Diagnosing Climate Change and Ocean Ventilation Using Hydrographic Data

Changes in atmospheric forcing can affect the subsurface water column of the ocean by three different mechanisms. First, warmed mixed-layer water that is subducted into the ocean interior will cause subsurface warming; second, the subducted surface water can be freshened through changes in evaporation and precipitation; and third, the properties at a given depth may be changed by the vertical displacement of isotherms and isohalines without changes of water masses. These vertical displacements of the water column can be caused either by changes in the rates of renewal of water masses or by dynamical changes (such as changes in wind stress). A method for analysing the subsurface temporal changes in hydrographic data is described in terms of these three processes: “pure warming,” “pure freshening,” and “pure heave.” Linear relations are derived for the relative strength of each process in terms of the observed changes of potential temperature and salinity in two different coordinate frames: (i) constant density surfaces, and (ii) isobaric surfaces. Inverse methods are applied to three realizations of the SCORPIO section at 43°S in the Tasman Sea. These sections were obtained in 1967, and in the austral winter and summer of 1989 and 1990, respectively. This data is used to explore the relative strengths of surface warming, surface freshening, and heave of the water column. The six-month differences for this region show small changes in Sub-Antarctic mode water (SAMW) and are not characterized by any one process, whereas below the mode waters the observed differences are well described by the heave process. In contrast, the 23-year differences show significant changes in the properties of the water that flows into the Tasman Sea: SAMW (300-700 db) is well described by pure warming of near- surface waters, while the changes observed at the depth of the salinity minimum are consistent with pure freshening. The observed changes in the interior of the ocean between adjacent seasons do not exhibit significant changes of water masses, consistent with the distance of this section from the outcrop region of the density surfaces of interest. For the 23-year differences, changed surface waters subducted into the ocean interior have sufficient time to influence the temperature-salinity correlations. The skill of our approach in discriminating between short-term changes (almost exclusively heave) and long-term changes associated with the subduction of changed surface waters is particularly encouraging. Although the observed changes could equally well be natural variability, they are qualitatively consistent with coupled numerical models of climate change in which surface waters are warmed and increased precipitation occurs south of the Sub-Antarctic Front.