Assessing and evaluating the ocean-colour footprint of a regional observing system

Abstract A recurring problem with point based marine observations is how to scale them up to infer local and regional dynamics, this is especially true of biogeochemical (BGC) variables such as chlorophyll-a (Chl-a) and dissolved inorganic nutrients. The recent study by Oke and Sakov (2012) uses a combination of historical satellite imagery, and numerical model output to assess the footprint of physical variables in regional observing system. However, in the absence of long integrations of marine biogeochemical models, there are very few tools to interpret the biological dynamics that may be captured by a regional observing system. In this study, a generic approach to assess the biological footprint of regional observing systems is developed. We used the recently reprocessed gridded chlorophyll-a product (NASA OC3M algorithm) to estimate the spatial footprint of the biological observations of the Australian Integrated Marine Observing System (IMOS) National Reference Station (NRS) network at intra-weekly, intra-monthly and intra-seasonal timescales. From this analysis, it is clear that the NRSs are only able to characterize variability at locations close (typically within 9–50 km) to the NRS sites at weekly timescales. However, at monthly and seasonal timescales, the NRS footprint is large enough to infer shelf and regional scale dynamics that are representative of substantial portions of the Australian shelf. This is a generic method that can be incorporated into the design and evaluation of regional observing systems where gridded ocean colour products are available.