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Li Y.,Institute of Plateau Meteorology | Yang S.,National Oceanic and Atmospheric Administration
Journal of Climate | Year: 2010

A new index measuring the East Asian winter monsoon is defined using the mean wind shears of upper-tropospheric zonal wind based on the belief that the physical processes of both higher and lower latitudes, and at both lower and upper troposphere, should be considered to depict the variability of monsoon. When the index is high (low), the westerly jet is strong (weak), the East Asian trough is deep (shallow), the Siberian high is strong (weak), and anomalous low-level northerlies (southerlies) prevail over East Asia. As a result, the surface and lower-tropospheric temperature over East Asia decreases (increases) and the cold surges over Southeast Asia and tropical western Pacific are more (less) active. The index, which exhibits distinct interannual variations, is also strongly correlated with the Arctic Oscillation and Niño-3.4 sea surface temperature (SST) index. Compared to previous indexes, this index takes into account more influencing factors and better elucidates the physical processes associated with monsoon, enhancing interpretations of the variability of monsoon and its effects on regional weather and climate. Furthermore, the monsoon index is significantly linked to antecedent tropical Pacific SST and is highly predictable in the NCEP Climate Forecast System, indicating the advantage of the index for operational predictions of monsoon. © 2010 American Meteorological Society.

Liu X.,National Climate Center | Yang S.,5830 University Research Court | Kumar A.,5830 University Research Court | Weaver S.,5830 University Research Court | And 2 more authors.
Climate Dynamics | Year: 2013

Biases of subseasonal prediction of the Asian summer monsoon are diagnosed using daily data from the hindcasts of 45-day integrations by the National Centers for Environmental Prediction Climate Forecast System version 2. The retrospective forecasts often show apparent systematic biases, which are mostly represented by the underestimation of the whole Asian monsoon. Biases depend not only on lead time, but also on the stage of monsoon evolution. An abrupt turning point of bias development appears around late June and early July, when ensemble spread and bias growth of winds and precipitation show a significant change over the northwestern Pacific (NWP) and the South Asian summer monsoon (SASM) region. The abrupt turning of bias development of winds, precipitation, and surface temperature is also captured by the first two modes of multivariate empirical orthogonal function analysis. Several features appear associated with the abrupt change in bias development: the western Pacific subtropical high (WPSH) begins its first northward jump and the surface temperature over the Tibetan Plateau commences a transition from warm bias to cold bias, and a reversal of surface temperature biases occurs in the eastern tropical Indian Ocean and the SASM region. The shift of WPSH position and the transition of surface thermal bias show close relationships with the formation of bias centers in winds and precipitation. The rapid growth in bias due to the strong internal atmospheric variability during short leads seems to mainly account for the weak WPSH and SASM in the model. However, at certain stages, particularly for longer-lead predictions, the biases of slowly varying components may also play an important role in bias development of winds and precipitation. © 2012 Springer-Verlag Berlin Heidelberg.

Jiang X.,Institute of Plateau Meteorology | Yang S.,National Oceanic and Atmospheric Administration | Li Y.,Institute of Plateau Meteorology | Kumar A.,National Oceanic and Atmospheric Administration | And 3 more authors.
Journal of Climate | Year: 2013

The NCEP Climate Forecast System (CFS) is an important source of information for seasonal climate prediction in many Asian countries affected by monsoon climate. The authors provide a comprehensive analysis of the prediction of the Asian summer monsoon (ASM) by the new CFS version 2 (CFSv2) using the hindcast for 1983-2010, focusing on seasonal-to-interannual time scales. Many ASM features are well predicted by the CFSv2, including heavy monsoon rainfall centers, large-scale monsoon circulation patterns, and monsoon onset and retreat features. Several commonly used dynamical monsoon indices and their associated precipitation and circulation patterns can be predicted several months in advance. The CFSv2 hasbetter skill in predicting the Southeast Asian monsoon than predicting the South Asian monsoon. Compared to CFS version 1 (CFSv1), the CFSv2 has increased skill in predicting large-scale monsoon circulation and precipitation features but decreased skill for the South Asian monsoon, although some biases in the CFSv1 still exist in the CFSv2, especially the weaker-than-observed western Pacific subtropical high and the exaggerated strong link of the ASM to ENSO. Comparison of CFSv2 hindcast with output from AtmosphericModel Intercomparison Project (AMIP) and Coupled Model Intercomparison Project (CMIP) simulations indicates that exclusion of ocean-atmosphere coupling leads to a weaker ASM. Compared to AMIP, both hindcast and CMIP show a more realistic annual cycle of precipitation, and the interannual variability of the ASM is better in hindcast. However, CMIP does not show any advantage in depicting the processes associated with the interannual variability of major dynamical monsoon indices compared to AMIP. © 2013 American Meteorological Society.

