Zonal shift in the cold airmass stream of the East Asian winter monsoon

The East Asian winter monsoon exhibits long-term variations in intensity and spatial pattern, though the latter one is less understood. To investigate the long-term spatial variations of the EAWM and their possible causes, we propose a new position index of the EAWM by quantifying the low-level East Asian stream (EAS) of cold airmass in the Lagrangian sense. Based on the new-defined index, we find that the EAS undergoes an evident zonal shift between two channels over the land and coast. At interdecadal timescale, the peak location of the EAS is displaced eastward, with an increasing southward cold airmass flux at the coast since the mid-1960s. The interannual shift of the EAS presents not only the zonal oscillation of peak location between two channels but also the width changes of coastal channel over the northwestern Pacific. These shifts in the EAS are related to the strength changes of two source cold airmass streams from Siberia or Bering Sea, which are associated with the phase changes in the upper-tropospheric atmospheric teleconnections. At interdecadal timescale, the phase change in the North Atlantic Oscillation modulates the zonal shift in the EAS via the East Atlantic-West Russia teleconnection. At interannual timescale, the Pacific/North American teleconnection becomes the dominant factor altering the zonal shift and width change of the EAS.

Co-evolutions of terrestrial temperature and seasonal precipitation from the latest Pleistocene to the mid-Holocene in Japan: carbonate clumped isotope record of a stalagmite

Quantitative paleotemperature reconstruction is a challenging and important issue in terrestrial paleoenvironmental studies, for which carbonate clumped isotope (Δ47) thermometry is a promising approach. Here we analyzed Δ47 values from 68 layers of OT02 stalagmite from Ohtaki Cave in central Japan, covering two separate time intervals (2.6–8.8 and 34.8–63.5 ka) to reconstruct temperature and meteoric d18O records. The average Δ47 temperature of the Holocene portion of this stalagmite was 16.3℃ ± 5.6℃, 6.6℃ ± 7.2℃ higher than the average of the latest Pleistocene portion, which was 9.7℃ ± 4.6℃. Δ47 thermometry also revealed that the coldest intervals (5℃–10℃) correspond to the Heinrich cooling events H4–6, and the warmest interval (up to 19.9℃ ± 6.0℃) in middle Holocene (approximately 6–5 ka) accompanied by the Hypsithermal climate optimum. We also reconstructed past meteoric δ18O by subtracting the temperature effect from stalagmite δ18O. Average meteoric δ18O was less negative in the Holocene (8.22‰ ± 0.99‰ VSMOW) than in the latest Pleistocene (8.81‰ ± 0.84‰). Over centennial timescales, meteoric δ18O was more negative during colder periods, such as Heinrich cooling events and the cooling event around 7 ka, and less negative in warmer periods, such as Hypsithermal warming. These relations indicated co-evolution of terrestrial paleotemperature and paleoprecipitation. A temperature dependency of 18O fractionation from water to vapor is a likely reason for the negative correlation between temperature and meteoric δ18O. Additionally, it is possible that increasing lower δ18O precipitation from East Asian winter monsoon (EAWM) has decreased the averaged meteoric δ18O in colder periods. These temperature effects on meteoric δ18O occur in opposite directions to fractionation between water and the stalagmite δ18O, explaining the small amplitudes of changes observed in the δ18O of Japanese stalagmites.

Characteristics of Subseasonal Winter Prediction Skill Assessment of GloSea5 for East Asia

In this study, the characteristics of systematic errors in subseasonal prediction for East Asia are investigated using an ensemble hindcast (1991–2010) produced by the Global Seasonal Forecasting System version 5 (GloSea5). GloSea5 is a global prediction system for the subseasonal-to-seasonal time scale, based on a fully coupled atmosphere, land, ocean, and sea ice model. To examine the fidelity of the system with respect to reproducing and forecasting phenomena, this study assesses the systematic biases in the global prediction model focusing on the prediction skill for the East Asian winter monsoon (EAWM), which is a major driver of weather and climate variability in East Asia. To investigate the error characteristics of GloSea5, the hindcast period is analyzed by dividing it into two periods: 1991–2000 and 2001–2010. The main results show that the prediction skill for the EAWM with a lead time of 3 weeks is significantly decreased in the 2000s compared to the 1990s. To investigate the reason for the reduced EAWM prediction performance in the 2000s, the characteristics of the teleconnections relating to the polar and equatorial regions are examined. It is found that the simulated excessive weakening of the East Asian jet relating to the tropics and a failure in representing the Siberian high pressure relating to the Arctic are mainly responsible for the decreased EAWM prediction skill.

