Interannual variability of the carbon dioxide system in the southern Indian Ocean (20°S–60°S): The impact of a warm anomaly in austral summer 1998

Abstract

The interannual variations of the carbon dioxide system and air‐sea CO2 fluxes are analyzed in the southwestern Indian Ocean from both in situ (Océan Indien Service d'Observations cruises in 1998 and 2000) and simulated oceanic CO2 fugacity dissolved inorganic carbon, total alkalinity, and nutrients. During austral summer of 1998, the ocean was warmer from 20°S to 60°S. In the Subtropical Zone (20°S–37°S), the warming, associated with the subtropical dipole pattern, creates an oceanic CO2 source around 2 mmol m−2 d−1 in January 1998 where all previous observations, included in 2000, indicated that this region was a small sink in summer. In the Sub‐Antarctic and Polar Front Zones (37°S–50°S), the interannual signal is not clearly detected because of the complex coupling between the horizontal and vertical dynamics and the biological activity that creates very high mesoscale variability in each summer. For the austral region, south of the Polar Front, we observe large variability in the CO2 sources and sinks between summer 1998 and 2000. In the Seasonal Ice Zone (south of 58°S) the interannual variation of the CO2 flux was mainly controlled by a warmer ocean, from 1.1°C, in summer 1998 (CO2 source of 3.2 mmol m−2 d−1) and an enhanced primary production occurring during summer 2000 (CO2 sink of −3.8 mmol m−2 d−1). In the Permanent Open Ocean Zone (POOZ, latitude 50°S–57°S), despite the half‐degree warmer sea surface in 1998, we observe lower (−15 μatm (1 atm = 105 × 1.01325 N m−2) on average) during this period, and consequently, we estimate a double of the oceanic CO2 sink in this region in 1998 than 2000, from −3.2 mmol m−2 d−1 in 1998 to −1.3 mmol m−2 d−1 in 2000. The strong oceanic CO2 sink in 1998 is associated with a dramatic decrease of nutrients (silicates less than 2 μmol kg−1). The enhanced biological activity in summer 1998, probably diatoms bloom, is also apparent in Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) chlorophyll a estimates. Using a one‐dimensional biogeochemical model applied in the POOZ and SeaWIFS time series, we predict a stronger oceanic sink during summer season than the winter, which is contrary to previous studies. The model suggests that biological activity controls most of the sink anomaly in summer 1998 (for about 70%) and dominates the warming effect.

Summer and winter distribution of δ¹³C_DIC in surface waters of the South Indian Ocean [20°S-60°S]

“…In this work, we increased the surface data set by including the surface bottle data (<20 m) collected during CTD stations (conductivity–temperature–depth, Seabird 911 and rosette equipped with 24 Niskin bottles). Except for δ 13 C DIC , instrumental techniques for seawater properties have been previously described (Jabaud‐Jan et al, 2004; Metzl et al, 2006; Metzl, 2009).…”

Sea–air CO&lt;sub&gt;2&lt;/sub&gt; 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).…”

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

“…Due to the relative scarcity of pCO 2 field data in the Southern Ocean, the inter-annual variability of air-sea CO 2 fluxes has seldom been investigated from a purely observational based approach. The impact of warm anomalies on air-sea CO 2 fluxes in the Southern Ocean has to some extent been investigated using cruise-to-cruise comparisons (Jabaud-Jan et al, 2004;Brévière et al, 2006). These two studies show that warm anomalies lead to significant, but opposing, effects on air-sea CO 2 fluxes in different regions of the high latitude Southern Ocean.…”