Sea Surface Salinity Change since 1950: Internal Variability versus Anthropogenic Forcing

Using an eastern tropical Pacific pacemaker experiment called the Pacific Ocean-Global Atmosphere (POGA) run, this study investigated the internal variability in sea surface salinity (SSS) and its impacts on the assessment of long-term trends. By constraining the eastern tropical Pacific sea surface temperature variability with observations, the POGA experiment successfully simulated the observed variability of SSS. The long-term trend in POGA SSS shows a general pattern of salty regions becoming saltier (e.g., the northern Atlantic) and fresh regions becoming fresher, which agrees with previous studies. The 1950-2012 long-term trend in SSS is modulated by the internal variability associated with the interdecadal Pacific oscillation (IPO). Due to this variability, there are some regional discrepancies in the SSS 1950-2012 long-term change between POGA and the free-running simulation forced with historical radiative forcing, especially for the western tropical Pacific and southeastern Indian Ocean. Our analysis shows that the tropical Pacific cooling and intensified Walker circulation caused the SSS to increase in the western tropical Pacific and decrease in the southeastern Indian Ocean during the 20-yr period of 1993-2012. This decadal variability has led to large uncertainties in the estimation of radiative-forced trends on a regional scale. For the 63-yr period of 1950-2012, the IPO caused an offset of similar to 40% in the radiative-forced SSS trend in the western tropical Pacific and similar to 170% enhancement in the trend in the southeastern Indian Ocean. Understanding and quantifying the contribution of internal variability to SSS trends helps improve the skill for estimates and prediction of salinity/water cycle changes.

Automated summary

The POGA experiment successfully simulated the observed variability of sea surface salinity (SSS) in the eastern tropical Pacific. The long-term trend in SSS shows salty regions becoming saltier and fresh regions becoming fresher, modulated by the interdecadal Pacific oscillation (IPO). Decadal variability in the tropical Pacific has led to regional discrepancies in the estimation of radiative-forced trends, highlighting the importance of understanding internal variability in SSS trends.