Journal of Geophysical Research: Atmospheres最新文献

The mechanism driving the lower thermospheric meridional circulation is analyzed using the high-resolution Whole Atmosphere Community Climate Model with thermosphere/ionosphere extension (WACCM-X). In the winter lower thermosphere, eastward forcing is dominant around z=120 km, consistent with equatorward circulation. Following previous studies, the vertical flux of the zonal momentum is examined for small-scale waves with zonal scales smaller than 2,000 km through the depth of the middle atmosphere. Wave decomposition analysis reveals that eastward waves with a wide range of phase speeds are generated around the polar vortex height. This result differs from the conventional understanding in gravity wave parameterizations, which states that eastward waves slower than the strong mesospheric wind cannot propagate to higher altitudes. Quasi-stationary and westward waves are also active in the mesosphere, some of which originate in the troposphere. However, these waves more often dissipate in the upper mesosphere at critical levels and easterly shear, which results in the dominance of eastward waves in the lower thermosphere. The horizontal and temporal variations of gravity wave activity in the lower thermosphere are associated with the structure of the mesospheric jet. These results suggest a possible method for diagnosing wave generation that could improve gravity wave parameterizations. In the summer hemisphere, westward forcing is expressed by the semidiurnal tide, as well as by filtered gravity waves. Thus, the driving mechanisms of the lower thermospheric circulation are the contribution of gravity waves, mainly generated in the mesosphere and selectively filtered in the upper mesosphere, and the semidiurnal tide.

In this study, we report the response of migrating diurnal tides (DW1) in the mesosphere and lower thermosphere (MLT) region to the Madden‒Julian oscillation (MJO). The DW1 amplitudes are decomposed from the neutral horizontal wind observed by four meteor radars in the equatorial region. The DW1 and zonal wind in the equatorial MLT region show consistent intraseasonal variations, implying that the upward propagating DW1 can affect the mesospheric zonal wind. By jointly analyzing the real-time multivariate MJO (RMM) indices and mesospheric DW1, we found that DW1 responds strongly to the MJO during both boreal winter (difference relative to seasonal means: ∼20%–25%) and summer (∼25%). The MJO convection affects the mesospheric DW1 by modulating the solar radiative absorption by water vapor and latent heat release in the troposphere. The seasonal difference in DW1–MJO response can be attributed to a slight weakening of the DW1 tidal heating sources during MJO phases 1–4 and a significant enhancement during phases 6–7 during summer, driven by the moisture transport from the Indian Ocean and Pacific to the East Asian continent. This finding provides an opportunity to the understanding of the coupling between the troposphere and the mesosphere through migrating tides.

Synoptic weather typing is a common technique for categorizing regional circulation patterns over a region. Defined and commonly used sets of synoptic weather types (SWTs) have the potential to simplify the analysis and interpretation of the regional circulation on the weather time and spatial scales and create a common language between the often segregated weather and climate research communities. However, circulation classifications are potentially complicated over Australia owing to its position in both the tropics and extratropics. Here we present a set of 30 SWTs over Australia by k means clustering the 850 hPa wind field in the ERA5 data set. We show that the Australian SWTs represent recognizable weather patterns over the continent throughout the year. Importantly, the SWTs capture features in the extratropics together with characteristics associated with the tropical monsoon. An initial analysis of the relationship of the SWTs with the modes of climate variability, high impact weather and renewable energy related variables showing the utility of the SWTs in a wide range of applications across disciplines within earth system sciences and beyond.

The Tropospheric Emissions: Monitoring of Pollution (TEMPO) instrument enables an unprecedented assessment of diurnal and community-scale variations in tropospheric nitrogen dioxide (NO₂) across North America. This study presents the first exploratory analysis of NO₂ patterns in eastern Canada, including Ontario, Quebec, and the Atlantic provinces, using TEMPO observations. We analyzed TEMPO data (V03) gridded at 0.02° × 0.02° from September 2023 to August 2024 and compared it with the Tropospheric Monitoring Instrument (TROPOMI) and surface-level measurements from Canadian national regulatory monitors. With the hourly resolution of TEMPO, we observed diurnal trends and hotspots that were not recognized by once-per-day TROPOMI measurements and pinpointed undermonitored areas. NO₂ in eastern Canada's eight major metropolitan areas, ports, and industrial cities similarly peaked in early morning and declined in later hours. Still, TEMPO detected variations in their hours of peaks and spikes, seasonal, and weekday-weekend distributions. In Atlantic Canada, correlations between TEMPO and TROPOMI column densities, as well as column-surface alignments, were lower (Spearman's ρ = 0.41–0.53) compared to the Quebec City-Windsor Corridor (Spearman's ρ = 0.81–0.90), primarily due to a wider dynamic range of pollution in the latter region. The two regions' TEMPO-TROPOMI mean absolute differences were 19.3% and 17.1%, respectively. Temporal variations (e.g., a later weekday morning peak in Ontario cities) and TEMPO's identification of additional undermonitored hotspots provide insights into air quality control planning. Our findings motivate future research using multiyear TEMPO data to investigate atmospheric NO₂ sources, transport, exposure, and associated population health impacts in Canada.

