Global fields of sea surface dimethylsulfide predicted from chlorophyll, nutrients and light

The major difficulty in estimating global sea–air fluxes of dimethylsulphide (DMS) is in interpolating measured seawater DMS concentrations to create seasonally resolved gridded composites. Attempts to correlate DMS with variables that can be mapped globally, e.g. chlorophyll, have not yielded reliable relationships. A comprehensive database of DMS measurements has recently been assembled by Kettle et al. [Global Biogeochem. Cycles 13 (1999) 399]. This database, which contains chlorophyll as a recorded variable, was extended by merging nutrients and light from globally gridded fields. A new equation was developed whereby DMS is predicted from the product of chlorophyll (C, mg m−3), light (J, mean daily shortwave, W m−2) and a nutrient term (Q, dimensionless) using a “broken-stick” regression: DMS=a, log10(CJQ)≤s DMS=b[log10(CJQ)−s]+a, log10(CJQ)>swhere Q=N/(KN+N), N is nitrate (mmol m−3) and KN is the half saturation constant for nitrate uptake by phytoplankton (0.5 mmol m−3). Fitted parameter values are: a=2.29, b=8.24, s=1.72. Monthly maps of global DMS were generated by combining these equations with ocean color data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). The resulting high DMS concentrations in high latitude, upwelling and shelf areas are consistent with observed patterns. Predicted global seasonally averaged mean DMS is 2.66 nM. The further application of gas transfer equations to these fields leads to estimates of globally integrated DMS fluxes from ocean to atmosphere of 0.86 and 1.01 Tmol S year−1 for two formulations of piston velocity. The simplicity of the new relationship makes it suitable for implementation in global ocean general circulation models. The relationship does not however resolve DMS variability in low-DMS areas, which constitute large tracts of the open ocean, and should therefore be used with caution in localized studies.