Changing world climatic conditions causing experts to think again

Press, 17 March 1977, Page 21

Changing world climatic conditions causing experts to think again

It is inevitable that New Zealanders, being British in orgin, should feel that the great English drought of the 1975-76 years should be repeated in New Zealand as of right. However, the atmosphere is no respecter of loyalties, and it comes as a bit of a shock to find that the analogy for New Zealand is probably to be found in the cool, wet summer of Eastern Europe and the Mediterranean Basin. In June, 1973, there was an aritcle in “The Press” entitled, “Does Canterbury have to live with drought.” This was prompted by an earlier article in “The Press” by Professor Lamb entitled, “Prolonged droughts tied to major climatic change.” At that time Canterbury was experiencing its driest five years on record, with rainfall departures well in excess of 25 per cent over the five-year period. Today, we have to consider the opposite. In April, 1974, the drought in Canterbury broke with a vengeance; the city was flooded and the dry climate regime which had prevailed in previous years gave way to one of a much cooler and wetter type. Increasingly, we have seen cool east-to-south airstreams over the country resulting from persistent, slow-moving pressure anomalies based on high pressure to the south-east, south, • and south-west of the country. All last summer this weather type persisted, and throughout the last winter and into the spring and now summer, no change has come about until February and early March has brought some sun and warmth. Has the spell of bad weather ended? There is much literature today which outlines the increasing tendency of weather patterns to persist. Professor Lamb, at the Climatic ■ Research Unit in England, has coined the term “block” to describe the breakdown in the normal circulation pattern.

The most recent, and a well-known example, is the persistent high pres-<. sure “block” over northwest Europe . which has been developing for many years, and culminated in the exceptional 18-month drought and heat-wave summers there in 1975 and 1976. The situation in Britain was ameliorated during September and October by a shift of this “blocking” situation to a position 20 degrees of longitude further east, allowing persistent wet weather to invade the western regions of the previous drought areas. Meanwhile, on the other side of the “Block,” conditions in Eastern Europe and the Mediterranean have been quite the re-

verse, during the same period, with record rainfalls and low temperatures. For instance, Malta, in the Mediterranean, had its wettest June on record; admittedly only 9mm fell. At Algiers in July, 42mm occurred compared with the normal 2mm, and other parts of Algeria recorded rainfalls 3200 per cent of normal rainfall. Many other areas of Greece, Italy, and Turkey also had rainfalls eight times the normal level, with temperatures up to three degress below the average.

More recently we have had the cold spell to the east of the Rocky Mountains of North America with, as usual, the related persistent mild conditions in Alaska and probably re-

lated continued drought conditions over and to the west of the Rocky Mountains.

The type of weather associated with a “block” therefore depends on which side you are located, the departure from normal dependent on the length of time the block persists. - As Professor Hare, the visiting climatologist from Canada said last year in Christchurch: “It makes it very difficult to determine whether the world is getting colder or warmer because extremes of climate in both the positive and negative direction could easily cancel each other out.”

A very good example of this occurred during the northern hemisphere winter of 1962-63. Mr Gabites of the New Zealand Meteorological Service investigated the phenomenal cold in Europe and Eastern North America that winter and found that although temperatures in Europe were -10 degrees C below the average for long periods, these departures were matched by temperatures of plus 12 degrees C elsewhere in the Northern Hemisphere. The result was a winter for the Northern Hemisphere progably a little warmer than usual.

Similar situations are indicated for the European summers of 1975 and 1976 and the situation at present in the Eastern United States. How does New Zealand fare in this situation? It appears we have been on the very unfavourabel side of such a persistent anomaly — unless, of course, you live on the West Coast of the South Island. For many months now meteorological reports have recorded the persistence of the easterly to southerly component

weather which has brought persistent cold, damp conditions to the east, and bright, dry weather to the west. The weather office recorded a complete absence of the north-westerly conditions in Christchurch during November, and both October and November were among the coldest on record, and January the coldest since 1953, with very low sunshine totals. Clearly the New Zealand area has been in an analogous situation to that experienced in Europe over the last two years, and possibly longer. At Lincoln College, the present low spring and summer temperatures are not without precedent. In 1946, both October and November appear to have

been very similar to 1976. In that year the summer continued to be cold and miserable and was, in fact, one of the coldest of a series of 12 relatively miserable summers, between 1942-53. Winters were also colder with, of course, the great snowfall of 1945.

From the evidence one could be very pessimistic about the chances of an early change back to the conditions of the last decade. During the history of climatology at Lincoln College rainfall has generally been higher during the cooler summers. The whole 20-year, 1934-53 period, in which the very cool period occurred, averaged 11 per cent above the long term mean for rainfall, with some very wet years indeed. Temperatures during the growing season for this whole 20-year period were about 12 per cent lower when compared to the previous 20 years when rainfall

Since those months the situation has improved, although in Canterbury January was the coldest since 1953 and the second dullest on record. Recent dry warm weather in February is perhaps an indication of a return to more normal weather conditions in Canterbury — but a return to the warmth and drought of the 1969-74 period would appear unlikely if past records have any meaning. It appears that Canterbury, in particular, has had quite definite periods of drought followed by

was also 13 per cent below the long term mean. In more recent years the period between 196975 gave not only the lowest rainfalls on record, but also the most consistently warm summers since those earlier y ears.

Canterbury has. therefore, a history of fluctuating climatic types, and the present departures are not unique, even though they may be unpleasant, and disastrous in some cases to agriculture when warmth-requiring crops, such as beans, tomatoes, cucurbits, corn, and grapes, have become common outside crops in response to the improving temperatures of recent years. But what must be more important in aaaodatfon with the current weather pattern is the persistent rainfall departures on the Alps and West Coast from which much of the hydroelectric potential is derived. If the present situation is to continue then rainfall in these areas will remain below normal, or at least show greater departures from the mean with consequent less reliability. And what does fall is likely to be in the form of snow due to the relatively low temperatures associated with the system.

The situation for November was unprecedented over the Alpine region of the South Island. For the five months, July to November, the Hermitage received only 42 per cent of its normal rainfall; the Fox and Franz Josef gla cier region only 48 per cent; while Milford Sound, which could conceivably be related to inflows into Manapouri and Te Anau, received only 36 per cent of the normal rainfall. The full figures (in millimetres) are as follows:

wetter periods, and that temperatures are probably related to these rainfall variations. The present persistant climatic departures in New Zealand are not without precedent but this does not mean that they could not become so. The next few months into winter will be watched with interest by everybody as New Zealand is particularly dependent upon the reliability if its west coast and alpine rains, further failure of which could spell disaster for power generation.

By

R. A. CROWDER,

, senior lecturer

at Lincoln College

July Aug. Sept. Oct. Nov. Ttl % Mean mm mm mm mm mm mm HERMITAGE .. Mean 221 244 128 282 61 330 197 371 83 395 690 1622 42.5 WEST COAST GLACIERS . Mean 18S 314 175 374 110 428 311 429 212 517 994 2062 48.2 MILFORD SOUND Mean 274 378 147 424 55 538 281 546 162 919 648 2534 36.3