East Asian Winter Monsoon Index Research Articles

Can climate indices forecast daily variations of wintertime PM2.5 concentrations in East Asia?

Synoptic meteorological variability plays an important role in determining air quality. In East Asia, the expansion and contraction of the Siberian high-pressure system is an essential mechanism for determining surface particulate matter concentrations (PM2.5) during the winter season. Here, we selected four climate indices that reflected the variability of the Siberian high-pressure system and analyzed their correlation with the daily variability of the observed winter PM2.5 concentrations in China and South Korea over the past six years (2014/15–2019/20). Siberian High Intensity (SHI) and East Asian Winter Monsoon (EAWM) indices were good indicators of daily PM2.5 concentration changes. Two to four days after the daily SHI and EAWM indices exceed the threshold (±1), the daily PM2.5 concentrations in East Asia significantly increased or decreased, up to 40 % compared to the mean winter PM2.5 concentrations. The climate indices associated with the Siberian high-pressure system thus potentially effectively forecast the daily PM2.5 concentrations in East Asia within a period of one week.

Influence of Kamchatka Blocking and East Asian winter monsoon on the winter 2m‐temperature over South Korea

AbstractThis study examines the factors affecting the 2m‐temperatures (T2m) in South Korea in winter (DJF) from 1979/1980 to 2018/2019. For this purpose, we performed an empirical orthogonal function (EOF) analysis of the geopotential height at 500 hPa in the region 15°–75°N, 70°E–180° centred around the Korean Peninsula. The first EOF mode, which accounted for 31.1% of the total variance of the winter T2m in the region, is related to the KamChatka Blocking Frequency (KCBF). The second mode, which accounted for 18.3%, is associated with East Asian winter monsoon (EAWM). In addition to EOF analysis, the partial correlation (PC) analysis also confirmed that KCBF Index and EAWM Index, which are highly correlated with the winter T2ms in South Korea, are independent of each other. The individual and combined effects of the KCBF and EAWM are examined because they independently affect the winter T2ms in South Korea. According to the composite analysis, in the years when KCBF is higher than normal (H_KCBF), negative and positive Z500 anomalies are located over the south Sea of Japan and the Kamchatka Peninsula, respectively. In other words, the positive geopotential anomaly over the Kamchatka Peninsula lowers the T2ms of South Korea by blocking the flow of cold air moving eastward from the Korean Peninsula. In the years when a strong EAWM occurred (S_EAWM), negative and positive Z500 anomalies are located over the Manchuria region and the northwestern regions of Russia, respectively. Consequently, cold air in the north moves southward, resulting in lower T2ms in South Korea. For the combined effects of KCBF and EAWM on the T2ms, winters of 40 years are categorized into L_KCBF‐W_EAWM, L_KCBF‐S_EAWM, H_KCBF‐W_EAWM and H_KCBF‐S_EAWM years. The analyses show that when KCBF and EAWM are in phase, the T2ms in South Korea are more prominent than when they are out‐of‐phase.

Response of Spatial and Temporal Variations in the Kuroshio Current to Water Column Structure in the Western Part of the East Sea

Using geographic sea surface current data, long-term changes in spatial and temporal variations in the Kuroshio Current 1993–2021 were analyzed, and the relationship between the Kuroshio Current and oceanic conditions, such as water column structure and intensity of East Korea Warm Current (EKWC) in the western part of the East Sea (WES), was investigated. Long-term changes in the Kuroshio Current intensity were positively correlated with the Pacific Decadal Oscillation and East Asian Winter Monsoon indices. When the Kuroshio Current was strong, its main axis passing around the Ryukyu Islands moved eastward, and the intensity of EKWC separated from the Kuroshio Current and flowed into the WES, indicating weakened conditions. When the intensity of the EKWC was weakened, its main axis moved away from the inshore area of the WES. As a result, the vertical distribution range of the cold and low saline water mass located in the bottom layer extended to shallower depths in the inshore area of the WES with increasing chlorophyll-a.

