Oceanic microstructure observed near Bermuda using a towed sensor

Abstract

Many hypotheses have been advanced to explain the formation of mixed layers in the ocean; the salt finger type of double-diffusive convection, in particular, has received much attention. Because of their uniquely ordered nature, salt fingers should be readily identifiable in the deep thermocline, if in fact they exist there. A relatively limited experiment could thus produce a definitive evaluation of the importance of salt finger convection in the ocean, at least in certain places and at certain times. Such an evaluation, which would help to direct future work on oceanic microstructure, was the primary objective of this thesis. A secondary and more general objective was to measure the intermittency of mixing events in the ocean; and also, by measuring the RMS gradients of temperature and salinity in such events, to evaluate directly the intensity of vertical mixing. Since a horizontally-towed sensor is essential if salt fingers are to be observed directly, a new instrument has been designed and built which can resolve fractional-centimeter structures of electrical conductivity while being towed at speeds of several knots. The design of this instrument is described in this report. The instrument has been towed in deep water near Bermuda. It is believed that salt fingers were observed in the main thermocline on several occasions; but they were so rare as to be negligible in the total vertical mixing. An analysis of one such possible salt finger event is presented. Numerous other small-scale fluctuations of electrical conductivity were observed, which can be reconciled only with a turbulent model. Intermittency statistics for these events are presented. The eddy diffusivity has been calculated from the data to be approxima tely 0.075 cm2/sec.