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

Munk’s Solution

Wind-Driven Equatorial Circulation

Equatorial Surface Currents

Wind-Driven Equatorial Circulation

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

Equatorial Convergence Photograph from the Space Shuttle Atlantis, at an altitude of 230 km on 7 August, 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 observed

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

Equatorial Divergence

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

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

Equatorial Undercurrent

150W

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ADCP Observations

FrequencyRangeResolution 38 kHz m 24m 75 kHz m 16m 150 kHz m 8m

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

Direct Observations Johnson et al. [2002]

Direct Observations ADCP velocity sections across Pacific NECC EUC SEC EUC

Direct Observations Temperature sections across Pacific

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

Current Location Mid-Basin Transport EUC - On equator NECC - 4 to 6 o N NECC EUC SEC(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 (ENSO)