Coupled physical and biological modeling of the spring bloom in the North Atlantic (I): model formulation and one dimensional bloom processes

Abstract

This is the first of two papers that introduce a mesoscale eddy resolving coupled physical and biological model system. The physical model consists of a quasigeostrophic interior with a fully coupled surface boundary layer. The nitrogen based biological model includes nitrate, phytoplankton, heterotroph and ammonium fields. This interdisciplinary model system is used to examine aspects of the 1989 JGOFS North Atlantic Bloom Experiment data set. This paper deals mainly with one dimensional processes and a companion paper addresses three dimensional phenomena. The data set consists of two time series of observations taken from different water masses in the mesoscale environment. The general features of the two time series are well represented by a one dimensional model when the mesoscale spatial variability in the initial condition is treated explicitly within the one dimensional framework. However, a significant bias is evident in the first time series as the sampling pattern began in a warm feature and moved toward colder ones. Mistaking spatial for temporal variability in this case results in an apparent sink of heat and source of nitrate in the data. Removing this bias with the one dimensional model results in an f-ratio that is almost a factor of two higher (0.64) than computed by other authors based on nutrient inventories and primary productivity measurements (0.37). The second time series was conducted in the interior of a mesoscale feature and spatial biasing is minimal. The model forms a seasonal thermocline and nitracline that compare quite well with the data in both magnitude and vertical extent. A subsurface ammonium maximum is generated by the model from an initially homogeneous profile that also agrees well with the data. Simulated primary productivity profiles match 14C incubations except on the final day of the simulation when surface nutrients appear in to have been exhausted slightly prematurely. Computed f-ratios are consistent with independent estimates based on uptake measurements. A systematic parameter dependence and sensitivity analysis is carried out on these results. The most sensitive parameters are the phytoplankton and heterotroph maximum growth rates. Detailed analysis of the behavior of the system indicates tight coupling between phytoplankton production and heterotrophic consumption even in the early stages of the bloom.