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    Atmospheric and hydrospheric sciences

    Response of the atmospheric hydrological cycle over the tropical Asian monsoon regions to anthropogenic aerosols and its seasonality

    Takahashi H, Watanabe S, Nakata M, Takemura T

    Atmospheric hydrological cycle, Asian monsoon, Aerosol, Cloud, Precipitation

    a Latitude-vertical (in pressure) cross-section in meridional and vertical winds over the Asian monsoon region (60° E–110° E) during JJA. b Same as Fig. 5a, but over the Pacific Ocean (120° E–150° W) during JJA. c Longitude-vertical (in pressure) cross-section in zonal and vertical winds along the equator (10° S–10° N) during JJA. As we used ω, we multiplied ω by − 1 for vectors to understand vertical motions. The plotted vectors are statistically significant for horizontal or vertical components, or both. The red (blue) color indicates ascending (descending) motion. The unit of zonal and meridional winds is m s −1. The unit of ω is 10 −4 Pa s −1. The plotted colors are statistically significant for the vertical component

    This study investigates the impact of anthropogenic aerosols on the atmospheric hydrological cycle over the tropical Asian monsoon region (South Asian, Southeast Asian, and western North Pacific monsoons), using a coupled atmosphere-ocean global climate model (CGCM), Model for Interdisciplinary Research on Climate-Earth System Models. Three-ensemble historical (HIST) and sensitivity (piAERO) experiments for the period 1985–2005 are conducted. The piAERO experiment is the same as HIST, but with anthropogenic aerosol emissions kept at preindustrial values. The results show that, as a whole, the Asian monsoon precipitation is reduced by the increase in aerosol loading during boreal summer and winter. This decrease in precipitation corresponds to a decrease in precipitable water due to the cooling in surface air temperature (SAT), mainly over adjacent oceans. The cooling is explained by the sum of the direct and indirect effects of aerosols. A modulation of the Walker circulation occurs, which can be explained by the east-west horizontal SAT gradient over the tropics due to the spatially heterogeneous increase in aerosols. Concurrent with the modulation of the Walker circulation, the anomalous descending motions over the tropical Asian monsoon region are consistent with the decrease in precipitation. In addition, the changes in local Hadley circulation (or a shift of the inter-tropical convergence zone) are unclear over the Asian monsoon region and thus cannot explain the decrease in precipitation. Moreover, the detailed spatial pattern of the response of the atmospheric hydrological cycle over the Asian monsoon region has distinct seasonality. Interestingly, signals are distinct in regions where tropical disturbance activity is vigorous during both boreal summer and winter. However, uncertainties regarding aerosol-cloud-precipitation interactions in current climate models and internal variability in the climate models may have affected the results.