The El Nino – Southern Oscillation (ENSO) phenomenon is another natural mode of climate variability with distinct, well-defined scales. ENSO refers to both El Nino events and their counterparts, La Nina events, in which the eastern equatorial Pacific ocean becomes either unusually warm or unusually cold. These ocean changes occur simultaneously with atmospheric changes. Rainfall, wind, and temperatures shift from their typical patterns over large regions of the earth, especially (but not only) in the tropics. The intrinsic spatial scale of an El Nino event is basically the size of the tropical part of the Pacific ocean. The intrinsic time scale, the time between successive El Ninos or La Ninas, is 2-7 years. We call this time range interannual, and say that ENSO is the dominant mode of interannual variability.
Again, as with every mode of climate variability (other than those set directly by the earth's orbit and rotation: the diurnal, annual, and tidal cycles) the period of ENSO has to be expressed as a range (2-7 years), not a single number. If it were a single number – say, 3 years - it would mean that El Ninos were perfectly predictable, since once one happened we would know the next would be exactly three years later. No such luck. Still, the time scale is well enough defined to stand out above the “noise” of the random year-to-year fluctuations of the climate. As with synoptic weather, we do have a body of theory for ENSO which explains its scales in terms of the underlying fluid dynamics. Since about the 1980s, we also have some ability to predict El Nino and La Nina several months to a year ahead of time, using computer models derived from that same fluid dynamics.
The MJO occupies the range of time scales between ENSO and day-to-day weather variability – between a few days and a couple of years. Arguably it is the only truly coherent mode of variability with a well-defined periodicity in that time scale range, weeks to months.
Understanding these coherent modes, with known space and time scales, enables us to predict the behavior of the atmosphere and ocean further in the future than we would otherwise be able to. Ed Lorenz showed in the 1960s that because the atmosphere is chaotic, weather – the specific weather that will occur on a specific day – can’t be predicted very far in advance; probably not more than about two weeks. However, because ENSO influences the climate – the average weather over a period of weeks, months, or longer – and because El Nino and La Nina events tend to take a year or so to develop, mature, and then decay, knowing the state of ENSO allows us to make seasonal climate forecasts. These are predictions about how the weather over the next several seasons will differ from normal, though they don’t say anything about any particular day. Because we have computer models that simulate the evolution of the climate system, sometimes we can even go further and predict the appearance of an El Nino or La Nina event before it starts.
Understanding of the MJO allows us (just since pretty recently) to make intraseasonal forecasts. These are also predictions of the average weather over a period, rather than for a single day. Because the MJO time scale is so much shorter than that of ENSO, however, the we are predicting the average weather just for a period of a week or two. These forecasts are somewhere in between weather and climate prediction. Because their time scales are not very much longer than those of weather forecasts, though, in some circumstances they can almost allow us to predict the weather further in advance than Lorenz told us was possible.