8.6: Salinity over Short Timescales

At shorter timescales—weeks to decades—oceanographers have a much clearer picture of the factors that change salinity in the ocean. For much of the ocean, two processes dominate: precipitation and evaporation. Because these processes occur at the surface of the ocean, they have their greatest effect on sea surface salinity, designated as SSS.

Unlike changes over geologic timescales, changes in SSS don’t involve addition or removal of salts. Salinity varies by addition or removal of water at the ocean’s surface. Precipitation and evaporation dilute and concentrate, respectively, the salts in seawater. Within any span of time, the ocean may experience both precipitation and evaporation. For this reason, oceanographers (and meteorologists, as it turns out) express the change in SSS as the difference between these two processes, or

∆ SSS = E – P

(Eq. 11.3)

where SSS is sea surface salinity, E is evaporation, and P is precipitation.

Now, here’s where it gets interesting. We can imagine three different scenarios:

E > P

E < P, or

E = P.

What happens to SSS in each of these?

If E > P, then SSS will increase.

If E < P, then SSS will decrease.

If E = P, then SSS will remain the same.

From this analysis, we may predict that differences in rates of precipitation and evaporation control surface salinity at a given location in the ocean. Observational data support this prediction. Recently compiled data on ocean salinity measured from ships, buoys, Argo floats, and satellite sensors demonstrate that the “freshest” regions of the ocean occur where precipitation dominates—that is, where E < P. Evaporation dominates in the saltiest regions, where E > P.

Of course, freshwater runoff modifies ocean salinity as well. For example, the Amazon River—the world’s largest river by volume of discharge at about 55 million gallons per second (e.g., Giffard et al. 2019)—lowers SSS to a distance some 100 miles out to sea. Meltwater from glaciers and ice caps can decrease SSS too, though these effects tend to be highly localized.

Seasonal changes in sea ice extent also modify SSS. When seawater freezes in fall and winter (at a temperature of about -2°C), the salts come out of solution as brine—super-salty seawater. This process, called brine rejection, increases SSS. Of course, when sea ice melts in spring and summer, it releases freshwater into the surrounding ocean, lowering SSS.

Hydrothermal vents play a role in salinity variations, though their effects are not well established. Circulation of water within the region beneath the seafloor—the subseafloor environment—removes some elements and adds others. Black smokers and white smokers provide visual evidence of precipitation of elements as the warmer vent fluids interact with the cooler surrounding seawater. Seafloor observatories (Chapter 4) have begun to reveal the extent to which hydrothermal vents contribute heat, salts, and gases to the water column. These new findings suggest a more dynamic and significant role for vents than once thought (e.g., Spietz et al. 2018; Seyfried et al. 2022; Evans et al. 2023).

Finally, SSS data over the past half century reveal trends associated with a warming climate. Because warming amplifies certain climate signals, wetter regions have been getting wetter and drier areas have been getting drier. Ocean salinities are decreasing and increasing, respectively, in response to these changes (e.g., Durack and Wijffels 2010).

Though we’ve focused on the surface here, changes in SSS affect the entire ocean. Increases in surface salinity cause an increase in seawater density. And if surface waters become more dense than the waters beneath them, the surface waters sink, in some cases, all the way to the bottom. These sinking motions contribute to the abyssal circulation of the ocean.