El Nino: What is it and will it influence our Summer weather?

You may have heard whisperings recently about the possible development of a climatic phenomenon known as ‘El Nino’ later this year, and its potential impacts on weather conditions on a global scale.

El Nino events typically bring an increase in rainfall over the both the North and South American continents, and a decrease in rainfall over southern and southeastern Asia, as well as the Australian continent as a whole.

But what is ‘El Nino’, and why does it have such a significant influence on global weather and climate? Moreover, what effects, in any, does it have on weather conditions here in Ireland?

El Nino, and its counterpart ‘La Nina’ are names to describe the fluctuation in sea surface temperature (SST) in the easternmost part of the Equatorial Pacific. These variations in SSTs in this particular area is known as the El Nino Southern Oscillation (ESNO).

El Nino — meaning ‘Christ Child’ – is the ‘warm phase’ of the ESNO and develops, on average, every 4 years or so. It is associated with anomalously warmer SSTs in the eastern regions of the tropical Pacific Ocean and can last from anything to 6 to 18 months.

El Nino develops when prevailing easterly winds (known as the ‘easterly trade winds) over the equatorial and tropical region of the Pacific considerably weaken and break down. Under normal ESNO conditions, the easterly trade winds are steered by a semi-permanent high pressure zone over the western region of the equatorial Pacific diverging air outwards towards a large-scale trough zone situated over the eastern Pacific region. These easterlies have the effect of piling up a vast body of warm water over the western Pacific region; this, in turn, causes cooler waters to ‘upwell’ in the eastern Pacific to replace the water that the trade winds have forced to cumulate on the western side. This extensive area of warm SST’s inevitably created more moisture and instability the atmosphere, resulting in the formation of a semi-permanent area of low pressure over the west Pacific region, which in turn brings ample and regular rainfall places such southern Asia and Australia.

This animation nicely demonstrates the various phases and subsequent effects of the ESNO on weather patterns over the Pacific Basin.  Image © BOM
This animation nicely demonstrates the various phases and subsequent effects of the ESNO on weather patterns over the Pacific Basin.
Image © BOM

 

Occasionally, these easterly winds slow down enough to allow all this warm water, which has piled up by as much as 1.5 ft on the western most side of the Pacific, to collapse and spill back out over the central Pacific, where it may warm even further before spreading east towards the Americas. This shift in the distribution of warm sea surface temperatures across the entire tropical Pacific in turn has a direct influence on broad-scale weather patterns over the region, with high pressure over the eastern Pacific being replaced by low pressure, and vice versa in the eastern Pacific.

Does El Nino have an impact on Irish Weather?

As discussed above, El Nino events can change broad-scale pressure patterns over the Pacific, which help to redistribute rainfall over the immediate area, which in turn can jar established patterns over the entire sub-tropical belt around the world. The question here though do these extensive changes in tropical weather patterns caused by El Nino have a director even indirectinfluence on synoptic patterns over the north Atlantic, and more specifically, Ireland?

Looking at historical weather data both in Ireland and the north Atlantic, and then comparing this with data recorded during the various phases of the El Nino Southern Osculation, we found that, overall, El Nino events tend to have little impact, at least directly, on weather patterns in this part of the world. What little effects a warm phase ENSO may bring to northwest Europe, will, according to many researchers who have studied this, (See List Below) most likely occur during the winter months, due to the fact that El Nino events are, on average, more likely to be at their strongest during the Winter season. We’ll deal with possible El Nino effects on the Irish Winter later in the year and focus instead on its perceived effects on our Summer, which is of more immediate interest to most of us right now. 

The Statistics.

Before moving on to actual examples to assess if Irish Summer weather is dominated by any particular type during an El Nino Summer, we’ll give a small look to a few statistics. The below table shows mean anomalies (from the 1981-2010 climate period) for Northern Ireland summer rainfall and Valentia Observatory summer mean temperature during specific El Nino episodes between the years 1931 and 2014:

Nino Table

From this alone, it would appear that, overall, the Summer temp anomalies at Valentia tend to favor near or below average values during weak or moderate El Nino events, while rainfall values tend to come in slightly higher than normal. On the other hand, strong summer El Nino episodes appear to favour drier and slightly warmer conditions during the summer season in Ireland on the whole.  What these statistics tell us, however, may or may not reflect in the actual reality as apparent correlation and averages between seemingly random variables within statistics will always show some sort of correlation or average, whether there be an actual association between them or not. In the next chapter we’ll look at actual examples to see any connection between El Nino and Irish Summer weather exist.

Next Chapter:

The summer of 1995 is regarded as being one of the warmest  in Ireland, and this occurred when NINO conditions were negatively neutral. The broadly similar summer of 1976, however, occurred when the NINO index was positively neutral, as did the warm summers of 2003, 2006 & 2013.  Of some interest is that of all the top ten warmest summer months at Valentia since 1869, all but two of them occurred while the NINO state was negatively neutral.

Looking at unusually wet summer months since 1931 over Northern Ireland, six out the ten all time wettest occurred while the NINO state was either positively neutral or positive; while the remained occurred while negatively neutral or negative. Focusing in even further to individual events. August 1986 saw a severe rain event affect south and east coast with 24 hour totals exceeding 80-100 mm within a 24 hour period. The NINO state during August 1986 was negatively neutral. Interestingly, a very similar storm affected the same region – on the very same date! – back in 1905 which gave similar rainfall totals and wind speeds, yet this event happened during a strongly positive NINO state.  Countless similar examples exist where similar weather events during the summer occurred whilst differing NINO states were dominant.

In summary, there appears to little evidence to suggest that the forecast El Nino this summer will have any substantial affect on our weather on way or the other. As we have seen, the statistics do suggest a small tendency for weak to moderate El Nino’s over the summer period to bring slightly cooler and wetter conditions to Ireland, but as we have also seen, warm, wet, cold or dry conditions are almost equally likely to occur whether El Nino, neutral or La Nina conditions are in force.

To keep up to date on the current El Nino forecasts over the coming months, be sure to check out the regularly updated diagnostic discussion from NOAA.

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.html

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2 thoughts on “El Nino: What is it and will it influence our Summer weather?

  1. […] El Nino occurs when the ‘El Nino Southern Oscillation (ESNO for short) enters a warm phase, which occurs on an irregular basis in the eastern regions of the tropical Pacific. This ‘warm phase’ occurs when very warm seas, which are normally distributed over the central and western belts of the central Pacific Ocean, are transported eastwards because of changing wind directions and pressure patterns over the wider region. These persistent westerly winds force the warmer waters the cumulate on the eastern side of the tropical Pacific which in turn forces cooler waters from below the surface to up-well over the western tropical Pacific. For a more detailed analysis of the ESNO and the factors that contribute to its formation and its subsequent impacts on global weather patterns, click HERE […]

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