Anomalous Subsidence Research Articles

The changing characteristics of torrential rainfall in the Huaihe River Basin from 1961 to 2020

Abstract The Huaihe River Basin (HRB) is one of the most severely affected regions by torrential rain disasters in China. However, with climate warming, it is unclear what new features torrential rain exhibits and how the main modes of torrential rain change in this region. This study analyzes the first two modes of torrential rain in the HRB from 1961 to 2020 and their driving factors. Results show that in the last 30 years, the frequency of torrential rain events in the HRB has clearly increased, especially in southern HRB region during the flood season. The first mode of torrential rain is an entire basin-wide mode, with a period of 2–3 years. The mode is primarily influenced by the low-level westerly jet stream and Jianghuai cyclones, which are closely linked to the stratospheric Quasi-Biennial Oscillation (QBO) through influencing planetary wave propagation and convective activity under different phases of QBO. The second mode is a north-south distribution mode, with a period of 4–6 years. The leading circulation system is the frontal systems. Both the East Asia/Pacific (EAP) teleconnection wave train and ENSO drive the second torrential rain mode. During El Niño years, anomalous subsidence and anticyclone in the western tropical Pacific, induced by sea surface temperature warming in the eastern tropical Pacific through the East Asia/Pacific teleconnection, can trigger the EAP wave train poleward in the East Asian region. This wave train leads to the cold-warm confrontation conducive to the second torrential rain mode.

Controls on the Stratigraphic Architecture of the US Atlantic Margin: Processes Forming the Accommodation Space

AbstractAccommodation space governs the spatial and temporal distributions of sediments in continental margins. Mapping the sedimentation patterns, therefore, offers insights into the solid‐Earth processes that shape accommodation space. We assembled an unprecedented amount of seismic and borehole data along the Eastern North American Margin and used it to divide the margin's sedimentary package into eight chronostratigraphic intervals, identifying temporal shifts in depocenters under the continental shelf, slope, and rise. The Jurassic depocenters follow the syn‐rift structure and its thermal subsidence loci. The Long Island Platform is the only margin segment where the early post‐rift sediment thickness matches subsidence predictions from uniform‐stretching models, whereas in Georges Bank Basin (GBB) and Baltimore Canyon Trough (BCT), sediment thickness is 1.5–3 times higher than predicted, pointing to other factors at play. A margin‐wide Jurassic transient shoulder uplift is inferred from the occurrence of stratigraphic onlaps above thinned crust. Unlike the Jurassic, the Cretaceous and Cenozoic depocenters disregard the inherited subsidence pattern. The accommodation space over the shelf and coastal plain during the Cretaceous was affected by regional isostatic compensation of the sedimentary loads accumulated on the shelf and rise. Accommodation space development in the GBB was interrupted during the Cretaceous after the margin crossed the Great Meteor Hotspot track, resulting in a widespread permanent uplift, erosion, and sediment redistribution. The distribution of anomalous Neogene subsidence in the BCT challenges previous suggestions of mantle dynamic control on the accommodation space and favors flexural downwarping of the shelf by sediment accumulation on the rise.

Process-Oriented Compound Long-Duration Dry and Hot Events in China: Atmospheric Conditions, Moisture, and Heat Budget Analyses

Abstract Summertime compound dry and hot events pose severe threats to agricultural production and human health, especially those with a long duration. Considering the joint evolution of such hazards in space and time, spatiotemporal compound long-duration dry and hot (SLDDH) events in China during 1961–2022 are identified with a process-oriented method. Here, we investigate the associated large-scale atmospheric circulation patterns and the physical processes causing their precipitation and temperature anomalies. In all regions of China, this persistent dry–hot compound extreme is accompanied by anomalous high pressure systems along with enhanced descending motion, increased net surface solar radiation, and decreased water vapor flux convergence. Moisture budget diagnosis shows that precipitation deficits during the SLDDH events are produced primarily by the suppressed vertical moisture advection associated with the dynamical contribution of anomalous subsidence, while the thermodynamic process due to the anomaly in atmospheric moisture content makes a small contribution. Horizontal temperature advection generally plays a negative role in sustaining SLDDH events, while it helps trigger the events in North China. In most regions, adiabatic warming due to abnormal subsidence plays a dominant role in determining the near-surface high temperatures during the long-lasting warm and dry periods, whereas diabatic heating has a cooling or small effect therein. However, in some northern areas such as North China and northern Xinjiang, hot extremes during SLDDH events arise from a combination of diabatic heating and adiabatic warming. This study thus quantifies and reveals the crucial factors leading to the severity of compound dry and hot events.

