Transcript Water Properties - College of Sciences

General Ocean
Circulation
75% of the Earth’s surface
Couples atmospheric processes with
tectonic processes
Important in regulating atmospheric
CO2
Important in global heat transport
Effect of differential heating and
cooling of the earth’s surface
• Temperature and salinity (density)
gradients (thermohaline circulation)
• Atmospheric circulation, wind cells and
surface ocean currents (drift currents)
Oceans
• 75% of Earth’s surface
• Important for heat transport
• Cycling in the ocean important for elements
– Couples shorter atm-ocean cycles with longer
tectonic cycles
– Ultimately involve burial in marine sediments
• Regulation of atm CO2
Ocean layers
• Thin surface mixed layer (~50 – 100 m) box
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Sunlight penetrates
Net primary prod/ps drives biogeochem cycles
OM produced here sinks out and is remineralized below
Important for surface heat transport
• Large –dark, cold and deep water box
– Most of the ocean volume (~90%)
– Isolated from surface for long periods of time (~500 –
1000 yr) = average mixing time for bottom waters
– Important for understanding oceanic CO2 uptake
• Processes within and between boxes
• Ocean zones defined by density differences
Surface currents
• Move large volumes of water at basinscale
• Transport heat
• Work simultaneously with thermohaline
circulation
– Some independence
– Some interaction (complex 3D circulation
Wind driven circulation
• About 10% of the water is moved by
surface currents
• Surface currents are primarily driven by
the wind and wind friction
• Move fast relative to thermohaline
circulation (1 to 2 m/s)
• Most water moved is above the pycnocline
• Reflect global wind patterns and Coriolis
effect!
Surface circulation
• But, surface flow is NOT parallel to wind
• Why?
– Coriolis force
– Ekman flow
Coriolis Effect
• Earth rotates on its axis 1x/day (CCW with N at
top)
• Radius of earth = 6371 km
• Circumference of earth = 2pr = ~40,000 km
• Speed of object at equator is ~40,000 km/d
(~1668 km/h)
• At 60o, radius is smaller by factor of 2 (cos 60 =
0.5)
• So speed is half that at equator (834 km/h)
• At 30o speed is 1442 km/h (cos 30 = 0.866)
Coriolis force
• Apparent
deflection of
things moving
long distances
due to rotation of
the earth
Variations in rotational speed.
Coriolis force
Consequences of Coriolis deflection.
• Apparent
deflection of
things moving
long distances
due to rotation of
the earth
Relative speeds of objects at different
radii moving at the same angular speed
• Inertia keeps
objects moving
at their original
speed
• We could see
this if viewed
from space but
we’re moving
as well
• Has an effect
when things
move between
latitudes
Result for wind cells is you
go from 2 cell model
to 6-cell model
Surface Ocean Circulation
• Nansen first connected wind with currents
• Showed his measurements to Ekman who
formulated a mathematical explanation of
surface currents
Ekman spiral
• Wind flows over surface and creates drag on
water
• Wind driven flow is deflected to right in N
hemisphere by Coriolis effect
• Water flows at only about 3% of the speed of the
driving wind.
• Current flows at 45o to the right of the wind
direction in the northern hemisphere
• But, only the surface feels the wind
• Each layer down only feels the layer above so is
deflected based on the layer above
• Each layer down moves more slowly than the
layer above
•Wind creates a drag on surface waters and successive layers
exert drag on each successive layer below.
