Chapter 5. SPATIALLY-COHERENT STRUCTURES
the upwelling zones. The typical distance between the Langmuir vortices is
from a few meters to a few hundred meters.
Langmuir circulations appear to be a transient and variable process. A
hierarchy of Langmuir circulations is often observed with smaller, more
irregular, and less well-defined streaks occurring between stronger and more
widely spaced streaks. In the hierarchy of spacings between the lanes, the
small cells continually form and gradually merge into the more permanent
larger cells (Thorpe, 1992). Evidence for the adjacent streaks merging and
individual streaks terminating can also be found in the infrared imagery
shown in Figure 5-52 (two circled areas).
According to observations, the speed of near-surface current in the wind
direction is maximal in convergence zones, reaching 0.2 m s
-1 with respect to
the current speed in divergence zones. The typical speed of downwelling in
the convergence zones is of the order of several cm s
-1 near the surface
according to some investigators. At the same time, downward vertical
velocities estimated by Weller and Price (1988) appear to be even larger,
perhaps because the vertical velocity increases with depth to some point
~h/2. Interestingly, the D’Asaro et al. (1996) attempt to identify motions
below the surface as Langmuir circulations using Lagrangian tracking did
not provide clear evidence for the presence of circulatory flows.
The Langmuir circulations form within a few minutes of onset of a wind
of 3 m s
-1 or faster (Pollard, 1979). Langmuir circulations are observed
regardless of surface heating or cooling. Nevertheless, under conditions of
surface cooling (unstable stratification), the Langmuir circulations may form
at a lower wind speed. For wind speeds exceeding 3 m s
-1 , surface fluxes
(cooling or heating) do not appear to appreciably modify their strength and
form.
Under high wind speed conditions the Langmuir cells are not clearly seen
on the ocean surface because of wave breaking though they may still exist
(Leibovich, 1983). Most observations of Langmuir circulations are reported
in the wind speed range from 3 to 10 m s
-1 , although there are observations
of the Langmuir type streaks at wind speeds less than 3 m s
-1 .
Weller and Price (1988) found that Langmuir cells rapidly destroy the
surface thermal stratification in shallow diurnal mixed layers. They,
however, could not find evidence that Langmuir vortices directly participate
in mixing in the deeper parts of the oceanic mixed layer.
Craik and Leibovich (1976) developed a list that abstracts a minimum of
qualitative features than any viable theory of Langmuir circulations must be
able to reproduce:
1) A parallel system of vortices nearly aligned with the wind.
2) A means must be given by which these vortices are driven by the
wind.
373
the upwelling zones. The typical distance between the Langmuir vortices is
from a few meters to a few hundred meters.
Langmuir circulations appear to be a transient and variable process. A
hierarchy of Langmuir circulations is often observed with smaller, more
irregular, and less well-defined streaks occurring between stronger and more
widely spaced streaks. In the hierarchy of spacings between the lanes, the
small cells continually form and gradually merge into the more permanent
larger cells (Thorpe, 1992). Evidence for the adjacent streaks merging and
individual streaks terminating can also be found in the infrared imagery
shown in Figure 5-52 (two circled areas).
According to observations, the speed of near-surface current in the wind
direction is maximal in convergence zones, reaching 0.2 m s
-1 with respect to
the current speed in divergence zones. The typical speed of downwelling in
the convergence zones is of the order of several cm s
-1 near the surface
according to some investigators. At the same time, downward vertical
velocities estimated by Weller and Price (1988) appear to be even larger,
perhaps because the vertical velocity increases with depth to some point
~h/2. Interestingly, the D’Asaro et al. (1996) attempt to identify motions
below the surface as Langmuir circulations using Lagrangian tracking did
not provide clear evidence for the presence of circulatory flows.
The Langmuir circulations form within a few minutes of onset of a wind
of 3 m s
-1 or faster (Pollard, 1979). Langmuir circulations are observed
regardless of surface heating or cooling. Nevertheless, under conditions of
surface cooling (unstable stratification), the Langmuir circulations may form
at a lower wind speed. For wind speeds exceeding 3 m s
-1 , surface fluxes
(cooling or heating) do not appear to appreciably modify their strength and
form.
Under high wind speed conditions the Langmuir cells are not clearly seen
on the ocean surface because of wave breaking though they may still exist
(Leibovich, 1983). Most observations of Langmuir circulations are reported
in the wind speed range from 3 to 10 m s
-1 , although there are observations
of the Langmuir type streaks at wind speeds less than 3 m s
-1 .
Weller and Price (1988) found that Langmuir cells rapidly destroy the
surface thermal stratification in shallow diurnal mixed layers. They,
however, could not find evidence that Langmuir vortices directly participate
in mixing in the deeper parts of the oceanic mixed layer.
Craik and Leibovich (1976) developed a list that abstracts a minimum of
qualitative features than any viable theory of Langmuir circulations must be
able to reproduce:
1) A parallel system of vortices nearly aligned with the wind.
2) A means must be given by which these vortices are driven by the
wind.
373