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Tornado activity in the United States varies over multiple time scales. The Madden–Julian oscillation (MJO) and global wind oscillation (GWO) are sources of climate variability at the seasonal and subseasonal scales. The univariate distributions of tornado-days across the phases of these oscillations have been documented, but the bivariate distribution of tornado-days across both oscillations has not been. Here, the bivariate distribution of tornado-days across the phases of the MJO and GWO is documented and analyzed. Results herein refine the previously reported univariate distributions and highlight variability across the phases of one of the oscillations while holding the phase of the other constant (e.g., tornado-days are more likely during phase 2 of the GWO, but the likelihood varies across the phase of MJO).

Globally, 2023 was the hottest year on record and saw the development of a strong El Niño with widespread impacts. This El Niño event was unusual for its strong oceanic warming yet muted Southern Oscillation and wind anomalies over the tropical Pacific. This discrepancy is perplexing given the historically close coupling of El Niño and the Southern Oscillation. Atmospheric model experiments show that warming in the Atlantic and Indian Oceans in 2023 and the slow background sea surface temperature trend reduced the surface wind response over the tropical Pacific by modulating the Walker circulation. We develop a hindcast system that reproduces 87% of the June–December El Niño warming even without wind stress feedback after April 2023. The intense oceanic warming was primarily driven by the strong build-up of western Pacific heat content during the preceding prolonged La Niña. This indicates that the 2023–2024 El Niño primarily arose from oceanic processes, independent of the classic positive Bjerknes feedback mechanism. Due to the strong ocean memory, this event was highly predictable at long time leads. Climate model simulations suggest that such 2023-like El Niños may become more frequent in a warming climate. Model simulations suggest that the 2023–2024 El Niño was mainly driven by oceanic processes and that this type of El Niño may become more frequent with warming.

Variations of global wind are important in changing the atmospheric structure and circulation, in coupling of atmospheric layers, and in influencing the wave propagations. Due to the difficulty of directly measuring zonal wind from the stratosphere to the lower thermosphere, we derived a global balance wind (BU) dataset from 50∘ S to 50∘ N and during 2002–2019 using the gradient wind theory and SABER temperatures and modified by meteor radar observations at the Equator. The dataset captures the main feature of global monthly mean zonal wind and can be used to study the variations (i.e., annual, semi-annual, ter-annual, and linear) of zonal wind and the responses of zonal wind to quasi-biennial oscillation (QBO), El Niño–Southern Oscillation (ENSO), and solar activity (F10.7). The same procedure is performed on the MERRA-2 zonal wind (MerU) to validate BU and its responses below 70 km. The annual, semi-annual, and ter-annual oscillations of BU and MerU have similar amplitudes and phases. The semi-annual oscillation of BU has peaks around 80 km, which are stronger in the southern tropical region and coincide with previous satellite observations. As the increasing of the values representing QBO wind, both values of representing BU and MerU (short for BU and MerU) change from increasing to decreasing with the increasing height and extend from the Equator to higher latitudes. Both BU and MerU increase with the increasing of the values of multivariate ENSO index (MEI) and decrease with increasing F10.7​​​​​​​ in the southern stratospheric polar jet region below 70 km. The responses of winds to ENSO and F10.7 exhibit hemispheric asymmetry and are more significant in the southern polar jet region. While above 70 km, BU increases with the increasing of MEI and F10.7. The negative linear changes of BU at 50∘ N are absent in MerU during October–January. The discussions on the possible influences of the temporal intervals and sudden stratospheric warmings (SSWs) on the variations and responses of BU illustrate the following: (1) the seasonal variations and the responses to QBO are almost independent on the temporal intervals selected; (2) the responses to ENSO and F10.7 are robust but slightly depend on the temporal intervals; (3) the linear changes of both BU and MerU depend strongly on the temporal intervals; (4) SSWs affect the magnitudes but do not affect the hemispheric asymmetry of the variations and responses of BU at least in the monthly mean sense. The variations and responses of global zonal wind to various factors are based on BU, which is derived from observations, and thus provide a good complement to model studies and ground-based observations.

