Salinity and Density Differences

VERTICAL STRUCTURE, THERMOHALINE CIRCULATION

& WATER MASSES

Three Effects of SalinityThree Effects of Salinityon Wateron Water

1. 1. Freezing PointFreezing Point Pure water freezes at Pure water freezes at 0 0 ºC. ºC.

Salt lowers its freezing Salt lowers its freezing point:point:Seawater with a Seawater with a salinity of 35salinity of 35ºº//ºº ºº freezes freezes at at

-1.91 ºC-1.91 ºC

2. Density2. DensitySalt molecules have a greater atomic mass Salt molecules have a greater atomic mass than Hthan H22O molecules, so the density of O molecules, so the density of water increases with salinity.water increases with salinity.As a result fresh water will “float” on salt As a result fresh water will “float” on salt water.water.

3. Vapour Pressure3. Vapour Pressure

As salinity increases, vapour pressure As salinity increases, vapour pressure drops. drops.

This means that freshwater evaporates This means that freshwater evaporates at a faster rate than seawaterat a faster rate than seawater

Density of HDensity of H22OO

The density of water depends mainly on The density of water depends mainly on salinitysalinity and and temperaturetemperature

Cold salty water is more dense then less Cold salty water is more dense then less salty warm watersalty warm water

Density of seawater is 1.020 to 1.030 Density of seawater is 1.020 to 1.030 g/cmg/cm33

Seawater’s density Seawater’s density increasesincreases when:when:

1. salinity increases1. salinity increases 2. pressure increases2. pressure increases 3. temperature decreases3. temperature decreases

DensityDensity Structure of the Ocean Structure of the Ocean

Much of the ocean is divided in to three Much of the ocean is divided in to three density zonesdensity zones

1. surface (mixed) zone1. surface (mixed) zone2. pycnocline2. pycnocline3. deep zone3. deep zone

1. Surface (mixed) Zone1. Surface (mixed) Zone

Consists of water in contact with the Consists of water in contact with the atmosphere and exposed to sunlightatmosphere and exposed to sunlightLeast dense zoneLeast dense zoneSurface to approx. 1000mSurface to approx. 1000m

2. Pycnocline2. Pycnocline

The zone where density increases with The zone where density increases with depthdepthThis zone isolates surface waters from the This zone isolates surface waters from the denser zone belowdenser zone below18% of ocean waters18% of ocean waters

3. Deep Zone3. Deep Zone

Lies below the pycnoclineLies below the pycnoclineDensity does not change much with Density does not change much with increasing depth in this zoneincreasing depth in this zone80% of oceans water80% of oceans water

More on Density DifferencesMore on Density Differences

Density varies within the world’s oceans Density varies within the world’s oceans depending on the location.depending on the location.

Factors that cause differences in density:Factors that cause differences in density:- High rates of evaporationHigh rates of evaporation- High inputs of freshwater (river run-off)High inputs of freshwater (river run-off)- High inputs of freshwater (ice melting)High inputs of freshwater (ice melting)- Temperature differencesTemperature differences

Thermocline – graph representing change in temperature

Halocline – graph representing change in salinity

Pycnocline – graph representing change in density

Done!Done!

A A WATER TYPEWATER TYPE is a body of water which is a body of water which has uniform temperature and salinityhas uniform temperature and salinity

The deep and bottom waters vary little in The deep and bottom waters vary little in temperature and salinity, and so are temperature and salinity, and so are represented almost completely by water represented almost completely by water types. types. They are not influence by wind and local They are not influence by wind and local climate changes and therefore reveal climate changes and therefore reveal much about their life history. much about their life history. 

SEASONAL THERMOCLINE SEASONAL THERMOCLINE In the subtropical gyres, at latitudes where seasonal In the subtropical gyres, at latitudes where seasonal

variations in wind strength and solar heating occur, a variations in wind strength and solar heating occur, a SEASONAL THERMOCLINE develops in addition to the SEASONAL THERMOCLINE develops in addition to the permanent or main thermocline.  permanent or main thermocline. 

During winter, strong winds typically mix the upper ocean During winter, strong winds typically mix the upper ocean layers down to 100 m or more, but then in spring when layers down to 100 m or more, but then in spring when winds moderate and heating increases a thin warm layer winds moderate and heating increases a thin warm layer develops at the surface with a sharp seasonal thermocline develops at the surface with a sharp seasonal thermocline beneath.  beneath. 

Later periods of storms may mix the seasonal thermocline Later periods of storms may mix the seasonal thermocline down but not usually as deep as in winter. Subsequent down but not usually as deep as in winter. Subsequent periods of heating and mixing may form step like periods of heating and mixing may form step like structures in temperature, forming a seasonal thermocline structures in temperature, forming a seasonal thermocline continuous with the permanent one.  continuous with the permanent one. 

Come winter, vigorous mixing distributes the heat added at Come winter, vigorous mixing distributes the heat added at the surface over the upper few hundred meters.  During the surface over the upper few hundred meters.  During the course of the winter, this heat is released slowly to the the course of the winter, this heat is released slowly to the atmosphere warming the local climateatmosphere warming the local climate

DEEP SALINITY LAYERING DEEP SALINITY LAYERING The bulk of the deep ocean is relatively uniform, especially in the Pacific. The bulk of the deep ocean is relatively uniform, especially in the Pacific.

