Lines drawn on a map joining points of equal salinity in the ocean are called:

An isohaline (or isohaline line) is a line on a map or chart that joins points in the ocean with the same salinity. The word comes from "iso" (equal) and "halos" (salt) in Greek.

What salinity is:

• Salinity is the total amount of dissolved salts in sea water.
• It is usually expressed in parts per thousand (written as per mille), meaning grams of salt per 1,000 grams of sea water, or in practical salinity units.
• The main dissolved salts are sodium chloride (common salt), magnesium chloride, magnesium sulphate, calcium sulphate and potassium sulphate.

Factors affecting salinity:

• Evaporation: High evaporation in hot, dry regions increases salinity.
• Precipitation: Heavy rainfall lowers salinity by adding fresh water.
• River inflow: Large rivers bring fresh water and reduce salinity near their mouths, for example near the Ganga and Amazon.
• Freezing and melting of ice: Freezing leaves salt behind in the water, raising salinity; melting ice lowers it.
• Ocean currents and winds: They mix water and redistribute salt.

Distribution of salinity:

• Salinity is generally highest in the subtropical regions (around 20 to 40 degrees latitude), where evaporation is high and rainfall is low.
• It is lower near the Equator, because of heavy rainfall, and near the poles, because of melting ice.

Enclosed seas show extremes:

• The Red Sea and the Persian Gulf have very high salinity, about 40 or more.
• The Baltic Sea has very low salinity, because of fresh water from rivers and low evaporation.
• Inland water bodies such as the Dead Sea and Lake Van have extremely high salinity.

Halocline:

In the ocean, a layer where salinity changes rapidly with depth is called a halocline.

Importance:

• Salinity, together with temperature, controls the density of sea water, which drives deep ocean circulation (the thermohaline circulation).
• Salinity affects the freezing point of water and marine life.

Why not the other options?

• ✗Isotherms join places of equal temperature.
• ✗Isobars join places of equal atmospheric pressure.
• ✗Isogonals join places of equal magnetic declination.