Abstract:
The goal in constructing a precipitation-based measure of ENSO was to
estimate the gradient of rainfall anomalies across the Pacific basin
and ensure a good relationship with SST- and pressure-based
indices. Areas were selected that represent the Maritime Continent
(MC) (100 N - 100S; 900 E - 1500 E) and central to eastern Pacific (P)
(100 N - 100 S; 1600 E - 1000 W). These regions capture the
... largest
precipitation anomalies associated with the interannual variations of
the Walker circulation and contain the largest correlations between
GPCP and Nino 3.4 and SOI. Within P and MC the absolute magnitude of
the largest correlation is over +0.6.
Because of the spatially varying nature of rainfall, it was decided to
use a moving block average which would capture the strongest zonal
gradients within the equatorial Pacific. This procedure is unlike many
fixed area average indices, and allows for a realistic meridional
component of the precipitation gradient and migration of the ascending
and descending branches of the Walker circulation.
A 100 latitude by 500 longitude box is moved in 2.50 increments (grid
block by grid block) throughout the P and MC domains. The maximum and
minimum average precipitation anomalies are found for P (Ap+ and Ap-
respectively) and MC (Amc+ and Amc- respectively). To create a
homogeneous record, Ap+, Ap-, Amc+, and Amc- were separately averaged
over 1991-97 for GPCP and GPROF. Then, the GPROF values were
subtracted from the GPCP values to create adjustments that were
applied to the GPROF part of the record (usually the last few
months). Significant adjustments are made to Ap+, and especially in
the winter and spring months. However, it is planned to replace GPROF
values with the more accurate GPCP analyses as they become available,
about three months after the observation time. Amc- is subtracted from
Ap+ and normalized to create the El Nino Index (EI). Ap- is subtracted
from Amc+ and normalized to create the La Nina Index
(LI). Normalization is achieved for each month by subtracting off the
1979-98 means and dividing by the standard deviations as shown in
Eq. 1: where i = (January, February, ... , December). Positive EI (LI)
values would indicate that the ENSO cycle was in its warm (cold)
phase. There are advantages to quantifying the evolution of the warm
and cold phases of ENSO through separate indices. However, there is
also an advantage to having one index describe the ENSO cycle. This
was accomplished by taking the difference and normalizing to create
the ENSO Precipitation Index (ESPI):
ESPI = normalized EI - LI (2)
Positive (negative) values indicate the warm (cold) phase of the ENSO
cycle. Applying a two month running mean to the ESPI data reduces the
effect of the 30-60 day oscillation signal.
View the Precipitation-Based ENSO Indices on-line:
http://precip.gsfc.nasa.gov/
[Summary Extracted from the NASA/GSFC/MAPB Home Page] 
