1. Wind-driven currents and upwelling
On timescales longer than a few days:
Earth’s rotation introduces Coriolis force
flow turns to the right (northern hemisphere) or left (southern hemisphere)
wind stress balances Coriolis force = Ekman transport Oceanographer’s rule: Ekman transport is toward the right of the wind stress (in northern hemisphere)
Adjacent to a coast…
Alongshore wind produces Ekman transport across-shore …
causes upwelling or downwelling of a few meters per day

6. Wind speed m s-1
Wind speed and along-shelf currents at various depths along the continental shelf off northwest Africa

7. m2 s-1 uE Northwest Africa values: Typical τ = 0.1 Pa
Depth (z)
Northwest Africa values:
The Ekman transport can be thought of as some average velocity acting over a layer.
At lower latitudes the Ekman transport becomes larger, yet the Ekman depth also becomes larger.
What about the average Ekman current?
Ue becomes larger at LOW latitudes because its inversely proportional to the square root of f
Typical τ = 0.1 Pa
f = 5x10-5 at 20N
typical DE ~ 30 m
uE ~ 0.1 / (1027 * 5x10-5 ) / 30 = 6.5 cm s-1

8. Strong wind toward south
Weak or no wind
Strong wind toward south
Weak or no wind

9. typical τ = 0.1 Pa 50/8 = 6.25 km day-1 f = 5x10-5 at 20N, DE ~ 30 m
Why do we care about the speed: Consider upwelling events – the speed at which water moves offshore will be related to the Ekman current.
Once transported away from the upwelling region, the supply of nutrients ceases. So the primary production that results will have to be sustained with the transient input of nutrient, not from a continuous source.
This has consequences for successions of trophic levels, nutrient recycling, and export.
typical τ = 0.1 Pa
f = 5x10-5 at 20N, DE ~ 30 m
50/8 = 6.25 km day-1
= 5.6 km/day

10. Alongshore flow shaded into page i.e. poleward
Notice that there is a strong ALONGSHELF current. Its magnitude is larger than the across-shelf Ekman flow, and it extends over a much greater vertical range.
Across-shore flow shaded to left (offshore)
6 cm/s
Observational data supporting the 2-dimensional pattern of upwelling.
Contrast weak and strong winds.
Notice that there is a strong ALONGSHELF current. It’s magnitude is larger than the across-shelf Ekman flow, and it extends over a much greater vertical range.
Density (in sigma units = rho –1000 kg m-3)

11. Along-shelf velocity (positive toward north) depth (m)
Temperature
wind (m/s)
Vertical profiles of temperature and along-shelf velocity, through time, in the California Current system.
Winds toward the north cause warming. Toward the south cause cooling.
Along-shelf velocity reverses with the wind.
How can the influence of the wind extend so deep, and drive an ALONGSHELF current, when we have previously argued that because of the earth’s rotation, wind stress is balanced by the Coriolis force associated with ACROSS-SHELF currents, and that the momentum from the wind is mixed into a relatively shallow Ekman layer of depth 20 m.
Along-shelf velocity (positive toward north)
depth (m)

12. It does NOT depend on the magnitude of the wind.
The width of the upwelling front is set by the “Rossby radius” which depends on depth, stratification, and Coriolis parameter (latitude).
It does NOT depend on the magnitude of the wind.
Upwelling changes density which creates pressure forces – these become balanced by … Coriolis … and create a “COASTAL JET”
Upwelling in a density stratified ocean leads to horizontal variations in the water pressure in the across-shelf direction.
This pressure force has to balanced by an equal and opposite force, or the ocean currents will accelerate. What typically happens in sustained upwelling regimes is that a steady balance is established between the across-shelf pressure force and an across-shelf Coriolis force. An across-shelf Coriolis force implies ALONG-SHELF current, because Coriolis force always acts toward the right of the current direction.
This alongshelf current is termed the COASTAL JET and is strongest where the pressure force is strongest: the upwelling front of steepest isopycnal slopes.

