APPENDIX 30
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.
| Station | Max 10 min Mean Speed Knots
| Beaufort Classification |
| | |
| | |
| Hillsborough | 40 | Gale Force 8
|
| Ballykelly | 40 | Gale Force 8
|
| Glenanne | 34 | Gale Force 8
|
| Castlederg | 29 | Near Gale Force 7
|
| Ballypatrick | 37 | Gale Force 8
|
| Aldergrove | 42 | Severe Gale Force 9
|
| St Angelo | 39 | Gale Force 8
|
| Killough | 58 | Violent Storm Force 11
|
| Station | Max Gust Knots |
Max Gust MPH |
| | |
| | |
| Hillsborough | 67 | 77
|
| Ballykelly | 56 | 64
|
| Glenanne | 56 | 64
|
| Castlederg | 59 | 68
|
| Ballypatrick | 50 | 57
|
| Aldergrove | 66 | 76
|
| St Angelo | 56 | 64
|
| Killough | 83 | 95
|
Duration
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.
STORMS IN NORTHERN IRELAND—POSSIBLE IMPACT OF CLIMATE
CHANGE
NOTE BY THE HADLEY CENTRE FOR CLIMATE CHANGE
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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