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