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Global warming is amplified in the Arctic. However, numerical models struggle to represent key processes that determine Arctic weather and climate. To collect data that help to constrain the models, the HALO–(𝒜𝒞)3 aircraft campaign was conducted over the Norwegian and Greenland seas, the Fram Strait, and the central Arctic Ocean in March and April 2022. The campaign focused on one specific challenge posed by the models, namely the reasonable representation of transformations of air masses during their meridional transport into and out of the Arctic via northward moist- and warm-air intrusions (WAIs) and southward marine cold-air outbreaks (CAOs). Observations were made over areas of open ocean, the marginal sea ice zone, and the central Arctic sea ice. Two low-flying and one long-range, high-altitude research aircraft were flown in colocated formation whenever possible. To follow the air mass transformations, a quasi-Lagrangian flight strategy using trajectory calculations was realized, enabling us to sample the same moving-air parcels twice along their trajectories. Seven distinct WAI and 12 CAO cases were probed. From the quasi-Lagrangian measurements, we have quantified the diabatic heating/cooling and moistening/drying of the transported air masses. During CAOs, maximum values of 3 K h−1 warming and 0.3 g kg−1 h−1 moistening were obtained below 1 km altitude. From the observations of WAIs, diabatic cooling rates of up to 0.4 K h−1 and a moisture loss of up to 0.1 g kg−1 h−1 from the ground to about 5.5 km altitude were derived. Furthermore, the development of cloud macrophysical (cloud-top height and horizontal cloud cover) and microphysical (liquid water path, precipitation, and ice index) properties along the southward pathways of the air masses were documented during CAOs, and the moisture budget during a specific WAI event was estimated. In addition, we discuss the statistical frequency of occurrence of the different thermodynamic phases of Arctic low-level clouds, the interaction of Arctic cirrus clouds with sea ice and water vapor, and the characteristics of microphysical and chemical properties of Arctic aerosol particles. Finally, we provide a proof of concept to measure mesoscale divergence and subsidence in the Arctic using data from dropsondes released during the flights.

One of the most robust remote impacts of El Niño–Southern Oscillation (ENSO) is the teleconnection to tropical North Atlantic (TNA) sea surface temperature (SST) in boreal spring. However, important questions still remain open. In particular, the timing of the ENSO–TNA relationship lacks understanding. The three previously proposed mechanisms rely on teleconnection dynamics involving a time lag of one season with respect to the ENSO mature phase in winter, but recent results have shown that the persistence of ENSO into spring is necessary for the development of the TNA SST anomalies. Likewise, the identification of the effective atmospheric forcing in the deep TNA to drive the regional air–sea interaction is also lacking. In this manuscript a new dynamical framework to understand the ENSO–TNA teleconnection is proposed, in which a continuous atmospheric forcing is present throughout the ENSO decaying phase. Observational datasets in the satellite era, which include reliable estimates over the ocean, are used to illustrate the mechanism at play. The dynamics rely on the remote Gill-type response to the ENSO zonally compensated heat source over the Amazon basin, associated with perturbations in the Walker circulation. For El Niño conditions, the anomalous diabatic heating in the tropical Pacific is compensated by anomalous diabatic cooling, in association with negative rainfall anomalies and descending motion over northern South America. A pair of anomalous cyclonic circulations is established at upper-tropospheric levels in the tropical Atlantic straddling the equator, displaying a characteristic baroclinic structure with height. In the TNA region, the mirrored anomalous anticyclonic circulation at lower-tropospheric levels weakens the northeasterly trade winds, leading to a reduction in evaporation and of the ocean mixed layer depth, hence to positive SST anomalies. Apart from the dominance of latent heat flux anomalies in the remote response, sensible heat flux and shortwave radiation anomalies also appear to contribute. The “lagged” relationship between mature ENSO in winter and peaking TNA SSTs in spring seems to be phase locked with the seasonal cycle in both the location of the mechanism’s centers of action and regional SST variance.

