A time series of photosynthetically available radiation at the ocean surface from SeaWiFS and MODIS data

Abstract

A global, 13-year record of photo-synthetically available radiation (PAR) at the ocean surface (9-km resolution) has been generated from SeaWiFS, MODIS-Aqua, and MODIS-Terra data. The PAR values are essentially obtained by subtracting from the solar irradiance at the top of the atmosphere (known) the solar energy reflected by the oceanatmosphere system (satellite-derived) and absorbed by the atmosphere (modeled). Observations by individual instruments, combinations of two instruments, and three instruments are considered in the calculations. Spatial and temporal biases between estimates from one, two, or three instruments are determined and corrected, resulting in a consistent time series for variability studies. Uncertainties are quantified on daily, weekly, and monthly time scales for the various instrument combinations from comparisons with in situ measurements. The correlative behavior of PAR, sea surface temperature, and chlorophyll concentration in the Equatorial Pacific is examined. PAR monitoring will continue with current and future satellite ocean-color sensors, in particular VIIRS, and the methodology will be extended to generating UV-A and UV-B irradiance.