Sensitivity of the Atlantic Thermohaline Circulation to Global - - PowerPoint PPT Presentation

Sensitivity of the Atlantic Thermohaline Circulation to Global Freshwater Fluxes

Didier Swingedouw, Pascale Braconnot, Pascale Delecluse, Eric Guilyardi and Olivier Marti. Laboratoire des Sciences du Climat et de l’Environnement.

Background

IPCC THC Index (max meridional stream fonction)

Background

IPCC THC Index (max meridional stream fonction)

• IPCC scénarii

display a wide range

• f behaviors for the

future of THC

• Differences are due

to the intensified hydrological cycle, with ambivalent consequences

• Ex : Dixon (1999)

Background

IPCC THC Index (max meridional stream fonction)

• IPCC scénarii

display a wide range

• f behaviors for the

future of THC

• Differences are due

to the intensified hydrological cycle, with ambivalent consequences

• Ex : Dixon (1999) vs

Latif (2000)

This study:

• Aims: Quantify the impact of global

freshwater forcing on the THC, illustrating for each process: Time Scales and Magnitude Time Scales and Magnitude

• Tool: IPSL-CM4 coupled model

Sensitivity experiments CTRL EP0 EPR0 R0

SSS : CTRL SST : CTRL SSS : CTRL-Levitus SST : CTRL-Levitus

• Profile based

climatology by de Boyer (2004)

• No convection in the

Labrador Sea in CTRL

• Other sites are correct

MLD : CTRL MLD : Climato

MLD : CTRL MLD : Climato

Labrador Irminger GIN Labrador Irminger GIN

• Profile based

climatology by de Boyer (2004)

• No convection in the

Labrador Sea in CTRL

• Other sites are correct
• 3 boxes are defined

for the remainder of the study

THC response over 100 years

• EP0 decreases down to

3Sv

• R0 increases up to 30 Sv
• EPR0 is nearly the sum of

EP0 and R0 over 60 years = Linearity on this time scale

EPR0 = EP0 + R0 EPR0 = EP0 + R0

a) THC index

EPR0 R0 CTRL EP0

c) Sum of EP0 and R0 /EPR0 b) Difference with CTRL

EP0+R0

Convection sites changes

• Convection is enhanced in

R0

• Convection vanishes in EP0
• Convection sites shift in

EPR0: Labrador <-> Irminger

a) R0 b) EP0 c) EPR0

Different impact Different impact

• f Nsa and Ssa on
• f Nsa and Ssa on

the stratification the stratification

• f the convection
• f the convection

sites sites

Ssa Nsa

Mixed layer for the last 50 years

Stratification changes compared to CTRL in the Labrador Sea

b) EP0 c) EPR0

Mixed layer depth

a) R0

Stratification changes compared to CTRL in the Labrador Sea

• Ssa decrease => positive

impact in 10 years

• Nsa decrease => negative

impact in 30 years

• Labrador Sea mostly

sensitive to Ssa and local forcing 10 years 30 years

a) R0 b) EP0 c) EPR0

Mixed layer depth

Irminger

10 ans 15 ans 10 ans 20 ans

c) EPR0 b) EP0 a) R0 a) R0 c) EPR0 b) EP0

GIN

Mixed layer depth

Summary of the convection sites sensitivity / + +

• Global

Global

• (10 years)

(10 years)

• (10 years)

(10 years)

• (10 years)

(10 years)

Ssa

+ +

(20 years) (20 years)

+ +

(15 years) (15 years)

+ +

(30 years) (30 years)

Nsa

GIN Seas Irminger Sea Labrador Sea

+ +:

: enhances convection

• :

: limits convection

/ /: Neutral impact, processes balance each other

Conclusions

• Convection sites are sensitive to differents components
• f the hydrological cycle: Labrador / Irminger; (Wood

1999). 10 to 40 years time scales coherent with Vellinga (2002)

• Global freshwater forcing damps the THC, i.e. local

freshwater forcing dominates over a time frame of 100 years i.e during transient response (Saenko, 2003)

• Model dependent results, but gives a framework for

analysing THC spread due to changes in the freshwater forcing in IPCC scenarii.

Thank you

MailTo: didier.swingedouw@cea.fr

Background

IPCC THC Index (max meridional stream fonction)

• IPCC scénarii

display a wide range

• f behaviors for the

future of THC

• Differences are due

to the intensified hydrological cycle, with ambivalent consequences

Background

IPCC THC Index (max meridional stream fonction)

• IPCC scénarii

display a wide range

• f behaviors for the

future of THC

• Differences are due

to the intensified hydrological cycle, with ambivalent consequences

• Ex : Latif (2000)