Driving subsidence mechanisms during the formation of the intracratonic General Levalle basin, central Argentina

The Cretaceous-Cenozoic General Levalle rift basin (GLB) in Central Argentina, near 900 km east from the modern Chile-Peru trench, is an exploratory hydrocarbon frontier located within an intracratonic setting. The basin is 4.5 km deep and can be divided into three major evolutionary stages, synrift, intermediate and postrift. To date, the basin subsidence has not been yet addressed. We particularly made emphasis on the postrift sequence given that records of thousands of metres of Cretaceous and Cenozoic Strata and tectonic and thermal events are not known to account for these large accumulations. In this work we performed a backstripping analysis, in combination with a stretching-thermal model, which were then compared to different dynamic topography studies. From our backstripping models we found a typical rift basin curve, with the largest subsidence at the synrift stage followed by a decrease in values during the postrift. Assuming the synrift stage was driven by tectonic extension, we correlated the tectonic subsidence curve, obtained from backstripping studies, with a stretching-thermal subsidence model. The best fit among subsidence curves was obtained with a stretching factor β = 1.285. However, we observed a mismatch along the postrift part of both curves, resulting in a residual subsidence of ∼323 m. When this anomalous tectonic subsidence was compared to the latest six dynamic topography models, we found a good correlation with one of them. For the models that did not match and using simple isostatic computations, we estimated a lithospheric mantle thickening of 35.4 km to account for the residual subsidence and dynamic topography models. From our analysis it turns out that, in addition to stretching and thermal subsidence, mantle thickening in the lithosphere and in the asthenosphere, controlled the total subsidence in the General Levalle basin. The subduction history of Nazca Plate could have affected the postrift stage in this basin.

Flash Drought in the South of Yangtze River and the Potential Impact of North Atlantic Sea Surface Temperature

AbstractFlash droughts (FDs), a type of drought with rapid onset, occurred in growing season have damaging impacts on crops, ecosystem and hence livelihoods. However, the associated physical mechanism were not well understood, which prohibits a skillful early warning. Based on the reanalysis data for 1961–2022, we diagnosed the characteristics, antecedent meteorological conditions and large‐scale atmospheric circulation of FDs over the region south of Yangtze River (SYR), a hotspot region of FDs. The FDs tend to occur during the periods before (i.e., from April to mid‐June) and after (i.e., early July–September) Meiyu season (B‐Meiyu and A‐Meiyu periods). Large precipitation deficits and rapid elevated temperature caused by anomalous subsidence and moisture divergence are responsible for the onset of FDs during the two periods. The North Atlantic tripole‐like sea surface temperature anomaly (SSTA) modes have potential impact on the anomalous circulations via modulating Rossby wave trains, but with different locations among the two periods. During B‐Meiyu period, the SSTA mode is characterized by positive SSTA over subtropical North Atlantic and negative SSTAs over the tropical and mid‐latitude ocean, triggering a “+ − + − + −” circulation pattern over North Atlantic–Eurasia, which reinforces the East Asian trough (EAT). Anomalous northeasterly winds dominate the SYR suppressing moisture transport from the tropics. The SSTA mode locates farther to the north during A‐Meiyu period, inducing a “+ − + − +” wave train over mid‐latitude North Atlantic‐Eurasia, leading to the strengthening of the western Pacific subtropical high. The high‐pressure anomalies control the SYR, resulting in anomalous subsidence and hence negative precipitation anomalies.

