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The present study focuses on the mechanism that controls the transition of the Euro-Atlantic circulation responses to El Niño–Southern Oscillation (ENSO) from early (December) to late winter (February) for the period 1981–2015. A positive phase of ENSO induces a precipitation dipole with increased precipitation in the western and reduced precipitation in the eastern tropical Indian Ocean; this occurs mainly during early winter (December) and less so in late winter (February). It is shown that these interbasin atmospheric teleconnections dominate the response in the Euro-Atlantic sector in early winter by modifying the subtropical South Asian jet (SAJET) and forcing a wavenumber-3 response that projects spatially onto the positive North Atlantic Oscillation (NAO) pattern. On the contrary, during late winter, the response in the Euro-Atlantic sector is dominated by the well-known ENSO wave train from the tropical Pacific region, involving extratropical anomalies that project spatially on the positive phase of the Pacific–North American (PNA) pattern and the negative phase of the NAO pattern. Numerical experiments with an atmospheric model (an AGCM) forced by an Indian Ocean heating dipole anomaly support the hypothesis that the Indian Ocean modulates the SAJET and enforces the Rossby wave propagation to the Euro-Atlantic region in early winter. These phenomena are also investigated using the ECMWF SEAS5 reforecast dataset. In SEAS5, the ENSO interbasin tropical teleconnections and the response of the Euro-Atlantic circulation anomalies and their change from early to late winter are realistically predicted, although the strength of the early winter signal originated from the Indian Ocean is underestimated.

Extratropical disturbances are known to impact the genesis and intensification of Mixed Rossby‐Gravity waves (MRGW) in the Western Hemisphere (WH). The study provides observational evidence supporting the wave resonance (WR) theory which attempts to explain the intensification of MRGW by extratropical forcing. Wavenumber‐frequency cospectral analysis and a bulk measure of growth of MRGW estimated using reanalysis data reveal that the extratropical forcing manifested in the form of eddy momentum flux convergence can create eddy kinetic energy (EKE) and aid the intensification of MRGW via WR mechanism during boreal winter season. However, the WR mechanism does not hold during boreal summer season as the extratropical forcing tends to dampen the MRGW. The analysis also reveals that the Doppler‐shifted eastward propagating MRGW in the WH during boreal winter season are not maintained by extratropical forcing, marking another highlight of this study. Plain Language Summary Mixed Rossby‐Gravity waves (MRGW) are a key component of tropical dynamics and extratropical forcing has been considered as an important factor in governing the growth of MRGW. Wave resonance (WR) theory, as proposed by previous theoretical studies, offers a compelling framework for understanding how extratropical forcing influences MRGW. This study seeks to provide observational evidence supporting the intensification of MRGW by extratropical forcing using the WR framework. Analyses conducted using reanalysis data show that the interaction between MRGW and extratropical forcing generates eddy kinetic energy, which intensifies the MRGW during boreal winter. In contrast, this interaction leads to the attenuation of MRGW during boreal summer season, highlighting the seasonal variability in these dynamics. Interestingly, while Doppler‐shifted eastward propagating MRGW are observed over the Western Hemisphere during boreal winter season, their presence cannot be attributed to extratropical forcing. Overall, this study underscores the importance of extratropical forcing as a crucial factor in modulating the MRGW. Key Points The study provides observational evidences supporting the growth of Mixed Rossby‐Gravity waves by tropical‐extratropical wave resonance Extratropical eddy momentum forcing enhances Mixed Rossby‐Gravity wave growth in boreal winter while dampens its growth in boreal summer Extratropical eddy momentum forcing does not explain the existence of Doppler shifted eastward propagating Mixed Rossby‐Gravity waves