Variability of East Asian winter monsoon during Middle–Late Holocene: A study based on a crater lake on Jeju Island, South Korea

Understanding the natural variability of the East Asian monsoon (EAM) system, mainly consisting of summer and winter monsoons, is essential to predict future climate changes that will influence agricultural and industrial activities in the highly populated region. To date, the variability of the East Asian winter monsoon (EAWM) and its controlling factors is still unclear in downwind areas that experience the Siberian High, mostly due to the lack of a high-resolution winter monsoon index. Based on the correlation between diatom variations in a volcanic cone of Jeju Island, South Korea, with past regional climate changes (e.g., temperature, dust, and sea ice) and solar activity, in this study, we evaluated the changes in the climate from the sum of acidophilous species (SAS), known to be mainly controlled by the pH of the prevalent water pool, and inferred that the SAS increased during the cold and dry periods (e.g., Little Ice Age and Dark Age Cold Period). Furthermore, millennial-timescale fluctuation in the detrended SAS during the past 8000 years, with peaks at ~500, 1500, 3000, 5600, 6600, and 7600 cal yr B·P., corresponded to the ice-rafted events reported from studies conducted on subpolar ocean sediments and solar activity. In this study, we suggest that, historically, high-latitude climate change driven by solar activity has controlled the EAWM; additionally, we support the application of the SAS in Jeju Island as a high-resolution EAWM index.

Analysis of Synergistic Effects of Cold Source and East Asian Winter Wind on Air Pollution in Typical Regions of China in Winter

This paper collects and analyzes the 1954–2017 monthly average reanalysis data from the U.S. National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR), the 1954–2017 haze days observation data from the National Meteorological Information Center/China Meteorological Administration (NMIC/CMA) and the PM2.5 daily average mass concentration data in 2013–2017 from the China Air Quality Online Monitoring Platform. The atmospheric apparent heat source Q1 (negative Q1 means cold source in winter) over the Tibetan Plateau in December of 1954–2017 is estimated based on thermodynamic equations, and the East Asian winter monsoon index (EAWMI) is calculated. In addition, the discrepancies of the air quality among China’s five typical regions (Beijing–Tianjin–Hebei, Fen-Wei Plain, Yangtze River Delta, Pearl River Delta and Sichuan-Chongqing regions) under the joint influence of the Q1 and EAWMI are studied. Results show that when it is a strong cold source year, abnormal downdrafts and “temperature inversion covers” occur in areas far from the high terrain, resulting in increased pollution, while the opposite is true in weak cold source years. In strong EAWMI years, there is an abnormal northerly sinking cold flow in the lower layers of mid-high latitudes, which increases the pollution in the area south of 30° N, and the opposite is true in weak EAWMI years. Affected by the combined activities of the Q1 and the EAWMI, the meteorological conditions of the five typical regions are different, and thus present different air pollution characteristics.

Near-surface wind speed changes in eastern China during 1970–2019 winter and its possible causes

