Newly Compiled and Gridded Seasonal Sea Surface T and S for the Atlantic Ocean at the Last Glacial Maximum Christian Sch¨afer-Neth and Andr´e Paul

Newly Compiled and Gridded Seasonal Sea Surface T and S for the Atlantic Ocean at the Last Glacial Maximum

Christian Sch¨afer-Neth and Andr´e Paul

DFG Research Center Ocean Margins, University of Bremen, Germany

Purpose:

Compile a new global, seasonal, and consistent SST and SSS data set for the last glacial, 23–18 ky BP

Data base:

Numerous SST reconstructions and oxygen isotope measurements from deep-sea sediment cores

Methods:

Variogram analysis and kriging, paleo-T-S-δ

O Relations Application:

Forcing and assessing numerical ocean and atmosphere models

Motivation

Understand glacial climate — on its own and as a prerequisite for deglaciation 20 years after CLIMAP, the new GLAMAP

(Glacial Atlantic Mapping and Prediction) ef- fort provides considerably improved data sets:

• unique time slice definition employed by con- tributing institutes

• new or refined seasonal SST estimates for the entire Atlantic Ocean

• increased sampling density

0 20 40 60 80 100 120

Variogram

0 10 20 30 40 50

Distance, degrees

Atlantic Ocean, Summer SST

LGM before GLAMAP

GLAMAP Levitus, 1982

Levitus sampled at core sites

• supplemental information on seasonal ice covers in both hemispheres New opportunity to construct consistent monthly SST and SSS maps for:

• driving ocean and atmosphere circulation models

• validating coupled models

Temperature: CLIMAP 1981

02 2

468101214 16

18

18 20

20 22

22 2624

August

-80˚ -80˚

-60˚ -60˚

-40˚ -40˚

-20˚ -20˚

0˚ 0˚

20˚ 20˚

40˚ 40˚

60˚ 60˚

80˚ 80˚

02468101214 14

16 16

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20 22

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24 26

26

February

-80˚ -80˚

-60˚ -60˚

-40˚ -40˚

-20˚ -20˚

0˚ 0˚

20˚ 20˚

40˚ 40˚

60˚ 60˚

80˚ 80˚

˚C

-2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

CLIMAP temperature source: NOAA Paleoclimatology Program / World Data Center

Temperature: Data Sources for 23–19 ky BP

-80˚ -80˚

-60˚ -60˚

-40˚ -40˚

-20˚ -20˚

0˚ 0˚

20˚ 20˚

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60˚ 60˚

80˚ 80˚

August

-80˚ -80˚

-60˚ -60˚

-40˚ -40˚

-20˚ -20˚

0˚ 0˚

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40˚ 40˚

60˚ 60˚

80˚ 80˚ February

˚C

-2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

Isolines: Pflaumann et al. 2002. ⁴: Niebler et al. 2002. 2: De Vernal et al. 2000 (August), Gersonde et al. 2002 (February).

◦

Pflaumann et al. 2002; De Vernal et al. 2000; Gersonde and Zielinski 2000. Grey shade: SST data taken from CLIMAP (1981). +: artificial tie points.

Temperature: Gridding Procedure

For each season. . .

• digitize isolines

• split complete set of core and isoline data into overlapping zonal belts 30^◦ wide

• for each belt. . .

– compute empirical directional variograms in spherical coordinates

– fit variogram models

– interpolate core and isoline data to a regular 1^◦ × 1^◦ grid by kriging in spherical coordinates

• join belts to a new Atlantic-wide grid

• smoothly incorporate Atlantic data into the global CLIMAP SST fields

-80˚ -80˚

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Variogram

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Distance, degrees

N empirical model

Temperature: Gridded Fields

0246 6

810 12 1614

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22 24

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August

-80˚ -80˚

-60˚ -60˚

-40˚ -40˚

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0˚ 0˚

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80˚ 80˚

0 0

246810121416 18

18 20

20 22

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February

-80˚ -80˚

-60˚ -60˚

-40˚ -40˚

-20˚ -20˚

0˚ 0˚

20˚ 20˚

40˚ 40˚

60˚ 60˚

80˚ 80˚

˚C

-2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

Temperature: GLAMAP–Modern Anomaly

-8 -10 -6

-6 -4

-4

-4

-4 -4 -2 -2

-2

-2

-2 0

August

-80˚ -80˚

-60˚ -60˚

-40˚ -40˚

-20˚ -20˚

0˚ 0˚

20˚ 20˚

40˚ 40˚

60˚ 60˚

80˚ 80˚

-8 -10 -6

-4

-4 -4

-4

-2 -2 -2 -2

-2 0

0

February

-80˚ -80˚

-60˚ -60˚

-40˚ -40˚

-20˚ -20˚

0˚ 0˚

20˚ 20˚

40˚ 40˚

60˚ 60˚

80˚ 80˚

˚C

-11.5 -10.5 -9.5 -8.5 -7.5 -6.5 -5.5 -4.5 -3.5 -2.5 -1.5 -0.5 0.5 1.5 2.5

Modern temperature source: World Ocean Atlas 1998, 10 m depth.

