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An improved understanding of the environmental factors influencing tropical cyclones (TCs) is vital to enhance the accuracy of forecasting TC intensity. More than half of TCs that were substantially affected by environmental factors were predominantly affected by low-level environmental relative vorticity (hereafter, VOR TCs). In this study, the seasonal variation and related physical features of VOR TCs from 2003–2017 during TC seasons in summer and autumn over the western North Pacific were analyzed. Autumn VOR TCs exhibited the strongest intensity among all TCs over the western North Pacific. The enhanced environmental relative vorticity during the TC intensification period was larger and more favorably distributed for VOR TC development in autumn. The vorticity diagnostic analysis showed that the convergence was the positive source of environmental relative vorticity of VOR TCs, while the contribution of convergence was larger in autumn than in summer. The increased convergence was related to seasonal variation in larger-scale systems, especially the higher environmental pressure gradient, which reflected the larger subtropical high and the compressed East Asian summer monsoon trough in autumn. In addition, the East Asian summer monsoon trough was also somewhat stronger during the intensification period of VOR TCs, especially in autumn.

In recent decades, southeast Australia has experienced both extreme drought and record-breaking rainfall, with devastating societal impacts. Variations in the Australian polar-front jet (PFJ) and the subtropical jet (STJ) determine, for example, the location and frequency of the cool season (April–September) weather systems influencing rainfall events and, consequently, water availability for the southern half of Australia. Changes in jet stream wind speeds also are important for aviation fuel and safety requirements. A split jet occurs when the single jet separates into the STJ and PFJ in the early cool season (April–May). This study focusses on split jet characteristics over Australian/New Zealand longitudes in recent decades. During the accelerated global warming from the mid-1990s, higher mean wind speeds were found in the PJF across the Australian region during June–September, compared to the STJ. In contrast, significant wind speed increases occur in the early cool season (April–May) at STJ latitudes, which straddle the East Coast of Australia and the adjacent Tasman Sea. These changes are linked to major changes in the mean atmospheric circulation, and they include relative vorticity and humidity, both being vital for the development of rain-bearing weather systems that affect the region.

Winter atmospheric blocking circulations such as Ural blocking (UB) have been recognized to play an important role in recent winter Eurasian cooling. Observational analyses performed here reveal that the winter warming in the Barents–Kara Seas (BKS) related to the recent decline of sea ice concentration (SIC) has been accompanied by a large increase in the mean duration of the UB events. A new energy dispersion index (EDI) is designed to help reveal the physics behind this association and show how the BKS warming can influence the mean duration of UB events. This EDI mainly reflects the role of the meridional potential vorticity (PV) gradient in the blocking persistence and it characterizes the changes in energy dispersion and nonlinearity strength of blocking. The meridional PV gradient combines the relative vorticity gradient (related to the nonuniform meridional shear of the mean zonal wind) and the mean zonal wind strength. It is revealed that the BKS warming leads to a significant lengthening of the UB duration because of weakened energy dispersion and intensified nonlinearity of the UB through reduced meridional PV gradient. Furthermore, the duration of the UB is found to depend more strongly on the meridional PV gradient than the mean westerly wind strength, although the meridional PV gradient includes the effect of mean westerly wind strength. Thus, the meridional PV gradient is a better indicator of the change in the blocking duration related to Arctic warming than the zonal wind strength index.

