Detection of Forced and Natural Atlantic Multidecadal Variability in Coupled Models and Observations

Mingfang TingYochanan Kushnir

Cuihua Li

Lamont-Doherty Earth ObservatoryColumbia University

AMO Index (7.5W-75W, 0-60N, ocean only) for Models and Observations

Detrended AMO Index

Questions:

How much of the multi-decadal Atlantic SST variability is caused by internal dynamics and how much is externally forced?

Can model ensemble average be taken as forced signals?

How do one separate the natural and forced components in observations?

Focusing on IPCC models with multiple realizations…

NCAR CCSM – 8 members GFDL CM2.1 – 5 members GISS_EH – 6 members GISS_ER – 9 members MRI – 5 members NCAR PCM – 4 members

AMO index for NCAR model (dotted line – ensemble average)

AMO for GFDL model (dotted line – ensemble average)

How much of the ensemble average is forced signal?

If infinite number of realizations are available, then ensemble average represents true forced signal

If only a small number of realizations are available, ensemble average contains internal variability as well as forced signal

EOF analysis of NCAR model’s ensemble average 20th century simulations

Correlation between PC and surface temperature

85%

5%

1.5%

GFDL model with 5 ensemble members

Correlation between PC and surface temperature

73%

14%

3%

PC1 of the ensemble average 20th Century simulations may be taken as forced signal

Natural and Forced Variability of AMO in NCAR Model

Run 1 Run 4

Run 2

Run 3

Run 5

Run 6

Run 7

Run 8

Forced

Natural and Forced Variability of AMO in GFDL Model

Run 1

Run 2

Run 3

Run 4

Run 5

Forced

What about observations…

Only one realization, no ensemble average is available

But, we can identify regions where most of the variability is forced

Ratio of variance Total variance of all member

ensembles lumped together, T2

Variance of the ensemble average, a

2

Variance of internal variability,I

2= T2- a

2

Ratio of forced and total variance

2

22

1

1

T

Ia nr

)(

NCAR (8) GFDL (5)

MRI (5)GISS_ER (9)

GISS_EH (5) PCM (4)

Ratio of Variance for 20th Century IPCC Coupled Models

For Indian Ocean, almost all models show that about 85% of the total variance is forced

Indian Ocean SST Index (50E-100E, 20S-10N)

CCSM

GISS_EH

GFDL

MRI

PCM

GISS_ER

Indian Ocean SST index (ensemble mean + observations)

Correlation between IO index and global TS in observations

Forced versus Natural AMO in Observations

Forced Signal

Natural Signal

AMO Forced Signal

MDR Forced Signal

AMO Natural Signal

MDR Natural Signal

Summary Ensemble averages of a limited number of

realizations do not necessarily represent forced signal.

EOF analysis of the ensemble average can be a useful way to separate the forced and natural components.

Indian Ocean responds primarily to radiative forcing and a large percentage of the total variance is forced. Thus it can be used as the footprint of the forced signal in observations.

Using Indian Ocean SST index to separate the forced and natural AMO and MDR, we found that the forced and natural components are of comparable magnitude in both cases.

Regression of global SST on PC1

NCAR CCSM

GISS EH

GISS ER

GFDL CM2.1

MRI

PCM

Spatial structure of natural AMO