TEMPERATURE OF OCEAN WATERS

About

1. Latitude : the temperature of surface water decreases from the equator towards the poles because the amount of insolation decreases poleward.
2. Unequal distribution of land and water: the oceans in the northern hemisphere receive more heat due to their contact with larger extent of land than the oceans in the southern hemisphere.
3. Prevailing wind: the winds blowing from the land towards the oceans drive warm surface water away from the coast resulting in the upwelling of cold water from below. It results into the longitudinal variation in the temperature. Contrary to this, the onshore winds pile up warm water near the coast and this raises the temperature.
4. Ocean currents: warm ocean currents raise the temperature in cold areas while the cold currents decrease the temperature in warm ocean areas.
5. Gulf stream (warm current) raises the temperature near the eastern coast of North America and the West Coast of Europe while the Labrador current (cold current) lowers the temperature near the north-east coast of North America.

Horizontal and Vertical Distribution of Temperature

• The temperature-depth profile for the ocean water shows how the temperature decreases with the increasing depth.
• The profile shows a boundary region between the surface waters of the ocean and the deeper layers.
• The boundary usually begins around 100 – 400 m below the sea surface and extends several hundred of metres downward
• This boundary region, from where there is a rapid decrease of temperature, is called the thermocline.
• About 90 per cent of the total volume of water is found below the thermocline in the deep ocean.
• About 90 per cent of the total volume of water is found below the thermocline in the deep ocean.
• In this zone, temperatures approach 0° C.

3 layer system from surface to the bottom-

• The first layer represents the top layer of warm oceanic water and it is about 500m thick with temperatures ranging between 20° and 25° C.
• This layer, within the tropical region, is present throughout the year but in mid latitudes it develops only during summer.
• The second layer called the thermocline layer lies below the first layer and is characterised by rapid decrease in temperature with increasing depth. The thermocline is 500 -1,000 m thick.
• The third layer is very cold and extends upto the deep ocean floor.
• In the Arctic and Antarctic circles, the surface water temperatures are close to 0° C and so the temperature change with the depth is very slight.
• Here, only one layer of cold water exists, which extends from surface to deep ocean floor.
• The average temperature of surface water of the oceans is about 27°C and it gradually decreases from the equator towards the poles.
• The rate of decrease of temperature with increasing latitude is generally 0.5°C per latitude.
• The oceans in the N hemisphere record relatively higher temperature than in the southern hemisphere.
• The highest temperature is not recorded at the equator but slightly towards north of it.
• This variation is due to the unequal distribution of land and water in the northern and southern hemispheres.
• The heat is transmitted to the lower sections of the oceans through the process of convection.

SALINITY OF OCEAN WATERS

About

• Salinity is the term used to define the total content of dissolved salts in sea
• It is calculated as the amount of salt (in gm) dissolved in 1,000 gm (1 kg) of seawater.
• It is usually expressed as parts per thousand or ppt.
• Salinity of 24.7 %has been considered as the upper limit to demarcate ‘brackish water’.

Factors Affecting Ocean Salinity

• The salinity of water in the surface layer of oceans depend mainly on evaporation and precipitation.
• Surface salinity is greatly influenced in coastal regions by the fresh water flow from rivers, and in Polar Regions by the processes of freezing and thawing of ice.
• Wind, also influences salinity of an area by transferring water to other areas.
• The ocean currents contribute to the salinity variations. Salinity, temperature and density of water are interrelated.

Hence, any change in the temperature or density influences the salinity of water in an area.

1. Van in Turkey (330 o/oo ),
2. Dead Sea (238 o/oo ),
3. Great Salt Lake (220 o/oo)

Dissolved Salts in Sea Water (gm of Salt per kg of Water)–

• Chlorine 18.97
• Sodium 10.47
• Sulphate 2.65
• Magnesium 1.28
• Calcium 0.41
• Potassium 0.38
• Bicarbonate 0.14
• Bromine 0.06
• Borate 0.02
• Strontium 0.01

Horizontal Distribution Of Salinity

• The salinity for normal open ocean ranges between 33 o/oo and 37 o/oo .
• In the land locked Red Sea, it is as high as 41 o/oo , while in the estuaries and the Arctic, the salinity fluctuates from 0 – 35 o/oo , seasonally.
• In hot and dry regions, where evaporation is high, the salinity sometimes reaches to 70 o/oo.
• The salinity variation in the Pacific Ocean is mainly due to its shape and larger areal extent.
• Salinity decreases from 35 o/oo – 31 o/oo on the western parts of the northern hemisphere because of the influx of melted water from the Arctic region
• The highest salinity is recorded between 15° and 20° latitudes.
• Maximum salinity (37 o/oo) is observed between 20° N and 30° N and 20° W – 60° W.
• It gradually decreases towards the north.
• The North Sea, in spite of its location in higher latitudes, records higher salinity due to more saline water brought by the North Atlantic Drift. Baltic Sea records low salinity due to influx of river waters in large quantity.
• The Mediterranean Sea records higher salinity due to high evaporation.
• Salinity is, however, very low in Black Sea due to enormous fresh water influx by rivers.
• The average salinity of the Indian Ocean is 35 o/oo. The low salinity trend is observed in the Bay of Bengal due to influx of river water.
• On the contrary, the Arabian Sea shows higher salinity due to high evaporation and low influx of fresh water.

Vertical Distribution Of Salinity

• Changes with depth, but the way it changes depends upon the location of the sea.
• Salinity at the surface increases by the loss of water to ice or evaporation, or decreased by the input of fresh waters, such as from the rivers.
• Salinity at depth is very much fixed, because there is no way that water is ‘lost’, or the salt is ‘added.’
• There is a marked difference in the salinity between the surface zones and the deep zones of the oceans.
• The lower salinity water rests above the higher salinity dense water.
• Salinity, generally, increases with depth and there is a distinct zone called the halocline, where salinity increases sharply.
• Other factors being constant, increasing salinity of seawater causes its density to increase.
• High salinity seawater, generally, sinks below the lower salinity water. This leads to stratification by salinity.