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HOME > Expert Assessments > Climate Diagnostics Bulletin > Editor's Note
 

Editor's Notes

     The following changes have been made for calculating monthly and daily teleconnection indices.

MONTHLY:

The calculation procedure, data source, analysis level, and base period, have changed for the monthly Northern Hemisphere extratropical teleconnection patterns and indices shown in Table E1 and Fig. E7. This has been done to take full advantage of the NCEP/ NCAR Reanalysis, and to eliminate inconsistencies inherent in the previous data set.

The Northern Hemisphere extratropical teleconnection patterns are now calculated from the CDAS monthly 500-hPa standardized height anomalies obtained from the NCEP/NCAR Reanalysis in the analysis region 20N-90N.  Previously, they were calculated from the monthly 700-hPa standardized height anomalies, which were obtained from a variety of analysis procedures prior to 1998, and from the CDAS after 1998. The anomalies and teleconnection patterns are calculated with respect to the 1950-2000 base period, which is 21 years longer than the previous base period 1964-1993.

  The Rotated Principal Component Analysis (RPCA) remains the basis for calculating the teleconnection patterns and indices, with the analysis level now being 500-hPa instead of 700-hPa. In this analysis the ten leading unrotated EOFs are first determined for each of the twelve calendar months, and are based on the standardized monthly anomalies in the three-month period centered on that calendar month [i.e., The July patterns are calculated based on the June through August monthly anomalies]. The ten leading rotated modes are then determined by a Varimax rotation of the ten un-rotated EOFs. The teleconnection patterns are a subset of these rotated modes.

  Previously, the indices of all ten rotated modes were calculated simultaneously for each month in the record using the Least Squares regression approach outlined by Mo and Livezey (1986, Mon. Wea. Rev., p. 2488-2515). However, an examination of all twelve sets of leading rotated modes reveals ten dominant teleconnection patterns, of which eight to nine appear in each of the twelve calendar months. Therefore, one or two of the leading modes in each calendar month are spurious, with no apparent physical meaning.

  These spurious modes are now omitted from the Least Squares equations, whereas before they were retained. The resulting teleconnection indices are the solution to the Least Squares system of equations, such that they represent the combination of teleconnection patterns (instead of the combination of the ten leading rotated modes), which explains the most spatial variance of the observed standardized height anomaly field in a given month.

  Once the teleconnection indices for all months are obtained, they are normalized for each teleconnection pattern and calendar month independently based on their 1950-2000 monthly mean and standard deviation. The resulting standardized indices are then assembled into a continuous time series spanning the period 1950-present. The most recent part of these time series is shown in Table 1 and Fig. E7.

 

  DAILY

The calculation procedure and base period have changed for calculating the daily NAO and PNA teleconnection indices shown in Fig. a2.1. These changes have been made to eliminate inconsistencies in the way that the monthly and daily indices are calculated.

  A major change is that the monthly teleconnection patterns are now linearly interpolated to the day in question, and therefore account for the seasonality inherent in the teleconnection patterns. Previously, only annual mean teleconnection patterns calculated from monthly anomalies were used.

  A second major difference is that the daily teleconnection indices are now calculated using the Least Squares regression approach identical to that used for the monthly indices. Therefore, all of the teleconnection patterns valid for the day in question are now recognized when calculating the indices. The daily indices now represent the combination of teleconnection patterns that accounts for the most spatial variance of the observed anomaly map on any given day. Previously, the indices represented the spatial correlation between the annual mean loading pattern of the NAO or PNA and the daily height anomalies, and did not account for the spatial overlap that exists amongst the various teleconnection patterns.

  A third major difference is that the new teleconnection indices are calculated from standardized daily 500-hPa height anomalies, as opposed to non-standardized anomalies. The standardized anomalies are now calculated based on the 1950-2000 climatological daily mean and standard deviation, whereas the anomalies were previously calculated from the 1971-2000 base period daily means.


Key features and changes to monthly teleconnection indices shown in Table E1 and Fig. E7.

 

             New                                   Old

 

Data Source                   CDAS                 Blend of CDDB data

                                                                   and new CDAS

 

Climatology:                1950-2000                     1964-1993

 

Calculation Level:         500-hPa                          700-hPa

 

Analysis Technique          RPCA                                  RPCA

                                    (Rotated Principal Component Analysis)

 

Analysis Period         

For determining             1950-2000                     1964-1993

Teleconnection patterns                                              

 

Data Input                Standardized monthly    Standardized monthly

height Anomalies              height anomalies

 

Teleconnection    RPCA and Least Square        RPCA and Least Squares

Amplitudes           regression using leading          Regression using ten

                        teleconnection  patterns               leading rotated modes

                    valid for the month of interest          valid for the month of

                                                                              interest

                                   

 

Key features and changes to daily NAO and PNA teleconnection indices shown in Fig. a2.1.

 

                New                                 Old

 

Data Source              daily mean CDAS         daily mean CDAS

 

Climatology:             daily 1950-2000             daily 1971-2000

                         With 4-harmonic smoother     applied to daily 

                                                                 climo means

 

Calculation Level:         500-hPa                        500-hPa

 

Analysis Technique            RPCA               RPCA for patterns only

                                   (Rotated Principal Component Analysis)

 

Analysis Period         

For determining          1950-2000                          1971-2000

Teleconnection patterns                                              

 

Data Input                  Standardized daily                   daily height

Height Anomalies                       anomalies

 

Teleconnection     monthly teleconnection   DJF seasonal pattern

Patterns                patterns interpolated        used for all days

 to day of interest

 

Teleconnection Least Squares regression using       Spatial correlation      

Amplitudes      interpolated teleconnection               between DJF pattern 

                           patterns. Amplitudes represent          and daily 

    the combination of teleconnection     height anomalies

   patterns that explains the maximum 

   spatial variance of the observed 

   standardized height anomaly map

 

 

 

 

 

 

 

 

 

 

 


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