Large-scale drivers and ocean-land feedbacks contributing to extreme precipitation during the January 2021 South Kalimantan flood, Indonesia

Understanding long-term variability in storminess is essential for constraining future climate patterns in the eastern North Atlantic, a region shaped by complex ocean–atmosphere interactions. Here, we reconstruct storm, fire, and hydroclimate variability from grain size, inorganic geochemistry, plant macrofossil and molecular organic records since mid-Holocene at Glenties Bog, a coastal blanket bog in western Ireland. Comparing our results with existing palaeostorminess records, we provide new insights into a dynamic interplay between wind strength, fire incidence, and hydrological conditions throughout the Holocene. Long-term temperature evolution influenced the background wind and hydroclimatic state, while volcanic activity became a key forcing mechanism during parts of the late Holocene. The warm mid-Holocene coincided with lower wind strength and enhanced fire activity, while no assoication between storm periods and volcanic activity was inferred, indicating that the climate state at the time of the volcanic forcing affects the climatic response. These results indicate that future warming may lead to profound changes in wind strength, hydroclimate, and fire regimes. Notably, climate-change-induced lowering of peatland water tables may increase the susceptibility of blanket bogs to intense and deep peatland fires. This study provides new insight into Holocene wind and hydroclimate dynamics in the eastern North Atlantic, improving understanding of the mechanisms driving North Atlantic climate variability across contrasting climate states. • Multiproxy reconstruction of Holocene climate in the eastern North Atlantic • Holocene temperature trends influenced background wind and hydroclimate • Mid-Holocene warmth associated with dry conditions, fires, and low wind strengths • Late Holocene marked by wetter conditions and enhanced storminess • Volcanic activity associated with enhanced storminess during parts of the late Holocene

Abstract The dispersal of meltwater discharged from the Laurentian Channel (LC) is investigated from numerical experiments with an eddy-resolving model representing the western North Atlantic during the last ice age. Meltwater dispersal is simulated over a full summer, when glacial ablation rates were presumably the highest. In our experiments, meltwater forms a buoyant plume, which flows to the southwest along the continental slope owing to the Coriolis force. Four mechanisms of offshore export are identified. 1) Meltwater is carried seaward by Ekman currents driven by upwelling-favorable winds along the slope. 2) Part of it is entrained away from the slope by meander crests and warm-core rings of the Gulf Stream (GS) between the LC and Cape Hatteras. 3) The other part is generally diverted offshore by the GS near Cape Hatteras, where the GS leaves the slope. 4) Meltwater can be trapped in a GS meander trough that pinches off and produces a cold-core ring, leading to its penetration into the subtropical gyre. In turn, the buoyant plume has relatively small but noticeable effects on the GS. In the western, weakly meandering segment of the GS, the vertical shear in horizontal velocity is generally reduced due to the presence of melt (light) water along the inshore flank of the GS. Our results are discussed in light of (i) a two-layer theory of a surface density front subjected to background flow and wind stress and (ii) sediment records from the Laurentian Fan and the Sargasso Sea.

Atmospheric equatorial Kelvin waves—convective disturbances that manipulate tropical wind and rainfall patterns—can propagate eastward at speeds ranging from nearly stationary to 30 m/s, with variability determined by moist processes and advection by the background wind. Current studies on Kelvin waves lack a comprehensive climatology that explains how their structure and propagation speeds change in different background states. Thus, this work builds a variable regression model that uses ERA5 reanalysis data to reconstruct Kelvin waves during different background wind shear conditions and phases of the Madden–Julian Oscillation (MJO) and the El Niño–Southern Oscillation (ENSO) over the Pacific. Overall, Kelvin waves tend to speed up during background conditions that generate upper-tropospheric westerlies and slow down during upper-tropospheric easterlies. East Pacific Kelvin waves are faster than West Pacific Kelvin waves because of climatological westerly shear in the former and easterly shear in the latter. However, strong westerly shear over the East Pacific allows extratropical Rossby waves to impede on the Kelvin wave, while strong easterly shear over the West Pacific distorts classical Kelvin wave structure. The results provide references for weather prediction models to accurately resolve the interaction between Kelvin waves and background circulation.

