Geoscientific Model Development | 2021
Calibration of temperature-dependent ocean microbial processes in the cGENIE.muffin (v0.9.13) Earth system model
Abstract
Abstract. Temperature is a master parameter in the marine carbon cycle, exerting a critical control on the rate of biological transformation of a variety of solid and dissolved reactants and substrates. Although in the construction of numerical models of marine carbon cycling, temperature has been long recognised as a key parameter in the production and export of organic matter at the ocean surface, its role in the ocean interior is much less frequently accounted for. There, bacteria (primarily) transform sinking particulate organic matter (POM) into its dissolved constituents and consume dissolved oxygen (and/or other electron acceptors such as sulfate). The nutrients and carbon thereby released then become available for transport back to the surface, influencing biological productivity and atmospheric p CO2 , respectively. Given the substantial changes in ocean temperature occurring in the past, as well as in light of current anthropogenic warming, appropriately accounting for the role of temperature in marine carbon cycling may be critical to correctly projecting changes in ocean deoxygenation and the strength of feedbacks on atmospheric\n p CO2 . Here we extend and calibrate a temperature-dependent representation of\nmarine carbon cycling in the cGENIE.muffin Earth system model, intended for\nboth past and future climate applications. In this, we combine a\ntemperature-dependent remineralisation scheme for sinking organic matter\nwith a biological export production scheme that also includes a dependence\non ambient seawater temperature. Via a parameter ensemble, we jointly\ncalibrate the two parameterisations by statistically contrasting model-projected fields of nutrients, oxygen, and the stable carbon isotopic\nsignature ( δ13C ) of dissolved inorganic carbon in the ocean\nwith modern observations. We additionally explore the role of temperature in\nthe creation and recycling of dissolved organic matter (DOM) and hence its\nimpact on global carbon cycle dynamics. We find that for the present day, the temperature-dependent version shows\na fit to the data that is as good as or better than the existing tuned non-temperature-dependent version of the cGENIE.muffin. The main impact of\naccounting for temperature-dependent remineralisation of POM is in driving\nhigher rates of remineralisation in warmer waters, in turn driving a more\nrapid return of nutrients to the surface and thereby stimulating organic\nmatter production. As a result, more POM is exported below 80\u2009m but on\naverage reaches shallower depths in middle- and low-latitude warmer waters\ncompared to the standard model. Conversely, at higher latitudes, colder\nwater temperature reduces the rate of nutrient resupply to the surface and\nPOM reaches greater depth on average as a result of slower subsurface rates\nof remineralisation. Further adding temperature-dependent DOM processes\nchanges this overall picture only a little, with a slight weakening of\nexport production at higher latitudes. As an illustrative application of the new model configuration and\ncalibration, we take the example of historical warming and briefly assess\nthe implications for global carbon cycling of accounting for a more complete\nset of temperature-dependent processes in the ocean. We find that between\nthe pre-industrial era (ca.\xa01700) and the present (year 2010), in response to a\nsimulated air temperature increase of 0.9\u2009 ∘ C and an associated\nprojected mean ocean warming of 0.12\u2009 ∘ C (0.6\u2009 ∘ C in\nsurface waters and 0.02\u2009 ∘ C in deep waters), a reduction in\nparticulate organic carbon (POC) export at 80\u2009m of just 0.3\u2009% occurs (or 0.7\u2009% including a temperature-dependent DOM response). However, due to this increased recycling nearer the surface, the efficiency of the transfer of carbon away from the surface (at 80\u2009m) to the deep ocean (at 1040\u2009m) is reduced by 5\u2009%. In contrast, with no assumed temperature-dependent processes impacting production or remineralisation of either POM or DOM, global POC export at 80\u2009m falls by 2.9\u2009% between the pre-industrial era and the present day as a consequence of ocean stratification and reduced nutrient resupply to the surface. Our analysis suggests that increased temperature-dependent nutrient recycling in the upper ocean has offset much of the stratification-induced restriction in its physical transport.