Gaining insight into the seasonal variability of CO2 at ocean station P using an upper ocean model

Abstract

An eddy-kinetic energy model for simulations of the oceanic vertical mixing (Gasparet al., 1990, Journal of Geophysical Research, 95, 16179–16193) is used to investigate the seasonal cycle of dissolved total CO₂ and CO₂ partial pressure at Station P (Gulf of Alaska) during the years 1971 and 1972.

The model simulates relatively weak seasonal variations of surface seawater pCO₂. All processes appear to contribute to the same order of magnitude: interaction of gas exchange and biology tends to compensate for the thermodynamical effect on pCO₂. The influence of various model parameters (total annual production of O₂, P, depth of photic zone, z₀, gas transfer velocity, K) on the seasonal variability of seawater pCO₂ and air-sea CO₂ flux is examined. Total annual production, P, seems to be the most influential.

The model also suggests a strong short-term variability of pCO₂ during episodic events. Model results show that the area around Station P behaves as a weak or neutral CO₂ source during summer and as a net sink during other seasons. The net annual CO₂ flux is an invasion flux, with values of 0.88 and 0.70 mol m⁻² y⁻¹ for 1971 and 1972, respectively, in agreement with the flux of 0.7 mol m⁻² y⁻¹ derived from observations during 1973–1978 (Wong and Chan, 1991, Tellus, 43B, 206–223).