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Intense mesoscale eddy activity has been observed off the southern Java coast (SJC), yet its impact on local ecosystems remains largely unknown. To investigate this, we examined remotely sensed altimetry, chlorophyll-a (Chl-a), and sea surface temperature (SST) data, focusing on their response to eddies in the region. Our eddy detection and tracking analysis revealed a unique cyclonic frontal eddy near the SJC coast and a large anticy-clonic eddy offshore, active from July to September 2019. The cyclonic frontal eddy induced water transport through eddy filaments, upwelled subsurface cold water, and enhanced Chl-a concentrations by horizontally entraining Chl-a-rich shelf water offshore. The anticyclonic eddy then contributed to further distributing this enriched water southward. The mean cross-shelf transport associated with the frontal eddy was estimated at 1.80–2.33 Sv offshore, exporting approximately 1.87–2.40 × 10³ tons of Chl-a to the Indian Ocean during its lifetime. Additionally, the spatial cross-correlation analysis of zonal and meridional wind stress with Chl-a revealed relatively high correlation values (0.6–1) and short lag times (<5 days) in offshore areas, indicating that the role of wind in the Chl-a advection cannot be ignored. We propose a three-stage mechanism to explain the presence of high Chl-a offshore:1) Wind-driven upwelling intensifies coastal nutrients, elevating Chl-a concentrations in coastal waters, 2) Frontal cyclonic eddy facilitates the retention and offshore export of these upwelling-enriched waters. and 3) Anticyclonic eddy advects these nutrient-rich waters further south. The combination of enhanced coastal upwelling and eddies can explain nutrient-rich coastal waters in offshore regions. [ABSTRACT FROM AUTHOR]

Both composition and temperature play a crucial role in determining the NO radiative cooling in lower thermosphere as observed by TIMED/SABER. In this work, we present a detailed investigation of seasonal variation in thermospheric NO radiative cooling. We have carried forward the investigation of Li et al. (2018) regarding the variations in local nighttime peak NO radiative cooling and its altitude during solar maximum and solar minimum conditions. By analyzing latitudinal changes over quiet times for each month in year 2018, it is evident that both the investigated parameters exhibit summer-winter variability. The qualitative contribution of different species (i.e., NO, and O), and temperatures in determining the vertical profile of NO radiative cooling for different latitudes is investigated by utilizing the NRLMSISE-00 estimated parameters, and SNOE observed NO density. The temperature, NO density, and compositional variations due to asymmetrical solar heating and associated summer-to-winter circulation in both the hemispheres during solar minimum conditions seem to be the dominating factor in controlling the NO radiative cooling during different seasons. The altitudes at which maximum cooling by NO occurs exhibits an inverse correlation with the amount of radiative cooling. The region of enhanced NO densities (polar and summer hemispheric low-mid latitude regions) have larger NO radiative cooling with lower peak altitudes in comparison to other regions (equatorial to winter hemispheric low-mid latitude regions), where NO radiative cooling is low with higher peak altitude values. [ABSTRACT FROM AUTHOR]

Elevated surface ozone levels are often detected in the New York metropolitan area during summertime. Moreover, surface ozone in this region exhibits sharp spatial gradients and distinctive diurnal cycles under the influence of complex boundary layer circulations induced by the intricate coastal geometry. This study examines how surface ozone is impacted by local circulations spatially and temporally under different temperature scenarios (all summer days, hot summer days, and extreme heat days) with the help of cluster‐based meteorological conditions during the summertime of 2017–2019. The most polluted days are found to be highly associated with hot sea breeze days with weak background flow. When sea breeze development in the New York Bight is delayed and its penetration north is intercepted by the dominant westerlies during hot summer days, daily maximum 8‐hr average ozone (DMA8) in some ozone hot spots of New York City (NYC) and the south shore of Connecticut (CT) typically drops 9–10 ppb under comparable temperature levels. The average regional decrease of DMA8 for NYC and coastal CT is 6.7 and 8.3 ppb, respectively. Furthermore, we conclude that a change in early morning meridional wind direction is the most critical meteorological characteristic in controlling sea breeze onset type and helping modulate ozone exceedances in the region during extreme hot days when ozone exceedances are expected to be very common. The conclusion is further demonstrated with two case studies during the Long Island Sound Tropospheric Ozone Study 2018 field campaign. Plain Language Summary: The New York metropolitan area is classified as a moderate 8‐hr ozone nonattainment area by the U.S. Environmental Protection Agency. Moreover, surface ozone in this region exhibits sharp horizontal gradients and distinctive diurnal cycles under the influence of complex boundary layer circulations. This study examined three summers of meteorological (wind and temperature) and chemical (ozone and its precursors of nitrogen oxides and formaldehyde) observations to investigate the impacts of local circulations on coastal ozone pollution in the area, with a focus on hot summer days when high concentrations of ozone are more likely. Aiming to provide insights into future scenarios amidst the projected intensification and prolongation of extreme heat in the region throughout the 21st century, we identified and analyzed the meteorological characteristics most critical in modulating ozone exceedances in ozone hot spots during extreme hot days with case studies from the Long Island Sound Tropospheric Ozone Study 2018 field campaign. Key Points: K‐means clustering is applied to group three summers of local circulations and temperature diurnal cyclesHot summer daily maximum 8‐hr average ozone drops 9–10 ppb in ozone hot spots when sea breeze is impacted by the dominant westerliesEarly morning meridional wind direction controls sea breeze onset type and modulates ozone exceedances during extreme conditions [ABSTRACT FROM AUTHOR]

The Chukchi Sea is an open estuary in the southwestern Arctic. Its near-surface salinities are higher than those of the surrounding open Arctic waters due to the key inflow of saltier and warmer Pacific waters through the Bering Strait. This salinity distribution may suggest that interannual changes in the Bering Strait mass transport are the sole and dominant factor shaping the salinity distribution in the downstream Chukchi Sea. Using satellite sea surface salinity (SSS) retrievals and altimetry-based estimates of the Bering Strait transport, the relationship between the Strait transport and Chukchi Sea SSS distributions is analyzed from 2010 onward, focusing on the ice-free summer to fall period. A comparison of five different satellite SSS products shows that anomalous SSS spatially averaged over the Chukchi Sea during the ice-free period is consistent among them. Observed interannual temporal change in satellite SSS is confirmed by comparison with collocated ship-based thermosalinograph transect datasets. Bering Strait transport variability is known to be driven by the local meridional wind stress and by the Pacific-to-Arctic sea level gradient (pressure head). This pressure head, in turn, is related to an Arctic Oscillation-like atmospheric mean sea level pattern over the high-latitude Arctic, which governs anomalous zonal winds over the Chukchi Sea and affects its sea level through Ekman dynamics. Satellite SSS anomalies averaged over the Chukchi Sea show a positive correlation with preceding months' Strait transport anomalies. This correlation is confirmed using two longer (>40-year), separate ocean data assimilation models, with either higher- (0.1°) or lower-resolution (0.25°) spatial resolution. The relationship between the Strait transport and Chukchi Sea SSS anomalies is generally stronger in the low-resolution model. The area of SSS response correlated with the Strait transport is located along the northern coast of the Chukotka Peninsula in the Siberian Coastal Current and adjacent zones. The correlation between wind patterns governing Bering Strait variability and Siberian Coastal Current variability is driven by coastal sea level adjustments to changing winds, in turn driving the Strait transport. Due to the Chukotka coastline configuration, both zonal and meridional wind components contribute. [ABSTRACT FROM AUTHOR]

Eastern boundary upwelling systems (EBUS) host very productive marine ecosystems that provide services to many surrounding countries. The impact of global warming on their functioning is debated due to limited long-term observations, climate model uncertainties, and significant natural variability. This study utilizes the usefulness of a machine learning technique to document long-term variability in upwelling systems from 1993 to 2019, focusing on high-frequency synoptic upwelling events. Because the latter are modulated by the general atmospheric and oceanic circulation, it is hypothesized that changes in their statistics can reflect fluctuations and provide insights into the long-term variability of EBUS. A two-step approach using Self-Organizing Maps (SOM) and Hierarchical Agglomerative Clustering (HAC) algorithms was employed. These algorithms were applied to sets of upwelling events to characterize signatures in sea-level pressure, meridional wind, shortwave radiation, sea-surface temperature (SST), and Ekman pumping based on dominant spatial patterns. Results indicated that the dominant spatial pattern, accounting for 56%-75% of total variance, representing the seasonal pattern, due to the marked seasonality in along-shore wind activity. Findings showed that, except for the Canary-Iberian region, upwelling events have become longer in spring and more intense in summer. Southern Hemisphere systems (Humboldt and Benguela) had a higher occurrence of upwelling events in summer (up to 0.022 Events/km²) compared to spring (<0.016 Events/km²), contrasting with Northern Hemisphere systems (<0.012 Events/km²). Furthermore, long-term changes in dominant spatial patterns were examined by dividing the time period in approximately two equally periods, to compare past changes (1993-2006) with relatively new changes (2007-2019), revealing shifts in key variables. These included poleward shifts in subtropical high-pressure systems (SHPS), increased upwelling-favorable winds, and SST drops towards higher latitudes. The Humboldt Current System (HumCS) exhibited a distinctive spring-to-summer pattern, with mid-latitude meridional wind weakening and concurrent SST decreases. Finally, a comparison of upwelling centers within EBUS, focusing on changes in pressure and temperature gradients, meridional wind, mixed-layer depth, zonal Ekman transport, and Ekman pumping, found no evidence supporting Bakun’s hypothesis. Temporal changes in these metrics varied within and across EBUS, suggesting differential impacts and responses in different locations. [ABSTRACT FROM AUTHOR]

The Three-Rivers Headwater (TRH) region has experienced notable dry-to-wet shift in summer precipitation in recent decades. Despite the widespread efforts of scientific community, the causes of such climate shift remain not well-understood. Building upon an ensemble of high-resolution Lagrangian simulations and moisture source diagnostic, this study investigated the long-term changes in moisture sources with the ERA-Interim reanalysis from 1980 to 2017, particularly aiming to pose a complimentary understanding on this shift from a perspective of moisture source changes. The results show that the variability of moisture sources for the water vapor reaching the TRH region is closely and quantitatively linked to its regional summer precipitation. The dry-to-wet shift in summer precipitation in TRH is regulated by the changes in moisture sources across multiple transport time scales (from 1 to 10 days), and could be attributed to the enhanced contribution by the moisture sources originated from the Tibetan Plateau (TP), the Central Eastern Asian region (CEA), and the Arabian Peninsula (ARP), of which regional accumulative moisture contribution increased by 11.3 (0.33%), 8.61 (0.27%) and 8.53 (0.52%) mm per year, respectively. This result implies that the terrestrial source ranks the leading role in regulating the decadal shift of TRH summer precipitation, rather than that directly from the oceanic regions. Although the moisture sources are across multiple transport timescales, the relatively shorter transport distance and steady contribution (lasting 1 to 10 days) from the TP highlight the significance of strengthened local precipitation recycling process in this dry-to-wet shift. Further analysis indicates the variations in mid-latitudinal meridional winds and the low-latitudinal SST play a role of 'bridge' connecting the changes in moisture sources to enhanced precipitation during the summer season. Key points: The decadal variation of regional precipitation over the TRH region is closely and quantitatively linked with the changes in moisture sources. The moisture sources responsible for the dry-to-wet shift in summer rainfall over the TRH have been detected. The weakened mid-latitudinal meridional winds and the warming low-latitudinal sea surface temperature bridge the moisture source changes and summer precipitation over the TRH region. [ABSTRACT FROM AUTHOR]

