Multiscale Spatiotemporal Variability of the East Asian Summer Monsoon Stationary Frontal System: Observation versus Its Representation in the GFDL HiRAM

The Meiyu-Baiu front (MBF) is the primary weather system modulating the monsoonal rainfall in East Asia, and it exhibits pronounced variability across multiple temporal and spatial scales. A suite of new objective indices based upon daily reanalysis data are developed to characterize such variability, which include an “intensity” index effectively measuring the meridional gradient of equivalent potential temperature across the frontal zone, a “location” index documenting the central latitude and longitude of the front, and a “stationarity” index corresponding to the standard deviation of the daily central latitude. All these indices are characterized by strong interannual to interdecadal variations. Specifically, the MBF has shown a tendency of southwestward shift since the mid-1990s and become more stationary over southern China with reduced intraseasonal meandering. The western North Pacific subtropical high (WNPSH) is the main circulation feature affecting the location of the MBF on interannual timescales. On interdecadal timescales, enhanced activity of high latitude blocking over East Asia is often accompanied with a stronger but less stationary MBF while a negative phase of the Arctic Oscillation favors a southward-shifted and more stationary MBF.

This study further examines the representation of the multi-scale temporospatial variability of MBF in the High-Resolution Atmospheric Model (HiRAM) of the NOAA Geophysical Fluid Dynamics Laboratory (GFDL). Compared with the observed variability of the MBF in the ERA-Interim, HiRAM reproduces reasonably well the seasonal mean precipitation pattern and the seasonal migration of MBF. However, wet biases are found over the northern and eastern China and northern Japan, and dry biases extend from the southern China to the western North Pacific. These rainfall biases are directly tied to a northwestward bias in the model simulated seasonal mean location of MBF and this location bias is most pronounced in the month of May. In general, the MBF in HiRAM is more intense, located more northwestward, and more stationary with weaker interannual variations compared to the observed. A pronounced positive bias in the ocean-land surface pressure contrast over East Asia, largely manifested as the westward expansion of WNPSH, is hypothesized to be the main cause of the northwestward location bias of MBF in HiRAM. This bias in surface pressure contrast likely results from the missing of realistic air-sea interactions in the HiRAM simulations.