JGR–Atmospheres

A large reservoir of saturation deficit air is known to exist over the northern Arabian Sea and the adjoining land regions during the peak of Indian summer monsoon (ISM). The strengthening of monsoon low-level jet (LLJ) in the northern parts of the Arabian Sea during the break phase of ISM helps in transporting this dry air toward northwestern India. Here, we show that, a weakening (strengthening) of the zonal flow over the northern Arabian Sea can reduce (enhance) the influx of the unsaturated air to the Northwest India and thereby enhance (reduce) precipitation there. The variability in the zonal flow over the northern Arabian Sea is a direct geostrophic response to the variability in the meridional pressure gradient over the Northwest India. The interannual variability in the mean sea level pressure over the region explains the inter-annual variability of ISM precipitation during July–August over northwestern India. The contribution of El Niño Southern Oscillation in the interannual variability of precipitation over this region is not significant.

The atmospheric river (AR) is a long, narrow, and transient corridor of strong horizontal water vapor transport. Various AR detection methods have been proposed, which have introduced significant uncertainty to the identified AR characteristics. This study has designed a data fusion algorithm to merge 12 data sets of different global and regional AR identification algorithms published by the Atmospheric River Tracking Method Intercomparison Project (ARTMIP) covering the period from 1980 to 2016. It aims to conduct frequency statistics to further research the global distribution, interannual variation, the trends in poleward shifting, and the impacted factors of AR occurrence. The quantitative results indicate an overall increasing trend in interannual variation, with a more pronounced growth trend observed in the oceanic region between 40 and 60ºS. Additionally, the study identifies a poleward shift in the peak latitude of AR occurrence frequency, with speeds of 0.589° and 0.769° per decade in the Northern and Southern Hemispheres, respectively. This shift may be associated with the tropical poleward expansion. Upon examining the relationship between AR frequency and sea surface temperature (SST) as well as zonal wind, the study finds that distinct dominant factors influence AR in different regions. AR events near the 30°N/S ocean are influenced more significantly by zonal wind than by SST. These findings shed light on the global characteristics of AR occurrences and provide insights into the factors governing their variability across different areas.

This study focuses on the interannual variability of winter mean surface air temperature (SAT) and subseasonal SAT reversal in North China, which have profound impacts on national life and economic activities. The analysis explores the temporal and spatial distribution characteristics of these variations and their relationship with the Eurasian Zonal Circulation (EZ). The findings reveal distinct interdecadal changes in North China’s winter mean SAT, along with a general subseasonal reversal of cold and warm conditions. In comparison to the North Atlantic oscillation/Arctic Oscillation, the EZ has a more significant influence on the interannual variability of winter and subseasonal reversals. Strong EZ years are associated with the weakening of the Siberian High and the Ural Blocking High, causing the East Asian Trough to move eastward, which leads to the retention of cold air from the north and increased SAT in North China. Conversely, weak EZ years exhibit opposite circulation patterns. The winter mean EZ is influenced by preceding autumn Arctic sea ice concentrations (SIC). Reduced autumn SIC results in decreased heat flux and reduced eddy energy, leading to substantial attenuation of the winter storm track across Scandinavia and Western Asia. This attenuation, mediated through the interaction between eddies and mean flow, triggers a slowdown of Eurasian zonal circulation and intensification of the Siberian High. These conditions favor a positive phase of the winter Scandinavia pattern, facilitating the transport of cold Arctic air into North China. Likewise, subseasonal reversals in SIC influence corresponding alterations in the EZ.

