Hi Muhammad, 

The appropriate representation of  "El Nino" is ENSO (El Nino Southern Oscillation). The "El Nino" only represents the Oceanic component and the Southern Oscillation indicates the Atmospheric component. Hence the name ENSO. In the atmospheric science community people use multiple indexes to define ENSO. The El Nino is defined as an increase of Eastern Tropical Pacific's sea surface temperature (SST) of 0.5 degree C from long term average (normally 30 years). And the La Nina is defined as a decrease of SST over the same area by -0.5 degree C from long term average.        

The ENSO has a recurrence interval of 2 to 7 years. You can use multiple tools to compute the return period or recurrence interval. I had used the Wavelet analysis tool to calculate amplitude and frequency of ENSO event in one of my graduate level course project.   

Beside the above link that Dr. Kenneth given, I would like to suggest you to visit the following web link for a series of articles on ENSO including different ENSO indexes; role of ENSO to global weather and climate  by renown atmospheric scientist professor Anthony G. Barnston's of Columbia University.  

Multivariate ENSO Index (MEI)

http://www.esrl.noaa.gov/psd/enso/mei/ 

Why are there so many ENSO indexes, instead of just one?

https://www.climate.gov/news-features/blogs/enso/why-are-there-so-many-enso-indexes-instead-just-one 

https://www.climate.gov/author/anthony-barnston  

Hope the above link will help you to find your answer more broadly 

Thank you all for your such valuable information.

Hi Muhammad,

Dr Kemal from University of Waterloo has provided above good links for the proxy datasets of El Niño, ENSO, etc...

Just a comment above the period and trend: nor trend has been clearly shown yet on any of the El Niño time-series, neither a clear periodicity. El Niño phenomenon is recurrent, this means that it will eventually happen sooner or later but is not periodic. Something similar happens with floods or droughts, you know they will happen, but not exactly when. From previous data you can know that this phenomenon happens in average every x number of years, but you cannot predict the occurrence just from the time-series itself. There has been periods when El Niño temporal behavior has been close to periodic but this is not the norm

Right now El Niño prediction is based in the detection of geophysical signatures in the region or around that might eventually lead some months later to the fully development to one of this episodes

 Like with almost any climate-relevant phenomena, the observations to detect or analyze it are either quite excellent but short, or longer and not so excellent. In the near term there's satellite data for SST anomalies, a whole suite of buoys along the tropical Pacific and other satellite data that probes the atmosphere; this allows a detailed view but there's not enough data for trend analysis. On the longer term, there's the pressure difference data, that is like a proxy for the real thing; long enough for a trend analysis but not really enough to neither follow the development of ENSO nor understand the processes. Either way, there hasn't ben any analysis that I've seen that indicates a trend - natural or driven by climate change, and not for lack of trying.

Forecasting anything in the atmosphere that is beyond a week (or theoretically a few weeks) requires a coupled component of the modeling system that has a longer memory - here the ocean - and still it can only be done in a probabalistic sense. All these forecasts are hence "ensemble forecasts". They are surprisingly good out at 6 months (sometimes even a year); much better than any other method. Still, it is fundamentally impossible to generate a perfect ensemble and of course there are forecast busts. But the method and the results cannot be compared with the outcome in the same way as with a deterministic weather forecast. A statistical forecast is never entirely wrong nor ever entirely right...

Michael Tjernström and  Miquel Angel Rodriguez Arias

Thanks a lot for your valuable information and enhancing my understanding to the topic.

Data is available on the site Climate Prediction Center ElNino/LaNina

kulkarni

Towe; this is interesting because even if there is no effect by global warming on ENSO, one could think there could an effect on CO2 by SST. Higher SSTs would mean less CO2 uptake, right? 

The "weather" impacts of ENSO are present but in many cases overrated; living in Europe I often get the question from journalists, and the statistical impact of ENSO on weather in Europe (temperature or precipitation), especially northern Europe, is marginal or none. But being in California January-February 1998 I can testify that it just wouldn't stop raining(!) - and the desert was in bloom. Fantastic...

Being a science nerd, there's a twist to that also; along the equator the rotation of the Earth doesn't matter much and information on whatever happens on one longitude is quickly transferred around the globe - on the equator. Further away from the equator, rotation makes atmospheric dynamics "stiff" and information has a harder time propagating away from the tropics. I imagine that even in the US, the NE states have a much more mellow response than the SW states, no?

As far as I know there's no real "theory" that predicts that ENSO would necessarily be affected by global warming (i.e. by CO2); just a big question. Some models do and others don't - in general not all even do a decent ENSO in the first place.

But it struck me; about 50% of each new kilo CO2 emitted is taken up by either as biomass or is resolved in sea water. But the solubility goes down with increasing temperature. ENSO cover a sizable area of ocean and in a El Nino its is substantially warmer than average, so shouldn't CO2 in the atmosphere go up a little when less is taken up by ocean water? I don't have a clue; it was just a thought.

ENSO effects are large near by; in the Americas when El Nino and in Australia when La Nina. But the further away you are, the effect disappears quite fast especially away from the tropics, and in Europe most of it is gone. Around the tropics the effect travels quite far and fast, but away from the tropics and farther downstream the correlations are weak. But is makes for good and spectacular news...

Kenneth, Mostafa, Michael, Muhammad, Miguel Angel and Jeevanprakash, I thank you for your honest discussions, and thought I am a palaeontologist, your comments are very instructive and I learn a lot about these variations of these "children" of the Pacific, thanks, FH

Hi Muhammad,

ENSO is monitored using mainly sea surface temperature measurements in the equatorial pacific region.  There are several common indices such as Nino3.4 which are just area average temperatures as defined here: http://www.metoffice.gov.uk/research/climate/seasonal-to-decadal/gpc-outlooks/el-nino-la-nina

You can easily download sea surface temperatures for over a century of observational records here: http://www.metoffice.gov.uk/hadobs/hadisst/

By looking at the series of monthly averaged ENSO indices it is easy to see that there is a lot of variation in the 3-5 year range.  This is far from perfectly periodic but shows that ENSO varies with an average period of about that number of years.

Finally, you asked about trends.  These are much smaller than the ENSO variability from an El Nino or La Nina.  For example we have 2-3 degrees warming at the moment due to a very strong El Nino, whereas the trend is a few tenths of a degree over the last few decades in the same tropical Pacific regions.

Best regards

Adam

It is reported that ENSO events 2-7 years variability almost in a periodic manner. Our recent study with a ENSO model shows, the events could occur in a chaotic fashion with wider variability of amplitude and time. We are in search of data to validate our observation.