Natural Decadal Climate Variability
Normalized 1st EOF of the barotropic stream function. Calculated for daily output of the integration over 10,000 years of the 2-layer LOM. Explained variance: 24%.
One of the central problems in understanding man-made climate changes is to improve our knowledge of the natural variability of the atmospheric climate and its causes. Flows observed in the atmosphere consist of a hierarchy of structures with different spatial and time scales ranging from small scale turbulence to the planetary scale patterns. There is a great demand to understand climate variability on the time scale of decades up to centuries. In order to estimate the natural variability on this mentioned time scales we use a hierarchy of climate models.
There is evidence from both theoretical and observational investigations, that the long-term behaviour of the atmosphere is controlled by few degrees of freedom. This gives the justification to investigate the long-term behaviour of the atmosphere by nonlinear models with a limited number of variables, so called low-order models (LOM). We performed long-term runs over 1,000 years with a 1-layer, a baroclinic 2-layer and in future also with a 3-layer LOM. Additionally, similar investigations are made with a Simple General Circulation Model (SGCM) using the full set of the thermo-hydrodynamical equations of the atmosphere and ocean.
From the results of these model runs we determine characteristic circulation patterns and their temporal behaviour by means of statistical methods. The figure shows the most dominant spatial structure of the barotropic stream function of the 2-layer model. It reflects the mean zonal flow influenced by the model's orography.
Our analyses of the power spectra of LOM results reveal evident maxima of internal variability on the decadal as well as on the seasonal time scale. These maxima on the decadal scale are confirmed by first investigations with the SGCM.
This multiscale natural variability of the atmosphere is caused by deterministic nonlinear interactions between the model components without any external forcing. The dominant mechanisms generating the long-term climate variability are internally driven by short time scale instabilities and nonlinearities connected with large scale atmospheric processes.
References
- Kurgansky, M. V., K. Dethloff, I. A. Pisnichenko, H. Gernandt, F.-M. Chmielewski, W. Jansen, Long-term climate variability in a simple, nonlinear atmospheric model, J. Geophys. Res., 101, 4299-4314, 1996.
- Weisheimer, A., K. Dethloff, A. Rinke, M. V. Kurgansky, W. Jansen, P. Hupfer, Zur niederfrequenten Variabilität der Atmosphäre in einem Low-Order-Modell, Annalen der Meteorologie, 34, 13-14, 1997.
- Dethloff, K., A. Weisheimer, A. Rinke, M. V. Kurgansky, W. Jansen, P. Maaß, P. Hupfer, Climate variability in a nonlinear atmosphere-like dynamical system, J. Geophys. Res., 103, 25,957-25,966, 1998.
- Handorf, D., V.K. Petoukhov, K. Dethloff, A.V. Eliseev, A. Weisheimer, I.I. Mokhov, Decadal climate variability in a coupled atmosphere-ocean climate model of moderate complexity, J. Geophys. Res., 104, 27,253-27,275, 1999.
- Weisheimer, A., Niederfrequente Variabilität großräumiger atmosphärischer Zirkulationsstrukturen in spektralen Modellen niederer Ordnung, PhD thesis at the University of Potsdam, 147 p., February 2000.
- Dorn, W., K. Dethloff, A. Rinke, M. Botzet, Distinct circulation states of the Arctic atmosphere induced by natural climate variability, J. Geophys. Res. - Atmospheres, 105, 29659-29668, 2000.
- Weisheimer, A., D. Handorf, K. Dethloff, On the structure and variability of atmospheric circulation regimes in coupled climate models, Atmos. Science Lett., doi:10.1006/asle.2001.0034, 2001.
- Weisheimer, A., M. V. Kurgansky, K. Dethloff, D. Handorf, Extratropical low-frequency variability in a three-level quasi-geostrophic atmospheric model with different spectral resolution, J. Geophys. Res.-Atmospheres, 108 (D5), 4171, doi:10.1029/2001JD001282.
- Dethloff, K., A. Rinke, D. Handorf, A. Weisheimer, W. Dorn, Nonlinear dynamics of the climate system, The climate in historical times : towards a synthesis of holocene proxy data and climate models / [GKSS-Forschungszentrum]. H.Fischer, T.Kumke, G. Lohmann, G. Flöser, H. Miller, H.v.Storch, J.F.W. Negendank (eds.) Berlin [u.a.] : Springer, 13-41 (GKSS School of Environmental Research), 2004.
- Handorf, D., W. Dorn, K. Dethloff, A. Rinke, A. Weisheimer, Internal climate variability in global and regional climate models, In H. Fischer, T. Kumke, G. Lohmann, G. Flöser, H. Miller, H. von Storch und J. F. W. Negendank (Eds.): The Climate in Historical Times: Towards a synthesis of Holocene proxy data and climate models, Springer Verlag, Berlin - Heidelberg - New York, 365-382, 2004.