This study investigated processes responsible for the acidification of the mixed layer water at station K2 ( \(47^{\circ }\) N, \(160^{\circ }\) E) in the western subarctic North Pacific by analyzing physical and biogeochemical variables during 1850–2014 in a Geophysical Fluid Dynamics Laboratory Earth system model of Coupled Model Intercomparison Project Phase 6 under historical carbon dioxide ( \(\hbox {CO}_{2}\) ) emissions. Analysis revealed an accelerated pH decline at a rate of −0.0015 \(\text{year}^{-1}\) , which began around 1960. An increase in dissolved inorganic carbon (DIC) is found to be the principal factor of the pH decline. DIC budget analysis of the mixed layer identified the primary physical process accelerating the DIC increase during 1960–2014: enhanced vertical entrainment of the DIC-rich deep water into the mixed layer during autumn–winter (rate of 3.1 \({{{{\upmu }}}} \text{mol}\,\text{L}^{-1}\,\text{month}^{-1}\) ). This enhancement is attributed to the equatorial Pacific frequent high sea surface temperature condition due to global warming and El Niño events through the model’s atmospheric teleconnection. \(\hbox {CO}_\text{2}\) gas exchange at the sea surface contributes secondarily to the DIC increase (0.16 \({\upmu }\text{mol}\,\text{L}^{-1}\,\text{month}^{-1}\) ). Meanwhile, horizontal advection (−0.6 \({\upmu }\text{mol}\,\text{L}^{-1}\,\text{month}^{-1}\) ) and biological consumption (−2.6 \({\upmu }\text{mol}\,\text{L}^{-1}\,\text{month}^{-1}\) ) partially counteract DIC input. These physical and biological processes collectively resulted in a simulated DIC increase of 0.043 \({\upmu }\text{mol}\,\text{L}^{-1}\,\text{month}^{-1}\) (i.e., 0.52 \({\upmu }\text{mol}\,\text{L}^{-1}\,\text{year}^{-1}\) ), consistent with the observed increase rate.