The reanalysis data suggest that recent surface warming over Antarctica start in 2016. In this study, using reanalysis data and numerical simulations, I attempt to determine the important mechanisms accounting for seasonal surface warming in Antarctica. The results suggested that seasonal surface warming in Antarctica is mainly determined by the surface energy budget over the Antarctic via horizontal heat advection. The surface energy budget anomaly over the Antarctic, which is mainly determined by anomalous solar radiation absorption, anomalous ocean heat content, and anomalous meridional atmospheric heat transport (AHT), is triggered by Antarctic sea-ice loss and thus determines the observational seasonality of recent warming in Antarctica via surface horizontal heat advection. In austral summer (December–January–February), additional solar radiation absorption induced by sea-ice loss and additional AHT from lower latitudes increase the energy budget over the Antarctic. Surface warming, more longwave radiation, and additional energy stored in the upper (deeper) ocean for short (long) time periods explain the additional energy sinks. During austral autumn-winter (March–August), additional seasonal heat storage (SHS; mainly stored in the upper ocean) is released to the atmosphere and warms the surface. Although the AHT anomaly contributes similarly to the solar radiation absorption/SHS anomaly during April–August, the poleward AHT largely decreased in June due to the weaker eddy activity induced by strong warming at Southern Hemisphere midlatitudes, which counteracts the additional SHS release and cools the Antarctic(a).

Anvil cirrus generated by deep convection covers large fractions of the tropics and has important impacts on the Earth's radiation budget and climate. In situ measurements made with high-altitude aircraft indicate a rapid transition in ice crystal size distributions and habits as anvil cirrus ages. We use numerical simulations to investigate the impact of high-frequency gravity waves on the evolution of anvil cirrus microphysical properties. The impacts of both monochromatic gravity waves and ubiquitous stochastic mesoscale temperature fluctuations are simulated. In both cases, the interplay between wave-driven temperature fluctuations, deposition growth/sublimation, and sedimentation causes accelerated removal of both small ice crystals (diameters less than about 10 μm) and large crystals (diameters larger than ≈30 μm). These changes are consistent with the observed evolution of anvil cirrus microphysical properties. The Kelvin effect (higher saturation vapor pressure over curved surfaces) is a critical factor in the anvil evolution, driving mass transfer from small to large ice crystals. The wave-driven decrease in ice concentration is much faster for typical anvil cirrus detrained at ≃11.5–12.5 km than for less frequent anvils at 15.5–17.5 km because of the strong temperature dependence of deposition growth and sublimation rates. The simulations also show that waves, along with the Kelvin effect, drive growth of mid-sized (5–20 μm) ice crystals, which is consistent with the observed transition to bullet rosette habits in aging anvil cirrus. We conclude that high-frequency gravity waves, which are generally not resolved in large-scale models, likely have important impacts on anvil cirrus microphysical properties and lifetimes.

To reconstruct the history of organic carbon (OC) aerosol over south-eastern Europe, dissolved organic carbon (DOC) and its 14C signature (DO14C) were investigated along an ice core drilled at the Mount Elbrus (ELB) in Caucasus. In summer, compared to pre-1945 levels, the DOC concentrations increased by 45% after 1960, the mean DO14C depletion in recent ELB ice relative to atmospheric 14CO2 of 32% being attributed to fossil-fuel sources. DO14C content of ice deposited during the bomb-peak era (1955–1980) closely followed atmospheric 14CO2 changes caused by atmospheric nuclear tests, suggesting the living biosphere as the main biogenic source of DOC in summer in this region. ELB data contrast with those previously obtained in summer Alpine (western Europe) ice in which a post-1950 doubling of DOC was observed and attributed to enhanced emissions of organic compounds from vegetation in France. This regional difference is discussed with respect to changes of biogenic organic compound emissions in response to past change of use-land and global warming. ELB data document, for the first time, changes of DOC and DO14C in winter mountain ice showing an increase by 44% of DOC levels associated with a 14C signature being 47% lower than that of atmospheric 14CO2 in ELB ice deposited after 1960. The 14C winter ELB ice record followed atmospheric 14CO2 changes with a delay of ∼3 years, suggesting that remaining emissions from the living biosphere, together with a small contribution from wood burning, are the main biogenic sources of DOC in winter in this region.

