A CMIP sub-project to study NAO variability (NAOMIP)

D. Stephenson
, P. Doblas-Reyes, R. Bojariu, J. Hurrell, T. Delworth(*), V. Pavan, F. Molteni

(*)=potential collaborator

The North Atlantic Oscillation (NAO) is a coherent vacillation in surface temperature and sea-level pressure showing substantial variability over periods up to centuries (Van Loon and Rogers, 1978). It plays a major role in controlling European climate and the North Atlantic ocean, and may play a role in modulating the North Atlantic ecosystems. Hence, it is of great interest to assess how well current coupled models can simulate the variability in the NAO. For more information on the NAO, consult the CLIVAR documentation . NAOMIP is a multi-national CMIP sub-project to compare the coupled ocean-atmosphere model simulations of the annual, interannual, and interdecadal variability in the North Atlantic Oscillation.

North Atlantic Oscillation (NAO) SLP Index

SLP Index

Lisbon minus Stykkisholmur Normalized December-March Average SLP Anomalies.
Values ascribed to the year of the January. This index was provided by Jim Hurrell (jhurrell@ra.cgd.ucar.EDU), and was employed in Hurrell (1995: Decadal trends in the North Atlantic Oscillation: regional temperatures and precipitation. Science, 269, 676-679). High index corresponds to strengthened westerlies with warmer than average conditions in western Europe and colder than average conditions in Greenland.

NAO Questions ?

  • Do coupled models reproduce the spectrum of NAO temporal variability ?

  • Are the major spatial patterns in surface temperature well-reproduced ?

  • Do realistic SLP-SST modes exist in some of the coupled model results ?

Plan of Action

The 80 years of monthly mean surface temperature data from CMIP1 for each model will be recuperated over the extended North Atlantic region (80W-30E,EQU-80N) and the spatio-temporal variability will be compared to that in the COADS observations. To shed light on the above questions, the following analyses will be made:
  • Preliminary CMIP1 results (Covey et al., 1996) indicate that the July-January difference in zonal mean surface temperature at 50N ranges widely between 17-30C for the different models (obs=23C). Advanced time series methods such as X-11 (Stephenson and Macias, 1997) will be used to examine the annual cycle in surface temperature and extract it efficiently.

  • A simple NAO index time series will be made for each model by area weighting the surface temperature anomalies over the Icelandic and Norwegian regions. The spectra of these time series will be compared to that of a similar NAO index made from the observations (Van Loon and Rogers, 1978).

  • An EOF analysis of the deseasonalised temperatures will be performed in order to extract a) the Principal Components, and b) the spatial patterns. The leading spatial pattern will be intercompared and the leading principal component will be compared with the simpler NAO index made using the area weighting.

  • There exists a possibility of obtaining time series of the SLP from certain groups and in these cases, the SLP will be cross-correlated with the surface temperatures using canonical correlation analysis, both for periods less than 5 years and for periods longer than 5 years, in order to verify the models' abilities to simulate the coupled scenarios presented in Bjerknes (1964).

  • [ More detailed plans on decadal variability to be added ? ]

The results of the intercomparison will be made freely available to all modelling groups via the World Wide Web and an article will be written summarising the major findings. Any other contributions from researchers interested in modelling aspects of the NAO would be openly welcomed as part of the sub-project.

Relevant publications

Bjerknes, J., 1964: Atlantic air-sea interaction. Advances in Geophysiscs. Academic Press, 1-82.

Hurrell, J. W., 1995: Decadal trends in the North Atlantic Oscillation: Regional temperatures and precipitation. Science, 269, 676-679.

Hurrell, J. W., 1996: Influence of variations in extratropical wintertime teleconnections on Northern Hemisphere temperatures. Geophys. Res. Lett., 23, 665-668.

Hurrell, J. W., and H. van Loon, 1996: Decadal variations in climate associated with the North Atlantic oscillation. Clim. Change, in press.

D.B. Stephenson, and J. Macias, (1997) Seasonality and regularity of the El Nino-Southern Oscillation from 1950-96, Submitted to Nature.

F. Valero, Doblas, F. J. and J. F. Gonzalez, 1996: On Long-Term Evolution of the Seasonal Precipitation in Soutwestern Europe Annales Geophysicae, 14, 976-985, 1996

Van Loon, H., and J.C. Rogers, 1978: The seesaw in winter temperatures between Greenlandand northern Europe. Part I: General Description. Mon.Wea.Rev., 106, 296-310.

Rogers, J. C., and H. van Loon, 1979: The seesaw in winter temperatures between Greenland and northern Europe. Part II: Some oceanic and atmospheric effects in middle and high latitudes. Mon. Wea. Rev., 107, 509-519.

Bojariu R., 1992: Air Temperature over Europe associated to certain oscillating type atmospheric phenomena, Meteorology and Hydrology, vol. 22, 2, 29- 31

Bojariu, R., C. Pop, 1993: North Atlantic Oscillation Effects on European Precipitation Variability, Proceedings of the International Symposium on Precipitation and Evaporation, Bratislava, vol. 1, 195- 197.

Bojariu, R.,1996: Climate Variability Patterns in the Upwelling Regions of North-western African Coast Derived with Multivariate Statistical Methods, Technical Note No.I.96.03, Institute for Remote Sensing Applications, Joint Reserch Centre, Ispra, Italia.

Bojariu, R., R. Cotariu,1996: Seasonal effects of Atlantic air-sea interaction on climate fluctuations over southern and central Europe, Romanian Journal of Meteorology, vol 3, 1, 1-7.

Bojariu, R., 1997: Climate variability modes due to ocean-atmosphere interaction in the central Atlantic, Tellus, in press.

Covey, C., B. D. Santer and E. Cohen-Solal, 1996: CMIP: A study of climate models and natural climate variability, Proceedings of the Workshop on Dynamics and Statistics of Secular Climate Variations, Miramare-Trieste, Italy, 4-8 December 1995, J. L. Kinter III and E. K. Schneider, eds., Report No. 26, Center for Ocean- Land-Atmosphere Studies, pp. 11-15.

... and some draft CMIP tables and Don Hooper's interface for reading netCDF format files into GrADS

For more details, please contact 

David B. Stephenson
Meteo-France CNRM,
42 av. Coriolis,
31057 Toulouse Cedex

Tel. : +(33)-5-61-07-96-98,
Fax. : +(33)-5-61-07-96-10,
e-mail : stephen@meteo.fr

Copyright (c) 1997 D. B. Stephenson 


This document reflects the authors' personal views and not those of any other organisation, and no liability is accepted for either errors or omissions. Permission is granted to make copies for individual use, not for redistribution.

This page was last updated on the 30th of September 1997.

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