Abstract The modulation of Madden–Julian oscillation (MJO) by El Niño–Southern Oscillation (ENSO) has been widely investigated, yet the mechanisms—particularly the combined influences of ENSO phase (El Niño/La Niña) and type (eastern Pacific/central Pacific)—remain elusive. Here, we unravel that differences in MJO characteristics (convective intensity, propagation, and structure) between El Niño and La Niña are asymmetric across two types of ENSO. These asymmetries are interpreted through established MJO frameworks, involving vertical wind shear, moisture–convection feedback, and structure–propagation nexus. Due to greater interphase differences in low-frequency background winds and humidity, eastern Pacific (EP) ENSO exerts stronger modulation on MJO intensity and northward propagation than central Pacific (CP) ENSO. Weaker MJO modulation of CP ENSO also stems from its distinct regulations of low- and high-frequency MJO components. Moreover, MJO eastward propagation is consistently faster during EP ENSO than during CP ENSO, primarily attributable to stronger premoistening and larger front Walker cell zonal scale leading MJO deep convection, independent of ENSO phases. The MJO propagation over the Maritime Continent also exhibits asymmetry: While MJO events propagate coherently into the central Pacific during both EP ENSO phases, they are frequently blocked during CP La Niña but not during CP El Niño. To explain further these propagation asymmetries, we propose five diagnostics based on winds, convection, stratiform heating, and humidity across intraseasonal and low-frequency time scales. Variations in the strength and behavior of these diagnostics under different ENSO backgrounds effectively explain the diversity of MJO propagation. This study advances toward a more comprehensive understanding of how diverse ENSOs modulate the MJO. Significance Statement The Madden–Julian oscillation (MJO) is the dominant mode of 20–100-day fluctuations in tropical winds and clouds and features significant year-to-year variations in its intensity and propagation. Among numerous factors that regulate MJO behaviors, El Niño–Southern Oscillation (ENSO) has been examined extensively in previous studies. Nevertheless, how the MJO varies across different phases and types of ENSO remains elusive, especially whether there exist asymmetric El Niño–La Niña differences in the MJO under two types of ENSO has received much limited attention. Using observation and reanalysis products, we have explored these issues comprehensively by diagnosing the convective intensity, propagation, and structure of MJO. Our findings promote the development of comprehensive paradigms for diverse ENSO-modulated MJOs.

Urban ventilation is an effective means of improving air quality and promoting sustainable development of urban areas. For Loess Tableland valley towns dependent on heavy industry, the ventilation characteristics of the town area are poorly understood. Therefore, it is important to first explore wind field characteristics over the negative terrain. In this study, the wind field over the negative terrain under the stable background wind was investigated by orthogonal experiments. Simulation results show that airflow patterns in the valley space can be classified into five categories, which are the unstructured flow, combination of unstructured flow and circulation organization, circulation organization, combination of circulation organization and background wind and background wind. The airflow pattern can affect significantly the vertical distribution of the velocity, temperature and air age in the valley space, and thus affecting the ventilation performance of the valley towns. Ventilation performance of valley towns was worse under the unstructured flow conditions, while it was optimal under the background wind conditions. Additionally, sensitivities of terrain factors influencing ventilation evaluation indices at the pedestrian level were analysed. The present study has provided a scientific basis for town planning and industrial emissions in the valley towns.

