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

Aquatic Sciences

John Middleton

South Australian Research and Development Institute, Aquatic

Sciences,

S.A., Australia

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

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/ ρ which is the viscous “limit” of upwelling. All upwelling occurs through the BBL and interior upwelling is shut-down (Allen et al 1995).

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)

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?

What can generate CTWs?

Column is displaced into deeper water – acquires cyclonic vorticity

Onshore Ekman transport causes return interior transport

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

Eg., Cape Leeuwin acts as a G.O. for zonal winds south of Australia – the

Ekman transport here is not blocked by a coast.

An idealised soln for Australia’s southern Shelf

Cape Leeuwin

At y=0 V=0

Southern Ocean

Steady wind stress

Southern Australia Coast y

x

Ekman transport

2D upwelling U=0CTW to y = c t

V

Frictional solns for V and U (nth CTW mode)

L1 ~ 600km

c1 ~ 3m/s

Sub-Ekman layer

transport

Alongshelf distance

• 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 ?

Application to Upwelling off Chile: a numerical study

Mean summer

Winds strong ~0.1 Pa

Mean winds vanish at 26oS – the geographical origin

Mean Summer Wind Stress

Coastal Sea Level

shows CTW

propagation from 26o

S

Soln grows in time and is

independent of y – 2D upwelling

CTW y = c1tCTW has arrived, solns

become indept of

time –shut down of

upwelling has

occurred.

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

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.

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

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

10 cm

1 cm

CTWs – perturbations in cross-shelf flow lead to vortex stretching/squashing and a material line will

propagate as shown.