Abstract. Circumpolar Deep Water (CDW), locally called Warm Deep Water (WDW), enters the Weddell Gyre in the southeast, roughly at 25 • E to 30 • E. In December 2002 and January 2003 we studied the effect of entrainment of WDW on the fugacity of carbon dioxide (fCO 2 ) and dissolved inorganic carbon (DIC) in Weddell Sea surface waters. Ultimately the fCO 2 difference across the sea surface drives airsea fluxes of CO 2 . Deep CTD sections and surface transects of fCO 2 were made along the Prime Meridian, a northwestsoutheast section, and along 17 • E to 23 • E during cruise ANT XX/2 on FS Polarstern. Upward movement and entrainment of WDW into the winter mixed layer had significantly increased DIC and fCO 2 below the sea ice along 0 • W and 17 • E to 23 • E, notably in the southern Weddell Gyre. Nonetheless, the ice cover largely prevented outgassing of CO 2 to the atmosphere. During and upon melting of the ice, biological activity rapidly reduced surface water fCO 2 by up to 100 µatm, thus creating a sink for atmospheric CO 2 . Despite the tendency of the surfacing WDW to cause CO 2 supersaturation, the Weddell Gyre may well be a CO 2 sink on an annual basis due to this effective mechanism involving ice cover and ensuing biological fCO 2 reduction. Dissolution of calcium carbonate (CaCO 3 ) in melting sea ice may play a minor role in this rapid reduction of surface water fCO 2 .

a b s t r a c tThe Weddell Sea plays an important role for the global oceans and climate by being one of the biggest production and export areas of Antarctic Bottom Water (AABW). Circumpolar Deep Water (CDW) enters the Weddell Gyre (WG) at its eastern boundary. Then called Warm Deep Water (WDW), it is a major contributor to the formation of deep and bottom waters due to ocean-ice shelf interactions in the southern and soutwestern Weddell Sea. Hydrographic data collected between 0 and 30°E on the RV Polarstern cruise ANT XX/2 reveals a two-core structure for the eastern inflow of warm water at roughly 20°E but not further downstream at the Greenwich meridian (GM). Model results and climatological fields suggest that the two cores represent two separate modes of warm inflow. One mode is driven by eddy mixing in the northeastern corner of the WG and the other one is an advective mode, forming the southern branch of the inflow which extends beyond 30°E before turning westward. Both pathways are likely to carry waters from different origins within the Antarctic Circumpolar Current ACC, where more ventilated CDW is found at the Southern Boundary SB compared to the centre. The southern route shows considerable interannual variability in the model. A variable inflow of two types of CDW together with admixed recirculated and cooler waters from the Weddell Sea can potentially contribute to the observed variability and warming trend of WDW over the last decade at the GM.

Circumpolar Deep Water (CDW), locally called Warm Deep Water (WDW), enters the Weddell Gyre in the southeast, roughly at 25 • E to 30 • E. In December 2002 and January 2003 we studied the effect of entrainment of WDW on the fugacity of carbon dioxide (fCO 2 ) and dissolved inorganic carbon (DIC) in Weddell Sea surface waters. 5 20 as well (provided the upward movement of WDW does not change significantly). modelling suggest that the Southern Ocean is particularly sensitive to changes, be From ice covered CO2-rich waters to a biological CO 2 sink Abstract Introduction Conclusions Abstract Introduction Conclusions Abstract Introduction Conclusions Abstract Introduction Conclusions Abstract Introduction Conclusions Abstract Introduction Conclusions Abstract Introduction Conclusions Abstract Introduction Conclusions Abstract Introduction Conclusions Abstract Introduction Conclusions Abstract Introduction Conclusions Abstract Introduction Conclusions Abstract Introduction Conclusions References Tables