Transcript Lecture 14b: Ocean Circulation

Surface circulation
Wind driven, Coriolis modified
Ekman and geostrophic flow
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
Change in
productivity rates
affects atm CO2?
Assume these
fluxes are balanced
Change in
burial rates
affects atm CO2?
Fig. 8-9
Why is this important? Processes in surface, wind-driven layers
are different but connected to processes in deep waters.
Short-term
Long term (organic)
Long term
(inorganic/tectonic)
Link with short term C cycle
In surface oceans
Onset of modern plate
tectonics “turns this on”
“adds” back CO2
Thermohaline
Circulation
Thermohaline circulation
• Vertical water movement
• Driven by density differences (can be very small)
– Remember temperature and salinity diagrams and the
properties of water
– Temperature and salinity profiles (with depth)
– Salty water is denser than fresh water
– Cold water is denser than warm water
• Density gradients with latitude (due to
temperature differences of surface waters)
– Polar water has the most uniform density (weakest
pycnocline) so is least stable
Fig. 5-6
•Polar water column is
least stable; most
uniform density
weakest pycnocline
•Surface is cold
•Deep water is cold
everywhere
•Easiest to rearrange
•Formation of ice
excludes salt
•Seawater freezes at
–2oC
Warming Cycle:
1. March
2. May
3. June
4. August
Cooling Cycle:
1. August
2. September
3. October
4. November
5. January
Temp. sea water
Wind
Most of the ocean is cold
Temperature degree C
Thermohaline circulation
• As for the atmosphere, there are
convergence and divergence zones where
water masses collide or diverge
• Important for global heat balance
• Deep circulation and basin exchange of
water, material, and heat
Thermohaline circulation
•
•
•
•
Deep circulation is driven by density differences
Horizontal movement along density surfaces
Movement is very slow (0.1 m/s)
Three layer ocean
– surface mixed layer
– Pycnocline
– Deep water
• Deep water formed at 2 places – N Atlantic and
Weddell Sea (Antarctica)
• Connection between surface and deep water
– Diffusion (slow and along density gradients)
– Mixing (e.g., storms)
– Upwelling (polar, equatorial and coastal)
Deep circulation is
like a conveyer belt that
moves heat and water
Water masses
• Possess identifiable properties
• Don’t mix easily – flow above or beneath each
other
–
–
–
–
–
Surface water – to about 200 m
Central water – to bottom of main thermocline
Intermediate water – to about 1500 m
Deep water – to about 4000 m (not the bottom)
Bottom water – in contact with seafloor
• Retain characteristics when the mass was
formed at the surface (heating/cooling,
evaporation/dilution)
Formation of deep water
• Antarctic bottom water – densest water in the
ocean – S (34.65%o), T (-0.5oC), dens (1.0279)
– Weddell Sea in winter – when ice freezes and get
brines
– Sinks and creeps North
– Pacific and Atlantic – 100’s to 1000’s of years to get
to northern basin
• North Atlantic deep water
– Formed in the Arctic but escapes only through
channels
– Warm, salty water chills (heat is transferred to air)
• Atlantic and Pacific deep water is less dense
than Antarctic bottom water
Mediterranean deep water
• Salty water (38 %o)
• Underlies central water mass in Atlantic
• Warmer than other deep water so not as
dense
Increasing
density
• Different
combinations of
temp and
salinity can
make water of
the same
density
• Isopycnal is a
line of constant
density
• Mixing of water
masses can
increase the
density
Age of a water mass
• Oxygen and C isotopes
• Water picks up oxygen and CO2 only at
surface by exchange with atmosphere
• Loses oxygen as it ages (respiration,
reaction with rocks); 14C decays as it
ages
• Water masses mix slowly
• Antarctic bottom water in the Pacific
retains its character for up to 1600 years
Fig. 8-11
-70‰
∆14C
-230‰
-110‰
-210‰
-150‰
-170‰
-150‰
-170‰
This diagram represents the flow at a depth of 4000 m; the strange-looking continent/ocean configuration is what
we would obtain if the oceans were drained to this depth. (After W.S. Broecker and T.-S. Peng, Tracers in the Sea,
New York: Eldigio Press, 1982, Figure 1-12.)
Water residence time