Climatic drivers of the Canadian wildfire episode in 2023

Extreme wildfires broke out in Canada from May 2023 and persisted four months. Tremendous fire emissions posed significant impacts on the air quality in both local and downwind regions. Here, the authors explored the meteorological factors driving this wildfire episode and the associated large-scale circulation. Relative to the mean value of 2001–2022, the 2023 Canadian fires released 0.37 Gt more CO2 (527.1%) during May–August. Such enhancement was strongly associated with the anomalous surface warming, especially in May–June, when the daily maximum temperature on average increased by 8.11°C, with regional hotspots up to 10°C in central and eastern Canada. Consistent with the surface warming, positive anomalies of geopotential height were observed at high levels in central and western Canada. The dispersion of Rossby waves led to the long-lasting dominance of high-pressure systems and the consequent warming through anomalous subsidence. The phase-locked wave pattern induced by favorable topography and the strong warming tendency in the pan-Arctic regions are expected to jointly escalate the probability of extreme wildfires over central and western Canada in the future.摘要2023年5月加拿大发生极端野火事件并持续4个月时间. 野火污染物排放对当地和下风向的空气质量造成了严重影响. 本文对驱动此次野火事件的气象因子和相关大尺度环流进行了探究. 研究表明, 此次极端野火排放的二氧化碳相较往年同期增长了0.37Gt (527.1%) . 受罗斯贝波频散影响, 同期加拿大中西部区域出现持续的位势高度正异常, 促进气流下沉并引发局地高温, 中西部区域5–6月最高温平均上升8.11°C, 部分区域甚至超过10°C, 导致野火排放的显著增长. 受有利地形和泛北极地区快速增暖的影响, 预期未来加拿大中西部发生极端野火的概率可能会显著上升.

Relative Contributions of Large‐Scale Atmospheric Circulation Dynamics and Anthropogenic Warming to the Unprecedented 2022 Yangtze River Basin Heatwave

AbstractThe unprecedented 2022 Yangtze River Basin (YRB) heatwave is a threat to human society and natural ecology, so the understanding of its underlying drivers is critical to regional climate adaptation and resilience. Here we conducted a multi‐method attribution analysis on the contribution of atmospheric circulation change and anthropogenic impacts to the occurrence probability and intensity of this extreme heatwave. Based on the nonstationary statistical analysis, the 2022 YRB heatwave is a 1‐in‐900‐year event and a 1‐in‐110‐year event with and without considering the 2022 YRB heatwave in the fitting, respectively. The large‐scale meteorological condition analysis shows that the 2022 YRB heatwave is featured with an anomalous high‐pressure system that favors a hot and dry atmospheric column, overlaid by anomalous subsidence and clear skies which leads to warming and greater solar heating. The ensemble constructed circulation analogue analyses show that the circulation anomaly fails to explain the observed 2022 YRB SAT anomalies fully. Specifically, 46% (0.132 ± 0.027°C decade−1) of the observed SAT trend during 1979–2022 (0.290 ± 0.048°C decade−1) is caused by anthropogenic warming and the associated thermodynamic feedback, while the remaining 54% (0.157 ± 0.038°C decade−1) of the trend is caused by changes in the large‐scale atmospheric circulation. Our findings on changes in atmospheric circulation patterns associated with YRB heatwave and anthropogenic contributions to YRB heatwave could provide valuable information for climate adaptation and mitigation strategies in the context of a warming climate.

Combined influence of El Niño, IOD and MJO on the Indian Summer Monsoon Rainfall: Case Study for the years 1997 and 2015

