Mixing and Entrainment in the Red Sea Outflow Plume. Part II: Turbulence Characteristics

Abstract

Abstract South of the Strait of Bab el Mandeb, saline Red Sea Water flows downslope into the Gulf of Aden mainly along the narrow 130-km-long “Northern Channel” (NC) and the shorter and wider “Southern Channel” (SC). In the NC, the Red Sea plume simultaneously exhibited weak entrainment into a 35–120-m-thick, weakly stratified bottom layer while a 35–285-m-thick interfacial layer above showed signs of vigorous mixing, overturns up to 30 m thick, and extensive zones of gradient Richardson numbers below 1/4. Turbulent overturning scales, or Thorpe scales, are extracted from regular CTD profiles and equated to Ozmidov scales. On this basis, interfacial mixing is quantified in terms of estimated turbulent dissipation rates, vertical turbulent salt flux, and interfacial stress. Even though these estimates are subject to significant uncertainty, they demonstrate the intensity of mixing during strong winter outflow in terms of eddy diffusivities Kρ on the order of 10−2 m2 s−1. The large Kρ occur in strong stratification such that vertical turbulent salt fluxes are also large. Along the NC, relative maxima of Kρ correspond to maxima in the bulk Froude number. Direct short-term measurements of the Reynolds stress just above the seafloor at two locations, one in the NC and one in the SC, allow comparisons of the bottom stress τb with the interfacial turbulent stress τi. The ratio τi/τb shows large scatter in a small sample, with maximum values on the order of 1. An appendix outlines procedures of making and reducing lowered acoustic Doppler current profiler measurements optimized for observing descending plumes.