WAP-1D-VAR v1.0: development and evaluation of a one-dimensional variational data assimilation model for the marine ecosystem along the West Antarctic Peninsula

Abstract

Abstract. The West Antarctic Peninsula (WAP) is a rapidly warming region, with\nsubstantial ecological and biogeochemical responses to the observed change\nand variability for the past decades, revealed by multi-decadal observations\nfrom the Palmer Antarctica Long-Term Ecological Research (LTER) program. The\nwealth of these long-term observations provides an important resource for\necosystem modeling, but there has been a lack of focus on the development\nof numerical models that simulate time-evolving plankton dynamics over the\naustral growth season along the coastal WAP. Here, we introduce a\none-dimensional variational data assimilation planktonic ecosystem model (i.e., the\nWAP-1D-VAR v1.0 model) equipped with a model\nparameter optimization scheme. We first demonstrate the modified and newly\nadded model schemes to the pre-existing food web and biogeochemical\ncomponents of the other ecosystem models that WAP-1D-VAR model was adapted\nfrom, including diagnostic sea-ice forcing and trophic interactions specific\nto the WAP region. We then present the results from model experiments where\nwe assimilate 11 different data types from an example Palmer LTER growth\nseason (October\xa02002–March\xa02003) directly related to corresponding model\nstate variables and flows between these variables. The iterative data\nassimilation procedure reduces the misfits between observations\nand model results by 58\u2009%, compared to before optimization, via an optimized set of\n12 parameters out of a total of 72 free parameters. The optimized model results\ncapture key WAP ecological features, such as blooms during seasonal sea-ice\nretreat, the lack of macronutrient limitation, and modeled variables and\nflows comparable to other studies in the WAP region, as well as several\nimportant ecosystem metrics. One exception is that the model slightly\nunderestimates particle export flux, for which we discuss potential\nunderlying reasons. The data assimilation scheme of the WAP-1D-VAR model\nenables the available observational data to constrain previously poorly\nunderstood processes, including the partitioning of primary production by\ndifferent phytoplankton groups, the optimal chlorophyll-to-carbon ratio of\nthe WAP phytoplankton community, and the partitioning of dissolved organic\ncarbon pools with different lability. The WAP-1D-VAR model can be\nsuccessfully employed to link the snapshots collected by the available data\nsets together to explain and understand the observed dynamics along the\ncoastal WAP.\n