Extreme heat waves with the Heat Wave Magnitude Index and their occurrence in the future

Alessandro Dosio, Simone Russo European Commission Joint Research Centre, Italy Jana Sillmann CICERO, Norway Alessandro.Dosio@jrc.ec.europa.eu

Our Common Future under Climate Change, Paris, 7-10 July 2015

www.dhw.wa.gov.au

Do we (really!) need a new extreme temperature index?

Heat wave: how it is defined

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World Meteorological Organization definition is "when the daily maximum temperature of more than five consecutive days exceeds the average maximum temperature by 5 °C, the normal period being 1961-1990

In the Netherlands, a heat wave is defined as period of at least 5 consecutive days in which the maximum temperature in De Bilt exceeds 25 °C, provided that on at least 3 days in this period the maximum temperature in De Bilt exceeds 30 °C. This definition of a heat wave is also used in Belgium and Luxembourg

In Denmark, a national heat wave is defined as a period of at least 3 consecutive days of which period the average maximum temperature across more than fifty percent of the country exceeds 28 °C

In the United States, definitions also vary by region; In the Northeast, a heat wave is typically defined as three consecutive days where the temperature reaches or exceeds 90 °F (32.2 °C), but not always as this ties in with humidity levels to determine a heat index threshold. The same does not apply to drier climates. A heat storm is a Californian term for an extended heat wave. Heat storms occur when the temperature reaches 100 °F (37.8 °C) for three or more consecutive days over a wide area (tens of thousands of square miles).

In Adelaide, South Australia, a heat wave is defined as five consecutive days at or above 35 °C, or three consecutive days at or over 40 °C.

The South African Weather Service defines a heat wave as “when for three days the maximum temperature is five degrees higher than the mean maximum for the hottest month.

Extreme temperature events: how they are measured

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Indices defined for heat wave purposes by WMO Expert Team on Climate Change Detection and Indices (ETCCDI) SU, Number of summer days: Annual count of days when TX (daily maximum

temperature) > 25oC TR, Number of tropical nights: Annual count of days when TN (daily minimum

temperature) > 20oC. TX90p, Percentage of days when TX > 90th percentile of reference period TN90p, Percentage of days when TN > 90th percentile of reference period WSDI, Warm spell duration index: Annual count of days with at least 6 consecutive

days when TX > 90th percentile of the reference period

Indices are based on absolute thresholds (SU and TR), are not suitable in some regions (e.g. tropics or regions with small temperature variability). Percentile-based thresholds, look at occurrences of events that exceed a prescribed percentile. Both TX90p and TN90p focus on the frequency of individual days (no information on duration of the spell).

The ETCCDI indices that measure extreme temperature only consider one aspect of a heat wave each, either event duration (WSDI) or frequency of days that may be (but are not necessarily) part of a heat spell (SU, TR, TX90p, and TN90p).

Perkins and Alexander, 2013

Do we really need a new HW index?

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Russo et al., 2014

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HWMI: how does it work?

6 Russo et al., 2014

Do we really need a new HW index?

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Russo et al., 2014

HWMI: does it work? Observations

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HWMI: CMIP5 GCMs - present

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HWMI ≥ 4

HWMI ≥ 8

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c.c. scenarios

Russo et al., 2014

HWMI: CMIP5 GCMs - future

WSDI vs. HWMI: CORDEX-Africa

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Dosio et al., Clim Dyn, submitted

WSDI

HWMI

WSDI vs. HWMI: how they differ

12 Dosio et al., Clim Dyn, submitted

High WSDI Low HWMI

Low WSDI Low HWMI

Low WSDI High HWMI

Conclusions

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Heat wave definition is very heterogeneous and it strongly depends on e.g., location ETCCDI indeces are not suitable in some regions (e.g. tropics or regions with small temperature variability) and only consider one aspect of a heat wave each, either event duration (WSDI) or frequency of days that may be (but are not necessarily) part of a heat spell The HWMI takes into account both intensity and duration of a Heat Waves and enables the quantification of the magnitude of heat waves across different time periods and regions of the world. HWMI works well when applied to detect observed heat waves Climate models are not always able to reproduce climate extreme (temperature) events. Projection show that in a warmer future climate, the probability of extreme events, such as those in Europe 2003, will increase and events considered rare today may be considered the norm under the most severe emission scenarios. However the extreme Russian heat wave in 2010 can still be considered a rare event in the future under the less severe scenarios RCP4.5 and RCP2.6 WHWMI and WSDI give complementary information