Interannual variability of the upper ocean carbon cycle at station ALOHA near Hawaii

Abstract

We investigate interannual variability of the upper ocean carbon cycle in the subtropical North Pacific on the basis of a 14‐year time series (1988–2002) of carbon parameters from Station ALOHA, the site of the U.S. JGOFS Hawaii Ocean Time series program (HOT). The data reveal substantial interannual variability in near‐surface concentrations of dissolved inorganic carbon normalized to constant salinity (sDIC, peak‐to‐peak amplitude of ±4 μmol kg−1), computed ocean surface partial pressure of CO2 (pCO2, ±6 ppm), and the 13C/12C ratio of DIC (±0.07‰). A strong anticorrelation (r = −0.50) between interannual anomalies in sea‐surface temperature (SST) and sDIC is found, which tends to suppress the correlation of either of these properties with pCO2. In contrast, no significant correlation (p < 0.05) is found between anomalies of the 13C/12C ratio of DIC and any other parameter. A diagnostic box model analysis reveals that interannual variability of near‐surface ocean sDIC is driven by air‐sea gas exchange, net community production, and lateral transport. In warmer than normal years the seasonal carbon cycle tends to be weakened, with a sDIC reduction in the mixed layer caused by diminished gas exchange and lateral transport outweighing the effect of less intense sDIC removal by net community production. This explains the observed anticorrelation between SST and sDIC. Interannual (peak‐to‐peak) variability of air‐sea gas exchange (±0.4 mol m−2 yr−1, i.e., 40% of the annual mean value) is primarily governed by strongly covarying changes in SST and wind speeds. Net community production varies interannually by up to ±0.9 mol m−2 yr−1 (39%) and tends to be associated with changes in horizontal transport. Less than 20% of the interannual variance in sDIC near Hawaii can be explained by the Pacific Decadal Oscillation (PDO), and an even smaller fraction (less than 5%) by the El Niño‐Southern Oscillation (ENSO). Because SST variations over a sizable fraction of the North Pacific subtropical gyre vary in concert with those at Station ALOHA, it is plausible that air‐sea fluxes in this region vary also synchronously, resulting in a variability of the atmospheric CO2 sink strength of the North Pacific subtropical gyre of up to ±0.2 PgC yr−1.