THE CLIMATE AND WEATHER OF WEST COAST 2nd edition G. R. Macara © 2016. 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 72 ISSN 1173-0382 Note to Second Edition This publication replaces the first edition of New Zealand Meteorological Service Miscellaneous Publication 115 (10), written in 1982 by J. W. D. Hessell. 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 WEST COAST 2nd edition G. R. Macara SUMMARY West Coast is New Zealand’s wettest region, and this may be attributed to its exposure to the predominant westerly airflow over the country, combined with the orographic effect of the Southern Alps. Annual rainfall totals at relatively high elevations regularly exceed 10,000 mm, with low elevation coastal locations typically recording between 2,000 and 3,000 mm of rainfall annually. Temperatures in lowland areas remain mild throughout the year, with temperatures less than 0°C and greater than 25°C occurring infrequently compared to most other regions of New Zealand. West Coast is not especially windy, and local wind regimes are strongly influenced by the southwest to northeast orientation of the Southern Alps. Photo: ©mychillybin.co.nz/Jingyi Tan ©mychillybin.co.nz/Jingyi Photo: CONTENTS SUMMARY 4 INTRODUCTION 7 TYPICAL WEATHER SITUATIONS IN WEST COAST 9 Disturbed westerly flows 9 Northerly flows 9 Southeasterly flows 10 Fine weather spells 10 CLIMATIC ELEMENTS 13 Wind 13 Rainfall 16 Rainfall distribution 16 Rainfall frequency and intensity 20 Recent extreme events in West Coast 22 Periods of low rainfall 22 Temperature 23 Sea surface temperature 23 Air temperature 24 Earth temperatures 27 Frosts 28 Sunshine and solar radiation 29 Sunshine 29 Solar radiation 30 UV (ultra-violet) radiation 30 Other elements 31 Thunderstorms and hail 31 Fog 31 Snow 31 DERIVED CLIMATOLOGICAL PARAMETERS 33 Vapour pressure and relative humidity 33 Evapotranspiration and soil water balance 34 Degree-day totals 36 ACKNOWLEDGEMENTS 38 REFERENCES 38 5 6 Net Images ©mychillybin.co.nz/Haupiri Photo: 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. The Southern Alps act as a barrier to the prevailing westerly airstream which is both deflected by them and forced to ascend; this causes rain which is often heavy and prolonged in West Coast. 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 effect of the oceans on air of tropical origin is to cool the lower layers, creating extensive sheets of stratiform cloud capable of producing large amounts of rain in West Coast, especially when entrained into cyclonic (low pressure) systems and subjected to orographic processes. Conversely, air originating from the far south typically reaches New Zealand as a southerly airstream, and the Southern Figure 1. Map of the West Coast region, showing the places mentioned in Alps protect West Coast from the this publication. instability showers formed when cold air is heated from below by a relatively warm sea surface. Under these circumstances, West Coast experiences fine weather and sunny skies with exceptionally good visibility. numerous glaciers. The Fox Glacier and Franz Josef Glacier are The West Coast region (Figure 1) is particularly renowned and contribute to the ongoing tourism of bounded in the west by the coast, and the region. These glaciers reach comparatively low altitudes for in the east by the main divide of the middle latitudes and their rates of flow are relatively fast due to Southern Alps which reaches 3,724 m at the steep topography of the western face of the Southern Alps the summit of Mount Cook. The region down which they descend. These glaciers respond relatively drains westwards to the coast, and the quickly to climatic forcings; their dynamic nature make these river systems are subject to remarkable glaciers useful indicators of climate variability and change. fluctuations in flow in response to rainfall events. The river catchments extend into Note that all numbers given in the following tables are calculated high alpine regions, where a proportion from the 1981–2010 normal period (a normal is an average of the considerable annual precipitation or estimated average over a standard 30-year period), unless falls as snow, enabling the formation of otherwise stated. 7 8 Meredith ©mychillybin.co.nz/Mark Photo: TYPICAL WEATHER SITUATIONS IN WEST COAST Surface winds on the West Coast tend to be light and variable, and are influenced both by the synoptic-scale circulation and by large frictional effects caused by the mountain barrier to the east. Nevertheless the weather is strongly typified by the wind flow in the free atmosphere above the friction layer, and the following sections refer to the direction of this wind rather than to local surface wind directions. Disturbed westerly flows Disturbed westerly flows over New Zealand are associated with depressions to the south of the country which usually move rapidly eastward (Figure 2). The flow may be intensified by development and southward movement of the belt of subtropical anticyclones which in turn are associated with changes in the principal upper-air hemispheric jet stream. The seasonal changes in the general circulation of the Southern Figure 2. Mean sea level pressure analysis for 0000 hours NZDT on 16 October 1977. Hemisphere as described above result in a maximum frequency of disturbed westerly situations in spring (Reid, 1980). The “disturbances” in the westerlies may have varied dynamics; most are usually depicted on synoptic charts as cold fronts. Winds just above the surface friction layer ahead of each front tend to be north of west and are almost perpendicular to the Alpine divide. The fronts are preceded by rain in West Coast which is often heavy due to orographic uplift. The change in wind direction to the southwest behind the fronts usually brings an initial dramatic clearance in the weather, though showers soon re-develop with the gradual change of winds back towards the west in the generally rather unstable conditions. Northerly flows Extensive northerly airflows (Figure 3) between an anticyclone (high pressure system) and a depression (low pressure system) are usually accompanied by prolonged rainfall which can reach torrential Figure 3. Mean sea level pressure analysis for 0000 hrs NZST on 15 August 1977. intensities in the Southern Alps. The skies over the West Coast typically remain overcast throughout these periods. The majority of West Coast’s rain occurs in winds having a northerly component, and the temperature during these rainfall events is relatively mild. The occurrence of thunder during the rain often 9 heralds the arrival of the front, which brings a change to south-westerly winds and clearing skies. Southeasterly flows Southeast winds are often accompanied by clear skies in West Coast as the air has lost moisture during uplift on the eastern side of the Southern Alps. Southeast airstreams over the South Island are usually caused by depressions (lows) over the North Island and anticyclones (highs) to the south of the country (Figure 4). The foehn effect occurs in West Coast under strong southeast gradients, just as they do in eastern parts of the South Island under strong northwest gradients. The highest temperatures recorded in West Coast occur during southeast airflows in summer, and southeast winds usually result in higher than average Figure 4. Mean sea level pressure analysis for 0000 hours NZST temperatures at all times of the year. on 12 April 1981. Fine weather spells Fine weather spells are usually associated with an anticyclone moving slowly eastwards over the South Island, or the area just south of it. If a depression develops to the north or north-east of the North Island a ridge of high pressure may still extend over the South Island when the anticyclone is centred far to the east or south-east. Prolonged dry spells are relatively infrequent in West Coast, and usually occur when an anticyclone becomes stationary over or very near New Zealand. Figure 5 shows a situation in which the air pressure over New Zealand is very high, with two intense anticyclones separated by a trough of relatively low pressure. The anticyclone east of New Zealand is forming a “block” to the progression of the trough which has become stationary, and the weather over the Figure 5. Mean sea level pressure analysis for 0000 hours NZST whole country including West Coast is fine and settled.