The Climate and Weather of Otago
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THE CLIMATE AND WEATHER OF OTAGO 2nd edition G. R. Macara © 2015. All rights reserved. The copyright for this report, and for the data, maps, figures and other information (hereafter collectively referred to as “data”) contained in it, is held by NIWA. This copyright extends to all forms of copying and any storage of material in any kind of information retrieval system. While NIWA uses all reasonable endeavours to ensure the accuracy of the data, NIWA does not guarantee or make any representation or warranty (express or implied) regarding the accuracy or completeness of the data, the use to which the data may be put or the results to be obtained from the use of the data. Accordingly, NIWA expressly disclaims all legal liability whatsoever arising from, or connected to, the use of, reference to, reliance on or possession of the data or the existence of errors therein. NIWA recommends that users exercise their own skill and care with respect to their use of the data and that they obtain independent professional advice relevant to their particular circumstances. NIWA SCIENCE AND TECHNOLOGY SERIES NUMBER 67 ISSN 1173-0382 Note to Second Edition This publication replaces the first edition of New Zealand Meteorological Service Miscellaneous Publication 115 (4), written in 1968 by J. F. de Lisle and M. L. Browne. It was considered necessary to update the first edition, incorporating more recent data and updated methods of climatological variable calculation. THE CLIMATE AND WEATHER OF OTAGO 2nd edition G. R. Macara CONTENTS SUMMARY 6 INTRODUCTION 7 TYPICAL WEATHER SITUATIONS IN OTAGO 9 Fine weather spells 9 Heavy rain 11 Showery weather 11 CLIMATIC ELEMENTS 13 Wind 13 Rainfall 16 Rainfall distribution 16 Recent extreme events in Otago 21 Periods of low rainfall 22 Temperature 23 Sea surface temperature 23 Air temperature 24 Earth temperatures 28 Frosts 29 Sunshine and solar radiation 31 Sunshine 31 Solar radiation 32 UV (ultra-violet) radiation 32 Other elements 33 Snow 33 Thunderstorms 33 Hail 33 Fog 33 DERIVED CLIMATOLOGICAL PARAMETERS 35 Vapour pressure and relative humidity 35 Evapotranspiration and soil water balance 36 Degree-day totals 36 ACKNOWLEDGEMENTS 42 REFERENCES 42 5 SUMMARY The climate of Otago is perhaps the most diverse of any region in New Zealand. The region is in the latitudes of prevailing westerlies, and exposed coastal locations often experience strong winds, but the winds are lighter inland. Winter is typically the least windy time of year, as well as for many but not all areas, the driest. Annual precipitation in Otago typically decreases with increasing distance from the western ranges and the east coast. Indeed Central Otago is the driest region of New Zealand, receiving less than 400 mm of rainfall annually. Dry spells of more than two weeks occur relatively frequently in Central Otago, but less so elsewhere. Temperatures are on average lower than over the rest of the country with frosts and snowfalls occurring relatively frequently each year. However daily maximum temperatures in summer can exceed 30°C, especially about inland areas of Otago. On average, coastal Otago receives less sunshine than many other parts of New Zealand. 6 INTRODUCTION New Zealand spans latitudes 34 to 47 degrees south, and so lies within the Southern Hemisphere temperate zone. In this zone, westerly winds at all levels of the atmosphere move weather systems, which may also be either decaying or developing, eastwards over New Zealand giving great variability to its weather. These prevailing westerlies sometimes abate, and air from either tropical or polar regions may reach New Zealand with heavy rainfalls or cold showery conditions respectively. The main divide of the Southern Alps acts as a barrier to the prevailing westerlies and has a profound effect on the climate of Otago, separating the New Zealand’s wettest region (the West Coast) from New Zealand’s driest region Central Otago. No area of New Zealand has a climate where the hot dry summers and cold dry winters more closely approximate a semi-arid ‘continental’ climate than does Central Otago. In coastal areas of eastern Otago, conditions are tempered by relatively cool sea surface temperatures nearby and by the absence of shelter from airflows moving over the area from the south and south-west. Otago is topographically Figure 1. Map of the Otago region, showing the places mentioned in this diverse east of the main divide, consisting publication. of large mountain blocks and relatively low elevation basins. The climate of these inland basins lends itself to a burgeoning viticulture industry, whilst horticulture has been the mainstay of the area for some time. Note that all numbers given in the following tables are calculated from the 1981-2010 normal period (a normal is an average or estimated average over a standard 30-year period), unless otherwise stated. 