Annual cycle of the density field in Bass Strait

Abstract

A study of the density field in Bass Strait has revealed an annual cycle ranging from well-mixed to vertically stratified conditions. Historical data of σ_t values have been converted into the potential energy, and monthly averages have been computed to show the seasonal variation of the stratification. Bass Strait is well mixed in winter but has some stratification during the rest of the year, largely controlled by surface heating and tidal stirring. The central part of Bass Strait is strongly stratified in late summer. The simple energy-budget model leading to the 'H/u³ criterion,' where H is the depth and u the tidal speed calculated from a numerical model, has been compared with the data and is successful in describing the onset of stratification in spring but not its break up in autumn. Contours of log ₁₀ H/u³ show maximum values in the same area as those of strongest stratification in the non-winter months.