The changes in near-surface wind speed (NWS) have a crucial influence on the wind power industry, and previous studies have indicated that NWS on global and China has declined continuously for decades under global warming. However, recently, the decreasing trend of global NWS has slowed down and even showed a recovery trend. Using the observation data of 831 weather stations of the China Meteorological Administration and the Japanese 55-year reanalysis data from 1970 to 2019, NWS changes in eastern China were analyzed and the possible influencing factors were discussed. Results show that winter NWS presented a decreasing trend from −0.29 m s−1 per decade (p < 0.001) in 1970–1989 to −0.05 m s−1 per decade (p < 0.01) in 1990–2019. Moreover, NWS exhibited a significant upward trend of 0.18 m s−1 per decade (p < 0.1) in 2011–2019, resulting in a 19.6% per decade recovery of the wind power generation. A possible cause is asymmetric changes of the sea level pressure and near-surface air temperature differences between the mid-high latitudes (40°–60°N, 80°–120°E) and low latitudes (20°–40°N, 110°–140°E) altered the horizontal air pressure gradient. Furthermore, NWS changes were closely associated with the large-scale ocean-atmosphere circulations (LOACs). NWS at 77.4% of the stations in eastern China shows significant correlation (p < 0.05) with the East Asian winter monsoon index, besides, the inter/multidecadal variability of NWS was considerably correlated to four LOACs, including Arctic oscillation (AO), North Atlantic oscillation (NAO), Pacific decadal oscillation (PDO), and El Niño–Southern Oscillation (ENSO). The time-series reconstructed by a multiple linear regression model based on above five LOACs matches well with the NWS. Interannual variability of NWS were significantly correlated to AO (−0.45, p < 0.01) and NAO (−0.28, p < 0.05), while the correlation between NWS and ENSO was weak.

Reexamining the Decadal Variation in the ENSO‐East Asian Winter Monsoon Relationship

AbstractAlthough numerous studies have paid attention to the unstable relationship between the El Niño/Southern Oscillation (ENSO) and East Asian winter monsoon (EAWM), significant discrepancies have existed, especially regarding the modulation of the Pacific decadal oscillation (PDO) and Atlantic multidecadal oscillation (AMO). Here, we reexamine this issue by applying a new method that considers the changing relationship between ENSO and the spatial variations of the EAWM rather than a given EAWM index. We suggest a complicated spatial‐temporal variation in the ENSO‐EAWM relationship (EER), which consists of two major modes regulated by the PDO and AMO in distinct ways. In other words, the PDO and AMO modulate two different aspects of the decadal EER, which helps to clarify the disagreements among previous studies that are generally based on specific and regional EWAM indices.

Roles of sediment supply, geochemical composition and monsoon on organic matter burial along the longitudinal mud belt in the East China Sea in modern times

Terrestrial sediment supply and its composition, aquatic productivity and coastal hydrodynamics together control the nature of organic matter (OM) that buried in ocean margins. Distinguishing their individual roles in river-dominated continental margins is challenging, but crucial for understanding (i) the regional carbon cycle in modern times and (ii) the OM burial-climate link on shorter timescales. Here we investigate the contents of total organic carbon (TOC) and total nitrogen (TN) and stable carbon isotopic composition of TOC (δ13C), along with grain size and calcium carbonate, in three short, 210Pb- and 137Cs-dated, sediment cores (DH1-1, DH3-3 and DH5-1) from the Yangtze distal mud belt in the East China Sea to delineate the spatiotemporal distribution, origin and burial flux of sedimentary OM for the last ca. 130 years. We also use selected inorganic elements (Si, Al, Ca, Na, K, Rb and Sr) and geochemical proxies derived from them (i.e. chemical index of alteration-CIA, molar Al/Si, molar K/Na, K/Rb and Rb/Sr) to elucidate the importance of detrital composition and chemical weathering on the supply, transport and burial of OM.Bulk organic geochemical and grain size data suggest a longer transport of fine sediments-laden OM, indicating the role of China Coastal Current on hydrodynamic sorting in the study area. Spatial distribution of Al/Si, K/Na, Rb/Sr and CIA in all cores corroborates the selective transport of fine-grained minerals from the Yangtze estuary to the distal mud belt. Less variable molar C/N ratios (5.6–7.7) and δ13C values (–22.4 to –21.2‰) in our cores and the results of δ13C mixing model indicate the predominance of marine OM burial over the last century in the study area. Relationships of TOC, δ13C and mean grain size and spatial evolution of δ13C along the river-mud belt continuum show that adsorption of marine OM on the Yangtze-derived fine sediments during their southward transport. Estimated mean burial fluxes of terrestrial and marine OC (0.73 ± 0.21 and 1.88 ± 0.29 Tg C yr−1) indicate that the Yangtze distal mud belt, despite covering ~18% of the shelf area, acts as a key OM sequestration center (>50%) in the East China Sea. Comparison of records of riverine OC burial and geochemical weathering and grain size proxies of our cores with East Asian winter monsoon index, and sediment and water discharges in the Yangtze River confirms that hydrodynamics associated with the winter monsoon winds-driven China Coastal Current predominantly controlled the sediment accumulation and OM burial along the mud belt before 1970s. Nonetheless, human activities especially the construction of Three Gorges Dam have reduced the burial flux of terrestrial OC from 0.93 to 0.54 Tg yr−1, significantly affecting the supply and burial of OM and thus the regional carbon cycle since 1970s.

