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Lixia Zhang and Tianjun Zhou

Abstract

East Asia is greatly impacted by drought. North and southwest China are the regions with the highest drought frequency and maximum duration. At the interannual time scale, drought in the eastern part of East Asia is mainly dominated by two teleconnection patterns (i.e., the Pacific–Japan and Silk Road teleconnections). The former is forced by SST anomalies in the western North Pacific and the tropical Indian Ocean during El Niño decaying year summers. The precipitation anomaly features a meridional tripolar or sandwich pattern. The latter is forced by Indian monsoon heating and is a propagation of stationary Rossby waves along the Asian jet in the upper troposphere. It can significantly influence the precipitation over north China. Regarding the long-term trend, there exists an increasing drought trend over central parts of northern China and a decreasing tendency over northwestern China from the 1950s to the present. The increased drought in north China results from a weakened tendency of summer monsoons, which is mainly driven by the phase transition of the Pacific decadal oscillation. East Asian summer precipitation is poorly simulated and predicted by current state-of-the-art climate models. Encouragingly, the predictability of atmospheric circulation is high because of the forcing of ENSO and the associated teleconnection patterns. Under the SRES A1B scenario and doubled CO2 simulations, most climate models project an increasing drought frequency and intensity over southeastern Asia. Nevertheless, uncertainties exist in the projections as a result of the selection of climate models and the choice of drought index.

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Wenxia Zhang and Tianjun Zhou

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The global land monsoon region, with substantial monsoon rainfall and hence freshwater resources, is home to nearly two-thirds of the world’s population. However, it is overwhelmed by extreme precipitation, which is more intense than that on the rest of the land. Whether extreme precipitation has changed significantly, particularly in association with global warming, remains unclear for this region. This study investigates the presence of monotonic trends in extreme precipitation and its association with global warming over the past century over the global land monsoon regions, by employing the most comprehensive, long-running, and high-quality observational extreme precipitation records currently available. Based on a total of 5066 stations with at least 50 years of records, we found significant increases in the annual maximum daily precipitation and associations with global warming in regional monsoon domains, including the southern part of the South African monsoon region, the South Asian monsoon region (dominated by India), the North American monsoon region, and the eastern part of the South American monsoon region during the period of 1901–2010, with responses to global warming of ~10.4%–14.2% K−1, 7.9%–8.3% K−1, 6.4%–10.8% K−1, and 15.1%–24.8% K−1, respectively. For the global monsoon region as a whole, significant increases in extreme precipitation and associations with global warming are also identified, but with limited spatial coverage. The qualitative results on the significance of the changes on the regional scale are generally robust against different time periods, record lengths of stations, and datasets used. The uncertainty in the quantitative results arising from limited spatial and temporal coverages and use of different datasets deserves attention.

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Lixia Zhang and Tianjun Zhou

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By using 55-yr NCEP–NCAR reanalysis data, two dominant interannual variability modes of summer upper-tropospheric (500–200 hPa) temperature over East Asia are identified. The first empirical orthogonal function (EOF1) mode in its positive sign features a monopole cooling anomaly, while the second mode (EOF2) features a meridional dipole mode, with the positive (negative) center located south (north) of 35°N. The EOF1 (EOF2) mode is associated with ENSO developing (decaying) summers. They are the result of dynamical teleconnections remotely induced by ENSO and local moist processes. During the El Niño developing summer, the Indian summer monsoon precipitation decreases and forces the Silk Road teleconnection pattern at 200 hPa, featuring an anomalous cyclone over the East Asian continent. Coupled with the anomalous northerly wind in eastern China at 850 hPa, rainfall over north (south) China is suppressed (enhanced). The anomalous cyclone in the upper troposphere, associated vertical motion, and precipitation contribute to the heat and vorticity balance and maintain the monopole cooling. In the El Niño decaying summer, driven by the combined effects of a local SST anomaly and remote warm SST anomaly forcing from the Indian Ocean, precipitation is reduced over the western Pacific Ocean. Less latent heat is released and forces the Pacific–Japan teleconnection pattern along the East Asian continent, inducing a tripolar rainfall anomaly over East Asia. The tripolar precipitation and vertical motion anomalies and the zonal extended cyclonic anomaly in the upper troposphere provide the heating and momentum flux balance and maintain the temperature anomaly pattern during the ENSO decaying summer.

