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To deepen our understanding of the behavior of tropical cyclones (TCs) over the western North Pacific (WNP) and its response to various external forcings, we investigate the variation of TC genesis potential over the WNP during the past 20 ka that experienced several large forcings. Using a set of transient simulations and a genesis potential index, our results indicate that TC genesis potential in the storm season shows an overall increase averaged over the WNP during the Heinrich Stadial 1 (HS1; ∼17–16 ka) relative to the Last Glacial Maximum. Subsequently, there is a sharp decrease in genesis potential during the Bølling–Allerød (BA; ∼13.5 ka) relative to the HS1, which is followed by an increase during the Younger Dryas (YD; ∼12.9–11.7 ka). During the Holocene, TC genesis potential shows a slight decrease during the early–middle Holocene (∼11.7–6 ka) and a significant increasing trend afterward. Further analysis shows that the contribution of each genesis factor to the genesis potential change varies greatly with time, which is in turn tied to changes in external forcings. The increased meltwater fluxes dominate the increased genesis potential during the HS1 and YD, whereas all external forcings (i.e., changes in meltwater, ice sheets, CO2, and insolation) contribute to genesis potential change during the BA. The long-term increasing trend from the mid-Holocene to the present is controlled by orbital insolation. These results may help improve our understanding on past TC activity and shed lights on TC response to various external forcings in a long-term future.

To advance our knowledge of the response of midlatitude westerlies to various external forcings, we investigate the meridional shift of midlatitude westerlies over arid central Asia (ACA) during the past 21 000 years, which experienced more varied forcings than the present day based on a set of transient simulations. Our results suggest that the evolution of midlatitude westerlies over ACA and driving factors vary with time and across seasons. In spring, the location of midlatitude westerlies over ACA oscillates largely during the last deglaciation, driven by meltwater fluxes and continental ice sheets, and then shows a long-term equatorward shift during the Holocene controlled by orbital insolation. In summer, orbital insolation dominates the meridional shift of midlatitude westerlies, with poleward and equatorward migration during the last deglaciation and the Holocene, respectively. From a thermodynamic perspective, variations in zonal winds are linked with the meridional temperature gradient based on the thermal wind relationship. From a dynamic perspective, variations in midlatitude westerlies are mainly induced by anomalous sea surface temperatures over the Indian Ocean through the Matsuno–Gill response and over the North Atlantic Ocean by the propagation of Rossby waves, or both, but their relative importance varies across forcings. Additionally, the modeled meridional shift of midlatitude westerlies is broadly consistent with geological evidence, although model–data discrepancies still exist. Overall, our study provides a possible scenario for a meridional shift of midlatitude westerlies over ACA in response to various external forcings during the past 21 000 years and highlights important roles of both the Indian Ocean and the North Atlantic Ocean in regulating Asian westerlies, which may shed light on the behavior of westerlies in the future.

Investigation of the evolution of summer Asian westerly jet (AWJ) in Earth history may deepen our understanding of the behavior of AWJ in the present and future. Here, the variation of summer westerlies and the associated mechanisms over West and East Asia are examined using the last millennium simulations from the Paleoclimate Modeling Intercomparison Project Phase III (PMIP3). The multimodel ensemble mean of 11 PMIP3 models reproduces the spatial distribution of modern zonal wind at 200 hPa and captures the location and intensity of the summer AWJ well. The results illustrate that the location of summer westerlies over West and East Asia shows no clear long-term trend over the last millennium, whereas there is an enhancing trend for the intensity, but with significant interannual and multidecadal variability. Summer westerlies over West Asia mainly oscillate within ~25-year band in terms of the location and intensity, while the principal periodicity over East Asia oscillate within ~25- and ~15-year band, respectively. Moreover, summer AWJ broadly moves poleward and weakens during the ‘Medieval Climate Anomaly’ (MCA, AD 1000–1200) relative to the ‘Little Ice Age’ (LIA, AD 1400–1600), which is attributed to the reduced meridional temperature gradient during the MCA. This poleward shift is consistent with the reconstructed moisture conditions over Central Asia, but the lack of direct proxy for westerlies restricts detailed model–data comparisons. Nevertheless, a possible scenario is presented on the spatiotemporal variation of summer AWJ over the last millennium, which may shed light on the response of the AWJ to natural forcings.