A large-scale analysis of landfalling atmospheric rivers (ARs) along the west coast of North America and their association with the upper-tropospheric flow is performed for the extended winter (November–March) for the years 1979–2011 using Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis data. The climatology, relationship to the El Niño–Southern Oscillation and the Madden–Julian oscillation, and upper-level characteristics of approximately 750 landfalling ARs are presented based on the 85th percentile of peak daily moisture flux. AR occurrence along the West Coast is dominated by early season events. In composites of upper-level fields during AR occurrences, certain characteristics stand out irrespective of the tropical climate indices. This suggests that extratropical dynamical processes play a key role in AR dynamics. The influence of the large-scale circulation on AR intensity prior to landfall is examined by objectively selecting an extreme subset of 112 landfalling AR dates representing the 95th percentile of strongest cases. Each landfalling AR date that is identified is traced backward in time using a novel semiautomated tracking algorithm based on spatially and temporally connected organized features in integrated moisture transport. Composites of dynamical fields following the eastward progression of ARs show a close relationship of the location of the jet, Rossby wave propagation, and anticyclonic Rossby wave breaking in the upper troposphere of the eastern Pacific and moisture transport in the lower troposphere. Comparison between the strongest and the weakest ARs within the most extreme subset shows differences in both the intensity of moisture transport and the scale and development of anticyclonic Rossby wave breaking in the eastern Pacific.

After consecutive two years with the La Niña phenomenon in 2020–2021, cold sea surface temperature (SST) anomalies in the central-eastern equatorial Pacific persisted in 2022, known as a “triple-dip” La Niña event. These conditions have had a profound impact on global weather and climate. Understanding the underlying processes and mechanisms is crucial for improving ENSO prediction and has significant socioeconomic implications. In this study, we investigate the processes responsible for the evolution of the third-year La Niña event in 2022 based on related observations and an intermediate coupled model (ICM). The results show that two factors are essential for the development of the third-year La Niña event: surface easterly wind anomalies over the central-western equatorial Pacific in early 2022 and the induced cold SST anomalies by the former. In the first half of 2022, mainly due to the off-equatorial downwelling Rossby wave reflection at the western boundary, the equatorial downwelling Kelvin waves continuously induce warming effect in the upper ocean, acting to weaken the cold SST anomalies in the central-eastern Pacific. However, the easterly wind anomalies over the central-western equatorial Pacific during February–March 2022 induce upwelling Kelvin waves, whose cooling effect reverses the warming SST tendency. Afterward, despite the pronounced warming effect induced by the reflected downwelling Kelvin waves, the wind-induced cold SST anomalies persist on the equator during the equatorial cold season, which are further intensified by the Bjerknes feedback. Sensitivity experiments based on the ICM further confirm these outcomes. When the cooling effect induced by the easterly wind anomalies is included in ocean initial conditions, the ICM can accurately capture the reappearance of the La Niña phenomenon. These results highlight the importance of anomalous easterlies in the central-western equatorial Pacific during the decaying phase of the La Niña event.

Teleconnections from the tropics energize variations of the North Pacific climate, but detailed diagnosis of this relationship has proven difficult. Simple univariate methods, such as regression on El Niño–Southern Oscillation (ENSO) indices, may be inadequate since the key dynamical processes involved—including ENSO diversity in the tropics, re-emergence of mixed layer thermal anomalies, and oceanic Rossby wave propagation in the North Pacific—have a variety of overlapping spatial and temporal scales. Here we use a multivariate linear inverse model to quantify tropical and extratropical multiscale dynamical contributions to North Pacific variability, in both observations and CMIP6 models. In observations, we find that the tropics are responsible for almost half of the seasonal variance, and almost three-quarters of the decadal variance, along the North American coast and within the Subtropical Front region northwest of Hawaii. SST anomalies that are generated by local dynamics within the northeast Pacific have much shorter time scales, consistent with transient weather forcing by Aleutian low anomalies. Variability within the Kuroshio–Oyashio Extension (KOE) region is considerably less impacted by the tropics, on all time scales. Consequently, without tropical forcing the dominant pattern of North Pacific variability would be a KOE pattern, rather than the Pacific decadal oscillation (PDO). In contrast to observations, most CMIP6 historical simulations produce North Pacific variability that maximizes in the KOE region, with amplitude significantly higher than observed. Correspondingly, the simulated North Pacific in all CMIP6 models is shown to be relatively insensitive to the tropics, with a dominant spatial pattern generally resembling the KOE pattern, not the PDO.

