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Frequently Asked Questions Regarding CPC's Current Monthly Atmospheric and SST Index Values

Contact Name and Address?

Dr. Wei Shi, Department of Commerce, NOAA, email :

Climate Prediction Center/NCEP/NWS/NOAA, 

NCWCP Room 3116
5830 University Research Court
College Park, MD 20740
Ph : (301) 683 - 3440

When Are These Files Updated?

The index values are updated around the 10th of each month.

What is Included?

The Indices subdirectory contains current monthly atmospheric and SST index values including all those located in Table 1. of the Climate Diagnostics Bulletin (CDB).

What do the File Extensions Mean?

The starting dates of the index values varies with the earliest being 1951 for the filenames without extensions. All the files contain the original data, anomaly, and standardized data, except the ones with the (.his) extensions. The (.his) files contain data prior to 1951.

What are the Base Periods?

Starting January 2021:Base period means used by CPC to calculate anomalies:


Have any Base Periods been changed?

On August 1, 2001 , the base period for the monthly SST Nino regions changed and the CPC now uses 1971-2000 as the base period. This update has resulted in some variation in the calculated anomalies.

On January 1, 2000, the base period for the monthly SST Nino regions changed and the CPC now uses 1961-1990 as the base period. This update has resulted in some variation in the calculated anomalies. (Note: SST Nino filename - sstoi.indices)

Have the Nino 3.4 Region Values Ever Been Corrected?

In June 1997, the Nino 3.4 values were changed during the period 1950-1994. The climatology used to be inconsistent, but both the anomalies and indices have been adjusted.

What Changes Were Made to the Nino 3.4 Region Values?

Prior to 1995, the NINO 3.4 region values were calculated based on the blended analysis from 1950 to 1994 and the anomalies were computed using the COADS/ICE climatology with a base period of 1950-1979. Starting Jan.1, 1995, the NINO 3.4 region values were recalculated using an Optimum Interpolation (OI) scheme, which replaced the blended analysis. Also, the anomalies started being calculated using the new adjusted OI climatology with a base period of 1951-1979.

Reconstructed SST's Adjusted OI
1950                                     1980                           1994
Jan. 1                                  Dec.31                      Dec.31

OI Climatology
1950                       1979

What appears to have happened was that the old Blended Analysis values were still used after Jan. 1, 1995 as well as the old anomalies calculated from the COADS/ICE climatologies. The Reconstructed SST's between 1950and 1980 were changed when CPC went to the new OI procedure. There should be no difference. The Adjusted OI values, however, have since replaced the Blended Analysis from Jan 1, 1981 through Dec. 31, 1994,

How is the SOI Calculated?

Note the anomalies are departures from the 1981-2010 base period.

Standard Deviation Tahiti = SQRT( SUMMATION(1) / N )

SUMMATION(1) - is the sum of all ((TA) ** 2)
TA - Tahiti anomaly = (actual(SLP) - mean(SLP))
N - number of months

So, Standardized Tahiti = (Actual Tahiti (SLP) - Mean Tahiti (SLP))
                                                  Standard Deviation Tahiti

Standard Deviation Darwin = SQRT( SUMMATION(1) / N )

SUMMATION(1) - is the sum of all ((DA) ** 2)
DA - Darwin anomaly = (actual(SLP) - mean(SLP))
N - number of months

So, Standardized Darwin = (Actual Darwin (SLP) - Mean Darwin (SLP))
                                                     Standard Deviation Darwin

To calculate the monthly standard deviation:

Monthly Standard Deviation (MSD) = SQRT( SUMMATION(3) / N)

SUMMATION(3) - is the sum of ((Standardized Tahiti - Standardized Darwin) ** 2)
N - total number of summed months

The SOI equation looks as follows: SOI = (Standardized Tahiti - Standardized Darwin) / MSD

What is the NCEP/NCAR Reanalysis?

The NCEP/NCAR reanalysis for atmospheric fields for the1981-2010 period utilizes a state-of-the-art data assimilation system with a T62 resolution version of NCEP's medium range forecast (MRF) mode. This same system (climate data assimilation system - CDAS) is being run early each month in order to provide analyses that are consistent with those produced by the reanalysis project. This represents a major advance in climate monitoring and research efforts. Effective as of February 2011, the following indices are derived from the CDAS/Reanalysis project and the BASE MEANS used for these indices are now 1981-2010 for computing anomalies and standardized data values.

200 MB Zonal Winds Equator (165W-110W) Zonally Average
500 MB Temperature Anomalies
850 MB Trade Wind Index(135E-180W) 5N-5S West Pacific
850 MB Trade Wind Index(175W-140W) 5N-5S Central Pacific
850 MB Trade Wind Index(135W-120W) 5N-5S East Pacific
Outgoing Long Wave Radiation Equator (160E-160W)

The Singapore winds at 30 & 50 MB were also replaced with CDAS/Reanalysis zonally averaged winds. 

Where will I find Singapore Wind Values?

