Mid-latitude East Asia Research Articles

Interdecadal change of Eurasian snow, surface temperature, and atmospheric circulation in the late 1980s

AbstractBoreal winter and spring snow cover extent and snow water equivalent over Eurasia experienced an obvious decrease in late 1980s. A concurrent surface warming is observed over the North Atlantic and Eurasia. The present study documents the relationship among the interdecadal changes in snow, surface air temperature, and wind and the plausible reason for this change. Analysis shows that the snow decrease contributes to the temperature increase only in some limited regions and that the surface warming is largely related to the change in atmospheric circulation. The effect of atmospheric circulation change on surface warming is manifested in wind‐induced heat advection and cloud‐radiation changes. The wind changes over the North Atlantic and Eurasia feature a wave train with an anomalous anticyclone over Europe and anomalous southerly winds over midlatitude east Europe and Asia. The anomalous southerly winds contribute to surface warming through advection of warmer air from lower latitudes. The anomalous anticyclone contributes to surface warming over Europe through anomalous warm advection by bringing warmer air from ocean and enhanced downward shortwave radiation by suppressing upward motion. The wave pattern appears to be connected to equatorial Atlantic warming. Warmer sea surface temperature (SST) in the equatorial Atlantic Ocean enhances convection and induces a lower level anomalous cyclone to the northwest of the SST warming. The role of equatorial North Atlantic SST increase in the formation of the wave train over Eurasia is supported by numerical experiments with an atmospheric general circulation model.

Comparison of the impact of the Arctic Oscillation and Eurasian teleconnection on interannual variation in East Asian winter temperatures and monsoon

The large-scale impacts of the Arctic Oscillation (AO) and the Eurasian teleconnection (EU) on the East Asian winter climate are compared for the past 34 winters, focusing on (1) interannual monthly to seasonal temperature variability, (2) East Asian winter monsoon (EAWM), and (3) the Siberian high (SH) and cold surge. Regression analysis reveals warming by AO and cooling by EU over mid-latitude East Asia during their positive phase and vice versa (i.e., warm phase: +AO, −EU; cold phase: −AO, +EU). The EU impact was found to be comparable to the AO impact. For example, warm (cold) months during the warm (cold) AO phase are found clearly when the AO is in the same warm (cold) EU phase. No significant correlation was found between East Asian temperature and the AO when the warm (cold) AO coincided with the cold (warm) EU. The well-known relationship of strong (weak) SH during the cold (warm) AO phase was observed significantly more often when the AO was in the same cold (warm) EU phase. Also, the indices of EAWM, cold surge, and SH were more highly correlated with the EU than with the AO. The advective temperature change and associated circulation demonstrate that the large-scale field including the SH over the mid-latitude Asian inland is better represented by the EU, influencing the East Asian climate. These results suggest that the impact of EU should be considered more important than previously thought for a better understanding of East Asian winter temperature and monsoon variability.

Initiation of East Asia monsoon failure at the climate transition from the Medieval Climate Anomaly to the Little Ice Age

We have reconstructed decadally-resolved continuous sea surface temperature and seawater δ18O (hence salinity) records over the last 1300yr from alkenone and planktonic foraminiferal oxygen isotope ratio analyses of the East Sea/Japan Sea marine sediments to investigate East Asia monsoon variability. Comparisons of the records with other paleoclimate records indicate a possible connection between changes in the mid-latitude East Asia monsoon and Pacific Decadal Oscillation (PDO) over this period. The results show that during the Medieval Climate Anomaly (MCA) when the PDO index was negative, East Asia was characterized by surface warming with a strengthened summer monsoon. Summer monsoon-related precipitation increased and pluvials possibly dominated in the region at that time. Onset of Asia monsoon failure and severe drought occurred at the end of the MCA and extended to the Little Ice Age (LIA) when the PDO became positive.

