Ocean Salinities Show an Intensified Water Cycle; Large Salinity Changes Will Eventually Influence Ocean Circulation

The 50-year linear surface salinity trend (pss/50-years). Contours every 0.2 pss are plotted in white. Regions where the resolved linear trend is not significant at the 99% confidence level are stippled in grey. Source: Durack and Wijffels. Click to enlarge.

A new study co-authored by CSIRO (Australia) scientists Paul Durack and Dr. Susan Wijffels finds evidence that the world’s water cycle has already intensified. The stronger water cycle means arid regions have become drier and high rainfall regions wetter as atmospheric temperature increases. The study will be published in the Journal of Climate.

The surface ocean beneath rainfall-dominated regions has freshened, whereas ocean regions dominated by evaporation are saltier. The paper also confirms that surface warming of the world’s oceans over the past 50 years has penetrated into the oceans’ interior, changing deep-ocean salinity patterns.

The salinity changes reported in this study are large and have a significant impact on the ocean stratification and density structure (to be described elsewhere) and thus will eventually influence the ocean
circulation itself, especially at high latitudes where salinity is the
dynamically active variable. Thus, besides being a marker of changes in
the global hydrological cycle, salinity-driven impacts of ocean
sequestration of heat and carbon need to be examined in the context of
better projections of future climate change.

—Durack and Wijffels

The study analyzed more than 1.6 million profiles of salinity, potential temperature and neutral density from historical archives and the international Argo Program. The period of analysis extends from 1950-2008, taking care to minimize the aliasing associated with the seasonal and major global El Niño Southern Oscillation modes.

The study finds a clear link between salinity changes at the surface driven by ocean warming and changes in the ocean subsurface which follow the trajectories along which surface water travels into the ocean interior.

This is further confirmation from the global ocean that the Earth’s water cycle has accelerated. These broad-scale patterns of change are qualitatively consistent with simulations reported by the Intergovernmental Panel on Climate Change (IPCC). While such changes in salinity would be expected at the ocean surface (where about 80 per cent of surface water exchange occurs), sub-surface measurements indicate much broader, warming-driven changes are extending into the deep ocean

—Paul Durack, PhD candidate at the joint CSIRO/University of Tasmania, Quantitative Marine Science program

The ocean’s average surface temperature has risen around 0.4 ºC since 1950. As the near surface atmosphere warms it can evaporate more water from the surface ocean and move it to new regions to release it as rain and snow. Salinity patterns reflect the contrasts between ocean regions where the oceans lose water to the atmosphere and the others where it is re-deposited on the surface as salt-free rainwater.

Based on historical records and data provided by the Argo Program’s world-wide network of ocean profilers—robotic submersible buoys which record and report ocean salinity levels and temperatures to depths of two kilometers—the research was conducted by CSIRO’s Wealth from Oceans Flagship and partially funded by the Australian Climate Change Science Program. Australia’s Integrated Marine Observing System is a significant contributor to the global Argo Program.

CSIRO initiated the National Research Flagships to provide science-based solutions to Australia’s major research challenges and opportunities. The 10 Flagships form multidisciplinary teams with industry and the research community.

Resources

• Paul Durack and Susan Wijffels (2010) Fifty-year Trends in Global Ocean Salinities and Their Relationship to Broad-Scale Warming. Journal of Climate (early online release) doi: 10.1175/2010JCLI3377.1