On 4 November 2021, during the rising phase of solar cycle 25, an intense geomagnetic storm (Kp = 8−) occurred. The effects of this storm on the outer magnetospheric region up to the ionospheric heights have already been examined in previous investigations. This work is focused on the analysis of the solar wind conditions before and during the geomagnetic storm, the high-latitude electrodynamics conditions, estimated through empirical models, and the response of the atmosphere in both hemispheres, based on parameters from the ECMWF ERA5 atmospheric reanalysis dataset. Our investigations are also supported by counter-test analysis and Monte Carlo tests. We find, for both hemispheres, a significant correspondence, within 1–2 days, between high-latitude electrodynamics variations and changes in the temperature, specific humidity, and meridional and zonal winds, in both the troposphere and stratosphere. The results indicate that, in the complex solar wind–atmosphere relationship, a significant role might be played by the intensification of the polar cap potential. We also study the reciprocal relation between the ionospheric Joule heating, calculated from a model, and two adiabatic invariants used in the analysis of solar wind turbulence. [ABSTRACT FROM AUTHOR]

Based on two meteor radars in mid‐latitudes of China, the mid‐latitude lower thermospheric neutral wind responses to the 2015 St. Patrick's Day great storm are investigated. The AE and PCN indices presented the similar quasi‐5‐hour oscillations during the storm. Interestingly, the analogous and close‐correlated storm‐time quasi‐5‐hour oscillations were also observed in both the meridional wind differences at 90–102 km derived from meteor radars. The meridional wind disturbances in the lower thermosphere also showed the extension toward the lower latitudes. It has been found that the enhanced equatorward wind disturbances at 250 km estimated by the Horizontal Wind Model‐14 and Fabry‐Perot Interferometer (FPI) emerged accordingly with the increases of AE and PCN with a time delay. And the enhancements of equatorward (poleward) wind disturbances at 250 km were accompanied by the increments of equatorward (poleward) wind disturbances at 94 km with a time lag of a few hours. It is thus suggested that the multiple intensified Joule heating events with quasi‐5‐hour time intervals were triggered by the successive substorm expansions during the storm. Then the Joule heating events led to the vertical wind and temperature disturbances in the mid‐latitude lower thermosphere via disturbing the thermospheric meridional circulation, which consequently induced the quasi‐5‐hour meridional wind disturbances therein. Plain Language Summary: The neutral temperature and global circulation in the middle and upper atmosphere can be strongly disturbed by the solar and geomagnetic activity. However, the effects of geomagnetic activity on the neutral winds in the mesosphere and lower thermosphere region at middle latitudes have not been fully understood. Using the neutral wind measurements from two meteor radars located at latitudes of 53.5°N and 40.3°N around 120°E meridian in China, we found that the meridional winds at different latitudes showed the prominent and similar disturbance patterns during the 2015 St. Patrick's Day geomagnetic storm. There exhibited the obvious quasi‐5‐hour oscillations in the meridional wind differences at 90–102 km derived from meteor radars. And such wind oscillations appeared to be analogous and close‐correlated to the quasi‐5‐hour oscillations in the AE and PCN indices. The possible contributors and physical processes responsible for such wind disturbances in the mid‐latitude lower thermosphere are discussed. This study is helpful to understand the influences of geomagnetic activity on the neutral winds in the mid‐latitude lower thermosphere region as well as the possible dynamic processes therein. Key Points: Storm‐time quasi‐5‐hour oscillations emerged in both the AE/PCN indices and the meridional winds in the mid‐latitude lower thermosphereThe increments of equatorward/poleward winds at 94 km exhibited the close correspondence and correlation to those at 250 km with a time lagThe successive Joule heating events led to the periodic meridional wind disturbances via disturbing thermospheric meridional circulation [ABSTRACT FROM AUTHOR]

Over the past 30 years, the Arctic Dipole Anomaly (DA) has repeatedly led to record lows in summer sea ice extent, with cloud radiative effects (CRE) playing a crucial regulatory role. Here, we reveal the CRE variations between positive and negative DA events and elucidate the slowing impacts of CRE on sea ice thickness (SIT) changes. The DA triggers robust meridional winds and transpolar drift, markedly reducing SIT in the Beaufort Sea (BeS), Chukchi Sea (CS), and East Siberian Sea (ESS), while increasing it in the Greenland Sea (GS). CRE significantly slow SIT changes, contributing +14.4, +4.4, +16.4, and −26.7 cm to changes from June to August, against total changes of −55.9, −29.4, −39.8, and +42.8 cm in September over BeS, CS, ESS, and GS, respectively. This study underscores the key impacts of CRE on sea ice variation, emphasizing their significance in the polar climate system. Plain Language Summary: The Arctic Dipole Anomaly is one of the principal atmospheric circulation patterns in the Arctic region, significantly influencing the spatial distribution of sea ice through induced changes in polar wind directions. These wind changes also affect the fraction of Arctic clouds and, by modifying cloud‐surface radiative forcing, alter the surface energy balance. Utilizing 20 years of multi‐source observation data along with reanalysis meteorology, we observed that during the summer, a positive Arctic Dipole Anomaly significantly decreases the thickness of sea ice in the Pacific sector and increases it in the Atlantic sector. During this period, variations in cloud‐surface radiative forcing substantially slow the extent of sea ice thickness reduction/increase. In other words, without considering the impact of cloud radiative forcing, the changes in sea ice thickness would be more severe. Key Points: There is a significant reduction in both the sea ice concentration and thickness across the Pacific sector of the Arctic with Arctic Dipole Anomaly phases in summerThe Arctic Dipole Anomaly induces alterations in the wind and temperature fields, which subsequently influence the spatial distribution of Arctic cloudsVariations in clouds and surface albedo alter the surface energy balance and slow the sea ice changes caused by the Dipole Anomaly [ABSTRACT FROM AUTHOR]

In this study, we investigate the climatology and trend of explosive cyclones (ECs) over the Southern Ocean (50oS–70oS) during 1980–2020 by combining a method that is most suited for identifying and tracking cyclones in the Southern Ocean and a latest climate reanalysis. On average, approximately 50 ECs are generated annually over the Southern Ocean, with a significant increasing trend of 2.3 per decade during the studying period. This increasing trend is dominated by the trend of strong ECs, particularly in autumn. We analyze the dynamical and thermodynamical effects associated with multiple deepened strong ECs in autumn over an identified key region in the southern Pacific Ocean sector (155oW–170oW, 50oS–65oS), where the density of the initiation of ECs shows the largest increasing trend in autumn. The composite analysis reveals the general patterns and duration of the effects on the atmosphere, ocean, and sea ice associated with multiple ECs in the southern Pacific Ocean. The results indicate that the deepened strong ECs are associated with significant changes in meridional winds, downward longwave radiation, and sensible and latent heat fluxes. These changes lead to cold sea surface temperature anomalies in the northeast of the key region, reaching a maximum 5–7 days after the EC deepening, and the increased sea ice cover south of the key region, peaking 4–5 days after the EC deepening. Highlights: Explosive cyclones (ECs) over the Southern Ocean (50oS–70oS) have a significant increasing trend of 2.3 per decade during 1980–2020. The increasing trend is dominated by the trend of strong ECs, especially in autumn. The anomalies of sea surface temperature and sea ice cover can sustain about 2 weeks following the generation of strong ECs. [ABSTRACT FROM AUTHOR]

Globally available and highly vertically resolved wind fields are crucial for the analysis of atmospheric dynamics for the benefit of climate studies. Most observation techniques have problems to fulfill these requirements. Especially in the tropics and in the Southern Hemisphere more wind data are required. In this study, we investigate the potential of radio-occultation (RO) data for climate-oriented wind field monitoring in the tropics, with a specific focus on the equatorial band within ± 5° latitude. In this region, the geostrophic balance breaks down, due to the Coriolis force term approaching zero, and the equatorial-balance equation becomes relevant. One aim is to understand how the individual wind components of the geostrophic-balance and equatorial-balance approximations bridge across the Equator and where each component breaks down. Our central aim focuses on the equatorial-balance approximation, testing its quality by comparison with ERA5 reanalysis data. The analysis of the zonal and meridional wind components showed that while the zonal wind was well reconstructed, it was difficult to estimate the meridional wind from the approximation. However, we still found a somewhat better agreement from including both components in the zonal-mean total wind speed in the troposphere. In the stratosphere, the meridional wind component is close to zero for physical reasons and has no relevant impact on the total wind speed. In general, the equatorial-balance approximation works best in the stratosphere. As a second aim, we investigated the systematic data bias between using the RO and ERA5 data and find it smaller than the bias resulting from the approximations. We also inspected the monthly-mean RO wind data over the full example year of 2009. The bias in the core region of highest quality of RO data, which is the upper troposphere and lower stratosphere, was generally smaller than ± 2 ms-1. This is in line with the wind field requirements of the World Meteorological Organization. Overall, the study encourages the use of RO wind fields for regional-scale climate monitoring over the entire globe, including the equatorial region, and also showed a small improvement in the troposphere when including the meridional wind component in the zonal-mean total wind speed. [ABSTRACT FROM AUTHOR]

The equations governing the neutral wind velocities and temperature, which depict the oscillations of atmospheric tides, incorporate various elements of the atmosphere. These factors comprise background wind, temperature profiles, and the atmosphere's composition, including ozone, oxygen, carbon dioxide, water, etc. The current work describes the method of solution of the atmospheric tidal model and investigates the effects of background wind and dissipation processes on the migrating semidiurnal tides during the March equinox. Thermal heating due to water vapour absorption in the troposphere, ozone in the stratosphere and oxygen absorption in the thermosphere is considered in this model. Furthermore, the model takes into account the various dissipation processes (i.e. ion drag and Hall coefficients, Newtonian cooling, divergence of momentum and heat fluxes due to molecular and eddy diffusion) and background wind. The semidiurnal tidal features obtained from the present model match well with the Global Scale Wave Model (GSWM-98). The bar charts are presented to quantify the effects of different parameters on horizontal winds for altitudes below 100 km and above 100 km, separately. It reveals that the background wind plays a vital role in deciding the semidiurnal tidal amplitudes of zonal and meridional winds. Momentum and heat flux coefficients also have a significant influence on semidiurnal tides. At higher altitudes (>100 km), the ion drag reduces the amplitude of semidiurnal tides considerably. [ABSTRACT FROM AUTHOR]