북서태평양 몬순이 한국 영향태풍활동에 미치는 영향

In this study, the correlation between the frequency of summer tropical cyclones (TC) affecting areas around Korea over the last 37 years and the western North Pacific monsoon index (WNPMI) was analyzed. A clear positive correlation existed between the two variables, and this high positive correlation remained unchanged even when excluding El Nio-Southern Oscillation (ENSO) years. To investigate the causes of the positive correlation between these two variables, ENSO years were excluded, after which the 8 years with the highest WNPMI (positive WNPMI phase) and the 8 years with the lowest WNPMI (negative WNPMI phase) were selected, and the average difference between the two phases was analyzed. In the positive WNPMI phase, TCs usually occurred in the eastern waters of the tropical and subtropical western North Pacific, and tended to pass the East China Sea on their way north toward Korea and Japan. In the negative WNPMI phase, TCs usually occurred in the western waters of the tropical and subtropical western North Pacific, and tended to pass the South China Sea on their way west toward the southeastern Chinese coast and the Indochina peninsula. Therefore, TC intensity was higher in the positive WNPMI phase, during which TCs are able to gain sufficient energy from the sea while moving a long distance to areas nearby Korea. TCs also tended to occur more often in the positive WNPMI phase. In the difference between the two phases regarding 850 and 500 hPa streamline, anomalous cyclones were reinforced in the tropical and subtropical western North Pacific, while anomalous anticyclones were reinforced in mid-latitude East Asian areas. Due to these two anomalous pressure systems, anomalous southeasterlies developed in areas near Korea, with these anomalous southeasterlies playing the role of anomalous steering flows making the TCs head toward areas near Korea. Also, due to the anomalous cyclones developed in the tropical and subtropical western North Pacific, more TCs could occur in the positive WNPMI phase.

Nonstationary impact of the winter North Atlantic Oscillation and the response of mid-latitude Eurasian climate

The North Atlantic Oscillation (NAO), although a local atmospheric mode over the North Atlantic Ocean, plays an important remote role in the Eurasian climate. However, the link between the NAO and Eurasian climate might be unstable. Here, we present a study on the relationship between the winter NAO and mid-latitude Eurasian climate, especially the mid-latitude East Asian precipitation, mainly based on meteorological data and output from an 1155-year-long coupled ocean–atmosphere model simulation. The results show that the winter NAO exerts a remarkable effect on the changes in mid-latitude Eurasian climate; however, the impact of the NAO is nonstationary. According to the model output, the impact of the NAO varies synchronously with the NAO variance with a period of around 150 years. During the high NAO variance period, the NAO has significant correlation with mid-latitude East Asian precipitation; low NAO variance periods do not. The variation of the NAO–precipitation teleconnection may arise from the changing influence of NAO on the local temperature. The NAO signal moves eastward by a zonally oriented wave train, where it modulates the atmospheric circulation structure, and thus results in the nonstationary relationship between the NAO and mid-latitude East Asian precipitation.

A Comparative Study on Precipitation Climatology and Interannual Variability in the Lower Midlatitude East Asia and Central Asia

Abstract This study examines precipitation climatology and interannual variability in two regions in the lower midlatitude Asia to the east and west of the Tibetan Plateau, one located in monsoonal East Asia (the M region) and the other in semiarid central Asia (the W region). The focus is on the 5-month summer half year (May–September) for the M region and the winter half year (December–April) for the W region, corresponding to their respective rainy seasons. The main mechanism of moisture transport for the M region is the summer lower-tropospheric southerly winds, whereas the winter midtropospheric westerly circulation between 25° and 45°N is responsible for conducting moisture fluxes to the W region. It is further discovered that the winter precipitation series are positively correlated between the two regions (r = 0.47). There is also a weak cross-seasonal correlation between the winter W region precipitation and summer M region precipitation (r = 0.27). Winter westerly circulation over the W region is influenced by both the east Atlantic–western Russia and the polar–Eurasia extratropical teleconnection patterns, while El Niño–Southern Oscillation influences regional circulation patterns in both regions through teleconnections via the Indo-Pacific warm pool convection in winter and its lagged impact on the western North Pacific anticyclone over the Philippine Sea. In the meantime, responses of the regional winter circulation in the M region to the upstream westerly circulation intensity cause the correlation in winter precipitation between the two regions. Such linkages form the basis of the concurrent and cross-seasonal correlations in precipitation between the two remote regions.