Entrainment-mixing processes between clouds and their environment significantly impact the microphysical, thermodynamical, and dynamical properties of clouds. However, quantitative observational analysis of different entrainment-mixing mechanisms between cumulus cores and edges remains lacking. This study provides the quantitative analysis of such mechanisms in cumulus cores versus edges using aircraft observations from the Rain in Cumulus Over the Ocean project. Results show that cores have larger liquid water content, larger cloud droplets, and smaller entrained droplet-free air parcels, consequently yielding a larger homogeneous mixing degree (ψ) and a smaller Damköhler number ($��D_a�$, the ratio of mixing time scale to droplet evaporation time scale). ψ is 0.13 higher and $��D_a�$ is 3.3 lower in the cores compared to the edges. Furthermore, core-edge differences in ψ are analyzed under varying environmental and cloud conditions. Smaller cloud width, weaker in-cloud vertical velocity, lower environmental relative humidity, and lower aerosol concentration all reduce ψ and enhance the core-edge ψ contrast. The results offer insights into the theoretical understanding and parameterizations of entrainment-mixing mechanisms in the cores and edges of cumulus clouds.

Tropical precipitation biases have persisted since the very first generations of climate models. These biases are highly sensitive to the region and/or season of interest, with commonalities in the east Pacific and Atlantic Ocean basins, possibly due to their similar observed climatological northern hemisphere intertropical convergence zone (ITCZ). However, the colloquial name “double ITCZ bias” comes from a time and/or zonal mean, and may be missing important information about errors at smaller time and space scales. In this study, we explore daily characteristics of the ITCZ in observations, reanalyses, and 25 Coupled Model Intercomparison Project 6 (CMIP6) models over the east Pacific Ocean. We devise and apply an algorithm that determines a region's dominant daily ITCZ configuration, its “ITCZ state,” based on the daily mean precipitation field. The five ITCZ states include: northern hemisphere (nITCZ), southern hemisphere (sITCZ), double (dITCZ), equatorial (eITCZ), and absent (aITCZ). We find that nearly all CMIP6 models gravely underestimate nITCZs and overestimate sITCZs during January through May, in contrast with what “double ITCZ bias” suggests. Surprisingly, all reanalyses also underestimate nITCZs and overestimate eITCZs. Errors in ITCZ state interannual variability are consistent with mean errors in reanalyses, while sITCZ interannual variability is far too low relative to the mean in most CMIP6 models. Lastly, all reanalyses and CMIP6 models overestimate precipitation rates in the southern hemisphere ITCZ band for dITCZs and sITCZs, suggestive of errors with atmospheric origins.

Compound climate extremes, where temperature and precipitation extremes occur together are increasing globally and pose growing risks to human and natural systems. India is already experiencing more frequent extreme weather events, underscoring the urgency of assessing how such events may evolve with continued warming. This study uses high-resolution, statistically downscaled, and bias-corrected CMIP6 climate projections from 30 different models to assess the future compound extremes such as Cold-Dry (CD), Cold-Wet (CW), Warm-Dry (WD), and Warm-Wet (WW) under 1.5°C and 2.0°C global warming levels for two future climate scenarios (SSP2-4.5 and SSP5-8.5) during the pre-monsoon season (March-June). The projections indicate that cold-related extremes (CD, CW) will become rare while warm-related extremes (WD, WW) will increase significantly across India, with the largest changes emerging by the mid-21st century across the West Central India (WCI), North East India (NEI), and Central North East India (CNEI). The occurrence of WD events has approximately doubled while WW events have increased nearly threefold across India during this season. Temperature increases, reinforced by land-atmosphere coupling and thermodynamic constraints such as higher vapor pressure deficit and greater moisture holding capacity, intensify wet and dry extremes, while circulation-driven precipitation changes play a lesser role. Population exposure to WD and WW events is projected to rise sharply, particularly in densely populated and climate-sensitive regions. These findings highlight the need for targeted, region-specific adaptation planning and risk-informed decision-making to manage the escalating risks from compound extremes in a warming climate.

