Upwelling/Downwellingand

Related Processes

The 2nd Last Class

Global Primary Production (i.e. New Growth (g of Carbon / year))

Fall

Spring

Why is the growth not the same on both sides of the continents? Compare the coast of Peru with that of Northern Brazil, and Namibia with the coast near Madagascar.

What drives primary production? Many things, but usually a combination of increased nutrient and/or light input. Light is primarily a function of season, but what would increase nutrients?

Some coastally intensified increases in nutrients are due to rivers. Notice the increased productivity in the inland water ways where the Fraser and Skaget rivers enter the ocean. Also the Columbia river at the bottom of the map.

This isn’t the only factor.

Fraser

Skaget

P.S. What is Ocean Color?

Primarily satellite derived from multiple spectral band amplitudes in the visible wavelength band. It is best correlated with coccolithopore concentration (i.e. phytoplankton).

CongoAmazon

YangtzeMississippi

Some major river systems show increased productivity near their deltas, but not all.

From where else can nutrients come?

What does a typical cross-shelf profile look like?

There are lots of nutrients below the surface mixed layer.

If we can bring them to the surface (euphotic zone), where there is enough light, then phytoplankton can consume them and we will see the result in the satellite pictures.

Nitrate (purple means close to zero)

Monterrey

More Examples

Primary productivity, during the week of this snapshot, in the Adriatic is intensified along one coast, but not the other.

Not just in primary production. Look at the west side of Lake Michigan. It is 10 degrees colder than the rest of the lake. Does this give us a clue as to the mechanism?

How to bring deep water to the surface?

1) Mixing

Stir up the upper ocean

2) Upwelling

Physically lift the deep water surface to a higher level

Less denseNutrient depleted

Less denseNutrient depleted

More denseNutrient rich

More denseNutrient rich

Both methods require the input of energy to change the system (either from the wind or the tides).

Upwelling

Upwelling is the upward movement of water. Since deep water is colder and more nutrient rich, it is often linked to increased primary productivity and cooling of surface waters.

Upwelling is usually driven by the wind.

How? Remember Ekman transport.

If the wind blows along the coast so that the Ekman transport is offshore (like in the diagram) then we are losing water near the coast. In a steady state, water must come from somewhere to replace the water blown offshore. It comes from below via upwelling.

light water

dense water

No Wind Turn on Wind

wind

Consider a two-layer fluid

EkmanFlow awayFrom coast

surface

Water near coast goes down

Return flow underneath

Dense water is closer to the surface

Upwelling is the broad explanation, but doesn’t explain all the details. Look at the incredibly complicated structure of primary productivity, in this snapshot, off of California

Upwelling depends upon the wind, which fluctuates.

In some regions the wind is very steady (Trade winds) and blows along the coast in an upwelling favorable manner (coast of Peru).

If the wind fluctuates, how long does it take for the upwelling to start or respond?

An inertial period, with a near-steady state in a few days

See how Lake Michigan responds to synoptic weather variability with time scales of a few days to a week.

Length Scales? How far from the coast do you see upwelling?

An internal Rossby radius. (around 30km)

How deep does upwelled water come from?

Usually from the water that is just below the Ekman layer.Typically from 50 to 200m deep.

Higher sea level means more pressure offshore. This tends to drive a return flow in the lower layers (or the bottom boundary layer).

What if the winds blow the other way? Downwelling

light water

dense water

No Wind Turn on Wind

wind

Downwelling:Consider a two-layer fluid

Ekmanflow towardscoast

surface

Water near coast goes up

Offshore flow underneath

Dense water is farther from the surface

What about currents?

Sloping density surfaces should have associated geostrophic currents. Eastern boundary upwelling regions have equatorward currents (e.g. Peru-Chile current, California current) Sometimes there exist deep countercurrents (this is more complicated)

Schematic representations of the flow off of the Oregon coast during upwelling (left) and downwelling (right).

Associated currents are stronger near the surface and flow in the same direction as the wind.

