<p>The newly developed third-generation Beijing Climate Center Climate System Model Medium Resolution (BCC-CSM3-MR) exhibits pronounced and seasonally varying sea surface temperature (SST) biases across the North Pacific, characterized by a “sandwich” pattern with warm biases east of Japan and cold biases in both the Okhotsk Sea and the subtropical North Pacific. These SST biases are closely associated with systematic errors in surface ocean currents, surface heat fluxes, and wind forcing, including a northward-shifted and weakened Kuroshio Extension, misrepresented westerlies and easterlies, and erroneous seasonal cooling and heating. Regional diagnostics reveal that cold SST biases intensify from late spring through early autumn and are subsequently transported downward, driven by underestimated stratification, excessive vertical mixing, and enhanced vertical shear of both zonal and meridional currents. Mixed-layer heat budget analyses further indicate that surface heat flux errors dominate SST bias formation in the Okhotsk Sea and the subtropical North Pacific, whereas the residual term, including the vertical processes, plays a leading role in the northern North Pacific during late autumn to spring. Sensitivity experiments using a forced ocean model provide a heuristic demonstration that biases in atmospheric forcing can contribute to surface cold signals consistent with those simulated by BCC-CSM3-MR. In particular, shortwave radiation errors produce the largest cooling, while precipitation and sea-level pressure biases have relatively minor impacts. Overall, the SST biases in BCC-CSM3-MR likely arise from a combined influence of atmospheric forcing errors and excessive oceanic vertical processes, highlighting key pathways for improving future model performance in the North Pacific.</p>