Seasonal and interannual inorganic carbon dynamics in the Northeast Pacific

The Northeast Pacific is an important net sink of atmospheric CO2. However, substantial natural variability modulates the long-term increase of seawater partial pressure of CO2 (pCO2), potentially influencing the magnitude of the CO2 sink. In addition to the seasonal cycle, the Pacific Decadal Oscillation (PDO) is known to play a role in driving a large fraction of the non-seasonal variability in the region. Yet, the magnitude of this natural variability, especially periods of high surface dissolved inorganic carbon (DIC), are not well constrained. Here we quantify the seasonal and non-seasonal variability in DIC and pCO2 using observations from the Line P program, the longest marine carbonate system time-series transect in the NE Pacific (1990-2019), as well as an ensemble of historical simulations with an Earth system model (EC-Earth-CC). Preliminary results show that the mean amplitude of the DIC seasonal cycle is similar across our NE Pacific transect (23-30 µmol kg-1) and decreases with depth to less than 5 µmol kg-1 at 60 to 70 m. In contrast, the non-seasonal variability remains approximately constant with depth, ranging between 10 – 20 µmol kg-1. We quantify the role of the PDO in driving this residual non-seasonal variability, and analyse the contrasting impact of temperature and DIC changes in controlling surface pCO2 during opposite phases of the PDO.