Identifying Overturns in CTD Profiles

Abstract

The authors propose a scheme to test whether inversions in CTD density profiles are caused by overturning motions (from which mixing rates may be inferred) or by measurement noise. Following a common practice, possible overturning regions are found by comparing the observed profile ?(z) and an imaginary profile ??(z) constructed by reordering ?(z) to make. it gravitationally stable. The resulting ?reordering regions? are subjected to two tests. ? The ?Thorpe fluctuation? profile ??(z) = ?(z) ? ??(z) is examined for ?runs? of adjacent positive or negative values. The probability density function (PDF) of the run length is compared with the corresponding PDF of random noise. This yields a threshold value for rms run length within individual reordering regions that must be exceeded for adequate resolution of overturns, taking into account both CTD characteristics and local hydrographic properties. ? Temperature and salinity covariations with respect to density are screened for systematic CTD errors such as those caused by time-response mismatches in temperature and conductivity sensors. Such errors may occur as the CTD passes through water-mass boundaries, for example, in interleaving regions. Resultant spurious inversions are avoided by the requirement of tight relationships between ?, T, and S within reordering regions. The tests are calibrated with examples from coastal and deep-sea environments. The results suggest that a CTD may resolve overturns in coastal environments where mixing and stratification are large but that noise will prevent overturn detection for typical CTD resolution in the weakly mixed, weakly stratified, deep sea.