South Asian Summer Monsoon Rainfall: Origin of Multidecadal Variability/Trends and Investigation of Seasonal Predictability

印度恒河平原是10亿人口的家园，是南亚的农业中心地带。人口和经济的快速增长导致了土地利用、土地覆盖、气溶胶负荷和温室气体排放的显著变化。区域水文气候显示出相当大的--有时是不稳定的--数十年的趋势：例如，恒河平原的夏季降雨量自20世纪50年代以来下降了10%-15%。此外，季节性降水很少被准确预测；尽管在物理过程的表示、数值模式分辨率和数据同化方面取得了进展，但预测技术仍然有限和停滞不前。该项目有以下目标：(A)调查1950年后降雨量的急剧下降是人为影响还是数十年自然变率造成的，还是两者兼而有之。通过揭示长期变化和自然变异性之间干扰的本质，这种归因分析将有助于估计未来几十年降雨量更急剧下降或某种程度上缓解干旱的可能性。模式分析将揭示数十年海洋变率影响南亚季风降水的机制。(B)通过对20-21世纪海温和降雨观测的辨别时空分析，以及根据同期和海温主导的一个独立时期的降雨量回归重建降雨，确定南亚夏季季节性降雨的潜在可预报性--从而对不同季节的季风降雨量基于海温的可预报性进行定量评估。自然和历史模拟中关键的SST-季风降水联系的相互比较将推进在两个耦合气候模式中对季风水气候变率的模拟，即。共同体地球系统模型(CESM)和国家海洋和大气管理局地球物理流体动力学实验室(NOAA GFDL)模型。

The Indo-Gangetic Plain is home to a billion people and is the agricultural heartland of South Asia. Rapid population and economic growth have led to significant land-use land-cover change, aerosol loadings, and greenhouse gas emissions. Regional hydroclimate exhibits substantial - at times, precarious - multidecadal trends: For example, summer rainfall over the Gangetic Plain has declined by 10-15% since the 1950s. The seasonal rainfall is moreover seldom predicted accurately; prediction skill has remained modest and stagnant despite advances in the representation of physical processes, numerical model resolution, and data assimilation.This project has the following goals:(A) To investigate if the steep decline in post-1950s rainfall is from anthropogenic effects, multidecadal natural variability, or both. By revealing the nature of interference between secular change and natural variability, this attribution analysis will help estimate the likelihood of even steeper rainfall decline or some reprieve from drying in future decades. The modeling analysis will uncover the mechanisms through which multidecadal ocean variability influences the South Asian monsoon rainfall. (B) To determine the potential predictability of seasonal summer rainfall over South Asia from a discerning spatiotemporal analysis of 20th-21st century SST and rainfall observations, and rainfall reconstruction from contemporaneous and SST-leading rainfall regressions in an independent period - yielding a quantitative assessment of the SST-based predictability of monsoon rainfall at various seasonal leads. Intercomparison of the key SST-monsoon rainfall links in nature and historical simulations will advance the modeling of monsoon hydroclimate variability in two coupled climate models, viz. the Community Earth System Model(CESM) and the National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Lab (NOAA GFDL) model.