During the satellite era, Greenland's surface air temperature has experienced a rapid warming trend, potentially lengthening the melting season and enhancing surface melting, contributing to ice mass loss. While previous studies have primarily attributed this warming to greenhouse gas forcing, regional feedbacks, and remote forcing from the tropical Pacific, the role of the Atlantic Ocean remains less explored. Using both observations and numerical model simulations, this study identifies an atmospheric teleconnection pattern triggered by tropical North Atlantic (TNA) sea surface temperature (SST) variability, which significantly contributes to surface warming over Greenland. This teleconnection exhibits strong seasonality, with a more pronounced connection during boreal winter and spring. Anomalous TNA warming initiates a poleward-propagating Rossby wave train, inducing regional circulation adjustments that intensify thermal and moisture advection to Greenland. Enhanced downward long-wave radiation, associated with these moisture changes, further amplifies the warming trend. These findings provide valuable insights into the warming of Greenland and, by extension, its potential implications for global sea level rise. Moreover, the identified teleconnection may improve the predictability and future projections of Greenland's climate.

Changes in mesoscale convective system (MCS) spatial structure under climate warming are crucial, as steeper gradients combined with stronger storms significantly increase the risk of local record-breaking rainfall events. However, mechanisms driving the changes in spatial structure remain unexplored. Here we examine changes in the spatial structure of observed summer MCSs over East Asia from 2000 to 2021, and find that the spatial structure of MCSs defined by hourly precipitation fields has become sharper, dominated by enhancement in heavy precipitation grid cells. The MCS-maximum precipitation intensity increases at a rate of 16.7% decade⁻¹, contributed roughly equally by both the overall increase in MCS precipitation rates and the rising spatial sharpness. The observed sharper spatial structure arises from an enhanced moisture–convection feedback. Climate change during 2000–2021 intensifies synoptic moist static energy (MSE) forcing which stimulates enhanced shallow convection about 9 hr before MCS initiation. This enhanced shallow convection redistributes water vapor and ascents into heavy precipitation grid cells, thus sharpening MCS. Following MCS formation, intensified deep convection reinforces the mechanism, leading to a more pronounced sharpening of the MCS structure. The “stronger MSE and sharper MCS” mechanism identified here is tied to enhanced moisture–convection feedback, in which the increased convective latent heating peaks around 600 hPa before and during MCS life, and enhanced stratiform latent heating peaks around 400 hPa primarily during the development and maturation phases of MCS.

Tropospheric iodine plays a key role in ozone depletion and new particle formation. Understanding how iodine responds to climatic shifts is crucial for predicting future atmospheric composition. Here, we present high-resolution iodine fluxes in the EGRIP (East GReenland Ice Core Project) ice core in central Greenland spanning the last 15.7 kyr. By integrating these data with complementary proxies from other ice and marine sediment cores, we explore both the temporal and spatial patterns of iodine deposition across the Greenland ice sheet. We find lower iodine fluxes at EGRIP compared to RECAP and NEEM ice cores over the last 15.7 kyr, explained by the lower efficiency of iodine transport to the interior of the ice sheet. Our results show that during the cold periods of the Last Glacial Termination (15.7–11.7 kyr before present), iodine fluxes at EGRIP were higher than during the Holocene, likely due to enhanced long-range transport of iodine adsorbed onto dust and sea-salt aerosols under stronger glacial wind regimes. In contrast, during the Holocene, the sources and transport pathways of iodine shifted to marine sources. We infer that iodine was increasingly emitted from the expanded seasonal sea ice area and biologically active subpolar North Atlantic waters, though overall fluxes at EGRIP were lower than during the glacial period, reflecting the reduction in long-range transport. After rising until 5 kyr before present, iodine fluxes declined throughout the Neoglacial, in concomitance with regional cooling, increased sea ice extent, and reduced ocean productivity.