Hu H.,Institute of Plateau Meteorology | Liang L.,Institute of Plateau Meteorology
Acta Geographica Sinica | Year: 2014

Based on the data set of 60 weather stations in the east of Qinghai-Tibet Plateau for the period 1967-2012 and the methods of REOF, polynomial fitting, running t-test, the temporal and spatial variations of snowfall are analyzed. Meanwhile, combined with the variation of precipitation and air temperature, the relationship between snowfall and snow cover is discussed. The results indicate the spatial distribution of annual snowfall in the eastern Qinghai-Tibet Plateau is far from uniformity, varying in the range from 1.3 to 152.5 mm. The autumn snowfall is richer in the central part than in the surrounding area, the winter snowfall shows a gradual decreasing trend from southeast to northwest, the spring snowfall shows the same spatial distribution as the annual snowfall. Based on the REOF method, the field of snowfall could be classified into eight subregions: Southern Qinghai Plateau, Northern Tibet Plateau, Qaidam Basin, the southeast edge of Qinghai-Tibet Plateau, northwest of Western Sichuan Plateau, the south edge of Qinghai-Tibet Plateau, northeast of Qinghai Plateau and the valley in Southern Tibet Plateau. As a whole, in the eastern Qinghai-Tibet Plateau, except for autumn, the snowfall shows the obvious interdecadal change, namely, the snowfall was little before the mid-1980s, rich from the late 1980s to the late 1990s, and became little again after the end of the 20th century. By using running t-test method, winter snowfall shifted from the little stage to the rich stage in 1986, the snowfall in winter and spring shifted from the rich stage to the little stage in 1997. Also, in different subregions, the temporal variation of snowfall shows different characteristics. There is close relationship between snowfall and snow depth in the east of Qinghai-Tibet Plateau. The spring snowfall is influenced significantly by air temperature, followed by autumn and winter snowfall. Different from the shift of spring rainfall from the little stage to the rich stage at the end of 20th century, spring snowfall exhibited the shift from the rich stage to the little stage because of the rise of air temperature.

Jiang X.,Institute of Plateau Meteorology | Yang S.,Sun Yat Sen University | Li Y.,Institute of Plateau Meteorology | Kumar A.,National Oceanic and Atmospheric Administration | And 2 more authors.
Journal of Geophysical Research: Atmospheres | Year: 2013

The National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS) provides important source of information on seasonal climate prediction for many Asian countries that are affected by monsoon. In this study, the authors provide a comprehensive assessment of the prediction of East Asian winter monsoon (EAWM) by the CFS version 2 (CFSv2) using the hindcast for 1983-2010, with a focus on seasonal-interannual time scales. Output from the Atmospheric Model Intercomparison Project (AMIP) and the Coupled Model Intercomparison Project (CMIP) simulations is also analyzed to understand the physical process of monsoon. Several major features of the EAWM are well predicted by the CFSv2. Particularly, the EAWM-related atmospheric circulation and surface climate over oceans are well predicted several months in advance, and the prediction over oceans is better than that over land. While the CFSv2 has low skill in predicting the Arctic Oscillation (AO), it well predicts El Niño-Southern Oscillation (ENSO) and its impact on the EAWM, contributing to the decent prediction of EAWM. Comparisons among hindcast, AMIP, and CMIP indicate that ocean-atmosphere coupling is important for EAWM prediction. While the EAWM in AMIP is weaker, CMIP predicts more realistic monsoon features. The enhanced performance of CMIP is partly attributed to its better simulation of precipitation over the western Maritime Continent (MC). All three types of experiments fail to depict the relationship between EAWM and AO and simulate a stronger-than-observed response of EAWM to ENSO. Improving the simulation of convection over the MC potentially enhances the skill of CFSv2 in predicting the EAWM. Key PointsShowing skill of winter monsoon prediction by major climate forecast systemDiscussing air-sea interaction, tropical convection, and winter monsoonProviding useful information for monsoon prediction operation ©2013. American Geophysical Union. All Rights Reserved.

