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Natalia Pilguj, Mateusz Taszarek, John T. Allen, and Kimberly A. Hoogewind

Abstract

In this work, long-term trends in convective parameters are compared between ERA5, MERRA-2, and observed rawinsonde profiles over Europe and the United States including surrounding areas. A 39-yr record (1980–2018) with 2.07 million quality-controlled measurements from 84 stations at 0000 and 1200 UTC is used for the comparison, along with collocated reanalysis profiles. Overall, reanalyses provide signals that are similar to observations, but ERA5 features lower biases. Over Europe, agreement in the trend signal between rawinsondes and the reanalyses is better, particularly with respect to instability (lifted index), low-level moisture (mixing ratio), and 0–3-km lapse rates as compared with mixed trends in the United States. However, consistent signals for all three datasets and both domains are found for robust increases in convective inhibition (CIN), downdraft CAPE (DCAPE), and decreases in mean 0–4-km relative humidity. Despite differing trends between continents, the reanalyses capture well changes in 0–6-km wind shear and 1–3-km mean wind with modest increases in the United States and decreases in Europe. However, these changes are mostly insignificant. All datasets indicate consistent warming of almost the entire tropospheric profile, which over Europe is the fastest near ground whereas across the Great Plains it is generally between 2 and 3 km above ground level, thus contributing to increases in CIN. Results of this work show the importance of intercomparing trends between various datasets, as the limitations associated with one reanalysis or observations may lead to uncertainties and lower our confidence in how parameters are changing over time.

Open access
Chiyu Zhao, Xin Geng, Wenjun Zhang, and Li Qi

Abstract

Previous studies have demonstrated that the Atlantic multidecadal oscillation (AMO) could affect El Niño–Southern Oscillation (ENSO) through thermocline adjustment, with a stronger ENSO sea surface temperature (SST) amplitude during a negative AMO phase than during a positive phase. In this study, we find that the ENSO atmospheric anomaly amplitudes in the tropical Pacific during different AMO phases are not necessarily consistent with these ENSO SST changes. For El Niño episodes, the low-level wind and precipitation anomalies over the tropical Pacific in the boreal winter are more pronounced during the negative AMO phase than during the positive phase, corresponding well to the stronger SST anomalies. However, La Niña events during the negative AMO phase are accompanied by weaker atmospheric anomalies in the tropical Pacific, although their SST anomalies are stronger than those during the positive phase. We suggest that this mismatch between La Niña SST and atmospheric anomalies can be largely attributed to AMO decadal modulation. A positive AMO favors intensified trade winds and weakened precipitation in the central tropical Pacific by modifying Walker circulation. Therefore, when La Niña coincides with a positive AMO, the low-level easterly and negative precipitation anomalies are superimposed, which gives rise to stronger atmospheric perturbations. In contrast, under a negative AMO background, the atmospheric anomalies induced by La Niña anomalous SST are partly counteracted by the AMO remote decadal modulation, thereby resulting in weaker anomaly amplitudes. Here, we highlight that AMO decadal forcing needs to be considered when investigating ENSO atmospheric variabilities and related regional climate impacts.

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Julio C. Marín, Deniz Bozkurt, and Bradford S. Barrett

Abstract

We analyze the seasonal evolution and trends of atmospheric blocking from 1979 to 2018 using a geopotential-height-based method over two domains, one located to the west (150°–90°W, 50°–70°S) and the other over and to the east (90°–30°W, 50°–70°S) of the Antarctic Peninsula. Spatial patterns of geopotential heights on days with blocking feature well-defined ridge axes over and west of much of South America, and days with the most extreme blocking (above the 99th percentile) showed upper-tropospheric ridge and cutoff low features that have been associated with extreme weather patterns. Blocking days were found to be more frequent in the first half of the period (1979–98) than the second (1999–2018) in all seasons in the west domain, whereas they seem to be more common over the eastern (peninsula) domain in 1999–2018 for austral winter, spring, and autumn, although these differences were not statistically significant. West of the Antarctic Peninsula, blocking days occur most frequently when the Antarctic Oscillation (AAO) is negative, whereas they are more frequent over the peninsula when the AAO is positive. We propose that our blocking index can be used to indicate atmospheric blocking affecting the Antarctic Peninsula, similar to how the Greenland blocking index has been used to diagnose blocking, its trends, and impacts over the Arctic.

