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Jiamin Wang, Xiaodan Guan, Yuping Guan, Kaiwei Zhu, Rui Shi, Xiangning Kong, and Shuyang Guo

Abstract

Due to global warming, the lengths of the four seasons, which are always taken as constant values, have experienced significant variations with rising temperature. Such changes play different roles on regional climate change, with the most significant effect on drylands. To guarantee local crop yields and preserve ecosystems, the identification of the changes of the four seasons in drylands is important. Our results show that, relative to humid lands, changing trends in lengths of spring, summer and autumn were particularly enhanced in drylands of the Northern Hemisphere mid-latitudes during 1951-2020. In this period, summer length has increased by 0.51 day per year, while spring and autumn lengths have contracted by 0.14 and 0.14 day per year, respectively. However, the enhanced changes in drylands did not appear in winter length. Such changes of spring, summer and autumn in drylands are dominated by internal variability over the entire study period, with a stronger external forcing effect on drylands than on humid lands. In drylands, the external forcing contributed to the changes in lengths of spring, summer and autumn by 30.1%, 42.2% and 29.4%, respectively. The external forcing has become an increasingly important component since 1990, with the ability to dominate all seasons in drylands after 2010. Nevertheless, only one out of the 16 Coupled Model Intercomparison Project Phase 6 (CMIP6) models used in this study can capture the enhanced changes in the lengths of spring, summer and autumn in drylands. Further investigation on the local effects of changes in seasons on agriculture and ecosystem would be needed, especially for the fragile regions.

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Jacob Coburn and S.C. Pryor

Abstract

This work quantitatively evaluates the fidelity with which the Northern Annular Mode (NAM), Southern Annular Mode (SAM), Pacific-North American pattern (PNA), El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) and the first-order mode interactions are represented in Earth System Model (ESM) output from the CMIP6 archive. Several skill metrics are used as part of a differential credibility assessment (DCA) of both spatial and temporal characteristics of the modes across ESMs, ESM families and specific ESM realizations relative to ERA5. The spatial patterns and probability distributions are generally well represented but skill scores that measure the degree to which the frequencies of maximum variance are captured are consistently lower for most ESMs and climate modes. Substantial variability in skill scores manifests across realizations from individual ESMs for the PNA and oceanic modes. Further, the ESMs consistently overestimate the strength of the NAM-PNA first-order interaction and underestimate the NAM-AMO connection. These results suggest that the choice of ESM and ESM realizations will continue to play a critical role in determining climate projections at the global and regional scale at least in the near-term.

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QIAOLING REN, XINGWEN JIANG, YANG ZHANG, ZHENNING LI, and SONG YANG

Abstract

It is known that the Tibetan Plateau (TP) can weaken the transient eddies (TEs) transported along the westerly jet stream. This study investigates the effects of the persistently suppressed TEs by the TP on the East Asian summer monsoon and the associated mechanisms using the NCAR Community Earth System Model. A nudging method is used to modify the suppression of the TEs without changing the steady dynamic and thermodynamic effects of the TP.

The suppressed TEs by the TP weaken the East Asian westerly jet stream through the weakened poleward TE vorticity flux. On the one hand, the weakened jet stream leads to less (more) rainfall in northern (southern) East Asia by inducing anomalous moisture convergence, mid-tropospheric warm advection, and upper-level divergence, particularly in early summer when the eastward propagation of TE suppression by the TP is strong. On the other hand, the precipitation anomalies can shift the East Asian westerly jet stream southward and promote the moisture convergence in southern East Asia through latent heat release. Therefore, the persistent suppression of the TEs leads to a southward shift of the East Asian rain belt by a convective feedback, as previously found that the steady thermodynamic and dynamic forcings of the TP favored a northward shift of the rain belt. This study suggests that the anomalously weak TEs can lead to the south-more-north-less rainfall change over East Asia.

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Tao Lian and Dake Chen

Abstract

While both intrinsic low-frequency atmosphere–ocean interaction and multiplicative burst-like event affect the development of the El Niño–Southern Oscillation (ENSO), the strong nonlinearity in ENSO dynamics has prevented us from separating their relative contributions. Here we propose an online filtering scheme to estimate the role of the westerly wind bursts (WWBs), a type of aperiodic burst-like atmospheric perturbation over the western-central tropical Pacific, in the genesis of the centennial extreme 1997/98 El Niño using the CESM coupled model. This scheme highlights the deterministic part of ENSO dynamics during model integration, and clearly demonstrates that the strong and long-lasting WWB in March 1997 was essential for generating the 1997/98 El Niño. Without this WWB, the intrinsic low-frequency coupling would have only produced a weak warm event in late 1997 similar to the 2014/15 El Niño.

