Atlantic Meridional Overturning Circulation and thePrediction of North Atlantic Sea Surface Temperature

Mojib Latif, Helmholtz Centre for Ocean Research and Kiel University

Klöwer, M., et al. (2014) Earth and Planetary Science Letters, 406, DOI 10.1016/j.epsl.2014.09.001.

Scientific Questions

1. What is the role of the AMOC in decadal North Atlantic

SST variability?

2. How predictable is the AMOC-related North Atlantic

SST variability?

3. How can we deal with model bias which is particularly

large in the North Atlantic?

1. What is the role of the AMOC in decadal North Atlantic SST variability?

hypothesis: low-frequency variability of the NAO drives the AMOC (following Eden and Jung 2001) through anomalous surface heat fluxes

The NAO drives convection in the North Atlantic, which in turn drives the “AMOC”

Latif and Keenlyside 2011

The Kiel Climate Model (KCM)

1. El Niño/Southern Oscillation(ENSO), period ~4 yrs

2. Atlantic Multidecadal Oscillation (AMO/V), period ~60 yrs

3. Pacific Decadal Oscillation (PDO/V), period ~50 yrs

4. Southern Ocean Centennial Variability (SOCV), ~ 300 yrs

The Kiel Climate Model (KCM) simulates rich internal AMOC variability in a 4,000 years long control run

AMOC index, 30°N

Park and Latif 2008

Surface air temperature anomalies associated with multicentennial and multidecadal AMOC variability

largely independent and originating in different regionsmulticentennial multidecadal

Park and Latif 2008

The KCM (T31-L19, 2°) is used to assess the impact of the NAO-related heat fluxes on the AMOC

the anomalous heat flux forcing is applied to a coupled model, which distorts the thermodynamic feedbacks less than when forcing ocean

models in uncoupled mode

The Kiel Climate Model (KCM) is forced by NAO-related heat flux anomalies

Q’net

Hypothesis: NAO-related heat flux anomalies drive the AMOC which in turn drives North Atlantic SST

a positive phase of the NAO is associated with an enhanced heat loss over the subpolar North Atlantic

Kiel Climate Model (KCM) response when forced by NAO-related heat fluxes

NAO index is in phase with mixed layer depth and subpolar gyre, but leads the model’s AMOC by several years

11-year running means

2. Dynamical/statistical prediction of the decadal North Atlantic SST variability

Model bias is large. We can’t expect that the model realistically simulates North Atlantic SST variability linked to AMOC variability

The issue of model bias

AMOC variability in the KCM 1870-2000

model overturning variability

Canonical Correlation Analysis (CCA) was used to statistically relate the overturning variability to the observed North Atlantic SST variability

Klöwer et al. 2014

CCA has been performed between model AMOC and observed North Atlantic SST

CCA finds those patterns in two datasets, with time evolutions that are most strongly correlated

AMOC leads observed North Atlantic SST by 1-2 decades, use model (KCM) AMOC to predict observed SST

Klöwer et al. 2014

Leading CCA modesSST leads AMOC by 10 years

SST lags AMOC by 21 years

Klöwer et al. 2014

Link between model AMOC and observed SST at two leads/lags expressed by the leading CCA modes

SST leads AMOC by 10 years SST lags AMOC by 21 years

Klöwer et al. 2014

…suggests a rather high decadal predictability potential

Statistical hindcast/forecast of the observed AMO index using the model AMOC as a predictor

the present AMO warm phase will continue until 2030, but with negative tendency

r=0.69

Klöwer et al. 2014

3. How can we deal with model bias which is particularly large in the North Atlantic?

either by improving the models (tough!)or by correction methods: flux correction, flow field correction

Drews et al., in prep.

Conclusions1. The NAO is an important driver of the AMOC, which was

shown by forcing the KCM by NAO-related heat fluxes2. This method could be an alternative to initialize decadal

predictions, as climate models suffer from large biases 3. The KCM’s AMOC can be used as a predictor to statistically

predict with high skill decadal North Atlantic SST variability4. This study suggests a rather high decadal predictability

potential of North Atlantic SST, which solely arises from the history of the NAO

5. The AMO/V is predicted to stay in its warm phase until 2030, but with a negative tendency

Comparison of model SST with observed SST by means of Canonical Correlation Analysis (CCA)

time series, CCA-mode 2

Comparison of model SST with observed SST by means of Canonical Correlation Analysis (CCA)

patterns, CCA-mode 2

Verification of the Bjerknes hypothesis: atmosphere drives NA SST on short, ocean on long time scales

correlation SST/Q, low-pass correlation SST/Q, high-pass

cutoff at about 10 yearsGulev et al. 2013

The individual surface heat flux components from reanalysis

the turbulent fluxes matter, radiative fluxes are weak, which argues against aerosol forcing of multidecadal SST variability

QSHQLH

QSWQLW

Evolution of the overturning streamfunction anomaly with respect to the NAO in the KCM

the climate model acts as a complicated filter on the NAO-forcing

Hindcast of the AMOC 1900-2010

Kiel Climate Model forced by NAO-related heat flux anomalies

overturning at 48°Noverturning at 48°N, 1500m

Lag-regression of model SST w.r.t. AMOC index at 30°N

SST anomaly patterns are strongly influenced by model bias

The problem of model bias, or why we can’t use the predicted model SST

CMIP5 multi-model mean SST bias

courtesy S. Steinig

incorrect path of North Atlantic Current inhibitsrealistic simulation of SST

The role of wind stress forcingskill in hindcasting observed SST when prescribing observed wind stress anomalies to the KCM

correlations based on annual means

Analysis of North Atlantic turbulent surface heat fluxes since 1880

suggests that the ocean drives North Atlantic SST at decadal time scales

Gulev et al. 2013

Comparison of model SST with observed SST by means of Canonical Correlation Analysis (CCA)

time series, CCA-mode 1

Comparison of model SST with observed SST by means of Canonical Correlation Analysis (CCA)

patterns, CCA-mode 1

observed SST model SST

The individual surface heat flux components from reanalysis

the turbulent fluxes matter, radiative fluxes are weak, which argues against aerosol forcing of multidecadal SST variability

QSHQLH

QSWQLW

NAO-forced mixed-layer depth and AMOC variability

corr.: NAO with mixed-layer depth corr.: mixed-layer depth with AMOC

•The research leading to these results has received funding from the European Union 7th Framework Programme (FP7 2007-2013), under grant agreement n.308299•NACLIM www.naclim.eu