Based on the reanalysis data o£ the National Center for Environmental Prediction (NCEP) and the precipitation dataset of the U. S. Climate Prediction Center (CPC), the changing trend of summer precipitation in North China (35° -40° N, 110° - 125° E) during 1979 - 2020 was studied. By calculating the monthly climatic precipitation in North China, it is found that precipitation was mainly distributed from June to August, so the trend of precipitation in North China from June to August was mainly analyzed. Firstly, the five-point moving average of regional mean precipitation in North China from June to August during 1979 - 2020 was conducted. It is found that the fitting curve of the five-point sliding average was basically consistent with the changing trend of regional precipitation, and it showed a certain upward trend. Secondly, the cumulative anomaly of regional average summer precipitation in North China showed a significant upward trend after 2005, which was similar to the moving average result, indicating that the precipitation in the later period increased compared with the earlier period. The changing trend of summer precipitation in North China in the past 42 years was analyzed, and the results show that precipitation showed a significant increasing trend in most areas of North China, so that regional average precipitation also tended to increase significantly. By comparing the precipitation in the past five years (2016 -- 2020) and the last 36 years (1979 -2015), it is found that the increase of summer precipitation in North China was more obvious, so the reasons for the increase in precipitation were further analyzed. Since the occurrence of precipitation requires favorable thermal dynamic conditions, the one-dimensional linear regression of water vapor content at 850 hPa and meridional wind speed was conduced, and it is found that the two variables tended to increase obviously, which was consistent with the increasing trend of precipitation. Seen from both the results of regional average and the spatial distribution of trends, the lower atmospheric water vapor content and wind speed showed a significant positive trend, which led to the increase of summer precipitation. Therefore, it can be concluded that there was a certain changing trend of summer precipitation in North China in the past 42 years, which can provide certain reference for the future forecast of summer precipitation in North China. [ABSTRACT FROM AUTHOR]

The anomalous zonal wind moves southward during the ENSO mature phase in boreal winter. Previous studies suggest that it may be caused by the nonlinear interaction of annual cycles or the influence of background mean state changes. In this research, the ECHAM4.6 atmospheric model is used to confirm the mechanism of the anomalous zonal wind southward shifting. The annual cycle of solar radiation and SST are removed in the sensitivity experiments to avoid the interaction between the ENSO and annual cycle. The results show that the north–south asymmetry mode of the ENSO anomalous wind field is not the result of a nonlinear interaction between ENSO and the annual cycle. The mean v-winds in winter motivate the southward shifting of the ENSO anomalous wind field through advection. [ABSTRACT FROM AUTHOR]

The Arabian Sea (AS), a basin in the North Indian Ocean (NIO), possesses strong coastal upwelling in its eastern and western margins. The spatio-temporal variability of upwelling over Somalia, Oman and the southwest coast of India is analysed using observed datasets. The spectral analysis of upwelling indices highlights that the semi-annual and annual frequencies were dominant by two orders of magnitude compared with the inter-annual variability over these upwelling zones. The inter-annual variability of coastal upwelling was demarcated with variations in the causative forcing of upwelling associated with the positive modes of Indian Ocean Dipole (pIOD), El-Niño Southern Oscillation (El-Niño) and combined pIOD and El-Niño events over the southwest coast of India (SWCI) and Somalia. Along SWCI, the sea surface temperature and meridional wind stress weakened leading to warming associated with the suppression of upwelling during El-Niño, pIOD and combined events. Along Somalia, the upwelling favourable wind stress showed slight weakening due to pIOD and strong decline due to El-Niño and combined modes. Along Oman, the upwelling favourable wind stress indicated negligible changes due to pIOD, an increase due to El-Niño and a slight decline due to combined modes. [ABSTRACT FROM AUTHOR]

In this study, based on the OI135.6 nm night airglow data of the FY‐3D Ionospheric Photometer (IPM) during the 2018–2021 geomagnetically quiet period, the global wavenumber 4 longitudinal structure of the equatorial ionization anomaly (EIA) at 2:00 local time was discovered, and the component of the wavenumber 4 was extracted from these structures. Compared with the OI135.6 nm night airglow data of the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) F18 during 2011–2014, there were significant differences in the variation pattern of the relative amplitude of the two versus solar activity and the seasonal variation in the proportion of the component of the wavenumber 4. Based on the neutral wind speed observation results of the Michelson Interferometer for Global High‐Resolution Thermospheric Imaging on board the Ionospheric Connection Explorer (ICON) from 2020 to 2021, the longitudinal structures of the 4 ionospheric waves after midnight are related to the cross‐equatorial meridional wind. We believe that the wavenumber 4 longitudinal structures after midnight originate from the semidiurnal eastward‐propagating with zonal wavenumber 2 (SE2) nonmigrating tide in the cross‐equatorial neutral wind rather than the diurnal eastward‐propagating with zonal wavenumber 3 (DE3) nonmigrating tide in from the zonal wind, which modulates the daytime wavenumber 4 longitudinal structures. Plain Language Summary: The longitudinal structures of EIA have been extensively studied by using satellite data. However, there are few observations and studies, due to the weak ionosphere near midnight. In this paper, we studied the longitudinal structures of EIA at 02:00 local time during geomagnetically quiet period, benefitted from the satellite orbits and high sensitivity of FY‐3D IPM. We found that the wavenumber 4 longitudinal structures of EIA still exist at 02:00 local time and are obvious at equinoxes. Compared with SSUSI F18 data, FY‐3D IPM data showed different characteristics of wavenumber 4 component of EIA longitudinal structures. Because of the different local time of data between SSUSI F18 and FY‐3D IPM, we consider that the longitudinal wavenumber 4 structures of EIA after midnight originated from the cross‐equatorial neutral wind rather than the electric field modulated by zonal neutral wind in daytime. Key Points: The wavenumber 4 longitudinal structures of equatorial ionization anomaly (EIA) still exist after midnight and are more obvious at equinoxesThe wavenumber 4 component of EIA after midnight is positively correlated with the solar activity levelThe origin of the wavenumber 4 longitudinal structures of EIA after midnight is the modulation of the cross‐equatorial neutral wind [ABSTRACT FROM AUTHOR]

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In recent decades, the existence of a relationship between snow cover on the Tibetan Plateau (TP) and East Asian summer monsoon (EASM) rainfall has been emphasized. According to recently published studies this snow- monsoon relationship experienced a shift after 1990. Although the changing snow-monsoon relationship has been studied, the causes of the interdecadal changes remain unclear. This study assesses the associations of TP spring snow cover with EASM rainfall before and after 1990 and explores what possible mechanisms could be responsible for the interdecadal changes. Correlation and composite analyses were used to assess the strength of the relationship between TP spring snow cover and EASM rainfall and to analyze the atmospheric and land surface patterns associated with high snow cover. The outcomes suggest that the relationship between TP spring snow cover and EASM rainfall changes from partially negative to positive over all regions of the TP from 1968-1990 (P1) to 1991-2019 (P2), implying that more snow cover is associated with less (more) EASM rainfall during P1 (P2). In P1, years with high snow cover are associated with an anomalous cyclone southwest of the TP (positioned over Iran and Pakistan) in spring, which persists into the following summer, partly due to the underlying snow cover. The anomalous cyclone is accompanied by downstream anomalies over East Asia, which form a strong east-west oriented wave pattern and induce a northerly inflow of dry air over East Asia, reducing rainfall over the northern EASM domain. In P2, high snow cover years are associated with an anomalous cyclone over the western TP, which weakens and loses its significance in May-June and summer, partly due to a decline in snow forcing. Southeastward propagation of wave energy in May-June initiates the formation of an anomalous anticyclone over southeastern China and the western North Pacific. Concurrently, a meridional circulation develops over East Asia that enhances the southwesterly moisture inflow, resulting in increased EASM rainfall. The changing snow-monsoon relationship can be linked to different wave train patterns resulting from changes in the background zonal wind and meridional temperature gradients. This research contributes to a better understanding of the changing snow-monsoon relationship. [ABSTRACT FROM AUTHOR]

Temporal oscillations in the IMF Bz associated with Alfvén waves occur frequently in solar wind, with a duration ranging from minutes to hours. Using Swarm observations, Fabry–Pérot interferometer measurements at Mohe station, and Thermosphere–Ionosphere–Electrodynamic General Circulation Model simulations, the perturbations of zonal (ΔUN) and meridional (ΔVN) winds due to temporal oscillations in the IMF Bz on 23–24 April 2023 are explored in the following work. ΔUN is strong westward with a speed of greater than 100 m/s at pre-midnight on 23–24 April. This phenomenon is primarily driven by the pressure gradient, offsetting by the ion drag and Coriolis force. On 23 April, ΔVN is weak northward at the pre-midnight and strong southward at a speed of ~200 m/s at pre-dawn. On 24 April, ΔVN is strong (weak) northward at pre-midnight (pre-dawn). It is mainly controlled by a balance between the pressure gradient, ion drag, and Coriolis force. [ABSTRACT FROM AUTHOR]

This study investigates the downward motion of Intermediate E-F Layers (ILs) in the Brazilian low latitude sector through observation and modeling. Ionosonde data from São José dos Campos (SJC) and Palmas (PAL) were analyzed to investigate the seasonal variation of the IL parameters, including the virtual height (h'IL) and the top frequency (ftIL). The ILs primarily originated from F layer detachment followed by downward motion, peaking before 11 LT and disappearing well before sunset. Daily height variability ranged between 130 and 190 km, with peak frequencies around 4-5 MHz. Using meteor radar data as input, the Ionospheric E-region Model (MIRE) simulated diurnal and semidiurnal tides to analyze neutral wind effects on ILs descent. Model simulations for SJC (October 2008) and PAL (April and June 2009) revealed distinct wind oscillations influencing IL dynamics at heights below 140 km. In SJC, meridional wind shears controlled IL descent, with possible zonal wind interactions weakening ILs. Conversely, in PAL during April 2009, both zonal and meridional winds contributed to IL formation and altitude descent. However, discrepancies between observed and modeled descent rates suggest the need for considering additional atmospheric wave interactions in future modeling studies. June 2009 over PAL presented unique IL behavior, exhibiting a lower observed decay rate and daily height oscillations potentially linked to local modulations. Meanwhile, MIRE indicated that meridional wind shearing predominantly controlled IL descent in the morning, with zonal wind becoming relevant post-midday. These findings enhance our understanding of IL dynamics and their atmospheric drivers. [ABSTRACT FROM AUTHOR]

