1998 Annual Report
Biological and Environmental Research

Predictability of the Coupled Ocean/Atmosphere System

T. P. Barnett, N. Schneider, and D. W. Pierce, Scripps Institution of Oceanography

The strongest empirical orthogonal function (EOF) of sea surface temperature anomalies, 10-year low-pass filtered, in the coupled ocean/atmosphere general circulation model. This pattern is similar to observed conditions, and shows that the coupled model faithfully captures the large-scale patterns of low-frequency climate variability. This pattern is associated with changes in winter temperature and precipitation over much of North America.

Research Objectives

Variability of the climate on interannual to decadal time scales is of paramount importance to human society, encompassing such variations as persistent droughts and flooding or spans of unusually warm or cold years. Obviously, it would be of great benefit to be able to predict such changes in long-term precipitation or temperature patterns. This project is aimed at understanding how predictable the coupled ocean/atmosphere system is on such time scales.

Computational Approach

The investigations start with a 100-year run of a fully coupled ocean/atmosphere general circulation model. This provides the base "climate" that we seek to predict. We then perform ensemble studies by starting at specific times within that 100-year control run, slightly perturbing the initial conditions in various ways, and letting the system evolve forward in time. The perturbations mimic the effect of making predictions of the real Earth's climate with somewhat unknown initial conditions, and of the other unpredictable short-term influences of weather. To the extent that the system evolves in the same way as the control run despite the small perturbations, the system is potentially predictable. Finally, we are investigating the physics of the longer-term variability in the control run to see what processes contribute to that variability; an understanding of these mechanisms might lead to other avenues to explore for predicting such fluctuations.

Accomplishments

The investigations of the physical mechanisms of the variability have shown that in this model, the majority of the oceanic variance in the mid latitudes can be explained by stochastic atmospheric forcing. There remain, however, tantalizing peaks at a period of 20 years in the cross spectra of atmospheric forcing and ocean variables off the coast of Japan. This suggests that some part of decadal variance might indeed be governed by coupled interactions and hold some predictive potential. The exact dynamics involved are currently under investigation.

In addition, the coupled integration has yielded a strong decadal mode in the tropical Pacific Ocean that involves advection of anomalous temperature in the oceanic thermocline from 10-20N to the equator, and a shift of centers of deep convection and concomitant wind stress, thermal, and freshwater forcing in the atmosphere. The details of this new mode are currently being explored.

Significance

The significance of the work, from an end-user's perspective, is still evolving. To the extent that specific mechanisms of variability, such as the decadal mode described above, can be reliably identified in nature, an understanding of their workings might contribute to more accurate forecasts of seasonal to yearly rainfall and temperatures over various parts of the globe. It will, however, take more time and work before such an increase in predictive skill can be realized.

Publications

T. P. Barnett, D. W. Pierce, M. Latif, D. Dommenget, and R. Saravanan, "Interdecadal interactions between the tropics and midlatitudes in the Pacific basin," Geophysical Research Letters (submitted, 1998).

T. P. Barnett, D. W. Pierce, N. Schneider, and R. Saravanan, "On the origins of the Pacific decadal oscillation," Geophysical Research Letters (submitted, 1998).

D. W. Pierce, T. P. Barnett, N. Schneider, and R. Saravanan, "Interactions between the Pacific Ocean tropics and midlatitudes on decadal time scales," J. Climate (in press, 1998).

http://meteora.ucsd.edu/


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