Decoupling of Temperature and Water Vapor in Terrestrial Monsoon Regions Due To Decreasing Ocean Evaporation At Low Latitudes

Abstract

The relationship between air temperature and atmospheric water vapor is important for understanding the changes in the climate system as well as water, energy, and carbon cycles at the global scale. We investigated the changes in atmospheric actual water vapor (AVP) at seasonal and spatial scales from 1979 to 2018 and their sensitivity to air temperature (S_avp). Comparing two different periods, 1979–1998 and 1999–2018, large differences in S_avp were found over land during summer. The west coasts of low-latitude regions gained enhanced water vapor for each 1°C increase in temperature, while the east coasts showed a deceleration in additional water vapor for each 1°C increase in temperature in the latter period compared to the former one. Although there is spatial heterogeneity in changes in Savp after the 1990s compared to before the 1990s, overall summer Savp has decreased by 33% over land and 27% in the monsoon region. These shifts were well reproduced by the Community Earth System Model and are closely related to a shift in low-latitude ocean evaporation (E_ocean) from an upward trend before the late 1990s to a downward trend after the late 1990s (p < 0.05). Furthermore, we found that the significant decrease (p < 0.05) in S_avp over land in summer was due to (a) weaker increase in actual atmospheric water vapor and (b) reduced zonal winds from oceans to the land, rather than through the direct effects of global warming on the atmospheric water demand. These results highlight the urgent need for E_ocean monitoring and forecasting to accurately assess land S_avp. Significance statements understanding how water vapor responds to changes in temperature is crucial for understanding global climate dynamics and the movement of water, energy, and carbon throughout the Earth system. We investigated the variations in water vapor temperature sensitivity across different seasons and regions from 1979 to 2018. By comparing two time periods, we found great differences in water vapor temperature sensitivity over land during summer. Overall, there was a weakening of water vapor temperature sensitivity over land and in monsoon regions. These changes were predicted by a climate model and correlated closely with changes in ocean evaporation patterns. This highlights the importance of closely monitoring ocean evaporation to predict the response of water vapor to warming over land.