To find out how El Niño, the positive Indian Ocean Dipole (IOD), and the Madden-Julian Oscillation (MJO) affect the Indian Summer Monsoon Rainfall (ISMR), this study looks at two different monsoon years: 1997 (normal ISMR) and 2015 (below-normal ISMR). These two years were ideal for examining their combined effects on the performance of ISMR because they both experienced super El Niño conditions over the Pacific Ocean and positive IOD over the Indian Ocean. In these two years, spatial diversity associated with sea surface temperature (SST) was found over the Indian Ocean as well as the Pacific Ocean. The analysis of the large-scale circulation showed a distinct difference: the below-normal monsoon in 2015 was due to weak southwesterly flow toward the Indian landmass as well as anomalous subsidence compared to 1997. Analysis of rainy and dry days shows that 2015 had a greater (less) number of dry (rainy) days compared to 1997, contributing to a below-normal ISMR in 2015. The study also looked at the role of MJO in the observed rainfall anomalies over India during these two years, when there was both El Niño and positive IOD. This was done by looking at the composite of rainfall anomalies for different MJO phases. In 1997, during the monsoon season, MJO showed prolonged activity over the Indian Ocean (favorable MJO phases), which contributed to normal ISMR. However, in 2015, MJO was found active for many days over the western Pacific (unfavorable MJO phases), which contributed to below-normal ISMR. Overall, this case study highlights the complex interactions between El Niño, IOD, MJO, and monsoon dynamics, emphasizing the need for a comprehensive study of these factors to understand the behaviour of ISMR.

Structural Restorations of the Complete Conjugate US‐Mexico Eastern Gulf of Mexico Margin

AbstractWe present the first sequential structural restoration with flexural backstripping of the Gulf of Mexico US‐Mexico conjugate margin salt basin. We construct four large‐scale (100s of km) balanced, sequential structural restorations to investigate spatio‐temporal patterns of subsidence, geometry of the original salt basin, feedbacks between post‐salt structural and stratigraphic evolution, paleo‐bathymetry, and crustal configurations. The restorations are based on interpretations of 2D and 3D seismic data, and include sequential sedimentary decompaction, flexural isostatic backstripping, and thermal isostatic corrections. The spatially variable crustal thinning factor is directly measured from seismic data, and lithologic parameters are determined by well penetrations. We present a model for the original salt basin and discuss evidence for and implications of a deep water salt basin setting for the GoM. Our analysis suggests a salt basin that contained ∼1–2 km thick salt in a basin 175–390 km across with ∼1 km of bathymetry after salt deposition. The base of salt is mostly smooth with <1 km of local relief in the form of normal faults that disrupt a pre‐salt sedimentary section. We find that supra‐salt extension and shortening are not balanced, with measurable extension exceeding shortening by 18–30 km on each cross‐section. Our subsidence analysis reveals anomalous subsidence totaling 1–2 km during Late Jurassic and Early Cretaceous times that may reflect dynamic topography or depth‐dependent thinning. We offer an interpretation of crustal breakup invoking pre‐salt clastic sedimentation, salt deposition in a deep water syn‐thinning basin, and post‐salt lower‐crustal exhumation.

Neogene morphotectonic evolution of the East Asian Continental Shelf

The offshore basins in the East Asian Continental Shelf have undergone distinct tectono-geomorphic-sedimentary evolution since the Neogene, driven by complex interactions between tectonic and surface Earth processes. However, the contribution of the latter to the evolution of the offshore basins has been less studied. The landscape simulation software Badlands serves as an ideal tool to investigate the impacts of tectonic and surface processes on sedimentation as it simulates the sediment routing in a source-to-sink context. Focusing on the southern East China Sea Basin (ECSB) and the northern South China Sea Shelf Basin (SCSSB), two key components of the East Asian Continental Shelf, we used Badlands to reconstruct the tectono-geomorphic-sedimentary evolution since the Neogene. It revealed two-phase evolutionary processes: (1) the southern ECSB and the northern SCSSB were in the unified shelf environment and represented as an integral during the Early Miocene, though the northern SCSSB experienced anomalous subsidence due to the southward jump of the SCS mid-ocean ridge. The integrated southern ECSB and northern SCSSB experienced uplift and denudation since the Middle Miocene, under the influence of the Philippine Sea Plate extrusion. (2) Subsequently, the southern ECSB and the northern SCSSB were separated into two independent basins in the Late Miocene, when Taiwan Island was significantly uplifted due to the Philippine Sea Plate extrusion and the arc-continental collision. Then Taiwan Island experienced a multi-stage uplift characterized by southward and westward migration. It concluded that tectonics (i.e. Taiwan Orogeny and the subduction of the Philippine Sea Plate) primarily dominated the differential evolution of the southern ECSB and the northern SCSSB. Surface Earth processes (i.e. the intensification of the East Asian Monsoon and the sea-level fluctuation) are the secondary contributors.