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Veeshan Narinesingh, James F. Booth, and Yi Ming

Abstract

This study examines the climatology and dynamics of atmospheric blocking, and the general circulation features that influence blocks in GFDL’s atmosphere-only (AM4) and coupled atmosphere–ocean (CM4) comprehensive models. We compare AM4 and CM4 with reanalysis, focusing on winter in the Northern Hemisphere. Both models generate the correct blocking climatology and planetary-scale signatures of the stationary wave. However, at regional scales some biases exist. In the eastern Pacific and over western North America, both models generate excessive blocking frequency and too strong of a stationary wave. In the Atlantic, the models generate too little blocking and a weakened stationary wave. A block-centered compositing analysis of block-onset dynamics reveals that the models 1) produce realistic patterns of high-frequency (1–6-day) eddy forcing and 2) capture the notable differences in the 500-hPa geopotential height field between Pacific and Atlantic blocking. However, the models fail to reproduce stronger wave activity flux convergence in the Atlantic compared to the Pacific. Overall, biases in the blocking climatology in terms of location, frequency, duration, and area are quite similar between AM4 and CM4 despite the models having large differences in sea surface temperatures and climatological zonal circulation. This could suggest that other factors could be more dominant in generating blocking biases for these GCMs.

Significance Statement

Atmospheric blocks are persistent high pressure systems that can lead to hazardous weather. Historically, climate models have had trouble capturing blocks, but recent changes in the models might lead to improvements. As such, the work herein investigates the spatial distribution, prevalence, duration, size, and dynamics of wintertime blocking in recent NOAA climate models. Overall, these models capture the long-term-average spatial pattern of blocking, and properly reproduce key dynamical features. However, the models produce too much blocking in the western United States, and too little over the northern Atlantic Ocean and Europe. These blocking biases are consistent with atmospheric stationary waves biases, but not jet stream bias. This downplays the role of jet biases in the models being responsible for blocking biases.

Open access
Yanbo Nie and Jianqi Sun

Abstract

The interannual variability in summer precipitation intraseasonal oscillation intensity over southwest China (SWC) is investigated in this study. The results indicate that the 7–20-day period dominates the intraseasonal variability in summer SWC precipitation. The leading mode of summer SWC precipitation 7–20-day oscillation intensity (SPOI) is a north–south dipole pattern with prominent interannual variability. The atmospheric circulation anomalies from both tropics and mid- to high latitudes are responsible for the interannual variability in the dipole pattern. In the tropics, an enhanced local Hadley cell and an anomalous anticyclone over southern China and the northwest Pacific contribute to the north-positive–south-negative pattern of SPOI by inducing moisture convergence (divergence) over northern (southern) SWC in the background state. In the mid- to high latitudes, the 7–20-day Rossby wave trains along the subtropical jet are crucial for the 7–20-day precipitation over northern SWC. Further analyses suggest that the sea surface temperature (SST) anomalies over the Maritime Continent (MC) and the North Atlantic (NA) are associated with the SPOI dipole pattern. The MC SST warming causes convection anomalies over the tropical Indo-Pacific, consequently triggering a Matsuno–Gill-type atmospheric response conducive to the north-positive–south-negative pattern of SPOI. The NA SST tripole triggers a Rossby wave train across Eurasia that strengthens the East Asian westerly jet and enhances 7–20-day atmospheric variability, consequently favoring the variability of 7–20-day precipitation over northern SWC. Diagnoses of moisture and vorticity budgets further indicate the importance of the interaction between intraseasonal fluctuations and atmospheric background in the formation of the north–south difference in 7–20-day precipitation variability over SWC.

