Atmospheric and Oceanic Sciences

Abstract The Tomorrow.io microwave sounder (TMS) builds on a heritage of passive microwave sounders (PMWSs) that have well‐established value in numerical weather prediction (NWP). The TMS constellation of CubeSats uses a combination of sun‐synchronous and inclined orbits to fill gaps in current public PMWS data. This work establishes a data assimilation methodology for the TMS and demonstrates its real global weather forecast impacts with observing system experiments. Our assimilation methodology for the TMS encompasses all‐sky error modeling and quality control, and novel variational bias correction predictors. The all‐sky error model is based on techniques in earlier literature, and a new cloud impact filter enables more effective assimilation of surface‐sensitive TMS channels. TMSs in inclined orbits are exposed to a transient thermal environment that causes periodic biases in the current level 1 calibrated brightness temperatures. In response, we develop variational bias correction predictors related to the unique night–day cycle of each satellite. The new predictors partially mitigate the transient instrument biases and improve verification scores for upper‐level temperature forecasts. However, this is a partial solution, and remains an active area of investigation. In our research‐quality NWP setting, assimilating two TMS instruments in January 2025 achieves 6‐hr forecast accuracy improvements similar to two advanced technology microwave sounder instruments for water vapor, and 50% of two advanced technology microwave sounders for tropospheric temperature. Statistically significant forecast accuracy improvements from the TMS persist for water vapor up to 3 days, and for temperature and winds up to 2 days. The TMS provides complementary information content to public PMWSs. Additional efforts at operational forecasting centers are ongoing to conduct more complete investigations.

Abstract This study examines the covariance between the intertropical discontinuity (ITD) and African easterly jet (AEJ) over the West African Sahel in wet and dry year composites, using a decade of reanalysis data. ITD and AEJ positions are strongly correlated, with a more pronounced linear relationship across wet years due to the sensitivity to an intensified monsoonal flow. Whereas the AEJ's diurnal cycle shows little meridional displacement, its intensity and the ITD position show clear diurnal cycles. Considering land‐surface controls, we find surface heat flux anomalies around to modify low‐level temperature and sensible heat flux gradients, affecting the thermal wind and shifting the AEJ core to south of gradient maxima. More northward gradients, and hence a more northerly AEJ, occur in wet years, with a smaller shift in the ITD. Consequently, the ITD–AEJ distance narrows in wet years, driven by enhanced convective activity. Correspondingly, we find that peak frequencies of colder, more intense convective systems shift further north in wet years, whereas the southern location of warmer systems remains similar between composites. This is due to deep moist conditions prevailing south of the AEJ for both composites, favouring warm mesoscale convective system formation in a weakly sheared environment. In contrast, convective available potential energy and shear maxima shift north and align with the AEJ in wet years, displacing the environment for colder storm development. The ITD shows less sensitivity to anomalies in surface processes but may influence isolated storm events close to its positioning. These findings improve understanding of ITD–AEJ interactions and their sensitivity to soil moisture conditions, highlighting the need for high‐resolution modelling to capture local feedback mechanisms.

Abstract The water‐vapour‐sensitive channels of infrared imager instruments on board geostationary satellite instruments, such as the spinning enhanced visible and infrared imager (SEVIRI), deliver valuable information on humidity and clouds in the upper and mid‐level troposphere. In past decades it was common practice to assimilate these channels in clear‐sky only, due to a number of challenges of the all‐sky assimilation, such as the forward modelling (i.e., the computation of the model‐equivalent brightness temperatures) of cloudy scenes. In recent years, however, more efforts have been taken by research institutions and weather services to overcome these challenges. In this article we describe which steps were necessary to assimilate the two water‐vapour‐sensitive channels of Meteosat‐SEVIRI at 6.2 and 7.3 m in our convective‐scale regional forecasting system ICON‐D2 at the German Weather Service within a local ensemble transform Kalman filter framework in all‐sky conditions with positive forecast impact. After finding an optimal set‐up how to use these channels in combination with the already operationally assimilated visible channel at 0.6 m including vertical height assignment and localization, observation error modelling and data reduction, a clear positive impact is achieved. Upper air moisture, the cloud cover of high and mid‐level clouds, and thereby also direct and diffuse radiation at the surface and the 2‐m temperatures and humidity verified against SYNOP stations is improved in forecasts for more than 24 hr.

