<p>Heat is used as a proxy tracer for gases to study the transport processes<br>
across the sea-surface interface to obtain a detailed insight into<br>
the diffusive and turbulent processes controlling the transport. A carbon<br>
dioxide laser forces a periodically varying heat flux density onto<br>
the water surface and the amplitude damping and phase shift of the<br>
sea surface temperature is measured from infrared image sequences.<br>
The transport process can be treated by linear system theory and the<br>
relation between the input signal (periodically varying surface flux density)<br>
and the output (surface temperature) is estimated. Within the<br>
framework of the SOPRAN initiative three field experiments in the<br>
Baltic Sea were conducted. The locally derived heat transfer rates are<br>
scaled to gas transfer rates, which are in good agreement with empirical<br>
gas transfer wind speed relationships for moderate winds speeds.<br>
At high wind speed, the transfer rates are lower, which is explained<br>
by the fact that heat transport is insensitive to bubble-mediated gas<br>
transfer, i.e. it measures only a part of the transfer process directly at<br>
the water surface.</p
