Reply on RC2

Goulven Gildas Laruelle

doi:10.5194/os-2021-70-ac2

Abstract

The temporal variability of the sea surface partial pressure of CO2 (pCO2) and the underlying processes driving this variability are poorly understood in the coastal ocean. In this study, we tailor an existing method that quantifies the effects of thermal changes, biological activity, ocean circulation and fresh water fluxes to examine seasonal pCO2 changes in highly-variable coastal environments. We first use the Modular Ocean Model version 6 (MOM6) and biogeochemical module Carbon Ocean Biogeochemistry And Lower Trophics version 2 (COBALTv2) at a half degree resolution to simulate the coastal CO2 dynamics and evaluate it against pCO2 from the Surface Ocean CO2 Atlas database (SOCAT) and from the continuous coastal pCO2 product generated from SOCAT by a two-step neuronal network interpolation method (coastal-SOM-FFN, Laruelle et al., 2017). The MOM6-COBALT model not only reproduces the observed spatio-temporal variability in pCO2 but also in sea surface temperature, salinity, nutrients, in most coastal environments except in a few specific regions such as marginal seas. Based on this evaluation, we identify coastal regions of ‘high’ and ‘medium’ model skill where the drivers of coastal pCO2 seasonal changes can be examined with reasonable confidence. Second, we apply our decomposition method in three contrasted coastal regions: an Eastern (East coast of the U.S) and a Western (the Californian Current) boundary current and a polar coastal region (the Norwegian Basin). Results show that differences in pCO2 seasonality in the three regions are controlled by the balance between ocean circulation, biological and thermal changes. Circulation controls the pCO2 seasonality in the Californian Current, biological activity controls pCO2 in the Norwegian Basin, while the interplay between biology, thermal and circulation changes is key in the East coast of the U.S. The refined approach presented here allows the attribution of pCO2 changes with small residual biases in the coastal ocean, allowing future work on the mechanisms controlling coastal air-sea CO2 exchanges and how they are likely to be affected by future changes in sea surface temperature, hydrodynamics and biological dynamics.

Highlights

The ocean plays an important role in offsetting human-induced carbon dioxide (CO2) emissions associated with cement production and fossil fuel combustion (Friedlingstein et al, 2019)
The global sea surface temperature (SST) field simulated by the model reproduces the strong large-scale tropical 260 to polar SST gradients, with a global median bias of -0.2 °C (Fig. 2a-c), and biases at the scale of MARCATS regions ranging from 0 °C in the North East Atlantic (M17) to 1.3 °C in the East coast of the U.S (M10, Fig. 3a and Table S1)
In this study, an OGCM (MOM6-COBALT) which is primarily designed for the open ocean was used to examined sea surface pressure of CO2 (pCO2) seasonality in the coastal domain

Summary

Introduction

The ocean plays an important role in offsetting human-induced carbon dioxide (CO2) emissions associated with cement production and fossil fuel combustion (Friedlingstein et al, 2019). The ocean is a net sink that absorbs roughly one 35 quarter of the anthropogenic CO2 emitted into the atmosphere (-2.5 ± 0.6 Petagram of carbon per year (Pg C yr-1) for the 20092018 decade, Friedlingstein et al, 2019). In recent decades, significant progress have been made with regard to the quantification and analysis of the spatial distribution of the coastal air-sea CO2 exchange (FCO2) globally and regionally (e.g., Borges et al, 2005; Cai, 2011; Chen et al, 2013; Laruelle et al, 2010, 2014, Roobaert et al, 2019). A more in-depth analysis revealed that the majority of the coastal seasonal FCO2 variations stems from the air-sea gradient in partial pressure of CO2 (pCO2), changes in wind speed and sea-ice cover can be significant regionally

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