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Yong Sun and Tianjun Zhou

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Analyses of 30-yr four reanalysis datasets [NCEP–NCAR reanalysis (NCEP1), NCEP–Department of Energy reanalysis (NCEP2), Japanese 25-year Reanalysis Project (JRA-25), and Interim ECMWF Re-Analysis (ERA-Interim)] reveal remarkably interannual variability of the Hadley circulation (HC) in boreal summer (June–August). The two leading modes of interannual variability of boreal summer HC are obtained by performing empirical orthogonal function (EOF) analysis on the mass streamfunction. A general intensification of boreal summer HC is seen in EOF-1 mode among NCEP1, NCEP2, and JRA-25 but the corresponding EOF-2 mode in ERA-Interim, while a weakened northern Hadley cell and remarkable regional variation of a southern Hadley cell are captured by the EOF-2 mode (from NCEP1, NCEP2, and JRA-25) and EOF-1 mode (from ERA-Interim), as evidenced by the enhanced (decreased) southern Hadley cell in the southern tropics (the northern tropics and southern subtropics). Both modes are driven by El Niño–like SST forcing in boreal summer, but are relevant to different phases of El Niño events. The EOF-1 (or EOF-2 derived from ERA-Interim) [EOF-2 (or EOF-1 derived from ERA-Interim)] mode is driven by SST anomalies in developing (decaying) El Niño summers. The interannual variations of the northern Hadley cell in both modes are driven by El Niño through modulating the interannual variations of the East Asian summer monsoon, while anomalous local Hadley circulation (LHC) in the regions 30°S–20°N, 110°E–180° and 30°S–20°N, 160°E–120°W in response to El Niño forcing largely determine the interannual variations of southern Hadley cell in both modes, respectively. The different behaviors of the southern Hadley cell between two leading modes can be well explained by the southward shift of the tropical heating center from north of 10°N in developing El Niño summers to south of 10°N in decaying El Niño summers.

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Liwei Zou and Tianjun Zhou

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A flexible regional ocean–atmosphere–land system coupled model [Flexible Regional Ocean Atmosphere Land System (FROALS)] was developed through the Ocean Atmosphere Sea Ice Soil, version 3 (OASIS3), coupler to improve the simulation of the interannual variability of the western North Pacific summer monsoon (WNPSM). The regionally coupled model consists of a regional atmospheric model, the Regional Climate Model, version 3 (RegCM3), and a global climate ocean model, the National Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG)/Institute of Atmospheric Physics (IAP) Climate Ocean Model (LICOM). The impacts of local air–sea interaction on the simulation of the interannual variability of the WNPSM are investigated through regionally ocean–atmosphere coupled and uncoupled simulations, with a focus on El Niño’s decaying summer. Compared with the uncoupled simulation, the regionally coupled simulation exhibits improvements in both the climatology and the interannual variability of rainfall over the WNP. In El Niño’s decaying summer, the WNP is dominated by an anomalous anticyclone, less rainfall, and enhanced subsidence, which lead to increases in the downward shortwave radiation flux, thereby warming sea surface temperature (SST) anomalies. Thus, the ocean appears as a slave to atmospheric forcing. In the uncoupled simulation, however, the atmosphere is a slave to oceanic SST forcing, with the warm SST anomalies located east of the Philippines unrealistically producing excessive rainfall. In the regionally coupled run, the unrealistic positive rainfall anomalies and the associated atmospheric circulations east of the Philippines are significantly improved, highlighting the importance of air–sea coupling in the simulation of the interannual variability of the WNPSM. One limitation of the model is that the anomalous anticyclone over the WNP is weaker than the observations in both the regionally coupled and the uncoupled simulations. This results from the weaker simulated climatological summer rainfall intensity over the monsoon trough.