Abstract Fields of equatorial upper-tropospheric circulation data are lag regressed against wavelet-filtered indices of upper-tropospheric zonal wind anomalies over a range of phase speeds at 50-day periods to study the propagation mechanisms of the upper-tropospheric circulation signal of the Madden–Julian oscillation (MJO) over the Indian Ocean. Results show that the MJO upper-tropospheric zonal wind is accelerated by the geopotential gradient force in quadrature with the existing wind anomaly, yielding its eastward propagation. This effect is offset by Doppler advection of the MJO wind by the easterly background wind. Divergence of mass by the zonal wind propagates the associated geopotential height anomalies in concert with the winds. Results confirm that advection of background wind by the MJO wind amplifies MJO wind anomalies in phase with those anomalies where the background wind is zonally confluent and breaks it down in regions of diffluent background wind. Coincidence of the mass source driven by moist convection and zonally diffluent zonal wind anomalies with falling geopotential height relative to the regions east and west is consistent with planetary-scale Kelvin wave zonal wind signals providing favorable conditions for convection. Circulation data at 100 hPa confirm approximately dry Kelvin wave dynamics driving MJO-associated equatorial circulations eastward.

Abstract We present new detections of mesoscale stationary features, which are interpreted as gravity waves, on the dayside clouds of Venus. These come from an analysis of images from two instruments onboard different spacecrafts: Visible and InfraRed Thermal Imaging Spectrometer—Mapper (VIRTIS‐M) on Venus Express (VEx) and IR2 on Akatsuki. For VIRTIS‐M we selected the period from the first extension of the mission (roughly from August 2007 to May 2009). The wavelength range selected was based on best viewing conditions to characterize waves, centered at 600 nm. For IR2 we surveyed the entire available data at 2.02 μm targeting the dayside hemisphere. Basic morphological properties of the detected features were retrieved, namely their horizontal wavelength, packet sizes and orientations. Our measurements show that the horizontal wavelengths of these features are between 100 and 500 km, their widths between 500 and 3,000 km, and mostly oriented so that the crests align with the local meridian. We also retrieve the background wind in the vicinity of the detected structures to then calculate the vertical wavelength. The inclusion of the vertical wind shear of the zonal wind at the altitudes where these waves are observed proved significant in these calculations. For the features from Akatsuki data, we find values in good agreement with previous works however, for VIRTIS‐M their dimensions appear slightly reduced. This works aims to expand the current knowledge of the distribution of stationary waves with further applications to understand dynamical connections between the surface and the cloud layer in Venus' atmosphere.

Abstract During austral summer of 2002/2003, the TIMED/TIDI meridional wind observations show that after the attenuation of the climatological quasi‐2‐day wave (Q2DW) with westward zonal wavenumber 3 (W3) and a period of 47 hr (hrs) in January, a 53 hr abnormal W3 Q2DW event occurred in February with symmetric latitudinal structure about the equator. Our analysis indicates that this abnormal W3 Q2DW is generated by the nonlinear interaction between the climatological W3 Q2DW and a ∼ zonally symmetric oscillation associated with a sudden stratospheric warming (SSW). The SSW occurring in boreal winter modulated the meridional circulation through interhemispheric coupling, which influenced the background wind in the summer hemisphere and favored the amplification of a symmetric Q2DW mode in February consequently. The diagnostic analysis reveals that this February W3 Q2DW was mainly amplified near the stratopause at low latitudes of the Southern Hemisphere, where the background atmospheric conditions are suitable for the amplification of the W3 Q2DW. Thus, both the nonlinear interaction and favorable background atmospheric conditions contribute to the occurrence of the abnormal W3 Q2DW in February.

