Defining and Predicting Heat Waves in Bangladesh

Abstract

This paper proposes a heat-wave definition for Bangladesh that could be used to trigger preparedness measures in a heat early warning system (HEWS) and explores the climate mechanisms associated with heat waves. A HEWS requires a definition of heat waves that is both related to human health outcomes and forecastable. No such definition has been developed for Bangladesh. Using a generalized additive regression model, a heat-wave definition is proposed that requires elevated minimum and maximum daily temperatures over the 95th percentile for 3 consecutive days, confirming the importance of nighttime conditions for health impacts. By this definition, death rates increase by about 20% during heat waves; this result can be used as an argument for public-health interventions to prevent heat-related deaths. Furthermore, predictability of these heat waves exists from weather to seasonal time scales, offering opportunities for a range of preparedness measures. Heat waves are associated with an absence of normal premonsoonal rainfall brought about by anomalously strong low-level westerly winds and weak southerlies, detectable up to approximately 10 days in advance. This circulation pattern occurs over a background of drier-than-normal conditions, with below-average soil moisture and precipitation throughout the heat-wave season from April to June. Low soil moisture increases the odds of heat-wave occurrence for 10–30 days, indicating that subseasonal forecasts of heat-wave risk may be possible by monitoring soil-moisture conditions.

Global predictability of temperature extremes

“…In the extratropics, seasonality and predictability are almost uniformly available. In these regions, atmospheric blocking causes persistent heat and cold events, as does feedback from dry soils and snow, both of which increase predictability of extreme events, even offering the potential to extend lead-times into sub-seasonal timeframes that would be valuable for enhanced preparedness (Fischer et al 2007, Nissan et al 2017, Purich et al 2014.…”

“…Heatwaves tend to be confined to the pre-monsoon summer season, before the rains arrive, which suppresses high temperatures. Predictability on weather timescales arises from a characteristic atmospheric circulation pattern and a deficit of normal rainfall at the beginning of the monsoon, which is reflected in anomalously low soil moisture for this time of the year (Nissan et al 2017). Models that better represent land-surface interaction are likely to perform better in predicting heatwaves in this region.…”

“…Temperature extremes account for 75% of weather-related deaths in high-income countries (CRED et al 2015), as well as increases in morbidity (Astrom et al 2011, Li et al 2015. For example, the 2003 heatwave in France was estimated to cause 15 000 excess deaths, and in Bangladesh, mortality increases approximately 20% on heat-wave days (Fouillet et al 2006, Nissan et al 2017. In India, a coldwave in 2003 killed more than 900 people, and unusual cold temperatures were responsible for 370 excess deaths in Moscow in 2006 (De et al 2005, Revich andShaposhnikov 2008).…”

“…Even with access to health data, determining detailed temperature-mortality relationships is not straightforward, because of the time-varying nature of population vulnerability to heat and cold. Where such relationships have been assessed, simple indices of relative heat and cold tend to have equal association with local impacts as do more complicated heatwave and coldwave definitions (Barnett et al 2010, Dixon et al 2005, Nissan et al 2017. In fact, most heatwave and coldwave definitions that have been derived from local mortality relationships tend to show impact at the most extreme percentiles of the local temperature climatology (Astrom et al 2011, Gasparrini et al 2015.…”

Health consequences of climate change in Bangladesh: An overview of the evidence, knowledge gaps and challenges

“…Storm shelters installed by the local government have reduced mortality from cyclones (Adger, Huq, Brown, Conway, & Hulme, 2003). Nissan, Burkart, Coughlan de Perez, Aalst, and Mason (2017) tried to define and predict heat waves in Bangladesh and proposed an early warning system as an adaptation measure to the direct health impact of climate change. Tracking of different adaptation measures is important to check whether a certain measure is a good practice or a "mal-adaptation" leading to further vulnerability (Magnan et al, 2016).…”