冬季极赤温差年际变化及其与东亚气候异常的联系

Based on NCEP/NCAR reanalysis data, we develop an index of the temperature difference between polar and equatorial regions (TDPE) and investigate interannual variability of this index ( I TDPE-S) at 2 m height above the surface, in relation to winter rainfall and temperatures over East Asia. Results suggest that I TDPE-S is able to reveal the remarkable increase of winter surface temperature at boreal high latitudes during global warming, with 4-8 year periodic variation. I TDPE-S correlates well with winter rainfall and temperature in East Asia. As the index increases (decreases), winter precipitation increases (decreases) over Northeast China, Xinjiang, the Hexi Corridor, Sea of Japan, South China Sea, and the Philippines, corresponding to a drop (rise) of winter mean temperature in northern and northeastern parts of China. Further study reveals that in years of higher I TDPE-S, anomalous water vapor in mid-latitude East Asia originates mainly from the western North Pacific. Water vapor related to rainfall anomalies at the low latitudes has its origin mainly in tropical oceans. A horizontal temperature advection-caused anomalous reduction of temperature partially explains an abnormally low mean temperature in winter at mid-latitude East Asia. These results are important for in-depth understanding of the formation mechanisms and effects of atmospheric circulation anomalies.

Subseasonal and Interannual Temperature Variability in Relation to Extreme Temperature Occurrence over East Asia

AbstractThis study investigates interannual variability in the frequency of occurrence of daily surface air temperature (SAT) extremes over East Asia in summer and winter between 1979 and 2009. In particular, this study examines the dominant seasonal SAT patterns, as obtained through empirical orthogonal function (EOF) analysis, and the associated variability in SAT extreme occurrence. Overall, the authors find that changes in extreme temperature occurrence associated with these dominant patterns are impacted by both shifts and narrowing/broadening of the subseasonal SAT probability distribution functions (PDFs). In summer, the leading pattern features large SAT anomalies in midlatitude East Asia centered over Mongolia. Over this center of action, positive SAT anomalies are accompanied by decreased precipitation and soil moisture, which increases the ratio of sensible to latent heat flux. Consequently, subseasonal SAT variance increases, resulting in an enhanced occurrence of positive SAT extremes relative to a simple SAT PDF shift. In winter, the leading pattern, which is highly correlated with the Arctic Oscillation, features large loadings in high-latitude Siberia that decay southward. In contrast with summer, large-scale dynamics play a larger role in the leading pattern: positive SAT anomalies are accompanied by a weakened and northward-shifted storm track, reduced subseasonal SAT variance, and a more pronounced decrease of cold extreme occurrence relative to a simple PDF shift. Finally, a brief look at the secular trends suggests that both shifts and narrowing/broadening of the PDF may also impact long-term trends in SAT extreme occurrence over some regions of East Asia.

Impact of the dominant large‐scale teleconnections on winter temperature variability over East Asia

