Observed inter‐relations between 10m winds, ocean whitecaps and marine aerosols

Abstract

Low elevation aerosol spectra, ocean whitecap cover and 10m wind speeds measured during the 1978 JASIN experiment have been inter‐related and compared with previously published observations. The positive dependence of aerosol concentration upon whitecap cover was found to increase with droplet radius reflecting the expected higher correlation of the concentration of larger droplets, which have shorter effective residence time in the marine atmospheric boundary layer, with the immediate whitecap cover, which reflects the instantaneous rate of aerosol generation at the sea surface. The power‐law wind dependence, Uγ of the low elevation concentration of droplets larger than 8γm radius was determined to be similar to the wind dependence of whitecap cover, with γ values of 3.23 and 3.31 resulting from the respective application of the robust bi‐weight fitting technique. This observation is consonant with an aerosol generation model in which the instantaneous rate of production is simply proportional to the immediate whitecap cover. The large droplet end of the JASIN low elevation aerosol spectrum is seen to undergo a marked enhancement when the wind speed exceeds 10ms−1. This is a consequence of the onset at that speed of supplementary droplet production via the mechanical disruption of wave crests. The observed growth, with increasing wind speed, in the disparity in amplitude of near‐sea‐surface and near‐cloud‐base aerosol spectra is in part a consequence of the fact that the larger droplets, produced in relative abundance at the higher wind speeds, fall out before they can be mixed effectively through the boundary layer.

Unique ocean-derived particles serve as a proxy for changes in ocean chemistry

“…Before the campaign began, a rain event occurred on DOY 90 causing the percentage of reacted sea salt (indicated by the presence of nitrate ions) to decrease from 70 to 90% of the sea salt particles to 20%, whereas fresh sea salt increased to ∼60% and a subsequent increase in the percentage of Mg‐type particles also occurred. The increase in Mg and fresh sea salt particles mostly likely occurred because of an enhancement in the bubble bursting mechanism induced by high winds (reaching up to ∼20 m/s) [ Monahan et al , 1983] and raindrops impacting the ocean surface [ Marks , 1990]. To further probe whether a primary, oceanic source was responsible for the detection of Mg‐type particles, the time series of Mg‐type particles, fresh sea salt, reacted sea salt, and wind speed was compared for DOY 104–114 (Figure 4a).…”

“…Bursting bubbles result in the formation of jet drops, which result from the breakup of the vertical jet of water that forms after the bubble cavity collapses, and the more numerous film drops, which result from the collapse of the thin film surrounding the bubble [ Blanchard and Woodcock , 1957]. Bubble bursting can be enhanced by high wind velocities that increase whitecap production [ Monahan et al , 1983] and by the impact of raindrops on the ocean surface [ Marks , 1990]. In addition to sea salt, organic material produced by marine biota can be ejected to the atmosphere from the bubble bursting mechanism.…”

Spatial Variation of Sea-Spray Fluxes over a Mediterranean Coastal Zone Using a Sea-State Model

“…Since the work of Blanchard (1963), substantial improvements in knowledge of the variation of the whitecap coverage have been obtained (e.g., Monahan and O'Muircheartaigh 1980;Monahan et al 1983) using wind speed and friction velocity. In the coastal zone, the profile of the coast has to be taken into account in the whitecap evolution, as well as the variation of the fetch resulting from changes in the wind direction, which in turn influences the wave spectrum and the related integral sea-state characteristics.…”

“…Wave breaking is important to gas exchange at the air-sea interface (e.g., Liss and Merlivat 1986), and breaking waves induce the occurrence of whitecaps at the sea surface that disappear after a time delay that can vary from 2 to 5 s (Monahan et al 1983). Much effort has been made in the last few decades towards a model for the whitecap fraction (Wu 1979;Monahan and O'Muircheartaigh 1986;Piazzola et al 2002;Guan et al 2007).…”

Biological modification of mechanical properties of the sea surface microlayer, influencing waves, ripples, foam and air-sea fluxes

“…On a finer time scale, however, foam occurred only relatively little while chlorophyll and excess viscosity were increasing, but in large quantities when the bloom was declining, between the Monahan (1971); open squares, whitecap fraction from Toba and Chaen (1973); open circles, preliminary whitecap fraction from the 1978 JASIN experiment (Monahan et al, 1983 Figure 2 of Monahan and Ò Muircheartaigh (1980), and the relationships for W vs. U 10 are from Andreas and Monahan (2000). DOI: https:// doi.org/10.1525/elementa.283.f8 peak in both chlorophyll and 3D viscosity and the end of the bloom.…”