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The National Aeronautics and Space Administration (NASA) Atmospheric Waves Experiment (AWE) instrument, launched in November 2023, provides direct observation of small‐scale (30–300 km) gravity waves (GWs) in the mesosphere on a global scale. This work examined changes in GW activity observed by AWE during two major Sudden Stratospheric Warmings (SSWs) in the 2023 and 2024 winter season. Northern Hemisphere (NH) midlatitude GW activity during these events shared similarities. Variations in mesospheric GW activity showed an evident correlation with the magnitude of zonal wind in the upper stratosphere. NH midlatitude GW activity at ∼ $\\mathit{\\sim }$87 km was reduced following the onset of SSWs, likely caused by wind filtering and wave saturation. The upward propagation of GWs was suppressed when the zonal wind reversed from eastward to westward in the upper stratosphere. In regions where the zonal wind weakened but remained eastward, the weakened GWs could be due to their refraction to shorter vertical wavelengths.

Atmospheric flow of cold air over mountain barriers in the Alpine region often gives a rise to strong and gusty downslope wind, Bora. Such flows are often accompanied by atmospheric waves, generated by the flow passing an elevated barrier. Such phenomenon can only rarely be observed visually and can generally not be reliably reproduced by simplified numerical models. Orography‐induced atmospheric small‐scale waves were experimentally observed on 25 January 2019 during a Bora outbreak in the Vipava valley, Slovenia. A vertical scanning lidar, positioned at the lee side of the Trnovski gozd mountain and a fixed direction lidar, 5 km apart in the Vipava valley, were used to characterize the density field. The flow exhibited a stationary jump after the mountain ridge and, superimposed, an oscillatory flow pattern. High‐resolution numerical simulations complemented the observations and supported experimental results on the flow periodicity but also on the wave structures and propagation characteristics. Plain Language Summary Airflow passing over mountainous barriers often leads to the generation of atmospheric small‐scale waves. These waves play an important role in redistributing momentum and energy, impacting atmospheric dynamics on both local and regional scales. Studying mountain regions as sources of these waves has been challenging due to the highly nonlinear interactions between the airflow and often complex terrain orography, compounded by a lack of meter‐scale high‐resolution observations and efficient models. This study investigates the atmospheric waves appearing above Vipava valley, Slovenia, during a strong and gusty downslope wind event known as Bora. Using high‐resolution remote sensing observations and numerical simulations, we characterized the properties of these waves. Coordinated high‐resolution measurements of atmospheric flow were obtained using two lidar systems—one positioned at the mountain site and the other within the valley. Two‐dimensional lidar scans revealed a downslope Bora flow layer and its sudden rise after the mountain ridge, which may indicate an hydraulic jump and is associated with small‐scale waves observed further down the valley. High‐resolution numerical simulations of the atmospheric flow over the actual terrain complemented and supported the observations of atmospheric structures above the valley. Key Points Vertical and horizontal lidar measurements in and above the Vipava valley show orography‐induced atmospheric waves The horizontal propagation of such atmospheric waves is with constant speed and their wavelength is estimated High‐resolution numerical simulations of the flow over the valley support lidar measurement results on flow periodicity

The leading pattern of boreal spring 250-hPa meridional wind anomalies over the North Atlantic and mid-high latitude Eurasia displays an obvious wave train. The present study documents the structure, energy source, relation to the North Atlantic sea surface temperature (SST), and impacts on Eurasian climate of this wave train during 1948–2018. This atmospheric wave train has a barotropic vertical structure with five major centers of action lying over subtropics and mid-latitudes of the North Atlantic, northern Europe, central Eurasia, and East Asia, respectively. This spring wave train can efficiently extract available potential energy from the basic mean flow. The baroclinic energy conversion process and positive interaction between synoptic-scale eddies and the mean flow both play important roles in generating and maintaining this wave train. The North Atlantic horseshoe-like (NAH) SST anomaly contributes to the persistence of the wave train via a positive air–sea interaction. Specifically, the NAH SST anomaly induces a Rossby wave-type atmospheric response, which in turn maintains the NAH SST anomaly pattern via modulating surface heat fluxes. This spring atmospheric wave train has significant impacts on Eurasian surface air temperature (SAT) and rainfall. During the positive phase of the wave train, pronounced SAT warming appears over central Eurasia and cooling occurs over west Europe and eastern Eurasia. In addition, above-normal rainfall appears over most parts of Europe and around the Lake Baikal, accompanied by below-normal rainfall to east of the Caspian Sea and over central Asia.