Diverse relationship between the tropical night in South China and the water vapor transport over the South China Sea and the plausible causes

South China (SC) frequently suffers from tropical night (TN), which features extreme high nighttime temperature with abnormal high humidity usually. This study explores the relationship between the TN in SC and the water vapor transport (WVT) over the South China Sea (SCS), an important water vapor source. 64% and 36% of the TN occurs under abnormally increased and decreased SCS WVT (named positive and negative SCS-WVT cases), manifesting their diverse relationship. The positive SCS-WVT cases feature above-normal humidity and subsidence over SC, favoring TN via adiabatic heating, longwave radiation and sensible heat persistently from day to night. The negative SCS-WVT cases feature strong subsiding anomaly over SC, which not only induces adiabatic heating, but also reduces the cloud cover, increases the daytime solar insolation and provides pre heating for TN.The diverse relationship between the TN in SC and SCS WVT is modulated by an anomalous anticyclone over the western North Pacific. For positive SCS-WVT cases, the anticyclone is related to two suppressed convection originating from the southwest and south of the subtropical high and moves westward. The anticyclone is centered east of the SCS on TN onset, inducing anomalous southerly SCS WVT. For negative SCS-WVT cases, the anticyclone is related to a suppressed convection originating from the southwest of the subtropical high and moves northwestward. The anticyclone is centered northwest of the SCS on TN onset, inducing anomalous northerly SCS WVT. SC is dominated by the anomalous subsidence in the northwest (east) of the anticyclone for positive (negative) SCS-WVT cases, and the subsidence is stronger for negative cases due to the vertically upward increase of negative vorticity advection associated with a northward-tilting structure. It is highlighted that different structures of anticyclonic anomaly could result in distinct features of extreme high temperature.

Causes of the Unprecedented Month‐To‐Month Persistent Extreme Heat Event Over South China in Early Summer 2020: Role of Sea Surface Temperature Anomalies in the Tropical Indo‐Pacific Region

AbstractAn unprecedented month‐to‐month persistent extreme heat event attacked South China in early summer (May–July) 2020, with an extremely and persistently stronger‐than‐normal intensity and higher‐than‐normal number of extreme heat days. The underlying mechanisms were investigated from the month‐to‐month perspective using observations, reanalysis data, and numerical experiments. The results showed that a local anomalous anticyclone in the mid‐lower troposphere over South China and the South China Sea directly contributed to this persistent extreme heat event via anomalous warm advection heating and diabatic warming at near‐surface level. Further investigation revealed that the rapid transition from El Niño to La Niña with subsequent development in the central and eastern tropical Pacific, along with persistent warming in the tropical Indian Ocean in the early summer of 2020, were crucial to the maintenance and reinforcement of the local anomalous anticyclone, with greater importance of the latter. On the one hand, rapid cooling in the central and eastern tropical Pacific enhanced the Walker circulation and further triggered an abnormal local Hadley cell, leading to anomalous subsidence over South China and the South China Sea. On the other hand, the persistent warming in the tropical Indian Ocean resulted in an enhanced and westward‐extended western North Pacific subtropical high via eastward‐propagating Kelvin waves and an intensified and eastward‐extended South Asian high by heating the troposphere. These two aspects of atmospheric circulation configuration favored the maintenance and reinforcement of the local anomalous anticyclone in the mid‐lower troposphere over South China and the South China Sea.

