East Asian Trough Research Articles

Relationship between stationary planetary wave activity and the East Asian winter monsoon

The variability of both the stationary planetary wave activity and the East Asian winter monsoon is strongly associated with the thermal contrast between oceans and landmasses. In this study, we explore the interannual relationship between the monsoon and the wave activity defined by an index of the difference in the divergence of Eliassen‐Palm flux between 50°N at 500 hPa and 40°N at 300 hPa. It is found that, compared to the winters of low wave activity, the equatorward propagation of planetary waves in the middle and upper troposphere is stronger in the high wave activity winters. During these high activity winters, the upward wave propagation from the troposphere into the stratosphere becomes weaker. This is accompanied by a smaller perturbation in the polar vortex, which tends to be colder and stronger. In the meantime, the East Asian westerly jet stream, the East Asian trough, the Siberian high, and the Aleutian low all become weaker apparently. In particular, the weakening of the Siberian high and the Aleutian low decreases the northeasterly wind over East Asia, leading to a warming condition in the region especially in northeastern Asia. A further analysis reveals that the zonal wavenumber‐2 pattern of planetary waves contributes dominantly to the variability of the East Asian winter monsoon.

Simulated Interannual Anomalous Atmospheric Circulation Patterns over East Asia during Different Stages of an Enso Cycle

Response of global atmosphere to the prescribed realistic monthly-varying global sea surface temperature during 1948~1999 was simulated with an atmospheric general circulation model (CCM3/NCAR). Simulated interannual anomalous atmospheric circulation patterns over East Asia during different stages of an El Nino cycle were described, with SVD and composite analysis methods. During the summer when an El Nino event develops, there exists an anomalously negative geopotential height center over Northeast China, Korean peninsula and Sea of Japan, the subtropical high over the western Pacific is weaker and shifts eastward, and East Asian summer monsoon is strengthened. During the winter when an El Nino event matures, the East Asian trough is deepened, resulting in a strengthened winter monsoon over the north of East Asia but a weakened one over the south. During the summer when an El Nino event decays, the subtropical high over the western Pacific is stronger and shifts southward and westward, and East Asian summer monsoon is weakened. There is a closest relationship between the El Nino event and atmospheric anomalies in East Asia only during the summer when an El Nino event decays. Overall, the simulated atmospheric anomaly patterns over East Asia due to El Nino's impact may reasonably explain the observed precipitation anomaly patterns that were described in previous studies.

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.

Pacific–East Asian Teleconnection. Part II: How the Philippine Sea Anomalous Anticyclone is Established during El Niño Development*

The anomalous Philippine Sea anticyclone (PSAC) conveys impacts of El Niño to east Asian climate during the mature and decay of an El Niño (from the winter to ensuing summer). It is shown that the anomalous PSAC forms in fall about one season prior to the peak El Niño; its strength increases with the El Niño intensity and its sign reverses during a La Niña. The PSAC formation concurs with abnormal deepening of the east Asian trough and with increasing number of northward recurvature of tropical storms in the western Pacific. The PSAC establishment is abrupt, coupling with a swing from a wet to dry phase of an intraseasonal oscillation (ISO) and often concurrent with early retreat of the east Asian summer monsoon. The ISO becomes inactive after PSAC establishment. The development of the PSAC is attributed to combined effects of the remote El Niño forcing, tropical–extratropical interaction, and monsoon–ocean interaction. The developing El Niño induces off-equatorial ascending Rossby wave responses and land surface cooling in northeast Asia; both deepen the east Asian trough in fall and induces vigorous tropical–extratropical exchange of air mass and heat, which enhances the cold air outbreak and initiation of the PSAC. Through exciting descending Rossby waves, the El Niño–induced Indonesian subsidence generates low-level anticyclonic vorticity over south Asia, which is advected by mean monsoon westerly, instigating the anomalous PSAC. The ISO interacting with the underlying ocean plays a critical role in the abrupt establishment of PSAC. The wind–evaporation/entrainment feedback tends to amplify (suppress) ISO before (after) winter northeasterly monsoon commences, suggesting the roles of atmosphere–ocean interaction and the seasonal march of background winds in changing the Philippine Sea ISO intensity and maintaining PSAC.

Winter Arctic Oscillation, Siberian High and East Asian Winter Monsoon

In this note, we investigate the impacts of the winter Arctic Oscillation (AO) and Siberian High (SH) on the East Asia winter monsoon (EAWM). It is found that the winter AO and the SH are relatively independent of each other in influencing the EAWM. The winter AO influences directly surface air temperature (SAT), sea level pressure (SLP) and the East Asian Trough at 500 hPa over the region northwards of 35°N in East Asia rather than through its impact on the SH. Compared with influences of the winter AO, the SH shows more direct and significant influences on the EAWM, particularly on SLP and northerly wind along the East Asian Coast. Impacts of the SH on the SAT occur primarily in the southwards of 50°N over East Asia, the northwestern Pacific and the South China Sea, because the AO suppresses the SH's influences in high latitudes of Asian Continent and the some subarctic regions.

