Report 49:    Comparison of the 200 hPa circulation in CSM and CCM3 simulations and NCEP and ERA reanalysis:  Principal and Common Principal Components

James S. Boyle

December 1998, 38 pp. 

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In this paper the interannual variation of monthly mean vorticity and diver-gence at 200 hPa are compared from four data sources: The NCEP/NCAR reanalyses 1958 through 1994, the ECMWF (ERA) reanalyses, 1979 through 1994, a NCAR CCM3 integration using prescribed SSTs from 1979 through 1993, and the NCAR CSM 300 year integration. Four twenty year periods were taken from the 300 year simulation for analysis. The NCEP, ERA and CCM3 all provide data for the period 1979 through 1993. The techniques used are principal and common principal component analyses on the fields transformed to spherical harmonics. The seasonal cycle is removed.

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 file)


UCRL-MI-123395

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