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Deep ocean convection: observations from an adaptive network of underwater gliders

Deep ocean convection: observations from an adaptive network of underwater gliders. David Smeed, Gwyn Griffiths and Harry Bryden, and Lucas Merckelbach Southampton Oceanography Centre, Sonya Legg GFDL, Princeton. Outline. Convection Gliders Methodology Research programme

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Deep ocean convection: observations from an adaptive network of underwater gliders

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  1. Deep ocean convection: observations from an adaptive network of underwater gliders David Smeed, Gwyn Griffiths and Harry Bryden, and Lucas MerckelbachSouthampton Oceanography Centre, Sonya LeggGFDL, Princeton

  2. Outline • Convection • Gliders • Methodology • Research programme • Previous experience with real-time data assimilation • Summary

  3. Convection Deep water formation is a key process in the thermohaline circulation of the ocean. It occurs over relatively short periods in late winter at just a few locations, notably the Norwegian-Greenland Sea, (A) the Labrador Sea (B) and the northwest Mediterranean Sea (C). Inhospitable conditions make it difficult to take observations from ships at times of active convection. A B C (Marshall & Schott 1999)

  4. Key processes Preconditioning Convective mixing Lateral exchange and spreading Lateral scales d (Marshall & Schott 1999)

  5. Gliders - a new platform for oceanographic observations (Webb Research Corp.)

  6. Gliders - buoyancy driven AUVs ~1m (Webb Research Corp.)

  7. Gliders - key features • Speed ~ 0.3 ms-1 • Endurance 4-6 weeks • GPS navigation at surface • 2-way satellite communications (Iridium) • Measurements • Conductivity, temperature and pressure • Average current velocities from navigation • Vertical velocity

  8. Project objectives To develop a system for observing ocean processes and features using a network of ocean gliders directed by a real-time control system incorporating a numerical forecast model. To make novel observations of open-ocean deep convection that will enable us to determine the characteristics of plumes within convective patches and the mechanisms and rates of lateral spreading of the convective patches.

  9. Methodology

  10. Field program Two field experiments in February 2006 and February 2007 in the Gulf of Lions, Mediterranean Sea. Specific oceanographic objectives: To characterise the amplitude and horizontal and vertical scales of the plumes. To examine the exchange processes at the edge of the convection patch.

  11. Gulf of Lions (MODB / MEDAR)

  12. Gulf of Lions (Marshall & Schott 1999)

  13. Gulf of Lions - observations (Schott et al 1996)

  14. Numerical modelling (Legg, McWilliams, & Gao, 1998)

  15. The need for real-time data • Convection is driven by air-sea fluxes • Optimum use of the gliders requires the best possible model forecast. • Model requires accurate air-sea fluxes of momentum, heat and freshwater, and lateral boundary conditions. (Mertens & Schott 1998)

  16. Real-time data requirements • Daily analyses and 3-7 day forecasts for • Air sea fluxes of momentum, heat and freshwater from weather forecast model (e.g. ECMWF). • Ocean temperature, salinity and velocity on lateral boundary from ocean forecast model (e.g. Mediterranean FOAM). • Ability to automate access.

  17. Real-time data assimilation Real-time forecasting of biological and physical dynamics at the Iceland-Faeroes front in June 2001 (Popova, Srokosz & Smeed 2002)

  18. (Popova, Srokosz & Smeed 2002)

  19. Summary • Novel project to develop and test adaptive sampling strategies using undersea gliders and real-time data assimilation in an ocean forecast model. • Will apply to study deep ocean convection in the Mediterranean Sea in winter 2005/6 and 2006/7. • Require real-time forecasts of air-sea fluxes and ocean variables on boundaries for high resolution model forecasts. • Expect other applications to follow.

  20. Chl-a mg/m3 (Popova, Srokosz & Smeed 2002)

  21. Depth of  = 27.55 (Popova, Srokosz & Smeed 2002)

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