December-March temperature reconstruction from tree-ring earlywood width in southeastern China during the period of 1871-2016.

Extremely cold temperatures are a significant threat to agriculture and transportation in winter in southeastern China. However, due to the shortness of instrumental records and the scarcity of long-term temperature reconstructions, more high-quality temperature reconstructions are still needed to fully examine their spatial-temporal variability over the past several centuries. In this study, we built an earlywood width (EWW) chronology, a latewood width (LWW) chronology, and a tree-ring width (TRW) chronology using tree-ring samples of Pinus taiwanensis Hayata from the western Tianmu Mountains and the Xianyu Mountains in southeastern China. The tree growth-climate relationships were analyzed, and we found the strongest correlation between December and March mean temperature and the EWW chronology. The December-March mean temperature history was then reconstructed over the period of 1871-2016 using a linear regression model, which is the first EWW-based temperature reconstruction in southeastern China. With a higher explained variance (47.0%) than that (31.7%) of a previous reconstruction using a TRW chronology, the quality of the model has largely improved. This reconstruction was also comparable with other nearby records, further demonstrating the reliability of our new model. Furthermore, our reconstruction exhibits a significantly negative relationship with the East Asian winter monsoon index (EAWMI) since the 1920s, which may be attributed to the obviously enhanced EAWMI thereafter.

Influence of East Asian winter monsoon on particulate matter pollution in typical regions of China

This study uses the NCEP/NCAR monthly average reanalysis and number of haze day data during 1958–2017, and the average daily PM2.5 mass concentration data during 2013–2017, to calculate the East Asian winter monsoon index (EAWMI) and statistically analyze the correlation between the winter monsoon index and air quality in China, particularly for the five typical regions (Beijing-Tianjin-Hebei, Fen-Wei Plain, Sichuan-Chongqing Delta, Yangtze River Delta, and Pearl River Delta). Thereafter, the strong and weak winter monsoon years were classified based on the EAWMI, and the atmospheric circulation and temperature fields over China and the five regions in different winter monsoon years were spatially compared. Finally, the study also investigated the various distribution features of the climatic circulation background responsible for the strong and weak winter monsoons and their impact mechanisms on air quality the five typical regions in China. The results show that the effect of the winter monsoon on the air quality of China may be represented by a north–south boundary line located at approximately 30° N. During strong winter monsoon years, pollution was lower in the area north of the boundary but higher to its south. By contrast, the opposite phenomenon was observed during the weak monsoon years. During the strong winter monsoon years, the Beijing-Tianjin-Hebei region and Fen-Wei Plain to the north of the boundary line were less polluted, while the Yangtze River, Chengdu-Chongqing, and Pearl River delta regions to the south of the boundary were more polluted. Diagnostic analysis of the circulation field indicated that during the strong winter monsoon years, an abnormal downward airflow occurred to the south of the boundary, limiting convective diffusion and thereby causing the increased pollution. However, during the weak winter monsoon years, ascending airflows occurred, which favored pollutant diffusion. Furthermore, during the strong winter monsoon years, an abnormal southeast airflow with weak horizontal wind speed occurred in the lower atmosphere of the Chengdu-Chongqing region, causing localized pollutant accumulation, thereby aggravating the pollution. In the Pearl River Delta region, a descending abnormal westerly flow inhibited the local uplift and diffusion of air. Moreover, importing pollutants occurred from the north, aggravating the pollution in the region.

