Abstract. The North Atlantic is one of the major ocean sinks for
natural and anthropogenic atmospheric CO2. Given the variability of the
circulation, convective processes or warming–cooling recognized in the high
latitudes in this region, a better understanding of the CO2 sink
temporal variability and associated acidification needs a close inspection
of seasonal, interannual to multidecadal observations. In this study, we
investigate the evolution of CO2 uptake and ocean acidification in the
North Atlantic Subpolar Gyre (50–64∘ N) using
repeated observations collected over the last 3 decades in the framework
of the long-term monitoring program SURATLANT (SURveillance de
l'ATLANTique). Over the full period (1993–2017) pH decreases (−0.0017 yr−1) and fugacity of CO2 (fCO2) increases (+1.70 µatm yr−1). The trend of fCO2 in surface water is slightly less
than the atmospheric rate (+1.96 µatm yr−1). This is mainly
due to dissolved inorganic carbon (DIC) increase associated with the
anthropogenic signal. However, over shorter periods (4–10 years) and
depending on the season, we detect significant variability investigated in
more detail in this study. Data obtained between 1993 and 1997 suggest a
rapid increase in fCO2 in summer (up to +14 µatm yr−1)
that was driven by a significant warming and an increase in DIC for a short
period. Similar fCO2 trends are observed between 2001 and 2007 during
both summer and winter, but, without significant warming detected, these
trends are mainly explained by an increase in DIC and a decrease in
alkalinity. This also leads to a pH decrease but with contrasting trends
depending on the region and season (between −0.006 and −0.013 yr−1). Conversely, data obtained during the last decade
(2008–2017) in summer show a cooling of surface waters and an increase in
alkalinity, leading to a strong decrease in surface fCO2 (between −4.4
and −2.3 µatm yr−1; i.e., the ocean CO2 sink increases).
Surprisingly, during summer, pH increases up to +0.0052 yr−1 in the
southern subpolar gyre. Overall, our results show that, in addition to the
accumulation of anthropogenic CO2, the temporal changes in the uptake
of CO2 and ocean acidification in the North Atlantic Subpolar Gyre
present significant multiannual variability, not clearly directly associated
with the North Atlantic Oscillation (NAO). With such variability it is
uncertain to predict the near-future evolution of air–sea CO2 fluxes
and pH in this region. Thus, it is highly recommended to maintain long-term
observations to monitor these properties in the next decade.
Effects of ocean acidification, warming and melting of sea ice on aragonite saturation of the Canada Basin surface water