Abstract
Compound drought and heat event (CDHE) causes severe impacts on agriculture, ecosystem, and human health. Based on daily maximum surface air temperature and meteorological drought composite ...index data in China, changing features of CDHEs in warm season from 1961 to 2018 is explored at a daily time scale based on a strict and objective definition in this study. Results reveal that CDHEs have occurred more frequently and widely in China, especially since the late 1990s. Notably, such changes are more obvious in Southwest China, eastern Northwest China, northern North China, and the coastal area of southeastern China. A prominent feature is that persistent CDHEs on a daily scale have increased significantly. To better understand climate change of compound extreme events, further studies on the physical mechanism, especially attribution analyses at a regional scale, are urgently needed.
Summertime hot extremes in China are categorized into three distinct types, i.e., independent hot days, independent hot nights, and compound events, based on differing configurations between daily ...maximum and minimum temperature. Linear trends for multiple indictors of these subtypes and traditionally defined hot days/nights exhibited remarkable differences in significance, magnitude, and even sign, especially for events involving daytime extremes. Thus, some significant changes masked in conventional analyses are successfully uncovered. Particularly, the dominance of independent hot days has decayed significantly, accompanied by a rapid boom of compound events and/or independent hot nights in different regions. These nighttime‐accentuated hot extremes have exhibited significant increases in duration, intensity, and spatial extent, with much stronger trends detected in severest events.
Key Points
Meticulous categorization unearths some significant changes overlooked in previous analyses
Summertime hot extremes have experienced significant transitions of dominating types
Severest hot extremes have been becoming substantially severer
Compared to individual hot days/nights, compound hot extremes that combine daytime and nighttime heat are more impactful. However, past and future changes in compound hot extremes as well as their ...underlying drivers and societal impacts remain poorly understood. Here we show that during 1960-2012, significant increases in Northern Hemisphere average frequency (~1.03 days decade
) and intensity (~0.28 °C decade
) of summertime compound hot extremes arise primarily from summer-mean warming. The forcing of rising greenhouse gases (GHGs) is robustly detected and largely accounts for observed trends. Observationally-constrained projections suggest an approximate eightfold increase in hemispheric-average frequency and a threefold growth in intensity of summertime compound hot extremes by 2100 (relative to 2012), given uncurbed GHG emissions. Accordingly, end-of-century population exposure to compound hot extremes is projected to be four to eight times the 2010s level, dependent on demographic and climate scenarios.
The boreal summer intraseasonal oscillation (BSISO), including a 30–60 day component (BSISO1) and a quasi-biweekly component (BSISO2), is the most prominent form of subtropical intraseasonal ...variability. Influences of BSISOs on summertime precipitation and temperature extremes in China are examined. Results indicate that BSISOs can simultaneously facilitate precipitation extremes in central-eastern China and extreme high temperatures in South China-Southeast China. During phase 2–4 of active BSISO1, accompanying precipitation extremes in central-eastern China, there is a fourfold-fivefold increase in probability of extreme high temperatures in Southeast China. About 50% of such simultaneous extremes fall into phase 2–3. BSISO2’s influences are pronounced from phase 6 to the next phase 2, with about 58% simultaneous extremes clustered within phase 7 to the next phase 1. It is the BSISO-induced vertical cell, with ascending motion in the Yangtze-Huai River Valley and descending motion in the south, that contributes to simultaneous extremes. Enhanced low-level southwesterlies convey moist and warm air towards southern parts of China. Strengthened ascending branch loaded by anomalously abundant moisture produces precipitation extremes in the north. Concurrently, combined effects of warm advection and descent-triggered adiabatic heating anchors extreme high temperatures well located in South China. The northeastward propagation of the BSISO1 confines influenced regions to eastern-southeastern parts of China, with gradually narrowing spatial extents. The BSISO2-induced simultaneous extremes sweep much broader areas, from southeast coasts to the central inlands. Above analyses on BSISOs-simultaneous extremes relationship lay a crucial scientific basis for predicting these high-impact events on sub-seasonal to seasonal scales.
Concurrent position shifts of the mid-level western Pacific subtropical high (WPSH) and the upper-level South Asia high (SAH) are regarded as significant precursors for persistent extreme ...precipitation events (PEPEs) in the Yangtze River Valley (YRV). By performing composite analyses, accountable vorticity genesis and dissipation are diagnosed based on a potential vorticity–diabatic heating theory. The results indicate that about 1 week preceding precipitation onset, a wave-like pattern of anomalous diabatic heating (Q) initiates its northwestward propagation from equatorial central Pacific. Subsequently, this wave-like pattern induces substantial changes in both horizontal and vertical structure of local Q along the propagating route. Forced negative vorticities in key areas result in the zonal approach between the SAH and the WPSH. During PEPEs, two thermal-induced vertical circulation cells take shape, with common strong ascent centered in the YRV. These anomalous cells are capable of self-maintaining for a few days via positive feedback processes. The WPSH and the SAH are therefore anchored in respective favorable positions for PEPEs. Simultaneously, descending motion of these two cells increases local solar radiation and decreases upward latent heat flux from surface, facilitating warmer underlying surface and swift accumulation of lower-level moisture. Correspondingly, enhanced heating to the north and rapid developing cyclone over warmer sea surface to the south combine to terminate above positive feedback processes. Finally, both the WPSH and the SAH retreat to their normal positions, accompanied by a quick decay of PEPEs.
