19 November 2014

Long-Term Sea-Level Change Revisited: The role of salinity


Long-term sea-level change revisited: the role of salinity

Estimates of global mean sea-level changes are primarily driven by thermal expansion (thermosteric warming-driven ocean change) and mass contributions from the cryosphere. Regional sea-level change can also be caused by variations in ocean salinity that impact density and induce a local sea level variation. Although this “halosteric” effect is well established for short-term regional sea level variations, the impact of the halosteric effect on large-scale and long-term sea level changes has not been extensively investigated.

To investigate the role of salinity on sea level, we utilize two observed estimates of long-term (1950-2008) thermosteric and halosteric sea level changes in conjunction with a large suite of climate model simulations. We evaluate changes on successively larger scales and quantitatively compare the relative importance of salinity induced sea level changes to those resulting from temperature variations.

We find the spatial change patterns from two observational analyses yield very similar basin-scale features, although finer-scale patterns differ as a result of significant data sparsity. When the multi-model mean (MMM) is assessed, the basin-scale features are well replicated, however we note that inter-model differences are quite large. When we consider changes calculated to represent basin-scale features, we find that these changes are replicated by 22 of 26 Historical simulations, whereas the response from HistoricalNat simulations bears no resemblance to observed basin-scale estimates.

This suggests that changes found in observed estimates are due to anthropogenic influence, and that Historical simulations are reproducing the basin-scale processes that are driving observed changes. Observations and models suggest that ~25% of basin-scale sea level change can be accounted for due to the halosteric signal, and that these both mask (Atlantic) and enhance (Pacific) basin-scale warming signals.

This research provides evidence that halosteric sea level changes play a larger role than previously thought and cannot be excluded in future assessments of long-term sea level change.

Reference: Durack, P. J., S. E. Wijffels and P. J. Gleckler (2014) Long-term sea-level change revisited: the role of salinity. Environmental Research Letters, 9 (11), 114017. doi: 10.1088/1748-9326/9/11/114017

Paul J. Durack
Lawrence Livermore National Laboratory (LLNL)
Durack, PJ, SE Wijffels, and PJ Gleckler.  2014.  "Long-Term Sea-Level Change Revisited: The role of salinity."  Environmental Research Letters 9(11): 114017, doi:10.1088/1748-9326/9/11/114017.

The work of PJD and PJG from the Climate Program at Lawrence Livermore National Laboratory is a contribution to the US Department of Energy, Office of Science, Climate and Environmental Sciences Division, Regional and Global Climate Modeling Program under contract DE-AC52-07NA27344. The Australian Government Department of Environment, the Bureau of Meteorology and CSIRO through the Australian Climate Change Science Program supported the work of SEW. We thank J Durack of the University of California, San Francisco and J A Church from CSIRO, Hobart, Australia for helpful comments with early drafts of this manuscript. We acknowledge the efforts of F Fernandes in writing and maintaining the python seawater toolbox (Fernandes 2014) and D Williams, C Doutriaux and their team for developing and maintaining the UV-CDAT analysis package (Williams et al 2014). We acknowledge the sources of observed data used in this study: M Ishii and M Kimoto (Ish09) and the International Argo Program and the national programmes that contribute to it. We acknowledge the World Climate Research Programme?s Working Group on Coupled Modeling, which is responsible for CMIP, and we thank the climate modeling groups (listed in table 1) for producing and making available their model output. For CMIP the US Department of Energy?s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. The DW10 data presented in this study can be downloaded from the CSIRO Ocean Change website at www.cmar.csiro.au/oceanchange. LLNL Release #: LLNL-JRNL-653602.