Abstract The Arctic has experienced rapid sea ice loss and a substantial surface albedo decline, altering its radiation budget. CMIP6 (Coupled Model Intercomparison Project Phase 6) models capture these trends but show considerable inter‐model spread in the magnitude, distribution, and seasonality of Arctic surface albedo. Over land, spread in AMIP (Atmospheric Model Intercomparison Project) and CMIP6 is associated with snow cover variations, while over the ocean, where sea ice dominates, the sources are less clear. We compare CMIP6 simulations with observations from the Clouds and the Earth's Radiant Energy System (CERES) and develop a decomposition method to quantify contributions from sea ice albedo, concentration, and extent. Over the Arctic Ocean, all three factors contribute to inter‐model spread in CMIP6. In AMIP, despite prescribed sea ice concentrations, we were surprised to find an inter‐model spread in Arctic Ocean albedo similar to that in CMIP6, driven solely by sea ice albedo. Applying the decomposition to future projections shows that the largest decline occurs in the Central Arctic, driven primarily by reductions in sea ice extent. After 2045, sea ice extent emerges as the dominant driver, highlighting ice edge retreat as key to future albedo decline. These findings pinpoint key sources of inter‐model spread in Arctic surface albedo and offer insights into quantifying shortwave (SW) radiative effect associated with sea ice responses in future projections.

Abstract A diverse array of sulfur‐containing organic compounds (SOCs) was identified in seasonal snow samples collected from northwestern China, using high‐performance liquid chromatography interfaced with an electrospray ionization high‐resolution mass spectrometer (HPLC–ESI–HRMS). Hierarchical cluster analysis (HCA) of the HPLC–HRMS data set classified SOCs into distinct clusters based on their molecular characteristics. Substantial differences in SOC composition were observed between urban (U) and rural/remote (R) clusters. In ESI− mode, the SOCs in the U cluster exhibited higher unsaturation degrees, oxidation levels, and larger molecular sizes than the R cluster. These compounds were likely derived from anthropogenic sources, such as transportation and industry, whereas those in the R cluster exhibited signatures of mixed sources, including biomass burning, cooking‐related emissions, and local biogenetic inputs. In ESI+ mode, SOCs were scarcely detected in the R cluster but were abundant in the U cluster. These compounds were predominantly reduced and unsaturated SOCs, with more than 85% (by intensity) containing one or no oxygen atoms in their elemental formulas. Tentative assignments for these species include sulfoxides, polycyclic aromatic S‐heterocycles, thiols, and sulfides, likely originating from the use and production of heavy oils. Additionally, volatility estimates suggest that the SOCs identified in this study are more volatile than those found in other environmental media. The findings highlight the unique SOCs composition in snowpack of northwestern China, particularly those detected via positive ESI mode, offering new insights into the environmental, climatic, and biogeochemical roles of SOCs.

Abstract The dominant intraseasonal modes of winter surface air temperature in extratropical Northern Hemisphere are identified through an empirical orthogonal function (EOF) analysis. The first mode is characterized by a dipole temperature pattern over the North America-North Pacific sector. The upper-tropospheric circulation associated with this mode is a zonally oriented Rossby wave train propagating westward. The second mode exhibits an in-phase relationship between the temperature anomaly over Eurasia and North America. The large-scale circulation associated with this mode is featured by same-sign geopotential height anomalies moving at an opposite zonal direction (eastward in Eurasia and westward in North America). The third mode shows an out-of-phase relationship between the temperature anomaly in Eurasia and North America. This out-of-phase temperature pattern is linked to the similar low-frequency wave train but with opposite signals in geopotential height anomalies. The sum of the three leading modes explains 40% of the total intraseasonal temperature variance. The distinctive wave propagations in Eurasia and North America are rooted in barotropic Rossby wave dynamics and determined by both the zonal wavelength and the background flow, with the former playing a larger role. An anomalous temperature budget analysis shows that the southeastward (westward) shifting of the lower-tropospheric temperature tendency in Eurasia (North America) results primarily from anomalous meridional temperature advection.

