Southern Ossilation Index Statistical Correlation
Southern Oscillation Index Statistical Correlation with Spring
Runoff in the Western US
Natural Resources Conservation Service
National Water and Climate Center
October 15, 1997
The USDA, NRCS National Water and Climate Center (NWCC) has completed
an analysis of the correlation of the Southern Oscillation Index
(SOI) with spring and summer volume runoff in the western U.S.
The results are shown in Figure 1 and Table 1 attached. Basins
with a significant correlation (greater than 0.35 or less than
-0.35), may require additional monitoring and analysis during
Water Year 1998 depending on specific water management needs.
What is El Nino/Southern Oscillation (ENSO)?
"ENSO" stands for "El Nino / Southern Oscillation".
The acronym arose in the climate research community, and reflects
an attention bias toward the warm phase of the entire cycle.
El Nino is just one phase of an irregular fluctuation between
warmer than usual and colder than usual ocean temperatures in
the Eastern Pacific. The cold phase has recently come to be known
as "La Nina". The El Nino/La Nina "cycle"
does not occur with strict periodicity. Historically, an El Nino
usually recurs every 3-7 years, as does its (cold) La Nina counterpart.
The overlying atmosphere is tightly coupled to ocean temperatures
and circulation patterns. An atmospheric pressure signal is seen
throughout the tropics that is strongly linked to El Nino and
La Nina. When barometric pressure is higher than usual in the
western Pacific near Indonesia, pressure is lower than usual in
the subtropical Pacific near Easter Island and Tahiti. This global-scale
pressure signal, identified 70 years ago, is known as the "Southern
Oscillation." Surface barometric pressure at Darwin, Australia
and the island of Tahiti are strongly anti-correlated: when one
is higher than usual, the other is lower than usual. The difference,
Tahiti minus Darwin, suitably normalized, is referred to as the
Southern Oscillation Index (SOI), and is frequently used as a
convenient, simple and reasonably accurate tool to monitor the
status of El Nino/La Nina.
Because more attention has been devoted to El Nino, and noting
the association between the Southern Oscillation in the atmosphere
and El Nino (and La Nina) in the ocean, the research community
began to refer to the combination as ENSO (El Nino/Southern Oscillation).
This moniker is somewhat asymmetric: El Nino pertains to just
one of the two phases of the Southern Oscillation. It would be
perhaps more accurate to refer to El Nino as the warm phase of
the Southern Oscillation, and to La Nina as the cold phase of
the Southern Oscillation. The term "ENSO" is, however,
Southern Oscillation Index values were obtained from the NOAA
Climate Prediction Center at their Internet address http://nic.fb4.noaa.gov/data/cddb/cddb/soi
[from the second table labeled "Standardized Data, Sea Level
Pressure, (Standard Tahiti - Standard Darwin)"] available
from water year 1951 to September 1997. Streamflow volumes were
obtained from the NWCC Centralized Forecast System, from water
years 1951 - 1997.
Calculation Methodology and Results:
Single and multiple month (summed) SOIs were correlated with spring
streamflow volumes at key basin streamflow points. Representative
streamflow points, flow periods analyzed, and SOI-streamflow correlation
values are shown in Table 1. The values given are for the SOI
period (sequence of months) that gave the highest correlation
with streamflow. The table contains only those basins that have
a correlation greater than 0.35 or less than -0.35.
Figure 1 summarizes the results graphically. River basins with
correlations greater than 0.35 are shown in red, basins with correlations
less than -0.35 in blue, basins with little or no SOI-spring runoff
correlation are shown in yellow, and the white indicates areas
not analyzed and/or streams that are not water supply forecast
Basins with correlations less than -0.35 (blue) tend to have higher
than average streamflow during El Nino years (when the SOI is
negative, as it is now), and lower than average streamflow during
La Nina (when the SOI is positive). Basins with correlations
greater than 0.35 (red) tend to exhibit lower than average streamflow
during El Nino years and higher than average streamflow during
La Nina. Basins with significant SOI correlations (blue and red
areas) will require further monitoring as the water year progresses.
Figure 1. Correlation Map of the Southern Oscillation Index (SOI) with spring and summer volume runoff.
Table 1. Representative basins with significant SOI-Spring/Summer Runoff correlation.