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A global climatology of warm conveyor belts (WCBs) is presented for the years 1979–2010, based on trajectories calculated with Interim ECMWF Re-Analysis (ERA-Interim) data. WCB trajectories are identified as strongly ascending air parcels (600 hPa in 2 days) near extratropical cyclones. Corroborating earlier studies, WCBs are more frequent during winter than summer and they ascend preferentially in the western ocean basins between 25° and 50° latitude. Before ascending, WCB trajectories typically approach from the subtropics in summer and from more midlatitude regions in winter. Considering humidity, cloud water, and potential temperature along WCBs confirms that they experience strong condensation and integrated latent heating during the ascent (typically >20 K). Liquid and ice water contents along WCBs peak at about 700 and 550 hPa, respectively. The mean potential vorticity (PV) evolution shows typical tropospheric values near 900 hPa, followed by an increase to almost 1 potential vorticity unit (PVU) at 700 hPa, and a decrease to less than 0.5 PVU at 300 hPa. These low PV values in the upper troposphere constitute significant negative anomalies with amplitudes of 1–3 PVU, which can strongly influence the downstream flow. Considering the low-level diabatic PV production, (i) WCBs starting at low latitudes (<40°) are unlikely to attain high PV (due to weak planetary vorticity) although they exhibit the strongest latent heating, and (ii) for those ascending at higher latitudes, a strong vertical heating gradient and high absolute vorticity are both important. This study therefore provides climatological insight into the cloud diabatic formation of significant positive and negative PV anomalies in the extratropical lower and upper troposphere, respectively.

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.

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

The Kuroshio south of Japan is known to vacillate between a straight and a large meander (LM) path. Since 1950, eight LM events have been observed with different durations. The most recent/on‐going LM started in August 2017 and has become the longest event in record. By analyzing eddy‐resolving sea surface height data and by adopting a wind‐forced linear vorticity model, we demonstrate that the on‐going LM is maintained by an exceptionally stable dynamic state of the Kuroshio Extension (KE) forced by wind stresses across the Pacific basin. The highly‐stable KE system not only minimizes westward eddy perturbations from disrupting the upstream Kuroshio path, its strengthened southern recirculation gyre further helps to anchor the Kuroshio across the Izu Ridge. By contrasting the on‐going event to the 2004–2005 event, we argue that the LM duration is more sensitive to the wind‐forced KE dynamic state than the eastward Kuroshio transport south of Japan. Plain Language Summary Occurrence of large meanders (LMs) by the Kuroshio south of Japan is a unique characteristic among the 5 western boundary currents in the wind‐driven subtropical ocean gyres. The LM occurrence and durations have both been observed to be irregular and chaotic. While processes initiating a LM event have been widely examined, the factors controlling the LM duration remain less explored. Two LM events with drastically different durations took place after the global eddy‐resolving sea surface height data from satellite altimeters became available. In contrast to the 2004–2005 LM event with a duration of 14 months, the on‐going LM started in August 2017 and, by persisting for >67 months, has become the longest among the eight recorded events since 1950. This study investigated the processes responsible for the differing durations of these two LM events. It is found that the wind‐forced stability of the Kuroshio Extension system east of the Izu Ridge plays a critical role in setting the LM's durations. This investigation is important because the Kuroshio LM impacts not only on fisheries south of Japan, but also synoptic weathers, such as typhoon paths, and climate in coastal areas facing the North Pacific Ocean. Key Points The Kuroshio south of Japan is known to vacillate between straight and large meander (LM) paths Started in August 2017, the on‐going LM event has become the longest among the eight LM events detected since 1950 This longevity is argued to be caused by highly stable dynamic state of the Kuroshio Extension after 2018 forced by the Pacific basin winds

The insensitivity of the Antarctic Circumpolar Current (ACC)’s prominent isopycnal slope to changes in wind stress is thought to stem from the action of mesoscale eddies that counterbalance the wind-driven Ekman overturning—a framework verified in zonally symmetric circumpolar flows. Substantial zonal variations in eddy characteristics suggest that local dynamics may modify this balance along the path of the ACC. Analysis of an eddy-resolving ocean GCM shows that the ACC can be broken into broad regions of weak eddy activity, where surface winds steepen isopycnals, and a small number of standing meanders, across which the isopycnals relax. Meanders are coincident with sites of (i) strong eddy-induced modification of the mean flow and its vertical structure as measured by the divergence of the Eliassen–Palm flux and (ii) enhancement of deep eddy kinetic energy by up to two orders of magnitude over surrounding regions. Within meanders, the vorticity budget shows a balance between the advection of relative vorticity and horizontal divergence, providing a mechanism for the generation of strong vertical velocities and rapid changes in stratification. Temporal fluctuations in these diagnostics are correlated with variability in both the Eliassen–Palm flux and bottom speed, implying a link to dissipative processes at the ocean floor. At larger scales, bottom pressure torque is spatially correlated with the barotropic advection of planetary vorticity, which links to variations in meander structure. From these results, it is proposed that the “flexing” of standing meanders provides an alternative mechanism for reducing the sensitivity of the ACC’s baroclinicity to changes in forcing, separate from an ACC-wide change in transient eddy characteristics.

