Reply KIRSTY

Hanley et al. (2010, hereafter HBS) present a global climatology of wind–wave interaction based on the inverse wave age (U10 cosu)/cp, where U10 is the wind speed at a height of 10 m, cp is the phase speed of the waves at the spectral peak, and u is the angle between the wind and the direction of propagation of the dominant waves. Their paper examines the extent to which the local wave field is coupled to the local wind field and also seeks to determine which regions of the ocean are in a wind-driven wave regime and which are in a wavedriven wind regime; the latter region is most often dominatedby fast-moving swell. The commentary byHögström et al. (2011, hereafter HSSR) argues that, for climatological mapping, the effect of waves on the air–sea momentum exchange should instead be based on the parameter U10/cp, rather than (U10 cosu)/cp. HSSR motivate their commentary with a simple example choosing parameters with cp 5 12 m s ,U10 5 10 m s , and u5 808. In this case,U10/cp 5 0.83, which implies wind–wave equilibrium (see Csanady 2001), whereas (U10 cosu)/cp5 0.14 implies wind–wave disequilibrium with an air–sea momentum flux directed upward from the ocean to the atmosphere. To characterize the above situation as a fully developed sea seems an incorrect diagnosis: the winds and waves are clearly not in equilibrium simply because they are not aligned! The cases of winds parallel and perpendicular to monochromatic wave crests also illustrates the point that U10/cp would be identical, but (U10 cosu)/cp correctly characterizes the cases as very different. These arguments seem to rule out the use of the parameter U10 /cp as a general wind–wave interaction parameter. The wind–wave interaction parameter (U10 cosu)/cp does however need further motivation, and so here we summarize some of the theoretical support for using (U10 cosu)/cp.