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 isotherm s and isohalines without changes of water masses. These vertical displ acements of the water column can be caused either by changes in the ra tes of renewal of water masses or by dynamical changes (such as change s in wind stress). A method for analysing the subsurface temporal chan ges in hydrographic data is described in terms of these three processe s: ''pure warming,'' ''pure freshening,'' and ''pure heave.'' Linear r elations are derived for the relative strength of each process in term s 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 o f the SCORPIO section at 43-degrees-S in the Tasman Sea. These section s were obtained in 1967, and in the austral winter and summer of 1989 and 1990, respectively. This data is used to explore the relative stre ngths of surface warming, surface freshening, and heave of the water c olumn. 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 we ll described by the heave process. In contrast, the 23-year difference s show significant changes in the properties of the water that flows i nto the Tasman Sea: SAMW (300-700 db) is well described by pure warmin g of near-surface waters, while the changes observed at the depth of t he salinity minimum are consistent with pure freshening. The observed changes in the interior of the ocean between adjacent seasons do not e xhibit significant changes of water masses, consistent with the distan ce of this section from the outcrop region of the density surfaces of interest. For the 23-year differences, changed surface waters subducte d into the ocean interior have sufficient time to influence the temper ature-salinity correlations. The skill of our approach in discriminati ng between short-term changes (almost exclusively heave) and long-term changes associated with the subduction of changed surface waters is p articularly encouraging. Although the observed changes could equally w ell be natural variability, they are qualitatively consistent with cou pled numerical models of climate change in which surface waters are wa rmed and increased precipitation occurs south of the Sub-Antarctic Fro nt.