Seasonal effects of temperature and salinity on the partial pressure of CO₂ in seawater

Abstract

The exchange of CO₂ between the atmosphere and the sea is of major importance to our understanding of the climatic consequences of anthropogenic CO₂. Because the solubility of CO₂ and the dissociation of carbonic acid in seawater are modulated by temperature and salinity, the detection of long-term changes in the oceanic carbonate system, first requires that the comparatively large short-term variations in the partial pressure of CO₂ should be characterized. We present here the first direct observations of seasonal variations in the partial pressure of CO₂ in the subtropical gyres of the North and South Pacific Ocean. These variations compare well with the variations predicted from observed changes in temperature and salinity using empirical equations which do not require the determination of alkalinity or total inorganic carbon, and the agreement is improved by the inclusion of air–sea exchange in the model calculations. Thus we predict that specific alkalinity within the two gyres remains extremely constant, which is confirmed by carbonate equilibrium calculations based on our measurements of total inorganic carbon.