Xu L.,Institute of Plateau Meteorology | Xu L.,National Climate Center | Li Y.,Institute of Plateau Meteorology
Climate Dynamics | Year: 2010

Based on recent Moderate Resolution Imaging Spectroradiometer high-resolution snow retrieval data over the Tibetan Plateau, the connection of snow anomalies to the East Asian Summer Monsoon has been reexamined to eliminate the uncertainty of the previous snow mapping in this region. The snow monitoring shows relatively large interannual variations over the Tibetan Plateau during winter and spring. The snow anomalies, however, show relatively short persistence-usually less than 3 months-that is substantial shorter than previous studies. The anomalous snow pack in the spring shows a good correlation to the onset of the East Asian Monsoon but does not show a significant relationship with the intensity of the summer monsoon as suggested by previous studies. The sequential thermal forcing of snow anomalies to the general circulation and monsoon onset over East Asia is further investigated. A positive/negative snow anomalies during spring will change the energy and water exchange at the land surface, suppressing/enhancing the sensible heating over the Tibetan, delaying/accelerating the reversal of the meridional temperature gradient between the Eurasian continent and the tropical ocean and further impact the onset of East Asian monsoon. © 2009 Springer-Verlag.

Wang S.,Institute of Plateau Meteorology | Zhang X.,University of Sichuan
Theoretical and Applied Climatology | Year: 2012

On the basis of the mean annual and seasonal temperatures from 30 meteorological stations in the Jinsha River Basin (JRB) from 1961 to 2008, the temperature trends are analyzed by using Mann-Kendall test and linear trend analysis. There is an increasing trend in mean annual and seasonal temperatures during this period, and the increasing trends in winter seem more significant than those in the other three seasons. The mean annual temperature has increased by 0. 0158°C/year during the last 48 years. There are more than 70% of stations exhibiting increasing trends for annual and seasonal temperatures. The increasing trends in the headwater and upper reaches are more dominant than those in the middle and lower reaches. The largest increase magnitude occurred in the low temperature area, while the largest decrease magnitude occurred in the high temperature area. The decreasing trends are mainly characterized for the maximum temperature time series, and summer is the only season showing a slight and insignificant increasing trend. All the time series showed a statistically significant increasing trend at the level of α = 0. 05 for the minimum temperature time series. As a whole, the increasing magnitude of the minimum temperature is significantly greater than the decreasing magnitude of the maximum temperature. © 2012 Springer-Verlag.

Yang S.,Sun Yat Sen University | Jiang X.,Institute of Plateau Meteorology
Journal of Climate | Year: 2014

The eastern Pacific (EP) El Niño-Southern Oscillation (ENSO) and the central Pacific (CP) ENSO exert different influences on climate. In this study, the authors analyze the hindcasts of the NCEP Climate Forecast System, version 2 (CFSv2), and assess the skills of predicting the two types of ENSO and their impacts on East Asian climate. The possible causes of different prediction skills for different types of ENSOare also discussed. CFSv2 captures the spatial patterns of sea surface temperature (SST) related to the two types of ENSO and their different climate impacts several months in advance. The dynamical prediction of the two types of ENSO by the model, whose skill is season dependent, is better than the prediction based on the persistency of observed ENSO-related SST, especially for summer and fall. CFSv2 performs well in predicting EP ENSO and its impacts on the East Asian winter monsoon and on the Southeast Asian monsoon during its decaying summer. However, for both EP ENSO and CP ENSO, the model overestimates the extent of the anomalous anticyclone over the western North Pacific Ocean from the developing autumn to the next spring but underestimates the magnitude of climate anomalies in general. It fails to simulate the SST pattern and climate impact of CP ENSO during its developing summer. The model's deficiency in predicting CP ENSO may be linked to a warm bias in the eastern Pacific. However, errors in simulating the climate impacts of the two types of ENSO should not be solely ascribed to the bias in SST simulation. © 2014 American Meteorological Society.