Variability of non-migrating diurnal tides are investigated using Canadian Middle Atmosphere Model (CMAM30) in zonal and meridional wind data from 1979 to 2010. Diurnal stationary tide (DS0) is found to maximise during boreal summers in both hemispheres in the middle atmosphere while the diurnal westward propagating tide of wavenumber two (DW2) maximises during boreal winters in both hemispheres. Both tides show strong annual variations over sub-tropical regions centered over ± 25 ° latitudes along with significant short term variability from 20 to 100 days. The investigations of short term variations of DW2 show significant correspondence with the variability of stationary planetary wave of wavenumber one (SPW1) in the high latitudes in both hemispheres as well as with DW2 amplitudes from lower atmosphere. It is thereby concluded in the current study that DW2 in the mesosphere is not only produced by non-linear interactions of SPW1 and the diurnal migrating tide (DW1) but also has contributions from sources in the lower atmosphere. On the other hand, variability of DS0 does not show any similarity with that of SPW1 and thereby indicate sources only from the lower atmosphere. In the high-latitude stratosphere, DS0 and DW2 amplitudes are negligible and thereby the contribution of non-linear interactions to their generation in this region is minimal. [ABSTRACT FROM AUTHOR]

In the North West Shelf region of Australia is a surface current (Holloway Current), which flows southwestward along the shelf break. This paper describes a seasonal undercurrent below the Holloway Current. A 5-day climatology is constructed from the output of an eddy-resolving oceanic general circulation model (OGCM). A seasonal northeastward-flowing undercurrent is found on the upper continental slope during the climatological April-May. This undercurrent reverses during February-March. During its annual cycle, the phase of the undercurrent tends to propagate southwestward and upward. The annual frequency dominates, but the positive and negative phases of the undercurrent are not symmetric in the yearly cycle because of the contributions from the semi-annual and 1/3-annual components. We propose a hypothesis that this undercurrent is a beam of coastal trapped wave (CTW). As an initial attempt to assess the plausibility of this hypothesis, we construct an idealized linear coastal-trapped wave (CTW) solution driven by an idealized harmonic meridional winds at the annual frequency. The solution takes the form of a beam originating from the forcing region on the continental shelf and propagating offshore and southward. When it emerges on to the continental slope, it takes the form of an undercurrent. This idealized solution shares several properties with the undercurrent in the OGCM despite several discrepancies. [ABSTRACT FROM AUTHOR]

Sea surface winds influence shipping, fisheries, and coastal projects. However, the current sea surface wind forecast exhibits noticeable biases. This study introduces a deep learning (DL)‐based bias correction model, WindNet, to improve the Global Forecast System (GFS) sea surface wind field forecast in the Northwest Pacific Ocean (NWPO). WindNet reduces the Root Mean Squared Errors (RMSEs) of wind speed at lead times of 24, 48, and 72 hr from 1.41–1.95 to 1.11–1.55 m s−1, achieving percentage reductions of 20.51%–21.28%. Simultaneously, the RMSEs of wind direction are reduced from 29.67–41.45° to 25.38–36.81°, demonstrating percentage reductions of 11.19%–14.46%. During typhoon passages, the RMSEs of wind speed and direction at three forecast lead times after using WindNet are reduced from 1.57–2.42 to 1.24–1.95 m s−1 and from 30.31–42.35° to 25.88–37.64°, demonstrating percentage reductions of 19.42%–21.02% and 11.12%–14.62%. By integrating a Squeeze‐and‐Excitation Network into WindNet, we find that utilizing information from the circulation field, apart from the zonal and meridional wind components at 10 m height, is crucial for the correction of the sea surface wind speed. WindNet can effectively capture the non‐linear relationship between other low‐level‐circulation‐related variables and sea surface wind speed. Therefore, WindNet remarkably enhances sea surface wind field forecast accuracy in NWPO. Plain Language Summary: We have developed a deep learning‐based bias correction model, WindNet, to enhance the forecast accuracy of the sea surface wind field in the Northwest Pacific Ocean from the Global Forecast System. The results reveal that the application of WindNet effectively reduces forecast biases in both sea surface wind speed and direction. Notably, the WindNet model exhibits outstanding performance during typhoon passages. This model provides valuable insights for improving the accuracy of wind field forecasts in other regions in the future. Key Points: A deep learning (DL)‐based bias correction model for improving forecast accuracy of sea surface wind is proposedThe DL model enhances the forecast accuracy of sea surface wind speed, leading to >20% reduction in root mean square error at a lead time of up to 72 hrThe DL model maintains excellent performance even in extreme weather conditions such as typhoons [ABSTRACT FROM AUTHOR]

Using the fifth generation European Center for Medium-range Weather Forecasts (ERA5) reanalysis data, we present a detailed examination of the climatological features of convectively coupled Kelvin waves (CCKWs) over the Indian and Pacific basins. The composited horizontal structure of Indian CCKWs resembles the theoretical Kelvin waves, with a maximum wave response at the equator. In contrast, the Pacific counterpart exhibits a very different pattern, characterized by a significant northward shift of the convective center, along with enhanced meridional winds and a relatively stronger wave response. The moist static energy (MSE) budget analysis is conducted to elucidate the physical factors that control the energetics of CCKWs. Despite the marked contrast in horizontal structure between Pacific and Indian CCKWs, the energy cycle and the physical factors that maintain this cycle are rather similar. During the recharge period (days -2 and -1), the column process (including vertical MSE advection, apparent heat source and moisture sink) functions to destabilize the atmosphere by importing the MSE; while the horizontal MSE advection tends to destabilize the atmosphere on day -2 but starts to stabilize the atmosphere earlier on day -1. During the discharge and transition period (from days 0 to + 2), the column process functions to stabilize the atmosphere by exporting the MSE; while the horizontal MSE advection inclines to stabilize the atmosphere on days 0 and + 1 but again starts to destabilize the atmosphere earlier on day + 2. The leading of horizontal MSE advection to the recharge-discharge cycle clearly points out the importance of the former in driving the eastward propagation of CCKWs. Both the horizontal MSE advection and column process are vital in maintaining the energy cycle of CCKWs, as they often take turns leading the role in recharging and discharging the atmosphere. [ABSTRACT FROM AUTHOR]

In the mid–high latitude atmosphere, the instability of planetary waves characterizes enhanced planetary wave activity or amplified large‐scale waviness leading to increased regional weather extremes. In this paper, a nonlinear Schrödinger equation is derived to describe the evolution of planetary waves. Then the consequences of Arctic amplification (AA)‐induced meridional background potential vorticity (PVy) changes on the modulational instability of planetary waves are examined. It is found that the modulational instability of uniform planetary wave trains mainly results from the presence of high‐order dispersion and nonlinearity, even though such an instability depends on the amplitude, vertical structure and zonal wavenumber of uniform planetary waves and the atmospheric stratification. Because the nonlinearity and high‐order dispersion depend on the magnitude of PVy, the modulational instability of planetary waves is significantly influenced by the variation of PVy associated with AA. It is also revealed that stronger modulational instability of planetary waves tends to occur in the smaller PVy region or in higher latitudes due to both stronger nonlinearity and weaker high‐order dispersion for fixed background and planetary wave parameters, which is conducive to more intense large‐scale waviness. However, because AA can reduce PVy in the mid–high latitudes mainly in the lower troposphere via reductions of winter zonal winds and meridional temperature gradients, the reduced PVy under AA can significantly enhance the modulational instability. Thus, the role of AA is to amplify planetary wave activity in mid–high latitudes through strengthening the modulational instability of planetary waves due to reduced PVy, which further enhances large‐scale waviness. [ABSTRACT FROM AUTHOR]

The instabilities produced by a linear model of the tropical atmosphere coupled to a prognostic equation for water vapour are investigated. The basic model with no meridional wind supports unstable eastward‐propagating waves. For parameter regimes relevant to the Indo‐Pacific warm pool, the long‐time asymptotic behaviour of the unstable waves is found to be absolutely unstable, so that the amplitude of disturbances will grow in time at every point in the domain. The absolute instability of the system is realized at planetary length‐scales and intraseasonal frequencies. Other parameter choices for the system do not produce this same behaviour at these length‐ and time‐scales. It is shown that the resultant long‐time behaviour of the instability is characterized by roughly equal roles for temperature and moisture fluctuations in setting the thermodynamic tendency of the waves. With the inclusion of momentum damping, the phase speed of the absolutely unstable solution is about 7 m·$$ \cdotp $$s −1$$ {}^{-1} $$. Addition of a strong background meridional moisture gradient, as in recent studies on the Madden–Julian Oscillation (MJO), appears to remove the absolute instability from the system. In a background state that varies slowly in the zonal direction, it is shown analytically that localized regions of instability may be formed, again using parameter choices relevant to the warm pool. The dynamics and thermodynamics of these local instabilities show some correspondence with the observed development of the MJO as it propagates through the warm pool. [ABSTRACT FROM AUTHOR]

We explore the effects of lower thermospheric water vapor deposited by launch vehicle plumes on polar mesospheric cloud (PMC) frequencies at 80°N. We use July‐averaged PMC frequencies from 2007 to 2022 from the Cloud Imaging and Particle Size (CIPS) instrument on NASA's Aeronomy of Ice in the Mesosphere (AIM) satellite. Launch sites worldwide are typically located near northern mid‐latitudes. Using the orbital launch record for the same time period, we find that the number of launches correlates with PMC frequencies with a coefficient of r = 0.60, which increases to r = 0.75 when only selecting launches from 2.5 to 21.5 local time (LT), indicating a weak LT dependence on global‐scale transport to 80°N. To support our findings, we use meridional winds from the Michelson Interferometer for Global High‐resolution Imaging experiment on NASA's Ionospheric Connection Explorer satellite and winds from the Horizontal Wind Model climatology to interpret the northward motion of air parcels at 105 km. We find the launch LT window that maximizes the correlation coefficient to be consistent with the expected maximum northward motion from the diurnal variation of mid‐latitude meridional winds. Comparisons with Microwave Limb Sounder satellite observations of upper mesospheric temperature and water vapor reveal a strong dependence of cloud frequency on water vapor (r = 0.86) but not on temperature (r = −0.26), indicating that water vapor is the primary source of PMC variability for the bright PMCs at 80°N. We therefore find that launch vehicle plumes originating primarily from northern mid‐latitudes modulate PMC frequency at 80°N in July. Plain Language Summary: Formation of clouds in the mesosphere in the polar summer depends heavily on atmospheric water vapor. Main engine exhaust plumes from space shuttles and other launch vehicles contain many tons of water vapor which can be transported poleward at an altitude of ∼105 km. Over several days this water vapor descends to ∼80–85 km and aids in the formation of summertime polar mesospheric clouds (PMCs). This work shows how the number of launches per year correlates with the mean interannual PMC frequency averaged over July at 80°N latitude. The correlation maximizes for vehicles launched from 2.5 to 21.5 local time. To provide additional important evidence for the poleward transport of plumes launched at these local times, we use wind data to quantify the maximum northward motion of an exhaust plume over the diurnal cycle near 105 km altitude. We also use satellite temperature and water vapor data at PMC altitudes to show that bright PMCs vary more with the water vapor content than with the air temperature. We conclude that the total water vapor deposited by summertime space traffic worldwide modulates the PMC frequency at 80°N. Key Points: The correlation between orbital rocket launches and polar mesospheric clouds at 80°N in July from 2007 to 2022 is quantifiedMeridional wind, temperature, and water vapor data are used to support causal interpretation of the correlation coefficientsEvidence indicates that water vapor in space traffic exhaust plumes modulates frequencies of polar mesospheric clouds at 80°N [ABSTRACT FROM AUTHOR]

