Inter-annual variability of the carbon dioxide oceanic sink south of Tasmania

Abstract

Abstract.We compiled a large data-set from 22 cruises spanning from 1991 to 2003, of the partial pressure of CO 2 (pCO 2 ) in surface waters over the continental shelf (CS) and adjacent open ocean (43 • to 46 • S; 145 • to 150 • E), south of Tasmania. Climatological seasonal cycles of pCO 2 in the CS, the subtropical zone (STZ) and the subAntarctic zone (SAZ) are described and used to determine monthly pCO 2 anomalies. These are used in combination with monthly anomalies of sea surface temperature (SST) to investigate inter-annual variations of SST and pCO 2 . Monthly anomalies of SST (as intense as 2 • C) are apparent in the CS, STZ and SAZ, and are indicative of strong inter-annual variability that seems to be related to large-scale coupled atmosphere-ocean oscillations. Anomalies of pCO 2 normalized to a constant temperature are negatively related to SST anomalies. A reduced winter-time vertical input of dissolved inorganic carbon (DIC) during phases of positive SST anomalies, related to a poleward shift of westerly winds, and a concomitant local decrease in wind stress is the likely cause of the negative relationship between pCO 2 and SST anomalies. The observed pattern is an increase of the sink for atmospheric CO 2 associated with positive SST anomalies, although strongly modulated by interannual variability of wind speed. Assuming that phases of positive SST anomalies are indicative of the future evolution of regional ocean biogeochemistry under global warming, we show using a purely observational based approach that some provinces of the Southern Ocean could provide a potential negative feedback on increasing atmospheric CO 2 .

Carbonate system in the water masses of the Southeast Atlantic sector of the Southern Ocean during February and March 2008

“…North of the SAF, a mean pH Tis value of 8.102 ± 0.014 with f CO 2 of 335 ± 5 µatm was observed, while to the south, the pH Tis was 8.069 ± 0.008 and the f CO 2 increased to 365 ± 10 µatm. The undersaturation observed in the sub-tropical zone with f CO 2 values below atmospheric (378 µatm, see Speich and Dehairs, 2008 for details) is in concordance with the strong sink for atmospheric CO 2 previously described for the area (Siegenthaler andSarmiento 1993, Metzl et al, 1995;Brévière et al, 2006;Borges et al, 2008). The observed Chla concentration varied between 0.06 and 0.67 mg m −3 .…”

Sea–air CO<sub>2</sub> fluxes in the Southern Ocean for the period 1990–2009

“…The detection of a long-term trend is difficult as observations tend to be most common in the austral summer and studies have shown that changes in sea surface temperature and net production can cause considerable variability in sea-air fluxes at regional scales during the summer months (Jabaud-Jan et al, 2004;Brévière et al, 2006;Borges et al, 2008;Brix et al, 2012). While a few observational studies attempt to describe the longer-term change of the carbon system, they focus on oceanic pCO 2 rather than changes in sea-air CO 2 fluxes (Inoue and Ishii, 2005;Lenton et al, 2012;Metzl, 2009;Midorikawa et al, 2012;Takahashi et al, 2009) or are station studies that may have local influences (Currie et al, 2009).…”

Air-Sea CO<sub>2</sub> fluxes on the Scotian Shelf: seasonal to multi-annual variability

“…These investigations have been complemented by the application of satellite data (Lefèvre et al, 2002;Olsen et al, 2008;Chierici et al, 2009) and modelling studies to respectively increase both spatial and temporal coverage, and to gain a mechanistic understanding of the systems (Etcheto et al, 1999;Wanninkhof et al, 2007;Lefévre et al, 2008;Padin et al, 2008;Thomas et al, 2008;Ullman et al, 2009). However, while fundamental understanding of air-sea CO 2 exchange in coastal oceans has yet to be achieved, the few available studies provide evidence for highly variable CO 2 fluxes in these regions Thomas et al, 2007;Borges et al, 2008). Despite relatively large CO 2 fluxes in coastal oceans (Tsunogai et al, 1999;Thomas et al, 2004), the temporal and spatial variability of pCO 2 remains particularly poorly understood (Borges, 2005;Borges et al, 2005;Cai et al, 2006;Laruelle et al, 2010), and the reliability of assessments employing space-borne sensors in these regions has not been established (Lohrenz and Cai, 2006;Salisbury et al, 2008).…”