North Pacific and North Atlantic multidecadal variability:

Origin, Predictability, and Implications for Model Development

Thanks to: J. Ba, N. Keenlyside, W. Park, H. Pohlmann

Mojib Latif, Leibniz Institute of Marine Sciences at Kiel University

Deviations of 1999-2008 SAT from long-term warming trend 1950-2008

PDO and AMO can be easily identified

Hurrell et al. 2010

PDO and AMO index at a glanceThere is a lot of interannual variability (signal-to-noise)

PDO and AMO seem not be linked to each other

PDO seems to have a slightly smaller period than AMO

Scientific questions1. Can we explain the multidecadal variability by

a simple stochastic model? 2. Is North Pacific (PDV) and North Atlantic

multidecadal variability (AMV) linked?3. What provides the memory in the two basins?4. What are the implications for decadal

predictability?5. How did the Walker Circulation change during

the 20th century (external and internal factors)?

Hierarchy of stochastic models

Test of the stochastic mechanism1. ECHAM5-OZ: ECHAM5 (T31) coupled to a multi-level mixed layer ocean (depth 500m), 1000 years (Dommenget and Latif 2008)

2. KCM: ECHAM5 (T31) coupled to an OGCM (NEMO), 4000 years (Park et al. 2009)

SSTA pattern of leading mode and spectra in ECHAM5-OZ

Atmospheric teleconnections spread the extratropical signal to the Tropics. Once SST anomalies have developed there, the signal is spread globally (referred to as a “Hyper Mode”).

Dommenget and Latif 2008

aa aa

Observed EOF-1 (32%), exp. trend removed, P > 10 years

ECHAM5-OZ EOF-1 (29%), P > 40 years

Leading multidecadal mode of SST

Dommenget and Latif 2008

The spatial structure of observed Pacific multidecadal SST variability is consistent with

the “Hyper Mode” picture

Is PDV related to AMV in KCM?

Leading mode (40%) has loadings only in the Pacific

Park and Latif 2010, subm.

Regression patterns, PDV

Does show elements of a Hyper Mode

Is PDV related to AMV in KCM?

POP2 (20%) has loadings only in NA. North Pacific and North Atlantic multidecadal SST variability are basically unrelated in KCM. NA SST is not (!) stationary.

Park and Latif 2010, subm.

Regression patterns, AMV

Hyper Mode picture does not apply

ST

Jin Ba, IFM-GEOMAR

3-d T/S EOF1 leads AMOC by a decade

Stochastically forced AMOC variability

Most observational evidence points towards the “ocean-only” oscillator (see also Griffies and Tziperman 1995)

The North Atlantic Oscillation (NAO)NAO-related SAT pattern (°C), +1σ

The NAO can affect Labrador Sea convection

The NAOI spectrum is almost white, but there is multidecadal variability

AMOC may have responded to the multidecadal NAO changes (Eden and Jung 2003)

weak AMOC? strong AMOC?

The NAO: a key for decadal AMOC variability in recent decades?

Time lag is of the order of a decade

Latif et al. 2006

Overturning anomalies from forced OGCM

Forced ocean model (ORCA 0.5°)

Alvarez et al. 2008AMOC changes follow multidecadal NAO changes

Cold phase of multidecadal mode

Warm phase of multidecadal mode

What do the ocean analyses show?

Pohlmann et al. 2010

AMOC at 45°N and 1000m depth from the assimilation experiments of 10 decadal prediction systems (three year running mean are shown)

There seems to be some consistency in the multidecadal AMOC changes

raw values normalized values

Implications for decadal predictability

Pacific Decadal Variability is largely consistent with a simple stochastic model (hyper mode). Decadal predictability potential may be only modest.

Atlantic Multidecadal Variability is more consistent with a stochastically driven oceanic eigenmode. This implies a considerably larger decadal predictability potential.

20th Century Walker CirculationInter-basin zonal temperature gradient (160°W-80°W, 80°E-160°E; 5°S-5°N)

and inverted SLP gradient (as defined by Vecchi et al. 2006)

Forced experiments with ECHAM5 with observed SST1. Sensitivity to SST forcing (HadISST vs. ERSST)2. Sensitivity to the radiative forcing (with and without)3. Sensitivity to horizontal model resolution (T31 and T106)4. Comparison with 20th century integrations with coupled models5. Analysis of the link between SST gradient and SLP gradient in

control runs with coupled models6. Analysis of internal variability of centennial trends in global

warming simulations with one coupled model KCM

Trends in 20th century SST obtained from HadISST and ERSST and SLP response

All SST forced ECHAM5 runs yield a strengthening of the Walker Circulation during the 20th century

Meng et al. 2010

20th Century Walker Circulation change

strong circulation

weak circulation

Trends in inter-basin SST and SLP gradient

There is a consistent linear relationship between the inter-basin SST and SLP gradient in all climate models

Meng et al. 2010

ECHAM5

Obs.

Walker Circulation response in global warming (1%/year increase) runs

strong circulation

weak circulation

differences between the last 30 and the first 30 years of the 100 year integrations

There is no robust response of the Walker Circulation, not even in a single model (KCM)

20th century obs.

1. Can we explain the variability by a simple stochastic model? A large component in NP.

2. Is North Pacific and North Atlantic multidecadal variability linked? Not much.

3. What provides the memory in the two basins? Gyre (NP) and AMOC (NA).

4. What are the implications for decadal predictability? More potential in NA rel. to NP.

5. What are the relative roles of external and internal factors? Internal variability may mask global warming effects even on centennial timescales (e.g. Walker Circulation strength).

Summary

Thank you for your attention!