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Anmin Duan and Guoxiong Wu

Abstract

The trend in the atmospheric heat source over the Tibetan Plateau (TP) during the last four decades is evaluated using historical observations at 74 meteorological stations in the period of 1961–2003 and satellite radiation data from 1983 to 2004. It is shown that in contrast to the strong surface and troposphere warming, the sensible heat (SH) flux over the TP exhibits a significant decreasing trend since the mid-1980s. The largest trend occurs in spring, a season of the highest SH over the TP. The subdued surface wind speed contributes most to the decreasing trend. At the same time, the radiative cooling effect in the air column enhances persistently. Despite the fact that the in situ latent heating presents a weak increasing trend, the springtime atmospheric heat source over the TP loses its strength during two recent decades. Further investigation suggests that the weakened SH over the TP may be part of the global circulation shift.

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Anmin Duan and Guoxiong Wu

Abstract

In Part I the authors have shown that heating sources in spring over the Tibetan Plateau (TP), and in particular the sensible heat flux (SHF), exhibit a significant weakening trend since the mid-1980s that is induced mainly by decreased surface wind speed. The possible reason of such a change is further investigated in Part II by analyzing historical observations and the NCEP/Department of Energy (DOE) reanalysis. The steady declining trend in the surface wind speed over the TP after the 1970s arises mainly from the zonal component. Since the mean altitude of the TP is about 600 hPa and the surface flow is controlled by the East Asian subtropical westerly jet (EASWJ) for most parts of the year, the substantial tropospheric warming in the mid- and high latitudes to the north of the plateau results in a decrease of the meridional pressure gradient in the subtropics. As a result, the EASWJ and the surface winds over the TP are decelerated. Moreover, changes of the general circulation in the twentieth century simulated by 16 coupled climate models driven by natural and anthropogenic forcings are examined. Intercomparison results suggest that sulfate aerosol indirect effects and ozone may be important in reproducing the weakening trend in EASWJ. Although nearly half of the models can successfully reproduce the observed trends in the EASWJ during the last two decades, there is an obvious spread in simulation of the spatial patterns of twentieth-century tropospheric temperatures, suggesting significant room still exists for improvement of the current state-of-the-art coupled climate models.

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Zhixiang Xiao and Anmin Duan

Abstract

The relationship between Tibetan Plateau (TP) snow cover and the East Asian summer monsoon (EASM) has long been discussed, but the underlying mechanism remains controversial. In this paper, the snow–albedo and snow–hydrology feedbacks over the TP are investigated based on multiple sources of snow data for the period 1979–2011. The results indicate that winter snow cover plays an important role in cooling local air temperature through the snow–albedo effect; the TP surface net solar radiation in years with above-normal snow cover is approximately 18 W m−2 less than that in below-normal snow cover years. However, data analysis demonstrates that persistent effects of winter snow cover are limited to the period from winter to spring over most parts of the central and eastern TP. Therefore, the preceding snow cover over the central and eastern TP exerts little influence over either the in situ summer atmospheric heat source or the EASM, because of its limited persistence. In contrast, the effects of winter or spring snow cover anomalies over the western TP and the Himalayas can last until summer, and these anomalies further influence the EASM by modulating moisture transport to eastern China and favoring eastward-propagating synoptic disturbances that are generated over the TP. Generally, above-normal snow cover over the western TP and the Himalayas facilitates abundant summer precipitation between the Yangtze and Yellow River basins, which is confirmed by results from a regional Weather Research and Forecasting model simulation.

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Moran Zhuang and Anmin Duan

Abstract

This study uses an atmospheric general circulation model to examine the relative effects of Maritime Continent (MC) orography, surface roughness, and land–sea contrast on the three cross-equatorial flows (CEF) north of Australia, including the South China Sea (SCS), Celebes-Moluccas (CM), and New Guinea (NG) CEFs, and Asian monsoon precipitation during boreal summer. Four experiments are conducted: with islands, with islands without orography, with islands with ocean roughness and no orography, and with ocean only in the MC region. At the approximately 1° horizontal resolution of these sensitivity experiments, results indicate that the land–sea contrast and orography in the MC have complicated impacts on the CEFs. The land–sea contrast creates the three CEFs. The orography is dominant in deepening, concentrating, and strengthening the CM CEF and modulating the longitudinal location of the NG CEF. For the intensity and depth of the SCS and NG CEFs, the surface roughness over the flat MC and orography are both important. In addition, the MC modulates the monsoon rainfall in tropical Asia. The decreased rainfall (by roughly 57% and 21.4% over South Asia and the SCS, respectively) is dominated by the reduced moisture availability resulting from the presence of the land–sea contrast, thereby intercepting the westward propagating quasi-biweekly convection. The surface roughness over the MC is key in reducing precipitation through reducing moisture convergence over Sumatra, Borneo, and northeastern New Guinea. However, the orography controls the intense precipitation over southwestern New Guinea and the adjacent seas through enhancing the moisture transport carried by the CM and NG CEFs.

