IDENTIFYING OVERTURNS IN CTD PROFILES

The authors purpose a scheme to test whether inversions in CTD density profiles are caused by overturning motions (from which mixing rates m ay be inferred) or by measurement noise. Following a common practice, possible overturning regions are found by comparing the observed profi le rho(z) and an imaginary profile <(rho)over cap (z)> constructed by reordering rho(z) to make it gravitationally stable. The resulting ''r eordering regions'' are subjected to two tests. The ''Thorpe fluctuati on'' profils rho'(z) = rho(z) - <(rho)over cap (z)> is examined for '' runs'' of adjacent positive or negative values. The probability densit y 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 adequa te resolution of overturns, taking into account both CTD characteristi cs and local hydrographic properties. Temperature and salinity covaria tions with respect to density are screened for systematic CTD errors s uch as those caused by time-response mismatches in temperature and con ductivity sensors. Such errors may occur as the CTD passes through wat er-mass boundaries, for example, in interleaving regions. Resultant sp urious inversions are avoided by the requirement of tight relationship s between rho, T, and S within reordering regions. The tests are calib rated with examples from coastal and deep-sea environments. The result s suggest that a CTD may resolve overturns in coastal environments whe re mixing and stratification are large but that noise a will prevent o verturn detection for typical CTD resolution in the weekly mixed, weak ly stratified, deep sea.