Northeast China (NEC) suffered a severe drought that persisted from March to July of 2017 with profound impacts on agriculture and society, raising an urgent need to understand the mechanism for persistent droughts over midlatitudes. Previous drought mechanism studies focused on either large-scale teleconnections or local land–atmosphere coupling, while less attention was paid to their synergistic effects on drought persistence. Here we show that the 2017 NEC drought was triggered by a strong positive phase of the Arctic Oscillation in March, and maintained by the anticyclone over the area south to Lake Baikal (ASLB) through a quasi-stationary Rossby wave in April–July, accompanied by sinking motion and north wind anomaly. By using a land–atmosphere coupling index based on the persistence of positive feedbacks between the boundary layer and land surface, we find that the coupling states over NEC and ASLB shifted from a wet coupling in March to a persistently strengthened dry coupling in April–July. Over ASLB, the dry coupling and sinking motion increased surface sensible heat, decreased cloud cover, and weakened long wave absorption, resulting in a diabatic heating anomaly in the lower atmosphere and a diabatic cooling anomaly in the upper atmosphere. This anomalous vertical heating profile led to a negative anomaly of potential vorticity at low levels, indicating that the land–atmosphere coupling had a phase-lock effect on the Rossby wave train originating from upstream areas, and therefore maintained the NEC drought over downstream regions. Our study suggests that an upstream quasi-stationary wave pattern strengthened by land–atmosphere coupling should be considered in diagnosing persistent droughts, especially over northern midlatitudes.

Kinematic backward trajectories are used to globally quantify the contributions of temperature advection, adiabatic compression and diabatic processes to near‐surface temperature anomalies (hereafter T′${T}^{\\prime }$ ) during the coldest day of each year (TN1day events) based on ERA5. Diabatic cooling dominates TN1day anomalies in the climatologically coldest regions, while advection forms TN1day anomalies over most ocean regions. Over most extratropical land masses, TN1day anomalies arise from a combination of both processes. The mean age and formation distance of TN1day anomalies vary strongly in space, from one to 8 days, and 500–5,500 km, respectively. Five distinct types of TN1day events are identified from these physical and spatio‐temporal characteristics, and their geographical occurrence is investigated. Furthermore, advective, adiabatic and diabatic contributions typically cancel each other partially, but less so for the most intense TN1day events, which occur when the atmosphere's ability to dampen near‐surface temperature anomalies is limited. Plain Language Summary This study globally quantifies the relative importance of the two physical processes that lead to large negative temperature anomalies during cold extremes at the Earth's surface. These are, first, cold air advection, that is, the transport of air from climatologically cold to warmer regions and, second, so‐called diabatic cooling which refers to cooling of the air by radiation, turbulence and processes related to phase changes of water in the air. Here, their relative importance is quantified by tracing roughly 250’000’000 air parcels contributing to cold extremes backwards in time. Cold air advection is globally the dominant process for cold extreme formation, in particular over ocean regions. Over land regions, however, the importance of diabatic cooling gradually increases toward the climatologically coldest regions. The temporal and spatial scales over which these anomalies form vary from less than a day (in tropical Africa) to more than 8 days (in Siberia), and from less than 500 km (in tropical Africa) to more than 5,000 km (in subtropical regions), respectively. The results of this study for the first time provide a global picture of how cold extremes form and will enable us to better evaluate climate models with regard to cold extreme formation. Key Points Using kinematic backward trajectories, the physical processes contributing to near‐surface cold extremes are quantified globally Advection dominates over oceans, and over land the importance of diabatic cooling increases toward the climatologically coldest regions The most intense cold extremes form in situations with a limited ability of the atmosphere to dampen negative temperature anomalies

This study investigates and compares large-scale moisture and heat budgets over the eastern rainy sea area around Dongsha, the western rainless sea area around Xisha, and the northern coastland of the South China Sea. Ten-year (2011–20) surface, balloon-sounding, satellite measurements, and ERA5 reanalysis are merged into the physically consistent data to study annual and vertical variations of the budgets. It shows that the surface and column-integrated heat and moisture budgets have the smallest annual evolution over the coastland. The latent heat as a key heat contributor in summer is mainly offset by total cold advection and partially offset by net radiative cooling. The horizontal moisture advection below 700 hPa presents moistening over the sea whereas drying over the coastland during rainy months, in which the vertical moisture advection presents moistening up to 250 hPa for all three subregions. The horizontal temperature advection is weak throughout the year over the sea but displays strong top warming and bottom cooling in summer and nearly the opposite in winter over the coastland. The diabatic cooling with a peak at ∼700 hPa in winter is largely due to the enhanced radiative cooling and latent cooling. While the diabatic heating with a peak at ∼500 hPa in summer is largely due to the enhanced latent heating. The earliest atmospheric heating and moistening occur in spring over the coastland, inducing the earliest precipitation increase. The enhanced heating and moistening over Xisha have a 1-month lag relative to Dongsha, resulting in lagging precipitation.

