Optimizing models of the North Atlantic spring bloom using physical, chemical and bio-optical observations from a Lagrangian float

摘要

The North Atlantic spring bloom is one of the main events that lead to carbonexport to the deep ocean and drive oceanic uptake of CO₂ from theatmosphere. Here we use a suite of physical, bio-optical and chemicalmeasurements made during the 2008 spring bloom to optimize and compare threedifferent models of biological carbon export. The observations are from aLagrangian float that operated south of Iceland from early April to lateJune, and were calibrated with ship-based measurements. The simplest model isrepresentative of typical NPZD models used for the North Atlantic, while themost complex model explicitly includes diatoms and the formation of fastsinking diatom aggregates and cysts under silicate limitation. We carried outa variational optimization and error analysis for the biological parametersof all three models, and compared their ability to replicate theobservations. The observations were sufficient to constrain mostphytoplankton-related model parameters to accuracies of better than 15 %.However, the lack of zooplankton observations leads to large uncertainties inmodel parameters for grazing. The simulated vertical carbon flux at 100 mdepth is similar between models and agrees well with available observations,but at 600 m the simulated flux is larger by a factor of 2.5 to 4.5 for the model with diatomaggregation. While none of the models can be formally rejected based on theirmisfit with the available observations, the model that includes export bydiatom aggregation has a statistically significant better fit to the observations and moreaccurately represents the mechanisms and timing of carbon export based onobservations not included in the optimization. Thus models that accuratelysimulate the upper 100 m do not necessarily accurately simulate export todeeper depths.