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Man Yue, Minghuai Wang, Jianping Guo, Haipeng Zhang, Xinyi Dong, and Yawen Liu

Abstract

The planetary boundary layer (PBL) plays an essential role in climate and air quality simulations. Nevertheless, large uncertainties remain in understanding the drivers for long-term trend of PBL height (PBLH) and its simulation. Here we combinate the radiosonde data and reanalysis datasets to analyze PBLH long-term trends over China, and to further explore the performance of CMIP6 climate models in simulating these trends. Results show that the observed long-term “positive to negative” trend shift of PBLH is related to the variation in the surface upward sensible heat flux (SHFLX), and the SHFLX is further controlled by the synergistic effect of low cloud cover (LCC) and soil moisture (SM) changes. Variabilities in LCC and SM directly influence the energy balance via surface net downward shortwave flux (SWF) and the latent heat flux (LHFLX), respectively. The CMIP6 climate models, however, cannot reproduce the observed PBLH long-term trend shift over China. The CMIP6 results illustrate an overwhelming continuous downward PBLH trend during the 1979–2014 period, which is largely caused by the poor capability in simulating long-term variations of cloud radiative effect. Our results reveal that the long-term cloud radiative effect simulation is critical for CMIP6 models in reproducing the long-term trend of PBLH. This study highlights the importance of processes associated with LCC and SM in modulating PBLH long-term variations and calls attentions to improve these processes in climate models in order to improve the PBLH long-term trend simulations.

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X. R. Zhao, Z. Sheng, H. Q. Shi, L. B. Weng, and Y. He

Abstract

Using temperature data measured by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument from February 2002 to March 2020, the temperature linear trend and temperature responses to the solar cycle (SC), Quasi-Biennial Oscillation (QBO), and El Niño-Southern Oscillation (ENSO) were investigated from 20 km to 110 km for the latitude range of 50°S-50°N. A four-component harmonic fit was used to remove the seasonal variation from the observed monthly temperature series. Multiple linear regression (MLR) was applied to analyze the linear trend, SC, QBO, and ENSO terms. In this study, the near-global mean temperature shows consistent cooling trends throughout the entire middle atmosphere, ranging from -0.28 to -0.97 K/decade. Additionally, it shows positive responses to the solar cycle, varying from -0.05 to 4.53 K/100sfu. A solar temperature response boundary between 50°S and 50°N is given, above which the atmospheric temperature is strongly affected by solar activity. The boundary penetrates deep below the stratopause to ~ 42 km over the tropical region and rises to higher altitudes with latitude. Temperature responses to the QBO and ENSO can be observed up to the upper mesosphere and lower thermosphere. In the equatorial region, 40%-70% of the total variance is explained by QBO signals in the stratosphere and 30%-50% is explained by the solar signal in the upper middle atmosphere. Our results, obtained from 18-year SABER observations, are expected to be an updated reliable estimation of the middle atmosphere temperature variability for the stratospheric ozone recovery period.

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Linyuan Sun, Xiu-Qun Yang, Lingfeng Tao, Jiabei Fang, and Xuguang Sun

Abstract

The El Niño-Southern Oscillation (ENSO) events which generally mature in winter profoundly affect the following summer rainfall in eastern China (ECSR), but such an impact can change significantly with decadal background. This study examines how the impact changes since the 1950s by running correlation and regression analyses. It is found that the ENSO’s impact on ECSR has undergone two decadal shifts in the late 1970s and 1990s, respectively. Sequentially, three distinct ENSO-induced ECSR anomaly patterns are categorized, which exhibit both robust and changeable sides. The robust side manifests generally more precipitation in the Yangtze River basin affected by the anomalous tropical western North Pacific anticyclone (WNPAC) in the post-El Niño summer. The changeable side is reflected in the more variable ENSO-induced rainfall anomalies north of the Yangtze River, due to the ENSO-induced different East Asian midlatitude circulation anomalies. Meanwhile, the El Niño-induced drought in South China is enhanced since the late 1970s with the intensification of the anomalous WNPAC. The ENSO’s changing impact on the ECSR stems from the changes of ENSO-induced tropical and midlatitude circulation anomalies over East Asia, which are associated with different zonal (from tropical Pacific to Indian Ocean) and meridional (from tropical Pacific to Midlatitude North Pacific) teleconnections of ENSO-induced SST anomalies. The former affects the intensity and location of the anomalous WNPAC by affecting Indian Ocean capacitor effect and convection anomalies over the tropical Indo-western Pacific. The latter modulates the ocean-to-atmosphere feedback in the midlatitude North Pacific, contributes to different local geopotential anomaly sources, and then directly or indirectly through Rossby wavetrain affects the East Asian midlatitude circulation.

