Equatorial Circulation
description
Transcript of 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
Munk’s Solution
Wind-Driven Equatorial Circulation
Wind-Driven Equatorial Circulation
Equatorial Surface Currents
Wind-Driven Equatorial Circulation
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, 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
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
Equatorial Divergence
Equatorial Divergence
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
PelagicEcosystems
NUTS
CO2
Fixed Carbon
O2
h
Equatorial Divergence
Equatorial Divergence
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
Equatorial Undercurrent
EUC @ 150W
EUC @ 155W
ADCP Observatio
ns
ADCP Observations
Frequency Range Resolution 38 kHz 800-1000m 24m 75 kHz 560-700m 16m 150 kHz 380-425m 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
NECCEUC
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 6oN
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)