Abstract Recent studies have reported the marine heatwaves (MHWs) across the tropical Atlantic that peaked in boreal early-spring 2024. Using multiple observational and reanalysis datasets, we demonstrate that MHW metrics based on detrended data, averaged over the entire tropical Atlantic, reached their satellite-era maxima at that time, consistent with a prior equatorial study. A mixed layer heat budget analysis identified distinct regional drivers: 1) in the northern tropical Atlantic (20°N–3°N), a preconditional MHW during fall 2023 that was sustained through the following seasons by positive shortwave radiation anomalies associated with reduced total cloud cover; 2) in the equatorial Atlantic (3°N–3°S), warming was driven by the previously identified ocean waves, amplified by negative mixed-layer depth anomalies; 3) in the southern tropical Atlantic (3°S–20°S), the primary driver was wind-driven mixed layer shoaling from November 2023 to February 2024. Further analysis links the northern tropical Atlantic cloud cover anomalies to the previously emphasized remote forcing from El Niño, whereas the wind anomalies over the southern tropical and equatorial Atlantic were predominantly controlled by the newly revealed South Atlantic Subtropical Dipole. Our study reveals the multifaceted formation mechanisms of extreme MHWs in the tropical Atlantic, offering crucial insights for improving their seasonal prediction.

Abstract. A comprehensive in-situ dataset of low-level Arctic clouds was collected in the Fram Strait during the HALO-(AC)3 campaign in spring 2022 using the research aircraft Polar 6. The clouds observed at altitudes below 1000 m were frequently in a mixed-phase state. We demonstrate that despite comparable optical properties, classic mixed-phase clouds (MPC) and mixed-phase haze (MPH) can be distinguished on the basis of their microphysical properties, with MPH observed about 8 times more frequently than MPC. While the thermodynamic phases of the particles within the MPH are similar to those in the MPC, the supercooled droplets observed in MPC are replaced by large (> 3 µm) wet aerosol particles in MPH. Furthermore, the particle number concentration measured in MPH is reduced by approximately 3 orders of magnitude compared to MPC. MPH is observed in subsaturated air with respect to water, suggesting that the small liquid particles are haze droplets and are in equilibrium below the activation threshold to form cloud droplets. Chemical analysis suggested that the haze particles contained significant amounts of sea salt. Additional in-situ measurements with an optical particle counter indicated that their number concentration was 2 times larger over the sea ice compared to the open ocean. Furthermore, measurements of the vertical distribution of the thermodynamic phases in low-level Arctic clouds revealed a characteristic structure, with a liquid regime frequently occurring at the top of the atmospheric boundary layer, followed by MPCs, and an MPH layer below. The findings from this study enhance our understanding of the microphysical composition of clouds in mixed-phase conditions.

Abstract. Representing mixed-phase clouds (MPCs) is a long-standing challenge for climate models, with major consequences regarding the simulation of radiative fluxes at high-latitudes and uncertainties in future cryosphere melting estimates. Low-level boundary-layer MPCs that prevail at high-latitudes can be either coupled or decoupled to the surface, which modulates their dynamical and microphysical properties. This study leverages a recent physically-based parameterization of phase partitioning considering an explicit coupling between microphysics and subgrid-scale dynamics and involving direct interactions between the cloud and turbulent diffusion schemes. This parameterization makes it possible to capture the structure of the decoupled state of polar boundary-layer MPCs – with a supercooled liquid dominated cloud-top sitting on top of precipitating ice crystals – in single column simulations with the LMDZ Atmospheric General Circulation Model. The positive feedback loop involving cloud-top radiative cooling induced by supercooled liquid droplets, subsequent buoyancy production of turbulence as well as the supercooled liquid water production associated with turbulence, is captured by the model. However, the liquid and cloud ice water path remain underestimated and most of the turbulence is confined near cloud top which is probably due to a missing parameterization for convective downdrafts in the model. The study further shows that accounting for the detrainment of shallow convective plume's air when diagnosing the in-cloud supersaturation makes it possible to capture the overall vertical structure of surface-coupled clouds, with realistic liquid and ice water contents. Nonetheless, a parameteric sensitivity analysis emphasizes the importance of properly calibrating the parameter controling the supercooled liquid water production term by subgrid turbulence.

Abstract. A multi-year analysis of aerosol optical depth (AOD, τ) and Ångström exponent (α) was conducted using ground-based photometer data from 15 Arctic and 11 Antarctic sites. Extending the dataset of (Tomasi et al., 2015) through December 2024, the study incorporates stellar and lunar photometric observations to fill data gaps during the polar night. Daily mean values of τ at 0.500 µm and α (0.440–0.870 µm) were used to derive monthly means and seasonal histograms. In the Arctic, persistent haze events in winter and early spring lead to peak τ values. A decreasing trend in Arctic τ suggests the impact of European emission regulations, while biomass-burning aerosols are becoming more significant. In Antarctica, τ increases from the plateau to the coast. Fine-mode aerosols dominate in summer-autumn, while coarse-mode particles are more prevalent in winter-spring. Shipborne photometer data align well with ground-based measurements, confirming the reliability of mobile observations. Trend analyses using the Mann-Kendall test and Theil-Sen regression indicate a significant negative trend in τ at Andenes (−2.43 % per year), likely driven by reduced anthropogenic emissions. Antarctic stations such as Syowa and South Pole show positive trends (+3.84 % and +3.54 % per year), though these are subject to uncertainties from data limitations and instrument changes. This work contributes to the Polar-AOD network (https://polaraod.net/, last access: 15 May 2025), enhancing the understanding of aerosol variability and long-term trends in polar regions while promoting open data access for the scientific community.

