A collaborator and I have a paper in the Journal of Climate under review that (indirectly) involves the use of NCEP-NCAR reanalysis data. The problem is that we've come across a disagreement between the NCEP-NCAR reanalysis and an older reanalysis used by Leathers and Palecki (1992) and were wondering, basically, which one is right.
Our paper uses an approach to times series analysis that involves sampling data rankings over moving time windows . I won't go over the details of the method here (see, e.g., Mauget et al. ( 2012)), but in the paper it detected an abrupt regime shift in temperature over the eastern U.S. in the late 1950's. In the paper we show this shift in two Figures (see Figs. 1d and 2d,e,f ). This shift was also noted years ago by Namias (1972) , and Dickson and Namias (1976), so we also included figures from those two papers in our paper for comparison (see Figs. 1c and 2a,b,c ).
At the same time this shift to colder eastern U.S.temperatures was evident in the late 1950's, Leathers and Palecki (1992) noted an abrupt shift to positive PNA index values. This PNA shift was driven mainly by an abrupt drop in geopotential heights over the southeastern US around 1957. Since that decadal downward regime shift in 700 mb Z was consistent with the eastern U.S. cooling shift that Namias(1972) and Dickson and Namias(1976) noted, we included figures from Leathers and Palecki (1992) that illustrated that PNA and 700 mb Z shift (see Figs. 1a,b).
We sent the paper out to be reviewed, and one of the reviewers suggested that instead of showing the Leathers and Palecki (1992) results, we generate our own series for the PNA and the PNA centers of action from more recent data. So I generated those series from the NCEP-NCAR I reanalysis 700 mb Z data I had on hand, which ended in October 2009. I followed the same procedures for generating the series that was outlined in Leathers et al. (1991), i.e., 13 month running filter etc. Afterwards I compared my PNA index with a similarly filtered PNA index derived from the CPC's monthly PNA values. Both correlate pretty highly (r=.918), even though the CPC PNA is derived from Z anomalies at 4 centers of action and the Leathers et al. (1991) PNA is a 3 point index. So I'm fairly sure no dumb mistakes were made.
When you compare the Leathers and Palecki (1992) results and and our similarly organized PNA series in Fig. 3 below you can see that the abrupt PNA shift evident in the Leathers and Palecki PNA is not found in the current NCEP NCAR reanalysis data. This is mainly because the drop in 700 mb Z over the southeastern US in ~ 1957 that is evident in Leathers and Palecki's results (their Fig. 1d) is not evident in the reanalysis data we're using (our Fig. 3d).
Leathers and Palecki (1992) Fig.1
I always thought that the southeastern U.S. shift in 700 mb Z shown in Leathers and Palecki (1992) (Their Fig. 1b) seemed kind of exaggerated. On the other hand, it is completely absent in the NCEP-NCAR reanalysis even though our analysis and those of Namias (1972) and Dickson and Namias (1976) show a decadal shift to cooler temperatures over the southeastern U.S. during the 1960's. My guess is that kind of persistent shift to cooler temperatures would also lead to some kind of drop in lower atmosphere Z, but there is no trace of that in the NCEP-NCAR data.
So our question is: Why is this drop in geopotential clearly evident in the Leathers and Palecki PNA index - which was derived from NMC daily 700 mb octoganal grid analyses - but not apparent in NCEP-NCAR 700 mb Z? Could this late 1950's geopotential shift may have been identified as a spurious feature in the NCEP-NCAR reanalysis and adjusted out somehow? Was the shift in the Leathers and Palecki (1992) seen as some kind of reanalysis artifact, or a consequence of a shift in the radiosonde data? They consider this possibility in Leathers and Palecki (1992), but show similar shifts in radiosonde Z and surface temperature over the southeastern U.S.
I've taken a look at the reanalysis problems list at http://www.esrl.noaa.gov/psd/data/reanalysis/problems.shtml. I see that the daily times that data was collected changed in 1957 – about the same time we see the 700mb Z shift in Fig. 1b - but other than that I don't see anything related to this problem. Does anyone at Reanalyses.org have any ideas on this?
Dickson, R. R. and J. Namias (1976). "North American Influences on the Circulation and Climate of the North Atlantic Sector." Monthly Weather Review 104(10): 1255-1265.
Leathers, D. J., et al. (1991). "The Pacific/North American Teleconnection Pattern and United States Climate. Part I: Regional Temperature and Precipitation Associations." Journal of Climate 4(5): 517-528.
Leathers, D. J. and M. A. Palecki (1992). "The Pacific/North American Teleconnection Pattern and United States Climate. Part II: Temporal Characteristics and Index Specification." Journal of Climate 5(7): 707-716.
Mauget, S. A., et al. (2012). "Evaluating Modeled Intra- to Multidecadal Climate Variability Using Running Mann–Whitney Z Statistics." Journal of Climate 25(5): 1570-1586.
Namias, J. (1972). "Experiments in Objectively Predicting Some Atmospheric and Oceanic Variables for the Winter of 1971–72." Journal of Applied Meteorology 11(8): 1164-1174.