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Bian He and Song Yang

Abstract

Based on observational data, a linear baroclinic model, and an atmospheric general circulation model (AGCM), the major modes of spring precipitation over the tropical Asian and Pacific regions are identified. and the influence of latent heating over the tropical western Pacific (TWP) on global climate is investigated. Results show that the first mode of empirical orthogonal function analysis explains 20% of the total variance in March, the largest in spring, with the maximum center located over the TWP. The precipitation is highly positively correlated with local sea surface temperature (SST) in March, which suggests that the warming SST is the trigger for the precipitation over the TWP. Further analysis suggests that an increase in latent heating over the TWP, especially in March, can produce Rossby waves along the westerly jet, which causes an increase in surface temperature over North America. The propagation intensity decreases from March to May. The changes in location and strength of the westerly jet stream in the Northern Hemisphere are responsible for this decrease. Experiments with both a linear baroclinic model and an AGCM verify the above hypothesis. The study highlights that the spatial distributions of latent heating and westerly jet stream are the two key factors for the formation of teleconnection patterns from eastern Asia to North America.

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Yueqing Li and Song Yang

Abstract

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.

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Siyu Zhao and Song Yang

Abstract

The early season rainfall (ESR) over southern China, usually occurring from April to June, is a prominent meteorological phenomenon of the East Asian monsoon system. In this paper, output from the 45-day hindcast by the NCEP Climate Forecast System, version 2 (CFSv2), and various observational datasets are analyzed to assess the predictability of the ESR and associated atmospheric circulation. Results show that CFSv2 can successfully predict the ESR and associated circulation patterns over southern China. The lower-tropospheric convergence and upper-tropospheric divergence as well as the local upward motion over southern China lead to the formation of ESR. Analysis of bias shows small differences and close relationships between the predicted and observed ESR values when the forecast lead time is less than 2 weeks. The skill in the ESR predictions by CFSv2 decreases significantly when the lead time is longer than 2 weeks. Overall, CFSv2 has a higher level of skill when predicting the southern China ESR compared to the rainfall over other Asian regions during the same period of time.

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Hao Yan and Song Yang

Abstract

The Moderate Resolution Imaging Spectroradiometer (MODIS) dual spectral rain algorithm (MODRA) is developed for rain retrievals over the northern midlatitudes. The reflectance of the MODIS water vapor absorption channel at 1.38 μm (R 1.38 μm) has a potential to represent the cloud-top height displayed by the brightness temperature (TB) of the MODIS channel at 11 μm, because of an excellent negative relationship (correlation coefficient ≤−0.9) between R 1.38 μm and TB11 μm for optically thick clouds with reflectance (R 0.65 μm) greater than 0.75. With a training rainfall dataset from the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) aboard the same Aqua satellite platform, two MODIS channels (R 1.38 μm and R 0.65 μm) are applied to form multiregression curves to estimate daytime rainfall. Results demonstrate that the instantaneous rain rates from MODRA, independent AMSR-E rainfall products, and surface rain gauge measurements are consistent. This study explores a new way to estimate rainfall from MODIS water vapor and cloud channels. The resulting technique could be applied to other similar satellite instruments for rain retrievals.

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Yuan Yuan and Song Yang

Abstract

Using multiple datasets and a partial correlation method, the authors analyze the different impacts of eastern Pacific (EP) and central Pacific (CP) El Niño on East Asian climate, focusing on the features from El Niño developing summer to El Niño decaying summer. Unlike the positive–negative–positive (+/−/+) anomalous precipitation pattern over East Asia and the equatorial Pacific during EP El Niño, an anomalous −/+/− rainfall pattern appears during CP El Niño. The anomalous dry conditions over southeastern China and the northwestern Pacific during CP El Niño seem to result from the anomalous low-level anticyclone over southern China and the South China Sea, which is located more westward than the Philippine Sea anticyclone during EP El Niño. The continuous anomalous sinking motion over southeastern China, as part of the anomalous Walker circulation associated with CP El Niño, also contributes to these dry conditions.

During the developing summer, the impact of CP El Niño on East Asian climate is more significant than the influence of EP El Niño. During the decaying summer, however, EP El Niño exerts a stronger influence on East Asia, probably due to the long-lasting anomalous warming over the tropical Indian Ocean accompanying EP El Niño.

Temperatures over portions of East Asia and the northwestern Pacific tend to be above normal during EP El Niño but below normal from the developing autumn to the next spring during CP El Niño. A possible reason is the weakened (enhanced) East Asian winter monsoon related to EP (CP) El Niño.

