Long-term variations in ocean acidification indices in the Northwest Pacific from 1993 to 2018

Miho Ishizu,Yasumasa Miyazawa,Xinyu Guo

doi:10.1007/s10584-021-03239-1

Abstract

Long-term variations in ocean acidification indices in the Northwest Pacific were examined using observational data and a biogeochemical model with an operational ocean model product for the period 1993–2018. The model and observational data for the surface ocean (< 100-m depth) exhibit consistent patterns of ocean acidification in the subtropical and Kuroshio Extension regions and relative alkalinization (i.e., reduced acidification) in the subarctic region of the Northwest Pacific. Below 100-m depth, acidification dominated in the subtropical regions and alkalinization in the subarctic regions. We attribute the excess acidification in the subtropical and Kuroshio regions to the vertical mixing of dissolved inorganic carbon (DIC) exceeding the DIC release by air–sea exchange. These regional differences in acidification and alkalinization are attributed to spatially variable biological processes in the upper ocean and horizontal and vertical physical redistribution of DIC. Our model and observational results have implications for the spatial extent and pattern of ocean acidification, along with the strength of the ocean carbon sink, which are key aspects of global climate change.

Highlights

The atmospheric partial pressure of CO2 (pCO2, air) has been increasing at a rate of ~ 1.8 parts per million by volume per year in recent decades, which is a consequence of human activities such as fossil fuel burning, deforestation, and cement production (Takahashi et al 2009; IPCC 2013)
The surface pHin situ declined by 0.002 yr–1 over most of the study area, which means that ocean acidification is occurring over most of the surface Northwest Pacific (Fig. 1a)
A biogeochemical model with an operational ocean product was used to examine longterm three-dimensional variations in ocean acidification indices and state variables related to ocean acidification from 1993 to 2018

Summary

Introduction

The atmospheric partial pressure of CO2 (pCO2, air) has been increasing at a rate of ~ 1.8 parts per million by volume (ppmv) per year in recent decades, which is a consequence of human activities such as fossil fuel burning, deforestation, and cement production (Takahashi et al 2009; IPCC 2013). The contemporary global ocean acts as a net sink of anthropogenic CO2, but this net flux reflects an imbalance between regions of C O2 outgassing and uptake. The subtropics act as a source for C O2 and the subarctic is a sink (Takahashi et al 2009), but there is strong regional and temporal heterogeneity in the air–sea CO2 flux, because the sea surface CO2 concentration is affected by ocean circulation (subduction and obduction; Toyama et al 2017; Ono et al 2019; Ishii et al 2020). Interannual and decadal variations in the climate system can alter the patterns of mean-state CO2 flux between ocean and atmosphere, and on longer time scales, anthropogenic perturbations to the ocean’s physical and chemical state may adjust the spatial patterns of C O2 flux. The results 0.0018 ± 0.0002 yr–1 revealed that pH25 decreased in winter from 1983 to 2007

Results

Discussion

Conclusion