The OI 630.0 nm airglow emission variability provides salient information on the dynamical changes taking place in the upper atmosphere at around 250 km. The emission rates vary with the changes in the ambient electron densities and the neutral constituents that are associated with these emissions. On several occasions, enhancements in these emissions are observed during post‐sunset hours, around 21 local time (LT), as measured from Mt. Abu (24.6°N, 72.7°E, 19°N Mag), a low‐latitude location at Indian longitudes. These enhancements occur following the typical monotonic decrease in emission intensity after sunset. The presence of poleward meridional wind preceded by cessation and reversal of equatorward wind at the post‐sunset hours was shown to be the cause for such observed emission enhancements in an earlier study. In this study, the cause of such reversal in meridional winds during post‐sunset hours has been investigated using the variation in electron densities and meridional winds simulated by the Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCM‐X), which also shows enhancements in electron densities similar to those observed in the post‐sunset OI 630.0 nm nightglow emissions, and simultaneous reversal in meridional winds as well. The amplitudes and phases of different components of tides obtained from WACCM‐X meridional winds reveal a significant contribution of higher‐order tides, especially, quarter‐diurnal tides, to the observed reversal in the meridional winds during post‐sunset hours. Plain Language Summary: Airglow emissions occur when atoms and molecules are deexcited after being excited by the solar energetic photons. OI 630.0 nm emissions (redline), which peak around 250 km and have a width of emissions around 100 km, act as tracers of that altitudinal region. Typically, the redline emissions decrease monotonically with time after sunset in the absence of solar radiation. However, on several occasions, an enhancement in redline emissions has been measured during post‐sunset hours over a low‐latitude location in the Indian longitudes. A previous study attributed this enhancement to the increase in electron density at the emission altitudes in the presence of poleward meridional winds, despite usually being equatorward during such times. The cause of such reversal in the equatorward winds and simultaneous increase in electron density at post‐sunset hours has been examined in this study using the variation in electron densities and winds simulated by the Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCM‐X). Remarkably, the simulation results also show enhancements in electron density similar to those observed in redline emissions. A strong tidal contribution, especially quarter‐diurnal component, has been found to be the cause behind the poleward reversal of meridional winds after sunset that causes an enhancement in redline emissions. Key Points: OI 630.0 nm post‐sunset emission enhancement over low‐latitudes is consistent with the presence of poleward meridional windsOn some days, WACCM‐X simulated meridional winds show a poleward wind reversal during post‐sunset hoursQuarter‐diurnal tides seem to play a significant role in reversing the meridional winds after sunset [ABSTRACT FROM AUTHOR]

A general decline in foundation species at the rocky intertidal has been observed during the last decades all around the world and primarily related to climate change. In agreement with that trend, the mussel aquaculture sector in Galicia (NW Spain), the main production area in Europe, has warned over the last years about a decline in the availability of wild mussel seed from the rocky coast. Here we compile for the first time, mussel seed collection reports by mussel farmers in Galicia for the period 2006–2021. We employed that dataset as a proxy of mussel recruitment evolution in the rocky shore for the last 16 years. Temporal analysis of our data confirmed the reported decline (-148 t yr-1), particularly pronounced from 2012 onwards. The data base also allowed us to analyze inter-annual variability according to both, climatic variations and management scenarios. Since cultivated mussels conform a meta-population with wild mussels from the rocky shore, alterations on the market preferences towards smaller individuals at harvest, could also contribute to a reduction in reproductive output. Our results show a decrease in life-time egg production under certain scenarios. Nonetheless, coastal upwelling seems to be the largest factor conditioning recruitment abundance, explaining as much as 60% of the variability observed. Decline on recruitment abundance was highly modulated by the observed increment in frequency of intense upwelling events, exceeding 500 m3 km-1 s-1 between July and November. Meridional winds also determined the spatial recruitment patterns, pointing to the large role of wind forcing on mussel larval dispersal. Our results highlights how alterations on upwelling regimes related to climate change can interact with mussel population dynamics and also condition aquaculture sustainability and food security. [ABSTRACT FROM AUTHOR]

We report results from a global simulation of the September 2017 geomagnetic storm. The global model comprises the ionospheric code SAMI3 and the atmosphere/thermosphere code WACCM‐X. We show that a train of large‐scale EPBs form in the Pacific sector during the storm recovery phase on 8 September 2017. The EPBs are associated with storm‐induced modification of the zonal and meridional winds. These changes lead to an eastward electric field which in turn causes an upward E × B drift in the post‐midnight sector. A large decrease in the Pedersen conductance caused by meridional equatorward winds leads to an increase in the growth rate of the generalized Rayleigh‐Taylor instability that causes EPBs to develop. Interestingly, several EPBs reach altitudes above 3,000 km. Plain Language Summary: The uppermost layer of the atmosphere, the thermosphere, is heated at high latitudes during geomagnetic storms by energy inputs from the magnetosphere. This heating significantly modulates the thermosphere winds on a global scale that results in the modification of the electrodynamics of the ionosphere at low‐ to mid‐latitudes. Using the coupled SAMI3/WACCM‐X model, we show that equatorial plasma bubbles (EPBs) (large‐scale depletions of the electron density in the ionosphere) can develop because of these stormtime changes to the winds and electric field. This is significant because EPBs can adversely impact space‐based communication and navigation systems by degrading the reception of electromagnetic signals that pass through them. Key Points: Stormtime modulation of the zonal and meridional winds increase the eastward electric field at night in the Pacific sectorEquatorial plasma bubbles subsequently develop in the Pacific sector during the September 2017 storm on September 8Several equatorial plasma bubbles rise to over 3,000 km with upward velocities exceeding 300 m/s [ABSTRACT FROM AUTHOR]

Sporadic E (Es) layers are thin enhanced ionization patches. The electron density of Es layer is very high, up to 100 times that of the regular E layer. Therefore, Es layer has a significant effect on radio wave propagation. The formation mechanisms of Es layers is very complicated, so it needs multi-observations and simulations research. Combined observations using an ionosonde, very high frequency (VHF) coherent backscatter radar, VHF broadband backscatter radar, and meteor radar were performed. The height and irregular structure of mid-latitude Es layers observed by ionosonde, VHF coherent backscatter radar and VHF broadband backscatter radar were compared with the zonal and meridional winds observed by meteor radar, the role of wind shear on the formation of Es layer was confirmed, and examples were also found where wind shear theory could not explain and apply to the formation of Es layer. We simulated the formation of Es layers based on wind shear theory. The results of simulations and observations revealed other potential role for Es layer formation besides wind shear. By comparing the height of turbopause with that of Es layer, it was found that the variation of the altitudes of turbopause and Es layer was consistent over different latitude stations, and the effect of the turbopause on the formation of Es layer was proposed. The combined effects of eddy diffusion below and molecular ion collision above turbopause can prevent metal ions from drifting downward and promote their convergence to form thin plasma layers at the turbopause height. The turbopause accumulation effect is considered as a potentially important mechanism for the formation of Es layers besides wind shear theory. The study in the paper is helpful for analysis of physical origin and modeling of Es layer. [ABSTRACT FROM AUTHOR]

Impact of cloud vertical structure (CVS) on a northward-progressing rainfall episode of the East Asian summer monsoon (EASM) is explored using the Weather Research and Forecasting model, in which CloudSat observation-based vertical structure of cloud liquid water content (LWC) can be imposed. Composite LWC anomaly from CloudSat data shows a northward tilted structure from the upper to the lower troposphere. Compared to the control simulation (without modification of LWC), the one with LWC imposed, but without tilted structure, doesn't show significant changes. When LWC is introduced and northward tilted, the geopotential height (HGT) decreases in the north of the convective center, which increases the meridional wind and provides favorable conditions for the northward shift of the precipitation belt. When LWC is southward tilted, HGT decreases in the middle and lower troposphere in the south of the convective center and increases in the north, which slows down the northward shift of the precipitation belt. Adding cloud water leads to increase in humidity and decrease in temperature, causing significant increase in stratiform clouds and related precipitation. In the configuration of northward tilted LWC, low-temperature and high-humidity area is located on the north side of the convective center, favorable for the occurrence and northward shift of the precipitation belt. Deep convection is weakened with convective precipitation reduced, while shallow convection enhances the latent heat release in the lower troposphere. Therefore, more water vapor and energy are transported from boundary layer to free atmosphere, promoting the northward shift of the precipitation belt. [ABSTRACT FROM AUTHOR]

The investigation of the height variations of the westerly in the mid-latitudes of the Northern Hemisphere is crucial for monitoring changes in the intricate atmospheric system. This study examined the hypothesis of changes in the thickness of the westerly domain by analyzing the height of the 500 hPa pressure level. To this end, monthly data from the European Centre for Medium-Range Weather Forecasts (ECMWF) spanning from 1959 to 2021 was utilized to investigate three components: geopotential height (GH), zonal, and meridional wind components. The dataset underwent statistical tests, including trend analysis using Least Squares Error linear regression (LSE-LR) and shift (jump) analysis using the Standard Normal Homogeneity Test (SNHT). The results revealed a significant increasing trend in the height of the troposphere and the westerlies in the mid-latitudes of the Northern Hemisphere, accompanied by significant fluctuations, indicating an increase in the height of the 500 hPa pressure level and a decrease in the thickness of the westerly domain. The extent of these trends was observed to be greater in cold months, particularly in January and February, while the changes were minimal in November. Notably, the spatial distribution of upward jumps, indicative of change, was highest in January, July, and August in the eastern and southeastern regions of Asia. Furthermore, the convergence section of the westerlies trough exhibited a substantial number of jumps, particularly in March, spanning Central America and southern parts of North America. Regarding decadal coincidence onset, the highest number of jumps occurred in February, encompassing the western Pacific Ocean, western Atlantic Ocean, Eastern Europe, and western Asia. The end of these jumps coincided in February, and the spatial proximity of the end times was closer compared to the onset times in February, March, April, July, and August. [ABSTRACT FROM AUTHOR]

