Stability Of Mixing Layers Research Articles

Evaluation of thermal and dynamic impacts of summer dust aerosols on the Red Sea

The seasonal response of upper ocean processes in the Red Sea to summer-time dust aerosol perturbations is investigated using an uncoupled regional ocean model. We find that the upper limit response is highly sensitive to dust-induced reductions in radiative fluxes. Sea surface cooling of -1°C and -2°C is predicted in the northern and southern regions, respectively. This cooling is associated with a net radiation reduction of -40Wm−2 and-90Wm−2 over the northern and southern regions, respectively. Larger cooling occurs below the mixed layer at 75m in autumn, -1.2°C (north) and -1.9°C (south). SSTs adjust more rapidly (c. 30 days) than the subsurface temperatures (seasonal timescales), due to stronger stratification and increased mixed layer stability inhibiting the extent of vertical mixing. The basin average annual heat flux reverses sign and becomes positive, +4.2 Wm−2 (as compared to observed estimates -17.3Wm−2) indicating a small gain of heat from the atmosphere. When we consider missing feedbacks from atmospheric processes in our uncoupled experiment, we postulate that the magnitude of cooling and the timescales for adjustment will be much less, and that the annual heat flux will not reverse sign but nevertheless be reduced as a result of dust perturbations. While our study highlights the importance of considering coupled ocean-atmosphere processes on the net surface energy flux in dust perturbation studies, the results of our uncoupled dust experiment still provide an upper limit estimate of the response of the upper ocean to dust-induced radiative forcing perturbations. This article is protected by copyright. All rights reserved.

Polar Water Column Stability

Abstract An expression is derived for the surface salt input needed to induce complete convective overturning of a polar water column consisting of 1) a layer of sea ice, 2) a freezing temperature mixed layer, 3) a pycnocline with linearly varying temperature and salinity, and 4) deep water with fixed temperature and salinity. This quantity has been termed the bulk stability by Martinson. The bulk stability is found to consist of three components. The first two make up Martinson’s salt deficit and are the salt input needed to increase the density of the mixed layer and the pycnocline layer to that of the deep water (the mixed layer stability and pycnocline layer stability, respectively). The third component is Martinson’s thermal barrier: the potential for pycnocline heat to melt ice, reducing the surface salinity. It is found that when the pycnocline density gradient due to temperature offsets more than one half of that due to salinity, the pycnocline layer stability is negative. Consequently, it is poss...

TWO-DIMENSIONAL SPATIALLY DEVELOPING MIXING LAYERS

Two-dimensional, incompressible, spatially developing mixing layer simulations are per formed with two classes of perturbations applied at the inlet boundary: (1) combinations of discrete modes from linear stability theory, and (2) a broad spectrum of modes derived from experimentally measured velocity spectra. The discrete modes from linear theory are obtained by solving the Orr-Sommerfeld equation, and linear stability analysis is used to investigate the effect of Reynolds number on the stability of mixing layers. Two-point spatial velocity and autocorrelations are used to estimate the size and lifetime of the resulting coherent structures and to explore possible feedback effects. It is shown that by forcing with a broad spectrum of modes derived from an experimental energy spectrum, many experimentally observed phenomena can be reproduced by a two-dimensional simulation.

Linear stability of the compressible reacting mixing layer

We investigate the linear stability of mixing layers with special emphasis on the effects of heat release and compressibility. Multiple supersonic modes exist for both nonreacting and reacting flows when the disturbance phase velocity is supersonic relative to the freestream. These supersonic modes become less unstable with increasing Mach number but more unstable with increasing heat release. The most unstable supersonic modes are three-dimensional (oblique) for nonreacting flows but two-dimensional for reacting flows.

Deep and shallow water characteristics of mixed layer response to momentum and buoyancy fluxes off Bombay, west coast of India, during winter

Relative roles of vertical fluxes of momentum by wind and buoyancy in the turbulent kinetic energy (TKE) transfer across the air-sea interface and resulting short term (∼72h responses in mixed layer (ML) variability were examined at deep (1600 m) and shallow (80 m) sites. One of these sites was on the continental slope and the other on the shelf off Bombay, west coast of India (based on time series data collected during winter (17–27 January 1986). Under mild wind production (0–0.22 W m −2) ML responded most of the time to convectively driven turbulence (downward buoyancy flux, Bo ranging from −3 to −4 × 10 −7W kg −1) at the deep slope site, whereas under increased wind turbulence at the shallow shelf site, mixed responses of ML were effected from vertical fluxes of momentum (up to 1 W m −2) and buoyancy (−2 to −3 × 10 −7W kg −1). Variability of the mixed layer stability ratio, D/L, suggested convective deepening of ML to be moderate (up to 40) at the deep site and weak (up to 10) at the shallow site. Correlations between wind stress parameter U* 3, Bo and MLD (mixed layer depth) were poor at the slope site and better at the shelf site. One-dimensional simulation of MLT (mixed layer temperature) and MLD resulted in substantial disagreements with observations (RMS departures of 0.4°C and 27 m, respectively) at the deep site while marked agreements (RMS departures of 0.2°C and 6 m, respectively) occurred at the shallow site, owing to the contrasts in complex advective influences between the two locations. The inclusion of advection correction in simulations improved agreements.