Recent acceleration of the sea surface f CO 2 growth rate in the North Atlantic subpolar gyre (1993–2008) revealed by winter observations

• Published in: Global biogeochemical cycles Vol. 24; no. 4
• Main Authors: Metzl, Nicolas, Corbière, Antoine, Reverdin, Gilles, Lenton, Andrew, Takahashi, Taro, Olsen, Are, Johannessen, Truls, Pierrot, Denis, Wanninkhof, Rik, Ólafsdóttir, Solveig R., Olafsson, Jon, Ramonet, Michel
• Format: Journal Article
• Language: English
• Published: 01-12-2010
• ISSN: 0886-6236; 1944-9224
• DOI: 10.1029/2009GB003658

Recent studies based on ocean and atmospheric carbon dioxide (CO
2
) observations, suggesting that the ocean carbon uptake has been reduced, may help explain the increase in the fraction of anthropogenic CO
2
emissions that remain in the atmosphere. Is it a response to climate change or a signal of ocean natural variability or both? Regional process analyses are needed to follow the ocean carbon uptake and to enable better attributions of the observed changes. Here, we describe the evolution of the surface ocean CO
2
fugacity (
f
CO
2
oc
) over the period 1993–2008 in the North Atlantic subpolar gyre (NASPG). This analysis is based primarily on observations of dissolved inorganic carbon (DIC) and total alkalinity (TA) conducted at different seasons in the NASPG between Iceland and Canada. The
f
CO
2
oc
trends based on DIC and TA data are also compared with direct
f
CO
2
measurements obtained between 2003 and 2007 in the same region. During winters 1993–2003, the
f
CO
2
oc
growth rate was 3.7 (±0.6)
μ
atm yr
−1
, higher than in the atmosphere, 1.8 (±0.1)
μ
atm yr
−1
. This translates to a reduction of the ocean carbon uptake primarily explained by sea surface warming, up to 0.24 (±0.04) °C yr
−1
. This warming is a consequence of advection of warm water northward from the North Atlantic into the Irminger basin, which occurred as the North Atlantic Oscillation (NAO) index moved into a negative phase in winter 1995/1996. In winter 2001–2008, the
f
CO
2
oc
rise was particularly fast, between 5.8 (±1.1) and 7.2 (±1.3)
μ
atm yr
−1
depending on the region, more than twice the atmospheric growth rate of 2.1 (±0.2)
μ
atm yr
−1
, and in the winter of 2007–2008 the area was supersaturated with CO
2
. As opposed to the 1990s, this appears to be almost entirely due to changes in seawater carbonate chemistry, the combination of increasing DIC and decreasing of TA. The rapid
f
CO
2
oc
increase was not only driven by regional uptake of anthropogenic CO
2
but was also likely controlled by a recent increase in convective processes‐vertical mixing in the NASPG and cannot be directly associated with NAO variability. The
f
CO
2
oc
increase observed in 2001–2008 leads to a significant drop in pH of −0.069 (±0.007) decade
−1
.