Super El Nino 2015-2016. Will it have an impact on Ireland’s Winter?

Introduction

There has been much speculation in the media recently about this year’s ‘El Nino’ and its possible impacts on the winter weather both here in Ireland and in the UK. With sources now saying that the winter El Nino of 2015/2016 may possibly be the strongest on record, we will take a look to see if this Pacific Ocean climate phenomena does indeed have a direct impact on winter weather conditions here in Ireland

First things first, what is ‘El Nino’?

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 to accumulate 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

Does EL Nino affect Ireland’s Weather?

Over the last few months, there has been much speculation and meanderings about El Nino’s impacts on this coming winter for Ireland and the UK by various media outlets with newspaper headlines informing us that we are to expect ‘four months of white-out Arctic conditions’ or ‘six months of violent storms’ because of this Winter’s upcoming strong El Nino. But are such headlines based on actual statistical evidence? In the next section, we will take a look at the actual statistics to see if such claims are based on some semblance of truth.

To assess whether El Nino does have an influence on the Irish winter weather, we must first take a look at whether El Nino has a direct, or even indirect effect on a climate index known as the ‘North Atlantic Oscillation’ (or NAO for short) as this is, without question, the primary driver of Ireland’s ever changing weather patterns in all of the four seasons. Before we take a look at correlations between the El Nino and the NAO, we’ll just a quick overview of what the North Atlantic Oscillation is to help give a better understanding of how it influences the various weather patterns that affect the northeast Atlantic region.

What is the North Atlantic Oscillation (NAO)?

The so called ‘North Atlantic Oscillation’ is a climate phenomenon that occurs over the broad north Atlantic region and is devised by measuring the mean sea-level surface pressure (MSLP) differences between Reykjavik, Iceland in the very north of the area and/or the Azores Island/Gibraltar in the southern most regions of the north Atlantic. To put it simply, the greater the MSLP gradient between these two regions the more ‘positive’ the NAO index will be; on the other hand, smaller, or even reversed MSLP gradients between both regions will result in the NAO index becoming more ‘negative’. In the next sub-section we will briefly explain what exactly is a ‘positive’ and ‘negative’ NAO index and how each of them affect the weather, both here in Ireland and the north European region in general.

The positive NAO
As explained above, a ‘positive’ NAO state is when large MSLP differences occur between the far north and more southern regions of the north Atlantic. These strong pressure gradients occur when relatively rapid moving low pressure systems dominate the northern regions of the northern Atlantic, while slower moving high pressure systems – or anticyclones – prevail over the more southern outreaches. This general Atlantic pattern, which is by far the more frequently occurring of the NAO states in any the seasons, is initiated by strong temperature contrasts (thermal gradients) between northern and southern regions of the north Atlantic, which in turn helps to amplify the polar jet-stream, which in itself can fuel a stronger thermal gradient over central regions of the Atlantic.

Atlantic weather chart showing the typical synoptic pattern of a 'positive' NAO index. Image: BBC
Atlantic weather chart showing the typical synoptic pattern of a ‘positive’ NAO index. Image: BBC

Because wind circulates anticlockwise and inwards towards primary depressions to the north, while conversely circulating clockwise and outwards around high pressure ridges and anticyclones, westerly winds predominate over the greater north Atlantic region which help steer in a variety of air masses and weather systems in over Ireland and northern Europe, such as those dull rainy or bright and breezy days that we have all become acclimatised and accustomed to in all seasons.

The negative NAO

In contrast to the positive state NAO, a negative NAO is the result of slacker MSLP gradients over the north Atlantic region which are in themselves a consequence of weaker thermal gradients between north and south. While the NAO is in a negative state, the polar jet stream is less pronounced than normal, which results in a tendency for higher than normal pressure to the north of Ireland with Atlantic depressions tending to take a more southerly route.

Weather chart showing the typical synoptic pattern when the NAO is under a 'negative' state. Image BBC
Weather chart showing the typical synoptic pattern when the NAO is under a ‘negative’ state. Image BBC

When such broad-scale conditions occur over the Atlantic, the probability of air masses from the north and east affecting Ireland increases markedly, which during the Winter season can bring very cold conditions to our shores.

Do El Nino events affect the NAO?

