Title: UpwellingDownwelling
1Upwelling/Downwelling and Related Processes
The 2nd Last Class
2Global Primary Production (i.e. New Growth (g of
Carbon / year))
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.
Fall
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?
Spring
3Some 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 isnt 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).
4Some major river systems show increased
productivity near their deltas, but not all.
Yangtze
Mississippi
Amazon
Congo
From where else can nutrients come?
5What 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.
Monterrey
Nitrate (purple means close to zero)
6More Examples
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?
Primary productivity, during the week of this
snapshot, in the Adriatic is intensified along
one coast, but not the other.
7How to bring deep water to the surface?
2) Upwelling Physically lift the deep water
surface to a higher level
- Mixing
- Stir up the upper ocean
Less dense Nutrient depleted
Less dense Nutrient depleted
More dense Nutrient rich
More dense Nutrient rich
Both methods require the input of energy to
change the system (either from the wind or the
tides).
8Upwelling
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.
9Consider a two-layer fluid
No Wind Turn on Wind
Water near coast goes down
wind
surface
Ekman Flow away From coast
Dense water is closer to the surface
light water
dense water
Return flow underneath
10Upwelling is the broad explanation, but doesnt
explain all the details. Look at the incredibly
complicated structure of primary productivity, in
this snapshot, off of California
11Upwelling 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)
12How 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
13Downwelling Consider a two-layer fluid
No Wind Turn on Wind
Water near coast goes up
wind
surface
Ekman flow towards coast
Dense water is farther from the surface
light water
dense water
Offshore flow underneath
14What 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)
15Schematic 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.
16Upwelling 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/ind
ices/upwelling/NA/click_map.html
17We dont 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.
18What 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.
(a) and (c)
(b) And (d)
19Annual 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.
20So 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.
21Other Methods of Increasing Primary Productivity
Chlorophyll-a
SST
- Hurricanes (or storms in general)
- Strong winds will mix up the upper ocean.
- Results in cold and nutrient rich surface waters.
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
22Annual 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.
23Other 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?
24The 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.
25A 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
26Sea 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.
Wind
Ekman Transport
downwelling
27Review
- 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