Abstract

The output of a 20-year integration of an annual cycle (AC) version of the NCAR Community Climate Model in which the external conditions went through 20 prescribed identical annual cycles is used to study mouth-to-month persistence of anomalies in monthly mean atmospheric circulation fields on a global and a hemispheric scale. Of all fields considered, the height fields (1000–300 mb) are the most persistent and the transient eddy flux fields the least persistent. Persistence in height field anomalies is largest in winter and small throughout the rest of the year. For the area north of 20°N, a comparison is made with the persistence of months mean height and temperature fields observed in the real world (RW) during a 28-yeu interval. On a pooled all month-pairs basis, RW height anomaly fields are significantly more persistent than those appearing in AC but, from a practical point of view, the difference is small. The differences in persistence are larger for temperature anomalies (500–1000 mb thickness) than for height. Differences between RW and AC monthly persistence over the area north of 20°N are largest in summer when the RW has a local maximum in persistence. On the assumption that the model and atmosphere have the same internal dynamics, the differences just described can be attributed to the interaction of the atmosphere with external or boundary conditions (e.g., ocean surface temperature), which was purposely omitted from the AC integration. Interaction with the lower boundary in summer seems, therefore, to be quite important to explain the observed level of month-to-month persistence in circulation anomalies. In winter, however, the internal dynamics of the atmosphere alone produces the required observed level of month-to-month persistence. The output of a 15-year integration of the same model in which the sea surface temperature, on a global scale, had realistic interannual variability, is used to interpret further the differences between RW and AC.

As a by-product of this study we have calculated the spatial degrees of freedom (dof) associated with time mean anomaly fields. The dof for global monthly mean anomaly height fields in the AC model are quite low, i.e., 25–35 on a yearly pooled basis. Over the area north of 20°N, the dof associated with monthly mean anomaly height fields of the AC model and the RW are quite close, varying from 15–20 in winter months to about 40 in summer.

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