Having briefly established what the North Atlantic Oscillation is and how it drives Ireland’s weather patterns, we will next look at whether or not El Nino events have an influence on the NAO state, particularly during the Winter months as this is the main focus of this article. Note: The data we will be looking at from here derive from a variety of sources, such as the UK Met Office, NOAA & KMNI and Met Éireann.

The Statistics

Below are a series of graphs that show the statistical correlation between the Nino index when in a positive state (> 0.0) and the NAO during each of the winter months since 1950.

decninonao janninonao

febninonao

At first glance, these three graphs may look pretty meaningless, but what they can tell us is how strong the relationship between two variables is in a statistical sense, and in this case, how strong the statistical relationship is between positive El Ninos during the Winter months and the NAO since 1950, which is the primary driver of Irish weather. However, in order to better understand the data in the above graphs, it would be better to break down the data into table form to give a clearer insight into the ‘average’ conditions experienced in each of the three winter months in Ireland during El Nino events of varying degrees of strength.

nino table2

What these three above correlation graphs, as well as the following table tell us is that while statistically, there appears to be slight negative influence on the NAO and El Nino events, particularly with regards to February and more especially when the Nino index is moderately positive, there is little correlation overall between El Nino events and significant impacts on the North Atlantic Oscillation in all of the three winter months. Also to be considered in this case is that the sample rate of Winter El Nino events becomes much less the higher the El Nino index becomes as exceptionally strong El Nino events during the winter months are comparatively rare, which makes any correlation between strong El Nino events (index > = +1.8) and the NAO state less reliable. Another aspect to keep in mind is that correlations based solely on raw statistics may be down to nothing more than random chance, given that it is possible to find statistical relationships between seemingly unconnected variables in pretty much any statistical test.

Some Practical Examples:

With the Nino Index expected to average around a very high 2.2c over this coming winter, in this section we will look at how other winters panned out here in Ireland during strong El Nino events. As stated in the previous chapter, strong winter El Ninos on this scale are exceedingly rare, with only about four of five occurring over the last one hundred years. Listed below are some of these years.

Winter 1940-1941. Nino index: 1.8. This particular winter finished cooler than average over Ireland with more or less average rainfall. Though December and February were only slightly cooler and wetter than normal, the much colder and drier January made this particular winter stand out.

Winter 1972-1973: Nino index: 1.8. Overall, a warmer than average winter with all three of the winter months coming out with above average mean temperatures and near normal rainfall totals. January this year was especially warm.

Winter 1982-1983: Nino index: 2.6. This winter saw a ‘Super El Nino’ in place and one of the strongest El Nino’s on record. The weather in Ireland during this winter was changeable overall with a fairly cool and dry December and February which was offset by a very warm and wet January. Overall, this winter finished with near average temperatures and rainfall totals.

Winter 1991-1992. Nino index: 1.8. A warmer than average winter with all three of the winter months finishing with above average mean temperatures. Near average rainfall totals overall.

Winter 1997-1998. Nino index: 2.5. One of the strongest El Nino events on record. This winter finished much warmer than normal over Ireland with February in particular ending up one of the warmest on record. Near average rainfall totals overall.

To expand on this further we have compiled a few graphs showing the probabilities of finishing mean temperatures over Northern Ireland for each of the three winter months during El Nino events with a Nino index equal to or greater than 1.0c over the last one hundred years.

decchart  janchart

febchart

Summary:

Over the course of this article, we have looked at various data in an attempt to try and ascertain if recent claims by various media outlets that El Nino events increase the probabilities of severe weather in Ireland (particularly regarding cold weather events) are based on actual statistics and facts. Having looked at statistical correlations between strong El Nino events and the North Atlantic Oscillation, as well as some practical real world examples, we have found very little evidence to support such claims. Although, as we have seen, there does seem to be some small amount of statistical correlation for El Nino events to have a more negative impact on the NAO, and subsequently, temperatures here in Ireland during February in particular, these correlations cannot be deemed as significant due to the very small sample rate of such events relative to more neutral Nino conditions. We have shown that warm or average winters are ever bit as likely to occur during moderate to strong El Nino events as cold weather is here in Ireland. The only thing we can do now at this stage is see enjoy whatever weather this coming winter throws at us, because anything can happen really, and no more or no less so because of this winter’s potentially record breaking El Nino.

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