Lamb, H.H. (1972) British Isles weather types and a register of the daily sequence of circulationpatterns,

Marsh, T. and Monkhousc, R. (1990) Hydrological aspects of thc dcvclopmcnt and rapid dccay of thc

Maycs, J.C. (1988) Regional airflow patterns in ihe Britkh Isles and their influence on rainfallfluctua-

Murray, R. and Lewis, R.P.W. (1966) Some aspccts of the synoptic climatology of thc British Islcs as

Northcott, G.P. (1989) The winter of 19W89 in the Unitcd Kingdom. Meieorol. Mag., 118, pp. 265-267 Pcrry, A.H. (1968) The regional variation of climatological charactcristics with synoptic indiccs.

__ (1969) Thc P.S.C.M. indcx and regional anomalics of temperature, rainfall and sunshine.

Prenticc, R. and Prcntice, M. (1989) Thc Scottish Highland floods of 1989. Swansea Geogr., 26, pp. 3 6

Ratcliffc, R.A.S. (1989) Review of summer 1989 in the Northern Hemisphere. Weather, 44, pp. 451-

Roy, M. (1989) Hazardous wcathcr in Scotland. Weuther, 44, pp. 220 Whcclcr, D.A. (1990) Climaticchange in Sunderland? Weather, 45, pp. 229-231 Wright, P.B. (1976) Recent climaticchangc. In: T.J. Chandler and S. Grcgory (Eds.) Theclimateof the

1861-1971. Geophys. Mem., No. 116, London, HMSO

1989 drought. Weather, 45, pp.290-299

fions. Ph.D.thesis, University of Wales, 400 pp.

measurcd by simplc indiccs. Meteorol. Mug., 95, pp. 192-203

Weather, 23, pp. 325-330

Weaiher, 24, pp. 225-228

53

452

Britkh Isles, London, Longman, pp. 224247

MAJORCA’S SEVERE STORMS OF SEPTEMBER 1989: A REMINDER OF MEDITERRANEAN UNCERTAINTY

By DENN~S WHEELER

Geography Department, Sunderland Polytechnic

HE ‘Mediterranean’ climate as described in many textbooks is an enigma: the couplet TGG hot dry summers and warm wet winters” conveys little of the dynamism and uncer- tainty of its regime. It is popularly perceived as a climate of unremitting warmth, endless sun and pcrpetual dryness. Spain has turned this misconception to advantage by treating its climate as an economic resource that attracts over 40 million foreign visitors every year, principally to the Mediterranean resorts between the months of April and August. Their experiences of the Mediterranean summer and the holiday industry’s hyperbolc merely reinforce the climatic myth. This image notwithstanding, visitors to the region in Septem- ber 1989 were confronted with one of the less familiar and less acceptable aspects of its climate-the severe autumn storms that so frequently mark, in spectacular fashion, the close of summer and the onset of winter.

Such events are a recurrent hazard along the Mediterranean coast of Spain where sharp relief combines with late summcr sca surface temperatures and the approaching winter’s first invasion of cold air to create rain storms of an intensity unknown elsewhere in Europe. The partially enclosed nature of the Mediterranean and its limited exchange with the cooler waters of the Atlantic are critical features that allow unusually high sea surface temperatures to be reached. This is not a feature shared by othcr ‘Mediterranean’ regions such as California and south-west Australia where the open ocean waters are much cooler, and it lends a degree of distinctivencss to the prototype ‘Mediterranean’ climate.

The potential instability that results from advecting cold Polar Air over a relatively warm sea and humid lower atmosphere needs only to be realised by forced ascent over the mountains that characterise the Spanish Mediterranean coastline to draw the annual sum- mer drought to a conclusion of monsoon-like abruptness. Although acknowledged within Spain (Martin-Vide 1985), this phenomenon is less widely appreciated elsewhere.

Yet even among a population familiar with such events, the 1989 storms were uncom- monly severe and widesprcad, bringing heavy rain to all areas between the Spanish-French

21

border and the Strait of Gibraltar (Tout and Wheeler 1990). This review concentrates on the events on the island of Majorca where rainfall was most intense and provided some of the greatest totals ever to be recorded in that locality. In doing so the paper not only examines a meteorological event of importance in its own right, but also demonstrates how geographical elements have conspired to produce a distinctive climatological element. In general terms the event was an extreme member of a family of occurrences described in a recent paper in this journal (Redaiio and hrente 1990).