This study examines the impact of the winter Arctic sea ice concentration (ASIC) anomaly on the succedent summer tropical cyclone genesis frequency (TCGF) over the western North Pacific (WNP) and provides a new insight into the underlying physical mechanisms. There is a significant time-lagged relation between winter ASIC anomalies over Greenland-Barents-Kara (GBK) seas and the following summer TCGF over the southeastern part of the WNP. This delayed association is attributable to large-scale circulation anomalies and the air-sea interaction processes over the North Pacific induced by the winter ASIC anomalies. Specifically, a higher winter ASIC over the GBK seas induces an atmospheric wave train that propagates southeastward to the North Pacific. The associated cyclonic anomaly over the mid-latitude North Pacific is accompanied by southwesterly wind anomalies over the subtropics and results in sea surface temperature (SST) warming by reducing upward surface heat fluxes. This SST warming is maintained and further extends southward to the tropical Pacific in the following summer via a wind-evaporation-SST feedback, which in turn forces overlying atmospheric circulation via a Gill-type atmospheric response, including a pair of cyclonic and anticyclonic anomalies in the low- and upper-level troposphere, respectively, over the WNP. These atmospheric anomalies favor TC genesis over the southeastern part of the WNP by decreasing the vertical wind shear and increasing the convection, low-level vorticity and humidity. The above processes apply to the years when lower ASIC winters are followed by decreased TC genesis over the southeastern part of the WNP except for opposite signs of SST and atmospheric circulation anomalies. This study suggests that the winter ASIC anomaly over the GBK seas is a potential predictor for the prediction of the WNP TCGF in the following summer.

This study reveals that the Arctic Oscillation (AO) in boreal spring has a marked impact on the Indian Ocean Dipole (IOD) in the following autumn. When the spring AO is in its positive (negative) phase, a positive (negative) IOD tends to occur in the following autumn. Possible physical processes for the impact of the spring AO on the autumn IOD are further examined. Positive spring AO is accompanied by a dipole precipitation anomaly pattern over North Atlantic, with positive anomalies over high latitude and negative anomalies over mid-latitude. The associated atmospheric heating anomalies over mid-high latitudes North Atlantic further induces an atmospheric wave train from the North Atlantic to the Indian Ocean (IO), leading to pronounced easterly wind anomalies over the tropical northern IO. These easterly wind anomalies can cause warm sea surface temperature (SST) anomalies in the western tropical Indian Ocean (WTIO) by modulating surface heat fluxes and oceanic heat transport. The warm SST anomalies in the WTIO persist into the following autumn, which increase the zonal gradient of SST anomalies in the equatorial IO and lead to easterly wind anomalies over there. Moreover, the equatorial IO easterly wind anomalies can induce cold SST anomalies in the southeastern tropical Indian Ocean (SETIO) via increasing upwelling of cold water. In addition, previous studies have indicated that a positive spring AO could lead to significant positive precipitation anomalies in the tropical central Pacific in the following summer. Our results show that the associated atmospheric heating over the tropical central Pacific can enhance the southeasterly wind anomalies off the west coast of Sumatra via anomalous Walker circulation, which also play a role in contributing to cold SST anomalies in the SETIO. Therefore, the spring AO may exert a significant impact on the subsequent autumn IOD through the above processes and can be used as a potential predictor of the IOD event.

The present study reveals a close connection between the winter Arctic sea ice concentration (ASIC) change over the Greenland–Barents Seas (GBS) and the El Niño–Southern Oscillation (ENSO) in the following winter. When there is more winter ASIC over the GBS, an El Niño-like sea surface temperature (SST) warming tends to occur in the tropical central-eastern Pacific (TCEP) during the following winter. It is found that the winter ASIC increase over the GBS triggers an atmospheric wave train propagating southeastward from the high latitude Eurasia towards the subtropical North Pacific, with cyclonic wind anomalies over the subtropical North Pacific. A barotropic model experiment with anomalous convergence prescribed around the GBS reproduces reasonably well the atmospheric wave train. The induced spring SST warming and associated anomalous atmospheric heating over the subtropical North Pacific play an essential role in the formation and maintenance of lower-level westerly wind anomalies over the western tropical Pacific. These westerly wind anomalies induce SST warming in the TCEP during the following summer via triggering an eastward propagating equatorial warm Kelvin wave. The summer TCEP SST warming further develops into an El Niño event in the following winter via a Bjerknes-like positive air–sea feedback process. This result suggests that the winter ASIC change around the GBS is a potential predictor of the ENSO events with a lead time of 1 year.

