Analysis of mesoscale thermoclinicity with an example from the tropical thermocline during GATE

J. D. Woods, Peter J Minnett

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Abstract

A method for the extension of isopycnal analysis to the mesoscale is described and illustrated with a map of the variation of vertical separation between intersecting isothermal and isopycnal surfaces at a mean depth of 41 m in the tropical Atlantic. Data were collected from a Brown conductivity-temperature-depth recorder (CTD) mounted in a Hermes Batfish that undulated with vertical speed of approximately 1 m s-1 while being towed at approximately 4 m s-1. Used in this way, the CTD sensors exhibit significant errors arising from thermal lags that were only partially corrected during data processing after the cruise. Estimates of the residual errors due to these lags suggest that the uncertainty in the spacing between isothermal and isopycnal surfaces would be less than ± 15 cm, which is comparable with the standard deviation of the measured spacing of the surfaces selected for purposes of illustration. However, evidence of spatial coherence of features in adjacent legs of the map suggests that the distributions are environmental rather than an instrumental artifact. To be consistent with this conclusion, the uncertainty in the measured spacing would have to be less than ±5 cm. Further evidence in favour of this view is provided by a study of the distribution of multiple incidences of the chosen density surface, which are believed to represent encounters with billows. These occur at locations of enhanced horizontal gradient of static stability, i.e. of enhanced vertical geostrophic current shear. The major feature in the map is a mesoscale tongue of disturbed water in which the spacing between the isothermal and isopycnal surfaces fluctuates by more than 1 m over horizontal distances of less than 500 m. Billows are encountered 13 times more frequently in this disturbed tongue than in the surrounding undisturbed water. Mesoscale fluctuations of thermoclinicity limit the accuracy of internal wave investigations based on measurements of temperature or sound speed distributions, but they offer a possible signal for the investigation of turbulent motions on scales where internal waves dominate the velocity fluctuations in current meter records. The technique of isopycnal analysis described in this paper is being used to analyse this signal as recorded in the large data set (comprising over 10,000 Batfish-CTD profiles) collected by R.R.S. Discovery during GATE.

Original languageEnglish (US)
Pages (from-to)85-96
Number of pages12
JournalDeep Sea Research Part A, Oceanographic Research Papers
Volume26
Issue number1
DOIs
StatePublished - 1979
Externally publishedYes

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