Using an optical band index generated from Landsat 8 OLI data, an inventory of Antarctic supraglacial lakes (size >0.01 km2) during 2014-2023 is presented. Shaded snow, cloud and rock shadow were removed by using a band-based filter. Over the north Antarctica, we identified 639 lakes covering an area of 520.79 km2, 1025 lakes covering 272.82 km2 over the East Antarctica, and 1045 lakes covering 93 km2 over south Antarctica. The influence of air temperature, 10-m zonal/meridional wind components and climate indices on surface melt were investigated. Over the north Antarctica, air temperature promoted melt, while Föhn and katabatic winds warmed adiabatically along the leeward side of the elevated features and initiated melt in south Antarctica. Nearly all areas have exhibited a decrease in melt over the past few years, and the number of melt cases dropped after 2020. Negative SAM index favoured melt over the north Antarctica. This study identified new SGLs, which will be a benchmark for further studies. The undulating terrain posed a difficulty for collecting and interpreting satellite data, which should be addressed using advanced technologies and algorithms. [ABSTRACT FROM AUTHOR]

This study investigates contributions of solar radiation and geomagnetic activity of consecutive 27-day recurrent geomagnetic storms (RGSs) to the variabilities in the equatorial ionospheric F-region in American Peruvian sector during 2007. Results show the ionospheric responses to the RGSs are quasi-periodic and multifaceted with highly evolved in the summer months. In High-Intensity Long-Duration Continuous A E Activity (HILDCAA) events, the ionospheric responses are more variable than in non-HILDCAA. The critical frequency and peak height of the F-layer tend to increase during storm-time in summer months. The maximum density enhancements are more than 70% in the three RGSs and they are long-lasting in the summer months. A new classification of daily variations in the virtual height of the F-layer ( h ′ F ) is proposed: Mode A shows mixing of great height before noon and low height near midnight, Mode B shows moderate height near midnight, and Mode C shows mixing of high height before noon and great height near midnight. These h ′ F Modes efficiently characterize the ionospheric variabilities and processes. The great uplifts of h ′ F during night-time in the summer months coincide with the presence of strong disturbance dynamo electric fields and disturbed neutral winds generated by intensified Joule heating. The solar EUV plays a role in the uplifts of h ′ F during the daytime. Zonal electric field disturbances and perturbations in the neutral meridional winds critically contribute to the equatorial ionospheric responses and ESF variabilities. Most cases of inhibited/suppressed ESF were observed in Mode A and occurred under overshielding conditions. The inhibited ESF associated with h ′ F not raised in the recovery phase is mainly contributed by a cooling state after great uplifts by daytime thermospheric winds. [ABSTRACT FROM AUTHOR]

Based on the monthly average precipitation data from meteorological stations on the Tibetan Plateau (TP), this study reveals the spatial distribution characteristics of the north-south reverse early summer (May-June) precipitation during 1979–2019. Through analysis of the circulation patterns associated with early summer precipitation, we find that the interannual variation of the precipitation on the TP is closely related to the meridional wind around the 32.5°N (ITPv). The results indicate that during anomalously strong (weak) ITPv, the TP is dominated by southerly (northerly) wind anomalies, resulting in more (less) precipitation in the northern and less (more) precipitation on the southern TP. The pattern is attributed to water vapor convergence and divergence in the northern and southern regions of TP, respectively. The meridional wind anomaly is linked to anomalous strong (weak) southwesterly winds in the southern upstream region and anomalous weak (strong) westerly winds in the northern upstream region of the TP. [ABSTRACT FROM AUTHOR]

The Yangtze River Valley (YRV) experienced an unprecedented heatwave in midsummer of 2022. Still, the detailed physical processes involved in the influence of abnormal large-scale atmospheric circulation on the heatwave remain unexplored. Here, we show that the positive meridional gradient of anomalous atmospheric moisture at the middle-lower troposphere and associated extreme dry air advection over the YRV are vital prerequisites for forming the 2022 YRV heatwave. The 2022 YRV heatwave is dominated by interannual variability, contributing 72.7% to the total temperature anomalies. Diagnosis of the surface heat budget equation indicates that the surface cloud radiative forcing is the most critical process in driving the 2022 YRV heatwave, which is dominated by the positive surface short-wave cloud radiative forcing associated with the suppressed precipitation and the middle-low clouds. The suppressed precipitation is induced by the vertical dynamical processes of anomalous moisture advection caused by the abnormal descending flows over the YRV, which are driven by the negative advection of anomalous latent heat energy by climatological meridional wind (anomalous dry air advection) according to the atmospheric moist static energy equation. Simulations from the Lagrangian model FLEXPART further indicate that the moisture anomaly over the north of YRV mainly originated from the surface evaporation in the YRV, implying that there is a positive land-air feedback during the life cycle of the YRV heatwave. Our study enriches the mechanism understanding of the 2022 YRV heatwave from the perspective of surface energy budget and land-air feedback. [ABSTRACT FROM AUTHOR]

Using the High attitude Interferometer WIND observation balloon and Antarctic Jang Bogo station high latitude conjugate observations of the thermospheric winds we investigate the seasonal and hemispheric differences between the northern and southern hemispheres in June 2018. We found that the summer (northern) hemisphere dayside meridional winds have a double‐hump feature, whereas in the winter (southern) hemisphere the dayside meridional winds have a single hump feature. We attribute that to stronger summer, perhaps, northern hemisphere cusp heating. We also compared the observation with NCAR Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) model. The TIEGCM reproduced the double‐hump feature because of added cusp heating. The summer hemisphere has stronger anti‐sunward winds. This is the first time we have very high latitude conjugate thermospheric wind observations. Plain Language Summary: Using balloon instrument in the northern hemisphere and ground based instrument in the southern hemisphere, we study the conjugacy of the thermospheric winds of high latitudes. We found that the more summer hemispheric heating alters the thermospheric winds and resulted in a double‐hump feature on the dayside meridional winds. We also used model with cusp heating to simulate the winds and were able to reproduce the double‐hump feature. Key Points: The stronger summer (northern) hemispheric power could be the cause of double hump in the dayside meridional windsThermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) was able to reproduce the double‐hump features seen by the High attitude Interferometer WIND observation, and single hump in the winter hemisphere observed at Jang Bogo stationTIEGCM overestimates the poleward wind on the dayside and equatorward wind on the nightside in the summer hemisphere [ABSTRACT FROM AUTHOR]

The present study investigates wavenumber‐4 (wave‐4) structure in the longitude variation of zonal and meridional winds observed by the Michelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) instrument onboard the Ionospheric Connection Explorer (ICON) satellite. The amplitude of the wave‐4 pattern in meridional wind displays semi‐annual variation with equinoctial maxima whereas its seasonal variation in zonal wind shows maxima during August–October at the equatorial and low latitudes. The wave‐4 longitude variation maximizes at lower thermospheric heights (below 130 km) in zonal and meridional winds. It is considered primarily driven by the non‐migrating eastward propagating diurnal tide with zonal wavenumber‐3 (DE3) in the zonal wind. However, the amplitude of DE3 tide in the meridional wind does not show any enhancement during September–October. The seasonal variations of the wave‐4 amplitude and the DE3 tide are not similar in the zonal and meridional winds. The migrating ter‐diurnal tide (TW3) exhibits significant amplitudes during March–April and September–November in the meridional wind. In addition, the latitude variation of non‐migrating TE1 tide shows maximum amplitude during September–October. These results suggest that the non‐linear interaction between the TW3 and TE1 tides can serve as a potential source for the wave‐4 longitude variation in the meridional wind at lower thermospheric altitudes. Plain Language Summary: The four peaked longitudinal structure observed in ionosphere and thermosphere has long been investigated to determine its seasonal and inter‐annual variability in many parameters, as well as its sources, which can be of lower atmospheric origin. Earlier studies have shown that wave‐4 structure in the longitude variation of thermospheric zonal wind has large amplitudes between June and October. The investigation of ICON‐MIGHTI zonal wind confirms this. The wave‐4 pattern in the longitude variation of the meridional wind reveals that it has maxima during equinox different from the zonal wind. The DE3 tide is the primary contributor to the wave‐4 pattern in zonal wind. The DE3 shows large amplitudes during March–April however, it is weak during September–October in meridional wind. The source of wave‐4 pattern in the meridional wind is investigated. In meridional wind, the seasonal variation of migrating ter‐diurnal TW3 tide shows large amplitudes (40 m/s) during March–April and September–November. The non‐migrating ter‐diurnal TE1 tide shows enhanced amplitudes during September–October which suggests that the non‐linear interaction between the TW3 and TE1 tides can generate wave‐4 longitude structure in meridional wind at lower thermospheric altitudes. This study indicates the existence of other possible sources for the wave‐4 pattern observed in thermosphere. Key Points: Wave‐4 longitude structure in thermospheric meridional wind shows semi‐annual variation with equinoctial maxima at 100–110 kmThough DE3 tide does not show any enhancement during September–October, TW3 tide is found to be dominant in the meridional windIt is suggested that the non‐linear interaction between TW3 and TE1 tides can generate wave‐4 structure in thermospheric meridional wind [ABSTRACT FROM AUTHOR]

A number of the uncertainties in climate projections for the end of the century are due to the difficulty of general circulation models (GCMs) to represent large‐scale precipitating systems such as the Intertropical Convergence Zone (ITCZ) in the historical period. In this study, we evaluate the Coupled Model Intercomparison Project Phase 5 and 6 models based on the position of the ITCZ in the South Atlantic during the period in which it is climatologically further south (February–May). This period characterizes the rainy season of the Brazilian semiarid. The models were ranked according to two objective methods and validated through a qualitative analysis. The first method considered a success rate to evaluate the correct position of the areas with maximum precipitation and minimum pressure and meridional wind as representative of the mean position of the ITCZ. Among the 15 coupled GCMs analysed for each CMIP, those that best represented the position of the ITCZ were HADGEM.2‐AO, MIROC5 and CANESM2 from CMIP5 and CESM2 and MIROC6 from CMIP6. These models represented well the following characteristics of the ITCZ in February–May: maximum precipitation near the equator at the height of the coast of South America and with the wind convergence zone sloping north towards the African continent. The most common problem noticed in the other GCMs was to displace the ITCZ to the south of its observed mean position within the period February–May. Also, some GCMs presented a double ITCZ (GISS‐E2‐H, GDFL‐CM3 and IPSL‐CM5A‐LR from CMIP5 and INM‐CM5‐0 from CMIP6) following two high TSM cores. One GCM (ACCESS1‐3 from CMIP5) presented a relatively good performance, but the zone of greater precipitation was shifted to the east, being located in the centre of the Atlantic basin. Knowing how the GCMs represent the ITCZ is a criterion that must be considered when choosing the GCMs for climate change studies. [ABSTRACT FROM AUTHOR]

