| Summary: | Subtropical Mode Water (STMW) is a water mass formed in winter by convective mixing on the equatorward side of western boundary currents in the subtropical gyres. After the return of the seasonal stratification in spring, it is found at the stratification minimum between the seasonal and main pycnoclines. By characterizing STMW primarily at the density gradient minimum, previous studies were limited in their ability to describe STMW properties over large temporal and spatial scales. Rather than using a density-based characterization, the North Atlantic STMW layer was identified here by its much smaller temperature gradient relative to the more stratified seasonal and main thermoclines. By using a temperature-based characterization, this study was able to take advantage of the large volume of XBT data collected between 1968 and 1988 to examine STMW properties on large spatial and long temporal scales. There was considerable spatial and temporal variability in the renewal of the STMW layer's vertical homogeneity from 1968 to 1988. Basin-wide renewal occurred in 1969, 1970, 1977, 1978, 1981, and 1985, with more localized renewal, usually east of 55$\sp\circ$W, in other years. While STMW is nearly vertically homogeneous upon renewal by convective mixing, the temperature gradient through the layer increases after renewal. The annual rate of increase in the temperature gradient in the year following renewal is $\sim$5-$6\times10\sp{-4}\sp\circ$C per 100 m per day, while the interannual rate of increase following winters with no renewal of the STMW layer is N${\sim}2.0\times10\sp{-4}\sp\circ$C per 100 m per day. The STMW layer which is the remnant of the previous winter's convective activity is typically found between 175 and 450 m, has an average temperature near 18$\sp\circ$C, and has a mean temperature gradient of 0.5$\sp\circ$C per 100 m. STMW layers in years following winter renewal are 25 m shallower, colder, and less vertically stratified, but more horizontally stratified, than those STMW layers following winters of no renewal.
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