Meridional Wind Research Articles

Synoptic weather patterns during fire spread events in Siberia

Several recent studies have indicated a strong relationship between extensive wildfires in Siberia and synoptic-scale weather processes. In this study, we used the concept of fire spread events to investigate the relationships between synoptic and surface-level weather conditions and extensive wildfires in Siberia during 2001–2022 using the MODIS and ERA5 reanalysis products. We analyzed the spatio-temporal features and seasonality of fire spread events in the region and found that most of them occurred in the central part of Eastern Siberia (ES) during the summer months, following the wildfire season in the region. A significant positive trend in the annual count of fire spread events was found in ES, coinciding with non-significant negative trends in cloud cover and precipitation and non-significant positive trends in air temperature and the fire weather index. Results show that in the ES region, which accounts for 46 % of the total number of considered events, the main driver of fire spread events is the formation of a positive geopotential height anomaly, which, based on the pattern of the meridional wind component, indicates the presence of an anticyclone above the area of fire spread events. The presence of a high-pressure zone causes a decrease in cloud cover over regions with fires, leading to increases in the amount of incoming solar radiation and surface air temperature and a decrease in precipitation. These conditions contribute to the drying of fuel and an increase in the overall fire hazard level, which in turn leads to an intensification of the combustion process, as evidenced by an increase in the radiative power of fires.

Different Configurations of Cross-Equatorial Flows over the Indian Ocean–Western Maritime Continent and Their Implications for Improving Regional Climate Predictability

Abstract Low-level cross-equatorial flows (CEFs) over the Indian Ocean–western Maritime Continent (IO-wMC) play a crucial role in transporting energy, mass, and water vapor into the Northern Hemisphere during the Asian summer monsoon season (May–September). Utilizing ECMWF reanalysis data (ERA5), we investigate three CEFs over the IO-wMC: the Somali-CEF, Bay of Bengal CEF (BoB-CEF), and South China Sea CEF (SCS-CEF). The statistical independence of the Somali-CEF and BoB-CEF implies distinct formation processes on the interannual time scale. To examine the interannual variability of CEFs, we perform an empirical orthogonal function (EOF) analysis of vertically integrated meridional winds from surface pressure to 850 hPa over the equatorial IO-wMC. The first EOF mode reveals a weakening of the Somali-CEF and strengthening of the BoB-CEF and SCS-CEF, which is associated with the concurrent summer Indian Ocean dipole and quasi-biennial phase transition of El Niño events. The second EOF is related to the Indo-western Pacific Ocean capacitor (IPOC) mode that often emerges in post–El Niño summers. Despite the IPOC’s influence on meridional winds over the tropical southwestern Indian Ocean, three CEFs are uncorrelated with the EOF2. On the other hand, the Somali-CEF and BoB-CEF are significantly weakened in the EOF3. The EOF3 is an ENSO-unrelated internal IPOC mode. We further use a multilinear regression model based on preceding sea surface temperature (SST) over the Indo-Pacific and EOF3 to predict regional climate anomalies and compare the prediction skill with the SST-based models. Our results suggest that adding EOF2- and EOF3-related variability to the prediction model can improve Asian summer monsoon predictability. Significance Statement This study examines the interannual variability of low-level cross-equatorial flows (CEFs) over the Indian Ocean and western Maritime Continent, crucial components of the Asian summer monsoon. We identify three CEFs and investigate their relationships with interannual sea surface temperature modes in the Indo-Pacific region. The leading empirical orthogonal function (EOF) modes of equatorial meridional wind reveal diverse CEF configurations and post–El Niño summer wind variability. Also, the EOFs distinguishes between El Niño and El Niño–unrelated equatorial meridional wind variability. Utilizing a multilinear regression model based on both preceding Indo-Pacific SST modes and configurations of CEFs, we demonstrate the potential to enhance regional climate predictions, contributing to a more comprehensive understanding and forecasting of the Asian summer monsoon and its associated climate impacts.

A survey of westward‐propagating mixed Rossby–Gravity waves and quantification of their association with extratropical disturbances

AbstractIn 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.

