Air-Sea Flux Variability in the Eastern Mediterranean and its Influence on Deep Water Formation

1. Air-Sea Flux Variability in the Eastern Mediterranean and its Influence on Deep Water Formation Outline Background - The Eastern Mediterranean Transient Possible Causes of the EMT Recent Results from SOC & NCEP/NCAR Datasets Conclusions EGS/AGU Assembly Society, April 10th 2003, S. A. Josey

2. E Med Map 1 Early 1990s Picture

3. The Eastern Mediterranean Transient Salinity Sep 1987 Salinity Jan 1995 Repeat hydrographic surveys in 1987 and 1995 revealed sharp increase in salinity of deep water in Eastern Med (Roether et al., 1996). Analysis of T-S properties of water revealed that location of deep water formation had shifted from Southern Adriatic to Aegean Sea.

4. The Signature of the EMT in Chlorofluorocarbons 1987 Isolines of CFC-12 along a section across the Ionian basin in 1987 and 1995 (Roether et al., 1996). 1995 Signal of new deep water formation also present in elevated levels of CFC-12.

5. E Med Map 2 Late 1990s Picture

6. What Caused the Transient? Various possible causes suggested: Change in wind-driven circulation (Samuels et al., 1999). Increase in salinity due to multi-decadal reduction in river runoff (Boscolo and Bryden, 2001), reduced Black Sea outflow (Zervakis et al., 2000). Extreme heat loss during severe winters of early 1990s. Reduced precipitation during 1989-1993 (Theocharis et al. ,1999). Relative contributions from 3) and 4) resolved using SOC and NCEP/NCAR datasets (Josey, 2003, JGR-Oceans, accepted.)

7. Contribution of Wind Forcing to the EMT Samuel et al. (1999) examined effects of intensified winter wind stress over Aegean in late 80s/early90s in a model study. 1980-1987 Salinity cross-section across Levantine and Ionian south of Crete. 1988-1993 High salinity Aegean outflow. Found increased exchange of LIW at Cretan Arc Straits with enhanced deep water formation in the Aegean and outflow into the Levantine / Ionian basins.

8. Impact of Heat Flux and E-P Anomalies Prior to cooling After 8 years of cooling Temp. Salinity New (warmer and more saline) deep water Cross sections south of Crete Wu, Haines and Pinardi (2000) used Med version of the GFDL modular ocean model to show that deep water formation in E Med can be triggered by reduction in SST of 1-2o C and increased E-P of 10-15%.

9. Combined Effect of Surface Forcing and River Runoff Boscolo and Bryden (2001) stress the role of long term (1960s-1980s) reduction in river runoff which erodes low salinity intermediate waters. 1987 T-S Diagram for Aegean Sea deep water from 1987-1995: open squares-observations, filled circles-model. 1995 Find major deep water formation events in simple mixed layer model forced with observed fluxes and long term increase in E-P to simulate runoff changes.

10. Issues Raised at CIESM Workshop Origin and evolution of the EMT reviewed at a CIESM workshop in March / April 2000. Report noted that: a.) Relative contributions of anomalously dry years and cold winters in triggering EMT need to be resolved. b.) Connection of EMT to larger scale atmospheric anomalies (NAO?) needs to be established. Attempted to address both of these issues.

11. Climatological Mean Winter Forcing Climatological winter (Nov-Feb) mean net heat flux and net evaporation (E-P) in the Mediterranean Sea.

12. Components of the Winter Forcing Wind stress and specific humidity (g/kg) SOC Winter (Nov-Feb) Flux Components and Met Variables Latent heat flux (W/m2) Shortwave flux (W/m2) Evaporation (m/month) Precipitation (m/month)

13. Winter Heat and Freshwater Flux Anomalies Heat Flux Freshwater Flux 1987 Cruise 1995 Cruise Time series of the anomalous net heat flux and net evaporation (E-P) in the Aegean Sea during winter (Nov-Feb).

14. Density Flux Calculation Monthly mean estimates of the thermal and haline contributions to the density flux determined from: Thermal term Haline term Salinity fields from the MEDAR dataset (M. Rixen).

