Seasonal Prediction Skill and Biases in GloSea5 Relating to the East Asia Winter Monsoon

The simulation and prediction of the climatology and interannual variability of the East Asia winter monsoon(EAWM),as well as the associated atmospheric circulation,was investigated using the hindcast data from Global Seasonal Forecast System version 5(GloSea5),with a focus on the evolution of model bias among different forecast lead times.While GloSea5 reproduces the climatological means of large-scale circulation systems related to the EAWM well,systematic biases exist,including a cold bias for most of China’s mainland,especially for North and Northeast China.GloSea5 shows robust skill in predicting the EAWM intensity index two months ahead,which can be attributed to the performance in representing the leading modes of surface air temperature and associated background circulation.GloSea5 realistically reproduces the synergistic effect of El Niño–Southern Oscillation(ENSO)and the Arctic Oscillation(AO)on the EAWM,especially for the western North Pacific anticyclone(WNPAC).Compared with the North Pacific and North America,the representation of circulation anomalies over Eurasia is poor,especially for sea level pressure(SLP),which limits the prediction skill for surface air temperature over East Asia.The representation of SLP anomalies might be associated with the model performance in simulating the interaction between atmospheric circulations and underlying surface conditions.

The Interannual Variability of East Asian Winter Monsoon and Its Relation to the Summer Monsoon

Based on the NCEP/ NCAR reanalysis data the interannual variability of the East Asian winter mon-soon (EAWM) is studied with a newly defined EAWM intensity index. The marked features for a strong (weak) winter monsoon include strong (weak) northerly winds along coastal East Asia, cold (warm) East Asian continent and surrounding sea and warm (cold) ocean from the subtropical central Pacific to the trop-ical western Pacific, high (low) pressure in East Asian continent and low (high) pressure in the adjacent ocean and deep (weak) East Asian trough at 500 hPa. These interannual variations are shown to be closely connected to the SST anomaly in the tropical Pacific, both in the western and eastern Pacific. The results suggest that the strength of the EAWM is mainly influenced by the processes associated with the SST anom-aly over the tropical Pacific. The EAWM generally becomes weak when there is a positive SST anomaly in the tropical eastern Pacific (El Ni?o), and it becomes strong when there is a negative SST anomaly (La Ni?a). Moreover, the SST anomaly in the South China Sea is found to be closely related to the EAWM and may persist to the following summer. Both the circulation at 850 hPa and the rainfall in China confirm the connection between the EAWM and the following East Asian summer monsoon. The possible reason for the recent 1998 summer flood in China is briefly discussed too. Key words East Asian winter monsoon - Interannual variability - SST - Summer monsoon This study was supported by “ National Key Programme for Developing Basic Sciences” G1998040900 part 1, and by key project (KZ 952-S1-404) of Chinese Academy of Sciences.

How Well do Existing Indices Measure the Strength of the East Asian Winter Monsoon?

Defining the intensity of the East Asian winter monsoon （EAWM） with a simple index has been a difficult task. This paper elaborates on the meanings of 18 existing EAWM strength indices and classifies them into four categories： low level wind indices, upper zonal wind shear indices, east-west pressure contrast indices, and East Asian trough indices. The temporal/spatial performance and prediction potential of these indices are then analyzed for the 1957-2001 period. It reveals that on the decadal timescale, most indices except the east-west pressure contrast indices can well capture the continuous weakening of the EAWM around 1986. On the interannual timescale, the low level wind indices and East Asian trough indices have the best predictability based on knowledge of the El Nio-Southern Oscillation and Arctic Oscillation, respectively. All the 18 existing indices can well describe the EAWM-related circulation, precipitation, and lower tropospheric air temperature anomalies. However, the variations of surface air temperature over large areas of central China cannot be well captured by most indices, which is possibly related to topographic effects. The results of this study may provide a possible reference for future studies of the EAWM.

