With a grain of salt

By Rachel Berkowitz

If you bring water to the boil in a kettle, you create a phase change from liquid to gas (steam), and the heated air around the kettle becomes saturated with moisture. The warmer the air gets, the more water vapor it can hold. The same principle applies outside of the kitchen: In a warming world, increased temperature means that the atmosphere can hold and transport more water vapor—a 7% increase in atmospheric moisture content for every degree Celsius of warming in Earth’s lower troposphere.

This affinity for moisture affects the entire water cycle, throughout the global climate system. Change is especially reflected in increased patterns of evaporation and precipitation, and a corresponding increase in ocean surface salinity, since surface salinity patterns respond to water cycle changes.

On the surface

In the past 50 years, salinity differences—the marker of the oceanic water cycle—have intensified in the upper 700 m of the ocean.

“Think of the ocean as a big rain gauge,” says John Toole, physical oceanographer at Woods Hole Oceanographic Institution. Salty regions become saltier because more water is exported through evaporation, and fresh regions become fresher because increased rainfall dilutes those regions more.

The atmosphere responds to temperature changes at a faster rate than the ocean, so ocean circulation levels variations in temperature and salinity. “As a consequence of the longer ocean timescale, and the fact that ocean salinity captures and averages all the longer timescale variability, there are coherent changes to ocean salinity patterns clearly present in the data,” explains Paul Durack of the Program for Climate Model Diagnosis and Intercomparison at Lawrence Livermore National Laboratory.

However, recent ocean salinity changes are inconsistent with those that we could expect to occur due to natural variability in the climate system, irrespective of climactic response to increased greenhouse gases. A recent Geophysical Review Letters paper by David Pierce of Scripps Institute of Oceanography and his co-authors (Durack among them), compares observed trends in ocean salinity with effects from natural solar and volcanic forcing, and from natural climate cycles such as El Niño, using 11,000 years of model simulations. The modeled temperature and salinity patterns do not replicate those recorded in the past 50 years of observations from the National Oceanographic Data Center.

But when the additional greenhouse gases generated by human activity are also applied to the climate models, the warming trends and salinity change patterns begin to replicate recent observations. Although the model has weaker peak values than the observations, the signal strength is significant when human forcing is included. Temperature and salinity are not perfectly correlated, so it is less likely that internal climate variability alone could explain joint, multi-decadal changes.

“The modern ocean has much better data coverage than existed previously,” says Durack. “Using these modern data, along with data from the historical database we really do see patterns of long-term change very clearly.” The ARGO program has profiled the ocean temperature and salinity from surface to 2 km depth since 2000 using a fleet of underwater robots; that research has provided new understanding of how today’s ocean exhibits natural climate variability. But observational coverage still limits the ability to understand what drives change patterns and whether they are coherent from year to year.

Taking things deeper

Changing the ocean’s salinity and temperature also affects its water density, which in turn plays a key role in how water circulates.

Toole and his colleagues at the Woods Hole Physical Oceanography Department observe advective signatures of temperature and salinity anomalies carried southwards from the Arctic Ocean by the Deep Western Boundary Current. The Line W project combines an array of moored instruments with ship observations near 40° N, 70° W to measure the time dependence of volume transport, temperature and salinity anomalies, and wave propagation. These measurements demonstrate deep ocean response to variability in air-sea exchanges and the ocean’s influence in global climate variability through changes in its transport of heat and freshwater.

“One interesting hypothesis is that changes in [the current’s] strength could impact where the Gulf Stream separates from continent and starts flowing offshore,” says Toole. In the last 15 years, the surface Arctic Ocean is a lot fresher than it has been in the past. For the last 20 years, subtropical regions of the North Atlantic have become saltier. However, the extreme regions of highest and lowest salinity have a direct relationship to precipitation and evaporation. Therefore, it takes a long time for surface effects to change deep ocean circulation patterns.

The Pierce and Durack study suggests a large change that has not yet reorganized how the ocean works, but that will have ongoing influence as greenhouse gas emissions continue. Human-driven CO₂ forcing has clearly contributed to ocean surface and upper layer changes, but decomposing the processes by which these forcing agents lead to salinity changes, their rates of change, and variability in long-term effects is the next step in ongoing research to better understand Earth’s complex climate variability and change.