Seasonal estimates of global oceanic whitecap coverage

Abstract

Seasonal estimates of oceanic whitecap coverage are presented on global contour maps. The instantaneous fraction of the sea surface covered by whitecaps as a function of wind speed has been described previously using a power law fitting technique [Monahan and Muircheartaigh, 1980]. This relationship, coupled with a Gaussian wind speed frequency distribution, allows us to calculate the oceanic whitecap coverage accounting for the variance about mean wind speeds. We use monthly mean wind speed and variance information in 5° latitude × 5° longitude areas over the world ocean to estimate the global oceanic whitecap coverage distribution. The whitecap coverage in the northern hemisphere oceans displays a substantial seasonal dependence. The boreal summer average whitecap coverage in the high‐latitude North Atlantic is about 0.5% and increases by a factor of roughly 6 to over 3.0% for a boreal winter average. The whitecap coverage in the high‐latitude southern hemisphere oceans exhibits only a factor of 3 difference for the same seasons, from about 1% in the austral summer to over 3% for the austral winter. There are several high‐latitude areas with greater than 3% average whitecap coverage over several months of the year. The monsoonal winds in the Indian Ocean result in over 3% mean whitecap coverage for the June‐July‐August period.

Global distribution and seasonal dependence of satellite‐based whitecap fraction

“…In the Southern Hemisphere, W peaks around 50°S; here W 10 varies less than 30% and W 37 less than 20% over the year. This result is in agreement with the findings of Blanchard [] and Erickson III et al []. The asymmetric distribution in mean W is a consequence of the larger seasonal variations of temperature and winds in the Northern Hemisphere (driven by the stronger response of land surface temperature) and persistent high winds and long fetches in the Southern Ocean, both of which result from the asymmetric distribution of land masses between the hemispheres.…”

“…Over much of the low‐latitude ocean (equatorward of 30°N and 30°S), seasonal means of W 10 are usually <0.5%, while W 37 seasonal means are typically above 0.5%. Like Erickson III et al [], we find enhanced W in the Arabian Sea during summer, with mean W 10 ≈1.5% and mean W 37 ≈2%.…”

“…At low to middle latitudes (up to 40 • N and 40 • S) W never exceeds 1%. Erickson III et al [1986] used the W(U 10 ) function of Monahan and O'Muircheartaigh [1980, hereinafter MM80] and global monthly mean winds at 5 • resolution. They found a similar general seasonal distribution and also highlighted geographic regions of persistently high whitecap fraction over periods of months, such as the Indian Ocean during the monsoon season and high-latitude storm tracks.…”

Whitecap coverage from satellite measurements: A first step toward modeling the variability of oceanic whitecaps

“…While Figure 5a represent the first global whitecap coverage obtained from satellite measurements, the global distribution and seasonal changes of oceanic whitecaps have been previously investigated using W ( U 10 ) relations [ Blanchard , 1963; Spillane et al , 1986; Erickson et al , 1986]. In estimating global whitecap coverage, all authors discuss possible underestimation of W values for two reasons.…”

Contribution to the global air–sea CO₂ exchange budget from asymmetric bubble-mediated gas transfer

“…Observations tend to suggest that: (1) there appears to exist universal power laws for the size spectrum and vertical distributions of bubbles (Monahan and Zeitlow, 1969;Kolovayev, 1976;Cipriano and Blanchard, 1981;Koga, 1982;Crawford and Farmer, 1987;Hwang et al, 1990;Deane and Stokes, 2002); and (2) the rate of sea-surface coverage of whitecaps from breaking waves varies approximately with a cubic or higher powers of wind speed, u (Monahan and O'Muircheartaigh, 1980;Erickson et al, 1986;Monahan and Torgersen, 1990;Lamarre and Melville, 1991;Asher et al, 2002). These observational conclusions on bubble distributions infer that the bubble source function, Uðr i ; z i Þ, is proportional to the horizontal surface distribution because the vertical and size distributions of bubbles are all universal.…”