7 8 TYPICAL WEATHER SITUATIONS IN OTAGO The weather in Otago is dominated by the west-to- characteristic categories: (a) fine weather spells, (b) east passage of anticyclones and intervening troughs heavy rain, and (c) showery weather. of low pressure across the New Zealand region. The weather conditions associated with a particular type of Fine weather spells meteorological situation can vary widely from one part Prolonged spells of fine weather of six days or more of Otago to another. This is largely due to the extensive are usually associated with an anticyclone moving geographic spread of the region, which encompasses slowly eastwards over the South Island, or the area both the Southern Alps to the west and coastal areas just south of it. If a depression develops to the north to the east. The irregular topography of Otago, with its or north-east of the North Island a ridge of high many mountain ranges, has a considerable influence pressure may still extend over the South Island when on the weather experienced across the region. the anticyclone is centred far to the east or south- east. This type of situation is shown in Figure 2. Fine, Anticyclones and ridges of high pressure typically sunny weather and light winds typically prevail over result in fine and calm weather across Otago. More Otago during such periods, although there are some often, however, anticyclones are located to the north exceptions. For example, fresh or strong north-easterly of the region, which results in a disturbed westerly sea breezes can occur along the eastern coast of air stream over Otago. Cold fronts or depressions in Otago, especially during summer. Such winds may the troughs between anticyclones will thus traverse bring areas of low cloud to those parts. Additionally, Otago fairly frequently. Northerly or westerly winds anticyclones support the development of low cloud associated with an approaching front bring rain or fog in inland parts of Otago, chiefly during April to (which may be heavy and persistent) to western areas. October. However, little-to-no rainfall occurs over central and eastern parts of the region due to the rain-shadow effect of the western ranges. Additionally, foehn winds occur to the lee of the western ranges, bringing mild temperatures in winter, and warm or hot temperatures in summer. As a cold front moves across Otago, rainfall may become widespread for a period of time, before easing to increasingly isolated showers as a new anticyclone approaches from the west. The passage of a cold front is associated with a considerable drop in temperature, and often results in snow falling to relatively low levels, especially during May to October. When a cold front passes from the south, rain and showers chiefly affect areas east of the western ranges. Similarly, rain-bearing easterly winds may result in heavy rain about the eastern parts, but dry conditions farther west. Indeed, western-most parts of Otago often have their sunniest weather when winds over the Otago region are from an easterly direction. Central Otago is shielded from rain on many occasions due to its inland location, with substantial rainfalls only occurring there when depressions and fronts are unusually vigorous. Although many different weather situations are possible in Otago, they tend to fall into only a few 9 Figure 2. Mean sea level pressure analysis for 1200 hrs Figure 5. Mean sea level pressure analysis for 1800 hrs NZST on 19 November 1960. NZST on 26 July 1962. Figure 3. Mean sea level pressure analysis for 0600 hrs Figure 6. Mean sea level pressure analysis for 1800 hrs NZST on 19 May 1965. NZST on 26 April 1965. Figure 4. Mean sea level pressure analysis for 1800 hrs NZST on 20 July 1955. 10 Heavy rain Heavy rain may occur on western-most parts of the direction of the wind-flow. If it is from the west, Otago when a strong, moist northerly to westerly showers are typically confined to the west and south of air-stream is lifted over the western ranges. This the region, with few showers reaching sheltered Central orographic effect serves to intensify the rainfall along and North Otago. In contrast, a southerly air-stream the western ranges, and the heavy rain may spill typically brings showers to all parts of the region. In this over eastwards into the headwaters of the Otago instance, heaviest falls usually occur near the east coast lakes and rivers. An example of this type of situation and in the area south-west of Roxburgh, with lighter is shown in Figure 3. A deep depression moving falls occurring in the north. towards the south-south-east had an occluded front A common type of situation associated with showery which crossed Otago on 19 May 1965, and produced weather over the Otago region is illustrated in Figure 100 to 125 mm of rain in the Makarora valley (north 6.