Abstract Landsat‐derived products are the most prominent, publicly available sources of large‐scale surface water extent data. However, few studies have assessed the limitations of spatial scale on such products. Here, we mapped seasonal surface water extents utilizing high‐resolution (4.77 m) PlanetScope Basemap imagery and machine learning. We conducted a pixel‐wise comparison of these high resolution classifications with a set of classifications from a moderate resolution (30 m) Landsat product. The vast majority (93%) of areas classified as water by the Landsat product were similarly classified by PlanetBasemap; however, only 65%–75% of the PlanetBasemap water area was also classified by the Landsat classes. Of the Landsat classes, only the partial surface water class comparably detects smaller water bodies (widths 50–70 m) with PlanetBasemaps. Our results indicate that higher resolution imagery detects more small water bodies, which are instrumental to better understanding flood dynamics, methane emissions, and downstream water volume and quality.

Abstract A long‐term, consistent satellite record of cloud droplet number concentration ( N d ) is essential for understanding aerosol‐cloud interactions and their climate effects. However, the Aqua MODIS‐retrieved N d exhibits an unexpected and substantial increase over the near‐global oceans after 2022, contradicting the expected decline from continued emission reduction efforts. Here we demonstrate that this surge is not physical but largely an artifact of sensor orbital drift, which alters viewing geometry and solar illumination. By leveraging concurrent Suomi‐NPP VIIRS observations unaffected by drift, we developed an empirical correction that removes this artificial signal and quantified global mean N d artificial biases of +2.4 cm −3 in 2023 and +5.0 cm −3 in 2024. These biases substantially distort the N d trends, reversing the previously decreasing global N d trend into an apparent strong rise after 2022. These findings highlight the critical need to correct for such artifacts when constructing satellite‐based climate data records.

Abstract It remains unclear how changes in moisture supply drive droughts in the Agro‐Pastoral Ecotone of Northern China (APENC), where ecological restoration and food security are increasingly vulnerable under warming. Using a moisture‐tracking model, we quantify the moisture sources of APENC's precipitation and its trends, and reveal mechanisms linking anomalous upwind moisture transport to droughts. Terrestrial moisture sources (67.60%) dominate APENC's precipitation. Over 2000–2023, moisture sources from high‐latitude Eurasia and the Tibetan Plateau have increased, whereas those from oceanic sources have decreased significantly. During recent drought events, reductions in moisture inflow from key terrestrial source regions, especially East Asia and the South Asia–Indian Ocean region, primarily triggered rainfall deficits in the APENC. The combination of weakened moisture inflows and reduced local humidity amplified moisture scarcity, sustaining drought severity. These findings highlight the coupled roles of remote transport and land–atmosphere feedbacks in APENC droughts, providing new evidence for understanding semi‐arid hydroclimate risks.

Abstract Earthquakes in the lower crust, where high pressures and temperatures favor ductile deformation, are rare but occasionally occur beneath active volcanic centers, providing insights into deep magmatic processes. We analyze a deep crustal earthquake swarm (February–August 2025) of over 5,000 events beneath the Abu Volcano Group, Japan. Using spectral characteristics, we distinguish deep low‐frequency earthquakes (DLFEs) from volcano‐tectonic events and identify several anti‐repeating pairs. All events are relocated with cross‐correlation and a double‐difference algorithm, revealing a complex multi‐cluster fault network comprising a steeply dipping conduit and sub‐horizontal fault strands. DLFEs migrate upward at ∼0.3 km/day along the conduit, suggesting fluid or magma ascent that triggers overlying brittle failure. Anti‐repeating earthquakes indicate small‐scale stress heterogeneity within adjacent fault patches. These observations elucidate the interplay between deep fluid migration, stress variability, and faulting mechanisms, providing a framework to interpret precursory deep volcanic seismicity and the structural controls governing deep crustal earthquake swarms.

