The Southern Oscillation Index, or SOI, gives an indication of the development and intensity of El Niño or La Niña events in the Pacific Ocean. The SOI is calculated using the pressure differences between Tahiti and Darwin. The base period used in the SOI calculation is 60 years (1933–1992).

Sustained negative values of the SOI below −7 often indicate El Niño episodes. Sustained positive values of the SOI above +7 are typical of a La Niña episode.

Early monthly pressure readings from Darwin and Tahiti have been digitised for electronic use. Early daily pressure readings have not yet been digitised, so a shorter dataset is available.

To monitor for signs of Indian Ocean Dipole (IOD) events, climatologists survey the difference in ocean temperature between the western and eastern equatorial Indian Ocean. During positive IOD events the eastern equatorial Indian Ocean is warmer than normal and the western equatorial Indian Ocean is cooler than normal. During negative IOD events the opposite occurs, the eastern equatorial Indian Ocean is cooler than normal and the western equatorial Indian ocean is warmer than normal.
Details: About the Indian Ocean Dipole

Before the satellite era, the primary source of SST data was observations made by ships passing through the region. The frequency of these observations was too low to produce a useful weekly dataset, so it is shorter than the monthly dataset.

One way climatologists monitor the Pacific Ocean for signs of El Niño or La Niña is to survey the ocean temperature in several (NINO) regions located along the equatorial Pacific. During El Niño events, sea surface temperatures (SSTs) in the central and eastern Pacific NINO regions become warmer than normal. During La Niña events SSTs become cooler than normal in these regions.
Details: About El Niño and La Niña

Before the satellite era, the primary source of SST data was observations made by ships passing through the region. The frequency of these observations was too low to produce a useful weekly dataset, so it is shorter than the monthly dataset.

Daily data sets have a separate value for every day in their record. Similarly, weekly and monthly data sets have separate values for every week or month, respectively, in their record.