Temperature and salinity of ocean waters

The governing concept is the oceanic salt budget, in which salinity reflects the combined effects of evaporation, precipitation, river inflow, ice processes, local circulation, and exchange with neighbouring water masses. Strong exchange tends to dilute an unusually salty basin or redistribute its salt, but it does not automatically remove all salinity. A sea may still remain highly saline if evaporation is intense, freshwater input is small, or the incoming water is itself saline. Hence option A is correct: exchange is important, but the complete balance must be examined. Option B uses an absolute and false claim, option C denies a direct relationship, and option D reduces a multi-factor process to depth alone.

The governing concept is the temperature-salinity, or T-S, identity of an ocean water mass. Water formed in a particular source region acquires a characteristic combination of temperature and salinity through surface heating or cooling, evaporation, precipitation, ice formation, and mixing. Because this combination influences density and can persist while the mass is transported, oceanographers compare T-S observations at different locations to identify the water mass and infer its origin and pathway. Thus option A is correct. The signature does not mean the water is soil, does not remove density, and does not make currents impossible; instead, it provides evidence of transport by circulation.

The governing concept is stable density stratification. Warm water is less dense than cold water, and low salinity also lowers density, so the surface layer can remain strongly buoyant above colder, saltier deep water. If wind and other mixing forces are weak, this density contrast suppresses vertical turbulence and reduces communication between the layers. As a result, nutrients stored at depth may reach the sunlit surface more slowly, while oxygen and other gases may also be exchanged less efficiently. Option A is correct. Option B reverses the effect, and options C and D describe processes that do not follow from weak ocean mixing.

The governing concept is seawater density, which is controlled mainly by temperature and salinity. Higher salinity normally makes water denser, while cooling also increases density. However, exceptionally warm surface water expands and becomes less dense; this thermal reduction in density can offset the density increase caused by high salinity. If the combined density remains too low, buoyancy prevents or weakens sinking. Thus option A is correct. Options B and D describe cooling, with D adding salinity, so both generally favour denser water and stronger sinking. Option C also tends to increase the salinity and density of nearby water when salt is rejected during freezing.

Nutrients such as nitrate and phosphate can accumulate in deep water. Mixing or upwelling can bring them into the sunlit surface layer, where phytoplankton use them for growth. If stratification limits this upward supply, phytoplankton growth—and therefore marine productivity—can be limited. Thus, option A is correct; the size of the effect depends on local conditions.

Cooling generally increases seawater density, while lowering salinity generally decreases it. When both occur together, their effects oppose one another, so the net density change depends on their relative size and the water’s conditions. Neither variable alone is enough to determine the result. Option A is correct.

High surface temperatures can make the upper water less dense than the cooler water below, strengthening stratification. The thermocline is the layer where temperature changes rapidly with depth, and this density structure can resist vertical exchange. Weak winds provide less stirring to overcome it, so vertical mixing can remain limited. Therefore, option A is correct.

Melting polar ice adds freshwater to nearby seawater, which lowers salinity. The meltwater is cold, so nearby surface water may also show relatively low temperatures. Thus, low temperature together with low salinity can be a clue to ice melt, making option B the best choice. This pattern is not conclusive by itself, because other processes can also affect temperature and salinity.

A T–S diagram plots water temperature against salinity. If a water mass retains a recognisable combination of these properties as it moves, scientists can compare measurements from different places with its characteristic signature and infer its path. Mixing and exchange can gradually alter that signature, but it may remain useful for tracing the water mass. Therefore, option A is correct.

A warm ocean surface can transfer heat to the atmosphere and increase evaporation. Lower surface salinity makes seawater less dense and can strengthen stratification, which may limit vertical mixing. These temperature- and salinity-related effects can therefore influence exchanges between the upper ocean and atmosphere. Option A is correct; the other choices either describe unrealistic conditions or deny established physical relationships.