Seasonal and long‐term dynamics of the upper ocean carbon cycle at Station ALOHA near Hawaii

Abstract

Long‐term trends and average seasonal variability in the upper ocean carbon cycle are investigated at Station ALOHA, the site of the U.S. JGOFS Hawaii Ocean Time series program (HOT), on the basis of a 14‐year time series (1988–2002) of dissolved inorganic carbon (DIC), alkalinity, and 13C/12C ratio of DIC data. Salinity‐normalized DIC (sDIC) and computed oceanic pCO2 show distinct upward trends of 1.2 ± 0.1 μmol kg−1 yr−1 and 2.5 ± 0.1 μatm yr−1, respectively, while the 13C/12C isotopic ratio of DIC (expressed as δ13Coc) decreases at a mean rate of −0.027 ± 0.001‰ yr−1. More than half of the rates of change in sDIC and oceanic pCO2, and most of the change in 13C/12C, are attributed to the uptake of isotopically light anthropogenic CO2 from the atmosphere. The residual trends appear to be caused mainly by a regional change in the net freshwater budget, perhaps associated with a regime change of the North Pacific climate system near 1997. Computed oceanic pCO2 is below atmospheric pCO2 for nearly the entire year, leading to an annual mean surface ocean pCO2 undersaturation of about 18 μatm, and to an annual uptake of CO2 from the atmosphere, which we compute to be 1.0 ± 0.1 mol m−2 yr−1. We estimate that about 30% of this flux relates to the uptake of anthropogenic CO2, and the remainder to biologically mediated export of organic carbon. Using a modified version of the diagnostic model of Gruber et al. [1998], constrained by δ13Coc, we infer net community production of organic carbon (NCP) to be the dominant process generating the observed seasonal variability in sDIC. The annual integral of NCP, 2.3 ± 0.8 mol m−2 yr−1, is comparable to previous estimates of biological production in the subtropical North Pacific. Annually integrated fluxes of air‐sea gas exchange and NCP at Station ALOHA are each about two thirds of those computed for the upper ocean near Bermuda using similar methods of estimation [Gruber et al., 1998, 2002]. However, the seasonal amplitudes of sDIC and δ13Coc near Hawaii are only half as large as near Bermuda, because air‐sea gas exchange and NCP tend to oppose each other near Hawaii, but reinforce each other near Bermuda.