Equatorial CirculationEquatorial CurrentsMunk’s SolutionWind-Driven Equatorial CirculationSlide 5Equatorial Surface CurrentsSlide 7Slide 8Equatorial ConvergenceSlide 10Slide 11Equatorial DivergenceSlide 13Slide 14Slide 15Slide 16Slide 17The Equatorial UndercurrentEquatorial UndercurrentsEquatorial UndercurrentSlide 21PowerPoint PresentationEUC @ 150WEUC @ 155WADCP ObservationsSlide 26ADCP PrinciplesDirect ObservationsSlide 29Slide 30TransportCurrent LocationReviewEquatorial 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 OscillationEquatorial CurrentsMunk’s SolutionWind-Driven Equatorial CirculationWind-Driven Equatorial CirculationEquatorial Surface CurrentsWind-Driven Equatorial CirculationWind-Driven Equatorial CirculationEquatorial ConvergencePhotographs of the convergent front from R/V Thompson, taken by Edward Peltzer.Equatorial ConvergencePhotograph from the Space Shuttle Atlantis, at an altitude of 230 km on 7 August, 1992. Distance scale is ~ 100 km.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 observedEquatorial 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 tongueEquatorial DivergenceEquatorial DivergenceEquatorial DivergenceEquatorial DivergenceEquatorial DivergenceThe 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 AtlanticEquatorial 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 featureEquatorial UndercurrentEquatorial UndercurrentEUC @ 150WEUC @ 155WADCP ObservationsADCP ObservationsFrequency Range Resolution 38 kHz 800-1000m 24m 75 kHz 560-700m 16m 150 kHz 380-425m 8mADCP 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 currentDirect ObservationsJohnson et al.[2002]Direct ObservationsADCP velocity sections across PacificNECCEUCSECEUCDirect ObservationsTemperature sections across PacificTransportMid-Basin Transport EUC - 30 Sv NECC - 20 Sv SEC - 50 SvPeak month EUC - March NECC - December SEC(N) - April SEC(S) - February NECCEUCSEC(N)SEC(S)Current LocationMid-Basin Transport EUC - On equator NECC - 4 to 6oNNECCEUCSEC(N)SEC(S)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