TOGA PanPacific Surface Current Study

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TOGA Pan-Pacific Surface Current Study

• NOAA Atlantic Oceanographic and Meteorological
  Laboratory (AOML)
• 25-26 April 1988
• Miami, Florida

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Background WOCE and TOGA

• In the late 80s, both TOGA and the World Ocean
  Circulation Experiment (WOCE) were incorporating
  drifters as part of their programs.
• Since the programs were bound to overlap, it
  would be cost-effective to coordinate their
  strategies.
• In April 1988, the WOCE Planning Committee and
  all Principal Investigators of the TOGA
  Pan-Pacific Surface Current study met at
  AOML/NOAA in Miami, Florida.

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Proposed Plan

• Deploy 450 drifters over 2 years
• Spatial coverage 15N-15S, 80W-126E
• Desired sampling density 2 deg. latitude by 10
  deg. longitude

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General Scientific Objectives
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Technical Objectives
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Some specific questions

• What are the causes of SST warming? Why, in some
  years, does cold water fail to surface in the
  eastern tropical Pacific from September to
  January?

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Competing Hypotheses

• SST variations are caused by surface heat
  advection. Warming events occur when normal
  surface circulation patterns are disrupted.
• SST variations are driven by local air-sea
  interactions. Warming events occur when vertical
  mixing in a region is reduced, and colder, deeper
  waters cannot be brought to the surface.

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• Which hypothesis is correct?
• How can they be tested using drifter data?

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The surface advection hypothesis
Flow component parallel to T gradient advects
heat.
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• Actual flow in the region is in opposite
  direction to geostrophic flow.
• The accuracy of the ship-drift derived flow is
  unknown.
• Not enough data at the time to compute the heat
  advection during the warming episodes.

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• Time-varying flow component also advects
  significant amount of heat.
• Higher frequency and smaller spatial scales than
  the mean flow.
• Again, direct measurements needed to fully
  resolve the head advection.

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The vertical mixing hypothesis

• Look at heat content of very near-surface layer
  of depth h.
• Newell (1986) hypothesizes that warming events
  happen when, over a monthly time scale, net
  surface heat flux approaches 0, average
  temperature over h is close to constant, and
  turbulent flux at base of the layer is
  significantly reduced from its normal value.
• Accurate measurements of daily temperature change
  following the water are needed the temporal and
  spatial turbulent flux variability.

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Additional objectives Regional circulation
studies

• Testing the competing heat advection hypotheses
  requires a pan-Pacific data set.
• Same data can be used to study local phenomena.
• To address the potential issues in detail, the
  plan divides the Pacific basin into 3 parts
  eastern, central and western.

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Eastern Pacific

• Heat advection in the Cold Tongue
• Why does the Cold Tongue fail to develop during
  El Nino events?
• Equatorial divergence
• Develop a time series of estimates of Ekman
  transport divergence near the equator.
• Shear-instability of waves
• Shear instability between equatorial currents
  generates cusp-shaped waves that lead to an
  equatorially-convergent heat transport.
• Drifters can detect the presence or absence of
  these waves.
• Eastern Pacific warm pool
• Shallower and more variable than the Western
  Pacific Warm Pool.
• Generation region for eastern Pacific hurricanes.

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Central Pacific

• SST anomalies during ENSO events
• Can they be observed in the central Pacific?
• Equatorial mixing
• No data on horizontal eddy energy in central
  tropical Pacific
• Provide direct measurements to verify model
  results
• Wind-driven currents
• Subtracting geostrophic flow from
  drifter-measured actual flow produces estimates
  of wind-driven component.

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Western Pacific

• Large-scale circulation
• multiple current systems with large seasonal and
  interannual variations
• Historical ship drift data is insufficient to
  resolve the temporal variations and mesoscale
  spatial features. Drifters provide better
  resolution.
• Cross-equatorial flow
• What are the seasonal patterns? Where does the
  flow in the New Guinea Current and SECC go? Is
  there a mean flow across the equator in the West?
• Western Pacific Warm Pool
• Present (as of 1988) techniques only reserved the
  temperature structure in the pool to within 0.7o
  C. TOGA requirements are within 0.3o C.
• Westerly wind bursts and eastward jets
• How far east do these jets carry Western water?
  What is the equatorial convergence in the surface
  layer in these jets.

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Drifter design and calibration

• Two drogue designs to be assesseed during
  calibration stage.
• Test survivability and water-following
  characteristics.
• Determine which type of drifter will be used for
  the program in the 90s.

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Deployment strategy

• Area of interest 15oS-15oN, 80oW-126oE
• Area is divided into approx. 230 2o x 10o boxes
• Objective build an array of drifters over 2
  years
• Minimum of drifters 230 conservative maximum
  450
• Common drifter lifetime 9months-1year
• Start with 50-60 deployments a month, work to
  desired density.
• Australia, France and Japan also involved in
  contributing drifters.

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Ship of opportunity tracks available for drifter
deployment.
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Data management

• Main data center at AOML/NOAA in Miami
• Raw ARGOS data sampled daily. Maps based on
  weekly drifts will be regularly distributed to
  participants.
• Monthly data reported to Climate Analysis
  Bulletin and made available to all countries
  participating in TOGA.

http//www.aoml.noaa.gov/phod/dac/gdp.html
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Current status
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Discussion Questions

• Which of the two competing hypotheses for SST
  variability do you think is right? What other
  technologies developed over the past 20 years
  would you use to test them?
• In addition to the pan-Pacific and regional
  studies listed in the plan, what other purposes
  would the drifter data be useful (or has been
  useful for)?
• Before the plan was formed, there was strong
  debate about 15m vs. 100m drogue depth for the
  drifters. Why did the 15m drogue win? What are
  the advantages and disadvantages of the shallower
  drogue?