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Decadal fluctuations of the water mass properties in the Souther Ocean

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Fahrbach, E. , Boebel, O. , Hoppema, M. , Klatt, O. , Rohardt, G. and Wisotzki, A. (2009): Decadal fluctuations of the water mass properties in the Souther Ocean , The 16th International Symposium on Polar Sciences, Incheon, Korea, June 10-12 .
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DECADAL FLUCTUATIONS OF THE WATER MASS PROPERTIES IN ATLANTIC SECTOR OF THE SOUTHERN OCEAN E. Fahrbach1, Olaf Boebel1, Mario Hoppema1, Olaf Klatt1, Gerd Rohardt1 and Andreas Wisotzki11Alfred-Wegener-Institut für Polar- und Meeresforschungin der Helmholtz-Gemeinschaft, Bremerhaven, Germany [Eberhard.Fahrbach@awi.de] INTRODUCTIONThe Southern Ocean contributes through atmosphere-ice-ocean interaction processes to the variability of the global climate system (Rintoul et al, 2001). Atmosphere-ice-ocean interactions, occurring in the open ocean and on the shelves, lead to water mass conversions. Whereas the shelf processes affect a reservoir limited through the shallow water depth (Baines and Condie, 1998) and the cross frontal transports at the shelf edges, open ocean processes can affect deeper layers directly if the stability of the water column is weak. A major contribution to the global deep and bottom water formation occurs in the Weddell Sea (Carmack, 1977, Rintoul, 1998). It is controlled by the transport of source waters into the Weddell Sea, transformation processes within the Weddell Sea, and the transport of modified water out of the Weddell Sea (Gill, 1973). Figure 1 Schematic representations of the Weddell gyre circulation and the location of the transect displayed in figure 2 (dotted line). Red arrows indicate inflow and circulation of Warm Deep Water, light blue arrows sinking water masses at the continental slope. Dark blue arrows stand for deep and bottom water circulation and water masses leaving the basin. The green arrow indicates shelf water leaving the western Weddell Sea. In the Weddell Sea, Circumpolar Deep Water enters from the north and circulates as Warm Deep Water in intermediate layers within the large-scale cyclonic gyre (Figure 1). Heat and salt are transported from that water mass into the surface layer by means of upwelling and entrainment. The vertical transport of heat and salt compensates the heat loss and the fresh water gain at the sea surface. The delicate balance of buoyancy loss and gain controls the stability of the water column. The vertical transport can be significantly affected by vertical flow and enhanced mixing in the vicinity of topographical features like Maud Rise. Even relatively small-scale topographical structures may have a significant effect on the water flow and mixing due to the generally weak stratification in polar oceans.Under conditions of a relatively stable water column, shallow open ocean convection represents a preconditioning for the shelf processes through heat extraction and salt redistribution of the source waters which are involved in frontal processes over the continental slope. In the case of relatively unstable conditions, open ocean convection can reach deeper layers and contribute directly to the deep water formation. Unstable conditions enhance the heat transport from the ocean towards the surface to an extent that large areas of the winter sea ice are melted and an open-ocean polynya is formed which then allows large heat losses of the ocean increasing the water mass conversion. Recent observations indicate that the water mass properties of the Warm Deep Water are subject to significant variations. After an initial warming and salinity increase observed during the nineties a cooling followed during the last years (Robertson et al, 2002, Fahrbach et al, 2004). The variations are most likely due to changes in the inflow from the circumpolar water belt, in combination with changes in the ice-ocean-atmosphere interaction in the Weddell Sea induced by changes in the atmospheric forcing conditions. The time variability of the Antarctic Circumpolar Wave (White and Peterson, 1996), the Southern Annular Mode (Thompson and Solomon, 2002) or the Antarctic Dipole (Yuan, 2004) might affect the Weddell Sea and generate the observed variations. Whereas the properties of the Weddell Sea Deep Water have remained essentially constant, the Weddell Sea Bottom Water has been subject to significant changes as well. Since the Warm Deep Water is the source water of bottom water, the variations of the two water masses are likely to be related through the formation process.THE DATAData from ship-borne surveys over more than twenty years in the Atlantic sector of the Southern Ocean are used to construct time series of water mass properties from 1984 to 2008 (Fahrbach et al, 2007). The most recent data set was obtained during a cruise of RV Polarstern in the context of the IPY Climate of Antarctica and the Southern Ocean (CASO) project.In order to reduce the effect of small or mesoscale features on the detection of low period changes of water mass properties, cross-basin averages of temperature and salinity were calculated. Changes of the mean temperature of a water mass are a measure of changes in the heat content. Since the shift of the Weddell Front would affect the mean properties significantly, the calculations were limited to the area south of 56°S to exclude the front (Figure 2). In addition to the hydrographic transects which were repeated in intervals of 2 to 3 years, moored instruments were deployed which provided quasi-continuous time series. Our time series data allow to draw the conclusion that the longer-term changes detected from the repeat sections are not the result of aliased higher frequency signals in spite that variations of seasonal to interannual time scales are clearly visible in the quasi-continuous records. Figure 2 Vertical