•Each layer is subject to Coriolis deflection
Ekman flow
• Wind exerts frictional drag on water causing a
thin layer of water to move
– Transfer of momentum is not efficient; induced current
is about 2% of wind speed
– Coriolis force causes water to veer right or left of wind
• As the surface layer of water begins to move, it
exerts frictional drag on the layer below
• And so on, each layer moving slower and
deflected relative to the layer above
• Produces a pattern of decreased speed with
depth and increased angle between flow and
wind direction with depth (Ekman spiral)
Ekman spiral
• Velocity vectors at different depths trace out a
spiral around a line perpendicular to the surface
– Steady wind induces flow at depth at 90o and 180o or
more to wind direction
• We say the wind “penetrates” to a depth where
flow is 180o to the wind (flow at this depth is
about ~4% of flow at surface)
• Water above this depth is the Ekman layer
• Wind speed, water viscosity and the Coriolis
effect all affect the depth of wind penetration
• Winds penetrate deeper at low latitudes except
right at the equator
Flow in Ekman layer
• Surface current typically 20-40o
to wind direction
• By definition, current at base of
Ekman layer is 180o to wind
direction
• Average or net flow of water in
Ekman layer is 90o to wind
• Average or net flow in Ekman
layer is the drift current
Wind direction
Surface current
direction
Direction of net
Transport within the
Ekman layer
Ekman flow
• Water doesn’t really spiral downward
• At some depth water flow will be opposite surface
flow and at this depth friction dissipates horizontal
flow
• Effects of surface wind felt to approximately 100m
• The net motion of the water movement, after the
sum of the effects of the Ekman spiral is the Ekman
transport or flow
• In theory, Ekman transport is 90o to the right of the
wind in the N hemisphere
• In nature, it barely reaches 45o because of the
interaction between the Coriolis effect and pressure
gradient
Surface currents - wind
• First order control by predominant wind pattern –
friction between atm and surface ocean
• More complex in the real world
– Position of continents
• Ekman transport
– Friction plus Coriolis
– Pushes water to center of gyres
– Regions of convergence and divergence
• Geostrophic flow – interaction between pressure
gradient associated with Ekman
transport/convergence and Coriolis effect
• Broad general subtropical gyres
Fig. 5-1
Surface currents
• Moving water “piles up” in the direction the wind
is blowing
• Continents and land masses also deflect flow in
E-W direction
• Water pressure increases where its piled up so
tries to slide back along a pressure gradient
• Coriolis effect intervenes deflecting currents to
the right of wind direction (in N hemisphere)
Ocean gyres
• Circular flow around the periphery of an ocean
basin
• This flow is often broken down into
interconnected currents (e.g., North Atlantic
gyre)
• Why doesn’t flow spiral toward center because
of Coriolis force?
Pressure gradients develop in the ocean
because the sea surface is warped into
broad mounds and depressions with a
relief of about one meter.
• Mounds on the ocean’s surface are caused by
converging currents, places where water
sinks.
• Depressions on the ocean;s surface are
caused by diverging currents, places from
where water rises.
• Water flowing down pressure gradients on the
ocean’s irregular surface are deflected by the
Coriolis effect. The amount of deflection is a
function of latitude and current speed.
Downwelling of water
Creation of geostrophic currents as
a result of the pressure gradient
Fig. 5-4
Upwelling of deep water to replace
surface water in areas of divergence
- e.g., along the equator
In the center of gyres
water piles up (converges)
upper ~100 m
Fig. 5-3 (a) Ekman spiral
Fig. 5-3 (b) Ekman transport
Water piles up in the direction of flow so piles up in middle of
gyres due to Ekman transport and creates a pressure gradient in
the opposite direction.
Pressure gradient
Pressure gradient
Ocean gyres
• Circular flow around the periphery of an ocean
basin
n
Westerly-driven ocean currents in the trade winds, easterlydriven ocean currents in the Westerlies and deflection of the
ocean currents by the continents result in a circular current,
called a gyre.
• This flow is often broken down into
interconnected currents (e.g., North Atlantic
gyre)
Fig. 5-2
Gyre circulation
• To deflect further than 45o, water would have to
move uphill against a pressure gradient
• To deflect away from the pressure gradient
would defy the Coriolis effect
• So water circulates clockwise around the gyre
balanced between the pressure gradient in the
center of the gyre and the Coriolis deflection
- Coriolis deflection versus gravity
• Higher sea surface height at the center of gyres
and maintained by wind energy
Geostrophic gyres/flow
• Gyres in balance between pressure gradient and
Coriolis effect
• Their currents are geostrophic currents
• Because of wind patterns and positions of
continents, major gyres are largely independent
of each other in each hemisphere.
• Six great surface current circuits in the world,
one is technically not a geostrophic gyre
• The Antarctic circumpolar current (west wind drift) moves
eastward around Antarctica driven by westerly winds and
is never deflected by a continent
Sea surface height
Hill is offset to the western side of basins because of western intensification
Western intensification
• Earth turns CCW
• Water piles up against land on west side
of basins
• Less (or no) Coriolis force at equator so water
doesn’t turn until it hits and obstacle (land)
• More Coriolis at mid and high latitudes so
current turns sooner
Geostrophic Flow Around the North Atlantic Ocean
Sea surface height
Effect on gyres
• The geostrophic mound is deflected to the
western part of the ocean basin because of the
eastward rotation of the Earth on its axis.