Open access
Meirong Wang and Anmin Duan

Abstract

Intraseasonal variation (ISV) is especially prominent and unique in the Asian summer monsoon region. In this work, the dominant ISV mode over the Tibetan Plateau (TP) in the summer monsoon season (June–August), together with its structure and evolution, is identified using station observations, Global Precipitation Climatology Project precipitation data, and ERA-Interim during 1979–2011. Results indicate that quasi-biweekly oscillation (QBWO) is the dominant mode of ISV over the TP and is significant in terms of the circulation, precipitation, and diabatic heating fields. In particular, the QBWO is closely related to the onset and active/break phases of the TP summer monsoon. In most cases, the QBWO originates from the equatorial western Pacific and first propagates northwestward to the Bay of Bengal and northern India, then northward to the southeastern TP, and finally eastward to the East Asian area, showing a clockwise propagation pathway. Two main mechanisms are responsible for the northward propagation of the QBWO signals. The first, in operation when the QBWO signals are located to the south of 20°N, is the generation of barotropic vorticity induced by the easterly vertical shear, leading to the northward movement of the convection. The second mechanism, responsible for the propagation taking place farther north toward the TP, is a moisture advection effect that destabilizes the lower atmosphere ahead of the convection. Further analyses suggest that the QBWO plays a role in linking the ISV of the different subsystems of the Asian summer monsoon as a macroscale monsoon system.

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Yu Zhao, Anmin Duan, and Guoxiong Wu

Abstract

The atmospheric circulation changes dramatically over a few days before and after the onset of the South Asian monsoon in spring. It is accompanied by the annual maximum surface heating over the Tibetan Plateau. We conducted two sets of experiments with a coupled general circulation model to compare the response of atmospheric circulation and wind-driven circulation in the Indian Ocean to the thermal forcing of the Tibetan Plateau before and after the monsoon onset. The results show that the Tibetan Plateau’s thermal forcing modulates the sea surface temperature (SST) of the Indian Ocean and the meridional circulation in the upper ocean with opposite effects during these two stages. The thermal forcing of the Tibetan Plateau always induces a southwesterly response over the northern Indian Ocean and weakens the northeasterly background circulation before the monsoon onset. Subsequently, wind–evaporation feedback results in a warming SST response. Meanwhile, the oceanic meridional circulation shows offshore upwellings in the north and southward transport in the upper layer crossing the equator, sinking near 15°S. After the monsoon onset, the thermal forcing of the Tibetan Plateau accelerates the background southwesterly and introduces a cooling response to the Indian Ocean SST. The response of oceanic meridional overturning circulation is limited to the north of the equator due to the location and structural evolution of the climatological local Hadley circulation. With an acceleration of the local Walker circulation, the underlying zonal currents also show corresponding changes, including a westerly drift along the equator, downwelling near Indonesia, offshore upwelling near Somalia, and a westward undercurrent.

Open access
Anmin Duan, Jun Hu, and Zhixiang Xiao

Abstract

Temporal variability within the Tibetan Plateau summer monsoon (TPSM) is closely linked to both the East and South Asian summer monsoons over several time scales but has received much less attention than these other systems. In this study, extensive integrations under phase 5 of the Coupled Model Intercomparison Project (CMIP5) historical scenarios from 15 coupled general circulation models (CGCMs) and Atmospheric Model Intercomparison Project (AMIP) runs from eight atmospheric general circulation models (AGCMs) are used to evaluate the performance of these GCMs. Results indicate that all GCMs are able to simulate the climate mean TPSM circulation system. However, the large bias associated with precipitation intensity and patterns remains, despite the higher resolution and inclusion of the indirect effects of sulfate aerosol that have helped to improve the skill of the models to simulate the annual cycle of precipitation in both AGCMs and CGCMs. The interannual variability of the surface heat low and the Tibetan high in most of the AGCMs resembles the observation reasonably because of the prescribed forcing fields. However, only a few models were able to reproduce the observed seesaw pattern associated with the interannual variability of the TPSM and the East Asian summer monsoon (EASM). Regarding long-term trends, most models overestimated the amplitude of the tropospheric warming and the declining trend in the surface heat low between 1979 and 2005. In addition, the observed cooling trend in the upper troposphere and the decline of the Tibetan high were not reproduced by most models. Therefore, there is still significant scope for improving GCM simulations of regional climate change, especially in regions near extensive mountain ranges.