transect of potential temperature from the Southwest Indian Ridge (left) to the Antarctic continent (right) displaying the isotherms ascending to the South in the Antarctic Circumpolar Current and the doming related to the Weddell gyre as measured during Polarstern cruise ANT XXII/3 in 2005. The major water masses are the Circumpolar Deep Water (CDW), the Warm Deep water (WDW), the Weddell Sea Deep Water (WSDW) and the Weddell Sea Bottom Water (WSBW). The black vertical line indicates the northern limit for the calculation of the average properties of the water masses. The location of the transect is displayed in figure 1.THE VARIATIONS OF THE WATER MASS PROPERTIESThe most prominent patterns are variations of temperature and salinity of the Warm Deep Water and the Weddell Sea Bottom Water (Figure 3). In the bottom water of the Weddell Sea proper a temperature increase by 0.12°C was observed over 16 years from 1989 to 2005. At the Prime Meridian warming occurred in the Warm Deep Water from 1984 to 1996 followed by cooling. The warming trend in the bottom water is detected here as well and started in 1992. It is coinciding with the increase of salinity. This is in stark contrast to the situation in the Northwestern Weddell Sea where Weddell Sea Bottom Water is cooling and getting fresher since the early nineties (Heywood et al., in prep). The variations in the near-surface layers are more difficult to quantify because of the intensive annual cycle. Therefore the properties of the Winter Water were derived using combined winter data from profiling floats and the CTD transects: Properties measured at the CTD sections were corrected for the seasonal variation derived from float data thus resolving the annual cycle. The main result is that the salinity of the Winter Water was subject to significant increase until 2004, potentially reducing the stability of the upper water column, but since then it is decreasing again. Consequently, the stability of the water column increased again and a renewed occurrence of a large polynya becomes less likely. Figure 3 Variations of the watermass properties in the Weddell gyre from 1984 to 2008 observed on the meridional transect displayed in figure 1 and shown on the transect in figure 2. The red line stands for the Warm Deep Water, green for the Weddell Sea Deep Water and dark blue for Weddell Sea Bottom Water. The full lines stand for potential temperature the broken lines for salinity.DISCUSSION AND CONCLUSIONSPronounced variations on a multi-annual time scale occur in the water mass properties of the Warm Deep Water, the Weddell Sea Bottom Water, the Winter Water and the winter sea ice thickness. The Warm Deep Water temperature increased until the mid 1990s, decreased until 2005 and is warming again since then. Whereas the temperature of the Weddell Sea Bottom Water is still increasing on the Greenwich meridian, it cools in the Weddell Sea proper since the early 2000s. The salinity of the Winter Water decreased with increasing Warm Deep Water temperature and is increasing from the mid 1990s to the mid 2000s and decreasing since. Since the Warm-Deep-Water core was shrinking and the Winter-Water layer deepening, input of salt from Warm Deep Water is a likely cause increasing salinity in Winter Water and a loss of stability of the upper layers. However, winter sea ice thickness was increasing until early 2000s and decreasing since, suggesting that heat transport from the Warm Deep Water is not directly controlled by the density difference if the stability of the water column is above a certain threshold. The long-term change of the water mass properties corresponds to an increase of the meridional air pressure difference on the Greenwich meridian from the early 90s to the early 2000s and a later decrease. However the active mechanism is still under investigation.REFERENCESBaines P. G. and S Condie, Observations and modelling of Antarctic downslope flows: A review. In: Jacobs SS, Weiss RF (eds) Ocean, ice, and atmosphere: Interactions at the Antarctic continental margin, Antarctic Research Series 75, American Geophysical Union, pp 29-49.1998. Carmack E. C., Water characteristics of the Southern Ocean south of the Polar Front. In: Angel M (ed) A Voyage of Discovery, George Deacon 70th Anniversary Volume, Pergamon Press, pp 15-41, 1977.Fahrbach, E., G. Rohardt and R. Sieger, 25 Years of Polarstern Hydrography (1982-2007), WDC-MARE Reports 5, 94 pp, 2007.Fahrbach, E., M. Hoppema, G. Rohardt, M. M. Schröder and A. Wisotzki, Decadal-scale variations of water mass properties in the deep Weddell Sea. Ocean Dynamics, 54, 77-91, 2004. Gill A. E., Circulation and bottom water production in the Weddell Sea. Deep-Sea Res 20: 111-140, 1973.Heywood, K.J., Thompson, A .F., Fahrbach, E., Mackensen, A., and Aoki, S., Cooling and Freshening of the Weddell Sea Outflow, in prep.Rintoul S.R, On the origin and influence of Adélie Land Bottom Water. In: Jacobs SS, Weiss RF (eds) Ocean, ice, and atmosphere: Interactions at the Antarctic continental margin, Antarctic Research Series 75, American Geophysical Union, pp 151-171, 1998.Rintoul S. R., C. W. Hughes and D. Olbers, The Antarctic Circumpolar Current system. In: Siedler G., J. Church, J. Gould (Eds.) Ocean circulation and climate: Observing and modelling the global ocean, Academic Press, pp 271-302, 2001. Robertson R., Visbeck M., Gordon A.L., Fahrbach E. (2002) Long-term temperature trends in the deep waters of the Weddell Sea. Deep-Sea Res II 49: 4791-4806.Thompson, D. W. J., and S. Solomon, Interpretation of Recent Southern Hemisphere Climate Change. Science, 296, 895-899, 2002.White W. B. & R. G. Peterson, An Antarctic circumpolar wave in surface pressure, wind, temperature and sea-ice extent. Nature 380: 699-702, 1996.Yuan, Y., ENSO-related impacts on Antarctic sea ice: a synthesis of phenomenon and mechanisms. Antarctic Science, 16 (4), 415 426, 2004.

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