Sedimentary Dynamics of the Central South Yellow Sea Revealing the Relation Between East Asian Summer and Winter Monsoon Over the Past 6000 years

The mud areas of East Asian marginal seas record considerable information about regional environmental evolution. However, debate continues regarding the relative importance of the major factors in regional sedimentary dynamics, i.e., the East Asian summer monsoon, East Asian winter monsoon, and oceanic circulation. In this study, we investigated the characteristics of grain size from a gravity core obtained in the South Yellow Sea to reveal changes in sedimentary dynamics since 6,000 years BP, and to elucidate the relationship between the East Asian summer monsoon and the East Asian winter monsoon. We found that the mean grain size was in the range of 6.9–7.8 Φ, the sediment was poorly sorted within a small range (1.2, 1.5), and the M values from 4.7 to 6.7 μm and most of the C values from 24 to 65 μm suggested pelagic suspension transport. Results indicated that the intensity of both the East Asian summer monsoon and the East Asian winter monsoon showed a fluctuating trend of decrease after approximately 6,000 years BP, and that the relationship between them was generally anticorrelated. Based on these results, we suggest that positive correlation between the East Asian summer monsoon and the East Asian winter monsoon usually results in the fall or establishment of ancient dynasties in the Central Plains of China and that negative correlation between them is controlled by strong solar radiation. Weakening of solar radiation diminishes its control of the intensity of (and thus the correlation between) the East Asian summer monsoon and the East Asian winter monsoon, at which time the North Atlantic Oscillation plays a modulating role.

Biases and Improvements of the ENSO- East Asian Winter Monsoon Teleconnection in CMIP5 and CMIP6 Models

The influence of El Niño–Southern Oscillation (ENSO) on the East Asian winter monsoon (EAWM) is investigated based on the outputs of phase 6 of the Coupled Model Intercomparison Project (CMIP6) models and compared to that in phase 5 (CMIP5). Results show that the CMIP6 models generally reproduce the ENSO-EAWM teleconnection more realistically than the CMIP5 models, although they still somewhat underestimate the ENSO-EAWM teleconnection than observed. Based on the inter-model spread of ENSO-EAWM teleconnection simulated in the CMIP5/CMIP6 models, we reveal that the commonly underestimated ENSO-EAWM teleconnection among the models can be traced back to the excessive cold tongue bias in the equatorial western Pacific. A model with a stronger climatological cold tongue favors generating a more westward extension of the ENSO-related SST anomaly pattern, which in turn forces an anomalous cyclonic circulation over the Northwest Pacific (NWP). It offsets the anticyclonic anomalies in the NWP triggered by the warm ENSO-related SST anomalies in the tropical Indian Ocean and the central-eastern Pacific and weakens the ENSO-EAWM teleconnection. Compared with the CMIP5 models, CMIP6 models better simulate SST mean state and the resultant ENSO-EAWM teleconnection. The present results suggest that substantial efforts should be made to reduce the bias in the mean-state SST for further improving the simulation and projection of the East Asian-western Pacific winter climate.

Formation and evolution of mountainous aeolian sediments in the northern Tibet Plateau and their links to the Asian winter monsoon and westerlies since the Last Glacial Maximum

The production, transport, and deposition of dust has profound impacts and feedbacks on the regional and global environments. In parallel, the Tibet Plateau (TP) plays an important role in East Asian and global climate systems. Previous studies have shown that the dust can reach the plateau by long-distance transport from the westerlies and by short-distance transport from regional weathering. Here we present three new datasets in the Qilian Mountains of the northern TP, including two mountainous aeolian sedimentary sequences and a surface sample dataset. Proxies include grain size, magnetic susceptibility, mineral composition, total organic carbon, and total nitrogen. Data from surface samples indicate spatial gradient effects in altitude (from low to high) and latitude (from north to south), suggesting dust transport from the Gobi Desert to the plateau. The synthetic analysis of two aeolian sedimentary sequences and paleoclimate simulation data reveals that the northern TP can be a dust transport channel controlled mainly by the Asian winter monsoon and less influenced by the westerlies.