Most ecosystems have resistance to soil moisture (SM) deficit, which is termed drought resistance. Drought resistance can be invalid and global terrestrial carbon uptake losses can be aggravated when SM deficit exceeds a critical threshold. However, soil moisture thresholds (SMTs) that detrimentally impact global terrestrial carbon uptake are still unclear. We performed numerical simulations using the Community Earth System Model, and estimated the SMTs by the back propagation neural network method for the years 2004–2014. The SMTs represent the inflection point for vegetation changes from high to low drought resistance phase, and terrestrial carbon uptake losses from low to high rate. Soil moisture-limited ecoregions have higher SMTs than energy-limited ecoregions, indicating the increased vulnerability and sensitivity of SM-limited ecoregions to SM deficit and more easily aggravated terrestrial carbon uptake losses during drought. SMTs varied in different vegetation types and broadleaf deciduous trees displayed the highest SMTs and C3 arctic grasses have lowest thresholds. Humid and high vegetation coverage rate regions have lower thresholds. The SMTs increase with the increase of clay content and the decrease of sand content. In addition, land-atmosphere feedback caused by SM deficit has a large impact on terrestrial carbon uptake and may be one of the main reasons for the aggravation of vegetation carbon uptake losses. Our results provide a unique perspective for investigating the impact of drought on vegetation.

During 26–27 June 2022, mainly influenced by three mesoscale vortices, central-eastern China (particularly for Henan and Shandong) experiences the first widespread torrential rainfall event of the 2022 flood season (maximum 24-hr accumulated precipitation is ∼380.9 mm), resulting in severe social impacts. The three mesoscale vortices form and sustain under favorable background conditions, mainly including a strong upper-level divergence, an intense middle-level warm advection, and a powerful lower-level convergence associated with a low-level jet. Among the vortices, the vortex which forms over Shandong, lasts for ∼9 hr, and makes a much larger contribution than the other vortices to the accumulated precipitation, is defined as the primary vortex. More than a half of the hourly precipitation peaks in this event appear in the life span of the primary vortex, which is closely related to the variations of the vortex in its cyclonic-vorticity and vertical extent. Backward trajectory analysis indicates that air particles originating from the lower troposphere southwest of the primary vortex contribute the most to its formation (∼82.7%). These air particles mainly experience a notable increase in their cyclonic-vorticity due to convergence-related vertical stretching, which directly renders the formation of primary vortex. During the whole life span of the primary vortex, convergence-related vertical stretching is the most favorable factor for its development/sustainment, and the convection-related vertical transport of cyclonic vorticity ranks second; whereas, the horizontal transport is the most detrimental factor. Moisture budget shows that Southeast China is the most important moisture source for this event (accounting for ∼48.9%).

The North Pacific monsoon trough (NPMT) is an imperative large-scale circulation pattern influencing tropical cyclone (TC) activity in the western North Pacific (WNP), thus its future change has great implications for the WNP TC activity. Here future change in the NPMT and its uncertainty are examined by using 35 climate models from Phase six of Coupled Model Intercomparison Project (CMIP6). Due to the El Niño-like sea surface temperature (SST) pattern, the multi-model ensemble (MME) mean projects an eastward extension of the NPMT in the latter half of the 21st century. However, considerable inter-model uncertainty exists in the projection, which is associated with the diversity in the zonal SST gradient across the tropical Pacific. This diversity in the projected SST is largely rooted in the simulated precipitation biases in the tropical Pacific in the historical experiments of CMIP6 models, which can modulate future Pacific SST distribution through the shortwave-SST feedback. This linkage between biases in historical climate and future climate allows us to constrain the projection by using observational data sets. Emergent constraint using observational precipitation data set reduces the projection uncertainty by 48% and projects an eastward extension of the north Pacific NPMT by 6.2°. This eastward extension of the NPMT indicates an eastward migration of TC genesis location, and elongated TC track and thus strengthened TC intensity, suggesting an increased TC-related disaster for residents near the WNP.

