Seasonal and long-term dynamics of the upper ocean carbon cycle at station ALOHA near Hawaii

Charles D. Keeling, Scripps Institution of Oceanography, University of California, La Jolla.
Holger Brix, and Nicolas Gruber, IGPP and Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles

  Long term trends and average seasonal variability in the upper ocean
  carbon cycle are investigated at Sta. ALOHA, the site of the
  U.S. JGOFS Hawaii Ocean Time-series program (HOT), on the basis of a
  fourteen year time-series (1988-2002) of dissolved inorganic carbon
  (DIC), alkalinity, and 13C/12C ratio of DIC data.  Salinity
  normalized DIC and computed oceanic pCO2 show distinct upward trends
  of 1.2±0.1 µmol/kg/yr and 2.5±0.1 µatm/yr,
  respectively, while the 13C/12C isotopic ratio of DIC decreases at a
  mean rate of -0.027±0.001 o/oo/yr.  More than half of the
  rates of change in DIC 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/m2/yr. 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 the 13C/12C
  ratio of DIC, we infer net community production of organic carbon
  (NCP) to be the dominant process generating the observed seasonal
  variability in DIC. The annual integral of NCP, 2.3±0.8
  mol/m2/yr, is comparable to previous estimates of biological
  production in the subtropical North Pacific.  Annually integrated
  fluxes of air-sea gas exchange and NCP at Sta. ALOHA are each about
  two thirds of those computed for the upper ocean near Bermuda using
  similar methods of estimation (Gruber et al., 1998, Gruber et al.,
  2002).  However, the seasonal amplitudes of DIC and its 13C/12C
  ratio 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.

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