Annual and Interannual Variations of Hydrographic Properties Observed Air-sea Carbon Dioxide Exchange at Station ALOHA

Christopher D. Winn

Abstract

Total dissolved inorganic carbon (DIC) and titration alkalinity have been measured over a four-year period at Station ALOHA. These measurements comprise the longest time-series record of carbon system parameters in the central Pacific Ocean. DIC undergoes a regular seasonal oscillation with a maximum in winter and a minimum in summer. This annual cycle is tightly correlated with upper ocean temperature. I interpret the annual cycle in upper ocean DIC concentration to be a consequence of air-sea carbon dioxide exchange driven by temperature dependent changes in carbon dioxide solubility. Titration alkalinity shows no seasonal variability within the analytical precision of the analysis.

The inorganic carbon system time-series record has been used in combination with the atmospheric carbon dioxide partial pressure record at Mauna Loa, Hawaii and the temperature and barometric pressure data from NODC buoy #51001, to estimate the flux of carbon dioxide between the ocean and the atmosphere. My results show that flux of carbon dioxide between the ocean and the atmosphere oscillates with season with the ocean being a slight source of carbon to the atmosphere in summer and a larger sink for atmospheric carbon dioxide in winter. On an annual basis, the ocean at the HOT site is a sink for approximately 0.7 moles C m^-2 yr^-1. I hypothesize that the net flux of atmospheric carbon dioxide into the ocean at the HOT site is balanced by the transport of warm DIC depleted water from the tropical Pacific Ocean via Ekman transport. I further suggest that the carbon dioxide added to the ocean surface at the HOT site is returned to the equatorial ocean in the upper reaches of the thermocline as a result of the downwelling which occurs at this latitude. Support for this hypothesis is provided by a simple box model and by the latitudinal gradient DIC in the subtropical Pacific Ocean.