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Nan Sun, Yunfei Fu, Lei Zhong, Chun Zhao, and Rui Li

Abstract

In this paper, we examine convective overshooting and its effects on the thermal structure of the troposphere and lower stratosphere in the Tibetan Plateau in summer by matching the Tropical Rainfall Measuring Mission (TRMM) with Integrated Global Radiosonde Archive (IGRA), Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC), European Centre for Medium-Range Weather Forecasts 5th Reanalysis (ERA-5), the Japanese Meteorological Association 55-year reanalysis (JRA-55) and the National Aeronautics and Space Administration Modern-Era Retrospective analysis for Research and Applications, Versions2 (MERRA-2). It was found that convective overshooting mainly occurs in the central and eastern part of the Tibetan Plateau, and its frequency varies from 0.01 × 10−4 to 0.91 × 10−4. The convective overshooting warms the low middle tropopause and cools the tropopause nearby significantly, which can also makes air get wetter. The tropopause of the convective overshooting is substantially lower than the mean tropopause. Statistical results calculated from the five datasets are generally consistent; however, each dataset has its own strengths and weaknesses. The high spatiotemporal resolution temperature profiles from ERA-5 along with the high vertical resolution temperature profiles from COSMIC can be combined to accurately study convective overshooting in the Tibetan Plateau.

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Dapeng Zhang, Yanyan Huang, BoTao Zhou, and Huijun Wang

Abstract

The decadal intensification of the South Asian High (SAH) after the late 1970s, which is determined based on the geopotential height (H), is suspicious due to the lifting effect upon H caused by global warming. The updated reanalysis datasets of ERA5 and JRA55 indicate that the anticyclone in the upper troposphere over the Tibetan Plateau is relatively weak during 1980–2018 compared to that during 1950–1979. This decadal weakening of the SAH after 1979 can also be observed in the radiosonde observation data. Correspondingly, the SAH defined by eddy geopotential height (H’) reflects a consistent decadal weakening variation. The decadal weakening of SAH detected from H’ after the late 1970s matches with a decadal southward shift of the East Asian Westerly Jet, causing ascending motions over the Yangtze River Valley and descending motions over North China. Moreover, the decadal weakening and westward shift of the SAH is accompanied with the positive relative vorticity anomalies over the Northwest Pacific in the upper troposphere, which implies a declining and eastward shift of the western Pacific subtropical high (WPSH) and a weakened East Asian Summer Monsoon (EASM). Hence, the decadal weakening of the SAH after the late 1970s may contribute to the Yangtze-River-flooding-and-North-China-drought pattern through its connection with other circulation systems of EASM.

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Marcia T. Zilli and Neil C. G. Hart

Abstract

During austral summer, persistent tropical-extratropical cloud bands, such as the South Atlantic Convergence Zone (SACZ) over South America (SAm), link the tropical humid areas to the subtropics. In this study, we utilize an automatic object-based methodology to identify synoptic cloudband events occurring over SAm which are responsible for almost 60% of the precipitation during the rainy season (November to March). In addition to identifying SACZ events as cloud bands persisting four or more days, the framework also highlights the relevance of transient events (i.e., events persisting for three days or less) to the climatology. The location and persistence of the cloudband events are modulated by the propagation of synoptic-scale extratropical disturbances interacting with intraseasonal variability in the basic state upper-level zonal wind. During persistent events (i.e., lasting four or more days), upper-level westerly anomalies over the subtropics favour the propagation extratropical disturbances deeper into the tropics. Conversely, transient events occur when the Bolivian High is displaced/expanded southeastward, bringing upper-level easterly winds into subtropical latitudes and blocking the propagation of Rossby waves into lower latitudes. Subsequent anomalous subtropical convection from the cloud bands result in sources of Rossby waves that interact with the basic flow, resulting in downwind enhancement or damping of the extratropical disturbances. The adopted methodology proved to be a powerful framework in demonstrating this interaction between scales, with the basic state influencing and being modified by the synoptic disturbances.

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