What is El-Nino Southern Oscillation (ENSO)? What are the phases?
El-Nino is the warm phase of the ENSO and is a swath of warm equatorial water temperatures in the Central and Eastern Pacific. La-Nina is the opposite, it is the cold phase of sea surface temperatures in the Central and Eastern Pacific.
Discussion by the National Weather Service: ENSO
ENSO Discussion via Climate.gov Climate.gov
ENSO regions are as follows:
The ENSO is calculated using the sea surface temperature anomaly (SSTA) of Nino region 3.4, which corresponds with the east/central Pacific. The atmospheric circulation used to set-up the SSTA correspond with worldwide impacts, and that is why that is currently used as the standard for measuring ENSO phase and strength.
So why does it matter to us?
NOAA on ENSO climate ENSO Impact
The ENSO phase is never static, there is always warming/cooling of the ocean surface and there are always circulation changes. These large scale changes in the Pacific lead to sensible weather changes for us. The ENSO phase is calculated via the tri-monthly data series called the ONI. Below you will see a graphical interpretation of the values. These values are calculated using the sea surface temperature anomaly (SSTA) of Nino region 3.4. There is also a table with values and color-coded to indicate which phase the ENSO is in, found here.
Sub-seasonal changes do exist within the tri-monthly time-period, and that includes the MJO, which is a major driving force in the formation of where tropical activity occurs and where the jet streams set-up in the mid-latitudes. This can be observed in the Long Range menu with the various links I have set-up there.