Surprising significant underperformance of the polynomial complementary relationship (PCR) of evaporation (Szilagyi et al., 2017, https://doi.org/10.1002/2016jd025611) by Wang et al. (2023, https://doi.org/10.1029/2023jd038683) is caused by the (a) station-by-station application of a grid-based estimation procedure of the Priestley-Taylor parameter (α) value, and (b) choice of the wind function. Application of the Rome wind function in the Penman equation together with either a well-chosen single (constant) α or α as a function of the wet-environment air temperature, should result in much improved evaporation estimates by the PCR in line with previous studies.

This study explores seasonal forecast skill and signal-to-noise (SN) ratio for stratospheric polar vortex (SPV) variations during Northern winter using hindcast (HC) data of six systems initialized around early November in the Copernicus Climate Change Service database. Results show high skill for December-January-February (DJF) mean El Niño/Southern Oscillation and Quasi-Biennial Oscillation (QBO) variations, although it is suggested that some systems underestimate the QBO amplitude. The ENSO and QBO variations in the HC data are also suggested to be underdispersive having too high SN ratio due to too strong signal and/or too weak noise (spread). The skill for DJF mean SPV variations is limited, with four systems having marginal skill near the 95% level, whereas the SN ratio is suggested to be realistic. It is also shown that no system has skill (evaluated using the Brier Score) at the 95% level in forecasting if each DJF winter experiences one or more major sudden stratospheric warmings, when a bias of the polar vortex strength in each system is simply considered. A comparison of the ensemble members for each system, when divided into two groups by the strength of the QBO-SPV teleconnection, suggests an increase in the skill for SPV variations for the group of stronger teleconnection. This confirms the idea that the QBO-SPV teleconnection contributes to the skill for SPV variations, and also suggests that the skill for SPV variations can be improved as the HC QBO and QBO-SPV teleconnection are better simulated.

A time series analysis of Deep Convective Clouds (DCC) in Atmospheric InfraRed Sounder data between 2002 and 2023 shows that the mean latitude of the Inter Tropical Convergence Zone (ITCZ) has shifted northward for land at a rate of about 30 km/decade. The shift for ocean is north as well at about half of the rate for land. Comparisons of our results with the existing literature are complicated by the use of different methodologies and data sources: We used the daily observations of DCCs, which by their association with extreme rain rates are indicators of locally extreme vertical velocity on 15 km horizontal scales, while most of the literature used model or reanalysis gridded data with typically 100 km bins. The southern boundary of the ITCZ shifts north faster than the northern boundary, as qualitatively stated in the literature. The resulting contraction of the ITCZ width with global warming was qualitatively predicted based on theoretical consideration. Our observation of the rate of decrease in the width for ocean is statistically consistent with the contraction deduced from European Centre for Medium Range Forecast Reanalysis and Modern-Era Retrospective analysis for Research and Applications data. The observed northward shift of the ITCZ was not predicted and is not seen in climate models in the 21st century.

The expression “lightning never strikes twice” is questioned in this paper because, among the randomness of lightning impacts, some spots are hit even more than twice year after year. This article introduces the recurrent lightning spots (RLS) concept, which are locations periodically impacted by cloud-to-ground lightning every consecutive year over a certain period. RLS are investigated in two regimes, with markedly different lightning climatology but similar orography, for 10 consecutive years: Catalonia (North East of Spain, Europe) and Barrancabermeja (North Central Colombia, South America). Results revealed 148 and 916 RLS in Catalonia and Barrancabermeja, respectively. RLS in both regions are typically found to be related to tall structures, mountain peaks, and steep terrain. The method allowed us to identify those tall towers and orographic relief frequently affected by lightning that are not detected with the mere computation of the ground flash density. In the case of Catalonia, some RLS are found offshore. Besides the scientific interest in understanding lightning, the new concept of RLS provides additional and valuable information applicable to lightning protection engineering.

Flash 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.