Spatial variation of East Asian winter monsoon evolution between northern and southern China since the last glacial maximum

AbstractThe East Asian winter monsoon (EAWM) is one of the most dynamic components of the global climate system. Although poorly understood, knowledge of long-term spatial differences in EAWM variability during the glacial–interglacial cycles is important for understanding the dynamic processes of the EAWM. We reconstructed the spatiotemporal characteristics of the EAWM since the last glacial maximum (LGM) using a comparison of proxy records and long-term transient simulations. A loess grain-size record from northern China (a sensitive EAWM proxy) and the sea surface temperature gradient of an EAWM index in sediments of the southern South China Sea were compared. The data–model comparison indicates pronounced spatial differences in EAWM evolution, with a weakened EAWM since the LGM in northern China but a strengthened EAWM from the LGM to the early Holocene, followed by a weakening trend, in southern China. The model results suggest that variations in the EAWM in northern China were driven mainly by changes in atmospheric carbon dioxide (CO2) concentration and Northern Hemisphere ice sheets, whereas orbital insolation and ice sheets were important drivers in southern China. We propose that the relative importance of insolation, ice sheets, and atmospheric CO2for EAWM evolution varied spatially within East Asia.

Weather Generator–Based Downscaling of EAWM Strength Prediction to the Climate of a Korean Basin

AbstractIn this paper, we propose a downscaling method that statistically describes a local-scale climate from large-scale circulations using the case of a Korean basin during boreal winter. Specifically, since the East Asian winter monsoon (EAWM) affects the climate of the Korean Peninsula, we make a weather generator model describing the response of the basin climate to the monsoon strength. Moreover, it operates on the basis of a tercile probabilistic prediction of the EAWM strength to generate diverse scenarios of daily weather sequence during the season, which can be utilized in evaluation of the climate impact. We evaluate the prediction skills of operational hindcasts for several existing EAWM indices by applying a multinomial logistic regression method to choose the most suitable index for the downscaling. In the weather generator model, the precipitation model part is designed to be fully parametric. Its parameter values are allowed to vary according to the monsoon strength so that they can represent the climate variability of precipitation. In the temperature model part, the daily temporal variations of the temperature over the Korean basin are decomposed into several oscillations with different frequencies. Since the slowly varying oscillations significantly respond to the monsoon strength, the proposed downscaling scheme is based on the statistical simulation of oscillations according to the monsoon strength. The proposed downscaling scheme is evaluated in terms of the reproducibility of the climate characteristics for a given EAWM strength and the informativeness for predicting monthly climate characteristics.

Decadal variations of the East Asian winter monsoon in recent decades

AbstractObservational studies indicated that the East Asian winter monsoon (EAWM) showed significant decadal variations and experienced an interdecadal weakening in the mid‐1980s. How did the EAWM evolve thereafter? In this study, we investigate the decadal variations of the EAWM in the past three decades using five reanalysis datasets and one observational dataset. In total, five members of EAWM system are examined. In the lower troposphere, the Siberian high intensity becomes stronger around 2005 in all five reanalysis datasets, whereas the Aleutian low strengthens in the mid‐1990s and weakens in the mid‐2000s. The two subsystems show opposing changes in the past two decades. In the middle troposphere, the intensity of the East Asian trough remains weakening in recent decades only with some short‐time strengthening around the years of 1995 and 2010. In the upper troposphere, the EAWM index based on the meridional shear of the East Asian jet stream at the 300‐hPa level shows a weak positive trend since the 1990s. It has decadal variations similar to those of the East Asian trough. In the context of these members' changes, the low‐level northerly wind index exhibits two cycles since the year 1980. This index increases in the mid‐1980s, then decreases since the mid‐1990s, and finally increases in the mid‐2000s. Further analysis on the East Asian surface air temperature also reflects similar decadal variations. In summary, unlike the interdecadal shift in the mid‐1980s, the EAWM members (i.e., the Siberian high, the Aleutian low, the low‐level northwesterly wind, the East Asian trough and the East Asian jet stream) do not show consistent decadal variations over the past 30 years. The low‐level members show more significant changes.