Abstract In May 2024, extraordinary solar activity triggered a powerful solar storm, impacting Earth and producing the extreme geomagnetic storm of 10‐11 May, the most intense since 2003. This had significant effects on the magnetosphere, leading to the creation of a new long‐lasting component of relativistic electrons and to flux changes in the South‐Atlantic Anomaly. Here we present radiation‐belt observations made by the Calorimetric Electron Telescope (CALET) on the International Space Station. Specifically, we took advantage of the count rates from three layers of the CALET charge detector and imaging calorimeter. We show that the new electron storage ring extended to energies in the multi‐MeV range and down to McIlwain's L = 2.2, well below the nominal slot‐region barrier of L = 2.8, and persisted for several months, depending on energy. The evolution of the new radiation‐belt configuration over time was characterized by estimating the decay rates as a function of energy and L .

Abstract Arctic sea ice plays a critical role in Earth's climate system, and as it continues to thin and retreat, understanding the processes driving its variability is increasingly important. Using satellite data and a coupled ocean–sea ice model, we examined how freshwater from the Mackenzie River influences fall sea ice formation in the Beaufort Sea. An “ice bridge” between the coast and offshore ice edge consistently forms over the river's freshwater plume, with its location and extent varying interannually with freshwater distribution. Regions influenced by the plume experienced sea ice onset an average of 3 weeks earlier than adjacent, saltier waters. Earlier ice formation was associated with enhanced stratification, shallower mixed layers, and reduced upper ocean heat content, all of which promotes faster surface cooling. Our findings highlight the importance of river discharge in shaping sea ice formation and suggest continued Arctic freshening will impact future sea ice timing and extent.

Abstract MgFe 2 O 4 was probed to 74(1) GPa and 2,840(130) K as a low‐pressure analog to post‐post spinel Mg 2 SiO 4 predicted in super‐Earths using synchrotron multigrain X‐ray diffraction techniques in the laser‐heated diamond anvil cell. With high‐temperatures above 65 GPa the eight‐fold coordinated δ ‐MgFe 2 O 4 (Th 3 P 4 ‐type) is stable with Mg and Fe disordered into one site, analogous to that predicted in Mg 2 SiO 4 . While the phase‐boundary slope between the h ‐ and δ‐phases of MgFe 2 O 4 and Fe 3 O 4 differ, the substitution of Fe 2+ and Mg 2+ has little overall pressure effect on the stability of the post‐post spinel phase. This eight‐fold coordinated δ ‐MgFe 2 O 4 is stable at ∼400 GPa lower pressures compared to the predicted δ ‐Mg 2 SiO 4 , suggesting the need for further explorations into the conditions needed to substitute Fe 3+ into δ ‐Mg 2 SiO 4 and the effect of ferric iron on the onset of eight‐fold coordination in super‐Earth mantles.

Abstract Central American agriculture and ecosystems are acutely sensitive to rainfall that spans the spring‐summer growing window, yet most studies still evaluate each season in isolation. Here we demonstrate that, since the 2000s, Central American precipitation anomalies in spring have become more likely to persist into summer‐a shift that single‐season analyses overlook. This lengthened persistence arises from a regime change in which multi‐month “persistent events” now dominate over the previously common “transitional events.” Persistent events are anchored by coupling between warm tropical Atlantic (TA) and cold tropical Central‐Eastern Pacific (TCEP) sea surface temperature (SST) anomalies that sustains cyclonic circulation, whereas transitional events rely on rapid TCEP SST reversals that disrupt rainfall anomalies. Strengthened Atlantic influences on Pacific climate since 2000 have prolonged Pacific SST persistence, boosting persistent‐event frequency and extending springtime rainfall anomalies into summer. This cross‐seasonal persistence heightens risks to water, food and biodiversity in Central America.

Abstract Magnetic signatures preserved in rocks have long provided insight into Earth's evolution, revealing processes from plate tectonics to the habitability of Earth. While large impacts are known to impose extreme stresses (>1 GPa) and heat that fundamentally alters magnetic records, lower stresses typical of earthquakes have been considered magnetically undetectable. We show that magnetic responses to sub‐GPa stresses can be precisely calibrated, enabling three‐dimensional paleostress reconstructions in rocks—even stresses of just a few MPa can fully reset magnetic signals without heat or deformation. This newly revealed magnetic sensitivity to stress opens a powerful, non‐destructive pathway to detecting paleostress fields in the elastic crust, offering new opportunities for improving seismic hazard assessment, interpreting impact processes, and re‐evaluating magnetic records across Earth and Planetary sciences.