Water Mass DistributionOEAS 604

Lecture Outline

1) Thermohaline Circulation2) Spreading pathways in ocean basins3) T-S diagrams4) Mixing on T-S diagrams

Chapter 9 – KnaussChapter 14 – Talley et al.

Thermohaline Circulation

• Circulation below the wind-driven currents • Flow is determined by density distribution

which is from temperature and salinity • Circulation forced by sinking of cold dense

water at high latitudes • Return flow in upper water column• Referred to a Meridional Overturning

Circulation (MOC)

Water Masses

• Form in certain regions of the ocean

• Advected by ocean currents

• Form frontal regions were meet

• Water mass – a body of water with a common formation history, having its origin in a particular region of the ocean

• Source region – finite volume of ocean where water mass formation takes place

Water Mass Analysis

• Water type - point in n-dimensional parameter space – it is a combination of temperature, salinity, nutrients, oxygen and other tracer values

• Water mass – water formed in certain regions with particular temperature, salinity and other properties

Mixing of water masses

Two-point mixing• Determine the mixture along line between

two source waters • f = fraction of water mass 1, f=1 all from

water mass 1, f = 0 none from water mass 1

• Tmix = fT1 + (1-f)T2

• Smix = fS1 + (1-f)S2

• Know Tmix, Smix, T1, T2, S1, S, so solve for f

Track water masses

Core method

Gives rates ofmixing andtransport direction

Atlantic

Indian

Pacific

T-S distribution for world oceans

North Atlantic and Southern Ocean

NA – open to north, high latitude, limited seasonal sea ice, freshwater sources SO – bounded by continent, seasonal sea ice, no rivers, large polynyas

Atlantic separatedInto basins with littleconnections

Indian Ocean

Blocked to north

Limited connectionto Southern Ocean

Complex bathymetryand limited openingsto the east

Pacific Ocean

Western basinDeeper

Opening to northconstricted byBering Strait

North Atlantic Deep WaterGreenland Sea WaterLabrador Sea WaterMediterranean Sea Water

High salinity, low temp, high oxygen

Antarctic Intermediate WaterLow salinity, high oxygen

Antarctic Bottom WaterHigh salinity, low temperature,average oxygen

Subtropical UnderwaterLow oxygen

Antarctic Bottom WaterPathways

Different varieties along western and eastern sides of basin

Limited across basinexchange

Flow on eastern side blocked by Walvis Ridge

AAIW

NADW

AABW

East-west sectionin South Atlantic

T-S diagrams in East and West Atlantic – NADW, AAIWNorth Atlantic and South Atlantic Central WatersHigh salinity Mediterranean Sea Water

West East

Annual progression of sea ice concentration in 1991, computed from the Special Sensor Microwave Imager (SSM/I) carried on the Defense Meteorological Satellite Program satellites.

Antarctic CircumpolarCurrent

High speedcurrent cores

SubantarcticFront

Polar Front

Southern ACCFront

SubtropicalFront

Track of the Endurance (1914-1916) (from Stone 1914)

Bottom water formation

Spreading and direction of AABW

Intermediate and mode waters form along northern side of Polar and Subantarctic fronts

T-S diagram from Southern Ocean

AASW – Antarctic Surface Water

WW – Winter Water

SSW – Shelf Water

HSSW – High Salinity Shelf Water

ISW – Ice Shelf Water

WS and RS – Bottom WaterCDW - Circumpolar Deep Water

T-S properties ofwater masschange around the Southern Ocean

CDW – cools and freshens

Spreading ofBottom water

Limited exchange

90°E Ridge is abarrier

T-S diagram for Indian OceanAAMW – Australian Mediterranean Water

Spreading ofbottom waterin Pacific Ocean

North-southsection in westernPacific

T-S diagramfrom westernPacific

Next Class

• Ekman Dynamics

• Upwelling

• Chapter 7, Knauss

• Chapter 7, Talley et al.