Upwelling Indices (in m3/s per 100m of coastline) assuming a 30 km wide upwelling zone than 100 m3/s per 100m of coastline gives w = 3m/day

http://www.pfeg.noaa.gov/products/PFEL/modeled/indices/upwelling/NA/click_map.html

We don’t need a coast

A gradient (really a divergence or convergence) in the Ekman transport will result in water piling up or being drawn down. In the open ocean, on short time scales this is unimportant (except maybe for hurricanes). However, persistent winds can have an effect. Consider the equator:

Trade Winds blow westward and slightly equatorward.

Ekman transport is poleward on both sides of the equator.

Equatorial sea level is depressed and upwelling occurs.

(a) and (c)

(b) And (d)

What about a hurricane (or any other weather system)?

Winds swirl in a rough circle.

Ekman transport is either into, or out of, the center of the system.

Water adjusts and upwelling or downwelling occurs.

This is complicated if the system is fast moving. Recall that upwelling can take a few days to properly adjust.

Annual Average Global Primary Productivity (from MODIS)

The subtropical oceans (e.g. Sargasso Sea) are dead zones, but the mid-latitude oceans are mildly productive even away from the coast. Why? Recall the global wind fields.

Note the equatorial upwelling, which is intensified on the Eastern side of the basin.

So in the subtropics the anticyclonic winds produce a surface convergence and downwelling. In the subpolar gyre, the winds produce upwelling. Not a lot (50m/year), but enough to make a difference to the phytoplankton.

Other Methods of Increasing Primary Productivity:

1) Hurricanes (or storms in general)

Strong winds will mix up the upper ocean.

Results in cold and nutrient rich surface waters.

SST Chlorophyll-a

In the top panels we see the warm Gulf of Mexico before Hurricane Ivan.

The middle panels shows the cooling associated with the passage of the storm and the mixing up of deep water.

Chlorophyll increases greatly over the next 3-4 days in the places where there was intense mixing.

Walker et al., GRL, 2005

Annual Average Global Primary Productivity (from MODIS)

Mid-latitude storm tracks are visible in this picture:

The storms have to deepen the mixed layer, in order to bring up new nutrients.

Wintertime storms are not very effective in creating growth because there is not enough light.

Other Methods of Increasing Primary Productivity:

2 ) Rossby Waves.

The propagating disturbances cause the thermocline to heave up and down.

If the thermocline is lifted to a level where there is sufficient sun light then nutrients trapped below the thermocline are available to phytoplankton.

Patterns that move like Rossby waves have been seen in Ocean Color measurements. However, some scientists debate the meaning of the signals:

Are we seeing more growth, or could it be some form of concentration of the existing phytoplankton?

The Island – Mass Effect

The Marquesa Island group in the central Pacific.

1) Average biomass over several years.

2) What happens when strong winds blow by the islands.

A Partial Explanation

Observations from an island in the Kuroshio.

Flow separation around an island can cause a recirculation zone in the lee, in which water upwells.

The top figure shows vorticity around the island.

The middle figure shows inferred vertical velocity. Note the orange and red colored upwelling in the lee of the island.

The pink dots show measured chlorophyll concentration at the surface. Notice the small dots in front of the island, and the big circles behind in the wake.

current

Sea Ice Driven Upwelling

Ekman transport is due to the wind pushing on the surface of the water in a rotating reference frame.

If there is sea ice covering the ocean that transfer is changed.

At the ice edge, the Ekman transport in the open water will not match the Ekman transport under the ice (which is very small). Consequently, upwelling or downwelling can occur.

Ekman Transport

Wind

downwelling

Review

1) Upwelling/downwelling is driven by alongshore winds.

2) The Ekman transport moves water away from the shore (in upwelling), or towards the shore (in downwelling).

3) The water underneath the Ekman layer responds by trying to compensate for the movement of the surface layers.

4) Upwelling/downwelling systems generate along-shore currents which move in the same direction as the wind.

5) Upwelling (downwelling) can occur in the deep ocean if the Ekman transport is divergent (convergent). Water upwells at the equator and in the subpolar gyres, but downwells in the subtropical gyres. Open ocean upwelling is weaker than coastal upwelling