Towards a merged satellite and in situ fluorescence ocean chlorophyll product

[摘要] Understanding the ocean carbon cycle requires a precise assessment ofphytoplankton biomass in the oceans. In terms of numbers of observations,satellite data represent the largest available data set. However, as theyare limited to surface waters, they have to be merged with in situobservations. Amongst the in situ data, fluorescence profiles constitute thegreatest data set available, because fluorometers have operated routinely onoceanographic cruises since the 1970s. Nevertheless, fluorescence is onlya proxy of the total chlorophyll a concentration and a data calibration isrequired. Calibration issues are, however, sources of uncertainty, and theyhave prevented a systematic and wide range exploitation of the fluorescencedata set. In particular, very few attempts to standardize the fluorescencedatabases have been made. Consequently, merged estimations with other data sources(e.g. satellite) are lacking.

We propose a merging method to fill this gap. It consists firstly inadjusting the fluorescence profile to impose a zero chlorophyll aconcentration at depth. Secondly, each point of the fluorescence profile isthen multiplied by a correction coefficient, which forces the chlorophyll aintegrated content measured on the fluorescence profile to be consistentwith the concomitant ocean colour observation. The method is close to theapproach proposed by Boss et al. (2008) to correct fluorescence data of aprofiling float, although important differences do exist. To develop andtest our approach, in situ data from three open ocean stations (BATS, HOTand DYFAMED) were used. Comparison of the so-called "satellite-corrected"fluorescence profiles with concomitant bottle-derived estimations ofchlorophyll a concentration was performed to evaluate the final error(estimated at 31%). Comparison with the Boss et al. (2008) method, usinga subset of the DYFAMED data set, demonstrated that the methods have similaraccuracy. The method was applied to two different data sets to demonstrateits utility. Using fluorescence profiles at BATS, we show that theintegration of "satellite-corrected" fluorescence profiles inchlorophyll a climatologies could improve both the statistical relevance ofchlorophyll a averages and the vertical structure of the chlorophyll afield. We also show that our method could be efficiently used to process,within near-real time, profiles obtained by a fluorometer deployed onautonomous platforms, in our case a bio-optical profiling float. Theapplication of the proposed method should provide a first step towards thegeneration of a merged satellite/fluorescence chlorophyll a product, as the"satellite-corrected" profiles should then be consistent with satelliteobservations. Improved climatologies with more consistent satellite and insitu data are likely to enhance the performance of present biogeochemicalmodels.

[学科分类] 地球化学与岩石