Recently, I was given the question: "What effects does El Niño have on Summer/Autumn?". The problem with answering this question is that usually what happens in an El Niño event (La Niña also), the Pacific transitions from one phase to another during the Summer season most of the time and during Autumn, it's usually building up to the El Niño Southern Oscillation (ENSO) event though peaks towards the end into Winter. That's a usual occurrence of an ENSO event, you can get unique ones that begin in the Summer and end in the Autumn or you can get one that continues on from the previous Winter throughout the year - these are rare scenarios. For this blog post, we're going to be looking at all the Summers that preceded (or were during) every known El Niño event regardless of its strength (weak, moderate, strong or very strong). I will give some disclaimers if the previous Winter was a La Niña. To be looking at this, we're going to use the Central England Temperature (CET) dataset. Keep in mind, NOAA always revise their ENSO anomalies so some events will chop and change like the 2014-15 weak El Niño which was previously disregarded as an El Niño event and instead ENSO neutral. Also, ENSO (whether it's El Niño or La Niña) has unknown effects for any time of year on the British Isles' weather because of just how unique each event is and combined with other teleconnections. However, sometimes you may find some interesting correlations like cold Aprils that followed strong El Niño events.
This first table is of every Summer's CET for the Weak El Niño events that have occurred. The overall means of these Summers at the bottom aren't far from average indicating a split between some notable hot and cool Summers showing you there is no correlation with Weak El Niño events and Summers in the British Isles.
This second table is of every Summer's CET for the Moderate El Niño events that have occurred. The overall means are again not far from the averages for the months though June is relatively warm whilst July and August are relatively cool.
This third table is of every Summer's CET for the Strong/Very strong El Niño events that have occurred. The means overall for these Summers show that they tend to be cool with three notably cool Summers in particular in this, 1972, 1965 and especially 1888.
The last test of the above is to combine those years into a 500mb height anomaly reanalysis to get a clearer picture of how these Summers look overall as an average in terms of their patterns.
However, due to how the composite generator works, I could only do one reanalysis chart for the weak El Niño Summers or years. It's a mess of a chart ain't it? A big trough of below average heights over much of Europe with a vast area of blocking over Greenland and the north. This is a very cool and wet reanalysis. There will always be exceptions though to the "rule" such as 2006 or 1976.
The 500mb height anomaly reanalysis for moderate El Niño Summers or years looks even worse in terms of the below average heights over the British Isles though the blocking to the north is not nearly as intense. It's a very wet reanalysis however.
It couldn't get any worse could it? Oh yes it could actually. This reanalysis is of strong or very strong El Niño Summers or years and the trough of below average heights is even deeper over top of the British Isles. Powerful jet stream especially for the time of year across the Atlantic. This would ensure very wet and cool Summer conditions indeed. It's a very 2012-esque chart.
As I could not fit in 2015 to the above reanalysis, here's Summer 2015's 500mb height anomaly reanalysis and it's barely any different to what we just looked at above, cool and wet.
So overall going by the above reanalysis and data, El Niño Summers tend to be poor but as always, there are exceptions that you can't disregard and the poor Summers probably skew the reanalysis or data since poor Summers are more frequent than classic Summers in the British Isles.