Although both pH and calcium carbonate mineral saturation states (Ω) are good metrics for ocean acidification, in the global surface ocean their spatial patterns and seasonal cycles are often out of phase, which appears counter intuitive. To explain this, we separate pH and Ω changes into thermal and nonthermal components. Thermal components are mainly related to the temperature driven internal acid-base equilibrium of seawater CO₂ systems. Nonthermal components are the remaining changes, reflecting the effects of other non-temperature processes such as air-sea gas exchange, mixing and biology or a combination of these processes. We find that pH is controlled by the balance between thermal and nonthermal components, which are out of phase but comparable in magnitude, while Ω is almost always dominated by nonthermal components. These findings explain why surface ocean pH and Ω are often out of phase in spatial patterns and seasonal cycles. When pH is more controlled by nonthermal components than thermal components, pH and Ω will be in phase since their nonthermal components are intrinsically in phase. In contrast, when pH is more controlled by thermal components, pH and Ω will be out of phase because of the out-of-phase between thermal and nonthermal components of pH.