21 May 2024
 | 21 May 2024
Status: this preprint is open for discussion.

Hydrological cycle amplification imposes spatial pattern on climate change response of ocean pH and carbonate chemistry

Allison Hogikyan and Laure Resplandy

Abstract. Ocean CO2 uptake and acidification in response to human activities are driven primarily by the rise in atmospheric CO2, but are also modulated by climate change. Existing work suggests that this `climate effect' influences the uptake and storage of anthropogenic carbon and acidification via the global increase in ocean temperature, although some regional responses have been attributed to changes in circulation or biological activity. Here, we investigate spatial patterns in the climate effect on surface-ocean acidification (and the closely related carbonate chemistry) in an Earth System Model under a rapid CO2-increase scenario, and identify another culprit. We show that the amplification of the hydrological cycle, a robustly simulated feature of climate change, is largely responsible for the spatial patterns in this climate effect at the sea surface. This `hydrological effect' can be understood as a subset of the total climate effect which includes warming, hydrological cycle amplification, circulation and biological changes. We demonstrate that it acts through two primary mechanisms: (i) directly diluting or concentrating dissolved ions by adding or removing freshwater and (ii) altering the sea surface temperature, which influences the solubility of dissolved inorganic carbon (DIC) and acidity of seawater. The hydrological effect opposes acidification in salinifying regions, most notably the subtropical Atlantic, and enhances acidification in freshening regions such as the western Pacific. Its single strongest effect is to dilute the negative ions that buffer the dissolution of CO2, quantified as `Alkalinity'. The local changes in Alkalinity, DIC, and pH linked to the pattern of hydrological cycle amplification are as strong as the (largely uniform) changes due to warming, explaining the weak increase in pH and DIC seen in the climate effect in the subtropical Atlantic Ocean.

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Allison Hogikyan and Laure Resplandy

Status: open (until 10 Jul 2024)

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Allison Hogikyan and Laure Resplandy

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Allison Hogikyan and Laure Resplandy


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Short summary
Rising atmospheric CO2 influences ocean carbon chemistry leading to ocean acidification. Global warming introduces spatial patterns in the intensity of ocean acidification. We show that the most prominent spatial patterns are controlled by warming-driven changes in rainfall and evaporation, and not by the direct effect of warming on carbon chemistry and pH. This rainfall/evaporation effect opposes acidification in saltier parts of the ocean and enhances acidification in fresher regions.