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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.

From 17 November to 27 December 2022, extremely cold snowstorms frequently swept across North America and Eurasia. Diagnostic analysis reveals that these extreme cold events were closely related to the establishment of blocking circulations. Alaska Blocking (AB) and subsequent Ural Blocking (UB) episodes are linked to the phase transition of the North Atlantic Oscillation (NAO) and represent the main atmospheric regimes in the Northern Hemisphere. The downstream dispersion and propagation of Rossby wave packets from Alaska to East Asia provide a large-scale connection between AB and UB episodes. Based on the nonlinear multi-scale interaction (NMI) model, we found that the meridional potential vorticity gradient (PV y ) in November and December of 2022 was anomalously weak in the mid-high latitudes from North America to Eurasia and provided a favorable background for the prolonged maintenance of UB and AB events and the generation of associated severe extreme snowstorms. However, the difference in the UB in terms of its persistence, location, and strength between November and December is related to the positive (negative) NAO in November (December). During the La Niña winter of 2022, the UB and AB events are related to the downward propagation of stratospheric anomalies, in addition to contributions by La Niña and low Arctic sea ice concentrations as they pertain to reducing PV y in mid-latitudes.

Changes in tropical cyclone (TC) movement are commonly attributed to those in the steering flow, beta effect, or topographic influences. However, a series of idealized simulations suggest that significant track deflections can still occur even under a steady steering flow on an f plane. TCs embedded in easterly flows of varying strength systematically deflect southward from the expected westward track when radiative effects are included. The resulting track deflection reaches approximately 200 km in some experiments over a 144‐hr period, comparable to typical 72‐ to 96‐hr forecast errors in global numerical weather prediction model. A potential vorticity tendency analysis reveals that the deflection primarily results from the diabatic heating and horizontal advection terms, each linked to asymmetries in the convection and wind fields, respectively. These asymmetries are initially triggered by vortex–flow interactions and further enhanced by radiative diurnal cycles. Our findings highlight the role of internal vortex asymmetries in modulating TC motion.

Linkages between extreme precipitation events (EPEs) in the central and eastern United States and synoptic-scale Rossby wave breaking are investigated using 1979–2015 climatologies of EPEs and upper-level potential vorticity (PV) streamers. The investigation focuses on two domains over the central and eastern United States, respectively, and emphasizes widespread EPEs, events exhibiting exceptionally large precipitation volumes. The relative frequency of PV streamers is found to be significantly enhanced relative to climatology immediately upstream of each domain during widespread EPEs. Majorities of the widespread EPEs in the central (~79%) and eastern (~56%) U.S. domains co-occur with a PV streamer positioned immediately upstream. Odds ratios of EPEs for days when a PV streamer occurs upstream of each domain indicate a strong, statistically significant association between EPEs and Rossby wave breaking. The strength of the EPE–Rossby wave breaking linkage, as measured by co-occurrence fractions and odds ratios, tends to increase with increasing EPE precipitation volume, such that the strongest linkage exists for widespread EPEs. Composite analyses reveal that Rossby wave breaking can result in widespread EPEs by establishing a persistent high-amplitude synoptic-scale wave pattern, within which strong poleward water vapor transport and ascent are forced over the EPE region immediately downstream of an elongated upper-level trough. Additional analyses demonstrate that, compared to corresponding null cases, Rossby wave breaking cases resulting in widespread EPEs exhibit a significantly higher-amplitude wave pattern that favors greater poleward transport of moist, conditionally unstable air and stronger ascent over the EPE region.

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 three-dimensional connections between Eurasian cooling and Arctic warming since 1979 were investigated using potential vorticity (PV) dynamics. We found that Eurasian cooling can be regulated by Arctic warming through PV adaptation and PV advection. Here, PV adaptation refers to the adaptation of PV to forcing and coherent dynamic–thermodynamic adaptation to PV change. In a PV perspective, first, the anticyclonic circulation change over the Arctic is produced by a negative PV change through PV adaptation, in which the change means the linear trend from 1979 to 2017. The negative PV change is directly regulated by Arctic warming because the vertical structure of Arctic warming is stronger at lower levels, which generates a negative PV change through the diabatic heating effect. Second, the circulation change produces a change in horizontal PV advection due to the existence of climatological PV gradients. Thus, as a balanced result, both the circulation change and PV change extend to the midlatitudes through horizontal PV advection and PV adaptation. Eventually, Eurasian cooling at the surface and in the lower troposphere is dominated by PV changes at the surface through PV adaptation. Meanwhile, enhanced Eurasian cooling in the middle troposphere is dominated by top-down influences of upper-level PV change through PV adaptation. Nevertheless, the upper-level PV changes are still contributed to by horizontal PV advection associated with Arctic warming. Overall, the general dynamics connecting Eurasian cooling with Arctic warming are demonstrated in a PV view.

