Drivers of 21<sup>st</sup> Century carbon cycle variability in the North Atlantic Ocean

Abstract

The North Atlantic carbon sink is a prominent component of global climate, storing large amounts of atmospheric carbon dioxide (CO 2 ), but this basin's CO 2 uptake variability presents challenges for future climate prediction. A comprehensive mechanistic understanding of the processes that give rise to year-to-year (interannual) and decade-to-decade (decadal) variability in the North Atlantic's dissolved inorganic carbon (DIC) inventory is lacking. Here, we numerically simulate the oceanic response to human-induced (anthropogenic) climate change from the industrial era to the year 2100. The model dis-5 tinguishes how different physical, chemical, and biological processes modify the basin's DIC inventory; the saturation, soft tissue, and carbonate pumps, anthropogenic emissions, and other processes causing air-sea disequilibria. There are four 'natural' pools (saturation, soft tissue, carbonate, and disequilibrium), and an 'anthropogenic' pool. Interannual variability of the North Atlantic DIC inventory arises primarily due to temperature-and alkalinity-induced changes in carbon solubility (saturation concentrations). A mixture of saturation and anthropogenic drivers cause decadal variability. Multidecadal variability 10 results from the opposing effects of saturation versus soft tissue carbon, and anthropogenic carbon uptake. By the year 2100, the North Atlantic gains 66 Pg (1 Pg = 10 15 grams) of anthropogenic carbon, and the natural carbon pools collectively decline by 4.8 Pg. The first order controls on interannual variability of the North Atlantic carbon sink size are therefore largely physical, and the biological pump emerges as an important driver of change on multidecadal timescales. Further work should identify specifically which physical processes underlie the interannual saturation-dominated DIC variability documented here.15

Processes Driving Global Interior Ocean pH Distribution

“…In this paper, we consider the measured pH (pH 2002 ) to be the baseline and explicitly calculate how much the interior ocean pH is changed by each process, as well as the accumulation of anthropogenic carbon (ΔpH anth ). Our analysis is based on well‐established methods that have been used to decompose DIC and TA in several previous studies (e.g.,Cameron et al, ; Couldrey et al, ; Eggleston & Galbraith, ; Gruber et al, ; Sarmiento & Gruber, ; Schmittner et al, ). In addition, a more simplified way to decompose interior ocean pH is detailed in Millero (), following Park ().…”