It is shown that the dominant structure of the seasonal mean mid-latitude circulation (500 hPa height) pattern over the Pacific - North America region forced by tropical SST-related diabatic heating is distinctly different from the seasonal mean internal variability pattern that occurs in the absence of ENDO-related SST anomalies. The separation of these two patterns is accomplished by utilizing ensemble general circulation model (GCM) integrations in conjunction with reanalyses.
Ensemble simulations made with the GCM of the Center for Ocean-Land-Atmosphere Studies (COLA) are compared to the reanalyses of the National Centers for Environmental Prediction (NCEP) for the 16 winters 1981/82 - 1996/97. The GCM ensemble for each winter consists of nine integrations initialized from analyses, and utilizing the observed time varying sea-surface temperature (SST). A signal to noise ratio is defined and found to exceed 3.0 (4.0) in the eastern North Pacific (over Mexico).
A number of techniques are used to calculate the SST / heating forced patterns and the internal variability patterns. The SST-forced mid-latitude circulation pattern is calculated in seven ways, namely (1) as the leading empirical orthogonal function (EOF) of the ensemble mean GCM height field for the 16 winters; (2) as the leading mode of a singular value decomposition (SVD) analysis of height with tropical diabatic heating from the GCM, (3) as the leading EOF (as above) for NCEP reanalyses for the same 16 winters; (4) as the leading SVD mode (as above) for the NCEP reanalyses for the same 16 winters; (5) as the leading EOF of height from reanalyses for the 10 winters having the five strongest warm and five strongest cold tropical SST anomalies in the last 39 years; (6) as the leading SVD mode (as above) from reanalyses for these same 10 strong SST winters; and (7) as a regression of GCM simulated height on a tropical SST time series obtained from the first EOF mode of reanalysis tropical diabatic heating. It is found that the results of all of these techniques agree extremely well with each other, and that the leading modes in the EOF (SVD) analyses explain large amounts of variance (covariance), about 50% (90%). We draw two conclusions: first, that the GCM ensemble means simulate the observed anomalies with high accuracy; and second, that the observed and simulated anomalies were indeed forced by tropical diabatic heating.
The internal variability pattern was calculated in six different ways: (1) as the leading EOF of height of the deviations of each seasonal mean from the corresponding ensemble mean for that winter, (2) as the leading height EOF from a 26-year GCM integration forced by climatological, annually varying SST, and (3) as the leading height EOF from reanalyses for 29 winters not associated with strong warm and cold tropical SST, (4) - (6), as above but for a teleconnectivity analysis for each of the three data sets. The patterns derived from these analyses have a common structure. It is found that it is this internal variability pattern, and not the SST-forced pattern described above, that closely resembles the "PNA" pattern of Wallace and Gutzler (1981). The SST-forced pattern is characterized by strong northward wave action flux across the subtropics in the central Pacific, as well as strong northeastward flux across the northeastern Pacific into Mexico, both of which are not associated with the internal variability pattern.
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Center for Ocean-Land-Atmosphere Studies
last update: 9 June 1999
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