Equatorial Circulation

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
Munks 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
Equatorial Divergence
17
Equatorial Divergence
18
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

19
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

20
Equatorial Undercurrent
21
Equatorial Undercurrent
22
(No Transcript)
23
EUC _at_ 150W
24
EUC _at_ 155W
25
ADCP Observations
26
ADCP Observations
Frequency Range Resolution 38 kHz 800-1000m
24m 75 kHz 560-700m 16m 150 kHz 380-425m
8m
27
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

28
Direct Observations
Johnson et al. 2002
29
Direct Observations
ADCP velocity sections across Pacific
SEC
NECC
EUC
EUC
30
Direct Observations
Temperature sections across Pacific
31
Transport
EUC
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)
32
Current Location
EUC
NECC
Mid-Basin Transport EUC - On equator NECC - 4
to 6oN
SEC(N)
SEC(S)
33
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)