Geography · Ch 18 — Water (Oceans)
Horizontal Distribution of Salinity
Horizontal Distribution of Salinity
The map is a global view of the ocean surface, drawn with the Equator, Tropic of Cancer, and Tropic of Capricorn as dashed horizontal reference lines. Across the water, curved isopleth lines connect points of equal salinity, each labelled in parts per thousand (‰). The values run from 30 up to 37, and the pattern they trace is the whole lesson.
The highest readings, 36–37‰, sit in the subtropical belts — roughly between 20° and 30° latitude in both hemispheres. These are the horse latitudes, where high pressure brings clear skies, strong sunshine, and brisk evaporation, which concentrates the salt. The map shows this as a band of tightly packed, high-value isopleths on either side of the tropics.
Salinity drops as you move away from those belts. Near the Equator, the values fall to around 34–35‰, because the doldrums bring heavy rainfall and cloud cover that dilute the surface water even though temperatures are high. Toward the poles, the isopleths fall further, to 30–33‰, reflecting the input of meltwater and ice, and the weak evaporation in cold regions. So the map is not a simple equator-to-pole gradient — it is a two-humped curve, peaking in the subtropics and dipping at the equator and at high latitudes.
The isopleths are not straight east–west lines. They bend and shift with the continents, ocean currents, and the shape of the basins. Where warm, salty currents push poleward, the high-salinity tongues extend further from the tropics; where cold, fresh currents return toward the equator, the low values reach deeper into the subtropics. The Atlantic, being narrower and more evaporative, shows higher overall values than the Pacific, which is wider and receives more river and meltwater input. …
The salinity of normal open ocean water is not uniform — it sits within a fairly narrow band, but local conditions can push it far outside that range. For most of the open ocean, salinity ranges between 33‰ and 37‰. The symbol ‰ means parts per thousand, so 35‰ means 35 grams of salt per kilogram of seawater.
Some places break this pattern dramatically. In the landlocked Red Sea, salinity rises as high as 41‰ because the water is trapped and evaporation is intense. At the other extreme, in estuaries and in the Arctic, salinity fluctuates seasonally anywhere from 0‰ to 35‰ — estuaries receive huge pulses of fresh river water, while the Arctic gains and loses meltwater through the year. In hot, dry regions where evaporation is very high, salinity can sometimes reach 70‰, nearly double the open-ocean average.
The Pacific Ocean shows a clear horizontal pattern, and its shape and larger areal extent are the main reasons. On the western parts of the northern hemisphere, salinity decreases from 35‰ to 31‰ because of the influx of melted water from the Arctic region. Similarly, after 15° to 20° south, salinity decreases to 33‰. So the Pacific is generally less salty than the Atlantic, and its salinity drops in specific zones where fresh meltwater enters.
The Atlantic Ocean tells a different story. Its average salinity is around 36‰, higher than the Pacific. The highest salinity is recorded between 15° and 20° latitudes. Maximum salinity of 37‰ is observed between 20° N and 30° N and between 20° W and 60° W — a band in the subtropical North Atlantic where evaporation greatly exceeds precipitation. From this zone, salinity gradually decreases towards the north.
At the surface, salinity is changeable. It increases by the loss of water to ice formation or evaporation, and it decreases by the input of fresh waters, such as from rivers. At depth, however, salinity is very much fixed, because there is no way that water is 'lost' or salt is 'added' at those depths. This creates a marked difference between the surface zones and the deep zones of the oceans. …
All waters in nature, whether rain water or ocean water, contain dissolved mineral salts. Salinity is the term used to define the total content of dissolved salts in sea water. 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 (per mille) or ppt. Salinity is an important property of sea water. Salinity of 24.7 per mille has been considered as the upper limit to demarcate 'brackish water'.
Factors affecting ocean salinity are: (i) The salinity of water in the surface layer of oceans depend mainly on evaporation and precipitation. (ii) 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. (iii) Wind, also influences salinity of an area by transferring water to other areas. (iv) 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.
The salinity for normal open ocean ranges between 33 per mille and 37 per mille. In the land locked Red Sea, it is as high as 41 per mille, while in the estuaries and the Arctic, the salinity fluctuates from 0-35 per mille, seasonally. In hot and dry regions, where evaporation is high, the salinity sometimes reaches to 70 per mille.
The salinity variation in the Pacific Ocean is mainly due to its shape and larger areal extent. Salinity decreases from 35 per mille -31 per mille on the western parts of the northern hemisphere because of the influx of melted water from the Arctic region. In the same way, after 15-20 degree south, it decreases to 33 per mille.
The average salinity of the Atlantic Ocean is around 36 per mille. The highest salinity is recorded between 15 and 20 degree latitudes. Maximum salinity (37 per mille) is observed between 20 degree N and 30 degree N and 20 degree W-60 degree W. It gradually decreases towards the north. …