A Sensitivity Analysis of the Impact of Rain on Regional and Global Sea-Air Fluxes of CO2

Abstract

The global oceans are considered a major sink of atmospheric carbon dioxide (CO2). Rain is known to alter the physical and chemical conditions at the sea surface, and thus influence the transfer of CO2 between the ocean and atmosphere. It can influence gas exchange through enhanced gas transfer velocity, the direct export of carbon from the atmosphere to the ocean, by altering the sea skin temperature, and through surface layer dilution. However, to date, very few studies quantifying these effects on global net sea-air fluxes exist. Here, we include terms for the enhanced gas transfer velocity and the direct export of carbon in calculations of the global net sea-air fluxes, using a 7-year time series of monthly global climate quality satellite remote sensing observations, model and in-situ data. The use of a non-linear relationship between the effects of rain and wind significantly reduces the estimated impact of rain-induced surface turbulence on the rate of sea-air gas transfer, when compared to a linear relationship. Nevertheless, globally, the rain enhanced gas transfer and rain induced direct export increase the estimated annual oceanic integrated net sink of CO2 by up to 6%. Regionally, the variations can be larger, with rain increasing the estimated annual net sink in the Pacific Ocean by up to 15% and altering monthly net flux by > ± 50%. Based on these analyses, the impacts of rain should be included in the uncertainty analysis of studies that estimate net sea-air fluxes of CO2 as the rain can have a considerable impact, dependent upon the region and timescale.

Fig 1. Mean monthly CO₂ flux between January 1999 and December 2005 for a reference dataset (no rain components).

Fig 2. Annual (right axis, solid lines) and monthly (left axis, dashed lines) global net sea-air CO₂ flux, without the effects of rain, F_ref, and with the effects of rain, F_T = F_DIC + F_k-rain.

Fig 3. The monthly mean global CO₂ flux attributed to the enhancement of transfer velocity (both non-linear, F_rain-k and non-linear, F_{rain-k (linear)}) and Direct deposition, F_DIC, TgC month^-1.

Fig 4. The mean effect of wet deposition on monthly CO₂ flux between January 1999 and December 2005, F_DIC—F_ref.

Fig 5. The effect of rain on monthly CO₂ flux between January 1999 and December 2005, given a non-linear model of transfer velocity (eq 10), F_k-rain—F_ref.

Fig 6. The combined effect of wet deposition and non-linear gas transfer velocity on CO₂ flux between Jan 1999 and Dec 2005, (F_DIC + F_k-rain) —F_ref.

Fig 7. The mean effect of rain on monthly CO₂ flux between January 1999 and December 2005, given a linear model (Ho 2004), F_{k-rain(linear)}—F_ref.

Note different scale compared to Figs 4, 5 and 6.