Nested models of the Southland Current

Mark HadfieldNational Institute of Water & Atmospheric Research, Wellington, NZ

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).

The Southland Current/Front

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Southland Current

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

ROMS simulations on 3 grids

10 km

2.5 km

625 m

ROMS simulations on 3 grids

10 km

2.5 km

625 m

ROMS simulations on 3 grids

10 km

2.5 km

625 m

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)

Outer (10 km) model

2.5 km

625 m

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

Intermediate (2.5 km) model

2.5 km

625 m

Inner model

• Very similar nesting approach to intermediate model.

• Lateral boundaries from intermediate model (1-day average)

Inner (625 m) model

2.5 km

Model validation: SST mean

Model validation: SST annual amplitude

2.5 km

625 m

Model validation: current meters

2.5 km

625 m

Model validation: current meters

2.5 km

625 m

r2 ~ 0.4

Validation conclusion

• Shows promise, could do better??

Issues

• Boundary artefacts

• Surface pressure forcing

• Better wind datasets

Boundary artefactsAt 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

M3CLAMPED M3RADIATION

Standard deviation of w (100 m)

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???

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

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.