Does the thermohaline circulation weaken in the Adriatic Sea?

1. Does the thermohaline circulation weaken in the Adriatic Sea? Ivica Vilibić, Jadranka Šepić Institute of Oceanography and Fisheries, Split, Croatia Nicolas Proust Student at Institute of Engineering Sciences of Toulon and Var, La Valette, France

2. Motivation Overturning stream function Adriatic The Mediterranean thermohalinecirculation(MTHC) is strongly weakened at the end of thetwenty-first century. This behaviour is mainly due tothe decrease in surface density and so the decrease inwinter deep-water formation. At the end of the twenty-firstcentury, the MTHC weakening can be evaluatedas –40% for the intermediate waters and –80% for thedeep circulation with respect to present-climate conditions. Somot et al., Climate Dynamics, 2006

3. Motivation A hint is provided by winter of 2012: You are here

4. Motivation However, one harsh winter does not make a transition to the glacial period (regardless of media, some crackpot scientists and record-breaking seawater density in Croatian coastal waters). Our motivation is: To detect and assess long-term AdTHC changes, as being highly relevant to a number of issues (e.g., hydrological cycle in the Med, sea level studies, biogeoscience research, ...) Pag Channel, 29-03-2012 Courtesy of PMF-Zg

5. Outline: • About Adriatic thermohaline circulation (AdTHC) • Major AdTHC driving mechanisms and their trends • AdTHC trends visible in long-term ocean series • Some conclusions, consequences and perspectives

6. About AdTHC The Adriatic is a semi-enclosed basin characterised by both estuarine and anti-estuarine types of THC estuarine AdTHC (driven by freshwater balance) rivers EAC WAC SAG

7. About AdTHC anti-estuarine AdTHC (driven by dense water generation) Deep convection site (AdDW) Vilibić et al., DSR, 2004 Supić and Vilibić, ECSS, 2006

8. About AdTHC anti-estuarine AdTHC (driven by dense water generation) Courtesy of ISMAR Venezia, 16 m Feb 2012 Vilibić et al., DSR, 2004 26-04-2012 IOF-MHS Virtual Laboratory, 2012

9. About AdTHC anti-estuarine AdTHC – DWG by shelf densification (NAd) Proper quantification of DWG is dependable on model resolution, due to high spatial and temporal variablity of the generating force (bora). Beg Paklar et al., CSR, 2001 Pullen et al., JGR, 2007 Zore-Armanda and Gacic, Ann. Geo., 1987

10. About AdTHC anti-estuarine AdTHC – DWG by deep convection (SAd) mixed layer depth Vilibić and Šantić, OD, 2008 DW from NAd (NAdDW)

11. About AdTHC anti-estuarine AdTHC – DWG by deep convection (SAd) spring phytoplankton bloom 04-03-2006 30-03-2006 14-04-2006 08-05-2006 Vilibić and Šantić, OD, 2008

12. About AdTHC Italy Haline forcing Croatia Anti-estuarine AdTHC is thermally-driven through heat losses during bora outbreaks, and is lagged with respect to the forcing for a few months months. Pulsations of dense water outflow generates lagged pulsations of EAC and inflow of saline waters. Thermal forcing Thermohaline forcing Orlić et al., JGR, 2007

13. AdTHC driving mechanisms changes estuarine AdTHC – freshwater balance changes (E, P, R) Overall decrease in river runoff and precipitation, increase in evaporation CRU TS 3.1. precipitation trend, 1951-2002 northern Adriatic river discharges, Po (up) and Adige (down) Philandras et al., NHESS, 2011 ... the freshwater deficit increases in the 1988-2005 period. The water deficit is due to increases in evaporation driven by increasing SST, while precipitation does not show any consistent trends in the period. Romanou et al., J. Cli., 2010 Cozzi and Giani, CSR, 2011

14. AdTHC driving mechanisms changes anti-estuarine AdTHC – DWG by heat losses, winds Spatial downscalling is still a demand for proper quantification of long-term changes in heat fluxes and wind stress at the Adriatic dense water generation sites! anti-estuarine AdTHC – trends in salinity, high interannual variability Middle Adriatic salinity Supić et al., Ann. Geo, 2004

15. AdTHC driving mechanisms changes Variability at lateral boundary – BiOS Gačić et al., GRL, 2010 ... we show that surface flow in thenorthwestern Ionian co-varies with the density of watermasses formed in the Southern Adriatic ...

16. AdTHC ocean changes An observational study • Data: • Palagruža transect • Stations P1 (528), P2 (529), P3 (216), P4 (206), P5 (199) – number of vertical profiles in brackets • 1952-2010 • temperature, salinity, dissolved oxygen • Methods: • removal of 12- and 6-month cycle • linear trends and variability, separately for each station, depth and parameter • significance estimates

17. AdTHC ocean changes Croatia Italy Annual averages in T, S, st, DO 1 – NAdDW outflow current 2 – LIW inflow current 3,4 – freshened surface waters

18. AdTHC ocean changes EMT Interannual and decadal variability, especially in the last two decades P2, 100 m

19. AdTHC ocean changes Croatia Italy T, S, st, DO trends 1952-2010 • significant at 95% o insignificant

20. AdTHC ocean changes T, S, st, DO trends 1952-1989 (pre-EMT period) - The pattern is similar, although different in significance and numbers

21. Conclusions Q: Does the thermohaline circulation weaken in the Adriatic Sea? A: Probably :) Nature cannot be quantified deterministically. • lower NAdDW production, lower ventilation of deep layers, lower LIW advection today than before 60 years • salinization, heating of surface layers, strengthening of stratification • strong effects on biogeochemistry, ecology of living organisms (especially in deep layers), fisheries, ... Jabuka Pit area is very important area for fisheries and biodiversity niche

22. Perspectives Q: What can be expected in the future? SST difference between 2070-2090 and 1961-1990 simulated by AOGCM Air temperature and precipitation differences between 2070-2090 and 1961-1990 ensembled over RegCMs winter summer Somot et al., GPC, 2008 Zampieri et al., PCE, 2012