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Poster communications

Uses and limits of thermal indices: the case of Sahel

Abstract : Our main goal here is to analyse extreme heat waves (HWs) in the Sahel (13°N- 18°N; 16°W-30°E), using different thermal indices. In the ACASIS project funded by the French “Agence Nationale de la Recherche”, HWs analyses are characterized for the first time during the hottest season in the Sahel, using the Global Summary of the Day (GSOD) synoptic observations during April-May-June 1973-2013. Such extreme high temperatures are usually defined by 3 criteria: 1/ Their low probability of occurrence: less than the 10% of the days. The use of absolute thresholds, associated with heat budget and physiological impacts, could be an alternative (Seneviratne et al., 2012). 2/ Their intensity, i.e. their amplitude in terms of standard deviations from the local climatology (Goubanova, 2007), or otherwise excesses over absolute intensity thresholds. 3/ Their severity: extreme events causing socioeconomic or human losses, the notion of impact is thus of major importance for their definition (Beniston et al., 2007). According to prior IPCC assessments (TAR, AR4 and SREX), it is very likely that increased maximum temperatures and enhanced probabilities of hot days occurrences will occur at the global scale. There is also medium confidence that warm spells/heat weave frequencies, lengths or intensities will increase in many regions (Cubasch et al., 2013). Because of its latitude, Sahel is especially concerned, with diurnal temperatures often exceeding 40°C in boreal spring and, to a lesser extent, autumn. AR4 concluded that global land-surface air temperature (LSAT) had increased over the instrumental period of record, with the warming rate approximately double that reported over the oceans since 1979. AR5 confirms previous estimates: Global LSAT increased: 0.1005°C ± 0.01925°C per decade over 1901-2012 period while the rise reaches 0.262°C ± 0.05°C per decade over 1979-2012 time period, according to the observations of CRUTEM4.1.1.0, GHCNv3.2.0, GISS and Berkeley dataset average (Hartman et al., 2013). Changes in the length of the spells are observed and they are likely to increase but this needs to be investigated further over Sahel where severe heat can have damaging consequences for societies. For instance, in Niamey (Niger) in April 2010 whereas Tmax reaching 47°C (Ringard et al., 2015), increased morbidity and mortality rates were observed, especially among the elderly and young children. Moreover, in some places, HWs can be associated with increased rates of atmospheric pollution. Terminology is large: a “ hot spell” implies the hottest temperature during an extreme of temperature with a small duration (a few hours), small extension (around the station) and a high frequency; a “warm spell” should be less frequent and covers a larger area; a “heat wave” (HW) exceeds thresholds of temperature most frequently and covers at least a whole region (~ 500-1000 km²). These thresholds can be absolute (T° fixed locally) or relative, exceeding 5 °C as local standards for example for Expert Team on Climate Change Detection and Indices (ETCCDI, 2013). ACMAD (African Centre of Meteorological Applications for Development) classifies the extreme temperatures following five high-impact weather events; they broadcasts a mail alert when Tmax> 40°C. In ACASIS, Sahelian HW is defined (Rome et al. 2015) as a period of at least 3 consecutive days of extremely high (above the local 90th percentile) daily heat index. The Heat Index (HI) (Steadman 1979, 1994), combining temperature and relative humidity, appears as the most appropriate index, suitable for tropical climate, which takes into account the human-perceived equivalent temperature. The role of humidity is rather small in boreal spring across the Sahel, when high incoming solar radiation is combined with extremely dry soils, preventing cooling effect associated with latent heat flux. As for global scales, results show a clear warming trend over the last three decades, with a clear trend for HW events to become more frequent, last longer, cover larger areas and reach higher intensities (see also Oueslati et al. 2015). We then observe a decrease in cases of “Caution” HI and an increase in cases of “Danger” and “Extreme Danger” HI values.
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Poster communications
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https://halshs.archives-ouvertes.fr/halshs-01175704
Contributor : Sandra Rome <>
Submitted on : Thursday, July 16, 2015 - 5:57:41 PM
Last modification on : Tuesday, December 8, 2020 - 10:47:59 AM
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Sandra Rome, Vincent Moron, Boutheina Oueslati, Benjamin Pohl, Bernard Fontaine, et al.. Uses and limits of thermal indices: the case of Sahel. CFCC15 Scientific Committee, chaired by Chris Field. Our Common Future Under Climate Change (CFCC15) International Scientific Conference, Jul 2015, Paris, France. UNESCO, Future Earth, and ICSU. ⟨halshs-01175704⟩

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