El Nino Monitoring Indices

Climate Prediction Division
Japan Meteorological Agency

   The elnino/index/ directory contains Readme.txt (this file) and 
historical data on the Southern Oscillation Index (soi) in ASCII format, and 
one sub-directory for Sea Surface Temperature Indices (sstindex).


1. Southern Oscillation Index (SOI; file name: soi)

   This index represents the intensity of trade winds over the equatorial
Pacific, with negative values indicating weaker-than-normal winds, a typical 
characteristic associated with El Nino events.
   The data come from monthly mean sea-level pressure values reported
in CLIMAT messages from Tahiti in the South Pacific and Darwin in
Australia.
   The index is defined as
       SOI = X / SD(X),
where
       X = ANM(Ps(T))/SD(Ps(T)) - ANM(Ps(D))/SD(Ps(D)),
and Ps(A) is sea-level pressure for point A, ANM(B) and
SD(B) are the anomaly and standard deviation of element B, and
T and D represent Tahiti and Darwin, respectively.
The base period for the normal is 1991 - 2020. The latest values are
preliminary.


2. Sea Surface Temperature (SST) Indices

   SST indices consist of monthly mean SSTs, monthly mean SST anomalies
(for the 1991 - 2020 base period), monthly mean SST deviations (for the
latest sliding 30-year base period), and related five-month means 
averaged over the five El Nino Monitoring Regions listed below since
January 1949. The latest values are preliminary.

   Monthly mean SSTs are analyzed by JMA's Climate Prediction Division.

   The five El Nino Monitoring Regions are:

       NINO.1+2:   EQ. - 10 S   90 W - 80 W
       NINO.3:     5 N - 5 S   150 W - 90 W
       NINO.4:     5 N - 5 S   160 E - 150 W
       NINO.WEST: 15 N - EQ.   130 E - 150 E
       IOBW:      20 N - 20 S   40 E - 100 E

   The directory (elnino/index/sstindex) contains two sub-directories
corresponding to the base periods of 1991 - 2020 and the latest sliding
30-year period (directories: base_period_9120 and sliding_30year_period).

   SST indices are derived from MGDSST datasets (Kurihara et al. 2006) after June 2015 and those of COBE-SST2 (Hirahara et al. 2014) before this.


2-1. base_period_9120 directory

   This directory (elnino/index/sstindex/base_period_9120/) contains
five sub-directories corresponding to the five El Nino Monitoring Regions.

   There are three data files under these sub-directories:

       (1) Monthly mean SST (file name: sst)

       (2) Monthly mean SST anomaly (file name: anomaly)
           (The base period for the normal is 1991 - 2020)

       (3) 5-month running mean SST anomaly (file name: 5rmean)
           (The base period for the normal is 1991 - 2020)

   Data files are in text format. The first line in each shows
the month, and subsequent lines contain data from 1949 to the latest year. 
The unit is degrees centigrade, and entries of 99.9 (or 99.90) represent 
undetermined values.


2-2. sliding_30year_period directory

   This directory (elnino/index/sstindex/sliding_30year_period/) contains
three sub-directories corresponding to the Nino.3, Nino.West and IOBW
regions.

    There are three data files under each of these sub-directories:

       (1) Monthly mean SST climatological reference based on the latest sliding 30-year
           period (file name: sst)
           The SST climatological reference is defined as the SST averaged over
           the latest sliding 30-year period for Nino.3, and as a linear
           extrapolation with respect to the latest sliding 30-year period
           for Nino.West and IOBW.

       (2) Monthly mean SST deviation (file name: deviation)
           SST deviation is defined as the difference between the monthly
           mean SST and the climatological reference based on the latest sliding 30-year
           period.

       (3) Five-month running mean SST deviation (file name: 5rmean)

    Data files are in text format.  The first line in each shows 
the month, and subsequent lines indicate data from 1949 to the latest year. 
The unit is degrees centigrade, and entries of 99.9 (or 99.90) represent 
undetermined values.


Directory structure

TOP
 |
 +-- elnino
         |
         +-- index
               |
               +-- Readme.txt
               |
               +-- soi
               |
               +-- sstindex
                      |
                      +--- base_period_9120
                      |            |
                      |            +-- Nino_1+2 ---- sst, anomaly, 5rmean
                      |            |
                      |            +-- Nino_3   ---- sst, anomaly, 5rmean
                      |            |
                      |            +-- Nino_4   ---- sst, anomaly, 5rmean
                      |            |
                      |            +-- Nino_West---- sst, anomaly, 5rmean
                      |
                      +--- sliding_30year_period
                                   |
                                   +-- Nino_3   ---- sst, deviaiton, 5rmean
                                   |
                                   +-- Nino_West---- sst, deviaiton, 5rmean
                                   |
                                   +-- IOBW     ---- sst, deviaiton, 5rmean


  For inquiries relating to this data set, see
http://www.data.jma.go.jp/tcc/tcc/contact.html.

References

    Hirahara, S., M. Ishii, and Y. Fukuda, 2014: Centennial-scale sea surface temperature analysis and its uncertainty. J. Climate, 27, 57-75.

    Kurihara, Y., T. Sakurai, and T. Kuragano 2006: Global daily sea surface temperature analysis using data from satellite microwave radiometer, satellite infrared radiometer and in-situ observations. Weather Service Bulletin, 73, Special issue, s1-s18 (in Japanese).