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Marine Aspects of Abrupt Climate Change. William Curry Woods Hole Oceanographic Institution. NSF ACGEO April 28, 2004. Outline. Geological Evidence for Abrupt Climate Change Role of Ocean Circulation Changes in Tropical Salinity/Hydrology Recent Changes in Ocean Salinity.

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marine aspects of abrupt climate change

Marine Aspects of Abrupt Climate Change

William Curry

Woods Hole Oceanographic Institution

NSF ACGEO

April 28, 2004

slide2

Outline

Geological Evidence for Abrupt Climate Change

Role of Ocean Circulation

Changes in Tropical Salinity/Hydrology

Recent Changes in Ocean Salinity

slide3

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

slide4

NADW

GEOSECS

slide5

NADW

Kroopnick (1985)

slide6

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

slide9

Reduced Density Shear in Florida Current during glacial period

Lynch-Stieglitz et al. (1999)

Holocene

Glacial

Transport

slide11

Millennial-scale Variability

GISP2

H1

YD

NADW on

NADW off

Bermuda Rise

Grootes and Stuiver, 1997

Stuiver, et al., 1995

Boyle and Keigwin, 1987

slide12

231

230

Pa

/ Th Method

slide14

231

230

Pa

/ Th

GISP2

H1

YD

NADW on

0.093

NADW off

Bermuda Rise

McManus et al. (2004)

slide15

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 ]

slide17

Maximum in tropical salinity when North Atlantic is cold

GISP2

H1

YD

Atlantic

Pacific

Schmidt et al. (2004)

Stott et al. (2002)

slide18

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

global sss e p distributions
Global SSS E-P distributions

Source: R.Schmitt,WHOI

Source: HydroBase2

Sea surface salinity distributions are determined

in large part by E-P patterns

high latitude freshening
High Latitude Freshening

North Atlantic deep waters have

become progressively fresher

since the 1960s

Dickson et al. (2002)

slide21

Reduced salinity in Greenland Sea throughout the water column

Curry and Mauritzan, in preparation

slide22

Largest effects in the upper 1000 m

Curry and Mauritzan, in preparation

slide23

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

Curry and Mauritzan, in preparation

subtropical north atlantic
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

slide25

Pacific

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

atlantic ocean
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

atlantic salinity change
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

indo pacific
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

slide29

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