AbstractMonthly mean geopotential height for the past 33 DJF seasons archived in Modern Era Retrospective analysis for Research and Applications reanalysis is decomposed into the large‐scale teleconnection patterns to explain their impacts on winter temperature variability over East Asia. Following Arctic Oscillation (AO) that explains the largest variance, East Atlantic/West Russia (EA/WR), West Pacific (WP) and El Niño–Southern Oscillation (ENSO) are identified as the first four leading modes that significantly explain East Asian winter temperature variation. While the northern part of East Asia north of 50°N is prevailed by AO and EA/WR impacts, temperature in the midlatitudes (30°N–50°N), which include Mongolia, northeastern China, Shandong area, Korea, and Japan, is influenced by combined effect of the four leading teleconnections. ENSO impact on average over 33 winters is relatively weaker than the impact of the other three teleconnections. WP impact, which has received less attention than ENSO in earlier studies, characterizes winter temperatures over Korea, Japan, and central to southern China region south of 30°N mainly by advective process from the Pacific. Upper level wave activity fluxes reveal that, for the AO case, the height and circulation anomalies affecting midlatitude East Asian winter temperature is mainly located at higher latitudes north of East Asia. Distribution of the fluxes also explains that the stationary wave train associated with EA/WR propagates southeastward from the western Russia, affecting the East Asian winter temperature. Investigation on the impact of each teleconnection for the selected years reveals that the most dominant teleconnection over East Asia is not the same at all years, indicating a great deal of interannual variability. Comparison in temperature anomaly distributions between observation and temperature anomaly constructed using the combined effect of four leading teleconnections clearly show a reasonable consistency between them, demonstrating that the seasonal winter temperature distributions over East Asia are substantially explained by these four large‐scale circulation impacts.

Seasonal scale variability of the East Asian winter monsoon and the development of a two-dimensional monsoon index

This study investigates the seasonal scale variability of the East Asian winter monsoon (EAWM), which is distinguished from the seasonal cycle with temporal variation throughout winter. Winters lasting 120 days (Nov. 17–Mar. 16) for a period of 64 years from the NCEP daily reanalysis data set are used to study the seasonal scale variability of the EAWM. Cyclostationary empirical orthogonal function (CSEOF) analysis is adopted to decompose the variability of the EAWM. The second CSEOF mode of 850-hPa temperature exhibits a seasonal scale variation, the physical mechanism of which is explained in terms of physically consistent variations of temperature, geopotential height, sea level pressure, wind, and surface heat fluxes. The seasonal-scale EAWM exhibits a weak subseasonal and a strong interannual variability and has gradually weakened during the 64 years. In a weak EAWM phase, the land-sea contrast of sea level pressure declines in East Asia. Consistent with this change, low-level winds decrease and warm thermal advection increases over the eastern part of mid-latitude East Asia. Latent and sensible heat fluxes are reduced significantly over the marginal seas in East Asia. However, during a strong EAWM phase, the physical conditions in East Asia reverse. A large fraction of the variability of the EAWM is explained by the seasonal cycle and the seasonal scale variation. A two-dimensional EAWM index was developed to explain these two distinct components of the EAWM variability. The new index appears to be suitable for measuring both the subseasonal and the interannual variability of the EAWM.

The Impact of Tropical Indian Ocean Variability on Summer Surface Air Temperature in China

Abstract Evidence is presented that the boreal summer surface air temperature over south China and northeast China is remotely influenced by the Indian Ocean Basin mode (IOBM) sea surface temperature (SST) anomalies. Above-normal temperature in south China and below-normal temperature in northeast China correspond to a simultaneous Indian Ocean Basin warming. The teleconnection from Indian Ocean SST anomalies to China summer surface air temperature is investigated using observations and an atmospheric general circulation model (AGCM). The results herein indicate that the tropical Indian Ocean Basin warming can trigger a low-level anomalous anticyclone circulation in the subtropical northwest Pacific and an anomalous cyclone circulation in midlatitude East Asia through emanating a baroclinic Kelvin wave. In south China, the reduced rainfall and downward vertical motion associated with the anomalous low-level anticyclone circulation lead to above-normal summer surface air temperature. In northeast China, by contrast, upward vertical motion associated with the anomalous cyclone leads to below-normal summer surface air temperature.