Variations of the East Asian Jet Stream and Asian–Pacific–American Winter Climate Anomalies

In this study, the authors apply the NCEP-NCAR reanalysis and other observations to depict the association of the Asian-Pacific-American climate with the East Asian jet stream (EAJS). With an emphasis on boreal winter seasons and on interannual timescales, they analyze the variations of the EAJS and their relationships with El Nino-Southern Oscillation (ENSO) and extratropical North Pacific sea surface temperature (SST), and assess the relative connections of the EAJS and ENSO to the anomalies of atmospheric circulation, surface temperature, and precipitation in the Asian-Pacific-American region. It is found that the EAJS is coupled to a teleconnection pattern spanning the entire Asian-Pacific-American region with the strongest signals over east Asia and the western Pacific. This pattern differs significantly from that associated with ENSO, which influences the earth's climate extensively with a strongest impact on the climate over the central Pacific and east. A strong EAJS is associated with an intensification of the weather and climate systems in Asia and over the Pacific such as deepening of the east Asian trough and the Aleutian low and strengthening of the east Asian winter monsoon. It is linked to colder and drier conditions in east Asia and stronger convection over the tropical Asia-Australia sector. Compared with ENSO, the EAJS seems to link to the climate signals of Asia and the Pacific more strongly. An intensified EAJS is also associated with anomalies of temperature and precipitation in North America due to the related changes in stationary wave patterns. While the EAJS does not strongly link to the tropical central-eastern Pacific SST, it is significantly associated with the extratropical North Pacific SST, more specifically the second most dominant mode of the empirical orthogonal function analysis of the SST. In addition, a strong (weak) EAJS seems to follow a large (small) meridional gradient of the western Pacific SST associated with warming (cooling) in the Tropics-subtropics and cooling (warming) in the extratropics.

East Asian Winter Monsoon and Arctic Oscillation

In this study, the connection between Arctic Oscillation (AO) and variability of East Asian winter monsoon is investigated. Two indices are chosen to describe the winter monsoon. One is the intensity of the Siberian High, defined as the average SLP over the center region, and the other is the temperature of eastern China, averaged over 76 surface stations. These are two tightly related components, correlate at −0.62 for period 1951–99. Temperature drops by 0.64 degrees Celsius in association with a one standard deviation increase in Siberian High intensity. It is found that there are significant out‐of‐phase relationships between the AO and the East Asian winter monsoon. The correlation coefficient between the AO and the Siberian High intensity index is −0.48 for period 1958–98. AO is also significantly correlated with the temperature of eastern China at 0.34. However, when the linear trend is removed, the correlation between AO and temperature is no longer significant. But the strong connection between the AO and Siberian High, and between the Siberian High and temperature are still significant. These results reveal that the AO influences the East Asian winter monsoon through the impact on the Siberian High. Negative phase of the AO is concurrent with a stronger East Asian Trough and an anomalous anticyclonic flow over Urals at the middle troposphere (500hPa). Both the AO and the Eurasian pattern play important roles in changes of the Siberian High and/or East Asian winter monsoon. They account for 13.0% and 36.0% of the variance in the Siberian High respectively.

Potential analogues for paleoclimatic variations in eastern interior Alaska during the past 14,000 yr: atmospheric-circulation controls of regional temperature and moisture responses

The paleoclimatic history of a region can be viewed as a series of surface temperature and moisture anomalies through time. The effects of changes in large-scale climatic controls (e.g., insolation, major circulation controls) can be mediated by the influence of smaller-scale controls (e.g., topographic barriers, coastlines); this may result in heterogenous surface climatic responses at the regional and sub-regional scale. Divergent paleoclimatic trajectories between regions may be explainable in terms of such meso-scale patterns. Using modern analogues for paleoclimate we examine how the sequence of climatic variations in eastern interior Alaska during the interval 12,000–0 14 C yr BP could have been generated by specific atmospheric circulation patterns. Fossil-pollen and lake-level records document the long-term trends in temperature and effective moisture for the region. Water-balance modelling provides additional estimates of paleoprecipitation. Synoptic climatological patterns are described using the modern (instrumental) record of upper-level and sea-level pressure, surface temperature, and precipitation. At 12,000 14 C yr BP, eastern interior Alaska was cooler and drier than present, a situation generated today by a southward displacement of the jet stream. Conditions warmer and drier than present at 9000 14 C yr BP may have been generated by increased ridging north of Alaska and a weakened westerly circulation. Warmer, wetter conditions than present possibly prevailed in the late-middle Holocene; these might reflect ridging over Alaska and troughing further west. Cool, wet conditions feature enhanced westerly flow into Alaska through an eastward shift in the east Asian trough and positive pressure anomalies in the North Pacific; they may be analogous to cold periods of the Little Ice Age. The analogues demonstrate how surface conditions in other parts of Beringia may sometimes be similar to, while at other times different from those in the eastern interior. These broader spatial patterns provide hypotheses about past climates that can be tested with paleoclimatological data. For example, the widespread positive temperature anomalies associated with the warm/dry (9000 14 C yr BP) analogue fit with the expansion northward of the eastern Siberian treeline. The anomalously cool conditions in northeast Siberia associated with the warm/wet analogue may explain the continued (late-middle Holocene) treeline advance in Alaska while there was retreat in Siberia.

