The zonal oscillation of the western Pacific subtropical high (WPSH) significantly influences the weather and climate over East Asia. This study investigates characteristics and mechanisms of the zonal variability of the WPSH on subseasonal time scales during summer by using a subseasonal WPSH (Sub-WPSH) index. Accompanied with the Sub-WPSH index, strong anticyclonic (cyclonic) anomalies are found over East Asia and coastal region south of 30°N on both 850 hPa and 500 hPa. During the positive period of the Sub-WPSH index, the WPSH extends more westward with enhanced precipitation over the Yangtze–Huaihe river basin and suppressed precipitation over the south of the Yangtze River in China. These precipitation anomalies can last for at least 1 week. While the subseasonal zonal variability of the WPSH is found to be closely associated with atmospheric teleconnections and local air- sea interaction, the mechanisms of the variability are different before and after mid-July (early and late summer). In both early and late summer, the East Asia/Pacific (EAP) wave train pattern affects the zonal shift of the WPSH by inducing a low-level anomalous anticyclonic/cyclonic circulation over the subtropical western Pacific, and this mechanism is stronger in late summer. In constrast, the influence of the Silk-Road pattern wave train is more important in the early summer. Meanwhile, in late summer, a stronger SST forcing on the atmosphere and a faster cycle of subseasonal variations of the WPSH are observed before the westward stretch of the WPSH, which could be related to the colder local SST anomalies. The westward stretch of the WPSH is accompanied by stronger anticyclonic anomalies in late summer.

One long-standing issue in the paleoclimate records is whether East Asian Summer Monsoon peaked in the early Holocene or mid-Holocene. Here, combining a set of transient earth system model simulations with proxy records, we propose that, over northern China, monsoon rainfall peaked in the early Holocene, while soil moisture and tree cover peaked in the mid-Holocene. The delayed ecosystem (soil moisture and tree cover) response to rainfall is caused by the vegetation response to winter warming and the subsequent feedback with soil moisture. Our study provides a mechanism for reconciling different evolution behaviors of monsoon proxy records; it sheds light on the driving mechanism of the monsoon evolution and monsoon-ecosystem feedback over northern China, with implications to climate changes in other high climate sensitivity regions over the globe.

Using the Weather Research and Forecasting model, numerical experiments with reconstructed ideal zonally uniform sea surface temperatures were designed to discuss the midlatitude upper‐level zonal wind response to the change of subtropical front strength over the North Pacific in winter. The results show that the enhancement of the subtropical frontal zone (STFZ) of the North Pacific leads to the stronger upper zonal wind, without considering the lateral boundary barotropic and baroclinic disturbances in this regional model. The enhanced STFZ first results in positive meridional temperature gradient anomalies by affecting the sea surface sensible heat flux. Then the low‐level wind increases because of the increased thermal wind. In addition, the temperature gradient anomalies spread to middle and upper troposphere by baroclinic waves, causing the increase of Eady growth rate and enhanced storm tracks on the southern side of the STFZ and the decrease on the northern side. The strengthened storm tracks give rise to a stronger midlatitude westerly jet accompanied with a southward movement through wave‐mean flow interaction, including both barotropical energy conversion and the Rossby wave breaking.

To understand the individual influences of the land cover, sea temperature, sea ice and carbon dioxide concentration on the global climate, sensitive experiments using the General Atmospheric Circulation Model 4.0 are designed to compare with the observation in this study. Firstly, through the analysis of Liang–Kleeman information flow method, it is straightforward that the pronounced causal relationships exist from these forcing to air temperature. In numerical experiments, the temperature is influenced by the albedo and atmospheric dynamic process. More detailed, in winter, the changes of each forcing will cause the positive Pacific-North American Pattern (PNA) phase, which makes the North American colder. The negative North Atlantic Oscillation (NAO) phase caused by the changes of CO₂ and sea ice induces cold winter over the Europe. This coincides with the extreme cold weather in Europe and North America in 2018. Whereas in summer, all forcings cause positive Arctic Oscillation (AO) phase, resulting in the most northern hemisphere warmer. It is noteworthy that for precipitation, the changes of each forcing increase winds from the sea surface to the land in East Asia, so the precipitable water increases, thus the precipitation overall increases. However, when CO₂ changes, the precipitation decreases due to the lack of dynamic conditions in some areas.

The atmospheric responses to the intensity variability of subtropical front zone (STFZ) in the wintertime North Pacific are investigated in both observation and sensitivity experiments using weather research and forecasting model (version 3.4.1). The STFZ is artificially intensified by applying a sea surface temperature (SST) anomaly within STFZ in sensitivity experiments. Results from observation and experiments indicate that the enhanced STFZ strengthens the meridional temperature gradient and thus the atmospheric baroclinicity, inducing more active transient eddies. The stronger transient eddy vorticity forcing accelerates the polar front jet with equivalent barotropic structure. The results of sensitivity experiments suggest the atmospheric responses are linear and symmetrical to the variation of STFZ intensity within its natural variability. However, the anomalies of atmospheric upstream condition and large-scale SST also have great impacts on the midlatitude atmosphere over North Pacific, in comparison with the effect of STFZ intensity variability. Therefore, It suggests carefully verification of the atmospheric responses found in the observations and numerical experiments.