The tropical tropopause is important for understanding the future state of our climate system because tropospheric air, which includes water vapor and other trace gases, enters the stratosphere preferentially through this layer. It has long been recognized that the upper troposphere in the western Pacific is the coldest longitudinally. This upper tropospheric cold region has links to tropical deep convection because the western Pacific is a frequent deep convection area. However, the mechanism of TTL cooling, in relation to tropical deep convection, has been debated and there are several hypotheses. Additionally, one of most important unsolved questions is the water vapor transport process from the troposphere to the stratosphere, especially because the analysis of recent decadal records shows that water vapor in the stratosphere is increasing. Water vapor is a greenhouse gas, and long-lived stratospheric water vapor can affect the global energy budget, and it has been shown that increasing water vapor in the stratosphere cools the stratosphere but heats the troposphere. Temperature and water vapor variations due to clouds in the TTL are investigated using co-located MLS, CALISPO, and CloudSat data. Convective cooling occurs only up to cloud top heights, with warming above these heights in the TTL. Water vapor and ozone anomalies above cloud top heights support that warming anomalies occur due to downward motion. Thicker clouds cause a greater magnitude of the temperature anomalies. The cooling of cirrus clouds indicates that the radiative heating contributes only to the reduction of the magnitude of the cooling in thin clouds. Environmental water vapor below cloud tops can either increase or decrease, depending on the cloud top height. The critical factor, which divides these different water vapor variations below cloud tops, is the relative humidity. Clouds hydrate the environment below 16 km, where the air after mixing between cloud and the environmental air does not reach saturation, but clouds dehydrate above 16 km, due to the supersaturation because of the larger temperature drop and the high initial relative humidity. Water vapor above cloud tops has negative anomalies compared to clear skies and suggests another dehydration mechanism.
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