Auckland Region climate change projections and impacts Prepared for Auckland Council, Council Controlled Organisations, and District Health Boards Revised September 2020 Prepared by: Petra Pearce (Climate Scientist) Rob Bell (Principal Scientist – Coastal and Estuarine Processes) Helen Bostock (Marine Geologist) Trevor Carey-Smith (Climate Scientist) Daniel Collins (Hydrologist) Nava Fedaeff (Climate Scientist) Ayushi Kachhara (Air Quality Technician) Gregor Macara (Climate Scientist) Brett Mullan (Principal Scientist – Climate Variability) Ryan Paulik (Hazards Analyst) Elizabeth Somervell (Air Quality Scientist) Abha Sood (Climate Scientist) Andrew Tait (Principal Scientist – Climate) Sanjay Wadhwa (GIS Analyst) John-Mark Woolley (Climate Research Scientist) For any information regarding this report please contact: Petra Pearce Climate Scientist Weather and Climate Applications +64-9-375 2052 [email protected] National Institute of Water & Atmospheric Research Ltd Private Bag 99940 Viaduct Harbour Auckland 1010 Phone +64 9 375 2050 NIWA CLIENT REPORT No: 2018021AK Report date: September 2020 NIWA Project: ARC17103 Auckland Council Technical report: TR2017/030-3 ISSN 2230-4525 (Print) ISSN 2230-4533 (Online) ISBN 978-1-98-855520-1 (Print) ISBN 978-1-99-002256-2 (PDF) This report should be referenced using the following citation: Pearce, P., Bell, R., Bostock, H., Carey-Smith, T., Collins, D., Fedaeff, N., Kachhara, A., Macara, G., Mullan, B., Paulik, R., Somervell, E., Sood, A., Tait, A., Wadhwa, S., Woolley, J.-M. (2020). Auckland Region climate change projections and impacts. Revised September 2020. Prepared by the National Institute of Water and Atmospheric Research, NIWA, for Auckland Council. Auckland Council Technical Report, TR2017/030-3 © All rights reserved. This publication may not be reproduced or copied in any form without the permission of the copyright owner(s). Such permission is only to be given in accordance with the terms of the client’s contract with NIWA. This copyright extends to all forms of copying and any storage of material in any kind of information retrieval system. Whilst NIWA has used all reasonable endeavours to ensure that the information contained in this document is accurate, NIWA does not give any express or implied warranty as to the completeness of the information contained herein, or that it will be suitable for any purpose(s) other than those specifically contemplated during the Project or agreed by NIWA and the Client. Quality Assurance Statement Dr Andrew Lorrey Group Manager – Weather & Climate Internally reviewed by: Applications NIWA Auckland Professor James Renwick School of Geography, Environment and Externally reviewed by: Earth Sciences Victoria University of Wellington Petra Pearce Formatting checked by: Climate Scientist – NIWA Auckland Dr Sam Dean Chief Scientist – Climate, Atmosphere Approved for release by: & Hazards NIWA Wellington Dr Andrew Tait September 2020 revisions Chief Scientist – Climate, Atmosphere approved for release by: & Hazards NIWA Wellington Note: Section 4.6 (Extreme, rare rainfall events) was updated in September 2020. At the time of original publication (early 2018) the final High Intensity Rainfall Design System (HIRDS) v4 report (Carey-Smith et al., 2018) had not been completed. Hence, revisions to this report were required to update some values that changed between the draft and final HIRDS reports. Some edits were also made to Section 1.3 (Downscaling methodology). Contents Executive summary ........................................................................................................... 18 1 Introduction ............................................................................................................ 19 1.1 Global climate change ............................................................................................ 21 1.2 Representative Concentration Pathways ............................................................... 23 1.3 Downscaling methodology ..................................................................................... 27 1.4 Natural factors causing fluctuation in climate patterns over New Zealand ........... 30 1.4.1 The effect of El Niño and La Niña ............................................................ 30 1.4.2 The effect of the Interdecadal Pacific Oscillation.................................... 31 1.4.3 The effect of the Southern Annular Mode .............................................. 31 1.4.4 The influence of natural variability on climate change projections ........ 32 2 Present-day and future climate of the Auckland Region ............................................ 34 3 Temperature ........................................................................................................... 35 3.1 Mean temperature ................................................................................................. 36 3.1.1 Present .................................................................................................... 36 3.1.2 Future ...................................................................................................... 39 3.2 Maximum and minimum temperature ................................................................... 49 3.2.1 Present .................................................................................................... 49 3.2.2 Future ...................................................................................................... 55 3.3 Diurnal temperature range ..................................................................................... 69 3.3.1 Present .................................................................................................... 69 3.3.2 Future ...................................................................................................... 70 3.4 Temperature extremes ........................................................................................... 77 3.4.1 Present .................................................................................................... 77 3.4.2 Future ...................................................................................................... 80 3.5 Growing degree-days .............................................................................................. 85 3.5.1 Present .................................................................................................... 85 3.5.2 Future ...................................................................................................... 87 3.6 Temperature projections within RCPs .................................................................... 90 4 Rainfall .................................................................................................................... 94 4.1 Total rainfall ............................................................................................................ 95 4.1.1 Present .................................................................................................... 95 4.1.2 Future ...................................................................................................... 98 4.2 Rain days (>1 mm) ................................................................................................ 108 Auckland Region climate change projections and impacts 4.2.1 Present .................................................................................................. 108 4.2.2 Future .................................................................................................... 111 4.3 Heavy rain days (> 25 mm) ................................................................................... 118 4.3.1 Present .................................................................................................. 118 4.3.2 Future .................................................................................................... 119 4.4 Days with consecutive heavy rainfall .................................................................... 126 4.5 99th percentile of daily rainfall (1-2 wettest rain days of the year) ...................... 133 4.5.1 Present .................................................................................................. 133 4.5.2 Future .................................................................................................... 134 4.6 Extreme, rare rainfall events ................................................................................ 136 4.6.1 Present .................................................................................................. 136 4.6.2 Future .................................................................................................... 138 Analysis of extreme rainfall .................................................................................. 139 Future extreme rainfall ......................................................................................... 139 Climate change augmentation factors .................................................................. 141 4.7 Dry days (< 1 mm) ................................................................................................. 144 4.7.1 Present .................................................................................................. 144 4.7.2 Future .................................................................................................... 144 4.8 Potential Evapotranspiration Deficit .................................................................... 152 4.8.1