The present study investigates changes in the location, frequency, intensity, and dynamical processes of North Atlantic extratropical cyclones with warming consistent with the IPCC Fifth Assessment Report (AR5) representative concentration pathway 8.5 (RCP8.5) scenario. The modeling, analysis, and prediction (MAP) climatology of midlatitude storminess (MCMS) feature-tracking algorithm was utilized to analyze 10 cold-season high-resolution atmospheric simulations over the North Atlantic region in current and future climates. Enhanced extratropical cyclone activity is most evident in the northeast North Atlantic and off the U.S. East Coast. These changes in cyclone activity are offset from changes in eddy kinetic energy and eddy heat flux. Investigation of the minimum SLP reached at each grid point reveals a lack of correspondence between the strongest events in the current and future simulations, indicating the future simulations produced a different population of storms. Examination of the percent change of storms in the storm-track region shows a reduction in the number of strong storms (i.e., those reaching a minimum SLP perturbation of at least −51 hPa). Storm-relative composites of strong and moderate storms show an increase in precipitation, associated with enhanced latent heat release and strengthening of the 900–700-hPa layer-average potential vorticity (PV). Other structural changes found for cyclones in a future climate include weakened upper-level PV for strong storms and a weakened near-surface potential temperature anomaly for moderate storms, demonstrating a change in storm dynamics. Furthermore, the impacts associated with extratropical cyclones, such as strong near-surface winds and heavy precipitation, strengthen and become more frequent with warming.

Flow along isobaths of a sloping lower boundary generates an across-isobath Ekman transport in the bottom boundary layer. When this Ekman transport is down the slope it causes convective mixing—much like a downfront wind in the surface boundary layer—destroying stratification and potential vorticity. In this manuscript we show how this can lead to the development of a forced centrifugal or symmetric instability regime, where the potential vorticity flux generated by friction along the boundary is balanced by submesoscale instabilities that return the boundary layer potential vorticity to zero. This balance provides a strong constraint on the boundary layer evolution, which we use to develop a theory that explains the evolution of the boundary layer thickness, the rate at which the instabilities extract energy from the geostrophic flow field, and the magnitude and vertical structure of the dissipation. Finally, we show using theory and a high-resolution numerical model how the presence of centrifugal or symmetric instabilities alters the time-dependent Ekman adjustment of the boundary layer, delaying Ekman buoyancy arrest and enhancing the total energy removed from the balanced flow field. Submesoscale instabilities of the bottom boundary layer may therefore play an important, largely overlooked, role in the energetics of flow over topography in the ocean.

A novel method that quantitatively evaluates the development processes of extratropical cyclones is devised and applied to the explosive cyclones over the northwest Pacific in the cold season (October–April). By inverting the potential vorticity (PV) tendency equation, the contribution of dynamic and thermodynamic processes at different levels to explosive cyclone development is quantified. In terms of geostrophic vorticity tendency at 850 hPa, which is utilized to quantify cyclone development, the leading factors for the explosive cyclone intensification are upper-level PV advection by the mean zonal flow and the PV production from latent heating. However, explosive cyclones are also subject to hindrances from vertical and meridional PV advections. Quantitatively, the sum of thermodynamic contributions by the latent heating, vertical PV advection, and surface temperature tendency is about 1.6 times more important than the dynamical PV redistribution by horizontal advections on the explosive cyclone intensification. This result confirms the dominant role of thermodynamic processes in explosive cyclone development over the northwest Pacific. It turns out from further analysis that the interactions of lower-level anomalous flows are important for thermodynamic processes, whereas the advections by the upper-level mean flow are primary for dynamic processes.

Subthermocline eddies (SEs) influencing ocean circulation are progressively known, yet their extensive impact on the western Pacific undercurrent system remains largely unexplored and, in some regions, often underestimated. Okubo‐Weiss parameter analysis reveals a distinctive meridional pattern of cyclonic and anticyclonic SE distribution in the interior western Pacific basin that aligns with zonally elongated mean flows. These westward‐propagating SEs play a pivotal role in regulating the formation of zonal undercurrents, particularly off‐equatorial regions, through the convergence of eddy potential vorticity flux. Along the Pacific western boundary region, anticyclonic SEs typically enhance (reverse) the velocity of boundary currents flowing northward (southward), primarily through barotropic energy conversion, while cyclonic SEs do the opposite. To summarize, we provide a schematic map of the circulation system in the western Pacific and emphasize the interconnected framework of undercurrents, particularly in relation to SEs. Plain Language Summary Subthermocline eddies (SEs) are widespread in the western Pacific Ocean, varying in size, depth, and speed. We've learned quite a bit about SEs and their influence on ocean circulation. However, their significant impact on the undercurrent system in the western Pacific is still not well understood and, in some regions, often underestimated. Using an eddy identification method, we have uncovered a pattern of SEs in the western Pacific that matches the alternating eastward mean flows. These SEs appear to play a role in shaping the tropical‐subtropical eastward undercurrent system. Along the western boundary, the northward (southward) undercurrents are strengthened by the SEs spinning clockwise (counterclockwise). In summary, we provide a map of the western Pacific circulation system and highlight the interconnected relationships between undercurrents and SEs. Key Points A meridional pattern of Subthermocline eddies (SEs) that aligns with the western Pacific zonal undercurrents has been revealed Off‐equatorial eastward undercurrents arise from the convergence of cyclonic (anticyclonic) swirling motions from the north (south) Along the western boundary, SEs significantly modulate the boundary undercurrents