EVIDENCE OF DECADAL-SCALE CHANGES IN GYRE CLOSURE
Evidence of decadal-scale changes in and around the maritime continent is shown in Fig. 1 from Wajsowicz (1999a). The SEC has a salinity maximum between 100m and 300m, and the variation in salinity averaged over these depths along 137｡? from 1965 to 1995 is plotted in Fig. 1a. Features to note are the variation in the northern extent of the salinity contours around 4｡?, and the trend towards a reduced salinity between 2｡? and 4｡? over the last 30 years. These suggest variations in the latitude of the northern extent of the SEC retroflection; the axis of the retroflection has shifted southwards with time. Fig. 1b shows the change in salinity averaged between 100m and 300m from the epoch 1966-1980 to the epoch 1981-1995. The western equatorial Pacific has become fresher, as has the Halmahera, Seram, Banada and southern Celebes Seas, and northwestern Australian shelf. In contrast, the entrance to the Celebes Sea, the Maluku Sea, and the eastern Indian Ocean over the throughflow exit latitudes, have become saltier. The salinity data were obtained from the historical profile database at the National Oceanographic Data Center (NODC), Washington D. C. Corresponding precipitation minus evaporation data show an increase in fresh-water flux with time over the western equatorial Pacific, but as the plots in Fig. 1 are for subsurface salinity, the suggestion is that the salinity differences are due to circulation shifts.
Fig. 1 Salinity averaged between 100m and 300m along 137°E, is contoured as function of time and latitude in (a). The difference in salinity averaged between 100m and 300m between the epoch 1981-1995 and the epoch 1966-1980 for the maritime continent is mapped in (b).
Sverdrup streamfunction patterns derived from Hellerman & Rosenstein (1983) wind stress climatology, and from European Centre for Medium-range Weather Forecasting ECMWF 1987-1995 mean wind stresses are shown in Fig.2. The former shows a throughflow fed in part by the Mindanao Current and in part by the SEC, and the latter shows a throughflow fed wholly by the SEC. Although these maps were produced under the assumption of linear dynamics, nonlinear dynamics and the inclusion of the Halmahera and Mindanao Eddies do not alter a fundamental difference.