The Surface Water and Ocean Topography (SWOT) mission provides a good opportunity to study fine‐scale processes in the global ocean but whether it can detect balanced submesoscale eddies is uncertain due to the “contamination” by unbalanced inertial gravity waves. Here, based on concurrent observations from SWOT and a mooring array in the northwestern Pacific, we successfully captured two submesoscale cyclonic eddies with negative sea level anomalies (SLAs) in spring 2023. We find that the SLA amplitude and equivalent radius of the first (second) eddy are 2.5 cm and 16.0 km (2.0 cm and 18.8 km), respectively. For both eddies, their vertical scales are around 150 m and their horizontal velocities and Rossby numbers exceed 15.0 cm/s and 0.4, respectively. Further analysis suggests that similar submesoscale eddies can commonly occur in the northwestern Pacific and that SWOT is capable to detect larger submesoscale eddies with scales greater than ∼10 km. Plain Language Summary The Surface Water and Ocean Topography (SWOT) mission measures sea surface heights with a spatial resolution an order of magnitude higher than the prior nadir altimetry missions. However, whether SWOT can detect oceanic submesoscale eddies that play key roles in oceanic energy cycle and vertical material transports, is uncertain. To investigate this issue, we deployed a mooring array beneath a SWOT orbital swath in the northwestern Pacific. Based on the concurrent SWOT and mooring data, we successfully captured two submesoscale cyclonic eddies in spring 2023. Radii of the submesoscale eddies are found to be between 10 and 20 km and their horizontal velocities exceed 15.0 cm/s. The ratio between their vertical relative vorticity and the planetary vorticity exceeds 0.4. Further analysis suggests that similar submesoscale eddies can commonly occur in the northwestern Pacific. This study demonstrates the capability of SWOT to detect submesoscale eddies in the global ocean. Key Points Two submesoscale cyclonic eddies (SCEs) were observed by SWOT and moorings in the northwestern Pacific Kinematic and dynamic features of the SCEs were revealed It is feasible to use SWOT data to detect larger submesoscale eddies in the ocean

This study finds a significant positive correlation between the Pacific meridional mode (PMM) index and the frequency of intense tropical cyclones (TCs) over the western North Pacific (WNP) during the peak TC season (June–November). The PMM influences the occurrence of intense TCs mainly by modulating large-scale dynamical conditions over the main development region. During the positive PMM phase, anomalous off-equatorial heating in the eastern Pacific induces anomalous low-level westerlies (and cyclonic flow) and upper-level easterlies (and anticyclonic flow) over a large portion of the main development region through a Matsuno–Gill-type Rossby wave response. The resulting weaker vertical wind shear and larger low-level relative vorticity favor the genesis of intense TCs over the southeastern part of the WNP and their subsequent intensification over the main development region. The PMM index would therefore be a valuable predictor for the frequency of intense TCs over the WNP.

The total aviation effective radiative forcing is dominated by non-CO2 effects. The largest contributors to the non-CO2 effects are contrails and contrail cirrus. There is the possibility of reducing the climate effect of aviation by avoiding flying through ice-supersaturated regions (ISSRs), where contrails can last for hours (so-called persistent contrails). Therefore, a precise prediction of the specific location and time of these regions is needed. But a prediction of the frequency and degree of ice supersaturation (ISS) on cruise altitudes is currently very challenging and associated with great uncertainties because of the strong variability in the water vapour field, the low number of humidity measurements at the air traffic altitude, and the oversimplified parameterisations of cloud physics in weather models. Since ISS is more common in some dynamical regimes than in others, the aim of this study is to find variables/proxies that are related to the formation of ISSRs and to use these in a regression method to predict persistent contrails. To find the best-suited proxies for regressions, we use various methods of information theory. These include the log-likelihood ratios, known from Bayes' theorem, a modified form of the Kullback–Leibler divergence, and mutual information. The variables (the relative humidity with respect to ice, RHiERA5; the temperature, T; the vertical velocity, ω; the divergence, DIV; the relative vorticity, ζ; the potential vorticity, PV; the normalised geopotential height, Z; and the local lapse rate, γ) come from ERA5, and RHiM/I, which we assume as the truth, comes from MOZAIC/IAGOS (Measurement of Ozone and Water Vapour on Airbus In-service Aircraft/In-service Aircraft for a Global Observing System; commercial aircraft measurements). It turns out that RHiERA5 is the most important predictor of ice supersaturation, in spite of its weaknesses, and all other variables do not help much to achieve better results. Without RHiERA5, a regression to predict ISSRs is not successful. Certain modifications of RHiERA5 before the regression (as suggested in recent papers) do not lead to improvements of ISSR prediction. Applying a sensitivity study with artificially modified RHiERA5 distributions points to the origin of the problems with the regression: the conditional distributions of RHiERA5 (conditioned on ISS and non-ISS, from RHiM/I) overlap too heavily in the range of 70 %–100 %, so for any case in that range, it is not clear whether it belongs to an ISSR or not. Evidently, this renders the prediction of contrail persistence very difficult.

