Chemistry Reference
In-Depth Information
rapid and dramatic disruption of the thermal skin across the entire surface
of the container. By contrast, when we used the same pump and tubing to
pump water, there was only a significant surface effect if the flow was
strong and the outlet was close to the water surface.
5 Discussion and conclusions
Infrared imagery of the sea surface is a powerful technique for investigat-
ing surface renewal processes at the sea surface, including the modulation
of these phenomena by wind stress, wave breaking, slicks and other envi-
ronmental variables. Thermal imaging techniques have shown to be effec-
tive in the study of large and small breaking waves (Jessup
et al
. 1997
a,b
);
and Jaehne and co-workers (Jähne and Haussecker 1998; Schimpf et al.
1999) have exploited small-scale temperature fluctuations to estimate gas
transfer velocities. It should be noted in caution that infrared cameras are
not "ideal surface thermometers" and, therefore, interpretation of an image
is not always straightforward.
Night-time measurements with the thermal IR camera in a moderate
wind (6-8 m s
-1
) show evidence of the importance of fairly large-scale (~ 1
metre) organised turbulent structures, and suggest that the process of air-
sea exchange is remarkably dynamic. The amplitude of the metre-scale
fluctuations varies enormously on time scales of a few minutes. It seems
likely that the variations in metre-scale fluctuations have a dynamical ori-
gin within the water column - perhaps associated with wave breaking
which also varies greatly on these time scales. Images show evidence of
vortices aligned with the wind. These vortices may be part of a spectrum
of similar motions extending from large-scale Langmuir circulation down
to much smaller scales (Melville et al. 1998). The smaller scale motions
are likely to be more directly significant to surface renewal and air-sea gas
exchange, but it may be possible to investigate small-scale surface renewal
indirectly by measuring the related large-scale fluctuations (requiring less
spatial resolution). Patches at relatively high temperature may result from
the action of small breaking waves (with or without air entrainment, but
we can not quantify this process).
At the sea surface, we may expect surfacing bubble plumes to contribute
to surface renewal in the wake of breaking waves. This surface renewal
will necessarily enhance air-sea gas exchange. It is our experience that
only a small flux of bubbles is necessary to transform surface renewal, but
this is difficult to quantify. It seems likely that bubbles are significant to
surface renewal by small breaking waves, even when there is no obvious
“whitecap”.
