Increasing storminess is a serious concern

THE recent protracted period of wet weather has played havoc with many livelihoods and activities, and Storm Kathleen, the eleventh storm of the 2023-2024 storm season, accelerated the rate of coastal erosion. Increased rainfall and storminess are both known to be linked to the NAO, the primary driver of the weather we experience.

Air pressure used to be measured in millibars, but it is now measured in hectopascals (hPa). The name change honours the 17th century French mathematician Blaise Pascal and the changeover is pretty straightforward as one new hectopascal is equal to one old millibar.

Air pressure at sea level averages 1013.25hPa. That pressure is equivalent to 14.7 pounds per square inch, that is, the weight that the column of air above exerts a piece of ground at sea level measuring one inch long by one inch wide. Pressures above the average are said to be ‘high’ and those below are described as ‘low’.

There tends to be a more or less permanent stable area of low pressure air normally located over Iceland. A corresponding more or less permanent stable area of high pressure air tends to be normally located over the Azores.

The cyclical change in the relative air pressure strength of the Icelandic low and the Azores high is called the North Atlantic Oscillation, or NAO for short. When the difference between the two systems is high we are said to be in a positive phase and the weather in Ireland tends to be dominated by westerly winds and to be mild, wet, and windy.

When the difference between the relative strengths of the two systems is low we are said to be in a negative phase. Westerly winds from the ocean weaken allowing easterly winds from northern mainland Europe to dominate and the weather in Ireland tends to be cold, dry, and not very stormy.

Recent years have seen a strong upward trend in positive NAO resulting in an increase in both rainfall and storminess in Ireland. The trend mirrors global warming suggesting that the two are closely linked.

The bad news is that if the trend continues we are likely to experience more rain, heavier rain and flooding accompanied by stronger westerly wind, more storms, and increased coastal erosion.