Abstract
The ocean biogeochemistry components of the Canadian Earth System Model v. 5 are presented and compared to observations and other models. CanESM5 employs the same biogeochemistry module as CanESM2 whereas CanESM5-CanOE (“Canadian Ocean Ecosystem model”) is a new, more complex biogeochemistry module developed for Sixth Coupled Model Intercomparison Project (CMIP6), with multiple food chains, flexible phytoplankton elemental ratios, and a prognostic iron cycle. This new model is described in detail and the outputs compared to CanESM5 and CanESM2, as well as to observations and other CMIP6 models. Both CanESM5 models show gains in skill relative to CanESM2, which are attributed primarily to improvements in ocean circulation. CanESM5-CanOE shows improved skill relative to CanESM5 in some areas. CanESM5-CanOE includes a prognostic iron cycle, and maintains high nutrient / low chlorophyll conditions in the expected regions (in CanESM2 and CanESM5, iron limitation is specified as a temporally static ‘mask’). Surface nitrate concentrations are biased low in the subarctic Pacific and equatorial Pacific, and high in the Southern Ocean. Export production in CanESM5-CanOE is among the lowest for CMIP6 models; in CanESM5 it is among the highest, but shows the most rapid decline after about 1980. CanESM5-CanOE has relatively low concentrations of zooplankton and detritus relative to phytoplankton, and a high and relatively constant living phytoplankton fraction of total particulate organic matter. In most regions, large and small phytoplankton show decoupled seasonal cycles with greater abundance of large phytoplankton in the productive seasons. Cumulative ocean uptake of anthropogenic carbon dioxide through 2014 is lower in both CanESM5 models than in observation-based estimates or the model ensemble mean, and is lower in CanESM5-CanOE (122 PgC) than in CanESM5 (132 PgC).
Highlights
CanESM5 employs the same biogeochemistry module as CanESM2 whereas CanESM5-CanOE (“Canadian Ocean Ecosystem model”) is a new, more complex biogeochemistry module developed for CMIP6, with multiple food chains, flexible phytoplankton elemental ratios, and a prognostic iron cycle
In Canadian Model of Ocean Carbon (CMOC) calcification is parameterized by a temperature dependent “rain ratio” and 100% burial of calcite that reaches the seafloor is assumed. 98 In this paper we present a detailed model description for CanOE and an evaluation of both CanESM5 and CanESM5-CanOE relative to observational data products and other available models
In the Indian Ocean, both Canadian Earth System Model (CanESM) models show high oxygen concentrations in the Arabian Sea and deeper layers of the Bay of Bengal relative to observations and other CMIP6 models; these biases are somewhat smaller in CanESM5-CanOE than in CanESM5 (Figure 2). 410 Biases in the eastern boundary current regions are depth and model specific
Summary
42 43 The Canadian Centre for Climate Modelling and Analysis has been developing coupled models with an interactive carbon cycle for more than a decade (Arora et al, 2009; 2011; Christian et al, 2010). Among the less satisfactory aspects of CMOC biogeochemistry are, firstly, that Fe limitation is specified as a static 'mask' that does not change with climate (being calculated from the present-day climatological distribution of nitrate), and secondly, that denitrification is parameterized so that nitrogen (N) is conserved within each vertical column, i.e., collocated with N2 fixation in tropical and subtropical open-ocean regions (Zahariev et al, 2008; Riche and Christian, 2018). This latter simplification produced excessive accumulations of nitrate in Eastern Boundary Current regions where most denitrification occurs. NXS and FeXS are remineralization of "excess" (relative to grazer or detritus ratios) N or Fe and are defined below (equation 16)
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