Seasonal and Interannual Variations of Global Tides in the Mesosphere and Lower Thermosphere Neutral Winds: I. Diurnal Tides

AbstractIn the mesosphere and lower thermosphere, diurnal tides are responsible for the dynamics and structures at low latitudes since they have largest amplitudes there. Based on the 20‐year (2002−2021) observations from Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics Doppler interferometer (TIDI), we investigate the seasonal variations of three diurnal tidal components (DW1, DE3, DW2), and their responses to stratospheric quasi‐biennial oscillation (SQBO) and solar cycle globally. The results show that: (a) DW1, DE3 and DW2 show prominent semiannual (SAO) and annual oscillations (AO) in their peak regions where the ratio of their annual mean amplitude to their maximum annual mean amplitude is larger than 0.8. DW1 also exhibit strong terannual oscillations (TAO) especially in meridional wind. (b) The responses of the amplitudes of seasonal variations of DW1, DE3 and DW2 to SQBO and solar cycle are comparable in magnitude to those of their annual mean amplitudes (for response to solar cycle, even stronger in some cases), and thus cannot be neglected. (c) In their respective peak regions, the responses of annual mean amplitudes of these diurnal tidal components to SQBO are uniformly positive except DW2, and their responses to solar cycle are uniformly negative. For these diurnal tides, the amplitudes of the dominant seasonal variations exhibit consistent response patterns with those of annual means to the SQBO/solar cycle. (d) Empirical formulas are given, which well describe the seasonal and interannual variations of dominant diurnal tidal components in their peak regions.

Evening Solar Terminator Waves in Earth's Thermosphere: Neutral Wind Signatures Observed by ICON‐MIGHTI

AbstractThe moving solar terminator (ST) generates atmospheric disturbances, broadly termed solar terminator waves (STWs). Despite theoretically recurring daily, STWs remain poorly understood, partially due to measurement challenges near the ST. Analyzing Michelson Interferometer for Global High‐resolution Thermospheric Imaging (MIGHTI) data from NASA's Ionospheric Connection Explorer (ICON) observatory, we present observations of STW signatures in thermospheric neutral winds, including the first reported meridional wind signatures. Seasonal analysis reveals STWs are most prominent during solstices, when they intersect the ST about ∼20° latitude from the equator in the winter hemisphere and have phase fronts inclined at a ∼40° angle to the ST. We also provide the first observed STW altitude profiles, revealing large vertical wavelengths above 200 km. Comparing these observations to four different models suggests the STWs likely originate directly or indirectly from waves from below 97 km. STWs may play an under‐recognized role in the daily variability of the thermosphere‐ionosphere system, warranting further study.

The Response of Surface Temperature Persistence to Arctic Sea‐Ice Loss

AbstractWe investigate the response of surface temperature persistence, quantified using a lagged autocorrelation, to imposed Arctic sea‐ice loss in coupled model experiments. Sea‐ice loss causes increases in persistence over ocean in midlatitudes and the low‐Arctic, which are of a similar magnitude to the total response to climate change in these regions. Using an idealized model, we show that sea‐ice loss induces a slowing of meridional wind anomalies, which can drive the midlatitude persistence increase obtained in coupled models. Sea‐ice loss should induce persistence increases in the Arctic, through its effect on the surface heat capacity. However, in coupled models with imposed sea‐ice loss, persistence increase in the Arctic is essentially absent. We suggest that methods used to constrain sea‐ice in coupled models may spuriously reduce the effects of sea‐ice loss on persistence.