Investigation of climate change during the Last Interglacial (LIG, ~130–115 ka BP) may deepen our understanding of the response of climate to Earth's orbit. Here we examine climate change over China and associated mechanisms during the LIG using the lig127k experiments from the Paleoclimate Modeling Intercomparison Project Phase 4. We illustrate that annual mean surface air temperature over China decreases by 0.22 °C during the LIG relative to the preindustrial, with a colder climate in winter and spring, whereas a warmer climate in summer and autumn. Annual mean precipitation increases (up to 54%) over northern and southwestern China during the LIG, whereas it decreases (up to 11%) over the other parts. This pattern largely holds for precipitation change in summer during the LIG, but precipitation generally decreases in winter. According to the moisture budget equation, the nonlinear and horizontal dynamic terms contribute largely to the drier (wetter) conditions in summer over the south (north) of eastern China, whereas the horizontal dynamic term and the horizontal thermodynamic term is more important in regulating drier conditions over south and north of eastern China in winter, respectively. Furthermore, East Asian summer monsoon is intensified during the LIG manifested by anomalous southerly at 850 hPa, a westward shift of western North Pacific subtropical high, and poleward migration of East Asian westerly jet. East Asian winter monsoon also gets stronger during the LIG, with anomalous northerly at low-level troposphere, enhanced mid-tropospheric East Asian trough, and an equatorward shift of westerly jet at upper troposphere. Additionally, we note obvious data-model discrepancies in annual temperature change over China during the LIG, with cooling seen in the models but warming suggested by multiproxies. This highlights close collaboration between model and data communities in future to deepen our understanding of climate change during the LIG.

To quantitatively estimate the divergences in the natural and anthropogenic dust emission fluxes among different climatic regions, the total dust emissions at the global scale from 2007 to 2010 were simulated in this study. Despite the widely scattered anthropogenic dust distribution, the total area of potential anthropogenic dust sources was found slightly higher than that of natural dust sources. The anthropogenic dust distribution area was 1.61 × 107 km2 in January and 1.54 × 107 km2 in July, respectively. The natural dust sources contributed 81.0% of the global dust emissions and the anthropogenic contributed 19.0% of the residual. The natural and anthropogenic dust emission flux was 6.34 ± 0.31 μg m−2 s−1 and 1.01 ± 0.07 μg m−2 s−1, respectively. Especially, natural and anthropogenic dust emissions situated in different climatic regions. Natural dust emissions mainly located in hyper-arid and arid regions such as the Sahara, Arabian and Taklimakan Desert where dust emission fluxes range from 1 to 50 μg m−2 s−1, accounted for 97.3% of natural dust emissions at the global scale. While anthropogenic dust emissions concentrate in semi-arid, sub-humid and humid regions and generally fluctuated between 0.1 and 10 μg m−2 s−1. In addition, natural and anthropogenic dust proportions in semi-arid regions were the most complicated due to the complex land cover types, including grasslands, urban areas, croplands and open shrub lands, resulting in 42.99% of the anthropogenic dust emissions in semi-arid regions. The complex interplay of natural and anthropogenic dust emissions contributing to the total dust loadings may be a crucial factor enhanced the warming over semi-arid regions. This study provided confidence for the further investigation of the climatic impacts of natural and anthropogenic dust in different climatic regions under the background of global warming, especially the strengthening warming in semi-arid regions.

