Frequently Asked Questions Regarding CPC's
Current Monthly Atmospheric and SST Index Values
Contact Name and Address?
Shi, Department of Commerce, NOAA, email :
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?
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
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
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
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: ftp://ftp.cpc.ncep.noaa.gov/ 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 : Donald.Garrett@noaa.gov, Climate
Prediction Center, Camp Springs, MD 20746.
SST's Recomputed again in
- 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:
IF YOU HAVE ALREADY FOUND AND TRANSFERRED THESE DATA, PLEASE CHECK THAT
YOU HAVE THE LATEST VERSION. THE ORIGINAL FILES WERE INCORRECT. WEEKLY
FILES WERE REPLACED NOVEMBER 02, 2001. MONTHLY FILES WERE REPLACED
NOVEMBER 13, 2001.
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
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.
National Centers for Environmental Prediction, NWS, NOAA
What are the Actual Indices Filenames?
zwnd200 200 MB Zonal Winds Equator (165W-110W)
wpac850 850 MB Trade Wind Index(135E-180W) 5N-5S
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
darwin Darwin Sea Level Pressure
tahiti Tahiti Sea Level Pressure
easter Easter Sea Level Pressure (DISCONTINUED)
rapa Rapa Island Sea Level Pressure ( DISCONTINUED)
(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)
soi (Stand Tahiti - Stand Darwin) Sea Level Pressure
soi.his (Stand Tahiti - Stand Darwin) SLP 1882 - 1950
IV. Sea Surface Temperature
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)
z500t Zonally Average 500 MB Temperature Anomalies
VI. Outgoing Long Wave Radiation
olr Outgoing Long Wave Radiation Equator (160E-160W)<New>
VII. NH Teleconnection pattern indices
tele_index.nh Standardized Amplitudes of NH teleconnection patterns