Limited flexural control of fold-thrust belts on the Jurassic Sichuan Basin, South China

The northern part of the Jurassic Sichuan Basin has long been thought of as a foreland basin in relation to the post-collisional compression along the northern margin of the Yangtze block. However, the exact coupling mechanism between mountain building and basin formation remains unclear. Here, we integrate stratigraphic correlation, basin subsidence analysis and flexural simulation to quantitatively assess the extent to which the fold-thrust belts have controlled basin subsidence. Flexural backstripping of the stratigraphic record, spanning from 201 to 149 Ma, along two cross sections that are perpendicular to the Micangshan fold-thrust belt and the Dabashan fold-thrust belt, respectively, reveals a limited flexural control of mountain loading on basin subsidence. Owing to the short-wavelength nature of plate flexure, the basin-adjacent thrust belts exerted dominant control on basin subsidence only along its margin, with the width of the foredeeps not exceeding ca. 120 km, failing to drive subsidence in the forebulge and backbulge regions. Flexural modeling results suggest that crustal thickening was relatively weak during the Early to Middle Jurassic. This was followed by a more rapid and intense phase of crustal growth in the Late Jurassic, possibly extending into the earliest Early Cretaceous. Compared to the Micangshan region, the Dabashan region has experienced more intense compression during the Late Jurassic. Additionally, our results reveal spatial variations in plate rigidity along the northern margin of the Yangtze block, with greater plate stiffness in the Dabashan region. The presence of residual subsidence, an anomalous long-wavelength subsidence component corrected for both basin-adjacent thrust loading and associated sediment loading, highlights the necessity for an additional driving mechanism for basin subsidence. This residual subsidence was likely dynamic subsidence induced by the flat subduction of the Paleo-Pacific plate (the Izanagi plate) beneath East Asia as the flat slab progressively migrated inland.

Impact of intra‐decadal variability of meridional heat transport on the rainfall over Southern Africa during austral summer

AbstractThe present study examines the impact of intra‐decadal variability of Meridional Heat Transport (MHT) associated with Indian Ocean Shallow Meridional Overturning Circulation (SMOC) on the rainfall variability over Southern Africa (e.g., Zambia, Malawi, Mozambique, south‐eastern Congo, Zimbabwe, southern Tanzania and Madagascar) during austral summer (December to February). Century long ocean reanalysis, atmospheric reanalysis and rainfall observations are used for the study. Spectrum analysis of MHT index and rainfall anomaly displays intra‐decadal (5–7 years) variability significant at 95% confidence level. The oceanic and atmospheric conditions during strong and weak phases of intra‐decadal MHT variability are examined. Strong (weak) MHT years displayed reduction (increase) in Sea Surface Temperature (SST) in southwest Indian Ocean, which are out‐of‐phase with net heat flux but in phase with ocean circulation. The negative (positive) SST anomaly favours positive (negative) sea level pressure anomaly, which supports the low level moisture divergence (convergence) and anticyclonic (cyclonic) circulation in the lower troposphere. This circulation leads to anomalous subsidence (convection) and below (above) normal moist static energy over the study region resulting in below (above) normal convection, that causes negative (positive) rainfall anomaly over the Southern Africa. The association between intra‐decadal variability of MHT associated with SMOC and rainfall over Southern Africa during austral summer are reported for the first time. The study may pave the way for enhanced predictability of intra‐decadal Southern African austral summer rainfall.

Triple-Dip La Niña Contributes to Pakistan Flooding and Southern China Drought in Summer 2022

Abstract In the summer of 2022, a long-lasting La Niña entered its third year. In Asia, southern China was in the grip of a historic drought while heavy rainfall ravaged Pakistan. Using a climate model forced by observed sea surface temperatures (SST) over the equatorial Pacific, we show that the back-to-back La Niña events from 2020 to 2022 are a key contributor to the global SST pattern in 2022, including the negative-phase Pacific decadal oscillation and exceptionally strong negative Indian Ocean dipole. The model reproduces the observed precipitation pattern over South and East Asia, including enhanced rainfall over Pakistan–northwest India and reduced rainfall over southern China. Additional model simulations indicate that the negative Indian Ocean dipole combined with La Niña reduces southern China rainfall by causing anomalous subsidence and anticyclonic flows. These results highlight the dominant role of long-lasting La Niña in modulating rainfall over heavily populated monsoon Asia.