• Bottom currents are slow
• Antarctic bottom water ~1600 years
• Other bottom water – 200-300 years (time it
takes to rise to the surface)
• Fast bottom currents on bottom around objects
• Surface currents are faster
• Surface water – on the order of years
– N Atlantic gyre may take about 1 year to complete a
circuit
Deep water circulation
• Flow may be slow but still modified by
Coriolis
Convergence zones
• Two water masses meet
• Usually one will go under the other
(density)
• If the same density, may mix to create a
new and denser water mass (remember TS diagram) – this is caballing
• Formation of N Atlantic intermediate water,
Antarctic intermediate water and Antarctic
bottom water produced by mixing
•S-curve tracks density
with depth
•Points a and b on an
Isopycnal so are the
same density, despite
different temperatures
and salinities
•If the two water masses
mix, will result in
denser water!
Convergence zones
• Areas of downwelling
• Antarctic convergence zone at 50-60oS
– South of this is the Antarctic Circumpolar Current
• Subtropical convergence zone (40-50oS)
– N boundary of Subantarctic Surface Water
• Equatorward of subtropical convergence zone is
Central Water which is the warmest and saltiest
water
• Subtropical convergence (45-60oN)
• Arctic convergence – ill-defined because of land
mass in the N
Divergence zones
• Areas of upwelling
• Tropical divergence
• Antarctic divergence
Upwelling
• Sinking water must be offset by upwelling
of water
• Water sinks in a localized area, relatively
rapidly
• Water rises gradually over larger areas
• When water moves to surface, must be
transported to a pole to sink again
• Diffuse upwelling maintains a permanent
thermocline (1 cm/d)
Maintained by diffuse
upwelling
Main thermocline
Thermohaline flow
• Sinking of surface water most pronounced in the
North Atlantic
• Water moves at great depths toward Southern
hemisphere and wells up into the surface in the
Indian and Pacific Oceans (~1000 years)
• Slow circulation that crosses hemispheres
superimposed on rapid flow of surface water in
gyres
• Some heat travels from S Pacific, across Indian
Ocean and into the Atlantic
• Flow distributes gases, solids, nutrients and
organisms among ocean basins
Fig. 5-12
Fig. 8-9
The Redfield Equation
106 CO2 + 16 NO3- + HPO42- + 122 H2O 
(CH2O)106(NH3)16(H3PO4) + 138 O2
Can also develop similar ratios for trace elements required for
growth (e.g., Fe, Mn, Zn, etc.)
Box Fig. 8-1
Shallow upper mixed layer
and main thermocline
Uptake by phytoplankton
Short residence times
Regeneration during particle sinking
Ocean circulation and/or vertical mixing
Long residence times
Thermohaline circulation
Norwegian
Sea
POM
POM
POM
Atlantic
Indian
POM
Pacific
Nutrient regeneration and O2 consumption
O2 decreases, nutrients increase as deep water
is transported/ages
undersat’d.
supersat’d.
plus nutrients
fixed into the
organic matter
and pCO2
Coastal Upwelling
Major upwelling
zones
Wind-induced upwelling
Note that transport goes to the right in the N Hemisphere
Seasonal upwelling due to seasonally
reversing monsoonal winds
Seasonal upwelling
Summer
Wind direction resulting from differential
pressure over sea and land superimposed on
a SeaWiFS chlorophyll image for the
southwest monsoon. High chlorophyll
concentrations in the western Arabian Sea
are due to coastal upwelling
(see animation at
http://www.bigelow.org/climatechange/Animation.htm)
Winter
Schematic showing snow cover extent
and wind direction superimposed on a
SeaWiFS chlorophyll image for the
northwest monsoon season. High
chlorophyll concentrations are due to
nutrients inputs from of winter convective
mixing
Equatorial upwelling
Equator
SE trades
CO2 from the
atmosphere
O2 to the
atmosphere
Brings nutrients
back for production
(100 %)
(> 99%)
(< 1%)
Fig. 8-9
Net burial of CH2O in sediments
The Oceanic P Cycle (all fluxes are 1010 mole P/yr)
River input
Surface Ocean
[PO4]avg = 0.15 µM
Downwelling
For example phosphorus
in the whole ocean
35
Particle flux
7.5
215
187.5
Upwelling
Whole Ocean
[PO4]avg = 2.05 µM
Ocean
P concentration
(1.4 1021 l)(2.05106 M)
 whoc 
351010 m olP / yr
 8,200yr
Input = output

