December 1998, 38 pp.
For the common time period, 1979 through 1993, the ERA, NCEP and CCM3 display a close correspondence in their the leading PCs of the 200 hPa vorticity. This mode is closely related to the ENSO variations of the period but the agreement extends to the extratropics. All four CSM periods have leading modes similar to each other which are dominated by a PNA type pattern and lack any Equatorial Pacific ENSO signature.
The agreement between the leading PCs for the ERA, NCEP and CCM3 for the 200 hPa divergence was somewhat less than that of the vorticity. The CCM3 and ERA indicate a somewhat larger magnitude center in the Equatorial Pacific about the dateline than NCEP. The CSM has an intense center a farther east at 150E. There are indications in the vorticity and divergence fields that this center is at the source for waves propagating to the mid latitudes yielding the PNA structures.
Two twenty year periods of the 1958 to 1996 NCEP reanalyses show a distinct difference between the two periods. The variations are comparable in magnitude if not nature to the variations seen amongst the 20 year time sections of the CSM run examined.
A CPC analysis of the NCEP, ERA and CCM3 show a common ENSO type response
as the leading common component. The CCM3 model departs from the reanalyses
for the second component. Combining the CCM3, CSM, NCEP and ERA shows that
the CSM does share a common component ENSO related feature. Both the CSM,
and CCM3 depart in the same manner with regard to the second common component
which has a PNA like nature. (pdf