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

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

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

NADW GEOSECS

NADW Kroopnick (1985)

Benthic Foraminifera  13 C of  CO 2   13 C of CaCO 3 Cibicidoides spp. Cibicides spp. Planulina spp. Data sources: Duplessy et al., 1984 Curry and Lohmann, 1990 Mackensen et al., 1993 McCorkle and Keigwin, 1994

Glacial Period

HoloceneGlacialTransport Reduced Density Shear in Florida Current during glacial period Lynch-Stieglitz et al. (1999)

GISP2 Boyle and Keigwin, 1987Grootes and Stuiver, 1997Stuiver, et al., 1995 Bermuda Rise H1YD Millennial-scale Variability NADW on NADW off

Pa 231 / Th Method 230

GISP2 Bermuda Rise H1YD NADW on NADW off Pa 231 / Th 230 McManus et al. (2004)

How do you reconstruct past salinity? Using the oxygen isotopic and minor element chemistry of fossil organisms T = 16.5 – 4.3(  CaCO3 -  water ) (  CaCO3 -  water ) 2 T = ln( [Mg/Ca] / 0.38 ) / 0.09  water ~ [  CaCO3, Mg/Ca ]

 18 O w and Salinity

Atlantic Pacific GISP2 H1YD Maximum in tropical salinity when North Atlantic is cold Schmidt et al. (2004) Stott et al. (2002)

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

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

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

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

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

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

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 Subtropical North Atlantic

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

Comparing observations from with , 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 Atlantic Ocean

Salinity Difference minus Atlantic Salinity Change 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

Indo – Pacific Comparing the 1960s to , 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. Figure courtesy of A. Wong Pacific SectionsIndian SAMW

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