Abstract A strong westward zonal wavenumber‐2 quasi‐4‐day wave (Q4DW) during the 2018/2019 Northern Hemisphere sudden stratospheric warming (SSW) is both captured by Aura Microwave Limb Sounder (MLS) observations and our new whole neutral atmosphere data assimilation system. The Q4DW during this SSW is characterized by a double‐peak altitudinal structure in temperature, geopotential height, and neutral winds ranging from ∼40 km up to the lower thermosphere. The Eliassen‐Palm flux diagnostics show that the wave source at ∼55 km over 45°N–75°N, the excitation, propagation, and amplification of which are controlled by the critical layer and atmospheric barotropic/baroclinic instability in the polar stratosphere related to SSW. The first Hough‐mode decomposition analysis of Q4DW indicates that the enhancement of the Rossby (2, −3) normal mode is mainly responsible for the amplification of Q4DW, the latitudinal structure of which is distorted by the anomalous background winds during this SSW. In the ionosphere, a simultaneous quasi‐4‐day oscillation (Q4DO) is found in the Wuhan University total electron content (TEC) product near 10:00–12:00 LT at magnetic latitudes of ∼+15° and ∼−25° with maximum amplitudes of ∼1.1 TECU and 1.2 TECU, respectively. Besides, the Q4DO also displays significant interhemispheric asymmetry and longitudinal variations. Interestingly, the secondary wave components ( s = 4, T = 10.7 hr and s = 0, T = 13.7 hr) in neutral winds from the nonlinear interactions between the Q4DW and the migrating semidiurnal tide are detected in the dynamo region, which may play a dominant role in generating Q4DO in the F‐region ionosphere.

Abstract Observational estimates of gravity wave momentum fluxes obtained from analyses of super‐pressure balloon tracks show an approximate log normal distribution in space and time. One study has suggested that a non‐orographic gravity wave parameterization with a steady source could reproduce this distribution through variability in the background wind, which “dynamically filters” the source spectrum. We use an implementation of a simple steady source gravity wave parameterization to show that this observed distribution is not reproduced, the most prominent departure from it being a large negative skew. We perform a crude tuning of the scheme's parameters to show that the discrepancy in the distribution cannot be corrected. We observe that at higher levels, parameterized fluxes are still skewed but less so. We suggest that dynamical filtering alone cannot introduce enough variability to obtain fluxes that are log normally distributed.

Abstract This study focused on the impact of vertical wind shear (VWS) in the stratosphere on the asymmetry of stratospheric gravity waves (SGWs) generated by tropical cyclones (TCs), based on a set of numerical simulations of TC-SGWs completed under different stratospheric background wind conditions. Results showed that a constant background easterly wind leads to a slightly asymmetric distribution of TC-SGWs, with intense waves in the lower stratosphere located on the upwind side, attributable to the asymmetry in TC structure under the condition of the easterly wind. In contrast, VWS induces a notable asymmetric distribution of TC-SGWs through directional critical levels. Spectral and wavenumber–vector analyses revealed that the co-occurrence of radial and tangential propagation of TC-SGWs modifies the critical-level filtering effect, producing a final distribution with intense waves located mainly in the southeastern quadrant relative to a TC center and to a lesser extent in the northeastern and southwestern quadrants, rather than throughout the eastern quadrants as a whole.

Abstract Background Wind erosion can cause land degradation in semiarid ecosystems and is substantially increased following wildfire. Although plant regrowth is an important source of site stability following wildfire, plants differ in their ability to protect soils from wind erosion and differ in their distribution following wildfire. As a result, rates of wind erosion can span four orders of magnitude in the years following wildfire among sites. Results Using measurements of aeolian sediment flux and plant community development from seven sites following wildfire, we identified ordinal plant communities to develop a quantitative index of site stability post-wildfire. Using these plant communities, we modeled how management focusing on reducing a single plant functional group (e.g., fuel treatments) may impact wind erosion as plant communities redeveloped after wildfire. We found the outcome of management focused on a single functional group has different impacts on wind erosion based on its surrounding plant community and time since wildfire. Conclusions Elucidating the role of plant functional groups in mitigating wind erosion can guide managers on post-wildfire and posttreatment options to reduce wind erosion risk.

