The Key Influences of Atmospheric Transport on the Seasonal Variation of Atmospheric CO ₂ Across the Northern Hemisphere

Abstract Atmospheric transport plays a crucial yet poorly quantified role in shaping atmospheric CO 2 seasonality across the Northern Hemisphere. This study used GEOS‐Chem simulations and a statistical decomposition to explicitly quantify the import and export effects of atmospheric transport on CO 2 seasonal amplitude and phase in the troposphere. We found that atmospheric transport explained 28%, the second to terrestrial ecosystems, of the boreal CO 2 seasonality and exceeded terrestrial ecosystems in the northern mid‐to‐low latitudes (>50%). The import (export) enhanced (suppressed) the seasonal amplitude, with the largest enhancement (decrease) in the northern middle latitudes, and caused phase shifts, with the minimum phase showing the largest advances in the northern tropics. The net effects revealed that the northern middle latitudes served as the strongest sources of CO 2 seasonality, followed by the northern high latitudes, while the northern tropics acted as the sinks. These results provide new insights on how atmospheric circulation redistributes CO 2 seasonality, advancing our understanding of the coupling between surface carbon fluxes and atmospheric CO 2 .