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Circulation and Drift Pathways in the Northwest Atlantic Ocean

Circulation and Drift Pathways in the Northwest Atlantic Ocean. Jinyu Sheng Oceanography, Dalhousie University. Collaborators: Richard Greatbatch (DAL) Dan Wright (BIO/DAL) Peter Jones (BIO) Kumiko Azetsu-Scott (BIO) Liang Wang (DAL) HuaLin Wong (DAL).

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Circulation and Drift Pathways in the Northwest Atlantic Ocean

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  1. Circulation and Drift Pathways in the Northwest Atlantic Ocean Jinyu Sheng Oceanography, Dalhousie University

  2. Collaborators: Richard Greatbatch (DAL) Dan Wright (BIO/DAL) Peter Jones (BIO) Kumiko Azetsu-Scott (BIO) Liang Wang (DAL) HuaLin Wong (DAL)

  3. Financial support provided by: • Canada Foundation for Innovation (CFI) • Atlantic Canada Opportunities Agency (ACOA) • Department of Fisheries and Oceans (DFO) • Meterological Service of Canada (MSC) • NSERC • MARTEC

  4. Contents • Introduction • Semi-Monthly Composite SST Images • Circulation Determined from Drifters • Ocean Circulation Model: CANDIE • Diagnostic, Prognostic, and Semi-Prognostic Modes • Model Results • Seasonal Temperature and Circulation • Comparison with Observations • Pathways and Vertical Mixing of Tracers • Animations • Surface Temperature and Currents • Pathways of Passive Tracers 5. Summary and Conclusion

  5. Introduction • NSERC/MSC/MARTEC established two Industrial Research Chairs in “Regional Ocean Modelling and Prediction” at Dalhousie University. • The main objective of the Chairs is to develop ocean models that will beused for predicting atmosphere-ocean-ice conditions in the Atlantic region of Canada. • Eastern Canadian seaboard is one of the most challenging marine environments in the world Schematic showing major currents in the Northwest Atlantic Ocean. (By courtesy of Dr. Igor Yashayaev)

  6. Semi-Monthly Composite SST Images in 1999 (By courtesy of Biological Oceanography Section, BIO)

  7. Semi-Monthly Composite SST Images in 1999 (By courtesy of Biological Oceanography Section, BIO) February May August November

  8. Trajectories of Sub -Surface Drifters (Lavender et al., 2000. Nature, 407) • More than 200 subsurface floats (PALACE, SOLO) • Drifting at 400, 700, and 1500m for several days, then ascends to the surface to transmit data via Argos. • Measuring T and S upon ascent or descent (7400 TS profiles). Red,orange and green arrows are three floats drifting at 700m. Blue and purple arrows are two floats drifting at 1500m.

  9. Mean circulation at 700m (Lavender et al., 2000. Nature 407) Bluearrowsare speeds of less than 5 cm/s (distance traveled over 30 days).Red arrowsare speeds of greater than 5 cm/s (distance traveled over 8 days).

  10. Deep Convection in the Labrador Sea (Lilly et al., 1999. JPO, 29) Yearlong temperature record in the Western Labrador Sea contoured over ten instruments between 110 and 2510m. White lines show the pressures at the instruments depth.

  11. CANDIE: Primitive Equation Ocean Model • CANDIE stands for CANadianversion of Diecast. • Diecast was developed by Dietrich, C-grid CANDIE was developed by Sheng, Wright, Greatbatch, and Dietrich (1998), and A-grid CANDIE was primarily developed by Wright based on A-grid Diecast. • Other contributors include: Youyu Lu (free surface), Liang Wang (passive tracers), Bill O’Connor (CANDIE User’s Guide), David Brickman (partial cell),HuaLin Wong(CANDIE website), etc. • A three-dimensional (3D), fully non-linear, primitive equation, finite-difference, z-level model. • CANDIE has been subjected to several rigorous tests, using test problems with known solutions, an important process for building confidence in a numerical model.

  12. Major Applications • Wind-driven circulation over an idealized coastal Canyon (Sheng,Greatbatch, Wright & Dietrich, 1998). • Process studies of the Gaspe Current (Sheng 2000). • Seasonal circulation in the Northwest Atlantic Ocean (Sheng, Greatbatch & Wright, submitted in Oct., 2000). • Internal tide generation over topography (Lu, Wright & Brickman, 2001 (in press)). • Tidal Circulation and mixing in the Gulf of St. Lawrence (Lu, Thompson &Wright, 2001 (in press)). • Circulation in the Eastern Canadian shelf seas (Sheng, Thompson, Dowd &Petrie, revised in 2000).

  13. Governing Equations

  14. Diagnostic vs Prognostic Models Diagnostic Model: Calculates ocean currents from specified temperature and salinity fields. • Relatively easy and straightforward to run. • Robust in multi-year simulations. • Wrong model for studying the interaction of temperature/salinity fields with the flow field. • Wrong model for studying winter convective mixing. Prognostic Model: Calculates ocean currents, together with temperature and salinity fields. • Capable of simulating baroclinic instability. • Capable of estimating winter convective mixing. • Sensitive to subgrid-scale mixing parameterizations • Deteriorating model skill in longer simulations.