The Mesosphere and Lower Thermosphere (MLT) is a critical atmospheric region ranging from 80–140 km in altitude. A main driver of momentum transport, density perturbations, temperature variations, and background winds in this region are atmospheric gravity waves which are not well accounted for in many models of the thermosphere and near-space region. Additionally, global circulation models that do include GWs fail to resolve small-scale activity ( < 200 km). We utilize a hydroxyl (OH) airglow imager located in Poker Flat, Alaska (65°N 147°W) to leverage our active citizen science initiative, the Gravity Wave Zoo, expanding the breadth of available GW, aurora, and instability data over multiple seasons. We focus on a short-term study between 27 December 2023 and 4 February 2024 to statistically validate a subsample of this multi-year dataset, and report on recent Gravity Wave Zoo progress and accuracy, presenting a summary of overall participation and citizen classifications of GW, instability, and auroral events. We find citizen scientist classifications indicate GWs in 54.5%, aurora in 40.1%, and instabilities in 23.4% of subjects. We further propose future research directions enabled by this work and highlight the advantages of high temporal resolution data on the scale of weeks, months, and seasons.

Abstract. This study investigates the impact of very high frequency data assimilation on analysis and forecast accuracy with the local ensemble transform Kalman filter for idealized deep convection. Previous studies showed that assimilating every 30 s data from Phased Array Weather Radar (PAWR) alleviates the problem of strongly non-Gaussian error probability distribution due to rapid nonlinear evolution of deep convection in real-world cases. This study performs perfect model observing system simulation experiments to understand better the impact of assimilating radar reflectivity every 30 s focusing on non-Gaussianity. The idealized experimental settings have unique advantage in verifications for unobserved variables since it was unclear in the previous studies with real-world data. The results show that every 30 s data assimilation contributes to a significant improvement of the analysis accuracy, particularly for vertical velocity associated with strong convection, although the impact on the forecast accuracy is limited. We also find a significant reduction in the non-Gaussianity of first guess ensemble. The impact of assimilation frequency on reducing non-Gaussianity is enhanced when the uncertainty in background wind or stability is included in the initial ensemble perturbation.

Abstract Atmospheric gravity waves (GWs) are believed to transport energy and momentum between different regions of the atmosphere. Historically, observations of these waves from both ground and space have been relatively abundant at altitudes up to the lower thermosphere, and somewhat less abundant in the upper thermosphere and F‐region ionosphere altitudes. Much of what is known of the typical properties and occurrence of these waves at thermospheric altitudes has been inferred from their impacts on the ionospheric density and motion, as direct observations of the neutral atmosphere have been less prevalent. Gravity waves in the middle thermosphere, from ∼120–200 km altitude, have rarely been observed directly and as such, their properties at these altitudes are less well documented. NASA's Global‐Scale Observations of the Limb and Disk (GOLD) mission makes observation of the middle thermosphere during daytime. During dedicated campaigns, GOLD has been able to observe GWs in this region. This study leverages 22 such campaigns during quiet geomagnetic conditions and low to moderate solar activity levels. Waves were observed with typical periods ∼2–4 hr. Leveraging ground‐based observations, the wavelengths were identified to be between ∼1,500–5,000 km, with phase speeds ∼150–600 m/s. The waves observed were seen to propagate primarily meridionally, in agreement with prior daytime mid‐latitude observations. Using observations of the background wind, the energy and momentum fluxes carried by these waves were found. During the quiet conditions observed, the waves were seen to transport energy flux over a wide range of latitudes.

Effect of built-up area expansion on urban ventilation over Loess Tableland valley terrain under stable background wind

