On the variability of 18 Degree Water formation as observed from moored instruments at 55°W

The analysis of hydrographic time series from moored sensors from the eastern end of the Sargasso Sea has shown that in the late 1980s the Subtropical Mode Water was dominated by high temperatures. The 18 Degree Water observed during this period was warmer and saltier than the definition given by Worthington, with an increase in values over the observation period. Although severe winter conditions in 1988 led to the formation of colder types of 18 Degree Water at the eastern part of the Sargasso Sea, they were not incorporated in the subtropical gyre. Simultaneous current meter measurements indicated eastward flow within the “southern recirculation gyre” during spring that might have been responsible for the fact that these cold Mode Water types have not been entrained into the gyre. The meteorological forcing fields show considerable interannual variability that is, in part, mimicked by changes in the 18 Degree Water properties. Due to the strong influence of the advection field on the 18 Degree Water properties, the relationship between the surface forcing and 18 Degree Water properties remains qualitative. The continuing influence of further mild winters after 1989, however, is manifested in rising temperatures within the Mode Water range.