Predictability of the East Asian Winter Monsoon Interannual Variability as Indicated by the DEMETER CGCMS

The interannual variability of East Asian winter monsoon （EAWM） circulation from the Development of a European Multi-Model Ensemble （MME） System for Seasonal to Inter-Annual Prediction （DEMETER） hindcasts was evaluated against observation reanalysis data. We evaluated the DEMETER coupled general circulation models （CGCMs）＇ retrospective prediction of the typical EAWM and its associated atmospheric circulation. Results show that the EAWM can be reasonably predicted with statistically significant accuracy, yet the major bias of the hindcast models is the underestimation of the related anomalies. The temporal correlation coefficient （TCC） of the MME-produced EAWM index, defined as the first EOF mode of 850- hPa air temperature within the EAWM domain （20^-60~N, 90^-150~E）, was 0.595. This coefficient was higher than those of the corresponding individual models （range： 0.39-0.51） for the period 1969 2001; this result indicates the advantage of the super-ensemble approach. This study also showed that the ensemble models can reasonably reproduce the major modes and their interannual variabilities for sea level pressure, geopotential height, surface air temperature, and wind fields in Eurasia. Therefore, the prediction of EAWM interannual variability is feasible using multimodel ensemble systems and that they may also reveal the associated mechanisms of the EAWM interannual variability.

Relationship between Bering Sea Ice Cover and East Asian Winter Monsoon Year-to-Year Variations

In this study, the relationship between year-to-year variations in the Bering Sea ice cover （BSIC） and the East Asian winter monsoon （EAWM） for the period 1969-2001 was documented. The time series of total ice cover in the eastern Bering Sea correlated with the EAWM index at -0.49, indicating that they are two tightly related components. Our results show that the BSIC was closely associated with the simultaneous local and large-scale atmosphere over the Asian-northern Pacific region. Heavy BSIC corresponded to weaker EAWM circulations and light BSIC corresponded to stronger EAWM circulations. Thus, the BSIC should be considered as one of the possible factors affecting the EAWM variation.

Interference of the East Asian Winter Monsoon in the Impact of ENSO on the East Asian Summer Monsoon in Decaying Phases

The variability of the East Asian winter monsoon （EAWM） can be divided into an ENSO-related part （EAWMEN） and an ENSO-unrelated part （EAWMres）.The influence of EAWMres on the ENSO-East Asian summer monsoon （EASM） relationship in the decaying stages of ENSO is investigated in the present study.To achieve this,ENSO is divided into four groups based on the EAWMres：（1） weak EAWMres-E1Ni（n）o （WEAWMres-EN）; （2） strong EAWMres-E1Ni（n）o （SEAWMresEN）; （3） weak EAWMres-La Ni（n）a （WEAWMres-LN）; （4） strong EAWMres-La Ni（n）a （SEAWMres-LN）.Composite results demonstrate that the EAWMres may enhance the atmospheric responses over East Asia to ENSO for WEAWMres-EN and SEAWMres-LN.The corresponding low-level anticyclonic （cyclonic） anomalies over the western North Pacific （WNP） associated with El Ni（n）o （La Ni（n）a） tend to be strong.Importantly,this feature may persist into the following summer,causing abundant rainfall in northern China for WEAWMres-EN cases and in southwestern China for SEAWMres-LN cases.In contrast,for the SEAWMres-EN and WEAWMres-LN groups,the EAWMres tends to weaken the atmospheric circulation anomalies associated with E1 Ni（n）o or La Ni（n）a.In these cases,the anomalous WNP anticyclone or cyclone tend to be reduced and confined to lower latitudes,which results in deficient summer rainfall in northern China for SEAWMres-EN and in southwestern China for WEAWMres-LN.Further study suggests that anomalous EAWMres may have an effect on the extra-tropical sea surface temperature anomaly,which persists into the ensuing summer and may interfere with the influences of ENSO.

Relationship Between East Asian Winter Monsoon and Summer Monsoon

Using National Centers for Environmental Prediction/National Centre for Atmospheric Research（NCEP/NCAR） reanalysis data and monthly Hadley Center sea surface temperature（SST） data,and selecting a representative East Asian winter monsoon（EAWM） index,this study investigated the relationship between EAWM and East Asian summer monsoon（EASM） using statistical analyses and numerical simulations.Some possible mechanisms regarding this relationship were also explored.Results indicate a close relationship between EAWM and EASM：a strong EAWM led to a strong EASM in the following summer,and a weak EAWM led to a weak EASM in the following summer.Anomalous EAWM has persistent impacts on the variation of SST in the tropical Indian Ocean and the South China Sea,and on the equatorial atmospheric thermal anomalies at both lower and upper levels.Through these impacts,the EAWM influences the land-sea thermal contrast in summer and the low-level atmospheric divergence and convergence over the Indo-Pacific region.It further affects the meridional monsoon circulation and other features of the EASM.Numerical simulations support the results of diagnostic analysis.The study provides useful information for predicting the EASM by analyzing the variations of preceding EAWM and tropical SST.