The preceding summer had been extremely dry and warm, and at the start of Septem- ber sea surface temperatures were in the region of 23°C. However, in an area where airmass contrasts are limited and frontogenesis is not marked, clues to forecasting can rarely be sought in surface weather patterns, and the absence of strong fronts and steep pressure gradients disguises the weather’s true intentions.

Most of Majorca recorded rain of up to 35 mm on 1 September from a weak surface low that subsequently drifted eastwards on the 2nd. The period from the 2nd to the 4th was generally dry with only scattered outbreaks of rain. The situation changed on the 5th with the approach of a warm front from the south and the establishment of a cold pool over the Gulf of Cadiz as an upper trough approached from the west. Such cut-off features are typical of the region and occur most frequently at this time of year (Meteorological Office 1978) when they are often associated with outbreaks of severe weather.

The situation is summarised in Figs. 1 and 2 which emphasise the important contrasts between the low-level and upper-air circulations. The principal surface features are the high pressure over the British Isles and the shallow low over Algeria. Although the pressure gradient was weak, the consequent light easterly to south-easterly airflow was sufficient to encourage onshore winds along most of Spain’s eastern coast and the Balearic Islands. This hot and dry airstream, after crossing the north African coastline, had ample opportunity to acquire large amounts of water vapour in its long track across the western Mediterranean

Fig. I Surface chart (mbar) for 12mGMT on 5 September 1989 (adapted from Deutschen Wetterdienstes)

22

Fig. 2 Surface contours of the 300 mbar level (in decametres) for oo00 GMT on 5 September 1989. The extent to which westerlies meander north to south guided by the surface contours is clearly shown and the location of assockred jet streams is indicated by the arrows (adapted from Deutschen Wetterdienstes)

towards Iberia. Conditions aloft, from 700mbar upwards, presented a different, more Vigorous, picture. The low zonal index of the upper westerlies is clearly seen in Fig. 2 by their wide latitudinal wanderings and, while varying in detail over the period of greatest storm activity on the 5th and 6th, the circulation continued to advect cold Polar Air eastwards across the western Mediterranean at medium and high levels following its south- ward passage on the western side of the upper trough. Midday temperatures on Majorca were in the region of 25°C with a dew-point depression of 5degC. Conditions at higher levels were quite different. Dry-bulb temperatures had fallen to 4°C at only700mbar and to -39°C at 300mbar.

Several features then combined to create a critical coincidence of circumstances that was to lead to the heavy rain. At low level there was modest convergence along the frontal zone. At the same time, winds were onshore on the south-east side of Majorca, and although the relief does not exceed 500m in this part of the island (Fig. 3) it was sufficient to initiate uplift and realise the potential instability of the low-level warm and moist air as it rose through the cold air aloft which formed part of the upper westerly circulation. Analysis of the data to hand suggests that the air reached its dew point at 950mbar (approximately 500m) and instability would have set in just a little higher at 940mbar persisting then through the troposphere.

The resulting heavy rains lasted for two days. Figures 4 and 5 show the geography of the respective daily totals from which several points emerge. Firstly, the levels of rainfall were exceptional by any standards with some areas recording up to 200mm on the 5th, others with over 250mm on the 6th. Few places recorded heavy rain over both days, but mention must be made of Es Picot (Fig. 3) where the 48-hour total was 332mm. Secondly, thestorms were highly localised. Such behaviour is typical of severe instability when places at no great distance from one another may register strikingly different rainfall totals.

23

Fig. 3 Relief map of the bland of Majorcu. Contours are ai 400m inien7aI.v.

Fig. 4 Isohyets of daily rainfall (mm) for 5 Sepiember I989

24

I Fig. 5 Isohyeis of daily rainfall (mm) for 6 September I989

Thirdly, the protection offered by the modest heights of the hills of Majorca’s south-east coastline is most striking. Scarcely any rain fell over the major part of the island on the 6th and most of the available moisture was precipitated over the hills immediately inland of the coast.

It must be added that the distribution of rainfall summarised in Figs. 4 and 5 is not typical of the island. For the most part it is the higher ground along the north-west coast rising to over lOOOm that receives the heaviest rainfall, most commonly from northerly winds on the western flanks of lee cyclones forming over the Gulf of Lions. In that area the 100-year return period 24-hour rainfall is over 200mm and on some of the highest peaks attains over 300mm. Scarcely any part of the south-east of the island has a 100-year return daily fall of over 150 mm (Grimalt 1987). Nevertheless, for the unfortunate residents and holiday-makers of that region the consequences were real enough. Severe flooding de- stroyed several buildings, closed roads and swept at least three people to their deaths, as well as causing incalculable damage to crops.