The higher‐order turbulence scheme, Cloud Layers Unified by Binormals (CLUBB), is known for effectively simulating the transition from cumulus to stratocumulus clouds within leading atmospheric climate models. This study investigates an underexplored aspect of CLUBB: its capacity to simulate near‐surface winds and the Planetary Boundary Layer (PBL), with a particular focus on its coupling with surface momentum flux. Using the GFDL atmospheric climate model (AM4), we examine two distinct coupling strategies, distinguished by their handling of surface momentum flux during the CLUBB's stability‐driven substepping performed at each atmospheric time step. The static coupling maintains a constant surface momentum flux, while the dynamic coupling adjusts the surface momentum flux at each CLUBB substep based on the CLUBB‐computed zonal and meridional wind speed tendencies. Our 30‐year present‐day climate simulations (1980–2010) show that static coupling overestimates 10‐m wind speeds compared to both control AM4 simulations and reanalysis, particularly over the Southern Ocean (SO) and other midlatitude ocean regions. Conversely, dynamic coupling corrects the static coupling 10‐m winds biases in the midlatitude regions, resulting in CLUBB simulations achieving there an excellent agreement with AM4 simulations. Furthermore, analysis of PBL vertical profiles over the SO reveals that dynamic coupling reduces downward momentum transport, consistent with the found wind‐speed reductions. Instead, near the tropics, dynamic coupling results in minimal changes in near‐surface wind speeds and associated turbulent momentum transport structure. Notably, the wind turning angle serves as a valuable qualitative metric for assessing the impact of changes in surface momentum flux representation on global circulation patterns. Plain Language Summary: The Cloud Layers Unified by Binormals (CLUBB) scheme offers a promising way to model the complexities of cloud behavior, but its impact on winds and global circulation has been less explored. In our study, we investigate how different ways of representing the complex coupling between surface drag and the lowest kilometer of the Earth's atmosphere affect global wind speeds and circulation. We specifically examine two distinct approaches: a static approach, which feeds a constant surface drag to CLUBB, and a dynamic approach, which adjusts the surface drag based on the winds updates computed by CLUBB. Over a present‐day climate, we find that static coupling tends to produce excessively large wind speeds in certain regions, like the Southern Ocean and parts of the North Atlantic and North Pacific. Instead, dynamic coupling produces excellent near‐surface wind speeds in these regions, and also over the rest of the globe. Moreover, we discover that dynamic coupling reduces the downward turbulent transport of momentum, highlighting the enhancements in near‐surface wind speeds found with this approach are physically consistent. Lastly, we use the change in wind direction with height to qualitatively evaluate how the two coupling methods affect global circulation patterns. Key Points: Dynamic coupling between Cloud Layers Unified by Binormals (CLUBB) and surface momentum flux enhances global winds climate simulations bringing CLUBB in line with control atmospheric climate modelIn midlatitude regions, the dynamic coupling enhances the boundary‐layer momentum transport compared to the static couplingThe wind turning angle turns out a useful qualitative metric, linking changes in surface momentum flux to the changes in global circulation [ABSTRACT FROM AUTHOR]

The main objective of this study is to investigate the spatial–temporal variability and climate forcing influence during the last 21,000 years of the South American Monsoon System (SAMS) and its components. TraCE-21k simulations, including Full and Single Forcings experiments, were employed. The identification of spatial variability patterns associated with the core of the monsoon region (SAMS) and the South Atlantic Convergence Zone (SACZ), the main components of SAMS during the austral summer, is based on multivariate EOF analysis (precipitation, humidity, zonal and meridional wind). This analysis produces two main modes: the first one is the South American Large Scale Monsoon Index (LISAM), representing the primary oscillation mode for SAMS, and the second one is the SACZ mode, representing the SACZ oceanic portion variability. The TraCE-21k EOF modes demonstrated the ability to represent the SAMS and SACZ patterns when compared to the 20th Century reanalysis. LISAM time series proved to be an important tool for identifying monsoon precipitation variability, consistent with the regime changes recorded in climatic proxies. The freshwater pulses forcing in TraCE-21k are a determining factor for the observed changes in the precipitation regime, mainly during the periods between Heinrich Stadial 1 and the Younger Dryas. The results indicate that the observed and modeled SACZ southward shift in the Late Holocene is primarily modulated by insolation changes, with a stronger correlation observed since the Mid-Holocene period. Through wavelet analysis, it was noted that energy was transferred from low frequencies to high frequencies during the Bølling–Allerød period for the full forcing and freshwater pulse experiments in the Northern Hemisphere. The SAMS multidecadal variability increased from the early Holocene with direct influences of orbital forcing and ice cover. [ABSTRACT FROM AUTHOR]

The East Asian jet stream is an important component of mid-high latitude circulation systems, and its intraseasonal oscillation plays a crucial role in the formation and development of persistent climate anomalies and extreme events over East Asia–Pacific region. This study examines the decadal changes in the intraseasonal variability (ISV) of the East Asian jet stream and investigates the possible reasons with reanalysis and observational datasets. It is found that the zonal winds over East Asia show notable ISV with a significant period of 10–25 days. The variance of 10–25-day filtered East Asian zonal winds exhibits a significant decrease around 2000, which is mainly contributed by the weakened ISV of westerlies within the key region of East Asian polar front jet (EAPJ). Further analysis reveals that the EAPJ experienced remarkable ISV changes in both intensity and location. The ISV of EAPJ intensity was weakened around 2000 due to decreased frequencies of both the extremely strong and weak cases, whereas the ISV of EAPJ location was evidently enhanced, with the southward shifts increased more than the northward ones. Along with the ISV increase (decrease) of EAPJ intensity (location), the dominant mode of EAPJ intraseasonal variation has changed from its intensity change to position shift since the year 2000. The decadal ISV changes and mode transition of EAPJ are closely associated with the modulation of lower-level large-scale circulation over Eurasia. The weakened ISV of meridional winds over Ural region after 2000 is favorable for an intraseasonal circulation pattern change from a continental-scale cyclonic anomalous pattern to a meridional dipole structure across the Eurasia in lower troposphere, resulting in regime shifts of East Asian temperature and its meridional gradient through temperature advection. The intraseasonal temperature gradient patterns display ISV changes parallel to those of EAPJ around 2000, indicating that the thermal anomalies are effective in leading to corresponding intensity and location variations of EAPJ via thermal wind relationship. Furthermore, the modulation of lower-level circulation is closely related to strengthened Arctic warming and weakened temperature variability over Barents sector after 2000, which may further be linked to sea ice reduction in the Barents Sea. [ABSTRACT FROM AUTHOR]

In this study, we utilize ICON observations from 2019 to 2022 to analyze the variability of vertical plasma drift and its relationship with the neutral winds. The results reveal that there are 19% of downward plasma drifts at 13–17 LT, which changes with seasons and longitudes. The downward plasma drift occurs less frequently compared to the contemporaneous counter electrojet during solstices. We identify the relationship between vertical plasma drifts and north foot magnetic zonal and meridional wind profiles at 90–300 km altitudes. As the vertical plasma drifts become small or downward, the zonal winds display diverse variations at the altitudes; that is, the disturbances are eastward at 95–120 km altitudes, westward at 120–160 km altitudes, and eastward above 180 km altitudes, while the meridional winds present weak changes in all altitudes. Additionally, we discuss the possible roles of the E‐ and F‐region dynamos on the vertical plasma drifts. Plain Language Summary: The neutral wind in the Earth's upper atmosphere drives the ionospheric ions to cross the geomagnetic field lines and produces the dynamo effects in the E‐ and F‐regions. The electric field is created due to the divergence free of the total electrical current. The zonal electric field drives the ionospheric plasma to drift upward and downward due to the horizontal field lines, and influences the structures of low latitude ionosphere. One open question is that the vertical plasma drift shows the significant day‐to‐day variability and sometimes remarkably deviates from the climatological pattern during the quiet times. The ICON satellite provides the simultaneous observations of the plasma drifts and neutral winds in low latitudes, which provides us a good chance to investigate the day‐to‐day variability of the vertical plasma drift and its relationship to the neutral wind, as we discuss in the manuscript. Key Points: The equatorial vertical plasma drifts show large variability and 19% downward plasma drift at 13–17 LTThe occurrence of the downward plasma drift presents a significant longitudinal and seasonal variationsThe E‐ and F‐region zonal winds play vital roles on the large variability and the occurrence of the downward plasma drift [ABSTRACT FROM AUTHOR]

Biweekly sea surface temperature (SST) variability significantly contributes to over 50% of the intraseasonal variability in the eastern equatorial Pacific (EEP) and Atlantic (EEA). Our study investigates this biweekly variability, employing a blend of in–situ and reanalysis data sets. The research identifies biweekly signals in SST, meridional wind, and ocean currents, notably in September–November in EEP and June–August in EEA. Biweekly southerly (northerly) winds drive instantaneous northward (southward) ocean currents in EEP, but with a 1–2‐day phase delay in EEA. Consequently, these currents lead to SST anomalies with a 3–4‐day lag in both EEP and EEA due to the presence of the cold tongue. The study reveals the origin of biweekly wind fluctuations in the western Pacific for EEP and the subpolar Pacific for EEA, connected by atmospheric Rossby waves validated through a linearized non‐divergent barotropic model. This research affirms the influence of subtropical and subpolar atmospheric forcing on equatorial SST. Plain Language Summary: Our research focuses on understanding the regular changes in sea surface temperature (SST) occurring every 2 weeks, which significantly contribute to the seasonal variations in the eastern equatorial Pacific (EEP) and Atlantic (EEA). By analyzing a mix of direct and reconstructed data, we uncover the distinct patterns of these biweekly changes in SST, winds, and ocean currents. Our investigation shows that the movements of the ocean currents are closely linked to the shifts in wind direction, affecting the temperature of the ocean waters. Notably, we observe a delay in the relationship between wind and SST in both EEP and EEA. Through our analysis, we establish that the origins of these biweekly wind patterns can be traced to specific regions in the Pacific. Moreover, we identify the role of atmospheric Rossby waves in connecting these wind patterns to their source regions, which helps us understand how changes in atmospheric conditions in different parts of the ocean can impact the equatorial SST. Key Points: Over 50% of intraseasonal SST variability in the eastern equatorial Pacific and Atlantic is attributed to biweekly fluctuationsThe atmospheric winds play a crucial role in driving the biweekly SST variabilityBiweekly winds in the eastern equatorial Pacific and Atlantic stem from the subtropical and subpolar regions, respectively [ABSTRACT FROM AUTHOR]