Projections of the East Asian Winter Monsoon under the IPCC AR5 Scenarios Using a Coupled Model:IAP_FGOALS

Responses of the East Asian winter monsoon （EAWM） in future projections were studied based on two core future projections of CMIP5 in coordinated experiments with the IAP-coupled model FGOALS2-s. The projected changes of EAWM in climatology, seasonality, and interannual variability are reported here; the projections indicated strong warming in winter season. Warming increased with latitude, ranging from 1°C to 3°C in the Representative Concentration Pathways simulation RCP4.5 projection （an experiment that results in additional radiative forcing of 4.5 W m-2 in 2100） and from 4° to 9°C in the RCP8.5 projection （an experiment that results in additional radiative forcing of8.5 W m-2 in 2100）. The northerly wind along the East Asian coastal region became stronger in both scenarios, indicating a stronger EAWM. Accordingly, interannual variability （described by the standard deviation of temperature） increased around the South China Sea and lower latitudes and decreased over eastern China, especially in North China. The two EAWM basic modes, defined by the temperature EOF analysis over East Asia, were associated with the Arctic Oscillation （AO） and stratospheric polar vortex. The future projections revealed more total variance attributable to the secondary mode, suggesting additional influences from the stratosphere. The correlation between AO and the leading mode decreased, while the correlation between AO and the secondary mode increased, implying increased complexity regarding the predictability of EAWM interannual variations in future projections.

Atmospheric Circulation Cells Associated with Anomalous East Asian Winter Monsoon

Atmospheric circulation cells associated with anomalous East Asian Winter Monsoon （EAWM） were studied using the 1948/49 to 2002/03 NCEP/NCAR reanalysis and NCAR CAM3 AGCM simulations with monthly global sea surface temperatures from 1950 to 2000. Several atmospheric cells in the Pacific [i.e., the zonal Walker cell （ZWC） in the tropic, the Hadley cell in the western Pacific （WPHC）, the midlatitude zonal cell （MZC） over the central North Pacific, and the Hadley cell in the eastern Pacific （EPHC）] are associated with anomalous EAWM. When the EAWM is strong, ZWC, WPHC, and MZC are enhanced, as opposed to EPHC. The anomalous enhanced ZWC is characterized by air parcels rising in the western tropical Pacific, flowing eastward in the upper troposphere, and descending in the tropical central Pacific before returning to the tropical western Pacific. The enhanced MZC has characteristics opposite those of the enhanced ZWC in the central North Pacific. The anomalous WPHC shows air parcels rising in the western Pacific, as in the case of ZWC, followed by flowing northward in the upper troposphere and descending in the west North Pacific, as in the case of the enhanced MZC before returning to the western tropical Pacific. The anomalous EPHC is opposite in properties to the anomalous WPHC. Opposite characteristics are found during the weak EAWM period. The model simulations and the observations show similar characteristics and indicate the important role of sea surface temperature. A possible mechanism is proposed to link interannual variation of EAWM with the central-eastern tropical Pacific sea surface temperature anomaly （SSTA）.

A late Holocene winter monsoon record inferred from the palaeo-aeolian sand dune in the southeastern Mu Us Desert, northern China

