资源环境科技发展态势分析平台

Accurately simulating the Indo-Asian monsoon (IAM) using atmospheric general circulation models (AGCMs) is challenging but crucial. This study uses reanalysis products European Centre of Medium-Range Forecast Interim reanalysis, Japanese Reanalysis year 55, and High Asia Reanalysis to highlight an easterly, low-level barrier jet along the Indo-Gangetic Plain (referred from here as IG LLJ), which we identify as the primary moisture transport mechanism for the northeastern branch of the IAM. We show that the NCAR family of AGCMs (Community Atmospheric Model (CAM)) does not capture this circulation until 1/2 degrees or greater spatial horizontal resolution is used. The IG LLJ develops due to a persistent low-pressure system centered over the Ganges basin and is enhanced by the Himalayas. Using diabatic heating rates and the moist Froude number as diagnostics, we find that in CAM, this branch of the IAM displays two different dynamical regimes as a function of resolution. At low resolution, the atmosphere near the Himalayas is statically unstable, diabatic heating is strong, and the moisture flow is southwesterly from the Arabian Sea and moves over the terrain (unblocked). At high resolution, the moist static stability near the Himalayan Mountains is stable, diabatic heating is weak, and the flow primarily enters easterly from the Bay of Bengal and moves parallel to the terrain (blocked). During the summer season, the low-resolution CAM is locked into the unblocked mode, which has serious implications for interpreting topography-monsoon interactions. For a broader context, we demonstrate that more than half of the CMIP5 models do not capture the IG LLJ, which further highlights model-data mismatch across the IAM region.