Relationships between the Southwest Monsoon Surge and the Heavy Rainfall Associated with Landfalling Super Typhoon Rammasun

Based on the China Meteorological Administration (CMA) best-track data, the ERA5 reanalysis, and the Global Precipitation Measurement (GPM) precipitation data, this paper analyzes the reasons for the heavy rainfall event of Super Typhoon Rammasun in 2014, and the results are as follows: (1) Rammasun was blocked by the western Pacific subtropical high (WPSH), the continental high, and the mid-latitude westerly trough. Such a stable circulation pattern maintained the vortex circulation of Rammasun. (2) During the period of landfall, the southwest summer monsoon surge was reinforced due to the dramatic increase of the zonal wind and the cross-equatorial flow near 108° E. The results of the dynamic monsoon surge index (DMSI) and boundary water vapor budget (BWVB) show that the monsoon surge kept providing abundant water vapor for Rammasun, which led to the enhanced rainfall. (3) The East Asian monsoon manifested an obvious low-frequency oscillation with a main period of 20–40 days in the summer of 2014, which propagated northward significantly. When the low-frequency oscillation reached the extremely active phase, the monsoon surge hit the maximum and influenced the circulation of Rammasun. Meanwhile, the convergence and water vapor flux associated with the low-frequency oscillation significantly contributed to the heavy rainfall.

East Asian Monsoon Forcing And North Atlantic Subtropical High Modulation of Summer Great Plains Low-Level jet

Dynamic influences on summertime seasonal United States rainfall variability are not well understood. A major cause of moisture transport is the Great Plains low-level jet (LLJ). Using observations and a dry atmospheric general circulation model, this study explored the distinct and combined impacts of two prominent atmospheric teleconnections – the East Asian monsoon (EAM) and North Atlantic subtropical high (NASH) – on the Great Plains LLJ in the summer. Separately, a strong EAM and strong western NASH are linked to a strengthened LLJ and positive rainfall anomalies in the Plains/Midwest. Overall, NASH variability is more important for considering the LLJ impacts, but strong EAM events amplify western NASH-related Great Plains LLJ strengthening and associated rainfall signals. This occurs when the EAM-forced Rossby wave pattern over North America constructively interferes with low-level wind field, providing upper-level support for the LLJ and increasing mid- to upper-level divergence.

Assessing the Reliability of Global Monsoon Low Pressure System Track Datasets for the East Asian Monsoon

Limited by the lack of atmospheric observation data over the ocean and the absence of a comprehensive set of track data for monsoon low pressure systems (MLPSs), an in-depth understanding of the activity of East Asian MLPSs has not been acquired. In recent years, advancements in satellite remote sensing and data assimilation techniques have enabled the creation of numerous high-resolution global reanalysis datasets. Additionally, with the improvement of tracking algorithms, two sets of global MLPS track data (HB2015 and VB2020) have been published. This study seeks to understand the fidelity of the two datasets with respect to the East Asian monsoon. The genesis location, movement path, and three-dimensional structure of the East Asian MLPSs obtained using HB2015 and VB2020 are compared, and the atmospheric circulation conditions of typical MLPSs are analyzed. The results show that both datasets are able to generate MLPSs with identical structure for the East Asian Monsoon, and they provide similar results in terms of the location and monthly frequency. Compared to the HB2015, the VB2020 adopts a more stringent set of thresholds for the determination of the MLPS genesis and extinction and a more rigorous tracking algorithm. Therefore, it yields a lower count of MLPSs with significantly shorter lifetimes. However, the MLPSs identified by the VB2020 all have cyclonic circulations in the proximity of their central areas as they continue their movement. In this sense, the results generated by the VB2020 are more consistent with the observed MLPSs and hence are more reliable. However, the tracking can end prematurely with this dataset.

Temporal variations of stable isotopes in precipitation from Yungui Plateau: Insights from moisture source and rainout effect

Long-term continuous monitoring of precipitation isotopes has great potential to advance our understanding of hydrometeorological processes that determine stable isotope variability in the monsoon regions. This study presents 4–year daily precipitation isotopes from Yungui Plateau in southwestern China that is influenced by Indian summer monsoon and East Asian monsoon. The local meteoric water line (LMWL, δ2H=8.12 δ18O+11.2) was firstly established at the Tengchong (TC) site, which was close to the global meteoric water line (GMWL, δ2H=8 δ18O+10) indicating little secondary sub–cloud evaporation in the falling rain. Precipitation δ18O values exhibited significant inverse relationships with precipitation amount (r = −0.42), air temperature (r = −0.43), and relative humidity (r = −0.41) with lower correlation coefficients throughout the entire period, which indicated that precipitation isotopic variability in TC could not be well explained by the local meteorological factors but influenced by other combined factors of regional precipitation amount and upstream rainout. Precipitation δ18O values showed a clear V–shaped trend throughout the observation period, characterized by higher δ18O values during the pre–monsoon period whereas lower values during the post–monsoon period. This seasonal variation of precipitation δ18O values was associated with the seasonal movement of the Intertropical convergence zone and seasonal changes in moisture transport. Combined with backward trajectory analysis, precipitation δ18O values were estimated by a Rayleigh distillation model showing that upstream rainout processes from Bay of Bengal (BoB) towards land (Myanmar), and recycling moisture over land were key factors affecting the isotopic compositions of the TC precipitation. These findings could enhance our understanding of atmospheric dynamics and moisture source in the monsoon regions and will potentially facilitate the interpretation of numerous isotopic proxy records from this region.

Monthly insolation linked to the time-transgressive nature of the Holocene East Asian monsoon precipitation maximum

More than 10% of the world’s population lives in the East Asian monsoon (EAM) region, where precipitation patterns are critical to agricultural and industrial activities. However, the dominant forcing mechanisms driving spatiotemporal changes in the EAM remain unclear. We selected Holocene records tracking monsoon precipitation in the EAM region reconstructed from pollen data to explore the spatiotemporal patterns of monsoon precipitation changes. Our analysis shows a time-transgressive pattern of maximum precipitation, with earlier occurrence in the southern area and later occurrence in the northern area. The monthly insolation changes force monsoon precipitation in different parts of the EAM region through a shift in the Western Pacific Subtropical High. We conclude that low-latitude monthly insolation changes (rather than average summer insolation changes) were the main forcing mechanisms of the spatiotemporal patterns of the monsoon precipitation maximum during the Holocene.