By analyzing observation-based high-resolution surface air temperature (SAT) data over the Asian monsoon region (here called “monsoon Asia”) for 1981–2007, the modulation by boreal summer intraseasonal oscillation (BSISO) of heat wave (HW) occurrence is examined. Strong SAT variability and a high probability of HW occurrence on intraseasonal time scales are found consistently in the densely populated regions over central India (CI), the Yangtze River valley in China (YR), Japan (JP), and the Korean Peninsula (KP). The two distinct BSISO modes (30–60-day BSISO1 and 10–30-day BSISO2) show different contributions to HW occurrence in monsoon Asia. A significant increase in HW occurrence over CI (YR) is observed during phases 2–3 (8–1) of BSISO2 when the 10–30-day anticyclonic and descending anomaly induce enhanced upward thermal heating and sensible heat flux (warm advection) around the areas. On the other hand, the northeastward propagating BSISO1 is closely connected to the increased HW probability over JP and KP. During phases 7–8 of BSISO1, the 30–60-day subsidence along with the low-level anticyclonic anomaly moves into northeastern Asia, leading to enhanced diabatic (adiabatic) warming near surface in JP (KP). Analysis of a three-dimensional streamfunction tendency equation indicates that diabatic cooling induced by the BSISO-related suppressed convections is the main forcing term of anticyclonic anomaly although it is largely offset by the decreased static stability associated with adiabatic warming. The BSISO-related vorticity advection leads to an anticyclonic (cyclonic) tendency to the northwestern (southeastern) part of the center of anticyclonic anomaly, favoring northwestward development of the BSISO anomalous circulations and thus providing a favorable condition for HW occurrence over the western Pacific–East Asia sector.

The western North Pacific Subtropical High (WNPSH) significantly influences East Asian weather. In the Northwest Pacific where sea‐salt aerosols (SSAs) are abundant and the large‐scale environment is dominated by the dry subsidence of the WNPSH during summer, inhomogeneous SSAs form as a product of the environment. However, the extent to which inhomogeneous SSAs affect the WNPSH remains unclear. This study investigates the radiative effects of SSAs through numerical simulations, revealing a novel mechanism for the strengthening of the WNPSH. The results demonstrate that inhomogeneous SSAs enhance the WNPSH by generating diabatic cooling in the upper troposphere and associated unstable subsidence motion. Further considering the radiative hysteresis effects of inhomogeneous SSAs, the WNPSH further strengthens under the combined dynamic and thermodynamic influences associated with upper‐level radiative cooling. Inhomogeneous SSAs not only enhance the WNPSH but also influence the location where the central area of high pressure intensifies. Plain Language Summary Variations in the western North Pacific Subtropical High (WNPSH), including changes in its intensity and position, significantly impact weather patterns in East Asia, particularly during the summer months. At high relative humidity (RH) levels, sea‐salt aerosols (SSAs) appear as homogeneous droplets; however, in moderate RH environments (45%–70%), SSAs exist in inhomogeneous states characterized by water coatings on their core particles. The dry subsiding airflow of the WNPSH creates extensive areas of moderate RH over the western Pacific Ocean. Despite this, the potential effects of SSAs on the WNPSH through radiative processes have not been previously considered. This study demonstrates that inhomogeneous SSAs induce cooling in the upper troposphere, which contributes to an overall strengthening of the WNPSH. Additionally, these aerosols influence the position where the primary area of high pressure intensifies. This newly identified mechanism by which inhomogeneous SSAs strengthen the WNPSH offers a unique perspective for understanding the evolution of the WNPSH. Key Points Inhomogeneous sea‐salt aerosols (SSAs) contribute to strengthening the western North Pacific Subtropical High (WNPSH) Inhomogeneous SSAs also influence the position where the main body of the high pressure intensifies Direct radiative effects of inhomogeneous SSAs offer a distinctive perspective for exploring the dynamics of the WNPSH

In August 2022, an exceptionally long-lasting heat wave (HW) affected the middle and lower reaches of the Yangtze River basin. This study uses the JRA55 daily reanalysis datasets to elucidate the thermodynamic characteristics of the daily evolution of historical extreme HWs in this region via the heat budget equation. HWs are generally characterized by the occurrence of anticyclonic circulation anomaly throughout the troposphere and positive air temperature anomaly with the maximum amplitude in the boundary layer. The anticyclonic anomaly can induce compression heating in the entire troposphere and warm zonal advection in the boundary layer. Meanwhile, due to the reduced cloud cover, more shortwave radiation reaches the ground surface, and the sensible heat flux becomes an important source of diabatic heating before the onset of HWs. The accumulated excessive heat in the HWs is primarily damped through the emission of longwave radiation and meridional thermal advection. For the HW in August 2022, its extreme persistence is mainly caused by prolonged adiabatic heating, enhanced diabatic heating during the developing stage and weakened diabatic cooling during the decay stage. The upper-level portion of the anticyclonic circulation anomalies is linked to the strengthened South Asia High. After applying the state-of-the-art dynamic metric, i.e., local finite wave activity, we reveal that the formation of the anomalous South Asia High in August 2022 is associated with the Stokes drift flux rather than the dispersion of Rossby wave energy. This characteristic sets it apart from other extreme HWs.

