Introduction to the Lagrangian Isopycnal Dispersion Experiment in the North Atlantic

1. Introduction to theLagrangian Isopycnal DispersionExperiment in the North Atlantic Long Zhou

2. Outlines • Experiment Domain • Instruments – RAFOS floats • Objectives • Low oxygen tongue • Parameterize the eddy fluxes for numerical models • SLA analysis in LIDEX domain • Mesoscale eddy mixing • Peclet number • Summary

3. Experiment Domain 6 – 11 deg N, 25 – 32 deg W 500 km meridionally and 720 km zonally

4. Open circles: RAFOS float deployment • 2 isopycnal surfaces: and • Diamonds: Sound sources, spaced apart 720 km zonally and 500 km meridionally

5. Instruments • Standard RAFOS floats are isobaric floats made of glass which has a low thermal expansion coefficient (Rossby et al. 1986). • Isopycnal RAFOS floats have approximately the same compressibility as seawaterthrough the use ofa compressee (Rossby et al. 1985)

6. Isopycnal RAFOS floats with oxygen sensor: measure temperature, pressure and oxygen.

7. Sound Sources

8. Objectives 1. Dynamics of low oxygen tongue • The low-oxygen tongue is due to: • Isolation of the water (i.e., no direct ventilation) • Enhanced productivity and organic regeneration in the coastal upwelling off North Africa (Doney and Bullister 1992). Data is from the World Atlas 94 (Levitus and Boyer 1994)

9. Lowest values are found very near the coast • increasing as it is advected westward • This surface is well below the euphotic zone. biological activity will consume oxygen at these depths. Reason: isopycnal and diapycnal mixing of higher O2 waters • The mean flow of the floats can also prove the assumption that a mean westward zonal current exists.

10. Objectives 2. To better parameterize the eddy fluxes • The long-term goal of this project is to aid the non-eddy-resolving climate models • The method used to relate diapycnal/isopycnal mixing to the model’s coordinate system has to be accurate to predict the future state of the ocean.

11. Sea Level Anomalies in LIDEX Domain • What is SLA or SSHA ?

12. SLA can be used to study ocean currents • Surface current is proportional to the surface slope. • The current is deflected by 90 deg to the right of the direction of the slope in the northern hemisphere • Cyclonic eddy – SSH greater at the center (N.H.)

13. SLA derived from the merged TOPEX/POSEIDON and ERS-1/2 Altimeter data covering the LIDEX domain Eddies: Red – Cyclonic Blue – Anti-Cyclonic Movie: October 14, 1992 to January 6, 1993 with 7 days step

15. Mesoscale Eddy Mixing (from Marshall)

16. (from Marshall)

17. Peclet number • The Peclet number (Pe) is a measure of the relative importance of advection to diffusion. • A typical open ocean is characterized by velocities of order .01 m/s, lengths of order 2-3000 km (the size of ocean gyres), and turbulent diffusivities of order 1000 m2/s. This gives a Peclet number of order 20-30. • Different Peclet number regimes would give different levels of isopycnal stirring and mixing relative to the mean advection.

18. joey: The Peclet number is a dimensionless number used in calculations involving convective heat transfer. It is the ratio of the thermal energy convected to the fluid to the thermal energy conducted within the fluid. If Pe is small, conduction is important and in such a case, the major source of conduction could be down the walls of a tube. The Peclet number is the product of the Reynolds number and the Prandtl number. It depends on the heat capacity, density, velocity, characteristic length and heat transfer coefficient.

19. Summary • The objectives of LIDEX are to address the dynamics of low oxygen tongue off northern Africa and to parameterize the messoscale eddy mixing for numerical modeling. • SLA can be used to study the surface geostrophic flow structure in the LIDEX domain in order to provide the advection term of an idealized tracer(temperature or dissolved Oxygen) • Eddy diffusivity K can be derived from the evolution equation

20. Acknowledgement Peter Cornillon Dave Hebert Joe Kuel Vitalii Sheremet