Role of wave breaking in the air–sea interaction - Breaking and Dissipation of Ocean Surface Waves - page 354

therefore the limits of its contribution can be estimated and parameterised depending on
the wind speed.
The winds and waves are also non-stationary, which has been shown to have a major
effect on estimating the wind input. Uz et al. ( 2002 ) concluded that the wind stress tends to
be higher in decreasing winds than with increasing winds at a given wind speed, mainly due
to the delayed response of the short waves to varying wind forcing (see Donelan & Plant ,
2009 ). Skafel &Donelan ( 1997 ) demonstrated modulation of the wind stress by the passage
of wave groups. Makin ( 1988 ) and Agnon et al. ( 2005 ) found wind-input oscillations due
to nonlinear wind-wave interactions.
Another uncertain source of potentially significant stress variations is the swell present
on the ocean surface. Dobson et al. ( 1994 ) did not find noticeable influence of the swell
on the sea drag, whereas Donelan et al. ( 1997 ), Drennan et al. ( 1999 ) and Guo-Larsen
et al. ( 2003 ) revealed significantly enhanced drag coefficients for cross-wind and, partic-
ularly, for adverse-to-the-wind swell. These features of swell were theoretically explained
by Kudryavtsev & Makin ( 2004 ). Drennan et al. ( 1999 ), Smedman et al. ( 1999 ) and
Grachev et al. ( 2003 ) observed negative stress - momentum flux from the waves to the
wind - for swell following the wind. Potentially, swell can influence the dominant and
short wind-wave spectra through hydrodynamic interactions, and through the interaction
of the changed spectra with the wind - the sea drag (see also Babanin & Makin , 2008 ).
Even in the absence of swell, the directional spread of wind waves themselves can alter the
seadragbyupto60%( Ting et al. , 2010 ).
This brief review of the long-standing problem emphasises the need for a complex
approach to account for multiple phenomena that may simultaneously affect the sea drag.
Babanin & Makin ( 2008 ) used the Lake George field data (see Section 3.5 , Young et al. ,
2005 ) in combination with the wind-over-waves coupled (WOWC) approach ( Makin et al. ,
1995 ; Makin & Kudryavtsev , 1999 , 2002 ; Kudryavtsev et al. , 1999 ; Kudryavtsev &Makin ,
2001 ) to address the complex processes, or rather some of them, associated with small-
scale air-sea interactions, and therefore the sea drag.
Babanin & Makin ( 2008 ) outlined a list of physical properties and phenomena whose
effect on sea drag should be investigated and incorporated in the final parameterisation in
order to reduce the scatter. It includes, among other possibilities,
(1) mean wind speed;
(2) sea-state dependence;
(3) wave steepness;
(4) full flow separation for strongly forced wind-waves;
(5) enhancement of sea drag due to wave breaking;
(6) rising and falling winds;
(7) gustiness of the wind;
(8) temperature stratification in the atmospheric boundary layer;
(9) swell;
(10) nonlinear wind-wave interactions;
(11) wave horizontal skewness and vertical asymmetry;