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Song Yang and Xingwen Jiang

Abstract

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 ENSO are 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.

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Yana Li and Song Yang

Abstract

This study investigates the variations and feedback attributions of changes in surface temperature between strong and weak East Asian winter monsoons. The variations of winter-mean surface air temperature are dominated by two distinct principal modes that account for 70.9% of the total variance. The first mode features high correlation with the high-latitude atmospheric circulation, including a correlation coefficient of −0.53 with the Arctic Oscillation in January, and the second mode is significantly linked to El Niño–Southern Oscillation, with a correlation coefficient of −0.37. The surface temperature anomalies of each mode are decomposed into partial temperature anomalies resulting from radiative and nonradiative feedback processes by applying a coupled climate feedback–response analysis method to quantify contributions from thermodynamic and dynamic processes. Results indicate that the surface cooling associated with both modes is mainly attributed to the nonradiative feedback processes of atmospheric dynamics and surface sensible heating and to the radiative feedback processes of water vapor and clouds. The first mode exhibits a deep barotropic anomalous high that weakens the high-latitude westerly jet stream but strengthens the midlatitude westerly jet stream. This circulation feature traps cold and dry air over northern East Asia. For the second mode, the ocean and land heat storage processes induce a large thermal gradient over eastern China and the northwestern Pacific, resulting in a large pressure gradient. Northerly anomalies further reinforce the pressure gradient, which favors cold air intruding southward into the tropics.

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K-M. Lau and Song Yang

Abstract

In this paper, a description of the global and regional features of the Asian summer monsoon in the Goddard Laboratory Atmospheres (GLA) general circulation model (GCM) is presented based on a 10-yr (1979–1988) integration of the model under the Atmospheric Model Intercomparison Project. It is found that the earliest signal of onset of the Asian monsoon is represented by the emergence of convection over the Indo-China “land bridge” in early May, followed by the sudden jump of the equatorial ITCZ (intertropical convergence zone) to 10°N over the South China Sea and Indian Ocean in mid-May. This sudden jump is linked to an abrupt northward shift of the ascending branch of the local Hadley circulation, possibly stemming from symmetric instability of the basic flow in May. In agreement with observations, the model shows a pronounced intraseasonal oscillation along the equator and enhanced synoptic-scale activities identified as easterly waves in the tropical and subtropical western Pacific. These intraseasonal and synoptic activities strongly regulate rainfall variability over East Asia, especially during the premonsoon period.

While the model simulates a similar range of variability in the broad-scale structure and evolution of the Asian monsoon as observed, there is considerable scope for improvement in the model's ability to simulate regional features of the monsoon. Compared to observed climatology, the model produces deficient rainfall over northern India but excessive rainfall over the South China Sea and the western Pacific region due to an overactive ITCZ at 10°N. The intraseasonal transition between the equatorial ITCZ and the monsoon (land) convection to the north is weaker than in nature. Moreover, the East Asian monsoon trough and Mei-yu rainband are under-developed in the model. These discrepancies are related to the limited northward extension of the model monsoon circulation and the confinement of the upper-level easterly flow near the equator.

Overall, the GLA GCM provides a reasonably realistic description of the seasonal and subseasonal climatology of the Asian monsoon and yields important information that sheds new light on the dynamical underpinnings of the multiscale variabilities associated with the Asian summer monsoon.

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Peter J. Webster and Song Yang

Abstract

We seek relationships between the perturbation kinetic energy (PKE), the zonal wind, and outgoing longwave radiation. It is found that the mean low-level PKE extratropical maxima are located at the “atmospheric centers of action” (e.g., the Aleutian and Iceland lows), which lie beneath the high-level PKE-maxima downstream of the westerly jet streams. As expected, this means that the PKE maxima in the middle latitudes are closely related to the propagating disturbances. In the tropics, however, the high-level PKE maxima and minima are located in the strongest upper tropospheric westerlies and easterlies, respectively, while the low-level PKE maxima and minima are located in the monsoon systems and the easterly trade winds, respectively. It is also found that the maxima and minima of the PKE in the middle latitudes, at least in the Northern Hemisphere, are in phase in the vertical but completely out of phase in the tropics. These features are closely linked to the structure of the zonal wind component.

The hypothesis is posed that in the tropics, the PKE maxima in the upper troposphere are related to the tropical-extratropical atmospheric interaction, to the equatorially trapped transient modes which are presumably created in the convective regions, or to a combination of both processes.

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Song Yang and Eric A. Smith
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