We
investigated the interannual variability of the partial pressure of carbon
dioxide (pCO2) in the surface waters of the western subarctic gyre
(155°E to 165°E, 48°N to 53°N) and the Alaska Gyre (AG, 195°E to 210°E, 45°N to
52°N) for a period of 9 years. We used automated measurements of pCO2
in the surface water (pCO2sw) and the air (pCO2air) as
well as sea surface temperature (SST) and salinity (S) obtained from the
Japanese-Canadian joint Volunteer Observing Ship (VOS) program. We observed
annual trends in the pCO2sw and based on simple least square fit to
observed data, the surface waters in the WSG showed a gradual pCO2sw
increase of 0.4 ppm yr-1 which was three times larger than in the AG
(1.8 ppm yr-1) for the 9-year period. In the WSG, this was about
half of the estimated atmospheric pCO2 increase for the whole period
(10 ppm or 1.2 ppm yr-1), whereas gas exchange explained much of the
increase in the AG (pCO2air increased 1.6 ppm yr-1).
Interestingly, the two gyres showed opposite annual trends in the SST and
salinity and in the WSG we observed a salinity and SST increase of 0.018 yr-1
and 0.07°C yr-1 (0.56°C for the whole study period), respectively,
whereas we observed a small freshening of 0.015 yr-1 and a cooling
trend of about 0.11°C yr-1 in the AG. We examine the possible
mechanisms to explain the annual trends in pCO2, based on the
observed changes in SST and salinity as well as observations made by other
investigators.
Author: M. Chierici, A. Fransson, Y. Nojiri (melissa at chem dot gu dot se)
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