Forecasting warm‐sector rainfall (WR) remains a major challenge, primarily due to weak synoptic forcing. Through cloud‐permitting numerical simulations, in addition to direct triggering mechanism from low‐level jets, we identify the important role of gravity waves in a heavy WR event in South China via convective preconditioning. The preconditioning manifests as mid‐level moistening and destabilization with wave‐like variations. This process is driven by fast‐propagating (∼24 m s−1) n = 2 waves, associated with lower‐tropospheric ascents and upper‐tropospheric descents. Waves are generated during the evolution of northern frontal rainfall (FR). As FR intensifies, surges in low‐level diabatic cooling mainly resulting from microphysical processes, trigger n = 2 waves, which further precondition the environment along their path. In contrast, a sensitivity experiment involving stably developing FR fails to reproduce the preconditioning process by waves and the subsequent occurrence of WR. Overall, our study illuminates a new pathway through which FR significantly influences WR via gravity waves. Plain Language Summary During the rainy season, the coastal regions of South China frequently experience heavy rainfall, leading to significant socio‐economic losses. Unfortunately, understanding the mechanisms underlying this heavy rainfall and accurately predicting it remains a great challenge, as it often occurs without evident synoptic influences such as fronts and low‐pressure vortex. Another rainband occasionally forms to the north of the coastal heavy rainfall through frontal lifting. The northern inland frontal rainfall could be considered a potential contributor to the initiation of coastal heavy rainfall, yet no concrete connection has been established until now. Our research reveals that this frontal rainband can generate low‐frequency gravity waves that have a substantial influence on coastal heavy rainfall. These waves primarily result from the sharp cooling associated with the frontal rainband, featuring lower‐tropospheric upward motions. They are capable to travel considerable distances ahead of the frontal rainband at high speeds. Along their journey, they moisten and destabilize the cloud environment ahead of the frontal rainband, thereby favoring the occurrence of coastal rainfall. Key Points The initiation of warm‐sector heavy rainfall at the coast is related to the evolution of northern frontal rainfall that located 300 km away Variations in latent cooling resulting from the frontal rainfall produce gravity waves featuring ascents over the lower troposphere These gravity waves moisten and destabilize mid‐level environment along their paths, promoting warm‐sector rainfall

The present study distinguishes the different effects of the eastern Pacific-type (EP-type) sea surface temperature anomaly (SSTA) and the central Pacific-type (CP-type) SSTA on the tropical easterly jet (TEJ) by dividing the TEJ into the entrance (over the tropical western Pacific), core (over the tropical Indian Ocean) and exit regions (over equatorial Africa and the Atlantic). It is found that the EP-type SSTA can exert an inverse influence on the intensity of the TEJ between its exit-core region and the entrance region, while the CP-type SSTA mainly affects the entrance of the TEJ. This discrepancy can be attributed to the different magnitude and spatial extension of the diabatic heating associated with the enhanced precipitation induced by the two SSTA patterns. The EP-type SSTA could induce a stronger and wider-spread positive precipitation anomaly over the tropical Pacific than the negative precipitation anomaly over the Maritime Continent, while the CP-type SSTA can induce an enhanced precipitation over the tropical Pacific that is roughly equivalent in amplitude and spatial extension to the suppressed precipitation over the Maritime Continent. Thus, the EP-type SSTA can affect the entire TEJ through the dominant diabatic heating in the tropical Pacific and its subsequent Gill-Matsuno response, accelerating the entrance region and slowing the exit-core region. In contrast, the CP-type SSTA could only produce a significant acceleration in the entrance of the TEJ as the remote effect of the diabatic heating in the tropical Pacific is nearly neutralized by the diabatic cooling in the Maritime Continent. A series of numerical experiments based on a linear baroclinic model confirms the above conclusions and distinguishes between the independent contributions of the diabatic heating in the tropical Pacific and the diabatic cooling in the Maritime Continents to the TEJ.