SEC) • North Equatorial Counter Current (NECC) • North Equatorial Current (NEC) • Equatorial undercurrents are often found • Transient behavior is important (ENSO)

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