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Thermohaline Circulation and
Water Mass Formation
(in contrast to the wind-driven circulation of the upper ocean)
•Thermohaline circulation: deep circulation is controlled by temperature (thermo) and salinity (haline), thus by density distribution.
• Other names/related processes:
- abyssal circulation
- meridional overturning circulation
- global conveyor belt
Thermohaline Circulation
(THC)
• How do we study THC?
• Note: very deep ocean currents cannot be directly measured (e.g., current meters) or calculated (e.g., geostrophy)
• Therefore, flow patterns and deep water masses are inferred from T/S properties
• What factors affect the T & S structure in the deep ocean?
– water-masses are formed with particular T/S properties at their origin
– advection (mostly along isopycnals)
– diffusion (along and across isopycnals)
– other mixing processes such as double diffusion and interaction with topography
T-S Diagram
Why do the Atlantic water masses have
larger variations in temperature and
salinity than the much larger Pacific
Ocean?
• Pacific water masses have mainly one
(Antarctic) source
• Atlantic waters have many
T-S Diagram: ocean average
Greenland
Sea
Norwegian
Sea Labrador
Sea
Antarctic Bottom Water
Antarctic Intermediate Water
Pacific Subarctic Water
N. Atl. Deep Water
Med. Intermediate Water
Summary of all
Water Masses
SACW NACW
SACW - South Atlantic Central Water
NACW - North Atlantic Central Water
SPCW - South Pacific Central Water
NPCW - North Pacific Central Water
SICW - South Indian Central Water
ECW - Equatorial Central Water
AAIW - Antarctic Intermediate Water
AIW - Arctic Intermediate Water
MIW - Mediterranean Intermediate Water
RSIW - Red Sea Intermediate Water
NPIW - North Pacific Intermediate Water
AABW - Antarctic Bottom Water
NADW - North Atlantic Deep Water
AADW - Antarctic Deep Water
NABW - North Atlantic Bottom Water
CoW - Common Water (AAIW + NADW)
PSW - Pacific Subarctic Water
From Pinet (1998)
Atlantic Ocean 150m
300m
5000m
Pot. Temp.
Salinity
S N
AABW
AAIW
NADW
MIW
AABW
NADW
NACW
What is the source of the water masses:
• deep and bottom water masses originate from sinking at high latitudes
• intermediate water masses originate from:
• downwelling at convergence regions in mid-latitudes
• Mediterranean Sea
• Labrador Sea
Metiterranean Sea: water formation and effect on the Atlantic
Mediterranean overflow and
Meddies
Meddies: Isolated eddies of salty Mediterranean water that
separated from the Mediterranean plume; they drift westward into
the Atlantic Ocean at ~1000m depth
High salinity-core “Meddies”
Worthington & Wright (1970)
Armi & Zenk (1984)
Detecting the signature of subsurface Meddies from Altimeter data (2005)
Antarctic
Bottom
Water
(AABW)
From: Tomczak, Matthias &
J Stuart Godfrey: Regional
Oceanography: an
Introduction 2nd edn (2003)
Weddell
Sea
From: Tomczak, Matthias & J Stuart
Godfrey: Regional Oceanography: an
Introduction 2nd edn (2003)
to Cape
Basin
8.
Antarctic Bottom
Waters are formed
at the Weddell and
Ross Seas and
slowly move along
the bottom, as seen
in the bottom
potential
temperature
While cooling, the formation of sea-ice also releasing salt
Antarctic water formation
The mixing between different water masses will create
new water properties
South Atlantic deep
flows: strongly
affected by bottom
topography,
especially ridges
that block the flow
and fracture zones
that allow some
flows to go through
Walvis Ridge
Rom
anche F
ractu
re Z
one
Iceland
Newfoundland
Labrador
Sea
Denmark
Strait
Iceland-
Faeroes
Ridge
Ireland
U.K.
North Atlantic Deep Water (NADW) formation
From: Tomczak, Matthias & J Stuart Godfrey: Regional Oceanography: an Introduction 2nd edn (2003)
Greenland
Iceland
NADW going through Denmark Strait
Convective water mass
formation occurs in
special locations where
strong winds and cold
temperatures strong
surface heat loss,
evaporation
increased density
.
One example is the
Labrador Sea
Where annual and decadal
changes in convection
can be seen
Greenla
nd
Icela
nd
NA
DW
DSO
FBCO
UK
FaroeIsland
DSO- Denmark Strait Overflow FBCO- Faroe Bank Channel OverflowNADW- North Atlantic Deep Water
Efforts to model
the overflow
through straits
and the mixing
processes are
important for
studies of climate
change
Observations across the Faroe Bank Channel
(Price et al.)
Faroe Bank
Channel
Faroe Island
North Atlantic
Ocean
OBS MOD
Upstream of the sill
~100km from sill
~200km
~400km
Downstream of the sill
OBS MODEL at sill
~-200km
~-300km
Temperature
along the
channel
Observed
Model
Model simulations of bottom waters passing through the Faroe
Bank Channel (Ezer, 2006)
Meridional Overturning
Circulation (MOC)
is a useful quantity used in
ocean modeling and climate
studies; it shows the stream
function (transport) as a
function of latitude and depth
and calculated from velocities
integrated across basins
Mixing mechanisms that affect water masses:
• Isopycnal mixing
• Diapycnal mixing
• Convective mixing
• Mesoscale eddy mixing
• Double diffusion
Shear
instability,
entrainment
detrainment
Geostrophic eddies
Downslope
descent
Bottom friction
Physical processes in overflows
Double Diffusion and Salt Fingers:
• a salty warm layer over a fresher-colder layer (best example- Med. outflow)
• molecular diffusion of heat 100 times larger than diffusion of salt (KT/KS>>1)
• small “salt fingers”- ~cm in diameter, <1m in depth – cannot be observed directly
• evident in thermal “staircase” profiles
colder
warmer unstable
Temperature mixes fast Salinity mixes slowly
density
increases
density
decreases
Formation of salt fingers in the lab
finished with ocean circulation and water masses
(only 6 more classes left…)
Next Classes:
• Ocean Waves (2 classes)
• Tides (1 class)
• Coastal Ocean and Estuaries (2 classes)
• Last class: review for final – Dec. 4
• Final Exam- Monday, Dec. 9