The Mesospheric Airglow/Aerosol Tomography and Spectroscopy (MATS) satellite was launched in November 2022, carrying as its main instrument a limb-viewing telescope with six spectral channels designed to image atmospheric O.sub.2 airglow and noctilucent clouds. Although the main objective of the satellite mission is to observe structures in the airglow introduced by propagating smaller-scale waves, the airglow emissions are also subjected to large-scale dynamic disturbances, such as atmospheric tides and planetary waves. This work presents large-scale structures in the airglow field, as observed by the MATS limb imager from February 2023 to April 2023. The ascending (north-going) node in the satellite orbit, corresponding to the local sunset, is dominated by a strong equatorial maximum in the dayglow. In contrast, the descending (south-going) node, corresponding to the local sunrise, indicates an accompanying equatorial minimum. These characteristics align with the expected behaviour of atmospheric tidal movements. Specifically, a downwelling of atomic oxygen is expected over the Equator at local sunset, contributing to airglow chemistry and enhancing the emissions. Another distinct feature in the data is a westward propagating disturbance observed at high latitudes in the northern hemisphere, maximising in February, interpreted as the quasi-10 d planetary wave of zonal wavenumber 1.

El Niño–Southern Oscillation (ENSO) teleconnections have been recognized as possible negative influences on crop yields in the United States during the summer growing season, especially in a developing La Niña summer. This study examines the physical processes of the ENSO summer teleconnections and remote impacts on the United States during a multiyear La Niña life cycle. Since 1950, a developing La Niña summer is either when an El Niño is transitioning to a La Niña or when a La Niña is persisting. Due to the distinct prior ENSO conditions, the oceanic and atmospheric characteristics in the tropics are dissimilar in these two different La Niña summers, leading to different teleconnection patterns. During the transitioning summer, the decaying El Niño and the developing La Niña induce suppressed deep convection over both the subtropical western Pacific (WP) and the tropical central Pacific (CP). Both of these two suppressed convection regions induce Rossby wave propagation extending toward North America, resulting in a statistically significant anomalous anticyclone over northeastern North America and, therefore, a robust warming signal over the Midwest. In contrast, during the persisting summer, only one suppressed convection region is present over the tropical CP induced by the La Niña SST forcing, resulting in a weak and insignificant extratropical teleconnection. Experiments from a stationary wave model confirm that the suppressed convection over the subtropical WP during the transitioning summer not only contributes substantially to the robust warming over the Midwest but also causes the teleconnections to be different from those in the persisting summer.

Successive atmospheric river (AR) events—known as AR families—can result in prolonged and elevated hydrological impacts relative to single ARs due to the lack of recovery time between periods of precipitation. Despite the outsized societal impacts that often stem from AR families, the large-scale environments and mechanisms associated with these compound events remain poorly understood. In this work, a new reanalysis-based 39-yr catalog of 248 AR family events affecting California between 1981 and 2019 is introduced. Nearly all (94%) of the interannual variability in AR frequency is driven by AR family versus single events. Using k-means clustering on the 500-hPa geopotential height field, six distinct clusters of largescale patterns associated with AR families are identified. Two clusters are of particular interest due to their strong relationship with phases of El Ni˜no–Southern Oscillation (ENSO). One of these clusters is characterized by a strong ridge in the Bering Sea and Rossby wave propagation, most frequently occurs during La Ni˜na and neutral ENSO years, and is associated with the highest cluster-average precipitation across California. The other cluster, characterized by a zonal elongation of lower geopotential heights across the Pacific basin and an extended North Pacific jet, most frequently occurs during El Ni˜no years and is associated with lower cluster-average precipitation across California but with a longer duration. In contrast, single AR events do not show obvious clustering of spatial patterns. This difference suggests that the potential predictability of AR families may be enhanced relative to single AR events, especially on subseasonal to seasonal time scales.