A multi-group of strong atmospheric waves (wave packet nos. 1–5) over China associated with the 2022 Hunga Tonga–Hunga Ha′apai (HTHH) volcano eruptions were observed in the mesopause region using a ground-based airglow imager network. The horizontal phase speed of wave packet nos. 1 and 2 is approximately 309 and 236 m s−1, respectively, which is consistent with Lamb wave L0 mode and L1 mode from theoretical predictions. The amplitude of the Lamb wave L1 mode is larger than that of the L0 mode. The wave fronts of Lamb wave L0 and L1 below the lower thermosphere are vertical, while the wave fronts of L0 mode tilt forward above the lower atmosphere, exhibiting internal wave characteristics which show good agreement with the theoretical results. Two types of tsunamis were simulated; one type of tsunami is induced by the atmospheric-pressure wave (TIAPW), and the other type of tsunami is directly induced by the Tonga volcano eruption (TITVE). From backward ray-tracing analysis, the TIAPW and TITVE were likely the sources of wave packet nos. 3 and 4–5, respectively. The scale of tsunamis near the coast is very consistent with the atmospheric AGWs observed by the airglow network. The atmospheric gravity waves (AGWs) triggered by TITVE propagate nearly 3000 km inland with the support of a duct. The atmospheric-pressure wave can directly affect the upper atmosphere and can also be coupled with the upper atmosphere through the indirect way of generating a tsunami and, subsequently, tsunami-generating AGWs, which will provide a new understanding of the coupling between ocean and atmosphere.

Previous studies indicated that boreal spring North Atlantic horseshoe-like (NAH) sea surface temperature (SST) anomaly pattern has a significant impact on the surface air temperature (SAT) variation over the mid-high latitudes of Eurasia via an atmospheric wave train. This study investigates the connection of springtime NAH SST anomaly and the Eurasian SAT variation in thirty-two coupled climate models that participated in the fifth phase of the Coupled Model Intercomparison Project (CMIP5). Most of the CMIP5 models simulate well the spatial pattern of SST anomalies in the North Atlantic related to the spring NAH SST, but overestimate the spatial standard deviation. There exists large spreads in the connection of the spring NAH SST anomaly pattern and the Eurasian SAT variation among the CMIP5 models. The models that capture the observed spring NAH SST-Eurasian SAT connection can well reproduce the observed atmospheric wave train, in particular, with a marked anticyclonic anomaly over north Europe and a cyclonic anomaly over central Eurasia. This wave train results in above-normal SAT over west Europe and eastern Eurasia and below-normal SAT over central Eurasia. By contrast, the models that fail to reproduce the spring NAH SST-related Eurasian SAT anomalies show limitations in simulating the atmospheric wave train. Further analysis suggests that the models with larger climatological westerly winds over the mid-latitude North Atlantic and high-latitude Eurasia as well as lower climatological SST in the tropical northern Atlantic have a better ability in simulating the observed atmospheric wave train over the North Atlantic and Eurasia and thus perform better in simulating the spring NAH SST-Eurasian SAT connection.

Large volcanic eruptions produce various atmospheric wave perturbations. One of these wave manifestations is atmospheric gravity waves (AGWs), which can be observed through remote sensing satellite measurements. Based on multisensory instruments on board Aqua, Suomi NPP, and Thermosphere Ionosphere Mesosphere Energetics Dynamics satellites, we discuss the propagation of AGWs across stratospheric and mesospheric altitudes following three large volcanic eruptions of 15 January 2022 Hunga Tonga‐Hunga Ha'apai, 11 April 2021 La Soufrière, and 13 February 2014 Kelud. Following the events, the mechanical updraft of air, observed as tropopause overshooting and the enhanced H2O in Microwave Limb Sounder measurements, contributed to the convective generation of AGWs through mechanical oscillator effect and thermal forcing. The present study is an important and useful contribution in compiling the propagation of AGWs across various atmospheric layers and substantiating their convective generation, for large volcanic eruptions. Thereby, strengthening our understanding of lithosphere‐atmosphere coupling through wave‐dynamic pathways by reinforcing the existing knowledge.