a re ne ed ed to s ee th i s pi c tu re.
TIF F (Un co mpre ss ed )d ec omp res so r
Qu i ck Ti me ™a nd a
TOGA Pan-Pacific Surface
Current Study
NOAA Atlantic Oceanographic and
Meteorological Laboratory (AOML)
25-26 April 1988
Miami, Florida
Qu i ck Ti me ™a nd a
TIF F (Un co mpre ss ed )d ec omp res so r
a re ne ed ed to s ee th i s pi c tu re.
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 PanPacific Surface Current study met at
AOML/NOAA in Miami, Florida.
Proposed Plan
• Deploy 450 drifters
over 2 years
• Spatial coverage: 15N15S, 80W-126E
• Desired sampling
density: 2 deg. latitude
by 10 deg. longitude
General Scientific Objectives:
Technical Objectives:
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?
Competing Hypotheses:
1. SST variations are caused by surface heat
advection. Warming events occur when
normal surface circulation patterns are
disrupted.
2. 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.
• Which hypothesis is correct?
• How can they be tested using
drifter data?
The surface advection hypothesis
Flow component
parallel to T
gradient
advects heat.
• 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.
• 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.
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.
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.
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 equatoriallyconvergent 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.
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.
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.
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.
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.
•
•
•
•
Ship of opportunity tracks available for drifter deployment.
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
Current status
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?