20CR Climate Indices: Southern Oscillation Index (SOI)

Description

This mode describes variability of the tropical atmosphere. It is often used as a measure of El Nino/La Nina and is correlated highly with ENSO indices. A high value represe nts higher surface pressure in the eastern tropical Pacific with more typical easterlies along the equator and is associated with La Nina's. A low value is associated with tropi cal surface westerlies and El Ninos.

We used the definition in Ropelewski 1987 MWR. Surface pressure at the grid points nearest to Tahiti and Darwin represent the timeseries at those two locations. Each timeseries is first standardized using the 1981-2010 monthly climatology. The Tahiti minus Darwin timeseries is calculated. This timeseries is then re-normalized using 1981-2010 for the st andardization.

This version of the SOI is calculated using SLP from the 20CRV2c. (SOI) is defined as the difference of zonal mean sea level pressure between 40°S and 65°S. The zonal means at 40°S and 65°S are each calculated. Each month's zonal mean is standardized by the mean/standard deviation determined for the climatological time period (1981-2010). Then, the 65°S value is subtracted from the 40°S value for each month.

Temporal Coverage

• Monthly values: 1851/01 to 2011 (V2c)
• Monthly values: 1871 to 2012 (V2)
• Update Status:Static: Static

Data Notes

• Units: mb
• Missing data value: -99