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The South Asian summer monsoon strongly modulates regional temperature and humidity. While extreme dry heat peaks in the pre‐monsoon season, recent literature suggests that extreme humid heat can continue to build throughout the monsoon season. Here we explore the influence of monsoon onset and subseasonal precipitation variability on the occurrence of extreme wet bulb temperatures (Tw) across South Asia. We find that extreme Tw events often occur on rainy days during the monsoon season. However, the influence of precipitation on Tw varies with the background climatology of surface specific humidity. In climatologically drier areas, positive Tw anomalies tend to occur when precipitation increases due to either early onset or wet spells during the monsoon. In contrast, in climatologically humid areas, positive Tw anomalies occur during periods of suppressed precipitation, including both delayed onset and dry spells during the monsoon. Plain Language Summary: The combination of high heat and humidity poses greater risks to human health, productivity, and well‐being relative to elevated temperatures alone. South Asia has already experienced some of the most extreme humid heat observed on Earth. Typically, the highest temperatures in South Asia occur during the pre‐monsoon season in March‐May. While the precipitation and cloudiness associated with the summer monsoon reduce extreme air temperatures, the increased humidity can contribute to the occurrence of extreme humid heat events. Our research finds that a majority of extreme Tw events across South Asia happen on rainy days during the monsoon season. This is especially true for regions where surface humidity is typically low during the monsoon season, and Tw tends to be higher during heavier than usual precipitation spells and during years when the monsoon season starts earlier in the calendar year. In regions with high background surface humidity, however, Tw is more intense when there is less precipitation than usual or when the monsoon onset is delayed. These results help to identify additional times with high risk of heat stress for outdoor workers, and provide a new way of understanding how monsoon variations influence local humid heat. Key Points: In contrast to dry heat, most extreme wet bulb temperature events in South Asia take place on rainy days during the monsoon seasonLocal background specific humidity determines whether wet bulb temperatures are intensified by enhanced or suppressed precipitationBaseline moisture availability also influences whether early or delayed monsoon onset intensifies wet bulb temperature anomalies [ABSTRACT FROM AUTHOR]

Increasing population, urbanization, and associated demand for food production compounded by climate change and variability have important implications for the managed ecosystems and water resources of a region. This is particularly true for south Asia, which supports one quarter of the global population, half of whom live below the poverty line. This region is largely dependent on monsoon precipitation for water. Given the limited resources of the developing countries in this region, the objective of our study was to empirically explore climate change in south Asia up to the year 2099 using monthly simulations from 35 global climate models (GCMs) participating in the fifth phase of the Climate Model Intercomparison Project (CMIP5) for two future emission scenarios (representative concentration pathways RCP4.5 and RCP8.5) and provide a wide range of potential climate change outcomes. This was carried out using a threestep procedure: calculating the mean annual, monsoon, and non-monsoon precipitation and temperatures; estimating the percent change from historical conditions; and developing scenario funnels and synthetic scenarios. This methodology was applied for the entire south Asia region; however, the percent change information generated at 1.5° grid scale can be used to generate scenarios at finer spatial scales. Our results showed a high variability in the future change in precipitation (-23% to 52%, maximum in the non-monsoon season) and temperature (0.8% to 2.1%) in the region. Temperatures in the region consistently increased, especially in the Himalayan region, which could have impacts including a faster retreat of glaciers and increased floods. It could also change rivers from perennial to seasonal, leading to significant challenges in water management. Increasing temperatures could further stress groundwater reservoirs, leading to withdrawal rates that become even more unsustainable. The high precipitation variability (with higher propensity for localized intense rainfall events) observed in the region can be a key factor for managed ecosystems and water management and could also lead to more incidence of severe urban flooding. The results could be used to assess both mitigation and adaptation alternatives to reduce vulnerabilities in managed ecosystems (agricultural and urban) and water resources. [ABSTRACT FROM AUTHOR]