Questions: Salinity Distribution and Sources and Sinks

Question 1

Surface salinity is highest in which oceanic region, and why?

• A. Near the equator, because the sun heats the surface most intensely there
• B. In the subtropical ocean basins (~20–30° latitude), because dry descending air drives intense evaporation with little rainfall
• C. At the poles, because cold water holds dissolved salts better
• D. Near major river mouths, because rivers carry dissolved minerals into the ocean

Answer: (B)

Salinity reflects the freshwater budget — evaporation concentrates salt, precipitation dilutes it. Subtropical regions are dominated by the Hadley cell's descending dry air, producing high evaporation and sparse rainfall, so salinity exceeds 37 PSU there. Near the equator, heavy ITCZ rainfall depresses salinity despite intense sunlight. Temperature affects density, not salinity concentration directly — cold water is denser, but not saltier.

Question 2

When seawater freezes to form sea ice, what happens to the salt in the freezing water?

• A. Salt becomes incorporated into the ice lattice, increasing the ice's salinity
• B. Salt is expelled through brine rejection, leaving the surrounding water saltier and denser
• C. Salt dissolves into the atmosphere as water vapor carries it away
• D. Salt concentration stays the same because no water is lost from the ocean

Answer: (B)

During sea ice formation, the growing ice crystal expels most dissolved salt in a process called brine rejection. The surrounding seawater becomes colder AND saltier — both effects increase its density. This dense brine sinks toward the ocean floor and is a key driver of the thermohaline circulation's deep limb. The reverse happens when ice melts: relatively fresh meltwater caps the surface and freshens the upper ocean.

Question 3

The global pattern of surface ocean salinity is largely a mirror image of the pattern of evaporation minus precipitation.

• T. True
• F. False

Answer: True

This is the central organizing principle of salinity distribution. Where evaporation exceeds precipitation (subtropics), fresh water is removed and salinity rises. Where precipitation exceeds evaporation (equator, high latitudes), fresh water is added and salinity falls. Rivers and ice formation create local exceptions, but the E−P pattern explains the large-scale structure.

Question 4

Because the tropics are the warmest part of the ocean, surface salinity there is higher than anywhere else.

• T. True
• F. False

Answer: False

Warmth drives evaporation, but the tropics also receive the most rainfall — the ITCZ delivers heavy precipitation near the equator that more than offsets evaporative concentration. The result is relatively low equatorial salinity. The highest surface salinities occur in the subtropical gyres (~20–30° latitude), where descending dry air produces strong evaporation with minimal rainfall. Temperature and salinity are linked through density but do not directly co-vary in the way this statement implies.

Question 5

Why does a change in the ocean's freshwater budget — such as increased glacial meltwater — threaten to disrupt thermohaline circulation?

Model answer: Thermohaline circulation depends on density-driven sinking of cold, salty water in polar regions. Increased meltwater adds a large volume of fresh water to the surface, decreasing salinity and therefore density. If surface water becomes too fresh to sink even when cold, the density-driven downwelling that feeds the deep circulation weakens or stops. This can slow or redirect the global conveyor belt, altering heat transport between ocean basins and affecting regional climates.

The key link is salinity → density → vertical sinking. Brine rejection during ice formation currently creates water dense enough to sink to the ocean floor. Fresh meltwater added on top of this region stratifies the surface, preventing sinking. Thermohaline circulation is sensitive not just to temperature but to the salinity side of the density equation — which is why the freshwater budget is a critical variable in climate projections.