Gao H.,National Climate Center | Yang S.,National Oceanic and Atmospheric Administration | Kumar A.,National Oceanic and Atmospheric Administration | Hu Z.-Z.,National Oceanic and Atmospheric Administration | And 3 more authors.
Journal of Climate | Year: 2011

The East Asian mei-yu (EAMY), which includes the mei-yu over eastern China, baiu over Japan, and changma over Korea, is an important component of the Asia summer monsoon system. The EAMY rain belt jumps northward to the Yangtze and Huaihe River valleys (in China), Japan, and Korea from mid-June to mid-July, with remarkable interannual variability. In this study, the variability and predictability of EAMY are investigated using the retrospective ensemble predictions of the NCEP Climate Forecast System (CFS). The CFS reasonably captures the centers, magnitude, northward jump, and other features of EAMY over most regions. It also reasonably simulates the interannual variations of EAMY and its main influencing factors such as the western Pacific subtropical high, the East Asian monsoon circulation, and El Niño- Southern Oscillation (ENSO). The CFS is skillful in predicting EAMY and related circulation patterns with a lead time of one month. An empirical orthogonal function analysis with maximized signal-to-noise ratio is applied to determine the most predictable patterns of EAMY. Furthermore, experiments in which the CFS is forced by observed sea surface temperature (SST) exhibit lower skill in EAMY simulation, suggesting the importance of ocean-atmosphere coupling in predicting EAMY. The CFS, which exaggerates the precipitation over the southern-southeastern hills of the Tibetan Plateau, overestimates the relationship between EAMY and tropical-subtropical atmospheric circulation due to the overly strong ENSO signals in the model, whereas the experiments forced by observed SST produce a weaker relationship. On the contrary, the CFS underestimates the link of EAMY to higher-latitude processes. An increase in the horizontal resolution of the CFS is expected to reduce some of these errors. © 2011 American Meteorological Society.

Wu R.,Center for Ocean Land Atmosphere Studies | Wen Z.,Sun Yat Sen University | Yang S.,National Oceanic and Atmospheric Administration | Li Y.,Institute of Plateau Meteorology
Journal of Climate | Year: 2010

The present study documents a pronounced interdecadal change in summer rainfall over southern China around 1992/93 and explores the plausible reasons for this change. The summer rainfall is persistently below normal during 1980-92 and above normal during 1993-2002. Coherent changes in atmospheric circulation are identified over East Asia and the South China Sea (SCS)-western North Pacific (WNP). The increase in rainfall is accompanied by an increase in lower-level convergence, midtropospheric ascent, and upper-level divergence over southern China. The changes in lower-level winds feature two anomalous anticyclones: one over the SCS-subtropical WNP, and the other over north China-Mongolia. The outflows from the two anomalous anticyclones converge over southern China, leading to anomalous moisture convergence, enhanced ascent, and increased rainfall. The development of the northern anticyclone is related to an increase in the Tibetan Plateau snow cover in the preceding winter-spring that leads to a contrast in temperature change between the plateau and the surrounding regions. The relatively small temperature change over the plateau, coupled with increases in temperature to the west and the east, leads to an increase in surface pressure extending northward from the plateau. The development of the southern anticyclone is related to an increase in sea surface temperature in the equatorial Indian Ocean that enhances lower-level convergence and ascent. The accompanying upper-level divergent flows from the tropical Indian Ocean to the SCS-WNP lead to the development of anomalous descent and lower-level anomalous anticyclone over the SCS-WNP. © 2010 American Meteorological Society.

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