Interdecadal Variations of Meridional Winds in the South China Sea and Their Relationship with Summer Climate in China*

Abstract Analysis of the NCEP and 40-yr ECMWF Re-Analysis (ERA-40) data and the Xisha Island station observation indicates that the low-level meridional wind (LLMW) over the South China Sea (SCS) experienced an interdecadal variation since the late 1970s. The LLMW change is associated with the reduction of tropospheric temperature in midlatitude East Asia. A mechanism is put forward to explain the triggering and maintenance of the tropospheric cooling. The enhanced convective heating over the southern South China Sea results in a meridional vertical overturning circulation, with anomalous descending motion appearing over continental East Asia. The anomalous descending motion reduces the local humidity through both anomalous low-level divergence and dry vertical advection. The decrease of the local tropospheric humidity leads to the enhanced outgoing longwave radiation into space and thus cold temperature anomalies. The decrease of the temperature and thickness leads to anomalous low (high) pressure and convergent (divergent) flows at upper (lower) levels. This further enhances the descending motion and leads to a positive feedback loop. The fall in tropospheric temperature over continental East Asia reduces the land–sea thermal contrast and leads to the weakening of cross-equatorial flows and the LLMW over SCS. A further diagnosis indicates that the LLMW is closely linked to the summer precipitation and temperature variations in China on interdecadal time scales. A weakening of the LLMW after 1976 is associated with a “−, +, −” meridional rainfall pattern, with less rain in Guangdong Province and north China but more rain in the Yangtze and Huaihe River basins and northeast China, and a “+, −, +” temperature pattern, with increased (decreased) surface temperature in the south and north (central) China.

East Asian dust storm and weather disturbance: possible links to the Arctic Oscillation

AbstractIn this work, the authors investigated the spring dust storm frequency variations in northern China and its relationship with weather disturbances, cold high, eddy kinetic energy (EKE), as well as the impact of the Arctic Oscillation (AO) on the dust storm frequency variations. It is found that there is a significantly decreasing trend of −26%/10 years in the dust storm frequency variations. In addition, the year‐to‐year variations are also remarkable, which account for 58.3% of the total variance of the dust storm frequency variations. The synoptic variance of 850 hPa height, EKE, and near‐surface cold highs are highly correlated with the dust storm frequency variations. During the last 40 years or so, there have been significant trends in synoptic variance, cold‐high frequency, and kinetic energy over northern China and Mongolia, which are −3.2, −4.0, and −5.0%/10 years, respectively. The trends are fairly consistent with long‐term variations of dust storm frequency. The AO shows good correlation with the interannual variations of the dust storm frequency. This out‐of‐phase relation is supported by the significantly negative correlations between AO index and eddy growth rate between 850/700 hPa in midlatitude East Asia, implying that AO can modulate weather disturbances and dust storm activities through influencing the atmospheric instability. It is found, however, that the spring AO index shows no evident long‐term trend. Land cover changes, global warming, and other climate teleconnections may, at least partly, be responsible for the secular decreasing of dust storm frequency over northern China. Copyright © 2006 Royal Meteorological Society.

A New East Asian Winter Monsoon Index and Associated Characteristics of the Winter Monsoon

A new East Asian winter monsoon index, which reflects the 300-hPa meridional wind shear associated with the jet stream, was defined to describe the variability of the winter monsoon in midlatitude East Asia. This index represents very well the seasonal mean winter temperature over Korea, Japan, and eastern China. The National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis data from 1958 to 2001 were used to examine the composite structures of strong and weak winter monsoons based on this index. The composite strong winter monsoon is characterized by an enhanced upper-level jet stream south of Japan, a strengthened midtropospheric East Asian trough, a stronger than normal Aleutian low and Siberian high, and increased low-level northeasterlies along the Russian coast. This composite structure suggests that a cold winter in Korea and Japan depends critically on processes that control the pressure gradients between the Aleutian low and the Siberian high. Power spectral analysis of the index shows significant peaks occurring in 3–4, 6–8, and around 18 yr. The decadal peak is primarily due to a prominent cold period from 1980 to 1986 versus a warm period from 1987 to 1993. The regressed sea level pressure field for the interannual component resembles the composite strong winter monsoon pattern, whereas the sea level pressure pattern for the decadal component bears close similarity to that of the Arctic Oscillation. These conditions in the winter monsoon are associated with excess snowfall in October over the Siberian high, northeastern China, and far eastern Russia. The sensitivity experiments with the Seoul National University general circulation model suggest that the change in snow depth in autumn over the Siberian high and northeastern China may lead to the variability of the winter monsoon intensity. The teleconnection analysis confirms that development of the Siberian high and/or the Aleutian low is associated with an enhanced East Asian winter monsoon; the Arctic Oscillation is closely related to the winter monsoon intensity on the decadal time scale.