  15. The Semi-Prognostic Method Sheng, Greatbatch, and Wright recently proposed a semi-prognostic method to improve the utility of the ocean model. The main idea is to replace the hydrostatic equation by: (1) with temperature and salinity equations unchanged. The Semi-Prognostic method is: Better thanthe robust diagnostic approach proposed by Sarmiento and Bryan [1982], since the new method does not constrain the temperature and salinity equations. Different fromthe assimilative approachexamined by Woodgate and Killworth [1997], since the new method does not add any relaxation terms directly in the momentum equations.

  16. Application of the Semi-Prognostic Method to the Northwest Atlantic Ocean

  17. Model Parameters • Model resolution:1/3degree by1/3degree in horizontal and31unevenly spaced z-levels in vertical. • Fourth-order accurate numerical scheme for space differencing and Thuburn’s flux limiter scheme for T/S advection terms. • Smagorinsky [1963] scheme for the horizontal eddy viscosity co-efficient. • Csanady [1982] scheme for vertical mixing co-efficients in the surface Ekman layer. • Ri-dependent scheme [Large et al., 1994] for vertical mixing co-efficients below the Ekman layer. • Static instability was removed by an instantaneous convective adjustment scheme. • Quadratic bottom stress with CD set to 0.0015.

  18. Climatology o o • (1/6) X(1/6) monthly mean temperature and salinity climatologies constructed recently by Geshelin, Sheng, and Greatbatch (1999) for the Northwest Atlantic. o o • (1/2) X (1/2) monthly mean COADS wind stress (COADS: Comprehensive Ocean-Atmosphere Data Set). o o • (1/2) X (1/2) monthly mean COADS net heat flux. o o • 1 X 1 annual mean transport streamfunction diagnosed by Greatbatch, Fanning, Goulding, and Levitus (1991).

  19. Sea Surface and Lateral Boundary Conditions • CANDIE was driven by monthly COADS wind and annual depth-mean boundary flow calculated by Greatbatch et al. (1991). • Sea surface salinity was restored to the monthly mean climatology generated by Geshelin et al. (1991). • The net heat flux through the Sea surface is approximated by : (2) Where and are respectively monthly mean COADS net heat flux and Geshelin et al.’s SST. • Sommerfeld radiation conditions were applied to T, S and normal velocity at open boundaries

  20. Model Results • Semi-Prognostic modelresults: • Sea surface temperature and currents. • Subsurface temperature and currents. • Comparison with SST images. • Comparison with drift velocity data. • Passive tracer experiments: • Major pathways of tracers released in the Labrador Sea. • Vertical fluxes of tracers in the Labrador Sea.

  21. Sea Surface Temperature and Circulation Predicted by the Semi-Prognostic Model

  22. Sea Surface Temperature and Circulation Predicted by the Semi-Prognostic Model February May August November

  23. Sub -Surface Temperature and Circulation Predicted by the Semi-Prognostic Model February May August November

  24. Time-Depth Distribution of Temperature

  25. Predicted Observed Sea Surface Fields in February Sea Surface Fields in May

  26. Predicted Observed Sea Surface Fields in August Sea Surface Fields in November

  27. Comparison of Predicted and Observed Sub -Surface Circulation Predicted Observed Blue arrowsare speeds of less than 5 cm/s.Red arrowsare speeds of greater than 5 cm/s.

  28. Major Pathways of Passive Tracers • Major pathways of passive tracers released intheLabrador Sea over: • (a)Upper water columns of the slope-water region(Tracer2); • (b)Upper water columns of the deep-water region(Tracer 4). • Comparison of results produced by: • (a)Semi-Prognostic Model; • (b)Pure-Diagnostic Model; • (c)Pure-Prognostic Model • Vertical fluxes of tracers in the Labrador Sea

  29. Predicted by the Semi-Prognostic Model: Tracer 2 (a) Concentration in the Upper-Ocean (0-383m) (b) Concentration in the Lower-Ocean (383-5000m)

  30. Comparison of Model Results:Tracer 2 (a) Semi-Prognostic (b) Pure-Diagnostic (c ) Pure-Prognostic

  31. Comparison of Circulation at 61m Produced bySemi- and Pure-Prognostic Models

  32. Comparison of Model Results:Tracer 4 (a) Semi-Prognostic (b) Pure-Diagnostic (c ) Pure-Prognostic

  33. Winter Convective Mixing:Tracer 4

  34. Animations of Semi-Prognostic Results Movie 1:Near-surface temperature and currents. Movie 2:Normalized concentration ofTracer 2. Movie 3:Concentration ofTracer 2at 967 m.

  35. Summary and Conclusion • We developed a semi-prognostic method to improve the utility of the primitive equation ocean model. • The newly-developed semi-prognostic method adjusts the flow field towards climatology, while allowing the temperature and salinity fields to evolve freely with the flow. • Multi-year model results using the semi-prognostic method show a significant improvement over those produced by pure-diagnostic and pure-prognostic models. • The results show here represent our early progress of developing the next generation of DALCOAST.

  36. Thank you

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