This study revealed a significant interdecadal change in the impact of spring western Tibetan Plateau (TP) snow cover (TPSC) on subsequent summer compound heat waves (CHWs) in western Europe (WE) after 1998. This interdecadal change is attributed to a change in a western Europe high–western TP low (WEH–TPL) atmospheric circulation pattern. This pattern arises due to both the inherent variability of TPSC and the phase transition of the Atlantic multidecadal oscillation (AMO) after 1998. The increased magnitude and persistence of western TPSC from spring to summer after 1998 enhanced the snow–atmosphere coupling effect, intensifying ascent and decent motion over the TP and WE, respectively, and strengthening the WEH–TPL pattern. In addition, the post-1998 positive AMO phase favors continuous and stable downstream Rossby wave propagation, enhancing the WEH–TPL pattern and the TPSC–CHWs relationship. Further analyses reveal that the interdecadal changes in the TPSC and the AMO around 1998 contribute to the presence of “double jets” over the North Atlantic–central Eurasian sectors. The TPSC–related anomalous atmospheric circulation and AMO phase shift contribute to the southern and northern branches of the intensified westerly jet, respectively. These conditions create a favorable environment for the formation and persistence of summer CHWs in WE. Numerical modeling experiments with a linear baroclinic model confirm these findings. Our findings suggest that in the context of a changing climate, TPSC plays a pivotal role in the genesis of summer CHWs in WE and may serve as a valuable predictor for CHWs.Significance StatementThis study discovered that starting from 1998 there was a significant change in how spring snow cover on the western Tibetan Plateau affects summer compound heat waves in western Europe. After 1998, the snow cover on the western Tibetan Plateau increased in size and lasted longer from spring to summer, intensifying the interaction between the snow and the atmosphere. This led to more rising and sinking air over the Tibetan Plateau and western Europe, respectively. Also, after 1998 a positive phase of the Atlantic multidecadal oscillation (AMO) was favorable for the circulation connection between western Europe and the Tibetan Plateau. Further analysis showed that these changes in snow cover and the AMO after 1998 caused “double jets” in the North Atlantic and central Eurasia that created better conditions for summer heat waves in western Europe. Numerical models are used to confirm these findings. Our research indicates that, in a changing climate, the snow cover on the western Tibetan Plateau plays a crucial role in the development of summer heat waves in western Europe and can be a useful predictor for these heat waves.

Changes in the stratospheric polar vortex (SPV) can remarkably impact tropospheric circulation. Based on the diagnosis of reanalysis data, this study finds that the location shift rather than the strength change dominates the intraseasonal variability of SPV. Further analysis suggests that it couples well with the tropospheric circulation, forming an intraseasonal stratosphere–troposphere oscillation (STO). The STO shows periodic westward propagation throughout its life cycle and has a deep structure extending from the troposphere to the stratosphere. It reflects the movement of the SPV toward North America, then the North Pacific, Eurasia, and the North Atlantic, and causes significant changes in surface air temperature over North America and East Asia. The mechanism of the STO involves Rossby wave propagation between the troposphere and stratosphere and cross-scale interactions in the troposphere. Upward Rossby wave propagation from the troposphere over East Asia maintains the STO’s stratospheric component, and the reflection of these waves back to the troposphere contributes substantially to the STO’s tropospheric center over North America. Meanwhile, the linear and nonlinear processes explain the STO’s westward propagation in the troposphere, which facilities vertical wave propagation changes. The STO unifies the SPV shifts, the retrograding tropospheric disturbances, and the wave coupling processes into one framework and provides a holistic view for a better understanding of the intraseasonal stratosphere–troposphere coupling. Given its oscillating nature, time scale, and widespread surface response, the STO may be a potential source of predictability for the subseasonal-to-seasonal prediction.