Precipitation Characteristics in Mid-Latitude East Asia as Observed by TRMM PR and TMI

Precipitation radar and microwave radiometer data from the Tropical Rainfall Measuring Mission (TRMM) satellite during the spring and summer months of 1998 are used to study the precipitation characteristics in the mid-latitude East Asia region (20°-40°N, 100°-140°E), with particular emphasis on comparing the differences between stratiform and convective rains, and between rains during spring and summer. The characteristics in the mid-latitude are also compared with those in the tropics. The two-season mean results for the mid-latitudes show that the mean rainfall rate for stratiform clouds is ∼2 mm h-1, at least 6 times lower than its convective counterpart. The mean convective rainfall rate is higher over land than over ocean, and higher in the mid-latitude than in the tropics. Generally, convective clouds generate more rain total for both mid-latitudes and tropics although they cover less area than stratiform clouds. Comparing between mid-latitude and tropical regions, the contribution of stratiform rains is larger in the mid-latitudes than in the tropics. There are distinct differences between stratiform and convective precipitation profiles, as well as noticeable seasonal variations of these profiles. The mean vertical profiles showed that stratiform rains have a quasi-constant rainfall rate below freezing level and a sharp drop-off above. Convective profiles, on the other hand, often have a maximum rainfall rate somewhere below the freezing level. Above the freezing level, precipitation layer for convective rains is thicker than for stratiform rains. For convective rains, significant differences are found between profiles over land and ocean, with a significant deeper layer for land convections given the same surface rainfall rate. Consistently, microwave scattering signature over land, expressed by polarization-corrected temperature at 85 GHz, is nearly twice as strong as that over ocean for the same surface rainfall rate. Precipitation profiles are similar in shape and depth between mid-latitude and tropical regions during summer when the freezing level heights are also similar between the two regions. However, compared to being no noticeable variation in the tropics during a year, there is a significant seasonal change for the precipitation profiles in mid-latitudes. The seasonal difference of precipitation profiles arises from the deeper precipitation layer in the summer than in the spring for a given surface rainfall rate, being in consistent with the seasonal variation of the freezing level height. This change in precipitation depth applies to both stratiform and convective rains, and to rains over both land and ocean. It should be mentioned that all the aforementioned results, except for the 85 GHz scattering signature, are derived from TRMM precipitation radar data.

The Bimodality of the 100 hPa South Asia High and its Relationship to the Climate Anomaly over East Asia in Summer.

The NCEP/NCAR pentad mean reanalysis data from 1980 to 1994 are employed to examine the activities of the 100 hPa South Asia High (SAH) during the boreal summer. The results show that there exists bimodality in the longitude location of the SAH. According to the two preferred regions for the SAH, the SAH is classified into the Tibetan Mode (TM) and the Iranian Mode (IM), respectively. The studies on the maintenance mechanism, both from circulation structure and thermal structure, manifest that the SAH has the feature of warm preference. The diagnosis based on the thermodynamic equation further reveals that the TM is closely related to the diabatic heating of the Tibetan Plateau, whereas the IM is more associated with the adiabatic heating in the free atmosphere, as well as the diabatic heating near the surface. The statistical composites, and case studies, corresponding to the two modes show that the SAH bimodality is strongly related to the climate anomalies over East Asia. In the case of TM, the southerly airflow over the subtropics and the northerly airflow over the middle-high latitude at 850 hPa are enhanced, forming a convergence zone along 30°N and resulting in increased rainfall extending from the south Japan, Korea Peninsula, and Yangtze-Yellow river valley of China to the Tibetan Plateau. In the case of IM, at 850 hPa the mid-latitude East Asia is dominated by a huge anomalous anticyclone, thus results in the decreased rainfall over the area.