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Nested models of the Southland Current

Nested models of the Southland Current. Mark Hadfield National Institute of Water & Atmospheric Research, Wellington, NZ. Introduction.

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Nested models of the Southland Current

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  1. Nested models of the Southland Current Mark Hadfield National Institute of Water & Atmospheric Research, Wellington, NZ

  2. Introduction Simulations of flow on the continental shelf southeast of South Island, New Zealand, with a view to modelling the dispersal of larvae from intertidal organisms (sea urchin aka kina).

  3. The Southland Current/Front You are here Southland Current

  4. The Southland Current • Is a section of the Southern Hemisphere’s Subtropical Front • Is locked to the continental shelf break on the eastern coast of the South Island • Has a transport of ~ 10 Sv, predominantly Subantarctic Water • Is very persistent and unidirectional; mean surface current in the core has a magnitude ~ 0.2–0.3 m/s • Temporal variability (on the shelf) is predominantly wind-driven

  5. 10 km ROMS simulations on 3 grids 2.5 km 625 m

  6. 10 km ROMS simulations on 3 grids 2.5 km 625 m

  7. 10 km ROMS simulations on 3 grids 2.5 km 625 m

  8. Outer model • Driven my monthly climatology at surface and lateral boundaries. No time-stamped forcing, no assimilation of time-stamped data. • Lateral boundary data from the SODA reanalysis • Surface data from the NCEP Reanalysis • Spun up for 3 years; 5-day average fields saved for a further 3 years • Puts the major fronts and current systems in the right places (more or less)

  9. Outer (10 km) model 2.5 km 625 m

  10. Intermediate model • Lateral boundaries from outer model (5-day average), i.e. one-way, off-line nesting • Surface stresses 6-hourly from the NCEP Reanalysis • Climatological surface heat & salinity flux • LMD vertical mixing • Boundary numerics: • M2FLATHER • FSCHAPMAN • M3RADIATION+M3NUDGING (here be dragons) • TRADIATION+TNUDGING

  11. Intermediate (2.5 km) model 2.5 km 625 m

  12. Inner model • Very similar nesting approach to intermediate model. • Lateral boundaries from intermediate model (1-day average)

  13. Inner (625 m) model 2.5 km

  14. Model validation: SST mean

  15. Model validation: SST annual amplitude 2.5 km 625 m

  16. Model validation: current meters 2.5 km 625 m

  17. Model validation: current meters r2 ~ 0.4 2.5 km 625 m

  18. Validation conclusion • Shows promise, could do better??

  19. Issues • Boundary artefacts • Surface pressure forcing • Better wind datasets

  20. Boundary artefacts At the interface between the outer and intermediate models, there is a mismatch in the wind forcing. Wind-driven surface currents in the intermediate model impinge on the lateral boundaries and generate spurious upwelling and downwelling. This can generate spurious density pertubations that spread into the interior and degrade the solution Standard deviation of w (100 m) M3CLAMPED M3RADIATION

  21. Boundary artefacts Could ROMS velocity boundary conditions be formulated to be less resistive to currents impinging on the boundary? With the present ROMS boundary layout, the velocity at the boundary does not feel the interior forcing directly. Would a reduced-physics formulation work better???

  22. Surface pressure forcing Trial simulations suggest surface pressure forcing (the barometer effect) may be significant, maybe more so than wind stress. Better wind data? Scatterometer data available from 2001-ish (but not sure which dataset is best) Mesoscale model data available from May 2007

  23. Conclusions • Modelling strategy (one-way nested models) has been reasonably successful. • There is room for improvement in the treatment of velocity at the boundaries. • Location of Southland Current verified by comparison with SST climatology. • Comparison with 1993 current-meter data suggest model has some skill but underestimates temporal variability. • Surface pressure forcing may be significant. • Better wind forcing datasets exist but there are not yet any good validation data for the period they cover.

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