The optical properties of secondary brown carbon (BrC) aerosols are poorly understood, hampering quantitative assessments of their impact. We propose a new method for estimating secondary source of BrC using excitation-emission matrix (EEM) fluorescence spectroscopy, combined with parallel factor analysis (PARAFAC) and partial least squares regression (PLSR). Experiments were conducted on a collection of PM2.5 samples from urban areas in five Chinese cities during winter and summer. The humic-like component with long-emission wavelengths (L-HULIS) was identified as a secondary source tracer of BrC. This was confirmed by correlating PARAFAC components with secondary organic aerosol tracers and molecular oxidation indices obtained from Fourier transform ion cyclotron resonance mass spectrometry analysis. Using L-HULIS as a secondary tracer of BrC, it was determined that the contribution of secondary sources to water-soluble BrC (WS-BrC) in source emission samples is significantly smaller than in PM2.5 from five Chinese cities, supporting our method. In the five cities, secondary source derived via L-HULIS contributes a dominant potion (80% ± 3.5%) of WS-BrC at 365 nm during the summer, which is approximately twice as high as during the winter (45% ± 4.9%). Radiocarbon isotope (14C) analysis provides additional constraints to the sources of L-HULIS-derived secondary WS-BrC in urban PM2.5, suggesting that aged biomass burning is the dominant contributor to secondary WS-BrC in winter, and biogenic emission is dominant during summer. This study is the first report on identification of secondary sources of BrC using the fluorescence technique. It demonstrates the potential of this method in characterizing non-fossil source secondary BrC in the atmosphere.

Modeling tritium content in water presents a meaningful way to evaluate the representation of the water cycle in climate models as it traces fluxes within and between the reservoirs involved in the water cycle (stratosphere, troposphere, and ocean). In this study, we present the implementation of natural tritium in water in the atmospheric general circulation model (AGCM) MIROC5-iso and its simulation for the period 1979–2018. Owing to recently published tritium production calculations, we were able to investigate, for the first time, the influence of natural tritium production related to the 11-yr solar cycle on tritium in precipitation. MIROC5-iso correctly simulates continental, latitudinal, and altitude effects on tritium in precipitation. The seasonal tritium content peaks, linked to stratosphere-troposphere exchanges, are accurately simulated in terms of timing, even though MIROC5-iso underestimates the amplitude of the changes. Decadal tritium concentration variations in precipitation owing to the 11-yr solar cycle are well simulated in MIROC5-iso, in agreement with the observations at Vostok in Antarctica for example, Finally, our simulations revealed that the internal climate variability plays an important role in tritium in polar precipitation. Owing to its influence on the south polar vortex, the Southern Annular Mode enhances the effect of the production component on tritium in East Antarctic precipitation. In Greenland, we found an east-west contrast in the detection of the 11-yr solar cycle in tritium in precipitation owing to the influence of the North Atlantic Oscillation on humidity conditions.

Using 2-year platform observations, this study investigates seasonal characteristics of sea land breeze (SLB) and how it influences air-sea turbulent heat fluxes (THFs) in the coastal areas of East China Sea (ECS) in different seasons. Unlike other SLB studies, this study uses hourly observation on a sea platform to explore SLB's effect on both air-sea latent heat and sensible heat transferring. The results show that sea wind (SW) does not have an obvious seasonal variation pattern while land wind (LW) is stronger in autumn and winter. The SLB day number shows a clear seasonal variation pattern, which accounts for 38.04% and 18.23% of summertime and wintertime days, reaching its peak and bottom respectively. The latent heat flux (LHF) and sensible heat flux (SHF) are high in autumn and winter while low in summer. The SLB-contributed LHF and SHF reach peaks in autumn and winter, which are 61.07 and 7.39 W/m2 respectively. The contribution importance of SLB on air-sea sensible/latent heat transferring is highest in summer while lowest in winter. On SLB days, the SHF decreases significantly by at least about 50% while LHF decreases moderately in all seasons, among which spring witnesses an inversion of sensible heat transferring direction. The warming effect of SLB is mainly responsible for the slump of SHF on SLB days. Multiple factors including relative humidity (RH), background wind field and in situ radiation cause the LHF decrease together, whose changing range varies with season.