Wind power input to oceanic near‐inertial oscillations (NIOs) plays a crucial role in sustaining the global ocean conveyor belt. However, the impact of tropical cyclones (TCs) on wind power input to NIOs, despite being the most vigorous atmospheric dynamics capable of exciting NIOs, is often overlooked in global estimations due to their transient nature and a lack of observations. Utilizing hourly wind and ocean current records, we quantified the wind power on NIOs induced by TCs from 1990 to 2019. Our findings reveal that the wind power on NIOs due to TCs is estimated to be between 0.028 and 0.065 TW, which accounts for a significant proportion, that is, 8%–17%, of that over the globe. This study highlights the importance of incorporating the wind power induced by TCs when estimating the global wind power on NIOs, as its impact is non‐negligible. Our findings contribute to a better understanding of the global energy balance by improving the estimation of wind power on NIOs. Plain Language Summary Wind power input to oceanic near‐inertial oscillations (NIOs) is important for sustaining the global ocean conveyor belt. However, the influence of tropical cyclones (TCs) on wind power input to NIOs, despite being the most intense atmospheric dynamics that could easily excite NIOs, is often overlooked in global estimations due to their transient nature and a lack of observations. By analyzing wind and ocean current records from 1990 to 2019, the wind power on NIOs induced by TCs was quantified. We found that wind power on NIOs due to TCs is non‐negligible and accounts for a significant portion of that over the globe. This study highlights the importance of considering the wind power induced by TCs when estimating the global wind power on NIOs, which helps to achieve a more accurate estimation of wind power on NIOs, leading to an improved understanding of the global energy balance. Key Points Observed data shows tropical cyclones contribute 8%–17% of global wind power on near‐inertial oscillations Tropical cyclones could contribute up to 90% of wind power on near‐inertial oscillations in their prone regions Accounting for tropical cyclones is essential when estimating wind power on near‐inertial oscillations

An extreme Atlantic Niño developed in the boreal summer of 2021 with peak‐season sea surface temperature anomalies exceeding 1°C in the eastern equatorial region for the first time since global satellite measurements began in the early 1970s. Here, we show that the development of this outlier event was preconditioned by a series of oceanic Rossby waves that reflected at the South American coast into downwelling equatorial Kelvin waves. In early May, an intense week‐long westerly wind burst (WWB) event, driven by the Madden‐Julian Oscillation (MJO), developed in the western and central equatorial Atlantic and greatly amplified one of the reflected Kelvin waves, directly initiating the 2021 Atlantic Niño. MJO‐driven WWBs are fundamental to the development of El Niño in the Pacific but are a previously unidentified driver for Atlantic Niño. Their importance for the 2021 event suggests that they may serve as a useful predictor/precursor for future Atlantic Niño events. Plain Language Summary Atlantic Niño is the Atlantic counterpart of El Niño in the Pacific, often referred to as El Niño's little brother. It was previously thought to have only regional influence on rainfall variability in West Africa, but a growing number of studies have shown that Atlantic Niño also plays an important role in the development of El Niño–Southern Oscillation, as well as in the formation of powerful hurricanes near the coast of West Africa. This study investigates the development of an extreme Atlantic Niño in the summer of 2021. Here, we show that the 2021 event was preconditioned by warm waters piled up near the South American coast, and then directly triggered by a westerly wind burst event that drove the warm waters eastward. The westerly wind burst event was driven by a patch of tropical thunderstorms that formed across the Indian Ocean and moved slowly eastward across the Pacific, South America, and the Atlantic, also known as the Madden‐Julian Oscillation. Westerly wind bursts driven by the Madden‐Julian Oscillation are fundamental for the development of El Niño in the Pacific, but a previously unidentified driver for Atlantic Niño, and thus may improve our ability to predict future Atlantic Niño events. Key Points The extreme 2021 Atlantic Niño was preconditioned by a series of oceanic Rossby waves reflected into downwelling equatorial Kelvin waves One of the Kelvin waves was greatly amplified by an intense week‐long westerly wind burst event, initiating the 2021 Atlantic Niño The westerly wind burst was driven by the Madden‐Julian Oscillation, which is a previously unidentified driver for Atlantic Niño

