Marine Aspects of Abrupt Climate ChangeMarine Aspects of Abrupt Climate Change

NSF ACGEO

April 28, 2004

William CurryWilliam CurryWoods Hole Oceanographic InstitutionWoods 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 ForaminiferaBenthic Foraminifera

13C of CO2 13C of CaCO3

Cibicidoides spp.Cibicides spp.Planulina spp.

Data sources:Duplessy et al., 1984

Curry and Lohmann, 1990Mackensen et al., 1993

McCorkle and Keigwin, 1994

Glacial Period

Holocene Glacial Transport

Reduced Density Shear in Florida Current during glacial period

Lynch-Stieglitz et al. (1999)

GISP2

Boyle and Keigwin, 1987Grootes and Stuiver, 1997 Stuiver, et al., 1995

Bermuda Rise

H1 YD

Millennial-scale VariabilityMillennial-scale Variability

NADW on

NADW off

Atlantic Ocean

231Pa

230Th

Burial231Pa/230Th

= 0.093

235U234U

Production231Pa/230Th

= 0.093

Atlantic Ocean

231Pa

230Th

Burial231Pa/230Th

= 0.093

235U234U

Production231Pa/230Th

= 0.093

Pa231 / Th Method

230

NADW

Atlantic Ocean Southern Ocean

231Pa

230Th50%

<10%

Opal flux

50% >90%

231Pa/230Th<0.093

231Pa/230Th>0.093

235U234U

238U

>200 kyr

230Th

20 yr

231Pa 200 yr

N S

NADW

Atlantic Ocean Southern Ocean

231Pa

230Th50%

<10%

Opal flux

50% >90%

231Pa/230Th<0.093

231Pa/230Th>0.093

235U234U

238U

>200 kyr

230Th

20 yr

231Pa 200 yr

N S

GISP2

Bermuda Rise

H1 YDNADW on

NADW off0.093

Pa231

/ Th230

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) + 0.14(CaCO3- water)2

T = ln( [Mg/Ca] / 0.38 ) / 0.09

water ~ [ CaCO3, Mg/Ca ]

18Ow and Salinity

Atlantic

Pacific

GISP2

H1 YD

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,WHOISource: HydroBase2

High Latitude Freshening

North Atlantic deep waters havebecome 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

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

Integrated across 24°N – the latitudeof the Northern Hemisphere salinity maximum -- the changes translate to a 5-10% increase in net evaporation rates over the lastfour decadesCurry 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 from1955-69 with 1985-99, we find recent freshening at both ends of a long meridional section through the deep western basinscontrasted 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 Waterstoward the western boundary and into the South Atlantic.

Curry, Dickson and Yashayaev, Nature in press

Atlantic Ocean

Salinity Difference 1985-99 minus 1955-69

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 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.

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