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Physical processes of wind-forced upwelling: time and space scales

Aquatic Sciences. Physical processes of wind-forced upwelling: time and space scales. John Middleton South Australian Research and Development Institute, Aquatic Sciences, S.A., Australia.

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Physical processes of wind-forced upwelling: time and space scales

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  1. Aquatic Sciences Physical processes of wind-forced upwelling: time and space scales John Middleton South Australian Research and Development Institute, Aquatic Sciences, S.A., Australia

  2. 2D Ekman UpwellingConsider the numerical solns for 2D upwelling in a stratified ocean driven by a constant wind stress (0.1 Pa) • The density field is shown at time: • …. Day 6 • ___ Day 10 • - - Day 30 • After day 10, the interior upwelling becomes shut-down and upwelling occurs thru the BBL UE

  3. Alongshore Dynamical Balance accel = wind stress - bottom friction Vt = (ζ – rV/h)/ρ U= 0 = UE + Uu with a spin-up time scale T=h(x)/r - larger in deeper water. For r=CDv* =2.5X10-4 and h=100m, T=5 days. At very large times, V ζ T/ ρwhich is the viscous “limit” of upwelling. All upwelling occurs through the BBL and interior upwelling is shut-down (Allen et al 1995).

  4. Implications • Alongshore currents can be very large (60cm/s for 0.1Pa wind) • Cross-shelf divergence of Ub can lead to downwelling at shelf break and a two-cell circulation (Allen et al 1995; Mooers et al 1976) • Increased BBL upwelling will act to shut vertical mixing down at top of BBL – reduced re-suspension of benthic nutrients. • Anomalously cold BBL upwelling reduces benthic organism movement (eg Lobster)

  5. 3D wind-forced upwelling • We now consider upwelling by a steady wind, but over a semi-infinite shelf. • The “start” of the shelf acts as a “geographical origin” for generation of Coastal Trapped Wave (CTW) that are important to 3D set-up of upwelling. • CTWs?, geographical origin?

  6. Onshore Ekman transport causes return interior transport Column is displaced into deeper water – acquires cyclonic vorticity What can generate CTWs? Diagram illustrates how winds can drive CTWs through the interior return of the Ekman transport. When wind or coast vanishes, no interior return flow – a geographical origin Australian Coast

  7. Eg., Cape Leeuwin acts as a G.O. for zonal winds south of Australia – the Ekman transport here is not blocked by a coast.

  8. An idealised soln for Australia’s southern Shelf Cape Leeuwin At y=0 V=0 Southern Australia Coast y V x CTW to y = c t 2D upwelling U=0 Steady wind stress Ekman transport Southern Ocean

  9. Frictional solns for V and U (nth CTW mode) L1 ~ 600km c1 ~ 3m/s Alongshelf distance Sub-Ekman layer transport

  10. Application to Upwelling off Chile: a numerical study • The Gulf of Arauco can account for 4% world’s fish landings. • Summertime upwelling mean winds • Few studies • Wind forced upwelling t<10d • Cyclonic Meso & Headland eddy advection • Bio Bio Canyon ?

  11. Mean Summer Wind Stress Mean summer Winds strong ~0.1 Pa Mean winds vanish at 26oS – the geographical origin

  12. Coastal Sea Level shows CTW propagation from 26o S CTW has arrived, solns become indept of time –shut down of upwelling has occurred. CTW y = c1t Soln grows in time and is independent of y – 2D upwelling

  13. Upwelling off Punta Lavapie: rate of wind-forced upwelling drops markedly after CTW arrives (about day 4)

  14. Implications: • Upwelling is not a 2D process, the frictional length and time scales are important to determining degree of shut-down and degree of interior vs BBL upwelling. • Geographical origin must be allowed for in models to get correct degree of upwelling • Use of periodic and other ad-hoc b.c.’s will not necessarily allow for this • Correct degree of upwelling important for x-shelf exchange (downwelling as well) • Correct degree of upwelling needed since this provides source of P.E. for B.I. for filaments/jets/eddies.

  15. CTW scattering and 2D upwelling – the Bonney Coast Upwelling off the Bonney Coast is deep (300m) and has a most significant SST signal. Numerical studies suggest that the alongshore gradients of sea level are small and that the upwelling is 2D in character

  16. 10 cm The results show that sea level signal (and velocity) generated using a mode 1 CTW paddle is largely dissipated at the Bonney Coast. It is possible that the CTWs generated at Cape Leeuwin are largely scattered by the islands, peninsulas and gulfs and so are unable to shut-down the upwelling off the Bonney Coast. –2D upwelling and viscous limit 1 cm

  17. CTWs – perturbations in cross-shelf flow lead to vortex stretching/squashing and a material line will propagate as shown.

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