The reported decreasing trend of the annual tropical cyclone (TC) landfalls in southern China and increasing trend in southeastern China in recent decades are confirmed to be an abrupt shift occurring at the end of the twentieth century, based on a statistical analysis. The opposite trends in the two adjacent regions are often considered to be a result of tropical cyclone landfalls in southern China being deflected northward. However, it is demonstrated in this study that they are phenomenally independent. In fact, the abrupt decrease of TC landfalls in southern China occurs as a result of an abrupt decrease of the westward events in the postpeak season (October–December), which in turn is a consequence of a significant decrease of the TC genesis frequency in the southeastern part of the western North Pacific (WNP) Ocean basin. On the other hand, the abrupt increase of TC landfalls in southeastern China occurs because of an abrupt increase of the northwest events in the peak season (July–September), as the consequence of a statistically westward shift of TC genesis. The relevant variations of TC genesis are shown to be mainly caused by decreased relative vorticity and increased vertical wind shear, which, however, are intrinsically related to the accelerated zonal atmospheric circulation driven by a La Niña–like sea surface warming pattern over the WNP that developed after the end of twentieth century.

At midnight on 27–28 June 2016, a Tibetan Plateau (TP) Vortex (TPV) generated over the western TP that subsequently caused a downstream record-breaking rainstorm and extremely severe natural disaster. Based on reanalysis data and satellite imagery, this study investigates the formation of this TPV from a potential vorticity (PV) perspective. Results show that, in late June 2016, a remarkable circulation anomaly occurred over the TP and its peripheral area, with easterly flow in the middle and lower troposphere developing in the subtropical zone, replacing the normal westerly flow there. Its forefront merged with the southwesterly flow from the west and penetrated and converged over the western TP where the surface was warmer than normal, forming a low-level jet and downward slantwise isentropic surfaces in-situ. When the air parcel slid down the slantwise isentropic surface, its vertical relative vorticity developed owing to slantwise vorticity development associated with PV restructuring. At the same time, the penetrating southwesterly flow brought abundant water vapor to the western TP and induced increasing sub-cloud entropy and air ascent there. Low-layer cloud formed and the cloud liquid water content increased. The strong latent heat that was released in association with the formation of cloud produced strong diabatic heating near 400 hPa at night and strong PV generation below. The normal diurnal variation was interrupted and the vortex was generated near the surface. These results demonstrate that, against a favorable circulation background, both adiabatic and diabatic PV processes are crucial for TPV genesis.

The rainfall over the Yangtze River Valley (YRV) in June 2020 broke the record since 1979. Here we show that all three oceans of the Pacific, Indian and Atlantic Oceans contribute to the YRV rainfall in June 2020, but the Atlantic plays a dominant role. The sea surface temperature (SST) anomalies in three oceans are associated with the two vorticity anomalies: negative 200-hPa relative vorticity anomalies over North China (NC) and negative 850-hPa relative vorticity anomalies in the South China Sea (SCS). The rainfall anomalies in the YRV are mainly controlled by atmospheric process associated with the NC vorticity. The positive SST anomalies in May over the western North Atlantic induce positive geopotential height anomalies in June over the mid-latitude North Atlantic, which affect the rainfall anomalies in the YRV by changing the NC vorticity via Atlantic-induced atmospheric wave train across Europe. The Indian Ocean and tropical North Atlantic, as capacitors of Pacific El Niño events in the preceding winter, affect the SCS vorticity associated with the anomalous anticyclone over the SCS and also facilitate the YRV rainfall by providing favorable moisture conditions. This study suggests that the May SST over the western North Atlantic is a good predictor of June rainfall anomalies in the YRV and highlights the important impacts of three-ocean SSTs on extreme weather and climate events in China.

Ocean state estimates that represent mesoscale and submesoscale dynamics are crucial for climate research and ocean forecasting. Accurate submesoscale representation in ocean models is a current challenge due to rapidly evolving flow and lack of observations at suitable scales. The SWOT satellite provides a step change in ocean surface measurements at high spatial resolution (∼ ${\\sim} $2 km). We use advanced data‐assimilation to demonstrate the impact of SWOT observations on circulation estimates in a high‐resolution hydrodynamic model of the East Australian Current. We show that assimilating SWOT data improves representation and prediction of the mesoscale motions, and improves representation (but not prediction) of the fine‐scale dynamics. Increased relative vorticity variance with SWOT assimilation is widespread in space and time (away from SWOT observations) and projected to depth. Using data observed by SWOT in a realistic ocean model, we show enhanced representation of ocean processes across scales, essential for improved ocean forecasts and projections.