Abstract
In addition to high temperature, high humidity can have significant consequences on thermal comfort of human beings. The co-occurrence of high temperature and high humidity (so-called ‘oppressive hot days’) often results in heat stress events, but the extent to which it is affected by preceding surface moisture has not been fully understood to date. In this study, we examine the relations between preceding 3-month standardized precipitation index (SPI) and the number of hot days indicated by the surface air temperature (NHD-Tx) and the wet-bulb globe temperature (NHD-Wx) that combines both temperature and humidity in the hottest month in low latitudes. Results show that, in contrast with the negative correlations between SPI and NHD-Tx, which are associated with the previously reported precipitation deficit-temperature feedback, significant positive correlations between SPI and NHD-Wx are found in some low latitude areas. The probability of above-average NHD-Wx could be ∼30% higher after wet conditions than that after dry conditions in areas like southern South America, some parts of Africa, and West Asia. Hotspot analyses further show that abundant preceding rainfall has an asymmetric impact on oppressive hot days by favoring more above-average NHD-Wx. Our analyses imply that a local feedback may exist between surface moisture and oppressive hot extremes, via which the unbearable heat stress over some parts of the tropics is modulated, controlled, and/or caused by changes in the preceding near-surface humidity/soil moisture. The spatially heterogeneous patterns of the relations between preceding rainfall and heat stress confirm the precipitation deficit-temperature feedback in many areas and reveal the coexistence of surface moisture-oppressive heat stress in several low latitude areas. We emphasize the necessity of considering both feedbacks for a better understanding of the distinct roles of preceding rainfall in the consequent development of heat stress in low latitudes.
Highlights
We examine the correlations between preceding standardized precipitation index (SPI), which is a proxy of soil moisture conditions, and heat stress events in the hottest month for the period of 1979–2016
In contrast to NDH-Tx, positive, significant correlations account for a much larger proportion (27% and 40% for figures 1(c) and (d), respectively) of the stations. The pattern of these statistically significant correlations is remarkably distinct from that between number of hot days (NHD)-Tx and preceding SPI in figures 1(a) and (b), which are characterized by predominantly negative correlations
Our analyses show statistically significant and positive correlations between preceding 3-month SPI and oppressive heat stress in the hottest month in several low latitude areas
Summary
Heat stress occurs when the human body feels thermal discomfort or heat injury due to exposure to the ambient air. Intensifying heat stress events have been widely reported in many regions of the world during the past decades (Tang et al 2012, 2014, Sun et al 2014, Michael et al 2018). They are projected to be more frequent and intense in the future (Meehl and Tebaldi 2004, Schubert et al 2014, Dong et al 2015, Leng et al 2016, Mora et al 2017). The feedback shows that preceding soil moisture deficit can result in more frequent high temperature extremes and persistent heat stress than under normal or wetter conditions (Seneviratne et al 2010, Alexander 2011, Miralles et al 2012, 2014). As shown by Mueller and Seneviratne (2012), the impact of moisture deficits on the number of high temperature days is asymmetric, i.e. high frequency of hot days is most strongly influenced by the moisture conditions, while the influence on the smaller numbers of hot days is much weaker
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