Multi-scale Temporal-spatial Variability of the East Asian Summer Monsoon Stationary Frontal System in the observation and GFDL HiRAM

A suite of objective indices based upon daily data are developed to characterize the multi-scale temporospatial variability of the East Asian summer monsoon stationary fronts (MSF)-the Meiyu-Baiu-Changma front, which include the intensity, location, and stationarity indices. All these indices are characterized by strong interannual to interdecadal variations. Specifically, the MSF has shown a southwestward tendency since the mid-1990s and becomes more stationary over southern China with reduced intraseasonal meandering. Simulation of MSF by the High-Resolution Atmospheric Model (HiRAM) from the Geophysical Fluid Dynamics Laboratory with a resolution of 0.25°×0.3125° is also examined, which is capable to reproduce the seasonal mean precipitation pattern and MSF location and the seasonal march of MSF. However, wet biases exist over northern and eastern China and northern Japan, while dry biases expand from southern China to the western North Pacific, along with the more northward seasonal mean location of MSF in the model, especially in May. The simulated MSF is stronger, located more northwestward, and more stationary with weaker interannual variations. And HiRAM is better at reproducing interdecadal variations of MSF locations rather than that of MSF intensity. Due to lack of atmosphere-ocean coupling in the model, enhanced land-sea sea level pressure (SLP) gradient over East Asia and the westward expansion of the western North Pacific subtropical high in the tropics are involved in the more northward location of MSF, while zonal locations of MSF are mainly modulated by the north-to-south circulations, other than the northwest-to-southeast circulations in the observation. The simulated annular pattern of SLP with anomalous highs in the lower latitudes should be the root cause for the more northwestward shift of MSF, as more air masses over lower latitudes are easily intruding poleward, pushing MSF more northwestward.