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Shuyu Wang, Xiaohui Ma, Guangzhi Xu, Shenghui Zhou, Ping Chang, and Lixin Wu

Abstract

Atmospheric rivers (ARs) originating near the Kuroshio Extension and the Hawaiian Islands and making landfall onto the west coast of North America in the North Pacific in the boreal winter season (October–March) are detected and tracked using ERA5 reanalysis (2000–19), and are classified as Kuroshio Extension (KE) and Pineapple Express (PE) ARs, respectively. Compared with KE ARs, PE ARs are longer and wider with higher intensity and shorter duration. Although the total occurrence of PE ARs is lower, the occurrence of extreme ARs is substantially higher than KE ARs. PE (KE) ARs are oriented more meridionally (zonally) with more equatorward (poleward) landfalling positions and associated precipitation. The genesis, development, and decay of KE and PE ARs and their relationships with extratropical cyclones (ECs) are investigated. Along- and cross-section analyses show that PE ARs are associated with stronger, deeper low pressure systems with closer tropical connections. Compared with KE ARs, PE ARs originate from well-developed ECs with stronger southward intrusion of cold fronts, forming closer to the ECs’ centers along the sharp temperature/pressure gradient zone. They are accompanied by enhanced and deeper vertical motion and stronger low-level wind. The intensity difference between KE and PE ARs is largely determined by the orientation and the strengths of temperature/pressure gradients of associated ECs rather than the intensity of associated ECs themselves. Furthermore, the evolution of ARs and ECs is not always synchronized, suggesting complicated AR and EC interactions that require further investigations.

Open access
Weiguo Wang, Hongyi Li, Zeming Xie, Xiaofan Zhu, Linhong Xiao, Xiaohua Hao, and Jian Wang

Abstract

Atmospheric water vapor plays a key role in the water cycle, especially on the Tibetan Plateau, where precipitation is an invaluable contributor to water resources. To better understand which water vapor source areas influence precipitation on the northeastern Tibetan Plateau (NETP), we applied the flexible particle dispersion method (FLEXPART) to simulate water vapor trajectories and water vapor source contribution related to precipitation events during the snow season from 1979 to 2017 on the NETP. The results show that continental water vapor source areas contributed 92.33% of the water vapor to precipitation events on the NETP, which was obviously greater than the water vapor contribution from oceanic areas. One key continental water vapor source area, the Tibetan Plateau without the study area, contributed 66.13% of the water vapor to the precipitation, and central Asia supplied 8.69%, ranking second. Comparing the distributions of the water vapor contributions to extensive and regional precipitation events revealed that the only difference between extensive and regional precipitation events is in the magnitudes of the water vapor contributions, and the spatial distributions of the water vapor contributions are extremely similar. Central and southern China obviously supplied more water vapor to extensive precipitation events than to regional precipitation events. These results help us better understand the recent drastic precipitation changes on the NETP.

Significance Statement

We sought to understand how water vapor influences precipitation over the northeastern Tibetan Plateau and which water vapor source areas play a key role in the water vapor supply. Therefore, we applied a numerical model to investigate the relationship between water vapor and precipitation from 1979 to 2017 during the snow season. Continental water vapor source areas contributed considerably more water vapor than oceanic water vapor source areas. The most important continental water vapor contributor was the Tibetan Plateau without the northeastern Tibetan Plateau area, and the second highest contributor was central Asia. Future work should focus on how water vapor impacts the precipitation changes in this wetter and warming area.

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Callum J. Shakespeare and Michael L. Roderick

Abstract

Climate models predict large increases in downwelling longwave radiation (DLR) at Earth’s surface as atmospheric CO2 concentrations increase. Here we introduce a novel methodology that allows these increases to be decomposed into direct radiative forcing due to enhanced CO2 and feedbacks due to subsequent changes in atmospheric properties. For the first time, we develop explicit analytic expressions for the radiative forcing and feedbacks, which are calculable from time-mean fields of near-surface air temperature, specific humidity, pressure, total column water vapor, and total cloud fraction. Our methodology captures 90%–98% of the variance in changes in clear-sky and all-sky DLR in five CMIP5 models, with a typical error of less than 10%. The longwave feedbacks are decomposed into contributions from changes in temperature, specific humidity, water vapor height scale, and cloud fraction. We show that changes in specific humidity and height scale are closely linked to changes in near-surface air temperature and therefore, in the global average, that 90% of the increase in all-sky DLR may be attributed to a feedback from increasing near-surface air temperature. Mean-state clouds play a major role in changes in DLR by masking the clear-sky longwave and enhancing the temperature feedback via increased blackbody radiation. The impact of changes in cloud cover (the cloud feedback) on the DLR is small (∼2%) in the global average, but significant in particular geographical regions.