15. Impact on the Ocean Buoyancy Loss Total Density Flux Thermal Density Flux Haline Density Flux Time series of the total, thermal and haline density flux anomalies in the Aegean Sea in winter.

16. Thermal and Haline Components of the Density Flux SOC NCEP/NCAR Total Thermal Haline Anomalous density flux (units kg m-2 s-1) averaged over winter 1991-92 and 1992-93.

17. Is The Summer Important? Winter Annual Total Thermal Haline Time series of annual and winter mean density flux anomalies for the Aegean Sea….winter dominates.

18. Source of Heat Flux Variability Net Heat (Wm-2) Latent (Wm-2) Sensible (Wm-2) Longwave (Wm-2) Shortwave (Wm-2) Composite plots of heat flux anomalies for severe winters of 1991-92 and 1992-93.

19. Effects of Northerly Flow Latent (Wm-2) Wind stress and sea level pressure (mb) Sensible (Wm-2) Sea-air humidity difference (g/kg) Atmospheric humidity (g/kg) Composite plot of latent heat flux and meteorological variable anomalies for severe winters of 1991-92 and 1992-93. Strong latent heat flux anomalies driven by intense northerly flow which brings cold, dry air over the Aegean.

20. Can E-P Increase Explain EMT Salinity Change? Roether et al. (1996) require increase in E-P of 20 cm/yr over 1988-1994 to explain EMT salinity increase. Tsimplis and Josey (2001) found 10 cm/yr using reduced Cretan Arc Straits outflow. Annual Mean E-P for the entire Eastern Mediterranean Basin Observed change (2-4 cm/yr) is a small proportion of required change (10-20 cm/yr). Implies internal salinity redistribution important.

21. The Long Term Context Aegean Sea EMT Winters with intense cooling Winter heat flux anomalies from NCEP/NCAR for 1950-2002

22. The Long Term Context Aegean Sea EMT Winters with intense cooling Southern Adriatic Winter heat flux anomalies from NCEP/NCAR for 1950-2002

23. Recent Deep Water Formation in the Southern Adriatic Hydrographic section across S Adriatic, March 2002, Bruno Manca (personal comm.). S Adriatic in completely new state, with deep ventilation from open-ocean convection and water from the Northern Shelf.

24. Are the Anomalies caused by the NAO?

25. Are the Anomalies Caused by the NAO? r2=0.05 r2=0.52 Scatter of the anomalous net heat flux against the NAO index (right) and London-Black sea index (right). Answer: No. Possibly related to separate mode of variability instead.

26. Is a Different Mode Responsible? January SLP difference between years when the rotated principal component scores exceed 1 standard deviation from mean in period 1899-1986 (from Rogers, 1990). Rogers (1990) identified four main modes of sea level pressure variability in the North Atlantic. Some similarity between his second mode, termed the East Atlantic pattern (EATL), and the composite formed from extreme Aegean winter heat loss.

27. Summary Relative contributions of heat and freshwater flux anomalies to density changes at the time of the EMT analysed using the SOC climatology and NCEP/NCAR reanalysis. Density changes dominated by heat flux, freshwater flux term is an order of magnitude smaller. Driven by anomalously strong northerly airflow over the Aegean in winters 1991-92 and 1992-93 which is not related to the NAO. Possibility of earlier event in the mid-1970s (?).

28. SOC Climatology • Source Data: 30 million ship meteorological reports globally over period 1980-93. • Method: Fluxes estimated from the reports using various semi-empirical formulae . • Mediterranean Version : Revised fields generated using Med basin specific longwave formula (Bignami et al., 1995) and aerosol correction (Gilman and Garrett, 1994).

29. Basin Mean Heat Budget Use of Mediterranean radiative flux formulae leads to better agreement with basin mean heat flux (-3 to-7 Wm-2) from exchange at Gibraltar. Remaining difference most likely due to latent heat flux.

30. Winter Latent Heat Loss 2002-2003 Latent heat loss of 140 to160 Wm-2. NCEP/NCAR reanalysis shows strong latent heat loss in Aegean last winter. Possible consequences for deep water production….

31. Issues for CLIVAR

32. Theocharis

33. The Pre-EMT View of the Circulation • Schematic of Circulation from M-R?

34. Annual Mean Fields