C 3/C 4 vegetation evolution over the last 7.0 Myr in the Chinese Loess Plateau: evidence from pedogenic carbonate δ 13C

The stable carbon and oxygen isotopic compositions of soil carbonate were measured on an eolian loess and red clay sequence at Lingtai, the Chinese Loess Plateau. This sequence is composed of 130 m of Tertiary red clay deposits with a basal age of 7.05 Ma overlain by 175 m of Pleistocene loess. In the field we identified ca. 110 carbonate nodule horizons in the red clay and 27 nodule horizons in the loess. These carbonate nodule horizons are formed by leaching and re-precipitation of carbonate from the eolian material. The δ 13C record of soil carbonate indicates a major expansion of C 4 plants at ca. 4.0 Myr in the Loess Plateau. This event is comparable in timing with the expansion of C 4 plants in northern North America ( Cerling et al., 1997. Nature 389, 153–158) but is ca. 3 million years later than the C 4 biomass expansion in Pakistan ( Quade et al., 1989. Nature 342, 163–166). The pedogenic characteristics of the soils and the δ 18O record in the red clay suggest that the C 4 plant expansion in the Loess Plateau was not driven by local climatic changes, which may support Cerling et al.'s (1997) assertion that the decline of atmospheric CO 2 levels in the Neogene is responsible for this global vegetation change. Our record also implies that the Tibetan Plateau could have been uplifted to a critical height in the late Miocene, thus resulting in the formation of the atmospheric Great East-Asia Trough.

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 Nino), and it becomes strong when there is a negative SST anomaly (La Nina). 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.

Impact of global warming on the Asian winter monsoon in a coupled GCM

The Asian winter monsoon (AWM) response to the global warming was investigated through a long‐term integration of the transient greenhouse warming with the ECHAM4/OPYC3 CGCM. The physics of the response was studied through analyses of the impact of the global warming on the variations of the ocean and land contrast near the ground in the Asian and western Pacific region and the east Asian trough and jet stream in the middle and upper troposphere. Forcing of transient eddy activity on the zonal circulation over the Asian and western Pacific region was also analyzed. It is found that in the global warming scenario the winter northeasterlies along the Pacific coast of the Eurasian continent weaken systematically and significantly, and intensity of the AWM reduces evidently, but the AWM variances on the interannual and interdecadal scales are not affected much by the global warming. It is suggested that the global warming makes the climate over the most part of Asia to be milder with enhanced moisture in winter. In the global warming scenario the contrasts of the sea level pressure and the near‐surface temperature between the Asian continent and the Pacific Ocean become significantly smaller, northward and eastward shifts and weakening of the east Asian trough and jet stream in the middle and upper troposphere are found. As a consequence, the cold air in the AWM originating from the east Asian trough and high latitudes is less powerful. In addition, feedback of the transient activity also makes a considerable contribution to the higher‐latitude shift of the jet stream over the North Pacific in the global warming scenario.

Forced Planetary Waves in the Northern Hemisphere Winter: Wave-Coupled Orographic and Thermal Forcings

Abstract A more complete and new formulation of the orographic forcing and new thermal forcings are included in a steady state model of the Northern Hemisphere planetary waves. When both forcings are included, the simulation produces excellent results which are compared in detail with observations. In particular, the Siberian high, the tropospheric East Asian trough and subtropical tropospheric East Asian jet stream maxima are well reproduced even though the forcing is primarily extratropical in origin. The modes uses a lower boundary condition in which the orographic forcing is determined by the effects of the total flow, not just the zonal mean basic state. Consequently, the net orographic forcing changes when thermal forcing is added and the tow solution is not equal to the linear sum of the solutions with orographic and thermal forcings separately. The thermally induced orographic forcing is found to be very significant and, in the troposphere, there is strong interaction between the two forcings with...