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, firmly engrained.

Data Sources:

Southern Oscillation Index values were obtained from the NOAA Climate Prediction Center at their Internet address [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.

General Interpretation:

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 points.

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. Map showing regional correlation of Southern Oscillation Index in Western United States

Table 1. Representative basins with significant SOI-Spring/Summer Runoff correlation. Data table of individual basin correlations of Southern Oscillation Index in Western United States