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Fengfei Song and Tianjun Zhou

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The climatology and interannual variability of the East Asian summer monsoon (EASM) simulated by 34 coupled general circulation models (CGCMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are evaluated. To estimate the role of air–sea coupling, 17 CGCMs are compared to their corresponding atmospheric general circulation models (AGCMs). The climatological low-level monsoon circulation and mei-yu/changma/baiu rainfall band are improved in CGCMs from AGCMs. The improvement is at the cost of the local cold sea surface temperature (SST) biases in CGCMs, since they decrease the surface evaporation and enhance the circulation. The interannual EASM pattern is evaluated by a skill formula and the highest/lowest eight models are selected to investigate the skill origins. The observed Indian Ocean (IO) warming, tropical eastern Indian Ocean (TEIO) rainfall anomalies, and Kelvin wave response are captured well in high-skill models, while these features are not present in low-skill models. Further, the differences in the IO warming between high-skill and low-skill models are rooted in the preceding ENSO simulation. Hence, the IO–western Pacific anticyclone (WPAC) teleconnection is important for CGCMs, similar to AGCMs. However, compared to AGCMs, the TEIO SST anomaly is warmer in CGCMs, since the easterly wind anomalies in the southern flank of the WPAC reduce the climatological monsoon westerlies and decrease the surface evaporation. The warmer TEIO induces the stronger precipitation anomaly and intensifies the teleconnection. Hence, the interannual EASM pattern is better simulated in CGCMs than that in AGCMs.

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Fengfei Song and Tianjun Zhou

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The climatology and interannual variability of East Asian summer monsoon (EASM) are investigated by using 13 atmospheric general circulation models (AGCMs) from phase 3 of the Coupled Model Intercomparison Project (CMIP3) and 19 AGCMs from CMIP5. The mean low-level monsoon circulation is reasonably reproduced in the multimodel ensemble mean (MME) of CMIP3 and CMIP5 AGCMs, except for a northward shift of the western Pacific subtropical high. However, the monsoon rainband known as mei-yu/baiu/changma (28°–38°N, 105°–150°E) is poorly simulated, although a significant improvement is seen from CMIP3 to CMIP5. The interannual EASM pattern is obtained by regressing the precipitation and 850-hPa wind on the observed EASM index. The observed dipole rainfall pattern is partly reproduced in CMIP3 and CMIP5 MME but with two deficiencies: weaker magnitude and southward shift of the dipole rainfall pattern. These deficiencies are closely related to the weaker and southward shift of the western Pacific anticyclone (WPAC). The simulation skill of the interannual EASM pattern has been significantly improved from CMIP3 to CMIP5 MME accompanied by the enhanced dipole rainfall pattern and WPAC. Analyses demonstrate that the tropical eastern Indian Ocean (IO) rainfall response to local warm SST anomalies and the associated Kelvin wave response over the Indo–western Pacific region are important to maintain the WPAC. A successful reproduction of interannual EASM pattern depends highly on the IO–WPAC teleconnection. The significant improvement in the interannual EASM pattern from CMIP3 to CMIP5 MME is also due to a better reproduction of this teleconnection in CMIP5 models.

