Contrasting Controls on Air–Sea CO ₂ Fluxes in the Pearl River Estuary and Adjacent Coastal Waters Under Typhoon Intensification and Track Shifts: A Modeling Study

Abstract

Northwest Pacific typhoons frequently affect the northern South China Sea, but their impacts on air‐sea CO ₂ flux (FCO ₂ ) in the Pearl River Estuary (PRE) and adjacent coastal waters remain poorly constrained. We used a coupled physical‐biogeochemical model to simulate Typhoon Hato (2017) and conducted sensitivity experiments with intensified winds and northward‐shifted tracks to quantify their effects on FCO ₂ variability. Hato converted both regions into pronounced CO ₂ sources, with mean estuarine FCO ₂ increasing ∼15× relative to pre‐typhoon conditions. During the event, FCO ₂ variability was governed primarily by air‐sea partial pressure difference of CO ₂ (Δ p CO ₂ ); in coastal waters it arose mainly from vertical mixing of DIC, whereas in the estuary it reflected TA advection. Wind‐intensification experiments showed that FCO ₂ increased nonlinearly with wind speed. In coastal waters, the drivers of FCO ₂ anomaly (ΔFCO ₂ ) shifted toward a dominance of the gas transfer velocity (k), whereas estuarine anomalies remained jointly controlled by gas transfer velocity and non‐thermal p CO ₂ ( p CO _{2_noSST} ); sea surface temperature (SST) effects remained secondary. Track‐shift experiments redistributed process importance. In coastal waters, the contribution from the gas transfer ( k ) term declined and enhanced p CO _{2_noSST} moderated ΔFCO ₂ peaks, whereas in the estuary an increased SST contribution and reduced k‐term contribution amplified peak variability. This paper shows that extreme typhoons trigger brief, intense nearshore CO ₂ outgassing pulses, and that future intensity increased and track shifts may reshape FCO ₂ magnitude and controls, increasing coastal carbon‐budget uncertainty.