Thursday 27 November 2014

We can't ignore fact that winter storms are getting longer and more damaging

Dr Kieran Hickey

Published 04/01/2014 | 02:30

Waves crash on to the Coast Road in Malahide, Co Dublin, yesterday.  Brian Lawless/PA Wire
Waves crash on to the Coast Road in Malahide, Co Dublin, yesterday. Brian Lawless/PA Wire

In the past two weeks, Ireland has experienced three very large and damaging Atlantic storms. All three storms have generated gusts well in excess of 100km/h and have caused a very small number of fatalities and injuries along with significant power outages, structural damage, coastal flooding and some river flooding, leading to widespread disruption.

The recurrent coastal flooding is a result of a combination of high spring tides, a storm surge consisting of a wall of water being pushed in front of the storm by the high winds and the reduction in atmospheric pressure causing the water column to expand upwards -- and on top of that, big waves that easily crash over coastal defences.

Some of the river flooding is due to the high rainfall associated with big Atlantic gales but also due to rivers not being able to discharge their water into the sea as they are effectively blocked by the incoming storm surge causing the water to back up the river channels. In addition, these storms were of relatively long duration, with gale-force winds and gale gusts occurring for many hours. So what's going on?

At the weather level, the main culprit is the jet stream, which is situated just north of Ireland at present and is dragging large Atlantic storms across the ocean. The three recent storms have roughly followed the same path influenced by the jet stream and the storm that is to come on Sunday night and Monday is doing likewise, so there will be no let-up for now.

This has probably been the worst sequence of storms for the past eight to 10 years at least. Temperatures have also remained quite high for this time of the year.

But we must put this situation in context. Ireland sits out in the Atlantic at mid-latitudes, which means that we have always been exposed to big Atlantic gales especially around this time of year. The famous storm of 1839 known as 'The Night of the Big Wind' occurred on January 6 and was probably the worst storm to affect Ireland since AD 1500; it caused hundreds of fatalities and vast destruction.

So, big Atlantic storms should not be a surprise as barely a winter goes by without at least one. However, to have three severe, long-duration and damaging storms in such a short space of time is certainly unusual.

It is also the case that since the mid-1990s the number of storms affecting Ireland has dramatically reduced. In 1994, Malin Head had 87 days with gales whereas in 2012 it recorded only 39 days with gales. Dublin Airport had seven days with gales in 1994 as compared to just four days with gales in 2012.

This decline is true of all of Ireland and shows a dramatic change in the storminess of Ireland's weather over the past 25 years or so to reach a historical low. So, one of the surprises of this current sequence of storms is that we have grown unfamiliar with just how stormy Ireland can be in the right meteorological circumstances.

The main cause of this decline is that the average storm track across the Atlantic has shifted northwards by between 100 and 200km, meaning that Ireland, which was in the centre, is now somewhat towards the periphery. This does not mean, as we have seen in the past fortnight, that all storms will follow the average track, and it still means that Ireland is and always will be vulnerable to major Atlantic storms. What is at issue is the frequency and severity of these storms. The driver behind this average storm-track movement is changes in the climate.

The sequence of recent major weather disasters should not be ignored. Since November 2009, barely a six-month spell has gone by without some significant weather upheaval in Ireland. The sequence included major flood events (2009 west of Ireland, Cork city; 2011 Dublin and Wicklow) and localised severe and recurrent flooding, both river and coastal (in Cork and Galway and in many other places), the two major cold spells of 2009-2010 and November to December 2010 and, more recently, the cold spell that lasted for nearly most of the first half of 2013, leading to the fodder crises. Then we had the summer heatwave along with very active thunder and lightning.

LAST year was a year of extremes, but bizarrely the average figures for various meteorological parameters won't show anything significant -- but it certainly wasn't a normal year.

The projections for Ireland's wind regime into the future give us an insight into what may be coming our way from now up to the end of this century. The global circulation models downscaled to Ireland indicate that the generally historical low levels of gales will continue into the future with the usual year-to-year variability, but the models also show that some of the gales will be more severe in terms of wind speed and duration. It is these more severe gales that, as we have seen, cause loss of life, injuries, damage and coastal and river flooding.

Dr Kieran Hickey is a climatologist at the Department of Geography, NUI Galway and author of 'Five Minutes to Midnight? Ireland and Climate Change' and 'Deluge: Ireland's Weather Disasters 2009 - 2010'

Irish Independent

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