A high-resolution biogeochemical model (ROMS 3.4 + bio_Fennel) of the East Australian Current system

Colette Kerry,Paulina Cetina-Heredia,Carlos Rocha,Moninya Roughan,Christopher A. Edwards

doi:10.5194/gmd-12-441-2019

Abstract

Abstract. Understanding phytoplankton dynamics is critical across a range of topics, spanning from fishery management to climate change mitigation. It is particularly interesting in the East Australian Current (EAC) system, as the region's eddy field strongly conditions nutrient availability and therefore phytoplankton growth. Numerical models provide unparalleled insight into these biogeochemical dynamics. Yet, to date, modelling efforts off southeastern Australia have either targeted case studies (small spatial and temporal scales) or encompassed the whole EAC system but focused on climate change effects at the mesoscale (with a spatial resolution of 1/10∘). Here we couple a model of the pelagic nitrogen cycle (bio_Fennel) to a 10-year high-resolution (2.5–5 km horizontal) three-dimensional ocean model (ROMS) to resolve both regional and finer-scale biogeochemical processes occurring in the EAC system. We use several statistical metrics to compare the simulated surface chlorophyll to an ocean colour dataset (Copernicus-GlobColour) for the 2003–2011 period and show that the model can reproduce the observed phytoplankton surface patterns with a domain-wide RMSE of approximately 0.2 mg Chl a m−3 and a correlation coefficient of 0.76. This coupled configuration will provide a much-needed framework to examine phytoplankton variability in the EAC system providing insight into important ecosystem dynamics such as regional nutrient supply mechanisms and biogeochemical cycling occurring in EAC eddies.

Highlights

The basic framework for most marine biogeochemical (BGC) models has been in use for the last few decades (e.g. Fasham et al, 1990)
These models are highly empirical by nature and attempt to describe non-linear processes such as photosynthesis by phytoplankton, zooplankton grazing, or detrital remineralization through idealized formulations that critically depend on poorly constrained parameters (Doney et al, 2001)
ROMS is a free-surface, hydrostatic, primitive equation ocean model solved on a curvilinear grid with a terrain-following vertical coordinate system (Shchepetkin and McWilliams, 2005); it has been successfully used in many regional BGC studies (e.g. California Current System – Powell et al, 2006; Fiechter et al, 2018; North Pacific – Kishi et al, 2007; Middle Atlantic Bight – Fennel et al, 2006)

Summary

Introduction

The basic framework for most marine biogeochemical (BGC) models has been in use for the last few decades (e.g. Fasham et al, 1990). Pockets of high phytoplankton concentrations south of the EAC separation zone are consistently observed in remote sensing products and bio-physical models of the region (Baird et al, 2006b; Macdonald et al, 2009) These pockets are associated with persistently elevated concentrations of nitrate in the upper 100 m (> 4 μm, CARS – CSIRO Atlas of Regional Seas climatology; Ridgway and Dunn, 2003) caused by separation and upwelling events, which have been shown to deliver 10 times more nutrients to the shelf than river or sewage discharges (Pritchard et al, 2003). We compare the simulated spatial variability in subsurface nutrient concentration (nitrate) to a climatological dataset (CARS – CSIRO Atlas of Regional Seas climatology)

Physical ocean model

Biogeochemical model

Remotely sensed surface chlorophyll

Climatological nitrate observations

Model evaluation metrics

Model evaluation

Variability of surface chlorophyll concentrations

Skill metrics

Dominant spatial and temporal patterns

Vertical distribution of nitrate

Findings

Conclusions