Diurnal to interannual variability of low‐level cloud cover over western equatorial Africa in May–October

AbstractThis study examines the diurnal to interannual variations of the stratiform cloud cover in May–October (1971–2019) from a 3‐hourly station database and from ERA5 reanalyses over western equatorial Africa (WEA). The main diurnal variations of the local‐scale fraction and genus of stratiform clouds are synthesized into three canonical diurnal types (i.e., “clear,” “clear afternoon,” “cloudy” days). The interannual variations of frequencies of the three diurnal types during the cloudiest months (JJAS) are mostly associated with two main mechanisms: a meridional shallow overturning cell associating more “cloudy” and less “clear” and “clear afternoon” days to anomalous southerlies below 900 hPa over and around WEA, anomalous ascent around 5°–7°N, anomalous northerlies between 875 and 700 hPa, and anomalous subsidence over the equatorial Atlantic. This circulation is strongly related to interannual variations of the equatorial Atlantic upwelling (i.e., more clouds when the upwelling is strong) associated with a meridional shift of the Intertropical Convergence Zone over the Tropical Atlantic and adjacent continents. The second mechanism operates mostly in the zonal direction and involves again the coupled ocean–atmosphere system over the equatorial Atlantic, but also the remote El Niño–Southern Oscillation (ENSO). An anomalously cold equatorial Atlantic drives increased low‐level westerlies toward the Congo Basin. Warm ENSO events promote broad warm and easterly anomalies in the middle and upper troposphere, which increase the local static stability, and thus the local stratiform cloud cover over WEA. The present study suggests new mechanisms responsible for interannual variations of stratiform clouds in WEA, thus providing avenues of future research regarding the stability of the stratiform cloud deck under the ongoing differential warming of tropical ocean and land masses.

Synoptic characteristics of heatwave events in Australia during austral summer of 1950/1951–2019/2020

AbstractAustralia has experienced increasing number of heatwave events (HWEs) in recent decades. This study aims to examine key synoptic features of austral summer HWEs over Australia during 1950/1951–2019/2020. Based on ERA5‐reanalysis and rotated empirical orthogonal functions, HWEs in Australia are categorized into four types according to where they often occur together: East Australia (EA), North Australia (NA), West Australia (WA) and Southeast Australia (SEA). Our results reveal that while anomalous anticyclones at 500 hPa level are the typical synoptic circulations responsible for HWEs of each subregion, their specific mechanisms differ between subregions. The atmospheric heat budget at 850 hPa level shows that anomalous diabatic heating promotes the onset of HWEs in each subregion. While anomalous vertical advection and adiabatic heating due to the anomalous subsidence play an important role in maintaining HWEs till their demise in NA, they only help trigger HWEs in EA, SEA and WA. Our results also suggest that anomalous advection of climatological mean temperature is important to both the onset and persistence of HWEs in EA and SEA, while it only helps sustain (trigger) HWEs in NA (WA). The advection of anomalous temperature also acts to trigger HWEs in EA, SEA and WA. On the surface, upward long‐wave radiation and sensible heat flux contribute to the development of HWEs, whereas both of them are reduced in SEA due to the enhanced anomalous cyclones near the surface and the decreased land–air temperature difference. These findings help improve understanding of the synoptic characteristics and distinctive mechanisms of HWEs in different subregions of Australia.

