Oceans worldwide are undergoing acidification due to the penetration of anthropogenic CO2 from the atmosphere. The rate of acidification generally diminishes with increasing depth. Yet, slowing down of the thermohaline circulation due to global warming could reduce the pH in the deep oceans, as more organic material would decompose with a longer residence time. To elucidate this process, a time-series study at a climatically sensitive region with sufficient duration and resolution is needed. Here we show that deep waters in the Sea of Japan are undergoing reduced ventilation, reducing the pH of seawater. As a result, the acidification rate near the bottom of the Sea of Japan is 27% higher than the rate at the surface, which is the same as that predicted assuming an air–sea CO2 equilibrium. This reduced ventilation may be due to global warming and, as an oceanic microcosm with its own deep- and bottom-water formations, the Sea of Japan provides an insight into how future warming might alter the deep-ocean acidification.
Deep oceans may acidify faster than anticipated due to global warming
Ocean to Climate 
Given that the thermohaline circulation in the Atlantic (AMOC) is projected to reduce in the future climate, this research implies that the acidification rate may increase in the deep Atlantic Ocean.
I have not followed oceanography articles in the Sea of Japan – East Sea (JES) for a long time. But, it is very interesting to learn that the thermohaline circulation in JES has decreased so much during the last 40 years (http://onlinelibrary.wiley.com/doi/10.1029/2001GL013078/abstract). In the Atlantic, there is no clear evidence of a long-term MOC slowdown (although the observed AMOC at 26.5N has indeed decreased between 2004-2016). As this and other articles suggest, perhaps, the ongoing changes in JES may provide some insights as to what may happen in the North Atlantic Ocean in the future when the AMOC finally slows down due to global warming