However the subtropical surface layers have a higher salinity because of However the subtropical surface layers have a higher salinity because of excess evaporation, compared to the northern polar areas which are diluted excess evaporation, compared to the northern polar areas which are diluted (lower salinity) because of the effects of massive precipitation and runoff(lower salinity) because of the effects of massive precipitation and runoff

However on looking more closely, it is seen that there is significant salinity However on looking more closely, it is seen that there is significant salinity variation in the deep Atlantic layers. First, there is a clear tongue of high variation in the deep Atlantic layers. First, there is a clear tongue of high salinity stretching south through the equatorial regions.  Second, there is a salinity stretching south through the equatorial regions.  Second, there is a core of high salinity water at 1000 m depth evident especially in the eastern core of high salinity water at 1000 m depth evident especially in the eastern sector of the North Atlantic.  sector of the North Atlantic. 

These features represent These features represent water masseswater masses, which are distinct bodies of water , which are distinct bodies of water formed in a restricted location, spreading out through the ocean by being formed in a restricted location, spreading out through the ocean by being carried with the current (advection) and by mixing with surrounding waters carried with the current (advection) and by mixing with surrounding waters (diffusion).  (diffusion). 

The two in question here are the North Atlantic Deep Water and Mediterranean The two in question here are the North Atlantic Deep Water and Mediterranean Intermediate Water, which has escaped out through the Strait of Gibraltar. In Intermediate Water, which has escaped out through the Strait of Gibraltar. In the Pacific, there is no clear contrast of deep water masses but two tongues the Pacific, there is no clear contrast of deep water masses but two tongues of lower salinity water are seen to sink and extend towards the equator from of lower salinity water are seen to sink and extend towards the equator from the subpolar regions.  These are known as the Arctic and Antarctic the subpolar regions.  These are known as the Arctic and Antarctic Intermediate water masses.  Intermediate water masses. 

The Antarctic Intermediate Water is also visible in the Atlantic section.  The The Antarctic Intermediate Water is also visible in the Atlantic section.  The spreading of these water masses is evidence of the THERMOHALINE spreading of these water masses is evidence of the THERMOHALINE CIRCULATION . Which means that subsurface flow patterns are produced CIRCULATION . Which means that subsurface flow patterns are produced by density differences not by windby density differences not by wind

WATER MASSESWATER MASSES At this point we consider what is meant by the term WATER At this point we consider what is meant by the term WATER

MASS. MASS. There are 4 main water masses:There are 4 main water masses: CENTRAL, INTERMEDIATE, DEEP & BOTTOM CENTRAL, INTERMEDIATE, DEEP & BOTTOM     A water mass is a kind of fingerprint, which allows the origin A water mass is a kind of fingerprint, which allows the origin

of particular water parcels to be identified even when they of particular water parcels to be identified even when they are far removed from their source.  are far removed from their source. 

The predominant regions where the temperature and salt The predominant regions where the temperature and salt content of sea water are altered are at the surface.  Once content of sea water are altered are at the surface.  Once away from the sea surface the only mechanisms which away from the sea surface the only mechanisms which alter temperature and salinity are mixing processes. So in alter temperature and salinity are mixing processes. So in general water masses tend to maintain their general water masses tend to maintain their characteristics and can be identified even if they have characteristics and can be identified even if they have traveled far away from their original location. traveled far away from their original location.

CHARACTERIZING GLOBAL WATERCHARACTERIZING GLOBAL WATER

A A WATER MASSWATER MASS is an extensive body of water is an extensive body of water which has a limited range of temperature and which has a limited range of temperature and salinity salinity 

The water masses of the world's oceans can be The water masses of the world's oceans can be summarized by a few points:summarized by a few points:

   The warm, less dense surface waters make up The warm, less dense surface waters make up only 20% of the total volume, but they encompass only 20% of the total volume, but they encompass the layers of greatest variation.  the layers of greatest variation.  The upper layers vary significantly from one The upper layers vary significantly from one ocean to another because they are strongly ocean to another because they are strongly affected by heat and water fluxes through the affected by heat and water fluxes through the surface locally. surface locally. 

Although it would be possible that both temperature Although it would be possible that both temperature and salinity could have values independent of and salinity could have values independent of each other, in fact, it is observed that they vary each other, in fact, it is observed that they vary throughout the oceans in a very related manner.  throughout the oceans in a very related manner. 

Since density must increase with depth to maintain Since density must increase with depth to maintain stability within the water column, it is known that stability within the water column, it is known that density increases with depth.density increases with depth.

Since water with a higher salinity is more dense it Since water with a higher salinity is more dense it to will increase with depth to maintain a stable to will increase with depth to maintain a stable water column.water column.

Therefore as the depth increases so do both the Therefore as the depth increases so do both the temperature and the salinity. So the Halocline temperature and the salinity. So the Halocline and the Pycnocline will follow the same patterns.and the Pycnocline will follow the same patterns.