Low-level jets (LLJs), in which the wind speed attains a local maximum at low altitudes, have been found to occur in the U.S. mid-Atlantic offshore, a region of active wind energy deployment as of 2023. In contrast to widely studied regions such as the U.S. southern Great Plains and the California coastline, the mechanisms that underlie LLJs in the U.S. mid-Atlantic are poorly understood. This work analyzes floating lidar data from buoys deployed in the New York Bight to understand the characteristics and causes of coastal LLJs in the region. Application of the Hilbert–Huang transform, a frequency analysis technique, to LLJ case studies reveals that mid-Atlantic jets frequently occur during times of adjustment in synoptic-scale motions, such as large-scale temperature and pressure gradients or frontal passages, and that they do not coincide with motions at the native inertial oscillation frequency. Subsequent analysis with theoretical models of inertial oscillation and thermal winds further reveals that these jets can form in the stationary geostrophic wind profile from horizontal temperature gradients alone—in contrast to canonical LLJs, which arise from low-level inertial motions. Here, inertial oscillation can further modulate the intensity and altitude of the wind speed maximum. Statistical evidence indicates that these oscillations arise from stable stratification and the associated frictional decoupling due to warmer air flowing over a cold sea surface during the springtime land–sea breeze. These results improve our conceptual understanding of mid-Atlantic jets and may be used to better predict low-level wind speed maxima.

During the positive phase of the North Pacific Oscillation, westerly wind anomalies over the subtropical North Pacific substantially increase subsurface heat content along the equator by “trade wind charging” (TWC). TWC provides a direct pathway between extratropical atmospheric circulation and El Niño–Southern Oscillation (ENSO) initiation. Previous model studies of this mechanism lacked the ocean–atmospheric coupling needed for ENSO growth, so it is crucial to examine whether TWC-induced heat content anomalies develop into ENSO events in a coupled model. Here, coupled model experiments, forced with TWC favorable (+TWC) or unfavorable (−TWC) wind stress, are used to examine the ENSO response to TWC. The forcing is imposed on the ocean component of the model through the first winter and then the model evolves in a fully coupled configuration through the following winter. The +TWC (−TWC) forcing consistently charges (discharges) the equatorial Pacific in spring and generates positive (negative) subsurface temperature anomalies. These subsurface temperature anomalies advect eastward and upward along the equatorial thermocline and emerge as like-signed sea surface temperature (SST) anomalies in the eastern Pacific, creating favorable conditions upon which coupled air–sea feedback can act. During the fully coupled stage, warm SST anomalies in +TWC forced simulations are amplified by coupled feedbacks and lead to El Niño events. However, while −TWC forcing results in cool SST anomalies, pre-existing warm SST anomalies in the far eastern equatorial Pacific persist and induce local westerly wind anomalies that prevent consistent development of La Niña conditions. While the TWC mechanism provides adequate equatorial heat content to fuel ENSO development, other factors also play a role in determining whether an ENSO event develops.

Understanding eastward‐propagating mechanisms of the Madden–Julian Oscillation (MJO) is of great importance for the subseasonal prediction of extreme weather and climate worldwide. Using global satellite observations and reanalysis data, this study unravels that dual combinations of strong/weak westward‐ (ISOw) and eastward‐propagating intraseasonal oscillation (ISOe) can shape diverse MJO propagations documented previously using clustering analysis. The dry ISOw signals from the Central Pacific strengthen the leading suppressed convection over the Western Pacific (WP) and, on the contrary, weaken the moist ISOe convection over the Maritime Continent. Thus, when ISOe is weak over the WP, the strong (weak) dryness of ISOw likely causes a jump‐like (stand‐like) MJO mode. In contrast, a propagating MJO is supported when ISOe becomes strong over the WP, and a further strengthening of the ISOw dryness will presumably accelerate MJO; moreover, a weakening of ISOw might slow down the MJO speed. Plain Language Summary Owing to the eastward propagation of the Madden–Julian Oscillation (MJO), tropical winds and precipitation usually oscillate in a broad life cycle of 20–100 days. The subseasonal (longer than 2 weeks but shorter than one season) prediction of extreme events, such as tropical cyclones, droughts, and heat waves, relies closely on an adequate understanding of the propagation mechanisms of MJO. Recently, clustering analysis has revealed four MJO propagation patterns, including stand, jump, slow, and fast modes. However, the underlying mechanisms of the origin of MJO propagation diversity are still elusive. Herein, we offer a novel explanation from the perspective of atmospheric westward‐propagation disturbances (WPDs). These WPDs, which first appeared over the Central Pacific, can cause stand‐ and jump‐like MJO modes when the eastward‐propagating 20–100‐day dry signals are weak over the Western Pacific. Otherwise, fast and slow MJO modes will be supported when the WPDs are further strengthened and weakened, respectively. These results highlight that the WPDs and their triggering place, that is, the Central Pacific, might serve as key ingredients for better understanding the MJO dynamics and predictability. Key Points Dual combinations of strong/weak westward‐ and eastward‐propagating disturbances shape Madden–Julian Oscillation (MJO) diversity Westward‐propagating disturbances control the formation of stand and jump MJOs and distinguish fast and slow MJOs Eastward‐propagating disturbances primarily help MJO cross the Maritime Continent barrier