The variation of the Asian winter monsoonal strength has seriously affected the climate and environmental conditions in the Asian monsoonal region, and even in marginal islands and the ocean in the East Asian region. However, relevant under-standing remains unclear due to the lack of suitable geological materials and effective proxies in the key study areas. Here, we present a grain-size record derived from the palaeo-aeolian sand dune in the southeastern Mu Us Desert, together with other proxies and OSL dating, which reflect a relatively detailed history of the winter monsoon and abrupt environmental events during the past 4.2 ka. Our grain-size standard deviation model indicated that 〉224 μm content can be considered as an indicator of the intensity of Asian winter monsoon, and it shows declined around 4.2–2.1 ka, enhanced but unstable in 2.1–0.9 ka, and obviously stronger since then. In addition, several typical climate events were also documented, forced by the periodic variation of winter monsoonal intensity. These include the cold intervals of 4.2, 2.8, 1.4 ka, and the Little Ice Age （LIA）, and relatively warm sub-phases around 3.0, 2.1, 1.8 ka, and the Medieval Warm Period （MWP）, which were roughly accordant with the records of the aeolian materials, peat, stalagmites, ice cores, and sea sediments in various latitudes of the Northern Hemisphere. Combined with the previous progresses of the Asian summer monsoon, we prelimi-narily confirmed a millennial-scale anti-correlation of Asian winter and summer monsoons in the Late Holocene epoch. This study suggests that the evolution of the palaeo-aeolian sand dune has the potential for comprehending the history of Asian monsoon across the desert regions of the modern Asian monsoonal margin in northern China.

A Numerical Study on the Winter Monsoon and Cold Surge over East Asia

By using the improved regional climate model （RegCM_NCC）, a numerical study has been undertaken for the East Asia region over a period of 5 years （1998-2002） in an effort to evaluate the model＇s ability to reproduce the winter monsoon conditions that were observed. The results showed that the model can successfully simulate the basic characteristics of the winter monsoon circulations, including the location and intensity of the cold-surface, high-pressure system, as well as the wind patterns and the intensity of the winter monsoon. The simulated occurrence frequency and regions of the cold surge were consistent with the observations. The simulated rainfall distribution over China was consistent with the observations collected in South China. The features of the simulated moisture transport were also in good agreement with the observations that were derived from the NCEP reanalysis data, indicating that moisture transport coming from the Bay of Bengal trough plays a crucial role in supplying moisture needed for precipitation in South China. In addition, the moisture transport coming from the near-equatorial west-Pacific was also important. These two branches of moisture transport converged in South China, as a prerequisite for occurrence of the precipitation that was observed there. Heat budgets have shown that the development of a heat sink over the East Asian continent was remarkable and its thermal contrast relative to the neighboring seas was the important forcing factor for the winter monsoon activity. The simulation also indicated that the significant differences in circulation patterns and rainfalls during the winters of 1997/98 and 1998/99 were affected by cold and warm ENSO events, respectively. The above analysis demonstrated the model＇s ability to simulate the East Asian winter monsoon.

Relationships between intensity of the Kuroshio current in the East China Sea and the East Asian winter monsoon

Based on satellite altimeter and reanalysis data,this paper studies the relationships between the intensity of the Kuroshio current in the East China Sea（ECS） and the East Asian winter monsoon（EAWM）.The mechanisms of their possible interaction are also discussed.Results indicate that adjacent transects show consistent variations,and on an interannual timescale,when the EAWM is anomalously strong（weak）,the downstream Kuroshio in the ECS is suppressed（enhanced） in the following year from February to April.This phenomenon can be attributed to both the dynamic effect（i.e.,Ekman transport） and the thermal effect of the EAWM.When the EAWM strengthens（weakens）,the midstream and downstream Kuroshio in the ECS are also suppressed（intensified） during the following year from October to December.The mechanisms vary for these effects.The EAWM exerts its influence on the Kuroshio＇s intensity in the following year through the tropospheric biennial oscillation（TBO）,and oceanic forcing is dominant during this time.The air-sea interaction is modulated by the relative strength of the EAWM and the Kuroshio in the ECS.The non-equivalence of spatial scales between the monsoon and the Kuroshio determines that their interactions are aided by processes with a smaller spatial scale,i.e.,local wind stress and heating at the sea surface.

Predictability of the East Asian Winter Monsoon Indices by the Coupled Models of ENSEMBLES

The seasonal predictability of various East Asian winter monsoon （EAWM） indices was investigated in this study based on the retrospective forecasts of the five state-of-the-art coupled models from ENSEMBLES for a 46-year period of 19612006.It was found that the ENSEMBLES models predict five out of the 21 EAWM indices well,with temporal correlation coefficients ranging from 0.54 to 0.61.These five indices are defined by the averaged lower-tropospheric winds over the low latitudes （south of 30°N）.Further analyses indicated that the predictability of these five indices originates from their intimate relationship with ENSO.A cross-validated prediction,which took the preceding （November） observed Nifo3.4 index as a predictor,gives a prediction skill almost identical to that shown by the model.On the other hand,the models present rather low predictability for the other indices and for surface air temperature in East Asia.In addition,the models fail to reproduce the relationship between the indices of different categories,implying that they cannot capture the tropicalextratropical interaction related to EAWM variability.Together,these results suggest that reliable prediction of the EAWM indices and East Asian air temperature remains a challenge.

