Enhanced tropospheric biennial oscillation of the East Asian summer monsoon since the late-1970s

The tropospheric biennial oscillation (TBO) of East Asian summer monsoon (EASM) has major impacts on East Asian climate. Here it is shown that, since the late-1970s, the TBO signal of EASM has strengthened significantly. The EASM TBO in wind anomalies undergoes a transition from a cyclone over the western North Pacific (WNPC) in preceding summer to an anticyclone over the western North Pacific (WNPAC) in following summer, with the anomalies strengthening remarkably after the late-1970s. Correspondingly, the biennial component of precipitation anomalies in eastern China show different distributions. Both observational and numerical simulation analyses demonstrate that these changes are caused by the westward shift of El Niño warming and enhanced Indo-Pacific and Atlantic-Pacific coupling. The positive sea surface temperature (SST) anomalies associated with the TBO of EASM shift toward the central Pacific after the late-1970s, which favor the strengthening of the WNPC and cause a weakened EASM. In following summer, both the north Indian Ocean and tropical north Atlantic SST warming are closely coupled with El Niño since the late-1970s, which favor the strengthening of WNPAC and cause an intensified EASM. Together, these changes provide more favorable background state for the transition of circulation anomalies over the western North Pacific, giving rise to enhanced biennial variability in EASM in the late-1970s.

Tropical volcanism enhanced the East Asian summer monsoon during the last millennium

Extreme East Asian summer monsoon (EASM) rainfall frequently induces floods that pose threats to millions of people across East Asia. The intensified EASM rainfall has been generally attributed to internal modes of climate variability, while external volcanic forcing has been suggested to suppress the EASM. In contrast to the hydrological weakening theory of volcanic eruptions, we present convergent empirical and modeling evidence for significant intensification of EASM rainfall in response to strong tropical volcanic eruptions. Our paleoclimate proxy analyses show a significantly increased EASM in the first summer after large tropical eruptions from 1470 AD to the present. The multi-proxy ensemble mean demonstrates that the occurrence of an El Niño in the first boreal winter after a volcanic eruption is necessary for the enhanced EASM. The results from the last-millennium climate model simulations show that a volcano-induced El Niño and the associated warm pool air-sea interaction intensify EASM precipitation, overwhelming volcanic-induced moisture deficiency. This work offers a new perspective on the intertwined relationship between external forcing and internal variability in the complex climate system and potential flood disasters resulting from tropical volcanic eruption.

East Asian summer monsoon enhanced by COVID-19

Anthropogenic emissions decreased dramatically during the COVID-19 pandemic, but its possible effect on monsoon is unclear. Based on coupled models participating in the COVID Model Intercomparison Project (COVID-MIP), we show modeling evidence that the East Asian summer monsoon (EASM) is enhanced in terms of both precipitation and circulation, and the amplitude of the forced response reaches about 1/3 of the standard deviation for interannual variability. The response of EASM to COVID-19 is consistent with the response to the removal of all anthropogenic aerosols simulated by atmospheric component models, which confirms the dominant role of the fast response to reduced aerosols. The observational evidence, i.e., the anomalously strong EASM observed in 2020 and 2021, also supports the simulated enhancement of EASM. The essential mechanism for the enhanced EASM in response to COVID-19 is the enhanced zonal thermal contrast between Asian continent and the western North Pacific in the troposphere, particularly at the upper troposphere, due to the reduced aerosol concentration over Asian continent and the associated latent heating feedback. As the enhancement of EASM is a fast response to the reduction in aerosols, the effect of COVID-19 on EASM dampens soon after the rebound of emissions based on the models participating in COVID-MIP.

Rapid Northwestward Extension of the East Asian Summer Monsoon Since the Last Deglaciation: Evidence From the Mollusk Record

The magnitude and rate of the expansion of the East Asian summer monsoon (EASM) rain belt under future climatic warming are unclear. Appropriate ecological proxy data may provide an improved understanding of the spatial extension of the EASM during past warming intervals. We reconstructed the spatiotemporal pattern of the extension of the EASM since the Last Glacial Maximum (LGM), using six well-dated mollusk fossil sequences from Chinese loess sections located on the modern northern edge of the EASM. The abundance of typical dominant mollusk species indicative of EASM intensity shows a delayed response, from ∼17 ka in the southeastern sections to ∼9 ka in the northwestern sections, during the last deglacial warming. Isoline plots based on a mollusk data synthesis show that the mollusk EASM indicators have a northeast–southwest zonal distribution for both the present-day, the cold LGM, and the warm mid-Holocene, which is consistent with the spatial pattern of modern precipitation. The resulting estimated expansion rate of EASM intensity accelerated during ∼12–9 ka (∼50 km/ka), which corresponds to the early Holocene interval of rapid climatic warming, a northwestward shift of ∼150 km compared to today. This implies that the northern fringe of the EASM in northern China will become wetter in the coming century, under moderate warming scenarios.

Using deep learning to predict the East Asian summer monsoon

Accurate prediction of the East Asian summer monsoon (EASM) is beneficial to billions of people’s production and lives. Here convolutional neural networks (CNN) and transfer learning are used for predicting the EASM. The results of the constructed CNN regression model show that the prediction of the CNN regression model is highly consistent with the reanalysis dataset, with correlation coefficient of 0.78, which is higher than that of each of the current state-of-the-art dynamic models. The heat map method indicates that the robust precursor signals in the CNN regression model agree well with previous theoretical studies, and can provide the quantitative contribution of different signals for EASM prediction. The CNN regression model can predict the EASM one year ahead with a confidence level above 95%. The above method can not only improve the prediction of the EASM but also help to identify the involved physical predictors.

Color Reflectance of Coastal Sediments in the South Bohai Sea and its Implication to Orbital Forcing of East Asian Summer Monsoon

There are two distinct variabilities of the East Asian summer monsoon (EASM) on orbital timescales observed in different proxies, and the forcing mechanisms between them are hotly debated. One of the ways to reconcile the debate is to present a geological archive recording two cycles in dominance and somehow in equivalence. In this work, we retrieved an EASM record by studying color reflectance of coastal sediments in the south Bohai Sea, East Asia. The leading component of reflectance derivative spectra accounts for 58.9% variance in total and its loading spectrum can be well correlated to that of mineral assemblages of illite and goethite. For this monsoonal record, orbital variabilities in precession and eccentricity bands are highlighted. By comparing this monsoonal record to previously published proxies, it is speculated that the spectral difference in the sediments of the south Bohai Sea and between various proxies in the EASM domain may indicate an integrated forcing of solar insolation and ice-sheet evolution in the late Quaternary. Overall, the monsoonal record in the Bohai Sea offers an opportunity to fill the gap of the diverse periodicities between various proxies, which is critical to extending our understanding of the EASM on orbital timescales.