Warming amplification over the Arctic Pole (AP hereafter) and Third Pole (Tibetan Plateau, TP hereafter) can trigger a series of climate responses and have global consequences. Arctic amplification ...(AA) and Tibetan amplification (TA) are the most significant characteristics of climate change patterns over the two Poles. In this study, trends, mechanisms and consequences of both AA and TA are compared. Based on ERA5 reanalysis during 1979–2020, both AP and TP have undergone significant warming with an annual rate of 0.72 °C/decade and 0.34 °C/decade respectively, which exceeds the rates for the Northern Hemisphere (0.29 °C/decade) and the global means (0.19 °C/decade) over the same period. Based on 22 Coupled Model Intercomparison Project Phase 6 models, AA over the AP is warming at a rate almost four times than the global means and twice as fast over the TP. Although both AA and TA are projected to continue in the future, currently there is no consensus on the dominant mechanisms for AA or TA over the two Poles. Proposed mechanisms of AA can be divided into two types: local climate factors (sea ice-albedo feedback, Planck feedback, temperature gradient feedback, cloud feedback, and water vapor feedback); and poleward heat and moisture transport from lower latitudes (atmospheric circulation effect, ocean circulation effect, and modulation of Pacific and Atlantic SST). Consequences of AA include decline of sea ice cover, retreat of the Greenland ice sheet, permafrost degradation, accelerated disturbances in marine and terrestrial ecosystems, and influences on extreme climate events at lower latitudes. Anthropogenic greenhouse gas emission, snow/ice-albedo feedback, cloud-radiation interactions, water vapor and radiative flux feedbacks, local forcing and feedback processes, land use changes and reduction in total ozone, are generally considered to be the main mechanisms causing TA. TA has caused significant change within the atmosphere and cryosphere over the TP and its surroundings, such as changes in climate extremes, snow cover, the retreat of glaciers, and permafrost degradation. Similarities and differences of warming amplifications over the two Poles are proposed, and the relative contribution of each mechanism to the warming amplifications and how the specific consequences may compare over the two Poles remain unclear and under continuing investigation.
Compound extreme events always cause severe impacts on human society and the natural system, especially in the populated areas. However, studies on their changes depend on definitions of the related ...extreme events. In this study, a newly defined compound drought and hot extreme events (CDHEEs) index based on the day-night concurrent hot extreme index and daily drought monitoring index is defined to explore possible changing features in summer-time CDHEEs from 1961 to 2018 in the densely populated areas of eastern China. Results reveal that the occurrence of CDHEEs shows a significant increase in the densely populated areas with the strengthening interaction of drought and hot extreme events, especially since the late 1990s. Further analyses indicate that urbanization and regional climate also moderate changes in CDHEEs, as we found that CDHEEs increased more prominently in the cities, especially in the drier regions. As a result, the population exposed to CDHEEs has doubled since the late 1990s in the densely populated areas of eastern China.
Two characteristic circulation patterns responsible for persistent extreme precipitation (PEP) events in Central–Eastern China are identified using a composite analysis: a double‐blocking high type ...and a single‐blocking high type. The double‐blocking high type is characterized by the development of two blocking highs near the Ural Mountains and the Sea of Okhotsk, with a trough between them. This long‐lived pattern continuously steers cold/dry air from mid–high latitudes to Central–Eastern China, where it encounters anomalously warm/moist air from lower latitudes conveyed by intensified southeasterlies associated with a westward shift of the western Pacific subtropical high (WPSH). The single‐blocking high type is characterized by the appearance of a blocking high to the south of Lake Baikal. This blocking high is accompanied by a deep trough located to its east, which extends into the southern Yangtze River Valley. The anomalous anticyclone associated with the westward extension of the WPSH is located further south and slightly east of that observed during the double‐blocking high type. Accordingly, the intensified southeasterlies convey anomalously moist air to the southern and eastern parts of Central–Eastern China. In the upper troposphere, the displacement of the South Asian High and jets favours divergence for both typical circulation patterns.
The comparison between the composites and additional cases that share similar circulation patterns but do not result in PEP in Central–Eastern China suggests that PEP events typically result from concurrent combinations of persistent anomalies from lower to upper levels, rather than from isolated anomalies of specific weather systems.
East Asia is one of the world's largest sources of dust and anthropogenic pollution. Dust particles originating from East Asia have been recognized to travel across the Pacific to North America and ...beyond, thereby affecting the radiation incident on the surface as well as clouds aloft in the atmosphere. In this study, integrated analyses are performed focusing on one trans-Pacific dust episode during 12–22 March 2015, based on space-borne, ground-based observations, reanalysis data combined with Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT), and the Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem). From the perspective of synoptic patterns, the location and strength of Aleutian low pressure system largely determined the eastward transport of dust plumes towards western North America. Multi-sensor satellite observations reveal that dust aerosols in this episode originated from the Taklimakan and Gobi Deserts. Moreover, the satellite observations suggest that the dust particles can be transformed to polluted particles over the East Asian regions after encountering high concentration of anthropogenic pollutants. In terms of the vertical distribution of polluted dust particles, at the very beginning, they were mainly located in the altitudes ranging from 1 km to 7 km over the source region, then ascended to 2 km–9 km over the Pacific Ocean. The simulations confirm that these elevated dust particles in the lower free troposphere were largely transported along the prevailing westerly jet stream. Overall, observations and modeling demonstrate how a typical springtime dust episode develops and how the dust particles travel over the North Pacific Ocean all the way to North America.
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•One trans-Pacific transport episode of dust was systematically examined from multiple observations and WRF-Chem modeling.•The westerly jet stream along with Aleutian cyclonic circulation facilitate the trans-Pacific transport of dust.•Physical mechanisms underlying the long-range transport of dust episode were investigated.
One trans-Pacific transport episode of dust was examined from multiple observations and WRF-Chem modeling, and the underlying physical mechanism has been investigated.
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