“Response of the North Atlantic

Meridional Overturning Circulation

to climate change”

Rita Lecci

PhD student in Science and Management of Climate Change

Ca’ Foscari University, Venice

Outline

The North Atlantic Meridional Overturning Circulation

and its variability

MOC: sensitivity experiments and future scenarios

SINTEX-G model and the C20C (Climate of the 20°

Century) project

Conclusions

The North Atlantic MOC (1/2)

•The thermohaline circulation (THC)

refers to the part of the large-scale

ocean circulation driven by global

density gradients created by surface

heat and freshwater fluxes.

•The Atlantic MOC carries warm

upper waters into far-northern

latitudes and returns cold deep

waters southward across the Equator.

• Its heat transport makes a

substantial contribution to the

moderate climate of Europe

The North Atlantic MOC (2/2)

(Bryden et al.,2005)

•The decrease in net northward flow of warm upper waters and decrease in net southward flow of cold

deep waters across the 25°N section result in a reduction of the northward heat transport across 25°N

from 1.3–1.4PW for the 1957, 1981 and 1992 sections to 1.1PW for the 1998 and 2004 sections.

Transatlantic section at 25°N

(Wunsch et al., 2006)

• A global 1° model constrained by least squares to a multiplicity

of datasets over the interval 1992–2004 are used to describe

apparent changes in the North Atlantic Ocean MOC at 26°N.

•The estimate of an upper ocean volume transport decrease is

equivalent to a reduction of 2.3 Sv over 12 yr, and appears

roughly consistent with the Bryden et al.(2005)

NA seasonally averaged (3-month mean) velocity fields

MOC: sensitivity experiments• Different models studied the response of the MOC to

a specified external source of freshwater (0.1 Sv)

applied uniformly over the Atlantic between 50° and

70°N.

•The size of the forcing freshwater flux was chosen to

be of the order of magnitude predicted for a large

CO2-induced climate change.

•The MOC weakened in all the models in response to

the freshwater input. The ensemble mean gives a 30%

weakening after 100 yr of freshwater input.

•The freshwater perturbation of 0.1 Sv for 100 yr is

insufficient to shut down the MOC in any of the

models.

• After the termination of the freshwater perturbation,

the MOC starts recovering toward its control strength.

(Stouffer et

al., 2006)

MOC in future scenarios

• An ensemble of 16 climate models forced by the scenarios Twentieth- Century Climate in

Coupled Models (20C3M) and Special Report on Emission Scenarios, A1B (SRESA1B; 2000-2100)

was used to calculate weighted best estimates for the present and future development of the

MOC under enhanced greenhouse gas forcing.

• 20C3M considers the observed increase of greenhouse gas concentrations, whereas in SRESA1B

the future evolution of atmospheric CO2 is prescribed to double relative to present day (720

ppm in 2100).

• Almost all models predict a gradual weakening of the MOC during the present century (2000–

2100), but predicted changes in interannual and interdecadal variations as well as the amount of

future reduction differ largely between individual models.

(Schneider et al., 2007)

SINTEX-G model• SINTEX-G is an Atmosphere Ocean sea-ice General Circulation Model (AOGCM) developed at

INGV with the aim of investigating the features and the mechanisms of the climate variabilityand change. (Gualdi et al., 2008)

The model is composed of four parts:

◦ Atmosphere (ECHAM4.6)◦ HR: Gaussian grid at triangular truncation T106 (1.125°x1.125°), VR: 19 hybrid sigma-pressure levels

◦ Ocean (OPA 8.2)◦ HR: quasi-isotrope tri-polar grid, VR: 31 vertical levels

◦ Sea –Ice (LIM)◦ HR: as the ocean model, VR: 3 layers

◦ Coupler (OASIS 2.4)

20C SST JFM SXG climatology A1b-20C SST JFM SXG climatology difference

Climate of the 20° century project

• Climate of the 20th Century is a CLIVAR project born to monitor and detect changes in

climate system

• It has the main focus of simulate and understand the climate variability during the 20th

Century:

•impact of changes in SST,

•sea ice,

•land surface conditions

•atmospheric composition.

•The aim is to design numerical experiments that reproduce the air-sea coupling

maintaining the flexibility to attempt to simulate the observed climate of the 20th century.

Future perspective

• A number of research groups have begun numerical experimentation into regionally

coupled or “pacemaker” experiments where tropical Pacific SST is prescribed from

observations, but coupled air-sea feedbacks are maintained in the other ocean

basins (e.g. Lau and Nath, 2003)

• Current thinking suggests that the pacemaker approach works because the east

Pacific SST anomalies are such a strong forcing function, whereas in the other basins

the feedback from atmosphere is relatively more important.

• The aim of the research project will be to do experiments with the SINTEX-G

model choosing to maintain coupled air-sea feedbacks in the North Atlantic ocean

basin.

• The goal is to understand the simulated MOC and to investigate its possible future

variability.

CONCLUSIONS

• The Meridional Overturning Circulation is an important part of the

Earth’s climate system

• Most state-of-the art global coupled models simulate a gradual

weakening of the North Atlantic MOC during the 21° century due

to increasing levels of GHGs in atmosphere, but not a complete

shutdown

• The aim of the research project will be to understand the

simulated MOC and to investigate its possible future variability

using the SINTEX-G model