An intense summer dust storm over East Asia during June 24–27, 2010, was systematically analyzed based on the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) and a variety of in situ measurements and satellite retrievals. The results showed that the WRF-Chem model captures the spatial and temporal distributions of meteorological factors and dust aerosol in summer over East Asia well. This summer dust storm is initiated by the approach of a transverse trough in the northwestern Xinjiang. Because of the passage of the cutoff-low, a large amount of cold air is transported southward and further enhanced by the narrow valleys of the Altai and Tianshan Mountains, which results in higher wind speeds and huge dust emissions over the Taklimakan Desert (TD). Dust emission fluxes over the TD areas are high as 54 µg m−2 s−1 on June 25. The dust aerosol from the TD then sweeps across Inner Mongolia, Ningxia and Mongolia, and some are also transported eastward to Beijing, Tianjin, Hebei, and even South Korea and Japan. The simulations further show that summer dust over East Asia exerts an important influence on the radiation budget in the Earth-atmosphere system. Dust heats the atmosphere at a maximum heating rate of 0.14 K day−1, effectively changing the vertical stability of the atmosphere and affecting climate change at regional and even global scales. The average direct radiative forcing induced by dust particles over the TD at all-sky is −6.0, −16.8 and 10.8 W m−2 at the top of the atmosphere, the surface, and in the atmosphere, respectively. The discussion about radiative forcing induced by summer dust provides confidence for future investigation of summer dust impact on cloud properties and precipitation efficiency in the eastern China.

The Taklimakan Desert (TD) and Gobi Desert (GD) are two of the most important dust sources in East Asia, and have important impact on energy budgets, ecosystems and water cycles at regional and even global scales. To investigate the contribution of the TD and the GD to dust concentrations in East Asia as a whole, dust emissions, transport, and deposition over the TD and the GD in different seasons from 2007 to 2011 were systematically compared, based on the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). Dust emissions, uplift, and long-range transport related to these two dust source regions were markedly different due to differences in topography, elevation, thermal conditions, and atmospheric circulation. Specifically, the topography of the GD is relatively flat, and at a high elevation, and the area is under the influence of two jet streams at high altitudes, resulting in high wind speeds in the upper atmosphere. Deep convective mixing enables the descending branch of jet streams to continuously transport momentum downward to the mid-troposphere, leading to enhanced wind speeds in the lower troposphere over the GD which favors the vertical uplift of the GD dust particles. Therefore, the GD dust was very likely to be transported under the effect of strong westerly jets, and thus played the most important role in contributing to dust concentrations in East Asia. Approximately 35% and 31% of dust emitted from the GD transported to remote areas in East Asia in spring and summer, respectively. The TD has the highest dust emission capabilities in East Asia, with emissions of about 70.54 Tg yr−1 in spring, accounting for 42% of the total dust emissions in East Asia. However, the TD is located in the Tarim Basin and surrounded by mountains on three sides. Furthermore, the dominant surface wind direction is eastward and the average wind speed at high altitudes is relatively small over the TD. As a result, the TD dust particles are not easily transported outside the Tarim Basin, such that most of the dust particles are re-deposited after uplift, at a total deposition rate of about 40 g m−2. It is only when the TD dust particles are uplifted above 4 km, and entrained in westerlies that they begin to undergo a long-range transport. Therefore, the contribution of the TD dust to East Asian dust concentrations was relatively small. Only 25% and 23% of the TD dust was transported to remote areas over East Asia in spring and summer, respectively.