The files (30 MB Singapore Winds) and (50 MB Singapore Winds) were replaced with files
named qbo.30.index and qbo.50.index. The two new indexes are from the CDAS Reanlysis data and are the zonally averaged winds at 30 and 50MB and taken from over the equator. These files can be retrieved from our anonymous ftp site: within the directory: /wd52dg/data/indices

When and Why was Nino 3.4 Added?

In April 1996, the Nino 3.4 region was added to allow researchers gain a better understanding what is happening with the SST's in the critical regions between the Nino 3 and Nino 4 regions. For example, an El Nino episode is occurring and major changes appear to be taking place where the Nino 3 and Nino 4 regions meet (150 Degrees W). The Nino 3.4 region will allow researchers a better view of what SST's are doing between Nino 3&4 regions. The new SST region can be found in the file sstoi.indices.

When Were the Nino Region SSTs Recomputed?

In January 1995, the Nino Region SSTs values were recomputed. The numbers may vary slightly from the previous version. The SST blend product that previously produced the SST values was discontinued on December 31, 1994. An Optimum Interpolation (OI) procedure was developed that recalculated the Nino 1, 2, 3, & 4 region SST's, which replaces the old SST Blend product. The new OI file is called <sstoi.indices>. The values using the old method can still be found through anonymous ftp at: in file name <nino>.For further details on how the new SST values were calculated please contact: Don Garrett, Dept of Commerce, NOAA, email :, Climate Prediction Center, Camp Springs, MD 20746.

SST's Recomputed again in October 2001 - This change may slightly vary some of the numbers from the previous version - Please see memo below for details.

This message is intended to inform users of the NOAA Reynolds and Smith OI SST analysis that the OI fields have been recomputed for late 1981 onward. We will use below the abbreviation OI.v1 for the original OI SST and OI.v2 for the recent version.

The most significant change for the OI.v2 is the improved simulation of SST obs from sea ice data following a technique developed at the UK Met Office. This change has reduced biases in the OI SST at higher latitudes. Also, the update and extension of COADS has provided us with improved ship data coverage through 1997, reducing the residual satellite biases in otherwise data sparse regions.

The weekly analyses are available in:

Monthly interpolated fields are available in:


At the request of users, the sea ice concentration values used to simulate SST obs at higher latitudes are now included with the SST data. Relative error fields are also included with the weekly fields.

The format has changed from that of the OI.v1 files. A sample program to read the data is included in the README file in each directory.

Both the OI.v1 and OI.v2 use a Cressman interpolation to fill in values over land. These values have no real meaning and are there only to allow users to easily convert to other grids. The analysis land/sea mask has been changed slightly for the OI.v2 and is available in the above directories.

A new climatology reflecting the changes at high latitude and with the new base period 1981-2010 is available at:

For more details on the reanalysis, see the draft paper (accepted by Journal of Climate with minor revisions) in directory:

We will continue to update the OI.v1 fields for a time to allow regular users time to switch to the new analysis.

Please contact the following person if you have additional questions about the SST change.

Diane Stokes
National Centers for Environmental Prediction, NWS, NOAA

What are the Actual Indices Filenames?

I. Winds

Filename                                     Description
zwnd200        200 MB Zonal Winds Equator (165W-110W)
wpac850        850 MB Trade Wind Index(135E-180W) 5N-5S West Pacific
cpac850         850 MB Trade Wind Index(175W-140W) 5N-5S Central Pacific
epac850         850 MB Trade Wind Index(135W-120W) 5N-5S East Pacific
singa30          30 MB Singapore Winds (DISCONTINUED)
singa50          50 MB Singapore Winds (DISCONTINUED)

II. Sea Level Pressure

Filename                                  Description
darwin            Darwin Sea Level Pressure
tahiti               Tahiti Sea Level Pressure
easter             Easter Sea Level Pressure (DISCONTINUED)
rapa               Rapa Island Sea Level Pressure ( DISCONTINUED)
easrap             (Easter + Rapa) Sea Level Pressure (DISCONTINUED)
raptah             (Rapa - Tahiti) Sea Level Pressure (DISCONTINUED)
darwin.his        Darwin (SLP) 1882 - 1950
tahiti.his           Tahiti (SLP) 1882 - 1950

III. Southern Oscillation Index (SOI)

Filename                                   Description
soi                   (Stand Tahiti - Stand Darwin) Sea Level Pressure
soi.his              (Stand Tahiti - Stand Darwin) SLP 1882 - 1950

IV. Sea Surface Temperature

Filename                                   Description
sstoi.indices        Nino 1+2 (0-10S)(90W-80W) Nino 3 (5N-5S)(150W-90W)
                  <New>Nino3.4(5N-5S)(170-120W) Nino 4 (5N-5S)(160E-150W)
V. Temperatures

Filename                                    Description
z500t Zonally Average 500 MB Temperature Anomalies

VI. Outgoing Long Wave Radiation

Filename                                    Description
olr                     Outgoing Long Wave Radiation Equator (160E-160W)<New>

VII. NH Teleconnection pattern indices

Filename                                    Description
tele_index.nh       Standardized Amplitudes of NH teleconnection patterns

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Page last modified: March 27, 2007
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