There were, however, some pcculiarities in this storm. The upper cold pool or gotu fria was not as clearly defined on the upper-air charts as in many other situations of this general type. This may help to account for the failure to forecast the storms - a much publicised aspect in the Spanish Press’s coverage. The storms were also unusually early. The most recent events, for example that of 1987 (Wheeler and Martin-Vide 1988), have come in October. There is, however, evidence (Cueva and Serrano 1983) that$ suggests September storms to have been more frequent in the past.

Coming towards the end of a year (1989) in which world weather produced some remarkable events, the conditions in the western Mediterranean might not stand alone in their peculiarities. But, whatever the broader implications of the storms might be, they are offered here as a sharp reminder that the Mediterranean climate is not always as benign as is widely supposed.

ACKNOWLEDGEMENTS

The author would like to acknowledge the help given in the preparation of this paper by Professor Augustin Jansa of the Instituto Nacional de Meteorologia, Palma, Majorca.

25

REFERENCES Grimalt i Gclabcrt, D.M. (1987) Aproximacio a una geogrufia del risc a Mallorca. Les inrmdacions.

Cueva, A. and Serrano, R. (1983) El temporal de octubre de 1982 en el marco de las lluvias torrenciales

Martin-Vide, J. (1985) Pluges i inundacions a la Mediferrania. Kestres, Barcelona Meteorological Office (1978) Weaflrer in lzome wafers, Vol . f f f : The wafers offflie Azores and offsoufh-

Redaiio, A. and Lorentc. J. (1990) Analysis of instantaneous rainfall rate during flash floods in

Tout, D. and Wheeler, D.A. (1990) The early autumn stornis of 1989 in eastern Spain. J . Mefeorol.

Wheeler. D.A. and Martin-Vide. J. (1988) The Barcelona storm: 1-5th October 1987. J . Mefeorol.

Ph.D. thesis, University of the Balearin

en la cuenca baja del Jucar. Cuadernos lie Geugrafia, 32, pp.61-86

west Europe and offnorflr-wesf Africa, Part I . HMSO. London

Barcelona. Weather, 45, pp.215-222

(UK), 15, pp.238-248

(UK) , 13. pp.79-85

SCIENTIFIC AND POLICY STATEMENTS Members of the Society will know from the President’s message on the front page of the

first Society Newsletter (July 1990) that Council are aiming to raise the profile of the Society as an authoritative independent source for Statements on weather-related and other meteoro- logical issues.

These Statements will be carcfully drafted by experts and will aim to express a balance of views. They will be targeted at the general public, decision-makers and members of the meteorological community. As such they will be useful ammunition for our newly appointed Press Officer.

The first Statement is likely to be on the greenhouse effect, and the latest draft is shown below. If Members have any comments on the general style being adopted, please send them to the Society’s President before 15 February At its February meeting the Council will consider thc appropriate final form of this Statement for wider publication and perhaps use it as a model on which to base future Statements on other topics.

Not all readers of Weather are Members of the Society, but this Statement is included as an article which it is hoped that our many subscribers will be interested to read. Maybe some of them will wish to take on a morc active r6le by applying to join the Society!

THE GREENHOUSE EFFECT- D R A m OF A STATEMENT FROM THE ROYAL METEORO- LOGICAL SOCIETY

What is [he greenhouse effect? The natural temperature of the Earth is determined by a balance between incoming

solar energy and outgoing infra-red (invisible) radiation emitted from the Earth’s surface. Although solar energy can pass easily through the atmosphere, the outgoing infra-red energy is partially absorbed and re-eniittcd by certain gases in the atmosphere. This partial trapping of the energy leaving the Earth leads to its surface temperature bcing about 33 degC highcr than it would be otherwise, making the planet habitable. This is the natural greenhouse cffect.

The gascs which cause this greenhouse effect arc not the most common ones (nitrogen and oxygen) but several which an: present naturally in much smaller concentrations, in particular water vapour, carbon dioxide (CO,), mcthane, nitrous oxide and ozone.

How is man enhancing the greenhoirse effect? Although the natural greenhouse effect makes life possiblc on Earth, man is respons-

ible for increasing the concentration of thc greenhouse gases, and concern has been voiced that this will lead to the Earth‘s surface temperature inmasing- the ‘global warming’. Since prc-industrial times (thc late eighteenth century) human activities have caused the con- ccntration of CO, to increase by 25 per cent, and that of methane to double. Man has also manufactured cntircly new species of greenhouse gases - the chlorofluorocarbons (CFCS). Although increascs of all these gases are continuing, CO, is by far the most important single

26