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.

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.

Impact of Autumn-Winter Tibetan Plateau Snow Cover Anomalies on the East Asian Winter Monsoon and Its Interdecadal Change

The present study investigates the impacts of autumn-winter Tibetan Plateau (TP) snow cover anomalies on the interannual variability of the East Asian winter monsoon (EAWM). It is found that the northern component of EAWM is significantly associated with October-November-December-January (ONDJ) snow cover anomalies over the eastern TP, whereas the TP snow cover changes have little impact on the southern component of EAWM. However, the relationship of the northern component of EAWM to ONDJ TP snow cover experienced an obvious change in the mid-1990s. During 1979–1998, due to the high persistence of TP snow anomalies from autumn to winter, extensive ONDJ TP snow cover anomalies have a prominent influence on atmospheric circulation over Asia and the North Pacific, with more TP snow cover followed by an enhanced Siberian high and a deepened Aleutian low in winter, resulting in stronger EAWM. During 1999–2016, TP snow cover anomalies have a weak persistence. The atmospheric circulation anomalies display a different distribution. As such, there is a weak connection between the northern component of EAWM and the TP snow cover anomalies during this period.

Simulations of the East Asian Winter Monsoon on Subseasonal to Seasonal Time Scales Using the Model for Prediction Across Scales

The Model for Prediction Across Scales (MPAS) is used to simulate the East Asian winter monsoon (EAWM) over the 2011–2020 winter. The 45 day hindcasts are made with 30 km horizontal resolution and constructed to a time-lagged ensemble system. The climatology, the major modes of EAWM variability, and the blocking activities are examined. The evaluation results reveal that MPAS can simulate the climatologic characteristics of EAWM reasonably, with a surface cold bias of 4% and a positive rainfall bias of 9% over East Asia. MPAS can perform skillfully in the forecasts of surface temperature probability of East Asia and is more reliable in detecting below normal and above normal events. The features that influence the EAWM variability are also analyzed. MPAS simulates reasonably in the occurrence frequency of blocking high in both locations and duration time. The empirical orthogonal function analysis also shows that MPAS can capture the two major modes of the surface temperature of EAWM. On the other hand, it is also found that a biased sea surface temperature may modify the circulations over the Western Pacific and affect the simulated occurrence frequency of cold events near Taiwan during winter.