Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Observations of Turbulent Kinetic Energy Dissipation in the Surface Mixed
Layer of the Baltic Sea Under Varying Forcing
Hans Ulrich Lass and Hartmut Prandke
Baltic Sea Research Institute Warnemünde, Seestrasse 15, D-18119 Rostock, Germany, lass@io-warnemuende.de
1. Introduction
An understanding of turbulence is a key goal of the
dynamics of the surface layer of the ocean since turbulent
processes are crucial in controlling the exchange of
momentum, dissolved and particular matter between the
atmosphere and the ocean. Until recently our knowledge of
turbulence in the surface mixed layer has been severely
limited by the difficulties of making measurements of the
fluctuating velocity components near the sea surface remote
from a disturbing platform (ship) carrying the necessary
equipment.
Using free falling dissipation profiler the high wave number
range of the turbulence was accessible. Oakey and Elliott
(1982) could show that a constant fraction of the energy flux
3
8u
in the atmospheric boundary layer, namely * , appears as
dissipation in the mixed layer. Measurements below
breaking surface waves revealed a layer of enhanced
dissipation where dissipation could not be scaled by the
wall-layer scaling, Agrawal et al. (1992). Anis and Moum
(1995) observed by means of a rising dissipation profiler
vertical profiles of dissipation in the surface boundary layer.
They observed a depth dependence of the dissipation close
to exponential with a decay rate on the order of the inverse
wave number of the waves, suggesting wave-related
turbulence in the upper part of the ocean surface boundary
layer. Moreover, they suggest that high levels of turbulent
kinetic energy are produced in a thin surface layer with
thickness of the order of the height of the breaking waves.
Shallow (50 m deep) banks in the central parts of the tide
less Baltic Sea provide a suitable site for performing reliable
turbulence measurements with bottom mounted instrument
carrier. This gives a good opportunity to study the surface
mixed layer dynamics in relation to surface gravity waves in
a stratified water body remote from the shores.
2. Methods and Data
Measurements have been performed in the central Baltic
Sea, in September 2001, July and October 2002 aiming at
the estimation of the turbulent energy balance of the surface
mixed layer under varying atmospheric forcing. Time series
of profiles of turbulent kinetic energy dissipation were
measured by a rising dissipation profiler. The profiler was
positioned with bottom mounted idler pulley outside the area
where the ambient turbulence is disturbed by the anchored
ship, see Figure 1 (for details, see Prandke et al., 2000). The
dissipation profiler started from a depth well below the
seasonal thermocline and rose up to the sea surface. Six
profiles were taken every hour in a burst mode.
Complementary measurements comprised time series of
hourly CTD profiles extending from the sea surface to close
to the bottom, of hourly current profiles measured with a
bottom mounted ADCP, of hourly wave spectra measured
with a pressure recorder SBE 26 moored at about 5 m below
the sea surface, of momentum and of buoyancy fluxes
through the sea surface calculated from continuous time
series of the corresponding meteorological parameters
measured on board the research vessel.
Figure 1. Scheme of dissipation measurements with a
rising dissipation profiler from the anchored R/V Prof. A.
Penck
3. Results
The wind forcing at the sea surface usually was
characterised by a sequence of calm phases followed by
wind events of moderate strength during the observations.
The typical summer stratification of the Baltic Sea
consisted during all three experiments of warm brackish
surface water and 4°C cold intermediate winter water.
Both layers were separated by a strong thermocline
located at about 20 – 30 m depth. Wind events forced
groups of inertial waves, while weak inertial oscillations
where prevailing as a typical background motion.
Two physically different dissipation regimes could be
observed, see Figure 2. The first, the internal dissipation
regime, was quite independent of the local wind speed. It
featured a maximum located in the seasonal thermocline
and was most intense after wind events that generated
strong inertial waves. The second regime, the surface
dissipation regime, was closely correlated to the local
wind speed. It had a maximum at the sea surface. An
injection layer of turbulent energy was observed near the
sea surface which was roughly one significant wave height
thick. About one third of the total turbulent energy flux
through the sea surface was dissipated within the injection
layer.
A transport layer of turbulence was observed below the
injection layer. The dissipation rate of turbulent energy
decayed exponentially with depth within the transport
layer. The decay rate depends on the local wind speed.
The dissipation rate of the transport layer decreased to
values of the internal dissipation regime at a depth
zi=2U^2/g, where U is the wind speed at 10 m above the
sea surface and g is the gravity of the earth.