Consistently with the past decades observations, CMIP5 Earth System Models project highest acidification rates in subsurface waters. Using 7 ESMs, we find that high acidification rates in mode and intermediate waters (MIW) on centennial timescales (-0.0008 ± 4 × 10–5 yr–1 to -0.0023 ± 0.0001 yr–1 depending on the scenario) are predominantly explained by the geochemical effect of increasing atmospheric CO2, whereas physical and biological climate change feedbacks explain less than 10% of the simulated changes. MIW are characterized by a larger surface area to volume ratio than deep and bottom waters leading to 5 to 10 times larger carbon uptake. In addition, MIW geochemical properties result in a sensitivity to increasing carbon concentration twice larger than surface waters (Δ[H+] of +1.2 mmol.m–3 for every mmol.m–3 of dissolved carbon in MIW vs. +0.6 in surface waters). Low pH transported by mode and intermediate waters are likely to influence surface pH in upwelling regions decades after their isolation from the atmosphere.

Resplandy L., Bopp L., Orr J. C. & Dunne J. P., in press. Role of mode and intermediate waters in future ocean acidification: analysis of CMIP5 models.Geophysical Research Letters. Article.