The mechanisms of the increased precipitation seasonality in Amazon

The Amazon precipitation seasonality increases from 1979 to 2005 in observation. This change may result from three components: nature variabilities, local land use changes, and global warming. First of all, the Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO) are two of the main nature variabilities, which affect the long-term climate significantly in Amazon. Both of them switching to positive or negative phase can have different effects on precipitation in Amazon. Besides, deforestation over Amazon also causes local climate change, including the increase of precipitation in the wet season. As for global warming, the atmospheric CO2 concentration rises and can lead to precipitation changes by radiative forcing and biophysical processes. However, recent studies mainly focus on the individual component for how it influences precipitation seasonality. This study uses Community Earth System Model (CESM) to separate each element affecting precipitation seasonality in Amazon by removing AMO/PDO signals and fixing the land type and CO2 concentration after 1970. The result shows that precipitation seasonality trend over Amazon in experimental runs is lower than that in control run, especially in the removing AMO run, which shows negative precipitation seasonality trend. It indicates that each component results in wet gets wetter, dry gets drier; furthermore, nature variabilities contribute the most to the seasonality increasing.