Influence of El Niño-Southern Oscillation (ENSO) on long term trends of heat waves over Bangladesh

The El Niño Southern Oscillation (ENSO) is a periodic fluctuation in both winds and sea surface temperatures across the tropical eastern Pacific Ocean. This phenomenon has a substantial impact on the climate in a wide expanse of the tropics and subtropics and consequently influences the summer-time temperatures during March to May in Bangladesh. This study aims to investigate the impact of extreme phases such as El Niño and La Niña, of El Niño-Southern Oscillation on heatwave occurrences in Bangladesh. The country is divided into eight distinct climate zones, and the investigation at hand underscores the significant correlation between heat waves and El Niño occurrences over Bangladesh during the study period. To understand the effect of ENSO, the study analyzed various aspects of heatwave events, such as frequency, duration, and magnitude by calculating yearly number of HW events (HWN), yearly sum of HW days (HWF), length of the longest yearly event (HWD), the average length of all yearly events (HWL), hottest day of the hottest yearly event (HWA) and the average magnitude of all yearly events (HWM). For this analysis, maximum daily temperature records from 34 observation stations were used during the summer season (March to May or MAM) from 1991 to 2023. Heatwave events were identified using the 90th and 75th percentiles of the distribution of daily maximum temperature values for the reference period. An appreciable increase (decrease) in the number of HW days during El-Niño (La-Niña) events was observed in the study. Notably, severe Heat waves were more prominent (longest and hottest) during El-Niño years. The South-west and North-west regions are identified as the maximum temperature regions based on both the magnitude and frequency of heat wave events. The observed escalation in the occurrence of heat wave events is proportionate to the El Niño events that occur during these periods of extreme temperatures. The increasing warming rate demonstrates that more frequent El Nino events in the future may further lead to more frequent and longer-lasting heat waves over Bangladesh.