marine aspects of abrupt climate change nsf acgeo april 28, 2004 william curry woods hole...
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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