General information : Monthly mean sea surface diffuse attenuation coefficient at 490nm in m^-1 (log10 scalling) derived from the VIIRS sensor. Processing information: Kd(490) data is processed using SeaDAS 7.4 software and the standard algorithm for Kd(490) (Werdell 2005). Temporal characteristics: This dataset consists of standard monthly mean sea surface Kd(490) maps at 9km resolution (L3 product). Description of observation methods/instruments: The remote sensing of 'Ocean Colour' represents a measure of the spectral variations in the light leaving the water surface, subsequently interpreted in terms of concentrations of optically-significant constituents in the water. The electromagnetic signal collected by the sensor on-board the satellite is largely determined by photons that have never reached the water surface, but have been backscattered within the atmosphere through multiple interactions between gas molecules and aerosols. After removing the atmospheric contribution, the water leaving radiance recorded at a given time by the satellite reflects the optical properties of the water which, in turn, mirrors a specific structure and biogeochemical composition of the marine waters. The satellite-derived reflectance at the air-sea interface is related to apparent optical properties of the water constituents including Kd(490). Quality/accuracy/calibration information: The algorithm is an empirical band-ratio formula, with retrieval standard error and r^2 (for log-transformed data) of 0.28 and 0.89, respectively, as calculated over the development in situ data set (Werdell 2005). References: Werdell, P.J. (2005). Ocean color K490 algorithm evaluation. http://oceancolor.gsfc.nasa.gov/REPROCESSING/SeaWiFS/R5.1/k490_update.html Other contextual information: The product is stored in NetCDF data and available for download.