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Xianke Yang and Ping Huang

Abstract

The evolution of El Niño–Southern Oscillation (ENSO) is an essential predictor for global climate anomalies, but ENSO’s effect is regulated by the considerable diversity of post-ENSO evolution. This study identified that there coexist a decaying mode and a developing mode of ENSO in the ENSO decaying periods (from September to August) through empirical orthogonal function (EOF) and extended EOF analyses. The diversity of ENSO evolution during the decaying periods is actually determined by the ENSO developing mode, which is unsteady and susceptible to other signals, whereas the ENSO decaying mode is quite steady. Using multiple datasets, segmented datasets, and sea surface temperature anomalies (SSTAs)-residual EOF, the key signals related to the diversity of ENSO evolution are identified and verified in the South Atlantic and equatorial South Pacific. Based on the decaying and developing modes, a prediction model was constructed to predict the boreal summer Niño-3 index. The ENSO developing mode, explaining very small variance, plays a decisive role in determining the prediction of the summer SSTAs in the Pacific. The forecasting skill of summer ENSO can also be greatly improved when continuous SSTAs and interbasin interactions in the whole tropics are considered. This prediction result indicates that identified precursory disturbance signals and interbasin interactions for ENSO development are crucial for boreal summer ENSO prediction.

Significance Statement

ENSO often shows great diversity in the temporal evolution during post-ENSO periods, thus limiting the predictability of tropical SSTAs in the following spring and summer. Identifying the signal impacting the diversity of ENSO is crucial for predicting ENSO. Here, we offer a new understanding of the essence of ENSO’s temporal diversity from the perspective of interbasin interactions. We identified that the ENSO typical decaying period contains a steady decaying mode and an unsteady developing mode simultaneously, and the developing mode induces the great diversity in ENSO’s evolution during post-ENSO periods. Two signals located in the South Atlantic and equatorial South Pacific are recognized as the main sources influencing the diversity of ENSO evolution.

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L. Ruby Leung, William R. Boos, Jennifer L. Catto, Charlotte A. DeMott, Gill M. Martin, J. David Neelin, Travis A. O’Brien, Shaocheng Xie, Zhe Feng, Nicholas P. Klingaman, Yi-Hung Kuo, Robert W. Lee, Cristian Martinez-Villalobos, S. Vishnu, Matthew D. K. Priestley, Cheng Tao, and Yang Zhou

Abstract

Precipitation sustains life and supports human activities, making its prediction one of the most societally relevant challenges in weather and climate modeling. Limitations in modeling precipitation underscore the need for diagnostics and metrics to evaluate precipitation in simulations and predictions. While routine use of basic metrics is important for documenting model skill, more sophisticated diagnostics and metrics aimed at connecting model biases to their sources and revealing precipitation characteristics relevant to how model precipitation is used are critical for improving models and their uses. This paper illustrates examples of exploratory diagnostics and metrics including 1) spatiotemporal characteristics metrics such as diurnal variability, probability of extremes, duration of dry spells, spectral characteristics, and spatiotemporal coherence of precipitation; 2) process-oriented metrics based on the rainfall–moisture coupling and temperature–water vapor environments of precipitation; and 3) phenomena-based metrics focusing on precipitation associated with weather phenomena including low pressure systems, mesoscale convective systems, frontal systems, and atmospheric rivers. Together, these diagnostics and metrics delineate the multifaceted and multiscale nature of precipitation, its relations with the environments, and its generation mechanisms. The metrics are applied to historical simulations from phases 5 and 6 of the Coupled Model Intercomparison Project. Models exhibit diverse skill as measured by the suite of metrics, with very few models consistently ranked as top or bottom performers compared to other models in multiple metrics. Analysis of model skill across metrics and models suggests possible relationships among subsets of metrics, motivating the need for more systematic analysis to understand model biases for informing model development.

Open access