Springtime Southern Hemisphere Quasi‐Stationary Planetary Wave Activities Associated With ENSO/IOD

AbstractThe tropical climate variabilities, such as Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO), are accompanied by changes in the tropical deep convection which can influence the atmospheric circulation in the Southern Hemisphere (SH). To investigate each role of IOD and ENSO in the September‐November (SON) circulation, we examine teleconnection patterns associated with IOD and ENSO events using the ERA5 monthly averaged data from 1979 to 2020. Our approach is to calculate the power spectral density (PSD) of the sea level pressure (SLP) and meridional wind and geopotential height at 300 hPa that are decomposed by zonal wave numbers (ZWNs), and to compute their correlations with IOD and ENSO at each latitudinal band. The main results are that IOD (ENSO) is negatively (positively) correlated with PSDs of ZWN2 and ZWN3 (ZWN1) at 300 hPa in the SH middle latitudes. Considering the Rossby wave train, IOD (ENSO) considerably affects the variability of the ZWN3 (ZWN1) pattern, which influences the meridional exchange of momentum. Additionally, the relationship between IOD and ZWN3 has become tighter in recent years, which is not seen in that with ENSO. The IOD and ENSO events also modify the SLP patterns and meridional surface winds, modulating the sea ice extent in the Southern Ocean. During the highly positive 2019 IOD event, the variability of the middle latitudes atmospheric circulation was considerably larger than climatology, suggesting a higher chance of more extreme weather patterns associated with more frequent intense IOD events in the warming climate.

Improving Boreal Summer Precipitation Predictions From the Global NMME Through Res34‐Unet

AbstractGlobal warming and climate change have increased the frequency and intensity of floods and droughts, limiting economic development and threatening human survival. Therefore, accurate global forecasts well in advance of precipitation are essential to facilitate timely adaptation. Current seasonal forecasts are based mainly on numerical models, but raw forecasts suffer from systematic bias and under/overdispersion problems and cannot be directly used in applications. In addition, bias correction methods for global forecasts need to be further developed. Based on a fusion of ResNet34 and Unet, called Res34‐Unet, deep learning post‐processing is proposed to correct global precipitation forecasts of the North American Multi‐Model Ensemble (NMME). Compared with raw global NMME predictions, post‐processed precipitation predictions can be improved by up to 45%, which is significant at different latitudes. Feature importance analysis shows that precipitation itself, meridional wind, and sea surface temperature are key factors.

Multi-Year Behavioral Observations of Quasi-2-Day Wave Activity in High-Latitude Mohe (52.5°N, 122.3°E) and Middle-Latitude Wuhan (30.5°N, 114.6°E) Using Meteor Radars

The behavior of multi-year quasi-2-day wave (Q2DW) activity in the high and middle latitudes in the mesosphere and lower thermosphere regions during 2013–2022 is revealed, for the first time, using two meteor radars along the 120°E longitude, which are located at Mohe (52.5°N, 122.3°E) and Wuhan (30.5°N, 114.6°E). We first describe the interannual monthly mean characteristics of the Mohe and Wuhan winds. We then determine the extraction of the Q2DWs via a least-squares method and calculate the occurrence dates, amplitudes, periods, and phases of the zonal and meridional Q2DWs. We find that the summer zonal wind speed of Mohe reached ~35 m/s at ~94 km in 2022, and the meridional wind speed reached ~−20 m/s at ~88 km in 2017. Similarly, the zonal and meridional wind speeds in Wuhan reached ~48 m/s and ~−30 m/s at ~94 km and ~90 km, respectively, in the summer of 2020. Statistical analysis shows that, in Mohe and Wuhan, the highest frequency of Q2DWs is observed between days 200 and 220. The Q2DW is mainly associated with the background mean wind and is consistent with a selective filtering mechanism. We believe that the correlation between wind shear and Q2DW amplitude is higher in summer because wind shear reaches its maximum when Q2DW starts to amplify. The wave period of the Mohe zonal Q2DW is longer than that of the Wuhan zonal Q2DW, while that of the meridional Q2DW is shorter. In addition, the zonal and meridional Q2DW amplitudes are weaker in Mohe than in Wuhan. The vertical wavelength of the Q2DW in Wuhan is shorter than that in Mohe. Solar activity F10.7 does not appear to be strongly correlated with Q2DW behavior in Mohe and Wuhan.