Interdecadal correlation of solar activity with Tibetan Plateau snow depth and winter atmospheric circulation in East Asia

Studies on the impact of solar activity on climate system are very important in understanding global climate change. Previous studies in this field were mostly focus on temperature, wind and geopotential height. In this paper, interdecadal correlations of solar activity with Winter Snow Depth Index (WSDI) over the Tibetan Plateau, Arctic Oscillation Index (AOI) and the East Asian Winter Monsoon Index (EAWMI) are detected respectively by using Solar Radio Flux (SRF) , Total Solar Irradiance (TSI) and Solar Sunspot Number (SSN) data and statistical methods. Arctic Oscillation and East Asian winter monsoon are typical modes of the East Asian atmospheric circulation. Research results show that on interdecadal time scale over 11-year solar cycle, the sun modulated changes of winter snow depth over the Tibetan Plateau and East Asian atmospheric circulation. At the fourth lag year, the correlation coefficient of SRF and snow depth is 0.8013 at 0.05 significance level by Monte-Carlo test method. Our study also shows that winter snow depth over the Tibetan Plateau has significant lead and lag correlations with Arctic Oscillation and the East Asian winter monsoon on long time scale. With more snow in winter, the phase of Arctic Oscillation is positive, and East Asian winter monsoon is weak, while with less snow, the parameters are reversed. An example is the winter of 2012/2013, with decreased Tibetan Plateau snow, phase of Arctic Oscillation was negative, and East Asian winter monsoon was strong.

Comparing the impact of strong and weak East Asian winter monsoon on PM2.5 concentration in Beijing

We investigate the relationship between the wintertime PM2.5 concentration in Beijing from 2005 to 2016 and the East Asian winter monsoon (EAWM) strength based on PM2.5 observations and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data. The large-scale atmospheric circulations are compared and the differences in spatial distribution and source areas of Beijing PM2.5 concentration are studied in depth combined with GRAPES-CUACE atmospheric chemistry model and its aerosol adjoint model in strong and weak EAWM years. The results show that the monthly Beijing PM2.5 concentration in winter is significantly anti-correlated with the individual EAWM indices of winter month. According to three EAWM indices, December 2014 and 2016 are selected as the representatives of strong and weak EAWM years for further comparison. In December 2014, the strong EAWM circulation was characterized by the strengthened subtropical westerly jet stream at 300 hPa, the deepened 500 hPa East Asian trough, the strengthened northwesterly wind at 850 hPa in North China, and the developed Siberian High. Conversely, these anomalous patterns in December 2016 were much weaker, resulting in lower boundary layer height, more stable atmosphere stratification and more frequent inversion. The simulated PM2.5 concentration in the northern and southern part of Beijing were affected by the EAWM to different extents, with the values of 55–75 μg/m3 in the north and 95–125 μg/m3 in the south. There was no obvious difference in the locations of sensitive source areas between December 2014 and 2016. However, compared to December 2014, the accumulated sensitivity coefficients of Beijing, Tianjin, Hebei and Shanxi sources at 72 h ahead of the objective time period were 2.2, 5.9, 1.6, and 1.4 times higher, and the key discharge period was 2.6 times longer in December 2016. The contributions of Beijing and Tianjin sources had greater increase rates, stressing the importance of sensitive source areas in the direction of southeast Beijing in weak EAWM year.