Abyssal vorticity balance in the northeast South China Sea was assessed for over a year based on observations from 28 current- and pressure-recording inverted echo sounders distributed west of the Luzon Strait. The regional first-order balance was dominated by the planetary vorticity flux and bottom pressure torque, which reflect the external and internal dynamics of abyssal circulation. Vertical motion considerably contributed to the planetary vorticity flux, whereas the contribution of horizontal motion was negligible. Positive and negative planetary vorticity fluxes dominate the areas along the eastern and western boundaries, indicating upward and downward vertical transport, respectively. The opposite planetary vorticity fluxes in the different areas were accompanied by different current patterns; regional anticyclonic and cyclonic characteristics appeared near the western and eastern boundaries, respectively, owing to the deep topography as the abyssal current followed the boundary. The planetary vorticity flux near the eastern boundary was substantial in spring and autumn; in contrast, along the western boundary it was enhanced only in spring. Deep eddies played important roles in planetary vorticity flux and regional vorticity balance. The results of this study reveal the formation dynamics of abyssal circulation in the South China Sea as well as its spatiotemporal distributions, providing a more detailed description of abyssal circulation.

A major open issue in tropical meteorology is how and why some tropical cyclones intensify under moderate vertical wind shear. This study tackles that issue by diagnosing physical processes of tropical cyclone intensification in a moderately sheared environment using a 20-member ensemble of idealized simulations. Consistent with previous studies, the ensemble shows that the onset of intensification largely depends on the timing of vortex tilt reduction and symmetrization of precipitation. A new contribution of this work is a process-based analysis following a shear-induced midtropospheric vortex with its associated precipitation. This analysis shows that tilt reduction and symmetrization precede intensification because those processes are associated with a substantial increase in near-surface vertical mass fluxes and equivalent potential temperature. A vorticity budget demonstrates that the increased near-surface vertical mass fluxes aid intensification via near-surface stretching of absolute vorticity and free-tropospheric tilting of horizontal vorticity. Importantly, tilt reduction happens because of a vortex merger process—not because of advective vortex alignment—that yields a single closed circulation over a deep layer. Vortex merger only happens after the midtropospheric vortex reaches upshear left, where the flow configuration favors near-surface vortex stretching, deep updrafts, and a substantial reduction of low-entropy fluxes. These results lead to the hypothesis that intensification under moderate shear happens if and when a “restructuring” process is completed, after which a closed circulation favors persistent vorticity spinup and recirculating warm, moist air parcels.

The characteristics of subdiffusion are influenced by hydrodynamic fluctuations, not the scaling of the mean square displacement (MSD) at the long time limit, but rather the transition of this property from ballistic behaviour at early times to its final scaling at large times. Additionally, since a significant portion of the normalised velocity autocorrelation function (NVAF), a widely used motion descriptor, no longer adequately describes the antipersistent character that is typical of a subdiffusive motion, it is also impacted by these fluctuations. The combined findings can lead to misleading conclusions if hydrodynamic fluctuations are not taken into account. Diffusing and vorticity time scales are crucial for the way the motion turns into the final subdiffusion dynamics, whose exponent is determined by the scaling of the friction.

The source of potential vorticity (PV) for the global domain is located at the Earth’s surface. PV in one hemisphere can exchange with the other through cross-equatorial PV flux (CEPVF). This study investigates the features of the climatic mean CEPVF, the connection in interannual CEPVF with the surface thermal characteristics, and the associated mechanism. Results indicate that the process of positive (negative) PV carried by a northerly (southerly) wind leads to the climatologically overwhelming negative CEPVF over almost the entire equatorial cross-section, while the change of the zonal circulation over the equator is predominately responsible for CEPVF variation. By introducing the concept of “PV circulation” (PVC), it is demonstrated that the interannual CEPVF over the equator is closely linked to the notable uniform anomalies of spring cold surface air temperature (SAT) over the mid–high latitudes of Eurasia by virtue of the PVC, the PV-θ mechanism, and the surface positive feedback. Further analysis reveals that equatorial sea surface temperature (SST) forcing, such as the El Niño–Southern Oscillation and tropical South Atlantic uniform SST, can directly drive anomalous CEPVF by changing the zonal circulation over the equator, thereby influencing SAT in the Northern Hemisphere. All results indicate that the equilibrium linkage between CEPVF and extratropical SAT is mainly a manifestation of the response of extratropical SAT to tropical forcing by virtue of PVC, and that the perspective of PVC can provide a reasonably direct and simple connection of the circulation and climate between the tropics and the mid–high latitudes.