About the Pacific Climate Extreme Indices
Extreme Climate Indices used
Extreme climate events such as heat waves, cold snaps, floods and dry spells have significant impacts on society. To examine whether such extremes have changed over time a variety of extreme climate indices can be defined, such as the number of days per year which exceed, or fail to exceed, fixed thresholds. However, since people tend to adapt to their local climate, a threshold considered extreme in one part of the world could be considered quite normal in another. To overcome this problem, thresholds based on percentile values can also be defined.
Twenty-five of the total thirty extreme temperature and rainfall indices used here are based on those defined by the WMO Expert Team on Climate Change Detection Monitoring and Indices. The core temperature indices which count the number of days where maximum temperature and minimum temperature are less than 0°C have not been included.
| Extreme Temperature Indices | Definition |
|---|---|
| Highest maximum temperature (TXx) | Monthly maximum value of daily maximum temperature (TX) |
| Highest minimum temperature (TNx) | Monthly maximum value of daily minimum temperature (TN) |
| Lowest maximum temperature (TXn) | Monthly minimum value of daily maximum temperature (TX) |
| Lowest minimum temperature (TNn) | Monthly minimum value of daily minimum temperature (TN) |
| Summer days (SU25) | Annual count of days with maximum temperature (TX) > 25°C |
| Tropical nights (TR20) | Annual count of days with minimum temperature (TN) > 20°C |
| User-defined hot days (SUx) | Annual count of days with maximum temperature (TX) > 26 to 37°C (1°C intervals) |
| User-defined hot nights (TRx) | Annual count of nights with minimum temperature (TN) > 21 to 32°C (1°C intervals) |
| Cool days (TX10p) | Number of days with maximum temperature (TX) < 10th percentile |
| Cool nights (TN10p) | Number of nights with minimum temperature (TN) < 10th percentile |
| Warm days (TX90p) | Number of days with maximum temperature (TX) > 90th percentile |
| Warm nights (TN90p) | Number of nights with minimum temperature (TN) > 90th percentile |
| Warm spell duration (WSDI) | Annual count of days with 6+ consecutive days when TX > 90th percentile |
| Cold spell duration (CSDI) | Annual count of days with 6+ consecutive days when TN < 10th percentile |
| Diurnal temperature range (DTR) | Monthly mean difference between TX and TN |
| Growing Season Length (GSL) | Annual (1st July to 30th June in SH and 1st January to 31st December in NH) count between first span of 6+ days with TG > 5°C and first span of 6+ days with TG < 5°C, where TG is the daily mean T |
| Extreme Precipitation Indices | Definition |
|---|---|
| Max 1-day rainfall (Rx1day) | Monthly maximum 1-day precipitation (PRCP) |
| Max 5-day rainfall (Rx5days) | Monthly maximum consecutive 5-day precipitation (PRCP) |
| Simple daily intensity (SDII) | Annual total precipitation divided by the number of wet days in the year; wet days defined as PRCP ≥ 1.0 mm |
| Rain days ≥ 10 mm (R10) | Annual count of days with PRCP ≥ 10 mm |
| Rain days ≥ 20 mm (R20) | Annual count of days with PRCP ≥ 20 mm |
| User defined rain days (Rnn) | Annual count of days when PRCP ≥ 1, 5, 30, 40, 50, 60, 70, 80 and 100 mm |
| Consecutive dry days (CDD) | Maximum number of consecutive days with daily rainfall (RR) < 1 mm |
| Consecutive wet days (CWD) | Maximum number of consecutive days with daily rainfall (RR) ≥ 1 mm |
| Very wet days rainfall (R95p) | Annual total precipitation when daily rainfall (RR) > 95th percetile |
| Extremely wet day rainfall (R99p) | Annual total precipitation when daily rainfall (RR) > 99th percentile |
| Annual total wet day rainfall (PRCPTOT) | Annual total precipitation in wet days (RR ≥ 1 mm) |
| Extreme Mean Sea Level Pressure (MSLP) Indices | Definition |
|---|---|
| Low MSLP days (MSLP10p) | Annual count of days when MSLP 0900 hrs < 10th percentile |
| High MSLP days (MSLP90p) | Annual count of days when MSLP 0900 hrs > 90th percentile |
| Very low MSLP days (MSLPn) | Annual count of days when MSLP 0900 hrs < 995 hPa |
Percentile calculations
Percentile-based threshold levels are calculated for 5-day windows across the annual cycle using 1961 to 1990 as the base period i.e. daily rainfall and temperatures values are compared relative to varying thresholds throughout the year. So it is possible to have warm spells during winter, cold spells during summer, or extreme precipitation recorded at any time of the year.
Data used
The extreme climate analyses are calculated using "high-quality" daily temperature and rainfall datasets developed specifically for monitoring long-term trends and variability in the Pacific (see maps below). The development of these datasets involved identifying and removing problem records using statistical techniques, visual checks and station history information (metadata). The high-quality daily temperature and rainfall datasets are available at http://www.bom.gov.au/climate/pccsp/.
The FClimDex software package was used to calculate the indices (please see link for more details). Monthly indices are calculated if no more than 3 days are missing in a month, while annual values are calculated if no more than 15 days are missing in a year. No annual value will be calculated if any one month's data are missing. For threshold indices, a threshold is calculated if at least 70% of data are present. For spell duration indices (marked with a *), a spell can continue into the next year and is counted against the year in which the spell ends e.g. a cold spell (CSDI) beginning on 31st December 2000 and ending on 6th January 2001 is counted towards the total number of cold spells in 2001
Please note that except for the purposes of study, research, criticism and review, no part of these analyses may be reproduced, or redistributed for any commercial purposes. Any use of these data should be acknowledged to the COSPPac and the relevant Pacific meteorological service.
