The Role of Whitecap Bubbles in Air–Sea Heat and Moisture Exchange

Abstract

In high winds, the sea surface is no longer simply connected. Whitecap bubbles and sea spray provide additional surfaces that may enhance the transfer of any quantity normally exchanged at the air?sea interface. This paper investigates the role that the air bubbles in whitecaps play in the air?sea exchange of sensible and latent heat. Bubble spectra published in the literature suggest that an upper bound on the volume flux of bubbles per unit surface area in Stage A whitecaps is 3.8 ? 10?2 m3 m?2 s?1. This estimate, a knowledge of whitecap coverage as a function of wind speed, and microphysical arguments lead to estimates of the sensible (QbS) and latent (QbL) heat fluxes carried across the sea surface by air cycled through whitecap bubbles. Because QbS and QbL scale as do the usual turbulent or interfacial fluxes of sensible and latent heat, these bubble fluxes can be represented simply as multiplicative factors fS and fL, respectively, that modulate the 10-m bulk transfer coefficients for sensible (CH10) and latent (CE10) heat. Computations show, however, that even the upper bounds on the bubble heat fluxes are too small to be measured. For 10-m wind speeds up to 20 m s?1, fS and fL are always between 1.00 and 1.01. For a 10-m wind speed of 40 m s?1, fS and fL are still less than 1.05. Consequently, for wind speeds up to 40 m s?1?a range over which it should be safe to extrapolate the models of sea surface physics used here?the near-surface air heated and moistened in whitecap bubbles seems incapable of contributing measurably to air?sea heat and moisture transfer.