Seasonal variability in the inorganic ocean carbon cycle in the Northwest Pacific evaluated using a biogeochemical and c

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Seasonal variability in the inorganic ocean carbon cycle in the Northwest Pacific evaluated using a biogeochemical and carbon model coupled with an operational ocean model Miho Ishizu 1

1

2

& Yasumasa Miyazawa & Tomohiko Tsunoda & Xinyu Guo

1,3

Received: 9 January 2020 / Accepted: 25 June 2020/ # The Author(s) 2020

Abstract

Here, we investigate the seasonal variability in the dissolved inorganic carbon (DIC) cycle in the Northwest Pacific using a high-resolution biogeochemical and carbon model coupled with an operational ocean model. Results show that the contribution to DIC from air–sea CO2 exchange is generally offset by vertical mixing at the surface at all latitudes, with some seasonal variation. Biological processes in subarctic regions are evident at the surface, whereas in the subtropical region these processes take place within the euphotic layer and then DIC consumption deepens southward with latitude. Such latitudinal differences in biological processes lead to marked horizontal and vertical contrasts in the distribution of DIC, with modulation by horizontal and vertical advection–diffusion processes. Keywords Biogeochemical model . Inorganic carbon cycle . NPZDC . Northwest Pacific . Ocean acidification . JCOPE . pH . Aragonite saturation

1 Introduction The atmospheric partial pressure of CO2 (pCO2) has been increasing at a rate of ~ 1.8 ppm by volume (ppmv) per year in recent decades as a result of human activities such as fossil-fuel burning, deforestation, and cement production (Takahashi et al. 2009; IPCC 2013). In the preindustrial era, the ocean was generally a net source of CO2 emissions to the atmosphere because of the mineralization of land-derived organic matter in addition to that produced by in

* Miho Ishizu [email protected]

1

Japan Agency for Marine-Earth Science and Technology, Yokohama-shi, Kanagawa, Japan

2

The Ocean Policy Research Institute of the Sasakawa Peace Foundation, Tokyo, Japan

3

Center for Marine Environmental Studies, Ehime University, Matsuyama, Japan

Climatic Change

situ production, and CaCO3 precipitation (Mackenzie et al. 2004). Rising atmospheric CO2 concentrations caused by fossil-fuel combustion and land-use changes (Mackenzie et al. 2004; Bauer et al. 2013; IPCC 2013; IGBP, IOC, SCOR 2013) reversed the direction of the air–sea CO2 flux, leading the global ocean to become a net sink of anthropogenic CO2. The present thickness of the upper thermocline, where large amounts of anthropogenic CO2 emissions are stored, is estimated to be of the order of a few hundred meters (Mackenzie et al. 2004). The oceanic coastal zone changed from being a source to a sink during the industrial era (Mackenzie et al. 2004; Bauer et al. 2013). Several estimates of CO2 sinks and sources in ocean provinces (Cai et al. 2006) and/or spatially explicit typology (Laurelle et al. 2010) showed that marginal seas in the tropics are sources of CO2, whereas those in temperate regions and at high latitudes act as sinks (Cai et al. 2006; Laurelle et al. 2010). Data-based estimate