TOGA PanPacific Surface Current Study NOAA Atlantic Oceanographic
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TOGA Pan-Pacific Surface Current Study NOAA Atlantic Oceanographic and Meteorological Laboratory (AOML) 25 -26 April 1988 Miami, Florida
Background: WOCE and TOGA • In the late ‘ 80 s, 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.
Proposed Plan • Deploy 450 drifters over 2 years • Spatial coverage: 15 N 15 S, 80 W-126 E • 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. 7 o C. TOGA requirements are within 0. 3 o 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 90 s.
Deployment strategy • • Area of interest: 15 o. S-15 o. N, 80 o. W-126 o. E Area is divided into approx. 230 2 o x 10 o boxes Objective: build an array of drifters over 2 years Minimum # of drifters: 230; conservative maximum: 450 • Common drifter lifetime: 9 months-1 year • 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 15 m vs. 100 m drogue depth for the drifters. Why did the 15 m drogue win? What are the advantages and disadvantages of the shallower drogue?
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