Why are Surface Ocean pH and CaCO3Saturation State Often out of Phase in Spatial Patterns and Seasonal Cycles?

As two most important metrics for ocean acidification (OA), both pH and calcium carbonate mineral saturation states (Ω) respond sensitively to anthropogenic carbon dioxide (CO2). However, contrary to intuition, they are often out of phase in the global surface ocean, both spatially and seasonally. For example, during warm seasons, Ω is lowest at high‐latitude seas where there are very high pH values, challenging our understanding that high‐latitude seas are a bellwether for global OA. To explain this phenomenon, we separate spatial and seasonal variations of both pH and Ω into thermal components mainly associated with internal acid‐base equilibrium of seawater CO2systems, and nonthermal components mainly associated with external CO2addition/removal using a global surface ocean climatological data set. We find that surface pH change is controlled by the balance between its thermal and nonthermal components, which are out of phase but comparable in magnitude. In contrast, surface Ω change is dominated by its nonthermal components, with its thermal components in phase and significantly smaller in magnitude. These findings explain why surface ocean pH and Ω are often out of phase in spatial patterns and seasonal cycles. When pH is primarily controlled by nonthermal components e.g., gas exchange, mixing and biology, pH and Ω will be in phase because their nonthermal components are intrinsically in phase. In comparison, when pH is primarily controlled by thermal components for example, rapid seasonal cooling or warming, pH and Ω will be out of phase because thermal and nonthermal components of pH are out‐of‐phase in nature. 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 CO2systems. 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. pH is controlled by the balance of thermal components via acid‐base equilibrium and nonthermal components mainly via CO2addition/removalCarbonate saturation state (Ω) is dominated by nonthermal components mainly via external CO2addition/removalThese differences explain why surface pH and Ω are often out of phase in spatial patterns and seasonal cycles pH is controlled by the balance of thermal components via acid‐base equilibrium and nonthermal components mainly via CO2addition/removal Carbonate saturation state (Ω) is dominated by nonthermal components mainly via external CO2addition/removal These differences explain why surface pH and Ω are often out of phase in spatial patterns and seasonal cycles