Influence of Low-frequency Solar Forcing on the East Asian Winter Monsoon Based on HadCM3 and Observations

In this study, we investigate the influence of low-frequency solar forcing on the East Asian winter monsoon（EAWM）by analyzing a four-member ensemble of 600-year simulations performed with Had CM3（Hadley Centre Coupled Model,version 3）. We find that the EAWM is strengthened when total solar irradiance（TSI） increases on the multidecadal time scale. The model results indicate that positive TSI anomalies can result in the weakening of Atlantic meridional overturning circulation, causing negative sea surface temperature（SST） anomalies in the North Atlantic. Especially for the subtropical North Atlantic, the negative SST anomalies can excite an anomalous Rossby wave train that moves from the subtropical North Atlantic to the Greenland Sea and finally to Siberia. In this process, the positive sea-ice feedback over the Greenland Sea further enhances the Rossby wave. The wave train can reach the Siberian region, and strengthen the Siberian high. As a result, low-level East Asian winter circulation is strengthened and the surface air temperature in East Asia decreases. Overall,when solar forcing is stronger on the multidecadal time scale, the EAWM is typically stronger than normal. Finally, a similar linkage can be observed between the EAWM and solar forcing during the period 1850–1970.

Simple Metrics for Representing East Asian Winter Monsoon Variability:Urals Blocking and Western Pacific Teleconnection Patterns

Instead of conventional East Asian winter monsoon indices （EAWMIs）, we simply use two large-scale teleconnection patterns to represent long-term variations in the EAWM. First, the Urals blocking pattern index （UBI） is closely related to cold air advection from the high latitudes towards western Siberia, such that it shows an implicit linkage with the Siberian high intensity and the surface air temperature （SAT） variations north of 40°N in the EAWM region. Second, the well-known western Pacific teleconnection index （WPI） is connected with the meridional displacement of the East Asian jet stream and the East Asian trough. This is strongly related to the SAT variations in the coastal area south of 40°N in the EAWM region. The temperature variation in the EAWM region is also represented by the two dominant temperature modes, which are called the northern temperature mode （NTM） and the southern temperature mode （STM）. Compared to 19 existing EAWMIs and other well-known teleconnection patterns, the UBI shows the strongest correlation with the NTM, while the WPI shows an equally strong correlation with the STM as four EAWMIs. The UBI-NTM and WPI-STM relationships are robust when the correlation analysis is repeated by （1） the 31-year running correlation and （2） the 8-year high-pass and low-pass filter. Hence, these results are useful for analyzing the large-scale teleconnections of the EAWM and for evaluating this issue in climate models. Int particular, more studies should focus on the teleconnection patterns over extratropical Eurasia.

Evaluation of the Twentieth Century Reanalysis Dataset in Describing East Asian Winter Monsoon Variability

The Twentieth Century Reanalysis （20thCR） dataset released in 2010 covers the period 1871-2010 and is one of the longest reanalysis datasets available worldwide. Using ERA-40, ERA-Interim and NCEP-NCAR reanalysis data, as well as HadSLP2 data and meteorological temperature records over eastern China, the performances of 20thCR in reproducing the spatial patterns and temporal variability of the East Asian winter monsoon （EAWM） are examined. Results indicate that 20thCR data： （1） can accurately reproduce the most typical configuration patterns of all sub-factors differences in the main circulation fields over East Asia involved in the EAWM system, albeit with some in comparison to ERA-40 reanalysis data; （2） is reliable and stable in describing the temporal variability of EAWM since the 1930s; and （3） can describe the high-frequency variability of EAWM better than the low-frequency fluctuations, especially in the early period. In conclusion, caution should be taken when using 20thCR data to study interdecadal variabilities or long-term trends of the EAWM, especially prior to the 1930s.

