Inventories of anthropogenic carbon and their variability in the Atlantic sector of the Southern Ocean, 1984-2010
Final Report Abstract
The Atlantic sector of the Southern Ocean is a key area for the formation of deep and bottom water. It is, hence, an important component of the Meridional Overturning Circulation and a significant sink for atmospheric gases. Climate relevant (anthropogenic) carbon (Cant) is taken up at the atmospheric interface and exported from the atmosphere during Antarctic Bottom Water (AABW) formation. In turn, formation of these waters are influenced by climate change. Despite their importance, Cant inventories and related formation rates of AABW in that area are not well known, and estimates of the temporal variability are quite uncertain. Major aims of this project were to calculate formation rates of AABW in the Weddell Sea and to determine the Cant inventories in the Atlantic Southern Ocean, especially in AABW. The obtained results were essentially based on transient anthropogenic tracer (CFC) observations. Primarily, we used a 27 year long time series of repeated transient anthropogenic tracer observations to investigate the evolution of the ventilation time scales and the related content of anthropogenic carbon (Cant) in deep and bottom water in the Weddell Sea. This time series consisted of chlorofluorocarbon (CFC) observations from 1984-2008 together with first combined CFC and sulphur hexafluoride (SF6) measurements from 2010/2011 along the Prime Meridian in the Antarctic Ocean and across the Weddell Sea. Surprisingly, we found that all deep water masses in the Weddell Sea have been continually growing older and getting less ventilated during the last 27 years. The decline of the ventilation rate of Weddell Sea Bottom Water (WSBW) and Weddell Sea Deep Water (WSDW) along the Prime Meridian is in the order of 15-21%; the Warm Deep Water (WDW) ventilation rate declined much faster by 33%; lower Circumpolar Deep Water (lCDW) by 38%. However, about 88-94% of the age increase in WSBW near its source regions (1.8-2.4 years per year) can be explained by the age increase of WDW/lCDW (4.5 years per year). We found that Cant is highest in WSBW on the slope of the Antarctic Peninsula (19.1±1.3 μmol/kg in 2011). The WSBW further downstream from the sources, at the bottom of the basin and on the Prime Meridian has a Cant of 9.4±1.7 μmol/kg. WSDW has about 7.6–8.2 μmol/kg on both investigated sections. The lowest values were found in WDW (6.6±1.4 μmol/kg) and lCDW (7.7±1.5 μmol/kg). The increase of Cant is highest in WSBW on the slope of the Antarctic Peninsula with 0.15±0.09 μmol/kg per year. Further away from the source regions, along the Prime Meridian in the WSBW and in WSDW with additional easterly sources, the Cant increase is 0.06–0.10 μmol/kg per year. The Cant in the upper and warmer deep waters (WDW and lCDW) seems to stagnate during the period of observations. However, as a consequence of the aging and reduced ventilation, the Cant increase in the deep and bottom water formed in the Weddell Sea slowed down by 14-21% over the period of observations. Declining ventilation of deep Antarctic waters and slowing down of Cant uptake is a potentially very important phenomenon for the global oceans and climate. We have explained the AABW age increase near its sources by mixing with WDW/lCDW. The strongest age increase was found in lCDW/WDW, and a possible reason for that could be the increasing stratification by surface intensified warming. Whether this age increase will continue at its present rate or if it is only multidecadal variability can only be judged in the future and therefore it is absolutely necessary to extend the observational time series. Although we did not find evidence for changing WSBW formation rates, we cannot rule out that that may happen in the future due to changing environmental or climate conditions as observed or projected by models. Therefore, additional and detailed observations, including hydrographic and tracer observations at the deep and bottom water ventilation sites at the Antarctic continental margins and within the ACC may give more clues as to the causes of the observed aging and ventilation reduction and slowing down Cant storage. Oliver Huhn, Monika Rhein: Spurensuche im Südpolarmeer; Edelgase und FCKW im Ozeanwasser zeigen Bedeutung für das globale Klima. In: Impulse aus der Forschung, 2/2012, Universität Bremen
Publications
- 2011. Direct observation of increasing CO2 in the Weddell Gyre along the Prime Meridian during 1973-2008. Deep-Sea Research II, Topical Studies in Oceanography, 58, 25-26
Van Heuven, S., M. Hoppema, O. Huhn, H. Slagter, H. d. Baar
(See online at https://doi.org/10.1016/j.dsr2.2011.08.007) - 2011. Distribution of iodide and iodate in the Atlantic sector of the southern ocean during austral summer. Deep Sea Research Part II: Topical Studies in Oceanography, 58, 25-26
Bluhm, K, P. Croot, O. Huhn, G. Rohardt, K. Lochte
(See online at https://doi.org/10.1016/j.dsr2.2011.02.002) - 2011. On the freshening of the northwestern Weddell Sea continental shelf. Ocean Science, 7, 3, 305-316
Hellmer, H. H., O. Huhn, D. Gomis, and R. Timmermann
(See online at https://doi.org/10.5194/os-7-305-2011) - 2011. Short-term variations of deep water masses in Drake Passage revealed by a multiparametric analysis of the ANT-XXIII/3 bottle data. Deep Sea Research Part II: Topical Studies in Oceanography, 58, 25-26
Sudre, J, V. Garçon, C. Provost, N. Sennechael, O. Huhn, M. Lacombe
(See online at https://doi.org/10.1016/j.dsr2.2011.01.005) - 2011. The effects of continental margins and water mass circulation on the distribution of dissolved aluminium and manganese in Drake Passage. Journal of Geophysical Research, 117, C01019, pp. 19
Middag, R., H. Baar , P. Laan , O. Huhn
(See online at https://doi.org/10.1029/2011JC007434) - 2013. Decline of deep and bottom water ventilation and slowing down of anthropogenic carbon storage in the Weddell Sea, 1984-2011. Deep-Sea Research I, 76, 66-84
Huhn, O., M. Rhein, M. Hoppema, S. van Heuven
(See online at https://doi.org/10.1016/j.dsr.2013.01.005)