Temporal and spatial variations in the ocean surface mixed layer are important for the climate and ecological systems. During 1980–2019, the Southern Indian Ocean (SIO) mixed layer depth (MLD) displays a basin‐wide shoaling trend that is absent in the other basins within 40°S–40°N. The SIO MLD shoaling is mostly prominent in austral winter with deep climatology MLD, substantially weakening the MLD seasonality. Moreover, the SIO MLD changes are primarily caused by a southward shift of the subtropical anticyclonic winds and hence ocean gyre, associated with a strengthening of the Southern Annular Mode, in recent decades for both winter and summer. However, the poleward‐shifted subtropical ocean circulation preferentially shoals the SIO MLD in winter when the meridional MLD gradient is sharp but not in summer when the gradient is flat. This highlights the distinct subtropical MLD response to meridional mitigation in winds due to different background oceanic conditions across seasons. Plain Language Summary: The ocean surface mixed layer (ML) is a well‐mixed layer with uniform physical and chemical properties and is key for the ocean in exchanging materials and energy with the atmosphere. The present study shows that during 1980–2019, the Southern Indian Ocean (SIO) ML depth (MLD) displays a basin‐wide decreasing trend, which is absent in the other basins within 40°S–40°N. The SIO MLD shoaling primarily appears in austral winter when the climatology ML is deep but is insignificant in summer, substantially weakening the MLD seasonality. The SIO MLD changes are primarily explained by the ocean dynamical adjustment driven by the surface zonal wind changes. Specifically, the strengthened Southern Annular Mode in recent decades drives southward shifts of the subtropical anticyclonic winds and ocean gyre year‐round. However, the poleward‐shifted ocean gyre preferentially decreases the SIO winter MLD as the meridional MLD gradient is sharp and thus efficiently reduces the deep ML water converging from the Southern Ocean into the SIO. In contrast, the SIO MLD displays negligible change in summer when its meridional gradient is flat. The results highlight that despite under nearly identical southward‐shifted subtropical winds, the winter and summer MLD responses are distinct due to different background oceanic conditions. Key Points: The seasonality of the Southern Indian Ocean surface mixed layer (ML) depth prominently weakens during 1980–2019The weakened seasonality mainly results from a pronounced winter ML shoalingThe southward shift of the subtropical ocean gyre driven by the strengthened Southern Annular Mode dominates the ML shoaling [ABSTRACT FROM AUTHOR]

In this study, data from 17 ground‐based, continental Arctic observatories are used to evaluate the performance of the European Centre for Medium‐Range Weather Forecasts Reanalysis version 5 (ERA5) reanalysis model. Three aspects are evaluated: (i) the overall reproducibility of variables at all stations for all seasons at one‐hour time resolution; (ii) the seasonal performance; and (iii) performance between different temporal resolutions (one hour to one month). Performance is evaluated based on the slope, R2 value, and root‐mean‐squared error (RMSE). We focus on surface meteorological variables including 2‐m air temperature (temperature), relative humidity (RH), surface pressure, wind speed, zonal and meridional wind speed components, and short‐wave downward (SWD) radiation flux. The overall comparison revealed the best results for surface pressure (0.98 ± 0.02, R2mean ± standard deviation [σR2]), followed by temperature (0.94 ± 0.02), and SWD radiation flux (0.87 ± 0.03) while wind speed (0.49 ± 0.12), RH (0.42 ± 0.20), zonal (0.163 ± 0.15) and meridional wind speed (0.129 ± 0.17) displayed poor results. We also found that certain variables (surface pressure, wind speed, meridional, and zonal wind speed) showed no seasonal dependency while others (temperature, RH, and SWD radiation flux) performed better during certain seasons. Improved results were observed when decreasing the temporal resolution from one hour to one month for temperature, meridional and zonal wind speed, and SWD radiation flux. However, certain variables (RH and surface pressure) showed comparatively worse results for monthly resolution. Overall, ERA5 performs well in the Arctic, but caution needs to be taken with wind speed and RH, which has implications for the use of ERA5 in global climate models. Our results are useful to the scientific community as it assesses the confidence to be placed in each of the surface variables produced by ERA5. [ABSTRACT FROM AUTHOR]

We investigate the spatial and temporal variability of extreme precipitation events (EPEs) in the Dronning Maud Land (DML) sector of Antarctica using high‐resolution ECMWF ERA5 reanalysis data. This study examines the spatial occurrence of EPEs across DML, focusing particularly on six locations spanning the coastal and interior parts of the area. The largest snowfall amounts are usually found on eastward‐facing slopes in the coastal zone. EPEs occur predominantly in north‐easterly to easterly flows, leading to enhanced precipitation on the windward side of the orographic features with a steep gradient. Wind during EPEs was found to be more directionally consistent in the coastal area than in the interior. An east‐west couplet of a mid‐tropospheric ridge and low‐pressure center is essential for steering warm moist maritime airmasses into the DML region before EPEs. Approximately 40% of EPEs result from atmospheric rivers (ARs), narrow bands of moist air originating at subtropical latitudes, which provide the greatest daily precipitation amounts. From 1979 to 2018, much of the DML experienced a statistically significant (p < 0.05) increase in the number of EPEs per year, along with increased precipitation from the EPEs. These trends were associated with significant changes in moisture availability and poleward meridional winds in the Atlantic sector of the Southern Ocean. The inter‐annual variability in the number of EPEs is primarily dictated by regional atmospheric variability, while the influence of the Southern Oscillation Index and Southern Annular Mode is limited. Plain Language Summary: Extreme precipitation events are severe weather conditions that cause unusually large amounts of rain or snow, leading to natural disasters. In Antarctica, a substantial proportion of precipitation comes from these high‐magnitude precipitation events and they control the year‐to‐year variability in annual total precipitation, particularly in the coastal region. These events have a significant impact on different aspects of the Antarctic climate system and can make anomalous changes in meteorological parameters. We investigate the influence of these extreme events over the Dronning Maud Land (DML) region of Antarctica and show that the terrain of the region plays a significant role in their distribution. We found that most of these events are caused by the combined influence of low‐pressure cyclones and anticyclones, which aids moisture transport to the region. The strong, long, and narrow bands of moisture transport called atmospheric rivers (ARs), from far north of the region, are involved in over 40% of EPEs. Over recent decades, there has been an increasing trend in the occurrence of these high precipitating days, which is associated with increased moisture content and favorable winds in the South Atlantic sector of the Southern Ocean, north of Antarctica. Key Points: Spatial distribution of extreme precipitation events is linked to the complex orography of the Dronning Maud Land sector of AntarcticaInterannual variability of extreme precipitation events is controlled by blocking high‐pressure systems north of the locationIncreased moisture content and poleward winds in the South Atlantic Ocean contributed to an increasing trend in high‐precipitating days [ABSTRACT FROM AUTHOR]

Long‐term variabilities of monthly zonal (U) and meridional winds (V) in northern polar mesosphere and lower thermosphere (MLT, ∼80–100 km) are investigated using meteor radar observations during 1999–2022 over Esrange (67.9°N, 21.1°E). The summer (June‐August) mean zonal winds are characterized by westward flow up to ∼88–90 km and eastward flow above this height. The summer mean meridional winds are equatorward with strong jet at ∼85–90 km and it weakens above this height. The U and V exhibit strong interannual variability that varies with altitude and month or season. The responses of U and V anomalies (from 1999 to 2003) to solar cycle (SC), Quasi Biennial Oscillation at 10 and 30 hPa, El Niño‐Southern Oscillation, North Atlantic Oscillation, ozone (O3) and carbon dioxide (CO2) are analyzed using multiple linear regression. From analysis, significant regions of correlations between MLT winds and above potential drivers vary with altitude and month. The positive responses of U and V to SC (up to 15 m/s/100 sfu) indicates the strengthening of eastward winds in mid‐late winter, and poleward winds in late autumn and early winter. The O3 likely intensifies the eastward and poleward winds (∼100 m/s/ppmv) in winter and early spring. The CO2 significantly influence the eastward flow in late winter and summer (above ∼90–95 km) and strengthen the meridional circulation. The significant positive trend in U peaks in summer, late autumn and early winter (∼0.6 m/s/year), the negative trend in V is more prominent in summer above ∼90–95 km. Plain Language Summary: The transition region between middle atmosphere and thermosphere is known as the mesosphere and lower thermosphere (MLT). The dynamics and circulation in this region are significant for global transport of important trace chemical species. Further the MLT winds play crucial role for the dynamical coupling of the middle and upper atmosphere. In the present study, the long‐term variability and tendencies in monthly mean zonal and meridional winds are investigated in the Arctic MLT between ∼80 and ∼100 km from meteor radar observations during 1999–2022 over Esrange (67.9°N, 21.1°E) in Sweden. The ability of radar provided the unique, consistent, and long‐term data set of polar MLT winds for duration of about two solar cycles. The MLT winds show important characteristic features and significant interannual variability that vary with altitude and month or season. In addition, the possible influence of climate forcings viz., solar activity, Quasi Biennial Oscillation (at 10 and 30 hPa), El Niño‐Southern Oscillation, North Atlantic Oscillation, ozone, and carbon dioxide on the variabilities of polar MLT winds has been analyzed using multiple linear regression. The significant interannual variabilities and tendencies in northern polar MLT zonal and meridional winds can be attributed to the above potential drivers. Key Points: The long‐term variabilities in arctic mesosphere and lower thermosphere winds in response to potential climate forcings have been investigated for 1999–2022 over EsrangeThe variability in U and V significantly correlated with O3 in winter and early spring, and with CO2 in summer based on altitudeThe interannual variability in U and V is found to vary with altitude and month or season [ABSTRACT FROM AUTHOR]

In this study we have conducted a survey of westward‐propagating mixed Rossby–Gravity (MRG) wave events in the upper troposphere and quantified their association with the intrusions of extratropical disturbances for the period 1979–2019. MRG wave events are identified by imposing the meridional structure of theoretical MRG waves onto the equatorial meridional winds at 200 hPa. In all, 2390 MRG wave events are identified and the majority (61%) of them occurred during the months of May–October, and 65% of the total MRG wave events occurred over the central–east Pacific and Atlantic Ocean domains. Not only the frequency of occurrence but also the amplitude, wavenumber and trapping scale of the MRG wave events are found to exhibit a clear seasonality. MRG wave events associated with intrusions of extratropical disturbances are identified as when the potential vorticity on the 350‐K isentropic surface at 15° latitude exceeded 1 Potential Vorticity Unit (PVU) in the vicinity of the MRG wave events. We find that 37% of the MRG wave events are intrusion MRG wave events and a large majority (88%) of such events occurred over the central–east Pacific and Atlantic Ocean domains. It is also noteworthy that nearly 70% of such intrusions occurred in the winter hemisphere where the westerly wind ducts are well developed. Over the central–east Pacific during northern hemispheric (NH) winter, it is observed that the intrusion MRG wave events have a bigger amplitude and have a larger meridional extent compared to non‐intrusion MRG wave events. They also exhibit a similar spatial scale as the extratropical disturbances implying that resonant interactions may be a primary mechanism for the genesis of MRG wave events. During NH summer, on the other hand, MRG wave events are primarily triggered by convective processes and the extratropical disturbances may be instrumental in amplifying their amplitude. [ABSTRACT FROM AUTHOR]