An assessment of the effect of sea surface surfactant on global atmosphere-ocean CO2 flux

Abstract

[1] We assess the possible impact of the distribution of naturally occurring surfactants on the direct integration of the global atmosphere-ocean CO2 flux across the ocean surface. The global atmosphere-ocean CO2 flux is calculated using the monthly mean DeltapCO(2) climatology compiled by Takahashi et al. [1997] as well as satellite wind speed and sea-surface temperature data. In the absence of any global map of surfactant coverage and as it is known that phytoplankton exudates and degradation products are the major sources of marine surfactants, ocean primary productivity, which can be derived from the satellite-based estimate of chlorophyll concentration, is used as an indicator of the presence of surfactants as proposed by Asher [1997]. From the calculated results it is found that suppression of the upward and downward CO2 fluxes by marine surfactants exhibits an asymmetric effect: The average percent reduction of absorption flux by surfactants is about twice that of outgassing, which results in an overall decrease in the net global CO2 uptake by the oceans. For almost half of the year (between January and May) the presence of surfactants does not affect CO2 outgassing from global oceans. In contrast, throughout the entire year the presence of surfactants suppresses CO2 absorption by the oceans. The major reduction in absorption fluxes occurs in the northern Pacific and Atlantic (10degreesN to 70degreesN) in all seasons and in the Southern Ocean (south of 40degreesS) in austral spring and summer. However, the most significant decrease in outgassing fluxes occurs in the equatorial and southern Pacific Ocean (40degreesS to 10degreesN), particularly in the eastern equatorial and subtropical waters off the southern American coast, in the period of austral spring and summer. Annual net CO2 flux is reduced by approximately 20% under the surfactant coverage condition that the primary productivity is above a threshold value of 25 g-C m(-2) mom(-1) and by about 50% with a threshold of 15 g-C m(-2) mom(-1).