Deep Ocean
[PO4]avg = 2.2 µM
Ocean
volume
Burial in
sediments
35
The Oceanic P Cycle (all fluxes are 1010 mole P/yr)
Very different in its
component parts
River input
Surface Ocean
[PO4]avg = 0.15 µM
Downwelling
35
Particle flux
 sfc
187.5
(11020 l)(0.15 106 M)

222.5 1010 m olP / yr
 6.7yr

7.5
Deep Ocean
[PO4]avg = 2.2 µM
Whole Ocean
[PO4]avg = 2.05 µM
215
Upwelling
Burial in

sediments
35
(1.3 1021 l)(2.2 106 M)
 deep 
222.5 1010 m olP / yr
 1,285yr
(1.4 1021 l)(2.05106 M)
 whoc 
351010 m olP / yr
 8,200yr
Meanders and eddies
• Currents are water so, lack well-defined edges so
have friction with adjacent boundaries
• Meanders as it flows poleward
• Sometimes get rings (warm and cold core) pinched
off
• Warm-core eddies rotate CW (in the N hemisphere)
• Cold-core eddies rotate CCW (in the N
hemisphere)
• Can be 1000 km in diameter and persist for years,
effects may be felt to the bottom and may be
important for mixing up deep water (& nutrients)
Colder productive water
Warm, clear water
Eddies
• Meanders pinch off
• Trap warm or cold
water at their center
• Cold-core eddies in
the Gulf Stream
• Warm-core eddies N
of the Gulf Stream
• Cold-core eddies very
nutrient rich and
productive
• Warm-core
ring
• Cold-core
ring
Kuroshio eddies in
natural color!
Green is phytoplankton
Formation of Rings
• Retain
momentum of
current
• Warm-core rings
spin CW (N
hemisphere)
• Cold-core rings
spin CCW (N
hemisphere
Rings can penetrate to
great depth
To the seafloor?
Cold ring has low pressure “upwelling” in the center (Coriolis deflection to the right in N hemisphere)
while warm ring has high pressure downwelling in the center (same reason).
El Niño/La Niña
• Normally get surface winds across the tropical
Pacific moving East to West (easterlies or trade
winds)
• High pressure over eastern Pacific and Low
pressure over western Pacific maintains this
pattern
• But, these pressure systems reverse or collapse
with some periodicity (every 3-8 years)
• Winds reverse and trade winds weaken or
collapse
• Change in atmospheric pressure is Southern
Oscillation
• Serious effects on productivity
El Niño/La Niña
• Wind-driven equatorial currents weaken or
collapse
• Warm water “sloshes” or flows eastward
• Get east-flowing warm water around
Christmas-time and this is El Niño
• Combine terms and get El Niño Southern
Oscillation (ENSO)
El Niño
• El Niños occurred in the E Pac during
1986-87, 1991-92, 1993, 1994 and 19971998, 2002-2003. 2006-2007
• Vary in strength
Normal
air pressure
Normal
SST with
Upwelling
Storms over land increase (CA)
Deeper thermocline
Failed upwelling or downwelling
El Niño
SST with warm
current and little
or no upwelling
http://www.pmel.noaa.gov/tao/elnino/la-nina-story.html
Normal
Warm pool
Effects on productivity
ENSO events
• Thermocline deepens in the Eastern
Pacific (EP)
• Productivity decreases in EP
• Sea level rises in EP
• Increases rain and storms on adjacent
land
– Milder temps on west coast?
– This year’s CA storms?
La Niña
• Not always right after an El Niño even
• Colder-than-normal event
• Thermocline decreases from normal (more
intense upwelling)
• Resumption and strengthening of trade
winds
• Winter temps warmer in SE and cooler in
NW
• Effects opposite of El Niño
North Atlantic Oscillation
• Seesaw of atmospheric mass between subtropical
high and polar low
• Dominant mode of winter climate variability in the
North Atlantic
• Positive NAO index is stronger than usual subtropical
high and deeper than normal Icelandic Low
– stronger than average westerlies at mid-latitudes
• Increased pressure difference results in
– Stronger winter storms crossing Atlantic
– More northerly track to storms
• Warm and wet European winter and cold and dry N
Canada and Greenland winter
• Mild and wet winter in Eastern USA