The northern and southern modes of East Asian winter monsoon and their relationships with El Niño‐Southern Oscillation

The characteristics of the northern and southern modes of the East Asian winter monsoon (EAWM) and their relationships with El Niño‐Southern Oscillation (ENSO) are further investigated in this paper using reanalysis data. The results demonstrate that the northern mode shows strong inter‐decadal variations, whereas the southern mode is marked by noticeable inter‐annual variations. The west–east pressure gradient plays a leading role in influencing northern mode, and both the west–east and north–south pressure gradients have significant impacts on the southern mode. The two modes are closely related, but their relationship shows inter‐decadal variations. The relationship between the southern mode and ENSO is significant and stable, but the relationship between the northern mode and ENSO is unstable and shows obvious inter‐decadal variations. The relationship between the northern mode and ENSO is significant before the late 1960s and after late 1990s, but it is not significant during the period between the late 1960s and late 1990s. ENSO influences the activity of northern mode mainly by changing the west–east pressure gradient. During the periods of a weak relationship between northern mode and ENSO, the variations in west–east pressure gradient may be dominated by the climate systems at mid–high latitudes, and the effects of ENSO are confined or offset. The inter‐decadal variations in the relationship between northern mode and ENSO may be modulated by the Pacific Decadal Oscillation (PDO) and Atlantic Multi‐decadal Oscillation (AMO), but their roles and importance have varied over time. The relationship between EAWM and ENSO and its inter‐decadal variations vary with the EAWM index, which is induced by the differences between the northern and southern modes. When the EAWM index is more closely correlated with the southern (northern) mode, its relationship with ENSO is (not) significant and stable (unstable).

Prediction Skill for the East Asian Winter Monsoon Based on APCC Multi-Models

The prediction skill for the East Asian winter monsoon (EAWM) has been analyzed, using the observations and different climate models that participate in the APEC Climate Center (APCC) multi-model ensemble (MME) seasonal forecast. The authors first examined the characteristics of the existing EAWM indices to find a suitable index for the APCC seasonal forecast system. This examination revealed that the selected index shows reasonable prediction skill of EAWM intensity and well-represents the characteristics of wintertime temperature anomalies associated with the EAWM, especially for the extreme cold winters. Although most models capture the main characteristics of the seasonal mean circulation over East Asia reasonably well, they still suffer from difficulty in predicting the interannual variability (IAV) of the EAWM. Fortunately, the POAMA has reasonable skill in capturing the timing and strength of the EAWM IAV and reproduces the EAWM-related circulation anomalies well. The better performance of the POAMA may be attributed to the better skill in simulating the high-latitude forcing including the Siberian High (SH) and Artic Oscillation (AO) and the strong links of the ENSO to the EAWM, compared to other models.

East Asian winter monsoon forecasting schemes based on the NCEP’s climate forecast system

The East Asian winter monsoon (EAWM) is the major climate system in the Northern Hemisphere during boreal winter. In this study, we developed two schemes to improve the forecasting skill of the interannual variability of the EAWM index (EAWMI) using the interannual increment prediction method, also known as the DY method. First, we found that version 2 of the NCEP’s Climate Forecast System (CFSv2) showed higher skill in predicting the EAWMI in DY form than not. So, based on the advantage of the DY method, Scheme-I was obtained by adding the EAWMI DY predicted by CFSv2 to the observed EAWMI in the previous year. This scheme showed higher forecasting skill than CFSv2. Specifically, during 1983–2016, the temporal correlation coefficient between the Scheme-I-predicted and observed EAWMI was 0.47, exceeding the 99% significance level, with the root-mean-square error (RMSE) decreased by 12%. The autumn Arctic sea ice and North Pacific sea surface temperature (SST) are two important external forcing factors for the interannual variability of the EAWM. Therefore, a second (hybrid) prediction scheme, Scheme-II, was also developed. This scheme not only involved the EAWMI DY of CFSv2, but also the sea-ice concentration (SIC) observed the previous autumn in the Laptev and East Siberian seas and the temporal coefficients of the third mode of the North Pacific SST in DY form. We found that a negative SIC anomaly in the preceding autumn over the Laptev and the East Siberian seas could lead to a significant enhancement of the Aleutian low and East Asian westerly jet in the following winter. However, the intensity of the winter Siberian high was mainly affected by the third mode of the North Pacific autumn SST. Scheme-I and Scheme-II also showed higher predictive ability for the EAWMI in negative anomaly years compared to CFSv2. More importantly, the improvement in the prediction skill of the EAWMI by the new schemes, especially for Scheme-II, could enhance the forecasting skill of the winter 2-m air temperature (T-2m) in most parts of China, as well as the intensity of the Aleutian low and Siberian high in winter. The new schemes provide a theoretical basis for improving the prediction of winter climate in China.

