Heatwaves: How Little We Know About What is Happening in the Tropics

Almost all studies on heat-health relationships have been done in the extratropics, and many climate change projections have simply re-used heatwave definitions from the extratropics to define heatwaves in tropical regions. To assess the behavior of different heatwave definitions in the tropics, we carry out an intercomparison of the CHIRTS gridded station-satellite temperature dataset with three reanalysis products: ERA5, NCEP-DOE Reanalysis 2, and MERRA2 to assess biases in the absolute value of extreme heat events and the distribution of extreme events. We find close agreement between all four datasets in the magnitude of extreme temperatures, with a cold bias in the reanalyses over mountainous areas. However, there is little to no agreement between datasets on the timing of extreme heat events in the tropics, and the datasets do not even agree on which month is the hottest month climatologically in these regions. Second, we compare how the four datasets represent the frequency and timing of extreme heat events, using two different types of heatwave definitions: 5-day duration-based extreme temperatures and extreme temperature-humidity combinations (Heat Index). In the case of 5-day duration heatwaves, there are almost zero events recorded historically in tropical regions. While high Heat Index values are observed in some tropical regions, there is again little agreement between datasets on the timing of these extreme Heat Index events. Given these results, we highlight the need for locally-developed heatwave metrics for different parts of the world, and we urge against the use of a single heatwave definition in global studies. We also recommend that any studies assessing heat-health relationships in tropical regions beware of the lack of agreement between observational and reanalysis datasets, and compare results from different gridded dataset products to estimate uncertainty in heat-health relationships.