Large uncertainty exists in the sign of long-term changes in regional scale mean precipitation across the current generation of global climate models. To explore the physical drivers of this uncertainty for New Zealand, here we adopt a storyline approach applying cluster analysis to spatial patterns of future projected seasonal mean precipitation change across CMIP6 models (n = 43). For the winter precipitation change signal, the models split roughly into two main groups: both groups have a very robust wet signal across the west coast of the South Island but differ notably in terms of the sign of precipitation change across the north of the North Island. These far north winter precipitation differences appear related to how far the Hadley cell edge and regional eddy-driven jet shift across the models relative to their historical positions. In contrast, for summer, most models have a markedly weaker and spatially non-uniform response, where internal variability often plays a large role. However, a small group of models predict a robust wet signal across most of the country in summer. This “wet model” group is characterized by a regional La Niña-like increase in high pressure shifted further to the south-east of New Zealand, associated with more frequent north-easterly flow over the country and accompanied by significant warming of local sea surface temperatures. This regional circulation response appears related to changes in stationary Rossby wave paths as opposed to changes in La Niña occurrence frequency itself.

Accurate forecasting of atmospheric rivers (ARs) holds significance in preventing losses from extreme precipitation. However, traditional numerical weather prediction (NWP) models are computationally expensive and can be limited in accuracy due to inaccurate physical parameter settings. To overcome these limitations, we propose a deep learning (DL) model, called GAN-UNet, to forecast the AR occurrence, position, and intensity in East Asia. GAN-UNet can capture the complex nonlinear relationship between the inputs at the past moment, including the vertically integrated water vapor transport (IVT), zonal wind at 850 hPa (U850), and meridional wind at 850 hPa (V850), and the forecast output (IVT, U850, or V850), whose results are comparable to NWP models. In addition, the average model (AM) by integrating the results generated by GAN-UNet and European Centre for Medium-Range Weather Forecasts (ECMWF) outperforms all the NWP models selected in this study, demonstrating its potential to improve the performance of NWP through the DL method. Specifically, the 5-day average F1 scores of the AM are 0.777 and 0.845, whose values are significantly better than those obtained by ECMWF (0.712 and 0.794) in the two key regions of East Asia; The AM 5-day average intersection over unions are 0.706 and 0.688 while the values of ECMWF are 0.675 and 0.64; in terms of intensity forecast, GAN-UNet and AM exhibited lower differences in most of the intensity bins, except for the final bin with IVT more than 825 kg m−1 s−1. With this thorough analysis, GAN-UNet is shown as an effective model to forecast ARs.

The discovery of smoke-induced dynamical anomalies in the stratosphere associated with the 2019/2020 Australian New Year pyrocumulonimbus (pyroCb) super outbreak initiated a new field of study involving aerosol/weather anomalies. This paper documents the dynamical anomalies associated with the February 2009 Australian Black Saturday pyroCb outbreak. Positive potential vorticity anomalies (indicating anticyclonic rotation) with horizontal extent ∼1000 km and vertical thickness ∼2 km are associated with the plume, which we classify as a Smoke With Induced Rotation and Lofting (SWIRL). The SWIRL initially formed east of Australia, but then moved westward, crossing over Australia, and continuing to Africa. The SWIRL lasted for nearly three weeks (13 February–4 March), traveling ∼27,000 km and rising from potential temperatures of ∼410–500 K (altitudes ∼18–21 km). The altitude of the SWIRL is corroborated with coincident satellite-based profiles of H2O, CO, HCN, O3, and aerosol extinction. A vertical temperature dipole (±3 K) accompanied the PV anomaly, as verified with coincident Global Navigation Satellite System radio occultation temperatures. The SWIRL dissipated as it passed over Africa. Operational ECMWF forecasts with early initialization (13 February) and late initialization (21 February) are examined. In the early case, the forecasted PV anomaly disappeared within 4 days, as expected due to lack of smoke heating in the forecast model. In the late case, while the forecasted PV anomaly was weaker than in the reanalyzes, a remnant anomaly remained out to 10 days.