LINEAR-STABILITY ANALYSIS OF SALT FINGERS WITH SURFACE EVAPORATION ORWARMING

Oceanic observations [Atmos. Ocean 29, 340 (1991)] have revealed small -scale thermohaline plumes near the surface of a calm sea under warmin g conditions. The stratification was favorable for the double-diffusiv e salt finger instability, though a previously unreported up-down asym metry was found in which narrow downward cells are balanced by a broad er, weaker upwelling. The scales of the thermal structures are consist ent with asymmetric hexagonal salt-finger modes [J. Phys. Oceanogr. 24 , 855 (1994)], but no selection mechanism for the asymmetry has previo usly been identified. This paper explores the influence of nonlinear p rofiles of temperature and salinity, as might arise due to surface eva poration or warming, on the linear stability problem in a salt-fingeri ng regime. Three models are considered. In the first, a sharp, nonline ar solute-concentration gradient is applied at the upper boundary, as might arise by surface evaporation. A Benard mode appears, driven by t he destabilizing density gradient in the thin boundary layer and influ encing motion only within the boundary-layer thickness. In the second model, a weak salinity gradient is introduced below the boundary layer ; double-diffusive bulk modes influence the motion across the entire f luid. Nonlinear interaction of the boundary layer and bulk modes provi des a mechanism for maintaining salt fingers with up-down asymmetry. T he third model contains a large temperature gradient at the surface, a s might arise from warming by solar radiation, overlying a quasi-isoth ermal region above a region of moderate gradient. The largest-growth m odes are found to be salt fingers that extend throughout the middle re gion and disappear in the top and bottom regions. This vertical struct ure is close to that of the asymmetric salt fingers described in Osbor n [Atmos. Ocean 29, 340 (1991)]. The differing length scales of the re gions impress an up-down asymmetry on plumes; this is expected to yiel d a hexagonal pattern at the onset. (C) 1996 American Institute of Phy sics.