Watermass

1 ”Descriptive Physical Oceanography” by Reddy - chapter 10.

What is watermass?

Waters of the world oceans can be classified into divisions having specific valuesof properties.

Each of these divisions is given a name based on the location of formation.

Water bodies having a common formation history is called as watermass.

Physical properties that are used to identify watermass is temperature and salinity.

Watermasses is represented by a graphic plot by taking salinity along x axis andtemperature along y axis called as T-S diagram.

T-S Diagram

Introduced by Helland-Hansen in 1916.He stated that in a given area, if temperature (T) and salinity (S) of sea water isplotted against each other, the points fall on on a well-defined curve known asT-S curve.Surface values are usually ignored.T-S points are plotted according with depth and may be mentioned.T-S curve is drawn over the σt values.The stability of the water is estimated from the slope of the T-S curve with σtlines.A water mass is defined by its T-S curve.Water type.

Formation of Watermass

1 Thermohaline alteration

Majority of water masses are formed at mid and high latitudes.Seasonal cooling and hence freezing makes the surface water denser.Cold and dense waters sink until it reaches a level having same density of thesurrounding waters.

2 Mixing of water bodies.

Southern Ocean Water masses

Divisions of Southern Ocean

Based on temperature (surface)

Land mass of Antarctic continent as southern limit.

Open to other oceans towards north.

Towards north SST increases slowly.

SST increases rapidly (2 to 3 K) at 50-60S due to convergence and called as”Antarctic Polar Front (APF)”.

Area between Antarctic continent and APF is called ”Antarctic Zone”.

Towards north, SST increases again slowly.

At about 40 S, SST increases rapidly (about 4K) and also salinity. This region iscalled as ”Subtropical Convergence zone”.

Area between APF and Subtropical convergence is called as ”Subantarctic zone”.

Antarctic Surface Water

Found south of APF.

Water properties determined by ice melting in summer and cooling in winter(thermohaline alteration).

Salinity less than 34.5

Depth of 100 to 200m.

Characterized by low temperature and low salinity.

Antarctic Circumpolar Water

Extends upto 4000m.

Temperature varies from 0 to 2.5C.

Salinity is 34.7

Circulated all over Antarctic by Antarctic Circumpolar Current (ACC)

Antarctic Bottom Water

Formed in the Weddell and Ross Seas.

Mixture of Antarctic Circumpolar water and Shelf water (mixing).

Shelf water: T = -2.0C, S= 34.4 to 34.8. High density.

Flows from Weddell sea into south Atlantic and also eastward.

Subantarctic Upper water

Occupies upper 500m.

Temperature varies from 4 to 14C and Salinity 33.9 to 34.9

Antarctic Intermediate water

Formed in the APFZ.

Thickness of about 500m.

T = 2 to 3C, S = 34.2

Flows northward. Mixing with high salinity water above and below increases itssalinity towards north. This increases its density and sinks.

Identified as a tongue of low salinity. The core is at a depth of 800 to 1000m at40S.

Has high oxygen content.

Deep water

Found at a depth of 1500 to 3000m depth.

Have different properties in each ocean.

T = 2 to 3C, S = 34.7 to 34.9.

Flows southwards.

Arctic ocean water masses

Arctic water

Depth surface to 200m.

Divided into Surface Arctic, Sub-surface arctic, and Lower Arctic waters.

Arctic Surface waterUpto 50m.High seasonal variation due to freezing and melting of ice.T = -1.5 C, Salinity 28 to 33.5.

Arctic Subsurface waterDepth: 50 to 100mT = -1.5 to -1.0C, S = 31.5 to 43.Less seasonal variation

Arctic Lower surface waterDepth: 100 to 200m.T, S intermediate between Arctic Subsurface water and Atlantic water.

Arctic Deep/Bottom water

Extends from 800m (0 deg C isotherm) to the bottom.

Consists of 60% of Arctic sea water.

Salinity varies slightly (33.90 to 33.99)

Temperature: -0.2C to -0.6C.

Further classified asGreenland Sea Deep Water : cold and fresherNorwegian Sea Deep Water : warmer and more salineEurasian Basin Deep WaterCanadian Basin Deep Water.

Arctic water

Depth surface to 200m.

Divided into Surface Arctic, Sub-surface arctic, and Lower Arctic waters.

Arctic Surface waterUpto 50m.High seasonal variation due to freezing and melting of ice.T = -1.5 C, Salinity 28 to 33.5.

Arctic Subsurface waterDepth: 50 to 100mT = -1.5 to -1.0C, S = 31.5 to 43.Less seasonal variation

Arctic Lower surface waterDepth: 100 to 200m.T, S intermediate between Arctic Subsurface water and Atlantic water.

Arctic Deep/Bottom water

Extends from 800m (0 deg C isotherm) to the bottom.

Consists of 60% of Arctic sea water.

Salinity varies slightly (33.90 to 33.99)

Temperature: -0.2C to -0.6C.

Further classified asGreenland Sea Deep Water : cold and fresherNorwegian Sea Deep Water : warmer and more salineEurasian Basin Deep WaterCanadian Basin Deep Water.