frequency variations, and how might this knowledge of the relationships help in predicting shorter-timescale climate phenomena?

What are the magnitudes, spatial and temporal patterns, and mechanisms of midlatitude atmospheric responses to both midlatitude and tropical SSTs? Numerous mechanisms have been hypothesized to explain the observed teleconnections between the tropical and extratropical latitudes, many involving the slow propagation of anomalies via ocean processes back to the equator and fast atmospheric processes away from the tropics. These and other such hypotheses must be thoroughly evaluated to identify the dominant mechanisms of anomaly persistence and communication. Also, the full spatial extent of such teleconnections has not been thoroughly documented. This information also must be pursued to determine over just what scales local and regional anomalies and influences are communicated. Moreover, links to tropical sea surface temperature and the decadal variability of ENSO were also drawn, as well as links to variations in the annual cycle of the southern hemisphere. The origin and maintenance of these phenomena and their associations must be the subject of considerable future investigations.

What are the mechanisms of interaction between the atmosphere and land surface processes on dec-cen timescales?

Through what mechanisms does the planetary boundary layer mediate between dec-cen variability of the surface boundary layer and the free atmosphere? One particular issue is how to average over short time- and space scales to study dec-cen processes, in particular boundary layer and interface processes.

What are the mechanisms of region-to-region and basin-to-basin interactions on the dec-cen timescale?

How do dec-cen changes in atmospheric trace gases and aerosols affect radiative balance and atmospheric circulation, and vice versa?

The study of decadal to centennial variability of atmospheric circulation faces many challenges. Much of our current understanding of the issue derives from the intense interest in anthropogenic climate change. This interest motivated efforts to reorganize the available instrumental and proxy data and to increase the volume of the archives through data “archeology” and additions of new data. These efforts should proceed side by side with establishing clear guidelines for future atmospheric observations and careful planning of the observational networks so that adequacy, continuity, and homogeneity of the records are assured (as discussed earlier). The observational efforts should focus on describing both state variables (winds, pressure, temperature, humidity, and rainfall) and forcing and other related variables (solar radiation, clouds, aerosols, and chemical composition).

Models of the climate system are powerful tools for the study of climate. Such models must be developed to allow the simulation of ocean, atmosphere, cryosphere, and changes in continental surface conditions. Representation of the processes controlling the evolution of all of these important components must be



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