University Of Tasmania

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Global ocean salinity : a climate change diagnostic?

posted on 2023-05-26, 04:19 authored by Durack, PJ
This thesis aims to use historical and recent global ocean observations to ascertain changes to the water cycle, expressed by ocean salinity changes. The analysis is dependent on over 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, and takes care to minimise the aliasing associated with the seasonal and major global El Nino-Southern Oscillation modes. The thesis is structured in 5 chapters. Chapter 1 introduces the reader to ocean observations and observed changes over the 20th and early 21st century. It provides an introduction to the global water cycle and the ocean's role in its operation and the anticipated future, as well as observed changes in response to climate change. Chapter 2 presents new estimates of global ocean salinity changes in dual pressure and density analyses and attempts to tease out primary processes driving these changes. Chapter 3 focuses on the pattern of sea surface salinity changes, and compares these to current state-of-the-art climate models which comprise the Coupled Model Intercomparison Project phase 3 (CMIP3) database. A comparison of the spatial patterns of change and the explicit rates of salinity pattern amplification in the 20th century realisations (20C3M) is made against the new observational estimates of change (Chapter 2). Chapter 4 concentrates on the three dimensional changes expressed by these new estimates; by combining the concurrent temperature analysis, it provides new coherent estimates of regional (and global) sea level rise as expressed by halosteric (salinity-) and thermosteric (temperature-driven) changes. Chapter 5 summarises these results, reviews key new findings and suggests areas for future research. This research has uncovered large, robust and spatially coherent multi-decadal linear trends in salinity to 1800 dbar depth. Salinity increases at the surface are found in evaporation-dominated regions and freshening in precipitation-dominated regions. This spatial pattern of change strongly resembles the climatological mean sea surface salinity field, consistent with an amplification of the global water cycle. Recorded changes in the ocean subsurface suggest that subduction and circulation by the ocean's mean flow of surface salinity and temperature anomalies are driving regional changes on the 50-year timescales. A robust amplification of the mean surface salinity pattern of 8% is found globally, with 5-9% apparent in each of the 3 key independently analysed ocean basins. 20th century realisations (20C3M) from the CMIP3 model suite support the broad-zonal relationship between amplified patterns of surface freshwater flux driving an amplified pattern of ocean surface salinity. The warming response represented in realistic (when compared to observed estimates) 20th century realisations appear similar in their patterns to those of 21st century projected future realisations (these projections are strongly forced by greenhouse gases). New observed surface salinity change estimates suggest a pattern amplification of 8% (`16±7% K^-1`; associated with a 0.5K global surface temperature increase) has been experienced for 1950-2000. Using modelled relationships this equates to an inferred change of 4% (`8±5% K^-1`) for evaporation minus precipitation (E-P) replicating the theoretical response described by the Clausius-Clapeyron relation. While there is a large spread in the CMIP3 20th century comparison results, the ensemble best-estimate tends to underestimate observed salinity changes by 50%, with `%K^-1` rates also found to be similar in projected 21st century realisations. Considering the full three dimensional salinity and temperature changes yields new quantitative estimates of steric sea level rise for 1950-2000. Thermosteric linear trend estimates for 0-700m replicate the rates expressed by well documented time series, however, provide new insights into the spatial pattern of these counteracting steric contributions. Halosteric estimates indicate large contractions (enhanced salinity) in the Atlantic, with corresponding expansions (freshening) occurring in the Pacific and a near neutral globally integrated response. When considering the total steric changes, the Atlantic (the most dynamically changing basin over the analysis period) undergoes strong warming (expansion) and strong enhanced salinity (contraction) with these signals cancelling to provide a muted total steric response. These new estimates of ocean changes for 1950-2000 provide a globally coherent and stringent target for coupled modelling systems when undertaking 20th century hindcast simulations. A better understanding of observed changes will aid in the evaluation of the upcoming CMIP5 (phase 5) database, providing a benchmark by which to assess the poorly known water cycle intensification and ocean changes expressed in the 20th century and beyond.


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