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Overview of the ENSO System
Defining ENSO
How do we define El Niño and La Niña?Given that there are typical characteristics of El Niño and La Niña, how are specific "ENSO events" defined? How different from neutral conditions do things have to be to qualify as an event, since things will never be perfectly neutral? The definition of an El Niño or La Niña event is based on one (or more) of the standard indices used in monitoring and involves both the magnitude (value) of the index as well as how long it persists. (See ENSO Basics for typical characteristics and Monitoring ENSO for information on indices).But just how large must the value of the index be, and for how long must it persist in order for an El Niño or La Niña to be identified? On these points different researchers have different ideas and there is no single method used to identify events. However, a common method in use is based on the NINO 3.4 Index, which is the departure in monthly sea surface temperature from its long-term mean averaged over the NINO 3.4 region. In this method, an El Niño or La Niña event is identified if the 5-month running-average of the NINO 3.4 Index exceeds +0.4 deg. C (for El Niño; -0.4 deg. C for La Niña) for at least 6 consecutive months (see Trenberth, 1997 in Links and References). The 5-month running-average (data is averaged over 5-month, overlapping periods incrementing one month at a time) is used to smooth out variations in sea surface temperature not associated with ENSO. El Niño and La Niña Events since 1950Figure 1 below shows how the definition above is applied to the record from 1950.
The following El Niño (red in Figure 1) and La Niña (blue) events have been identified by the method we described earlier. Note that the numbers indicate the order they appear on the plot above and are not indicators of the strength of the events. Notice that many El Niño and La Niña events extend across different calendar years. Also notice that some El Niño and La Niña events persist for as many as two full years. Table 1. El Niño and La Niña
Years
The annual cycle of El Niño and La NiñaAs seen in the table above, not all El Niño and La Niña events are of equal duration nor do they all evolve in the same way. In fact, observations indicate a fair amount of variability in the life cycles of individual El Niño and La Niña events. Figure 2a and 2b below display how the value of the NINO 3.4 Index (in units of degrees C) varied over time for the 6 strongest El Niño's and La Niña's which have occurred since 1950. All of the plots start in January of the year in which El Niño or La Niña conditions were first observed (Jan(0)) and run through to January two years later (Jan (+2)). The charts show some interesting features of El Niño and La Niña events:
The top 10 El Niño and La Niña eventsAs was mentioned, there is no single definition of an El Niño or La Niña event and that each event evolves somewhat differently. These ideas are expressed in Table 2 and Table 3 below, which ranks the 10 strongest El Niño and La Niña events occurring since 1950 based on the NINO 3.4 Index for different seasons. For El Niño events, the NINO 3.4 Index values are plotted in bold red; for La Niña, bold blue. Also shown in the tables are corresponding values of the NINO 3 Index and the (negative) of the Southern Oscillation Index, or SOI . As these tables show, each index produces a different ranking.Table 2: Top 10 years years with the highest NINO 3.4 index as averaged over different seasons. Also shown are the NINO 3 Index and the Southern Oscillation Index (SOI) (which has been multiplied by -1 so as to have the same sign as the sea surface temperature indices).
Table 3. Top 10 years years with the lowest NINO 3.4 index as averaged over different seasons. Also shown are the NINO 3 Index and the Southern Oscillation Index (SOI) (which has been multiplied by -1 so as to have the same sign as the sea surface temperature indices).
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