Towards Dynamically Consistent Boundary forcing

Jeroen Molemaker (UCLA)Evan Mason (ULPGC)

Sasha Shchepetkin (UCLA)Francois Colat (UCLA)

One way nesting

• Obvious limitations:– One way will never be two way!

The round peg and the square hole

Testing lab: Canary Current system

Forcing at side boundaries

• Forcing of the outermost grid.– Something ROMS

• Forcing of (off line) one way nested grids– ROMS ROMS

Forcing the outermost grid

• Output from other (global) models

• Observations (such as World Ocean Atlas)

Observation based forcing

• World Ocean Atlas 2005 T, S monthly climatology

• Absolute SSH (Rio, 2005)– Now, annual mean, but we should include at

least monthly averaged perturbations

Our ‘truth standard’

Drifter data SSH variance

World Ocean Atlas 2005• Using level of no motion (1300 m)

World Ocean Atlas + absolute SSH

World Ocean Atlas + absolute SSH

‘crude’ Ekman layer transport

Assessing large scale, slow dynamics

-Subtract geostrophic flow

- Scale vertical profiles with mixed layer depth, f and wind stress vector:

z’ = z/Hbl, (u’) = (u Hbl f)/

Roms’ Ekman spiral

World Ocean Atlas + SSH ‘spiral’ Ekman transport

Impervious to baroclinic structure?

Patrick style Ekman KPP spiral Ekman

How well did we do?

Data:

Model:

Drifters SSH variance

ROMS ROMS

• Expected consistency much higher– Only regime transition is unavoidable

• Starting point:– Methods as existing in ROMS tools (Pierrick Penven, Patrick Marchesiello…. Many others)

ROMS ROMS

• Sigma z-levels Sigma coordinate

• No boundary mass flux correction

Or not so reasonable?

Horizontal-vertical interpolation

Matching boundary mass flux with parent grid

Matching grids at the boundary

Summary

• One way nesting can be good when:

– Solutions have consistent dynamics• roms roms• No enormous jumps in resolution

– Interpolation does not destroy said consistency