Abstract
AbstractThis paper presents the first decadal reanalysis simulation of the biogeochemistry of the North West European shelf, along with a full evaluation of its skill, confidence, and value. An error‐characterized satellite product for chlorophyll was assimilated into a physical‐biogeochemical model of the North East Atlantic, applying a localized Ensemble Kalman filter. The results showed that the reanalysis improved the model simulation of assimilated chlorophyll in 60% of the study region. Model validation metrics showed that the reanalysis had skill in matching a large data set of in situ observations for 10 ecosystem variables. Spearman rank correlations were significant and higher than 0.7 for physical‐chemical variables (temperature, salinity, and oxygen), ∼0.6 for chlorophyll and nutrients (phosphate, nitrate, and silicate), and significant, though lower in value, for partial pressure of dissolved carbon dioxide (∼0.4). The reanalysis captured the magnitude of pH and ammonia observations, but not their variability. The value of the reanalysis for assessing environmental status and variability has been exemplified in two case studies. The first shows that between 325,000 and 365,000 km2 of shelf bottom waters were vulnerable to oxygen deficiency potentially threatening bottom fishes and benthos. The second application confirmed that the shelf is a net sink of atmospheric carbon dioxide, but the total amount of uptake varies between 36 and 46 Tg C yr−1 at a 90% confidence level. These results indicate that the reanalysis output data set can inform the management of the North West European shelf ecosystem, in relation to eutrophication, fishery, and variability of the carbon cycle.
Highlights
Trends and patterns of biogeochemical variables that are relevant for marine policy and ecosystem understanding can be evaluated by merging numerical models and ocean color in extended ‘‘biogeochemical reanalysis,’’ using a consistent data assimilation algorithm [Lahoz and Schneider, 2014; Gehlen et al, 2015]
Skill in Matching the Ocean Color Data In the years 1998–2009, the distribution of chlorophyll observed in the North West European shelf was characterized by sharp gradients from the coastal areas toward the oceanic waters (Figure 3a), and the reanalysis matched this pattern quite closely (Figure 4)
The crucial implications of this supplementary information were evident in two case studies, where we assessed that: 1. An area as large as 325,000 km2 was vulnerable to oxygen deficiency at the bottom of the North West European shelf, but additional 40,000 km2 are included when using a strict 1% confidence criteria
Summary
Trends and patterns of biogeochemical variables that are relevant for marine policy and ecosystem understanding can be evaluated by merging numerical models and ocean color in extended ‘‘biogeochemical reanalysis,’’ using a consistent data assimilation algorithm [Lahoz and Schneider, 2014; Gehlen et al, 2015]. Such algorithm corrects the model estimates toward the observations, taking account of the errors in the model and in the observations [Kalman, 1960]. Reanalyses for years 1998–2012, using chlorophyll observations from SeaWiFS and MODIS and the NASA OBM, evaluated significant declining trends of chlorophyll in the Northern Hemisphere and Indian Oceans [Gregg and Rousseaux, 2014], and estimated declining trends of phytoplankton functional groups in part of the global oceans [Rousseaux and Gregg, 2015]
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