Satellite Water Colour Observations in African Seas

Abstract

Satellite water colour image data can now be used to show the spatial patterns of coastal and ocean water properties, especially surface chlorophyll concen-tration, on scales covering from kilometers to the entire globe. The repeated coverage shows weekly, seasonal and longer term variability, and provides a unique resource for regions where in-situ data sources are scarce, as is the case for most waters round the continent of Africa. Easy access to extensive data sets of satellite-measured chlorophyll, characterized through the average absorption and scattering properties of phytoplankton, and through natural fluorescence, are now available from Goddard Interactive Online Visualization ANd aNalysis Infrastructure (GIOVANNI), devel-oped by the US National Aeronautics and Space Administration (NASA), and other data systems. In addition, data such as the MCI index from MEdium Resolution Imaging Spectrometer (MERIS), available by the European Space Agency (ESA), derived using spectral bands near 700 nm in the near-infrared part of the spectrum, provide further information on blooms and aquatic vegetation. We present here re-sults of satellite observations of various African seas, showing some of the variety of observations and time series results possible. 2.1 Satellite Ocean Colour Data " Satellite water colour data " refers to the relatively low (typically 1 km) spatial resolution optical image data of oceans and coastal waters provided by specialized satellite imagers. Data are collected in carefully chosen spectral bands, typically 10–20 nm wide. Land imaging usually requires higher spatial resolution (to 30 m or less) and often uses broad spectral bands (100 nm) or even the full available optical window (panchromatic images). Important requirements for water imaging are frequent repetition, with the satellites providing near-daily coverage, though frequently interrupted by cloud, and precise short and long-term calibration.