. OCEANIC SUBMESOSCALE SAMPLING WITH WIDE-SWATH ALTIMETRY James C. McWilliams Department of Atmospheric & Oceanic Sciences Institute of Geophysics & Planetary Physics U.C.L.A.
Recall the long-standing mystery of spirals on the sea (Munk et al., 2000) seen in slick patterns and SAR images on a scale of kilometers (top) near Catalina Island; (bottom) in the Mediterranean (Scully- Power, 1986) but never measured in situ nor even remotely in u, η, or T.
mesoscale currents: 40 km horizontally, 1 km vertically, & 1 week temporally arising from instabilities of the general circulation dominance of horizontal velocity spectrum and isopycnal eddy fluxes energetically conservative and adiabatic in interior statistically well sampled by Jason/TOPEX/Poseidon altimetry relatively well understood submesoscale currents: 4 km horizontally, 100 m vertically, & 1 day temporally arising by frontogenesis from the messocale dominance of near-surface vertical velocity spectrum and diapycnal eddy fluxes energetically dissipative and diabatic an opportune target for wideswath altimetry a scientific frontier
ENERGY BUDGET FOR THE OCEANIC GENERAL CIRCULATION wind work buoyancy work general circulation energy: KE & PE boundary layers interior eddy instability internal wave induced balance breakdown & forward cascade "eddy viscous" tidal work 3d turbulence dissipation
SST image in (250 km) 2 off California with associated measurements in situ. Notice fronts and frontal instabilities (Flament et al., 1985).
Criterion: large T over x = 5 km (Castelao et al., 2006).
eddies T(z) 00 01 01 000000 111111 N E 0000000 1111111 0000000 1111111 000000 111111 000000 111111 01 undercurrent fronts upwelling00 11 surface current 11 wind current EASTERN BOUNDARY CURRENT Computational Simulations: (Capet et al., 2006) steady τ, Q, F flat bottom & straight coast embedded in equilibrium EBC domain with x = 12 km interior domain: (750 km) 2 with x = 12 0.75 km grid sizes [spanning the submesoscale transition] ROMS code (Shchepetkin & McWilliams, 2005)
KE (2m) [m 3 /s 2 ] 10 2 10 0 10 2 10 4 10 5 10 4 10 3 k [rad/m] Horizontal velocity u spectrum at the surface with x = 12 0.75 km grid sizes: convergent shape k 2 in submesoscale range (dashed; cf., Stammer, 1997) currents are mostly geostrophic not enstrophy cascade of geostrophic turbulence, k 3 (dot-dashed) not kinetic energy inertial-range cascade, k 5/3 (dotted)
20 600 19 500 400 18 17 16 y [km] 300 15 14 200 100 13 12 11 0 600 400 200 0 x [km] 10 Simulated SST [C] with mesoscale eddies and submesoscale fronts and frontal instabilities.
2 600 1.5 500 y [km] 400 300 1 0.5 200 0 100 0.5 0 600 400 200 0 x [km] 1 Simulated vertical vorticity normalized by f: submesoscale is most active between mesoscale eddies.
600 500 0.05 0 y [km] 400 300 0.05 0.1 200 0.15 100 0.2 0 600 400 200 0 x [km] 0.25 Simulated sea-surface height [m]: submesoscale is subtle finestructure.
Submesoscale Restratification Flux. x = 0.75 km x = 6 km Equilibrium buoyancy balance, b = g(αt βs): t b = (u b) z (wb) + z (κ z b). 0 mesoscale boundary-layer. submeoscale [τ, Q, F] Submesoscale vertical restratification flux is O(100) W m 2, largely balanced by boundary-layer turbulent flux.
5 385 24.2 385 380 24.1 380 y [km] 375 24 y [km] 375 0 370 23.9 370 365 365 360 490 485 480 475 470 465 x [km] 23.8 360 490 485 480 475 470 465 x [km] 5 Horizontal pattern for two adjacent fronts: b (left) & w (right). One is evidently stable and the other actively unstable.
PROCESS CLIMATE FORCING BAROCLINIC & BAROTROPIC INSTABILITY SUBMESOSCALE INSTABILITY FRONTOGENESIS ISOTROPIZATION (Kelvin Helmholz) DISSIPATION LARGE SCALE MESOSCALE SUBMESOSCALE MICROSCALE SCALE The submeoscale range connects the mesoscale to the microscale by downscale energy transfers, both potential and kinetic. It is an inherently different dynamical regime neither mesoscale (Ro = V/fL 1) nor microscale (Ro 1). Its primary effects are restratification, interior boundary-layer material exchange (ecosystems), frontogenesis, frontal instabilty & breakdown, partial geostrophic imbalance, and forward energy cascade toward dissipation.
Submesoscale Altimetric Measurements There are few measurements in situ because of difficult sampling requirements. Computational simulations are new but will proliferate. [OSSEs are possible.] Principle altimetric objective is documenting global variety, especially in regions with small SST where surface fronts are obscure and with differing mesoscale activity levels. Wide-swath sampling limit (Fu): δη = 1 cm over x = 1 km (cf., 2 cm over 7 km now) partial coverage of submesoscale range down to 4 km radian scale based on k 2 u g spectrum, with sparse temporal sampling. Issue of confusion with internal tide and other inertia-gravity waves. [Pattern discrimination is likely to work well.] Desirability of burst sampling at higher (x, t)-resolution, if feasible (Alsdorf).
. Summary Wide-swath altimetry is a great opportunity for exploratory measurements of submesoscale (geostrophic) currents that are probably important for their roles in oceanic general circulation and biological productivity......as well, of course, for its utility for mesoscale currents.