Unravelling rift propagation from Cenozoic subsidence patterns: Examples from the Dangerous Grounds, South China Sea

AbstractHow seafloor spreading in marginal basins controls subsidence patterns in continental margins under rift propagation settings remains poorly understood. Herein, based on detailed geological interpretation and backstripping of 37 high‐resolution multichannel seismic reflection sections, we calculate the Cenozoic tectonic subsidence across the Dangerous Grounds (DG), South China Sea, in order to study interactions between seafloor spreading and the propagation of rift basins. Our results show that rapid subsidence occurred in the eastern Dangerous Grounds (EDG) from the Palaeocene to early Oligocene, then migrated westwards to the middle Dangerous Grounds (MDG) during the early Oligocene to early Miocene, and finally reached the western Dangerous Grounds (WDG) during the early to middle Miocene. We interpret this western migration of intensive subsidence as the response of the inherited pre‐Cenozoic heterogeneous lithospheric structure of South China. Thermal re‐equilibration of the lithosphere then dominated the rapid post‐spreading subsidence throughout the whole DG from the middle Miocene. An anomalous subsidence deficit occurred in the EDG during the early Oligocene to early Miocene, and in the MDG during the early to middle Miocene; this phenomenon might be related to small‐scale mantle convection associated with the propagation of seafloor spreading from east to southwest.

Increased vertical resolution in the stratosphere reveals role of gravity waves after sudden stratospheric warmings

Abstract. Sudden stratospheric warmings (SSWs) have a long-lasting effect within the stratosphere as well as impacts on the underlying troposphere. However, sub-seasonal forecasts of the winter polar stratosphere fail to use their full potential for predictability as they tend to underestimate the magnitude and persistence of these events already within the stratosphere. The origin of this underestimation is unknown. Here, we demonstrate that the associated polar stratospheric cold bias following SSW events in sub-seasonal hindcasts can be halved by increasing vertical model resolution, suggesting a potential sensitivity to gravity wave forcing. While the predictability of the planetary Rossby wave flux into the stratosphere at lead times longer than a week is limited, the existence of a critical layer for gravity waves with a low zonal phase speed caused by the disturbed polar vortex provides predictability to the upper stratosphere. Gravity wave breaking near that critical layer can, therefore, decelerate the zonal flow consistently with anomalous subsidence over the polar cap leading to warmer temperatures in the middle polar stratosphere. Since the spectrum of gravity waves involves vertical wavelengths of less than 4000 m, as estimated by wavelet analysis, a high vertical model resolution is needed to resolve the positive feedback between gravity wave forcing and the state of the polar vortex. Specifically, we find that at a spectral resolution of TCo639 (approximate horizontal grid spacing of 18 km) at least 198 levels are needed to correctly resolve the spectrum of gravity waves in the ECMWF Integrated Forecasting System. Increasing vertical resolution in operational forecasts will help to mitigate stratospheric temperature biases and improve sub-seasonal predictions of the stratospheric polar vortex.

A physical analysis of summertime North American heatwaves

This study examines the dominant heatwave variability over North America (NA), extracted from an empirical orthogonal function (EOF) analysis of summertime monthly warm extreme index anomalies over 1959–2021. The principal mode features a dipole structure with a large area of anomaly over northwestern NA and an anomaly of opposite sign over the southern U.S. The corresponding principal component is associated with a large-scale atmospheric wave train extending from the North Pacific to North America (NP-NA) and a northeastward injection of moisture from the subtropical western Pacific towards western NA, which are key factors in supporting the NA heatwave variability. The NP-NA wave train can be systematically reinforced and supported by synoptic-scale eddies, and may also be forced by an anomalous convection over the tropical-subtropical western Pacific. Surface radiation heating directly contributes to surface temperature anomalies and is dominated by anomalous downwelling shortwave and longwave radiations. In association with a positive phase of the heatwave variability, the NP-NA wave train brings an anticyclonic anomaly over northern NA, leading to anomalous descent, reduced total cloud cover and below-normal precipitation and surface relative humidity over northern NA. Over northwestern NA, the anomalous subsidence causes air to warm through compression. Reduced cloud cover results in increased downward shortwave radiation that is a key contributor to surface radiation heating. In addition, increase in vertically integrated water vapour through the moisture injection from the North Pacific collocates with tropospheric warming. The atmosphere has more water vapor holding capability and acts as a greenhouse gas to absorb longwave radiation, leading to increased downward longwave radiation that is the second major contributor to surface radiation heating. Processes with circulation and surface radiation anomalies of opposite signs will likewise lead to the negative heatwave variability.