ENSO index and the Irish Winter: An ongoing study.

Over the last while, the question has been asked more than once here on Boards.ie if the El Nino/La Nina Southern Oscillation, or the 'ENSO' has any bearing on the overall weather patterns during the winter here in Ireland.

The truth is, it is hard to say. The primary driving force of the weather over Ireland during the winter (and at any other season for that matter!!) is the North Atlantic Oscillation (NAO), which a lot more often than not is in a postive state bringing a typical zonal westerly based pattern over Ireland with the usual sunshine and scattered showers and the odd wet dark day.

In this thread however, I am hoping to see if there is any connection between the ENSO index and the Irish weather/climate pattern during the winter months (that is, December/January/February). Can a pattern be established? I don't know but as this will be an ongoing study over the next month or so maybe I might find some connection, dealing with specific parameters, such as MSLP anomalies, temperature and rainfall deviations and so forth and how they might coincide with the Nino index in any given year or over a number of years.

Will welcome very much any criticisms or differing views from my own interpretations (because that is all they are, my intepretations) of the data & statistics that will be used in this thread. There are deeper, darker and more intricate scientific minds than mine here on Boards who may see differing data patterns to me!

Just to add, I will be using data from just 4 synoptic stations from around country, and primarily focus on just 2, that is Malin Head and Valentia, with data from Claremorris and Dublin Apt used in some cases just to give a sense of the whole. In all cases, the data used will be from the the winter of 1956 up to the winter 2010. Not nearly enough to establish a climitological pattern of how each postive/neutral/negative might affect Ireland but nontheless, might give an idea.

By the way. You can blame Nacho Libre for promting me to do this in the first place..


Sorry for the long winded introduction. Time to get to work!


Part One.

La Nina Winter (cold Nino phase) and mean sea level pressure patterns over Ireland.

As mentioned already, the primary driving force of the winter weather over Ireland is the Icelandic low to the north and the Azores high to the south with us inbetween somewhere under all that stratus gloom.

The 1961-1990 30 year winter season Mean Sea Level (MSLP) gradient pattern over Ireland reflects this nicely: (forgive bad artistry )


(Data on map based on Dec/Jan/Feb 61-90 MSLP averages of Johnstown Castle, Belmullet, Valentia, Dublin Apt, Claremorris & Malin Head)

A slightly west of South-west gradient pattern with typifies the predominant weather pattern over Ireland during the winter months.

Interestingly, when I averaged out the winter MSLP values that occurred during La Nina winters since 1956, a notable increase in the average MSLP values became evident:


(mean MSLP values based on winter cold nino years since 1956 to 2010 and based on MSLP values from Malin Head, Claremorris, Dublin Apt and Valentia)

As can be seen from the map above, the average MSLP values during the winter season appear rise a little during cold nino phase winter events (< -0.5c), by as much as 2.0 hPa in the south, and 1.5 hPa in the north, which despite the actual rise, suggests a slight tightening (from the 61-90 mean) of the gradient pattern over Ireland during the winter season as a whole.

As to the strength of winter La Nina events and MSLP deviations over Ireland, a further breakdown of averages was needed! (I have been writing this post for exactly an hour now an I is getting tired!!!!!!)

Below are maps showing mean winter MSLP averages and deviations when the winter nino index are between -2.0c and -1.5c (strong La Nina); -1.5c & -0.9c (moderate La Nina) and -0.9c & -0.5c (weak La Nina) respectively:





Going from the maps above, it is interesing to see that during strong La Nino events, that the mean gradient isobar pattern increases substantially, despite a positive MSLP value anomaly from the 61-90 norm. Also interesting is that this mean gradient pattern widens out a little during moderate La Nina winters and even more so during weak winter La Nina events.


Edit: Finish off:

Going from the maps above, it is interesting to see that during strong La Nino events, that the mean gradient isobar pattern increases substantially, despite a positive MSLP value anomaly from the 61-90 norm. Also interesting is that this mean gradient pattern widens out a little during moderate La Nina winters and even more so during weak winter La Nina events.

1st map shows the average mean sea level pressure that occurred over Ireland during strong winter La Nina events since 1956, of which there was 4: 1973-1974/1988-1989/1975-1976/1999-2000. Average out over these 4 years, the MSLP gradient pattern that emerges is remarkably tight. The isobar gradient between Malin Head and Valentia for example equated to 5.19 hPa, which is around 2.40 hPa above the actual 61-90 normal gradient difference between the 2 stations. Despite this apparent isobar gradient tightening, MSLP anomalies from the 61-90 norm also increased, and quite markedly at Valentia, by as much as 4.10 hPa and much less so at Malin Head, which sees an increase of just 1.0 hPa. Also of note is the more westerly alignment of the gradient pattern against the actual normal.

This would seem to suggest that strong La Nina winter events has the effect of increasing the isobaric gradient over the north Atlantic in general between a stronger than average Azores High and fairly vigorous Icelandic low, resulting in a fairly mobile zonal winter pattern over Ireland. In later chapters, we will see how this pattern coincides with winter rainfall and temperature deviations across the country.
Both Figure 2 and Figure 3, which show both the average MSLP patterns over Ireland of moderate to weak winter cold Niño phases during the winter months, suggest that the isobaric gradient slackens a good deal during moderate to weak winter La Nina events over the north Atlantic.
The mean MSLP of the moderate winter La Nina (nino index of between -0.9c &-1.4c) events since 1956, which equates to 5 years (1999-2000/2007-2008/1995-1996/1971-1972/1984-1985) does show slacker gradient pattern as compared to stronger La Nina winter events, with the mean gradient difference between Malin Head and Valentia of 2.98 hPa, which is over 2.00 hPa lower than the during strong cold phase winters. MSLP anomalies lower a good deal at Valentia but rise a little at Malin Head with the gradient pattern a little more to the SW, which is closer to normal.

This pattern may indicate that the Icelandic low is a little weaker than normal during moderate La Nina winters allowing more ridging from the Azores High during the winter months over Ireland. How this coincides with temperature and rainfall anomalies during these events I have yet to find out but will discuss in later chapters!!!

During weaker winter La Nina events (3rd pic), the average mean sea level pattern over Ireland becomes a little more interesting. Not only does it show a marked ‘backing’ to the SSW of the mean gradient flow, but an increased widening of isobars over the east of the country, but a tightening over the NW. Also interesting is that Malin Head sees an increase in its MSLP anomaly, by up to 2.00 hPa, while Valentia sees a further drop, if still a little above average. This may suggest that ridging to the north and east would be more likely to occur during weak winter La Nina’s than during moderate to strong cold nino phase winter events. Again, this is open to interpretation as I need to assess the mean temperature and rainfall patterns over Ireland during weak winter La Nina events and will discuss these later on in this thread, but it is interesting the note that the winter of 1962-1963 in Ireland occurred during a weak winter La Nina event!!! (-0.6c)

Chart showing gradient depth of mean sea level pressure between Malin Head and Valentia during winter La Nina years since 1956:



Out of the 20 La Nina winters since 1956, only 2 have had a winter mean where Malin Head's mean SLP value came out higher than Valentia. 1962-1963 and 1995-1996.



In Part 2, will look at winter rainfall averages and how they may deviate from the norm over the country during La Nina winters.