A machine‐learning and data assimilation forecasting framework for surface waves

In this article, we combine deep symbolic regression (DSR) and ensemble optimal interpolation‐based data assimilation (DA) methods to correct the error in forecasts from the numerical model WaveWatch III. In our experiments, DA and DSR training is performed on hindcasts and then the model is integrated forward in time using both the numerical model and the symbolic expressions generated from the DSR procedure to generate the forecasts. The DSR method is utilized in this article to generate the symbolic equations that correct the model error in the WaveWatch III/ DA system. The proposed algorithm takes the zonal () and meridional () wind components from Global Forecast System (GFS) forecasts, wave heights from WaveWatch III, and geographical coordinates (latitude and longitude) to model physical relationships not included in the original numerical model. The DA is performed using Jason‐2 and Satellite with ARgos and ALtiKa (SARAL) altimeter measurements, and the independent testing uses in situ buoys. The root‐mean‐squared deviation (RMSD) of the proposed method is better than that of the numerical model with/without DA for up to 42 hr with only 12 days of assimilation spin‐up cycle. The symbolic equation generated from the proposed framework can be used to correct the predictions from WaveWatch III for weather prediction.

Response of ICON/MIGHTI Measured Low‐Mid Latitude OI630.0 and OI557.7 nm Dayglow Emissions to the 27 August 2021 Geomagnetic Storm

AbstractObservations from the Michelson Interferometer for Global High‐Resolution Thermospheric Imaging onboard the Ionospheric Connection Explorer spacecraft are used to study the response of OI630.0 and OI557.7 nm dayglow to a moderate geomagnetic storm on 27 August 2021. The storm reaches a minimum Dst index of −82 nT, significantly impacting the dayglow within the latitudinal range of approximately 20°N–42°N, where the dayglow observations are of good quality. During the geomagnetic storm, the OI630.0 dayglow intensity slightly increases, while the peak volume emission rate (VER) decreases, and the peak height rises noticeably. The F‐layer intensity, peak VER, and the entire‐layer intensity of OI557.7 dayglow decrease significantly. The rise in peak height is not noticeable for the OI557.7 dayglow. The VERs of the both dayglow emissions respond differently to the geomagnetic storm at different altitudes. The OI630.0 dayglow layer as a whole extends upward and rises in altitude. For dayglow averaged above 35°N, the OI630.0 dayglow VER increases above approximately 225 km but decreases below this altitude. The largest increase occurs near 300 km, reaching approximately 82.8%, while the largest decrease occurs around 160 km, reaching about −22.0%. The OI630.0 dayglow intensity increases by approximately 6.3%, the peak VER decreases by about −8.0%, and the peak height rises by approximately 16.3 km, corresponding to a 7.8% increase. The F‐layer intensity, peak VER, and the entire‐layer intensity of OI557.7 dayglow decrease by approximately −27.5%, −32.4% and −17.4%, respectively. The response of the dayglow also depends on longitude and is accompanied by a southward meridional wind.

Documenting the Progressions of Secondary Eyewall Formations

Abstract Intense tropical cyclones can form secondary eyewalls (SEs) that contract toward the storm center and eventually replace the inner eyewall, a process known as an eyewall replacement cycle (ERC). However, SE formation does not guarantee an eventual ERC, and often, SEs follow differing evolutionary pathways. This study documents SE evolution and progressions observed in numerous tropical cyclones, and results in two new datasets using passive microwave imagery: a global subjectively labeled dataset of SEs and eyes and their uncertainties from 72 storms between 2016 and 2019, and a dataset of 87 SE progressions that highlights the broad convective organization preceding and following an SE formation. The results show that two primary SE pathways exist: “No Replacement,” known as “Path 1,” and “Replacement,” known as the “Classic Path.” Most interestingly, 53% of the most certain SE formations result in an eyewall replacement. The Classic Path is associated with stronger column average meridional wind, a faster poleward component of storm motion, more intense storms, weaker vertical wind shear, greater relative humidity, a larger storm wind field, and stronger cold-air advection. This study highlights that a greater number of potential SE pathways exist than previously thought. The results of this study detail several observational features of SE evolution that raise questions about the physical processes that drive SE formations. Most important, environmental conditions and storm metrics identified here provide guidance for predictors in artificial intelligence applications for future tropical cyclone SE detection algorithms.

