Marine Aspects of Abrupt Climate Change

1. Marine Aspects of Abrupt Climate Change William Curry Woods Hole Oceanographic Institution NSF ACGEO April 28, 2004

2. Outline Geological Evidence for Abrupt Climate Change Role of Ocean Circulation Changes in Tropical Salinity/Hydrology Recent Changes in Ocean Salinity

3. How do you reconstruct past deep water circulation? Using the chemistry of fossil organisms and the nutrient chemistry of the deep water Reconstructing the past gradients in sea water density – geostrophy Measuring the export of radionuclides by deep water currents

4. NADW GEOSECS

5. NADW Kroopnick (1985)

6. Benthic Foraminifera Cibicidoides spp. Cibicides spp. Planulina spp. d13C of SCO2 ~d13C of CaCO3 Data sources: Duplessy et al., 1984 Curry and Lohmann, 1990 Mackensen et al., 1993 McCorkle and Keigwin, 1994

7. Glacial Period

9. Reduced Density Shear in Florida Current during glacial period Lynch-Stieglitz et al. (1999) Holocene Glacial Transport

11. Millennial-scale Variability GISP2 H1 YD NADW on NADW off Bermuda Rise Grootes and Stuiver, 1997 Stuiver, et al., 1995 Boyle and Keigwin, 1987

12. 231 230 Pa / Th Method

14. 231 230 Pa / Th GISP2 H1 YD NADW on 0.093 NADW off Bermuda Rise McManus et al. (2004)

15. How do you reconstruct past salinity? Using the oxygen isotopic and minor element chemistry of fossil organisms T = 16.5 – 4.3(dCaCO3- dwater) + 0.14(dCaCO3- dwater)2 T = ln( [Mg/Ca] / 0.38 ) / 0.09 dwater ~ [ dCaCO3, Mg/Ca ]

16. d18Ow and Salinity

17. Maximum in tropical salinity when North Atlantic is cold GISP2 H1 YD Atlantic Pacific Schmidt et al. (2004) Stott et al. (2002)

18. Summary of Geological Evidence Three lines of evidence imply that North Atlantic circulation changes were linked to abrupt climate changes – tracers, geostrophy, radionuclides Reduced volume and mass flux of NADW, and most likely a change in poleward heat transport Freshening of subpolar North Atlantic linked to changes in circulation – generally believed to be catastrophic freshening Tropical Atlantic and Pacific Oceans (a lot) saltier while North Atlantic fresher – changes in hydrological cycle must have been involved

19. Global SSS E-P distributions Source: R.Schmitt,WHOI Source: HydroBase2 Sea surface salinity distributions are determined in large part by E-P patterns

20. High Latitude Freshening North Atlantic deep waters have become progressively fresher since the 1960s Dickson et al. (2002)

21. Reduced salinity in Greenland Sea throughout the water column Curry and Mauritzan, in preparation

22. Largest effects in the upper 1000 m Curry and Mauritzan, in preparation

23. Reduced N-S gradient in density across the Denmark Strait – 1990s gradient is about 90% of 1960s gradient Curry and Mauritzan, in preparation

24. Subtropical North Atlantic Integrated across 24°N – the latitude of the Northern Hemisphere salinity maximum -- the changes translate to a 5-10% increase in net evaporation rates over the last four decades Curry et al. (2003) Nature

25. Pacific Hawaii Ocean Time Series (HOTS) have increased since the late 1980s – continuing a trend first noted by Wong et al. (2001)

26. Atlantic Ocean Comparing observations from 1955-69 with 1985-99, we find recent freshening at both ends of a long meridional section through the deep western basins contrasted with salinification of the surface waters in the intervening tropics and subtropics of both hemispheres. Salinity increases at mid depths reflect the trans-ocean spreading of Mediterranean Outflow Waters toward the western boundary and into the South Atlantic. Curry, Dickson and Yashayaev, Nature in press

27. Atlantic Salinity Change Salinity Difference 1985-99 minus 1955-69 Salinity increases range 0.1 to 0.4 psu in all Atlantic waters ventilated in the high evaporation regions of the tropics and subtropics Curry et al. (2003) Nature

28. Indo – Pacific Comparing the 1960s to 1985-94, Wong et al (2001) found systematic salinity decreases in the intermediate waters of both hemispheres opposing salinity increases in the surface waters of the tropics and subtropics. Indian Pacific Sections SAMW Figure courtesy of A. Wong

29. Summary of Ocean Changes Tropical Atlantic increased in salinity by 0.3 psu in last 40 years; evidence - also observed in tropical Pacific (Wong et al., 2001) Subpolar regions of North Atlantic freshening throughout water column – 0.05 psu in 40 years. Subpolar freshening also in South Atlantic, as well as both hemispheres of the Indo-Pacific (Wong et al., 2001) Freshening can be traced in Atlantic western boundary deep waters to the equator Greenland Sea surface water freshening has increased density stratification and buoyancy – a fresh water lid? The same salinity changes seen in geological examples of abrupt climate change