East Asian Winter Monsoon record from the environmental sensitive grain size component of QF Old Red Sand, Haitan Island, China

The ＂Old Red Sand＂ is widely distributed along the coast of Fujian Province, China. Most studies have been carried out from as- pects of the origin, age and laterization of the ＂Old Red Sand＂, but this paper focused on reconstructing the history of the Asian Winter Monsoon change. On the basis of granulometric analysis of high-resohition samples, we have obtained environmental sen- sitive grain size component （ESGSC） from the Qingfeng （QF） profile by using the grain size-standard deviation method, which proves that the selected ESGSC is an important climate proxy. The mean grain size of this ESGSC could be used to reconstruct the East Asian Winter Monsoon （EAWM） intensity. As such, the history of the EAWM change since 44.0 ka reconstructed here reveals three main phases based on chronology dates of previous researches： （1） 44.0-25.5 ka B.P., the EAWM is relatively weak but increases gradually with fluctuations; （2） 25.5-15.5 ka B.P., relatively strong with high frequency fluctuations; （3） 15.5-7.1 ka B.P., with a weaker winter monsoon, but during 11-10 ka B.P. is remarkably enhanced. The EAWM recorded by mean grain size of the two neighboring sections have a better repeatability, so the millennial scales oscillation should be a reliable signal of the EAWM intensity. The climate recorded by ESGSC of the QF ＂Old Red Sand＂ compared to 6-80 of Huhi Cave stalagmites and Greenland GISP2 ice cores shows a good consistency, especially in detail, the YD event and four Heinrich events are all recorded, but the signal of D-O cycles was relatively weak.

The strong winter monsoon in East Asia and El Nino

The relationship between meridional wind in mid - latitudes of East Asia and SST in the Equatorial Eastern Pacific is analysed in this paper. It is pointed out that there exist close relations between the seasonal changes of the meridional wind in mid - latitudes of East Asia and SST in the Equatorial Eastern Pacific. The intensification of north winds over East Asia also plays an important role in the rise of SST in Equatorial Eastern Pacific one year later. The strong winter monsoon usually occurs in previous winter of El Nino and it causes low temperature to a great extent in China . The low temperature in China can be regarded as a precursor of El Nino.

Definition and characteristics of the south edge of the subtropical winter monsoon in East Asian

Based on observational daily data of 730 meteorological stations in China, the south edge of the subtropical winter monsoon is defined according to relevant criterion and its variation characteristics are analyzed. Results show that this south edge has obvious inter-annual variation characteristics and shows a northward moving tendency as a whole, but since the 21 st century it has moved southwards and date of the south edge entering winter becomes earlier. Wind fields of the anomalously northward south edge of the subtropical winter monsoon in East Asia has an obvious southerly wind component which prevents cold air from moving southward. The index of this south edge and winter temperature has a positive correlation. Climate warming might be the main reason for the northward movement of the south edge of the subtropical winter monsoon.

Low- and Mid-High Latitude Components of the East Asian Winter Monsoon and Their Reflecting Variations in Winter Climate over Eastern China

The present study defines a low-latitude component (regionally averaged winter 1000-hPa V-winds over 10 25°N, 105 135°E) and a mid-high-latitude component (regionally averaged winter 1000-hPa V-winds over 30 50°N, 110 125°E) of the East Asian winter monsoon (EAWM), which are denoted as EAWM-L and EAWM-M, respectively. The study examines the variation characteristics, reflecting variations in winter climate over eastern China, and associated atmospheric circulations corresponding to the two components. The main results are as follows: 1) the EAWM-L and EAWM-M have consistent variation in some years but opposite variations in other years; 2) the EAWM-M index mainly reflects the extensive temperature variability over eastern China, while the EAWM-L index better reflects the variation in winter precipitation over most parts of eastern China; and 3) corresponding to the variation in the EAWM-M index, anomalous winds over the mid-high latitudes of East Asia modulate the southward invasion of cold air from the high latitudes and accordingly affect temperatures over eastern China. In combination with the variation in the EAWM-L index, anomalous low-latitudinal winds regulate the water vapor transport from tropical oceans to eastern China, resulting in anomalous winter precipitation. These pronounced differences between the EAWM-L and the EAWM-M suggest that it is necessary to explore the monsoons' individual features and effects in the EAWM study.