Identification of cross-equatorial northerly surge (CENS) relationship to very heavy rain events in Jakarta

CENS is a meteorological phenomenon in the form of surface wind flow originating from the South China Sea (SCS), very strongly across the equator. CENS is closely related to the increase in average rainfall in parts of Indonesia. There has been very heavy rainfall on January 18, 2022 in the Jakarta area which resulted in flooding. Therefore, this study focuses on seeing if there is a CENS relationship that is factor in increasing rainfall in the Jakarta area on January 18, 2022. The results showed that the NCS began to form on January 16-17, 2022 with a Siberian High of 1040.6 hPa and the propagation of cold air masses from Asia to the south which was an indicator of CENS. The next day, the CENS phenomenon began to occur, characterized by meridional wind speeds in the 105˚- 115˚BT region of less than -5 m/s. This phenomenon triggers the strengthening of the northeast Asian monsoon flow so that the area south of the equator, including Jakarta, gets a large moisture transport from the SCS. This causes the potential for the formation of high convective clouds to trigger very heavy rain and cause flooding in the Jakarta area on January 18, 2022.

Thermospheric Responses to the 3 and 4 November 2021 Geomagnetic Storm During the Main and Recovery Phases as Observed by NASA's GOLD and ICON Missions

AbstractLeveraging observations by two NASA missions—GOLD (Global‐scale Observations of the Limb and Disk) and ICON (Ionospheric Connection Explorer), we investigate concurrent responses of thermospheric composition, temperatures, and neutral winds to the geomagnetic storm on 3–4 November 2021, as well as their interplay at low and middle latitudes. The synergetic observations reveal remarkable depletions up to 60%–70% in GOLD O/N2, along with large enhancements in GOLD temperatures poleward of 30° in the middle thermosphere. Meridional winds from ICON observations are altered by ∼100 m/s equatorward of 25°N latitude and at 250 km, characterized by a reversal of prevailing northward winds to geomagnetic storm‐driven southward winds. This study fills a need, after a decade‐long gap, for observing concurrent and co‐located responses of composition, temperatures, and neutral winds in the thermosphere to geomagnetic storms.

Upper‐level wind mode over the North Atlantic‐Arctic Ocean leading to coherent variations of Eurasian and North American winter temperatures and weakness in the CMIP6 models

AbstractThis study discovers that the interannual synchronization of winter temperature anomalies between northern Eurasia and North America is closely related to variation in the second leading mode of the 300 hPa meridional wind anomalies (i.e., V300‐EOF2) over the North Atlantic‐Arctic Ocean. The V300‐EOF2 mode modulates the large‐scale Rossby wave propagating across the mid‐high latitudes of the Eurasian and North American continents at the mid‐high level of troposphere, and it also interferes with the climatological quasi‐stationary wavenumber‐2 to affect the stratosphere circulation, which facilitates the southward intrusion of cold air into Eurasia and North America simultaneously. This study also evaluated whether the CMIP6 high‐top (HT) models could more accurately represent the V300‐EOF2 mode and the associated modulation mechanism on synchronized temperature variation between Eurasia and North America than the low‐top (LT) models. The results demonstrate that although the multi‐model ensemble means (MME) of both the HT and LT models can reproduce the V300‐EOF mode, they all underestimate its modulation mechanism on atmospheric circulation and coherent temperature changes over mid‐latitude Eurasia and North America, with minor differences in simulation skill capability. Most importantly, although containing a full stratosphere, the HT models do not significantly improve their simulation ability to depict the observed linear interference between the anomalous wave‐2 and the climatological stationary wave‐2 compared to the LT models. Therefore, comparing to the LT models, the CMIP6 HT models present no clear advantages in reproducing the V300‐EOF2 mode and its associated low‐frequency climate anomalies. These results also have implications for the evaluation of the predictability of concomitant Eurasian‐North American cooling in the Subseasonal‐to‐Seasonal Prediction Project.

