Select Committee on Northern Ireland Affairs Minutes of Evidence


Further Memorandum submitted by the Meteorological Office

  Thank you for your letter of 26 April.

Mean wind speed and gust speeds

  The tables below give the highest 10-minute mean wind speeds and highest gust speeds recorded at various anemometers stations in Northern Ireland on 24 December 1997.
StationMax 10 min Mean Speed Knots Beaufort Classification
Hillsborough40Gale Force 8
Ballykelly40Gale Force 8
Glenanne34Gale Force 8
Castlederg29Near Gale Force 7
Ballypatrick37Gale Force 8
Aldergrove42Severe Gale Force 9
St Angelo39Gale Force 8
Killough58Violent Storm Force 11

StationMax Gust Knots Max Gust MPH
St Angelo5664


  The 1997 storm was less severe and of shorter duration that the 1998 storm, lasting some two to three hours between 7 pm and 10 pm. The two storms also affected different parts of Northern Ireland, with the 1997 storm having greatest effect in the Southeast, while the 1998 storm had a greater effect on the Northwest. In the Republic of Ireland, the Southwest took the brunt of the 1997 storm; Roche's Point in County Cork recorded a highest gust of 89 knots and Valentia Observatory in County Kerry, a highest gust of 88 knots, whereas Malinhead, County Donegal, and Belmullet, County Mayo, had highest gusts of 65 and 66 knots respectively.

Return Period

  Given that the winds at Aldergrove on Christmas Eve 1997 were much less extreme than those on Boxing Day 1998—there are six other years since 1964 that equal or exceed the maximum gust at Aldergrove, recorded on 24 December 1997—the Return Period is considerably shorter. Using the technique described in my earlier letter, and bearing in mind its limitations, we estimate that the Return Period of the 1997 storm—based on the Aldergrove records—lies in the five to eight years range.

Climate Change

  You also asked about the possible effect of climate change of the frequency of future wind events. I attach a note, prepared by our Hadley Centre for Climate Change, discussing this issue.



  1. Storminess in Northern Ireland in winter is related, to a considerable extent, to the behaviour of the "North Atlantic Oscillation (NAO)" which is a pattern of pressure at mean sea level, or 500hPa height, related to the strength of the Azores high and the Iceland low. There is also a wider manifestation of the NAO right across the Arctic known as the Arctic Oscillation (AO). Time series of the AO and NAO are strongly correlated in the winter over the last 40 years, indicating that variations in storminess over the North East Atlantic often have a very large scale component.

  2. A reliable index of the NAO can be created well back to the last century. It shows that the stormiest periods in winter (strong NAO) were generally around 1900-30 and the period since 1970, particularly in the early 1970s and since 1988, though one recent winter actually had a record weak NAO pattern. Thus there have been multidecadal, multiannual and strong interannual variations in the storminess related to the NAO. This argues for a purely natural component of NAO variability on many time-scales. This is seen in long runs of coupled climate models where greenhouse gases are not changed and where multidecadal and intercentury variations occur.

  3. It is worth noting that winter storminess over the UK is also related to other patterns of pressure variation more concentrated over western Europe, but have been much less studied.

  4. Coupled climate models that incorporate changes in greenhouse gases often predict that storminess will increase in the next century over the Atlantic/North Europe region, though results are model dependent. An increase in storminess is plausible as the core region of the NAO or AO is a cold upper tropospheric region in polar latitudes. Cooling of this region caused by increasing carbon dioxide may occur, giving rise to an increased jet stream and therefore storminess. The extent to which this happens may depend on the strength of interactions with the lower stratosphere where the cooling due to increased carbon dioxide would originate. However it is observed that the pattern of the NAO and AO does extend or even amplify into the lower stratosphere. On this view, the current depletion of lower stratospheric ozone would amplify the cooling tendency, and help increase storminess over the North Atlantic.

  5. Recently published research in the Hadley Centre has indicated that sea surface temperature patterns in the North Atlantic affect the NAO. It is clear that the same sea temperature patterns are influenced by the NAO itself. The interaction between the atmosphere and the ocean in the North Atlantic is a now major research topic. Other patterns of sea surface temperature may affect the North Atlantic climate, and coherent advection of Sea Surface Temperature anomalies across the North Atlantic in winter over many years has recently been demonstrated.

  6. In summary, it appears that there are natural variations in winter storminess on multidecadal (and perhaps multi-century) time-scales, and there may be an emerging anthropogenic component. The ocean plays a role yet to be fully determined but work on this has started in the Hadley Centre. In the meantime, the complex natural variations of the NAO, and the possibility of climate change, makes the assumptions behind traditional calculations of the return periods of extreme winds in Northern Ireland quite uncertain.

17 May 1999

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