HWF (sum of days satisfying definition criteria)

Globally averaged trends

(trends in bold are significant, averaged across all regions with data)

HWA (amplitude of hottest event)

INDICES• A wide range of indices published in the scientific literature employed for heatwave/warm spell

measurement were surveyed (see Perkins and Alexander, 2012).

• By balancing a range of “definitions” and “components”, the framework seeks to satisfy the requirements

of many users, as well as enabling the measurement of events from a climatological standpoint.

• Definitions – a heatwave/warm spell exists when at least 3 consecutive days satisfy the below criterion:1) CTX90pct: Tmax exceeds the calendar day 90th percentile, based on a 15-day moving window.

2) CTN90pct: Tmin exceeds the calendar day 90th percentile, based on a 15-day moving window.

3) EHF: Positive Extreme Heat Factor based on average temperature, derived from: EHI(accl.) = (Ti + Ti−1 + Ti−2)/3 − (Ti−3 + …+ Ti−32)/30 EHI(sig.) = (Ti + Ti−1 + Ti−2)/3 − T95

EHF = max[1,EHI(accl.)]x EHI(sig.) Where Ti = (Tmaxi+Tmini)/2; T95=95th percentile of time period

• Aspects (annually per definition): HWN: Total number of events. HWD: Length of longest event. HWF: Sum of days satisfying definition criteria. HWM: Average magnitude of all events. HWA: Hottest day of hottest event.

• Trends estimated by Sens’ Kendall slope method, significant at 5% level.

Closing the Gap on Measuring Heat Waves and Warm SpellsSarah E. Perkins1,2 and Lisa V. Alexander1,2

1. Climate Change Research Centre, University of New South Wales, Sydney, Australia. 2. ARC Centre of Excellence for Climate System Science, University of New South Wales, Sydney, Australia.

Email: Sarah.Perkins@unsw.edu.au

INTRODUCTION• Due to a wide range of impacted sectors (e.g. Coumou and Rahmstorf, 2012), the definition of a heatwave

remains is very broad in describing a period of consecutive days where conditions are excessively hotter than normal.

• This has resulted in a plethora of metrics that seek to measure heatwaves, however are generally only suited to a specific sector or study (e.g. Matzarakis and Mayer, 1997; Deo et al., 2007).

• This inconsistency creates great difficulty in ascertaining observed changes in heatwaves.• Annual warm spells (which include non-summertime events) may also be of interest and need to be

measured appropriately

DATA Australia:

• AWAP gridded dataset of daily Tmin and Tmax at 0.5o X 0.5o resolution (Jones et al., 2009).• Time period 1951-2008.• All data consistent through space and time.• 5-month summer (i.e. heatwaves only)

Global:• Updated HadGHCND (Donat and Alexander, 2012) daily Tmin and Tmax at 3.75ox2.5o resolution.• Time period 1951-2011.• Grid points included with at least 65% of data for the entire period, 5% of which is after 2000. • 5-month summer for heatwaves (Austral=Nov-Mar; Boreal=Jun-Sep); all months for warm spells.

CONCLUSIONS• Increasing trends in HWF are driving increases in HWN and HWD.• Small-scale differences in changes of event magnitude (HWA, HWM).• Tmin (night-time) events increasing fastest, agreeing with other studies. • Warm spell trends larger than heatwaves – non-summertime events driving this annual trend more than summertime only events.• Qualitatively, all definitions agree on changes of aspects.• Framework allows for a consistent measure of events, while permitting the user to “cherry pick” the most appropriate definitions/aspects for their purpose.

KEY OBJECTIVETo define a framework in which heatwaves/warm spells can be measured, giving justice to

the geographical regions affected, the communities impacted, and the climatic fields involved.

REFERENCESCoumou, D. and Rahmstorf, S., 2012: A decade of weather extremes. Nature Climate Change, DOI: 10.1038/NCLIMATE1452Donat, M. G., and Alexander L.V., 2012:The shifting probability distribution of global daytime and night-time temperatures, GRL, DOI:10.1029/2012GL052459.Deo, R. C., McAlpine C. A., Syktus J., McGowan, H. A. and Phinn, S., 2007: On Australian heatwaves: time series analysis of extreme temperature events in Australia, 1950-2005. L. Oxley and D. Kwasiri (eds), Proceedings of the International Congress on Modelling and Simulation. Modelling and Simulation Society of Australia and New Zealand.Jones, D. A., Wang W. and Fawcett R., 2009: High-quality spatial climate data-sets for Australia. Australian Meteorological and Oceanographic Journal, 58, 233-248.Matzarakis, A. and Mayer, H. 1997: Heat stress in Greece. Int. J, Biometeorol., 41, 34-39.

For more information please see:Perkins, S.E., Alexander L.V. and Nairn J.R., 2012. Increasing intensity, frequency and duration of observed global heatwaves and warm spells. GRL, doi:10.1029/2012GL053361Perkins, S.E. and Alexander L.V., 2012. On the Measurement of Heatwaves, submitted to J. Climate.

%/decade

CTN90pct

EHF

HWD

HWA

Warm Spells

DefinitionHWN

Number/decade

HWDDays

/decade

HWF% days

/decade

HWAMagnitude/

decade

HWMMagnitude

/decade

Warm CTX90pct 2.53 0.22 0.42 0.18 0.05

Spell CTN90pct 3.39 0.3 0.54 0.36 0.19

EHF 0.56 0.43 0.28 0.34 0

Heat CTX90pct 1.4 0.08 0.28 0.12 0.11

Wave CTN90pct 1.93 0.14 0.37 0.2 0.18

EHF 1.38 0.42 0.64 0.34 0

Leng

th (d

ays)

Mag

nitu

de (d

egre

es C

/de

gree

s C2

Globally averaged time series of CTX90pct, CTN90pct and EHF warm spell HWA (top) and HWD (bottom).

Global (warm spells) Global (heatwaves)Australia (heatwaves)

CTX90pct

days/decade

CTN90pct

EHF

CTX90pct

Australia (heatwaves) Global (warm spells)

Degrees C/decade Degrees C (C2 for EHF)/decade