MIKE MOSS SAYS: Jason, There are a number of parameters that NOAA and other organizations monitor and attempt to predict in order to gauge the current and future intensity of El Nino or La Nina episodes. Principal among these is the sea surface temperature departure from normal across a series of rectangular areas in the central and eastern equatorial Pacific. These are known as Nino Regions, and you can see those regions defined at
and a time series of their average departures from climatological normals at
The greater the departure from normal, the stronger the El Nino episode is considered to be.
Another, closely correlated measure is the Southern Oscillation Index, which compares surface air pressures at Tahiti and Darwin, Australia. During El Nino, surface pressures tend to be higher than average at Darwin and lower than average at Tahiti, so keeping track of this index is another way to quantify the intensity of an El Nino episode. The index is computed by subtracting the Darwin value from the Takiti value, so that a negative index is usually indicative of an El Nino. You can see a current time series of the SOI at this address:
There are a variety of other pieces of data that experts on El Nino use to track the phenomenon, such as average winds and departures from normal of winds at about 5,000 and 35,000 feet above the surface, temperatures and temperature anomalies within the equatorial Pacific (subsurface temperatures), depth of the thermocline across the equatorial Pacific, and the topography of sea level in that region (yes, sea level can vary based on predominant wind patterns, along with variations in the temperature and therefore density of ocean water).