Some evidence for boundary mixing in the deep ocean

Also published as: Journal of Geophysical Research 83 (1978): 1971-1979Profiles of salinity and potential temperature in the deep ocean are presented which suggest the characteristic signature of two complementary mixing processes: vertical mixing within ~50-m-thick layers at boundaries and topographic features and lateral advection and eventual smearing of these mixed layers along iopycnal surfaces. The combined effect of these two processes is often parametrically disguised as a vertical eddy diffusivity in one-dimensional models. An estimate shows that the two processes can account for all the vertical mixing in the deep ocean without any vertical diffusion in the interior.Prepared for the Office of Naval Research under Contract N00014-76-C-0197; NR 083-400 and for the National Science Foundation under Grant OCE 76-81190

Effects of variations in eddy diffusivity on property distributions in the oceans

Also published as: Journal of Marine Research 37 (1979): 515-529The hypothesis that variations in eddy diffusivity may be responsible for some of the observed distributions of oceanic scalars is explored. A gradient in eddy diffusivity affects property distributions much as would an additional velocity field from regions of high to low eddy diffusivity. In support of such an interpretation, the cross-isopycnal distribution of density is compared with an eddy diffusivity prescribed by the combination of boundary mixing and isopycnal exchange. Since the surface area available for boundary mixing varies with depth, similar variations are reflected in property distributions. For isopycnal distributions, an eddy diffusivity field inferred from the eddy potential energy field description of Dantzler (1977) is compared with the salinity distribution from the Mediterranean Outflow.Prepared for the Office of Naval Research under Contract N00014-76-C-0197; NR 083-400 and for the National Science Foundation under Grant OCE 76-81190

SOFAR float Mediterranean outflow experiment data from the second year, 1985-86

In October, 1984, the Woods Hole Oceanographic Institution SOFAR float group began a three-year-long field program to observe the low frequency currents in the Canary Basin. The principal scientific goal was to learn how advection and diffusion by these currents determine the shape and amplitude of the Mediterranean salt tongue. Fourteen floats were launched at a depth of 1100 min a cluster centered on 32°N, 24°W, and seven other floats were launched incoherently along a north/south line from 24°N to 37°N. At the same time investigators from Scripps Institution of Oceanography and the University of Rhode Island used four other SOFAR floats to tag a Meddy, a submesoscale lens of Mediterranean water. In October, 1985, seven additional floats were launched, four in three different Meddies, one of which was tracked during year 1. This report describes the second year of the floats launched in 1984 and the first year of the ones launched in 1985. Approximately 41 years of float trajectories were produced during the first two years of the experiment. One of the striking accomplishments is the successful tracking of one Meddy over two full years plus the tracking of two other Meddies during the second year.Funding was provided by the National Science Foundation under grant Numbers OCE 82-14066 and OCE 86-00055

The Community Foehn Classification Experiment

Strong winds crossing elevated terrain and descending to its lee occur over mountainous areas worldwide. Winds fulfilling these two criteria are called “foehn” in this paper although different names exist depending on region, sign of temperature change at onset, and depth of overflowing layer. They affect local weather and climate and impact society. Classification is difficult because other wind systems might be superimposed on them or share some characteristics. Additionally, no unanimously agreed-upon name, definition nor indications for such winds exist. The most trusted classifications have been performed by human experts. A classification experiment for different foehn locations in the Alps and different classifier groups addressed hitherto unanswered questions about the uncertainty of these classifications, their reproducibility and dependence on the level of expertise. One group consisted of mountain meteorology experts, the other two of Masters degree students who had taken mountain meteorology courses, and a further two of objective algorithms. Sixty periods of 48 hours were classified for foehn/no foehn at five Alpine foehn locations. The intra-human-classifier detection varies by about 10 percentage points (interquartile range). Experts and students are nearly indistinguishable. The algorithms are in the range of human classifications. One difficult case appeared twice in order to examine reproducibility of classified foehn duration, which turned out to be 50% or less. The classification dataset can now serve as a testbed for automatic classification algorithms, which - if successful - eliminate the drawbacks of manual classifications: lack of scalability and reproducibility

The dynamics of the bottom boundary layer of the deep ocean

Also published as: Proceedings of the 8th International Colloquium on Ocean Hydrodynamics, 19?7, pp. 153-164Profiles of salinity and temperature from the center of the Hatteras Abyssal Plain have a signature that is characteristic of mixing up a uniformly stratified region: a well-mixed layer above the bottom, bounded by an interface. The penetration height of the mixed-layer varies from about 10 m to 100 m and has been correlated by Armi and Millard (1976) with the one day mean velocity, inferred from current meters located above the bottom boundary layer. Here the dynamics of such layers is discussed. A model of entrainment and mixing for a flat bottom boundary layer is outlined; this model is however incomplete because we find too little known of the structure of turbulence above an Ekman layer. An alternate model is suggested by the estimate, from the correlation of penetration height with velocity of the internal Froude number of the mixed layer, F~1.7. This value indicates that the large penetration height may be due to the instability of the well-mixed layer to the formation of roll waves.Prepared for the Office of Naval Research under Contract N00014-66-C-0241; NR 083-004 and for the International Decade of Ocean Exploration of the National Science Foundation under Grant GX 29054