• The Sargasso Sea is a large lens of warm water
encircled by the North Atlantic gyre and
separated from cold waters below and laterally
by a strong thermocline.
• Western boundary currents, such as the Gulf
Stream, form a meandering boundary separating
coastal waters from warmer waters in the gyre’s
center. Meanders can be cut off to form warmcore and cold-core rings.
• The current flow pattern in gyres is
asymmetrical with narrow, deep and swift
currents along the basin’s western edge and
broad, shallow slower currents along the
basin’s eastern edge.
Geostrophic Flow Around the North Atlantic Ocean
Western boundary currents
• Narrow, fast, deep currents at the western
margins of gyres
• Bring warm water poleward
• Gulf-stream is the largest
– 2 m/s (5mph), 450 m deep, ~70 km wide
• Large volume of water transported
– Expressed as a Sverdrup (1 Sv = 1 mil m3/s)
• Moves like a river (hose analogy) but moves a
lot more water
• Water within the current is warm, clear and blue
(not much by way of nutrients or life compared to
surrounding water)
Surface circulation of the N Atlantic with flow in Sverdrups
(1 Sv = 1 million cubic meters/s) – western boundary currents very important
for heat transport
Eastern boundary currents
• Wide, shallow, slow currents on the east
side of ocean basins (off the west coasts)
– 1000 km wide, 2 km/h (1.2 mph)
• Bring cold water equatorward
• Lack defined boundaries and lack eddies
Sea surface temperatures
Think about gyres and heat transport
Other notable currents
• Equatorial countercurrent – no Coriolis and not
much wind and so some water moves back east
– Pacific is wider so more pronounced
• Undercurrents – again, if water doesn’t turn, it
piles up, sinks as far as it can (density) and then
tries to return on a density surface
• High latitude currents – continental collisions
(tendency of water to flow around obstacles)
more important than Coriolis force at high N
latitudes; can also get polar easterly influence
• West wind drift or Antarctic circumpolar current –
unimpeded westerlies
Upwelling and downwelling
• Equatorial upwelling – water on either side of equator
moving westward is deflected slightly poleward and is
replaced by deeper water
• Some upwelling and downwelling induced by gyre
circulation
– Depression of thermocline on western side of basin
– Shallow current on eastern side of basin
• Along coastal areas Ekman transport can induce
downwelling or upwelling by driving water towards or
away from the coast, respectively.
– Wind blowing parallel to the shore or offshore
East versus west coast climates
Northern hemisphere coastal upwelling – eastern side of basin
• Surface
chlorophyll
• High from
nutrient-rich
deep water
upwelling
along CA
• Cold-water
upwelling
also chills
the air
Northern hemisphere coastal downwelling – eastern side of basin
Areas of upwelling – coastal upwelling has seasonality (as do winds)
Surface currents - patterns
• Similar in all basins
• At low latitudes, have large, “closed” gyres
– Gyres elongated in the E-W direction
– Gyres centered on the subtropics (~30oN or S)
• West-directed flow at N and S equatorial
currents
• East-directed flow ~ 45oN and S
• N-S directed flow at eastern and western
boundary currents
Surface currents - patterns
• West-directed flow driven by tradewinds (coming
from N or S-east
• East-moving equatorial countercurrents
• Western boundary currents are distinct, narrow
(< 100km), swift (>100 km/day) and deep (2 km)
• Eastern boundary currents are broad (>1000
km), weak (~10s km/day) and shallow (~500 m)
• Have smaller, less developed polar gyres in N
• Have circumpolar “gyre” in the S
Transverse currents
• E-W currents driven by the trade winds
(easterlies) and mid-latitude westerlies
• Link the boundary currents
• Equatorial currents
– Moderately shallow and broad
– Pile up water on west side of basin (W Atl is 12 cm
[8”] higher than Pac; W Pac is 1 m higher than E Pac)
• Eastward flowing currents at mid-latitudes are
weaker (wider and slower) than equatorial
currents
• Differences in land mass distribution in N and S
hemispheres affects flow
Fig. 8-9
Why is this important? Processes in surface, wind-driven layers
are different but connected to processes in deep waters.