Zeng-Zhen HuZhaohua Wu

Center for Ocean-Land-Atmosphere Studies4041 Powder Mill Road, Suite 302

Calverton, MD 20705, USAE-amil: hu@cola.iges.org

The Intensification and Shift of the Annual North Atlantic Oscillation

in a Global Warming Scenario Simulation

Question: How is the NAO Response to Global Warming ?

(1) Observed evidences:

(a) Low frequency trends

(b) Spatial pattern shift

(a) Low Frequency Trends

(b) Spatial Pattern Shift

(2) Model Simulations I

(a) Role of SST:

• Global SSTA Rodwell, Rowell, & Folland, Nature, 1999 Latif, Arpe, & Roeckner, GRL, 1999

• Tropical SSTA Hoerling, Hurrel, & Xu, Science, 2001

• Chaotic-like response of NAO to tropical SSTA Schneider, Bengtsson, & Hu, JAS, 2003

• Many other works

(2) Model Simulations II(b) Greenhouse gases:

• No significant change for the traditional NAO/AO Index ECHAM4/OPYC3: Ulbrich & Christoph, CD, 1998; Hu et al. CD, 2000 HadCM2: Zorita & Gonzalez-Rouco, GRL,2000; Gillett et al. JGR, 2002

• Significant change for the NAO/AO CCCma: Fyfe, Boer, & Flato, GRL, 1999 ECHAM3/LSG & ECHAM4/OPYC3: Zorita & Gonzalez-Rouco, GRL,2000 HadCM3, GISS, ECHAM3/LSG & ECHAM4/OPYC3: Gillett et al. JGR,

2002 GISS: Shindell, Miller, Smith, & Pandolfo, Nature, 1999

• The evident time-dependent shifting of the centers of action of NAO ECHAM4/OPYC3: Ulbrich & Christoph, CD, 1998

• Influence from stratosphere & Nonlinear response

What Are the Real NAO Response to 2*CO2 in ECHAM4/OPY3 CGCM??

(1) What causes the disagreement among various investigations?

(2) Is there any intensity change of NAO ?

(3) Is there any shift of the two centers of action of the NAO (the Icelandic low and the Azores high) ?

(4) Can the traditional NAO index reflect the change and shift ?

(5) Is there any effect on variances and frequencies of NAO variation at inter-monthly to interannual timescales ?

(6) What is the possible mechanism behind the change and shift ?

Main Results of This Work

(1) Annual NAO is significantly strengthened in global warming scenario;

(2) The two centers of action of the NAO, the Icelandic low and the Azores high, are intensified and shifted northeastward by 10 ° to 20 ° in latitude and 30 ° to 40 ° in longitude;

(3) The shift of the centers of action leads to a failure in capturing the NAO change with the traditional definition of the NAO index;

(4) The intensification of Icelandic low is tied up with annual zonal mean state change, and the strengthened Azores high is related to the annual stationary wave change. The shift of the centers of action of the NAO is associated with the annual stationary wave change;

(5) Variances and frequencies of NAO variation at inter-monthly to interannual timescales are not affected much by global warming.

Model and Experiments

• ECHAM4 AGCM, T42, L19 (Roeckner et al., 1996);

• OPYC3: L10+Mixed layer+Sea ice model (Oberhuber, 1993a, b);

• CGCM: Fluxes of heat and freshwater adjusted annually (Bengtsson, 1996);

• 240 year integration of global coupled atmosphere-ocean-sea ice-land surface climate model under the transient greenhouse gas warming scenario. In the simulation, the annual concentrations of the greenhouse gases are specified as observed from 1860 to 1990, and prescribed based on IPCC scenario IS92a (IPCC, 1992) after 1990. The integration starts at January 1860 and ends at December 2099;

• More details on the simulation can be found in Roeckner et al. (1999).

Distribution of Mean Ts & Differences(Warming over ocean is much weaker than that over land)

SLP Mean and Differences(SLP response at high & low latitudes is different)

Intensified NAO is Confirmed by the Change of Zonal Mean

No Trend in the Traditional NAO Index

No Significant Shift in Frequency Distribution of the Traditional NAO Index

Shift in Teleconnectivity (Absolute values of the strongest negative correlation)

Teleconnectivity in NCEP/NCAR Reanalysis

Shift of the Northern & Southern Centers(Teleconnectivity of monthly SLP with a 50 Years Window)

Trend Exists in a Modified NAO IndexSLP (43N,6E)-SLP(77N,0)

Significant Shift in Frequency Distribution of the Modified NAO Index

NAO Intensity (Iceland low) Change Results from Zonal Mean Change

Shift of the Centers is Associated with Change of Stationary Waves

(What is the role of COLD-LAND-WARM OCEAN pattern on the change ??)

Candidate I to Interpret the NAO Low-Frequency Variations: Tropical Warming

(Hoerling, Hurrel, & Xu, 2001, Science; Schneider, Bengtsson, & Hu, 2003, JAS)

Candidate II to Interpret the NAO Low-Frequency Variations: Downward Propagation of Stratosphere Vortex

(Baldwin & Dunkerton, JGR, 1999;Baldwin & Dunkerton, 2001,Science)

Further Information and Acknowledgements :

Further Information :Web page: www.iges.org/pub/ctr/ctr_127.pdf E-mail: hu@cola.iges.org; zhwu@cola.iges.orgPublication: Tellus, 56A, 112-124, 2004

Acknowledgements :This research was supported by the Center for Land-Atmosphere-Ocean Studies (COLA), NSF (ATM 98-14295), and DOE (De-FG02-01ER63256). The authors are indebted to Profs. E. K. Schneider, L. Bengtsson, and D. Straus for their discussion and suggestions. We would also like to thank the colleagues from Max-Planck Institute for Meteorology, Hamburg, Germany, who make the GCM integration available.

Main Results

(1) Annual NAO is significantly strengthened in global warming scenario;

(2) The two centers of action of the NAO, the Icelandic low and the Azores high, are intensified and shifted northeastward by 10 ° to 20 ° in latitude and 30 ° to 40 ° in longitude;

(3) The shift of the centers of action leads to a failure in capturing the NAO change with the traditional definition of the NAO index;

(4) The intensification of Icelandic low is tied up with annual zonal mean state change, and the strengthened Azores high is related to the annual stationary wave change. The shift of the centers of action of the NAO is associated with the annual stationary wave change;

(5) Variances and frequencies of NAO variation at inter-monthly to interannual timescales are not affected much by global warming.

Small seasonality for the SLP change pattern (DJF similar to annual)

Small seasonality for the SLP change pattern (JJA similar to annual)

Storm Track Means and Changes:Winter and Spring, transient energy of 2-6 Days