Roles of Intraseasonal Disturbances and Diabatic Heating in the East Asian Jet Stream Variabilities Associated with the East Asian Winter Monsoon

This study evaluates the relative importance of diabatic heating and intraseasonal disturbances with regard to the variabilities of the East Asian jet stream (EAJS) associated with the East Asian winter monsoon (EAWM). First, strong and weak monsoon years are selected based on the EAWM index of Jhun and Lee, which is highly correlated with the monsoon northerlies between the Eurasian continent and the Pacific. The EAJS is stronger and narrower in strong monsoon years and weaker and wider in weak monsoon years. Model experiments were performed to investigate the atmospheric response to the diabatic heating and the eddy–mean flow feedback from the intraseasonal disturbances. The diabatic heating is closely related to the convective activities. The intraseasonal disturbances include high-frequency components with periods of 3–10 days and low-frequency components with periods of 10–90 days. The model results indicate that the diabatic heating plays a major role maintaining the stronger and weaker EAJS in the strong and weak monsoon years, respectively, whereas the impacts of the eddy feedback are relatively small.

The influence of climate regime shifts on the marine environment and ecosystems in the East Asian Marginal Seas and their mechanisms

Step changes to seawater temperature (SWT) in the East Asian marginal seas (EAMS) are associated with three recent climate regime shifts (CRS) occurring in the mid-1970s, late 1980s, and late 1990s, but the responses of the ocean conditions and marine ecosystems had regional differences. A step change in SWT in the East China Sea (ECS) was detected after the CRS of the 1970s as were step changes in the North Pacific Index (NPI), Pacific Decadal Oscillation Index (PDOI), and East Asian Winter Monsoon Index (EAWMI). SWT in the ECS decreased with decreasing warm water volume transport into the EAMS and a strong monsoon, but step changes in SWT in other regions were not detected as clearly. After the CRS of the 1980s, SWT in all EAMS increased rapidly with step changes detected in all five climate indices examined. These changes were associated with a weak winter monsoon, increasing surface air temperature (SAT), and increasing warm water volume transport into the EAMS. However, after the CRS of the 1990s, a decrease in SWT around the EAMS was detected in the northern part of East China Sea (NECS), and the ECS with step changes also in the EAWMI and the Arctic Oscillation Index (AOI). In contrast, SWT in the East Sea/Sea of Japan (EJS) and the Yellow Sea (YS) continuously increased during this time. Long-term changes in zooplankton biomass were affected by regional differences in the responses of atmospheric and oceanic variability to CRSs. Specifically, long-term changes in the timing of peaks in zooplankton abundances exhibited differences. During warm periods (e.g. after the 1980s CRS) in the EJS, the amount of zooplankton biomass in October increased, while in February it decreased. On the contrary, in the YS and the NECS, the peaks of October and June in zooplankton biomass occurred during cold periods (after the 1970s and 1990s CRS). Major fisheries resources also responded to the three CRSs, although warm and cold water species responded differently to changes in oceanographic conditions in regional spawning grounds.