13. Consider first an ocean with a sloping sea surface (so never mind the density stratification for the moment).
The pressure at some depth z will be greater at B than A because the water column is taller and weights more.
Never mind density for now … but consider pressure gradients due to a sloping sea surface

14. Pressure force balancing Coriolis = Geostrophic currents
difference in pressure over distance Δx between A and B is Δp = ρ g Δz
horizontal pressure gradient force per unit mass (i.e. divide by density ρ) is where tanθ is the surface slope
Coriolis force per unit mass (i.e. acceleration) = fv is the geostrophic velocity
A giant beach ball on the ocean surface would roll downhill accelerating at this rate
The pressure force depends on the slope of the sea surface.
For the Coriolis force to balance this, the CURRENT must be INTO the page, NOT in the direction of the pressure force.

15. 
Vertical cross-section view
Plan view

16. 
Vertical cross-section view
Plan view

17. Barotropic and baroclinic conditions
Ocean pressure depends on the weight of the overlying water
If the water is relatively light (less dense), then pressure increases less rapidly with depth
A tall column of less dense water can weigh the same as a shorter column of more dense water
The pressure force due to a sloping sea surface can be cancelled out by density stratification that varies horizontally (such as in an upwelling region)
Now consider how the pressure force might alter is we include a density stratified ocean.

18. Oceanographer’s rule: “Light on the right”
heavy light
Blue shade shows water density Arrows show flow direction and strength.
(a) Density stratification is the same at A and B, pressure increases at same rate and pressure gradient remains constant. Geostrophic velocity is the same at all depths
(b) Density stratification is stronger at B than A. Isopycnal surfaces are inclined to pressure surfaces. Average density (weight) of column A is more than column B. As the isobaric surfaces become more horizontal, the geostrophic velocity becomes less.

19. Density (in sigma units = rho –1000 kg m-3)
time
Alongshore flow shaded into page i.e. poleward
Light on the right.
Below about 100 m depth the slope of the isopycnals changes and the direction of the alongshelf flow reverses.

20. so geostrophic coastal jet is out of the page
Upwelling favorable wind is out of the page
balanced by Coriolis force
sea level slope causes pressure force
sea level
Ekman
left has more water than right but on average is less dense so that the difference in pressure is the same
density contours show the upwelling
offshore
coast
constant pressure surface at depth
vertical
Width of the coastal jet?

21. The Rossby radius sets the scale of the width of an upwelling front
Surface gravity waves travel at speed: h is the water depth g is gravity (9.81 m s-2)
On the interface between two density layers, this speed decreases by a factor sqrt(Δρ/ρ)
Typical values in an upwelling region would be g’ = gΔρ/ρ = 10 x 0.5 / 1026 = ms-2 c’ = 1 m s-1 if the upper layer is about h = 200 m
What sets the width scale of the upwelling region?
The Rossby radius is a fundamental scale that appears all the time when considering length scales of large scale ocean motion.
In the atmosphere, the Rossby radius is much larger because the density variations are so much less. The Rossby radius sets the scale of geophysical turbulent motions (eddies, cyclones, meanders in the jet stream, meanders in the Gulf Stream etc)

22. The Rossby radius scale of the width of an upwelling front
The time scale on which the Coriolis force acts = 1/f seconds In this time water traveling at speed c’ would travel a distance: R = c’/f referred to as the Rossby radius
c' = 1 ms-1 f = 5 x 10-5 s-1 (at 25oN) R = 20 km
the “Coastal Jet”
R = 20 km
0 m
200 m
400 m

23. Water temperatures are colder nearer to shore than offshore for most of the California coastline north of Pt. Conception (the bend in the coastline at 34.5 N). But the strength of the upwelling, as evidenced by how far the cold water extends offshore, varies along the coastline. Pronounced filaments, where upwelled water is pushed hundreds of km offshore, are often associated with coastal promontories.
Enhanced upwelling at Capes

24. Phytoplankton bloom over Mauritanian coast Oct 2001 from SeaWiFS
Cap Blanc
Phytoplankton bloom over Mauritanian coast Oct 2001 from SeaWiFS

25. The Canary Current System Northwest Africa
across-shelf velocity
The Canary Current System Northwest Africa
During strong winds there is a two layer circulation
onshore flow occurs at about 100 m depth on the shelf
isopycnals slope steeply upward
Equatorward winds drive upwelling of cold nutrient rich waters
density