Two of the most basic parameters generated from a reanalysis are temperature and winds. Temperatures in the reanalyses are derived from conventional (surface and balloon), aircraft, and satellite observations. Winds are observed by conventional systems, cloud tracked, and derived from height fields, which are in turn derived from the vertical temperature structure. In this paper we evaluate as part of the SPARC Reanalysis Intercomparison Project (S-RIP) the temperature and wind structure of all the recent and past reanalyses. This evaluation is mainly among the reanalyses themselves, but comparisons against independent observations, such as HIRDLS and COSMIC temperatures, are also presented. This evaluation uses monthly mean and 2.5° zonal mean data sets and spans the satellite era from 1979–2014. There is very good agreement in temperature seasonally and latitudinally among the more recent reanalyses (CFSR, MERRA, ERA-Interim, JRA-55, and MERRA-2) between the surface and 10 hPa. At lower pressures there is increased variance among these reanalyses that changes with season and latitude. This variance also changes during the time span of these reanalyses with greater variance during the TOVS period (1979–1998) and less variance afterward in the ATOVS period (1999–2014). There is a distinct change in the temperature structure in the middle and upper stratosphere during this transition from TOVS to ATOVS systems. Zonal winds are in greater agreement than temperatures and this agreement extends to lower pressures than the temperatures. Older reanalyses (NCEP/NCAR, NCEP/DOE, ERA-40, JRA-25) have larger temperature and zonal wind disagreement from the more recent reanalyses. All reanalyses to date have issues analysing the quasi-biennial oscillation (QBO) winds. Comparisons with Singapore QBO winds show disagreement in the amplitude of the westerly and easterly anomalies. The disagreement with Singapore winds improves with the transition from TOVS to ATOVS observations. Temperature bias characteristics determined via comparisons with a reanalysis ensemble mean (MERRA, ERA-Interim, JRA-55) are similarly observed when compared with Aura HIRDLS and Aura MLS observations. There is good agreement among the NOAA TLS, SSU1, and SSU2 Climate Data Records and layer mean temperatures from the more recent reanalyses. Caution is advised for using reanalysis temperatures for trend detection and anomalies from a long climatology period as the quality and character of reanalyses may have changed over time.

The Quasi‐Biennial Oscillation and the Semiannual Oscillation have been identified to be the leading modes of variability in the tropical middle atmosphere. With reanalysis data and independent rocket soundings from a low latitude site, we report the existence of yet another variability in the tropical lower mesosphere which is primarily evident as easterly bursts in zonal winds during the months of May‐July. It occurs with a variable interval of 2–5 yrs in the late 20th century and 7–9 yrs in the early 21st century. These Quasi‐Periodic Easterly Bursts are found to have remote influences on the Antarctic polar vortex as well as residual circulation in the lower mesosphere. We identify a potential causative mechanism for the easterly bursts that involve enhanced cross equatorial advection of momentum as well as gravity wave drag. A close association with Quasi Biennial Oscillation winds is observed, however, cause of the observed periodicity remains elusive. Plain Language Summary Circulation in the tropical middle atmosphere is distinctively different from that of the higher latitudes in that there exists long‐period oscillations chiefly driven by a broad spectrum of atmospheric waves. These oscillations that dominate the wind variability here, include the Quasi‐Biennial Oscillation (QBO) and the Semiannual Oscillation (SAO) with approximate periods of 28 and 6 months, respectively. With the help of reanalysis data sets and independent rocket soundings from a low latitude site, we identify the existence of a hitherto unrecognized variability in the tropical middle atmosphere. The newly identified variability seems to have association with the Southern Hemispheric polar vortex as well as the transport circulation in the lower mesosphere. Further, an attempt is made to identify the causative mechanism using the framework of interaction of large‐scale planetary waves with the mean flow. Key Points A new pattern of variability in the tropical middle atmosphere is identified The observed variability is associated with stronger polar vortex and an enhanced residual circulation in the Southern Hemispheric mesosphere A close connection with the presence of an extended layer of westerlies in the Quasi Biennial Oscillation regime is observed