Moisture sources for precipitation over the Pamirs Plateau in winter and spring

AbstractThe abundant precipitation over Pamirs Plateau (PP) in spring and winter plays a vital role in the water resources over the arid areas of Central Asia. Understanding the moisture sources and water vapour transportation associated with precipitation are very important, but there were few studies investigating the moisture sources of PP. We used a Lagrangian model driven with the Weather Research and Forecasting output to analyse the moisture sources at Northern Pamirs Plateau (NPP) and Southern Pamirs Plateau (SPP) in spring 2016, 2009 and 2001, and in winter 2016, 2010 and 2017, as the seasonal precipitation in PP was the largest, median and lowest during 2001 to 2018, and the precipitation in spring and winter was much higher than that in summer and autumn. The moisture sources from four regions were quantified: Atlantic‐Europe‐Africa, Arctic‐Northern Asia, Indian Ocean, and moisture recycling from the PP. Atlantic‐Europe‐Africa and Indian Ocean are the dominant moisture source regions in spring and winter, which contribute more than 70% to the total moisture affecting the precipitation at PP. The contributions of Indian Ocean are higher at SPP than those at NPP in spring and winter. The contributions from Arctic‐Northern Asia and PP are generally low, except that the moisture from PP region contributed 19% to the spring precipitation at NPP, indicating the importance of a local moisture source in enhancing the spring precipitation at NPP. The moisture contributions originating from the different source regions show great differences between winter and spring. The moisture transportation is affected by westerlies, and the zonal winds in spring affect the moisture transportation, while the meridional winds over the Arabian Peninsula mainly affect the moisture transportation in winter.

Vintage Port prediction and climate change scenarios

The Douro region is renowned for its quality wines, particularly for the famous Port Wine. Vintage years, declared approximately 2–3 times per decade, signify exceptional quality linked to optimum climatic conditions driving grape quality attributes. Climate change poses challenges, as rising temperatures and extreme weather events impact viticulture. This study uses machine learning algorithms to assess the climatic influence on vintage years and climate change impacts for the next decades. Historical vintage data were collected from 1850 to 2014. Monthly climatic data for the same period were obtained, including temperature, precipitation, humidity, solar radiation, and wind components. Various machine-learning algorithms were selected for classification, and a statistical analysis helped identify relevant climate variables for differentiation. Cross-validation was used for model training and evaluation, with the hits and misses (confusion matrix) as the performance metric. The best-performing model underwent hyperparameter tuning. Subsequently, future climate projections were acquired for four regional climate models from 2030 until 2099 under different socio-economic scenarios (IPCC SSP2, SSP3, and SSP5). Quantile mapping bias adjustment was applied to correct future climate data and reduce model biases. Past data revealed that vintages occurred 23.6 % of the years, with an average of two vintage years per decade, with a slightly positive trend. Climate variables such as precipitation in March, air temperatures in April and May, humidity in March and April, solar radiation in March, and meridional wind in June were identified as important factors influencing vintage year occurrence. Machine-learning models were employed to predict vintage years based on the climate variables, with the XGBClassifier achieving the highest performance with 76 %/88 % hits for the vintage/non-vintage classes, respectively, and an ROC score of 0.86, demonstrating strong predictive capabilities. Future climate change scenarios under different socio-economic pathways were assessed, and the results indicated a decrease in the occurrence vintage years until 2099 (10.3 % for SSP2, 9.1 % for SSP3, and 5.8 % for SSP5). The study provides valuable insights into the relationship between climate variables and wine vintage years, enabling winemakers to make informed decisions about vineyard management and grape cultivation. The predictions suggest that climate change may challenge the wine industry, emphasising the need for adaptation strategies.