Supplemental material, westerlies_past1000_supplementary for Variation of the summer Asian westerly jet over the last millennium based on the PMIP3 simulations by Nanxuan Jiang, Qing Yan and Huijun Wang in The Holocene

Deciphering the variations in the subtropical westerly jet (STJ) and the subpolar westerly jet (SPJ) over the Southern Hemisphere in Earth’s history will help us to better understand how the westerlies may respond to various external forcings in the future. Using Simulation of Transient Climate Evolution over the past 21,000 years simulations, we demonstrate that the STJ migrated equatorward in the Last Glacial Maximum relative to the present, which followed by abrupt poleward, equatorward, and poleward shifts during the Heinrich Stadial I, Antarctic Cold Reversal, and Younger Dryas, respectively. These large-amplitude fluctuations are controlled by changes in meltwater fluxes and polar ice-sheet topography. During the Holocene, the STJ shows no obvious long-term trend, due to the offsetting effect between increased orbital insolation in austral summer and decreased Antarctic ice-sheet topography. The meridional shift of the SPJ shows an in-phase change with the STJ during the past 21 kyr. In terms of the dynamic mechanisms, the meridional shift of the STJ and SPJ is induced by changes in the meridional temperature gradient and the maximum eddy growth rate, respectively; and is also linked to changes in the mean meridional circulations (that is, the Hadley and Ferrel cells). The modeled meridional shift of the southern westerlies is supported by the results from the Paleoclimate Modelling Intercomparison Project Phase Ⅲ and is consistent with several reconstructions, but it is difficult to constrain robustly by geological evidence as the reconstructed westerlies vary across sites and proxies. Nevertheless, our study provides a possible scenario for the evolution of the southern westerlies during the past 21 kyr and identifies the dominant forcing and the associated mechanisms, which may advance our knowledge on the future behavior of the southern westerlies.

Dust emissions refer to the spatial displacement of dust particles from wind forcing, which is a key component of dust circulation. It plays an important role in the energy, hydrological, and carbon cycles of the Earth's systems. However, most dust emission schemes only consider natural dust, neglecting anthropogenic dust induced by human activities, which led to large uncertainties in quantitative estimations of dust emissions in numerical modeling. To fully consider the mechanisms of anthropogenic dust emissions, both indirect and direct anthropogenic dust emission schemes were constructed and developed in the study. Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) retrievals were used to constrain the simulations at global scale. The results showed that the schemes reasonably reproduced the spatio-temporal distributions of anthropogenic dust from 2007 to 2010. The high centers of anthropogenic dust emission flux appeared in India, eastern China, North America, and Africa range from 0.9 to 11 μg m−2 s−1. Compared with natural dust emissions, indirect anthropogenic dust emissions have indistinctive seasonal variation, with differences less than 3.2 μg m−2 s−1. Pasturelands contribute higher anthropogenic dust emissions than croplands, with emissions of approximately 6.8 μg m−2 s−1, accounting for 60 % of indirect anthropogenic dust emissions. Moreover, average anthropogenic dust emissions in urban areas have a value of 13.5 μg m−2 s−1, which is higher than those in rural areas (7.9 μg m−2 s−1). This study demonstrates that the environmental problems caused by anthropogenic dust in urban areas cannot be ignored.

The effect of aerosols is an important indicator of climate change. Sulfate aerosols, as the major scattering aerosols, which have attracted more and more attention in recent years. The Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) were utilized to investigate the spatial distribution of sulfate aerosols and their radiative forcing characteristics over East Asia in 2010. Results showed that sulfate aerosols were mainly distributed over eastern China (24&ndash, 43&deg, N, 101&ndash, 126&deg, E), especially in the Sichuan Basin. The concentration of sulfate aerosols decreased with increasing altitude over East Asia. It also exhibited obvious seasonal variations, where the largest range of sulfate aerosol concentrations was found in summer, with a maximum of 2.4 &mu, g kg&minus, 1 over eastern China. Although sulfate aerosol concentrations varied slightly during day and night, there was still a significantly difference in the sulfate aerosol radiative forcing. Specifically, the magnitude of the direct radiative forcing induced by sulfate aerosols at the surface was approximately &minus, 3.02 W m&minus, 2 in the daytime, while that was +0.24 W m&minus, 2 in the nighttime. This asymmetric change that was caused by the radiative forcing of sulfate aerosols between day and night would have significant impacts on climate change at the regional scale.