Temperature: Seasonal Cycle

Directly sine-interpolating between the reconstructed February and August fields leads to local overshots.

Therefore, use the PMIP approach. . .

• compute August and January LGM minus modern anomalies

• construct monthly anomalies by sine-interpolation

• add monthly modern SSTs to obtain monthly

glacial SSTs ⁰

1 2 3 4 5 6

∆T, °C

1 2 3 4 5 6 7 8 9 10 11 12 Month

...SST

...anomaly

LGM minus modern SST, NW Pacific sine-fitted to...

To complete...

• draw monthly ice edges from the reconstructed ex- tremes

• set monthly SSTs to freezing point in the ice- covered regions

Salinity: δ

Oc, Planktic Foraminiferal Oxygen Isotope Data Base

20˚ 20˚

40˚ 40˚

60˚ 60˚

80˚ 80˚

δ¹⁸O_C ^{‰ PDB}

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

5.0 δ¹⁸Oc Sources:

Bard et al. 1987 Lackschewitz 1991 Duplessy et al. 1991, 1992 Morris 1988

Jansen and Erlenkeuser 1985 Ruddiman and Mcintyre 1981 Jansen and Veum 1990 Sarnthein et al. 1995

Jones and Keigwin 1989 Veum et al. 1992

J¨unger 1993 Vogelsang 1990

Keigwin and Boyle 1989 Weinelt 1993

Kellogg et al. 1978 Weinelt et al. 1996 K¨ohler 1991 Zahn et al. 1985

Compute δ¹⁸Ow, the water oxygen isotope ratio, from δ¹⁸Oc and the sampled August SST using

• the Epstein et al. (1953) paleotemperature equation:

δ¹⁸Ow = δ¹⁸Oc −21.63 + √

310.61 + 10Tc

• species-dependent temperature corrections:

N. pachyderma sin. (Weinelt 1993)

Tc = SST − 2.5 if SST < 4.5^◦C Tc = 0.42 SST + 0.39 if SST > 4.5^◦C G. bulloides (Duplessy et al. 1991)

Tc = SST − 1

20˚ 20˚

40˚ 40˚

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80˚ 80˚

δ¹⁸O_W ^{‰ SMOW}

0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2

Salinity: LGM–Modern Anomaly

To estimate sea surface salinity. . .

• assume the modern S-δ¹⁸Ow relation:

S = A · δ¹⁸Ow + B

• compute August salinity at the core loca- tions from the glacial relation:

SSS = A · ^µδ¹⁸Ow − ∆ice

¶ + B + ∆S_∆h with ∆ice = 1.2⁰/₀₀ and ∆S_∆h = 1.07

• from the World Ocean Atlas 1998 10 m salinity, compute the glacial minus mod- ern anomaly, set to 1.07 over unsampled regions

• grid

• include the 0.5-1.0 Weddell Sea anomaly (Duplessy et al. 1996; Melles 1991)

40 50 60 70 80

Latitude 1.55

1.60 1.65 1.70 1.75 1.80 1.85 1.90

A

34.40 34.42 34.44 34.46 34.48 34.50

B

GEOSECS, Atlantic Ocean, 0-100 m

0 0

1 1 1 1 1 1

1 1

1 1 1

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white:

global 1.07 anomaly

August

-80˚ -80˚

-60˚ -60˚

-40˚ -40˚

-20˚ -20˚

0˚ 0˚

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40˚ 40˚

60˚ 60˚

80˚ 80˚

-1.00 -0.50 0.00 0.50 1.07 1.50 2.00

Salinity: Gridded Fields and Seasonal Cycle

To compute monthly glacial salinity. . .

• interpret the August salinity anomaly as representative for the entire year

• add the gridded anomaly to the modern 10 m salinity fields (World Ocean Atlas 1998)

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August

-80˚ -80˚

-60˚ -60˚

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February

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0˚ 0˚

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60˚ 60˚

80˚ 80˚

0.0 33.0 33.4 33.8 34.2 34.6 35.0 35.4 35.8 36.2 36.6 37.0 37.4 37.8 38.2 38.6 40.0

Density: What Drives the Model

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August

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February

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kg m^-3 - 1000

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Diagnosed from an OGCM run under T/S restoring

And Finally. . .

• The new GLAMAP data are much more consistent than any older reconstruction

• Spatial coverage of the Atlantic Ocean has been greatly improved

• This enables a compilation of inher- ently consistent seasonal glacial sea surface T and S maps suitable for driv- ing and validating numerical models

• The newly gridded SST and SSS fields will be made available online at www.pangaea.de

• First model application: glacial-to-modern contrasts of

– north-south density gradients and meridional overturning – deep and bottom water mass formation and characteristics – southwest African upwelling

• Stay tuned for the more detailed information provided in the following talk. . .