Present‐Day Regional Antarctic Sea Ice Response to Extratropical Cyclones

AbstractBoth atmospheric warming and poleward moisture transport increase the likelihood of sea ice surface melt. In the Southern Hemisphere, short‐lived extratropical cyclones (ETCs) are responsible for a bulk of total heat and moisture transport toward high latitudes. Although these storms form ubiquitously in the midlatitudes, moisture availability and temperature characteristics vary by source region. In this study, we assess atmospheric, oceanic, and sea ice concentration (SIC) anomalies associated with austral winter ETCs over different Antarctic regions using ERA5 reanalysis data. Between 1990 and 2019, we find a total of 514 ETCs, with greater storm frequency in the eastern hemisphere groups. Compared to the climatology, sea ice melts (grows) behind the warm (cold) front of each system and is negatively correlated with atmospheric poleward moisture transport, temperature, meridional winds, and sea surface temperature for all ETCs. We find that Bellingshausen storms move moisture and warm air furthest poleward over their lifespan. However, East Weddell and East Antarctic ETCs are responsible for greater absolute poleward moisture transport than Bellingshausen and Ross systems. More intense ETCs correspond to greater SIC through Day 1, suggesting that SIC impacts ETC strength, regardless of ETC region. From cyclogenesis to cyclolysis, sea ice extent declines underneath composite ETCs, trends are generally not significant. Overall, while sea ice response produced by ETC‐induced atmospheric and oceanic changes varies regionally, the long‐term impacts of ETCs on regional sea ice are negligible over the study period.

The Role of the North Atlantic for Heat Wave Characteristics in Europe, an ECHAM6 Study

AbstractThe recent severe European summer heat waves of 2015 and 2018 co‐occurred with cold subpolar North Atlantic (NA) sea surface temperatures (SSTs). However, a significant connection between this oceanic state and European heat waves was not yet established. We performed two AMIP‐like model experiments: (a) employing daily 2018 SSTs as observed and (b) applying a novel approach to remove the negative NA SST anomaly, while keeping SST daily and small‐scale variability. Comparing these experiments, we find that cold subpolar NA SSTs significantly increase heat wave duration and magnitude downstream over the European continent. Surface temperature and circulation anomalies are connected by the upper‐tropospheric summer wave pattern of meridional winds over the North Atlantic European sector, which is enhanced with cold NA SSTs. Our results highlight the relevance of the subpolar NA region for European summer conditions, a region that is marked by large biases in current coupled climate model simulations.

Long-term studies of the summer wind in the mesosphere and lower thermosphere at middle and high latitudes over Europe

Abstract. Continuous wind measurements using partial-reflection radars and specular meteor radars have been carried out for nearly 2 decades (2004–2022) at middle and high latitudes over Germany (∼ 54° N) and northern Norway (∼ 69° N), respectively. They provide crucial data for understanding the long-term behavior of winds in the mesosphere and lower thermosphere. Our investigation focuses on the summer season, characterized by the low energy contribution from tides and relatively stable stratospheric conditions. This work presents the long-term behavior, variability, and trends of the maximum velocity of the summer eastward, westward, and southward winds. In addition, the geomagnetic influence on the summer zonal and meridional wind is explored at middle and high latitudes. The results show a mesospheric westward summer maximum located around 75 km with velocities of 35–54 m s−1, while the lower-thermospheric eastward wind maximum is observed at ∼ 97 km with wind speeds of 25–40 m s−1. A weaker southward wind peak is found around 86 km, ranging from 9 to 16 m s−1. The findings indicate significant trends at middle latitudes in the westward summer maxima with increasing winds over the past decades, while the southward winds show a decreasing trend. On the other hand, only the eastward wind in July has a decreasing trend at high latitudes. Evidence of oscillations around 2–3, 4, and 6 years modulate the maximum velocity of the summer winds. In particular, a periodicity between 10.2 and 11.3 years found in the westward component is more significant at middle latitudes than at high latitudes, possibly due to solar radiation. Furthermore, stronger geomagnetic activity at high latitudes causes an increase in eastward wind velocity, whereas the opposite effect is observed in zonal jets at middle latitudes. The meridional component appears to be disturbed during high geomagnetic activity, with a notable decrease in the northward wind strength below approximately 80 km at both latitudes.