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Tianjun Zhou and Rucong Yu

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This paper examines variations of the surface air temperature (SAT) over China and the globe in the twentieth century simulated by 19 coupled climate models driven by historical natural and anthropogenic forcings. Most models perform well in simulating both the global and the Northern Hemispheric mean SAT evolutions of the twentieth century. The inclusion of natural forcings improves the simulation, in particular for the first half of the century. The reproducibility of the SAT averaged over China is lower than that of the global and hemispheric averages, but it is still acceptable. The contribution of natural forcings to the SAT over China in the first half of the century is not as robust as that to the global and hemispheric averages. No model could successfully produce the reconstructed warming over China in the 1920s. The prescribed natural and anthropogenic forcings in the coupled climate models mainly produce the warming trends and the decadal- to interdecadal-scale SAT variations with poor performances at shorter time scales. The prominent warming trend in the last half of the century over China and its acceleration in recent decades are weakly simulated. There are discrepancies between the simulated and observed regional features of the SAT trend over China. Few models could produce the summertime cooling over the middle part of eastern China (27°–36°N), while two models acceptably produce the meridional gradients of the wintertime warming trends, with north China experiencing larger warming. Limitations of the current state-of-the-art coupled climate models in simulating spatial patterns of the twentieth-century SAT over China cast a shadow upon their capability toward projecting credible geographical distributions of future climate change through Intergovernmental Panel on Climate Change (IPCC) scenario simulations.

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Tianjun Zhou and Jie Zhang

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Recent studies have identified different modes associated with two flavors of El Niño in terms of the three-dimensional structure of atmospheric temperature. The first is a deep-warm mode, which features a coherent zonal mean warming throughout the troposphere from 30°N to 30°S with cooling aloft. The second is a shallow-warm mode, which features strong wave signatures in the troposphere with warmth (coolness) over the central Pacific (western Pacific). The ability to simulate these two modes is a useful metric for evaluating climate models. To understand the reproducibility of these two modes, the authors analyzed the multimodel ensemble mean (MMEM) of 11 atmospheric general circulation models (AGCMs) that participated in the second phase of the Atmospheric Model Intercomparison Project (AMIP II). Each model was run in an AGCM-alone mode forced by historical sea surface temperatures covering the period 1980–99. The authors find that atmospheric temperature variability is generally well captured in the MMEM of AMIP II models, demonstrating that the observational changes documented here are driven by SST changes during the El Niño events and the variety of vertical temperature structures associated with two flavors of El Niño are highly reproducible. The model skill for the first mode is slightly higher than the second mode. The skill in the upper troposphere–lower stratosphere is lower than for the tropospheric counterpart, especially at high latitudes. The performances of individual models are also assessed. The authors also show some differences from previous data analyses, including the variance accounted for by the two modes, as well as the lead–lag relationship of the shallow-warm mode with the Niño-3.4 index.

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Rucong Yu and Tianjun Zhou

Abstract

A significant interdecadal cooling with vivid seasonality and three-dimensional (3D) structure is first revealed in the upper troposphere and lower stratosphere over East Asia. A robust upper-tropospheric cooling appears in March and has two peaks in April and August, but in June, a moderate upper-tropospheric warming interrupts the cooling, while strong cooling occurs in the lower stratosphere. The seasonally dependent upper-tropospheric cooling leads to a clear seasonality of interdecadal changes in the atmospheric general circulation and precipitation against their normal seasonal cycle over East Asia. In March, precipitation over southern China (south of 26°N) has increased in accordance with the strong upper-tropospheric cooling occurring in northeast Asia. In April and May, following the southward extension and intensification of the upper-tropospheric cooling, the normal seasonal march of the monsoon rainband has been interrupted, resulting in a drying band to the south of the Yangtze River valley in late spring. In June, the moderate upper-tropospheric warming and strong lower-stratospheric cooling over northeast Asia has suddenly enhanced the northward migration of the rainband and resulted in an increase of precipitation in the mid–lower reaches of the Yangtze River and farther north. During July and August, the return of upper-tropospheric cooling has weakened the northward progression of southerly monsoon winds, resulting in a mid–lower Yellow River valley (34°–40°N) drought and excessive rain in the Yangtze River valley. The change of surface temperature is well correlated with the change in precipitation, especially in the spring. The surface cooling is generally collocated with excessive rain, while the warming is generally collocated with droughts. Possible causes for the robust interdecadal change are discussed, and stratosphere–troposphere interaction is suggested to play a crucial role in seasonally dependent 3D atmospheric cooling over East Asia.

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