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Wenting Hu, Anmin Duan, and Guoxiong Wu

Abstract

The off-equatorial boreal summer intraseasonal oscillation (ISO) is closely linked to the onset, active, and break phases of the tropical Asian monsoon, but the accurate simulation of the eastward-propagating low-frequency ISO by current models remains a challenge. In this study, an atmospheric general circulation model (AGCM)–ocean mixed layer coupled model with high (10 min) coupling frequency (DC_10m) shows improved skill in simulating the ISO signal in terms of period, intensity, and propagation direction, compared with the coupled runs with low (1 and 12 h) coupling frequency and a stand-alone AGCM driven by the daily sea surface temperature (SST) fields. In particular, only the DC_10m is able to recreate the observed lead–lag phase relationship between SST (SST tendency) and precipitation at intraseasonal time scales, indicating that the ISO signal is closely linked to the subdaily air–sea interaction. During the ISO life cycle, air–sea interaction reduces the SST underlying the convection via wind–evaporation and cloud–radiation feedbacks, as well as wind-induced oceanic mixing, which in turn restrains convection. However, to the east of the convection, the heat-induced atmospheric Gill-type response leads to downward motion and a reduced surface westerly background flow because of the easterly anomalies. The resultant decreased oceanic mixing, together with the increased shortwave flux, tends to warm the SST and subsequently trigger convection. Therefore, the eastward-propagating ISO may result from an asymmetric east–west change in SST induced mainly by multiscale air–sea interactions.

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Bin Tang, Wenting Hu, and Anmin Duan

Abstract

A future projection of four extreme precipitation indices over the Indochina Peninsula and South China (INCSC) region with reference to the period 1958–2014 is conducted through the application of a multimodel ensemble approach and a rank-based weighting method. The weight of each model from phase 6 of the Coupled Model Intercomparison Project (CMIP6) is calculated depending on its historical simulation skill. Then, the weighted and unweighted ensembles are used for future projections. The results show that all four extreme precipitation indices are expected to increase over the INCSC region, both in the middle (2041–60) and at the end (2081–2100) of the twenty-first century, under three Shared Socioeconomic Pathway (SSP) scenarios. The increases in total extreme precipitation (R95p), extreme precipitation days (R95d), and the fraction of total rainfall from events exceeding the extreme precipitation threshold (R95pT) in the Indochina Peninsula are more significant than those in South China. The occurrence of extreme rainfall events may become more frequent in the future over the INCSC region, since the probability that R95pT increases is larger than 0.7 in the whole INCSC region. A comparison between the weighted and unweighted ensemble means shows that the uncertainty over South China is almost always reduced after applying the weighted scheme to future probabilistic projection, while the reductions in uncertainty over the Indochina Peninsula may depend on SSPs. The more extreme precipitation over the INCSC region in the future may be related to the larger water vapor supply and the more unstable local atmospheric stratification.

Open access
Bin Tang, Wenting Hu, and Anmin Duan

Abstract

Precipitation extremes over the Indochina and South China (INCSC) region simulated by 40 global climate models from phase 6 of the Coupled Model Intercomparison Project (CMIP6) were quantitatively assessed based on the skill score metrics of four extreme precipitation indices when compared with observational results from a high-resolution daily precipitation dataset for 1958–2014. The results show that it is difficult for most of the CMIP6 models to reproduce the observed spatial pattern of extreme precipitation indices in the INCSC region. The interannual variability of the extreme precipitation indices is relatively better simulated for South China than for Indochina. In general, most of the CMIP6 models perform better in South China compared with Indochina when taking both the simulations of spatial pattern and interannual variability into consideration. Only three models (EC-EARTH3, EC-EARTH3-Veg, and NorESM2-MM) can successfully reproduce both the spatial pattern and the interannual variability for the INCSC region. Through model ranking, the multimodel ensemble generated by a selection of the most skillful models leads to a more realistic simulation of the extreme precipitation indices both in South China and Indochina. Better simulation of the meridional wind component over South China and the water vapor convergence over Indochina can partly reduce the wet biases, resulting in a more realistic simulation of extreme precipitation indices over the INCSC region.

Open access