The global water cycle is becoming more intense due to climate change (Allan et al. 2020), which is also enhancing the differences between the wet and dry seasons and increasing the intensity of sub-seasonal precipitation while reducing its frequency (Konapala et al. 2020; Liu & Allan 2013; Schurer et al. 2020). Regional variations in atmospheric circulation, the tropical rain belt, and the timing of the seasons lengthen the dry season (Mamalakis et al. 2021). The total number of days during a dry spell that has daily precipitation amounts below a predetermined threshold (Agbazo et al. 2021) and is important to examine different features of drought periods (Lana et al. 2015; Wang et al. 2022). A dry spell is a period of unusually dry weather that is shorter than drought and not as severe (Mathugama & Peiris 2021). A dry spell is a stretch of three or more days during the wet season when there hasn't been any rain. A pentad dry period lasts for five days. (Sanchi et al. 2021). The frequency and severity of dry spells are rising, which is one effect of climate change in tropical areas (Sanchi et al. 2021). Dry spells are crucial for controlling the dynamics of soil moisture, terrestrial energy exchange, and vegetation development (Agbazo et al. 2021; Sanchi et al. 2021). Additionally, dry spells have an impact on the drainage basin's water quality, which could have an impact on society's health and hinder the production of energy in hydroelectric power plants (Whitworth et al. 2012; Mahbod et al. 2023). The greatest threat to food security in this region is the occurrence of dry spells and changing rainfall frequency and timing during the growing season which results in a lack of soil moisture, also fire risks (Zhang et al., 2018; Burton et al., 2021). A study on the regional and local-scale dry spell pattern is vital for the appraisal of hydrological results. In tropical regions, the success or failure of the crops, particularly under rainy conditions is largely connected to the distribution of dry spells. Understanding the distribution of dry periods throughout the course of a year helps get the most out of dry land agriculture. Dry spells have an impact on a variety of industries besides agriculture, including fishing, health, and electricity. In light of the aforementioned, the impacts of dry spells in various industries ultimately have a direct effect on a nation's economy (Mishra et al. 2011). The length of dry spells can be used to determine the best crop or variety for an area, as well as to breed varieties with different maturation times (Rajeev et al. 2022). In this regard, knowledge of wet and dry spell lengths is essential for managing water resources, for optimal planning, and for creating environmental and agricultural applications.
The South Asian summer monsoon season brings 85% of the subcontinent's yearly rainfall (Turner & Annamalai 2012; Fahad et al. 2022). In Bangladesh, the months of June and September saw 57.6% of the country's yearly rainfall (Monir et al. 2023a). The monsoon is crucial for the agricultural industry since more than 56% of the region's land is used for agriculture (Singh et al. 2014; Rawat et al. 2021). When prolonged dry periods occur in June and September during the crucial times for planting, soil preparation, and crop growth, the yields of monsoon crops are significantly reduced (Singh et al. 2014). Since 1951, the mean June-September rainfall has been dropping at a substantial (10% significance level) in the Indian subcontinent (Rawat et al. 2021; Verma et al. 2022). Also, in Bangladesh, rainfall is dropping by 75% during the monsoon season (Monir et al. 2023a). Since the 1970s, the worldwide mean dry spell length (DSL) has grown by 0.46 days each decade (He et al. 2022). Droughts are natural climatic catastrophes that occur in specific parts of the world during dry spells with minimal precipitation caused by a lot of ocean and local factors (Breinl et al., 2020; Faiz et al., 2020). El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), a planetary-scale ocean-atmosphere coupled system, have an impact on dry spell break conditions over the Indian subcontinent (Jha et al. 2016; Vengateswari et al. 2019; Vishnu et al. 2022). The summer monsoon's interannual variability is impacted by numerous forms of climatic variability, including an unusual seasonal sea surface temperature (SST) gradient, which is also related to a positive IOD phase (Zubair et al. 2003; Arrigo & Wilson 2008; Hussain et al. 2016). The amount of moisture carried by low-level winds from the western Indian Ocean determines the intensity of the summer monsoon rains in this area (Pathak et al. 2017). Due to the Himalayas, Karakorum, and Hindukush's (HKH) local geography, warm and moist monsoon air cannot be exchanged with cold and dry extratropical air, which determines how the summer monsoon precipitation is distributed over South Asia (Ashfaq 2020).
Several studies have been undertaken on the impacts of changing precipitation patterns in the Indian subcontinent, specifically in relation to climate change and its geographical distribution
(Ghosh et al. 2009; Rajeev et al. 2022). Few studies observed dry spell break patterns during the summer monsoon in this region (Singh et al. 2014; Rajeev et al. 2022; Ullah et al. 2023). These researches, however, didn't discuss the contributing variables to dry spell patterns. The ENSO-Monsoon relationship's unpredictability includes fluctuations in rainfall observed by Selvaraju (2003), Geethalakshmi et al. (2005), Arrigo and Wilson (2008), Kokilavani et al. (2015), Jha et al. (2016), Crétat et al. (2017), Yadav et al. (2018), Vengateswari et al. (2019), Terray et al. (2021), and Vishnu et al. (2022) in this region. Few researchers have studied the other factors north-south SST gradient of the Indian Ocean for the summer monsoon dry spell in the Indian subcontinent (Goswami et al. 2006; Roxy 2014; Noska & Misra 2016; Fahad et al. 2022; Weldeab et al. 2022; Zhang et al. 2023). Zubair et al. (2003), Arrigo and Wilson (2008), Cai et al. (2014), Jha et al. (2016), and Hussain et al. (2016) evaluated the impact of the formation of a positive Indian Ocean Dipole on summer monsoon rainfall variability. Land surface temperature (LST) (Garai et al. 2022), monsoon wind (Pathak et al. 2017), and topography (Ashfaq 2020; Fahad et al. 2022) were also observed for monsoon rainfall variability in this region.
Bangladesh is a disaster-prone country in South Asia. Almost every year, the country is hit by a natural disaster of some form, such as drought, resulting in significant losses in agricultural productivity (Dastagir, 2015; Thomas et al. 2023). The analysis of the dry spell during the monsoon season is crucial in Bangladesh since 56% of the land is used for agriculture during this period, and it also influences surface water storage and groundwater storage changes (Zhang et al., 2018; Rawat et al. 2021; Monir et al. 2023b). However, there has been no research on the pattern of dry spells, particularly in Bangladesh, despite some studies in this region (Singh et al. 2014; Rajeev et al. 2022; Ullah et al. 2023). Even though some study examines both ENSO and IOD for the variability of the summer monsoon rainfall in this region (Arrigo & Wilson 2008; Weldeab et al. 2022), no research has ever examined all of the ocean and land components together. Therefore, knowledge of the zonal long-term pattern of the summer monsoon dry spell and its relationship to ocean elements is essential for Bangladesh's agro-ecosystem sustainability. Such a research hasn't been conducted yet. So, to fill this research gap, the present novel study aims to analyze the long-term (1985–2022) trend of the dry spell with rainfall occurrences and changes during the summer monsoon in different climate zones of Bangladesh. To understand the variability of dry periods, this study additionally considers ocean elements IOD, SST, ENSO, and monsoon wind. This study also looked at the surface water changes due to the changing pattern of dry spells. This study used the Mann-Kendall trend test to detect dry spell trends and the Frontier Atmospheric General Circulation model to evaluate a relationship between monsoon rainfall variability and ocean and land factors.