26. The Canary Current System Northwest Africa
along-shelf velocity
The Canary Current System Northwest Africa
During strong winds get an equatorward coastal jet of about 30 cm s-1
Subsurface poleward undercurrent
density

27. High nitrate concentrations appear at the coast first, and then travel offshore
During low winds surface nitrate is around 2 mg/m^3.
After onset of strong winds concentrations increase to around 5-10 mg/m^3
A few days later near-shore values have fallen to below 5 mg/m^3, and mid-shelf values are still elevated.
During low winds surface nitrate is around 2 mg m-3.
After onset of strong winds concentrations increase to around 5-10 mg m-3
A few days later near-shore values have fallen to below 5 mg m-3, and mid-shelf values are still elevated.

28. The Canary Current System Northwest Africa
Cold water first appears close to shore, then moves offshore
There is a series of these events in response to varying winds
12 days
Speed: 50 km in 12 days = 4.2 km/day 50 x 103 / (12 * 86400) = 5 cm s-1

29. Productivity and upwelling events in the Canary Current system
High productivity results from the alternation of upwelling events and relatively calm periods
Upwelling brings nutrients to the surface
Calm wind periods allow the water column to stratify (mixed layer < critical depth)
Phytoplankton grow while held in the shallow mixed layer …there is a miniature spring bloom during each calm period
Evidence for this comes from studies of assimilation number… the amount of carbon fixed per unit time per unit of chlorophyll-a
high during calm periods
low during active upwelling (Huntsman and Barber, 1977; Deep Sea Res, v24, pp 25-33)
When phytoplankton are growing rapidly under the stimulation of high nutrient availability the assimilation number is high.

30. Nutrient recycling in upwelling systems
About 40% of primary production in the NW Africa / Canary Current system appears to be sustained by recycled ammonia released from bottom sediments
2-layer upwelling circulation acts as a nutrient trap
Nitrogen taken up by phytoplankton:
consumed by zooplankton … fecal pellets to seafloor
phytoplankton mortality … sink to seafloor
bacterial regeneration in seafloor sediments (and water column)
Regenerated nitrogen is carried into the upwelling circulation by the shore-ward lower layer currents

31. Neil Banas NPZ visualizer

32. Peruvian Upwelling System
Continental shelf is narrower than NW Africa
20 km rather than 50 km
Deep water has higher nutrient concentrations
20-25 mg m-3 vs 5-10 mg m-3 NO3
Wind stress is less on average, and more steady in time
0.08 Pa vs 0.15 Pa

33. Peruvian Upwelling System
Weaker winds …
Shallower 2-d upwelling circulation
offshore transport is in top ~20 m
onshore transport at intermediate depths m (not whole water column)
bottom waters are relatively still, favors accumulation of organic matter and reducing sediments
Shallower mixed layers with less vertical mixing …
MLD < critical depth
Primary production maintained at a relatively constant level
Steady upwelling favors formation of a distinct plume of cold water and elevated primary production extending well offshore

34. California Current Upwelling System
Average summer sea surface temperature anomaly with respect to latitude average
Cape Blanco, Cap Blanc ?
Upwelling is most intense from Cape Mendocino to Point Conception

35. Bakun’s Upwelling Index
Used average wind stress parallel to the coast to compute Ekman transport in m3 s-1 per 100 m of coastline
Seasonal variability, with summer maximum upwelling in the north
More year round upwelling to the south

36. Benguela Current System

37. 1-4 day lag occurs between nutrient input and phytoplankton bloom
As in the Canary Current system, optimal biological production occurs during an alternation of upwelling winds and quiescent conditions when the water column can re-stratify.
1-4 day lag occurs between nutrient input and phytoplankton bloom

38. so geostrophic coastal jet is out of the page
Upwelling favorable wind is out of the page
balanced by Coriolis force
sea level slope causes pressure force
sea level
Ekman
left has more water than right but on average is less dense so that the difference in pressure is the same
density contours show the upwelling
offshore
coast
constant pressure surface at depth
vertical
Baroclinic Rossby radius