Abstract. Transatlantic aviation is a major industry and even small flight time changes have major economic and environmental implications. While our ability to optimise these flights for background wind variations at day-to-day scales is excellent, at the longer timescales needed for sustainability planning and fuel cost hedging these capabilities are more limited. Here, we quantify the association between four climate indices (the El Niño-Southern Oscillation, the North Atlantic Oscillation, the Quasi-Biennial Oscillation and solar irradiance) and transatlantic flight times using thirty years of commercial flight data. This allows us to identify whether these indices can be used to identify systematic flight time shifts. We find that ENSO and the NAO are associated with statistically-significant changes in one-way flight times of up to 82.2 ± 3.5 min, and changes in round-trip times of 4.8 ± 0.5 and 4.0 ± 0.8 min respectively, while the QBO and TSI have weaker but significant effects. Together, these indices plus a linear trend explain up to 27 % of variation depending on season and direction, and are associated with month-to-month fuel cost & CO2 emission variations of up to 27MUSD & 120 kT for one-way trips and USD 5 million & 23 kT for round trips. We also show that westward, round-trip and non-winter-eastward flight times have increased by several minutes per decade since the 1990s. Our results provide the first observational quantitative basis for aviation fuel and carbon cost management at monthly and longer timescales.

Abstract An analysis of buoy data from the National Data Buoy Center (NDBC) reveals that El Niño significantly modulates subdaily variations in air–sea latent heat fluxes across the Hawaiian Islands. During El Niño winter months, the mean amplitude (standard deviation) of these variations reached up to 35 W m −2 —approximately 11 W m −2 greater than during La Niña winters—representing a 33% increase. To clarify the underlying processes, we examined the drivers of subdaily latent heat flux variability under differing boundary layer stability regimes. Results show that El Niño–Southern Oscillation (ENSO) phases alter the background air–sea humidity difference and wind speed, thereby influencing the amplitude of subdaily flux variations. By analyzing both daily mean and subdaily anomalies of wind speed and humidity difference during El Niño and La Niña events, we quantified their respective contributions. El Niño enhances subdaily flux variability primarily by increasing background humidity differences and wind speeds under near-neutral boundary layer conditions. Under unstable conditions, the elevated humidity difference continues to dominate the variability. During El Niño events, the contributions of background humidity difference and wind speed were 61% and 68%, respectively, under near-neutral stability, and 87% and 36%, respectively, under unstable conditions. These findings highlight the dynamic interplay between large-scale climate variability and high-frequency air–sea interaction processes in the central Pacific. Significance Statement Latent heat flux plays a critical role in ocean–atmosphere interactions and ocean circulation. Elucidating its subdaily variation mechanisms is essential for advancing our understanding of ocean dynamics, climate impacts, and the global heat budget balance, with direct implications for enhancing predictive models and simulations. This study provides the first observational evidence that El Niño amplifies subdaily latent heat flux variations near the Hawaiian Islands by up to 33%, driven by large background air–sea humidity differences and wind speeds under different boundary layer stability regimes. By linking small-scale flux variations to long-term climate signals, the findings advance the mechanistic understanding of ocean–atmosphere interactions, offering critical insights for improving the representation of subdaily processes in climate models.

Abstract Two quasi‐orthogonal nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) were observed by conjugate midlatitude all‐sky imagers in Sutherland (32.4S, 20.8E; magnetic latitude: −40.9) and Asiago (45.87N, 11.53E; magnetic latitude: ) on 4 October 2018. These MSTIDs had fronts elongated quasi‐orthogonally to one another as observed from each location. The first MSTID was aligned northeast‐southwest (NE‐SW) in the Southern Hemisphere (SH) and northwest‐southeast (NW‐SE) in the Northern Hemisphere (NH) and propagated equator‐westwards. These properties are typically attributed to MSTIDs generated through the coupled Perkins and sporadic E instabilities. This is supported by observed conditions in both hemispheres indicating the presence of sporadic E layers and reasonable Perkins instability growth rates. The second MSTID was aligned NW‐SE (SH) and NE‐SW (NH) and propagated equator‐eastwards and represents the first optical observations of conjugate equator‐eastward propagating MSTIDs. A possible linkage to gravity wave‐induced polarization electric fields in the NH (and mapped to the SH) is presented, as significant gravity wave activity was observed in OH and OI greenline observations by the Asiago imager. Their equator‐eastward propagation direction was favored by background winds at the hemisphere of origin, as determined from global model observations.