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In this paper, an extended nonlinear multiscale interaction model of blocking events in the equivalent barotropic atmosphere is used to investigate the effect of a slowly varying zonal wind in the meridional direction on dipole blocking that is regarded as a nonlinear Rossby wave packet. It is shown that the meridional gradient of potential vorticity (PVy=∂PV/∂y) prior to the blocking onset, which is related to the background zonal wind and its nonuniform meridional shear, can significantly affect the lifetime, intensity, and north–south asymmetry of dipole blocking, while the blocking dipole itself is driven by preexisting incident synoptic-scale eddies. The magnitude of the background PVy determines the energy dispersion and nonlinearity of blocking. It is revealed that a small background PVy is a prerequisite for strong and long-lived eddy-driven blocking that behaves as a persistent meandering westerly jet stream, while the blocking establishment further reduces the PVy within the blocking region, resulting in a positive feedback between blocking and PVy. When the core of the background westerly jet shifts from higher to lower latitudes, the blocking shows a northwest–southeast-oriented dipole with a strong anticyclonic anomaly to the northwest and a weak cyclonic anomaly to the southeast as its northern pole moves westward more rapidly and has weaker energy dispersion and stronger nonlinearity than its southern pole because of the smaller PVy in higher latitudes. The opposite is true when the background jet shifts toward higher latitudes. The asymmetry of dipole blocking vanishes when the background jet shows a symmetric double-peak structure. Thus, a small prior PVy is a favorable precursor for the occurrence of long-lived and large-amplitude blocking.

Sheddings of Kuroshio Loop Current (KLC) eddies in the northeastern South China Sea (SCS) are investigated using mooring arrays, multiple satellite data, and data-assimilative HYCOM products. Based on altimeter sea surface heights between 1992 and 2014, a total of 19 prominent KLC eddy shedding (KLCES) events were identified, among which four events were confirmed by the concurrent moored and satellite observations. Compared to the leaping behavior of Kuroshio, KLCES is a relatively short-duration phenomenon that primarily occurs in boreal autumn and winter. The KLC and its shedding anticyclonic eddy (AE) trap a large amount of Pacific water with high temperature–salinity and low chlorophyll concentration in the upper layer. The corresponding annual-mean transport caused by KLCES reaches 0.24–0.38 Sv (1 Sv ≡ 10 6 m 3 s −1 ), accounting for 6.8%–10.8% of the upper-layer Luzon Strait transport. Altimeter-based statistics show that among ~90% of the historical KLCES events, a cyclonic eddy (CE) is immediately generated behind the AE southwest of Taiwan. Both energetics and stability analyses reveal that because of its large horizontal velocity shear southwest of Taiwan, the northern branch of KLC is strongly unstable and the barotropic instability of KLC constitutes the primary generation mechanism for the CE. After CE is generated, it quickly grows and gradually migrates southward, which in turn facilitates the detachment of AE from KLC. The intrinsic relationship between KLC and CE explains well why eddy pairs are commonly observed in the region southwest of Taiwan.

We develop elementary weather prediction models using deep convolutional neural networks (CNNs) trained on past weather data to forecast one or two fundamental meteorological fields on a Northern Hemisphere grid with no explicit knowledge about physical processes. At forecast lead times up to 3 days, CNNs trained to predict only 500‐hPa geopotential height easily outperform persistence, climatology, and the dynamics‐based barotropic vorticity model, but do not beat an operational full‐physics weather prediction model. These CNNs are capable of forecasting significant changes in the intensity of weather systems, which is notable because this is beyond the capability of the fundamental dynamical equation that relies solely on 500‐hPa data, the barotropic vorticity equation. Modest improvements to the CNN forecasts can be made by adding 700‐ to 300‐hPa thickness to the input data. Our best performing CNN does a good job of capturing the climatology and annual variability of 500‐hPa heights and is capable of forecasting realistic atmospheric states at lead times of 14 days. Although our simple models do not perform better than an operational weather model, machine learning warrants further exploration as a weather forecasting tool; in particular, the potential efficiency of CNNs might make them attractive for ensemble forecasting. Key Points Deep convolutional neural networks trained to predict gridded weather from historical reanalysis significantly outperform basic benchmarks Unlike the dynamical barotropic vorticity model, the neural networks can predict amplification and decay of weather systems The neural networks produce realistic 14‐day weather forecasts despite having no explicit knowledge of atmospheric physics

Barotropic flow along depth contours is found in accordance with standard geostrophic theory. A numerical model is developed that studies the deviation from such a flow. The model gives a good approximation of the dynamical processes on the West Spitsbergen Shelf (WSS) and shows that the West Spitsbergen Current (WSC), the main gateway of Atlantic water (AW) toward the Arctic, connects more easily to the Isfjorden Trough than anywhere else along the shelf. The circulation of AW in the troughs along the WSS is here named the Spitsbergen Trough Current (STC). From hydrographical and ocean current observations it is evident that the STC is primarily barotropic and driven by the sea surface height. A connection between the along-coast wind stress and the STC is established, and it is demonstrated how the increased occurrence of winter cyclones in Fram Strait during January–February accelerates and widens the WSC. Ultimately, this results in a strengthened STC and dominance of AW on the WSS. The STC represents a slower route of AW toward the Arctic Ocean and a large heat transport toward the West Spitsbergen fjords during winter (0.2–0.4 TW toward Isfjorden). Heat flux estimates show that half of the AW heat loss in the Isfjorden Trough is due to heat loss to the surrounding water masses, while the rest is lost to the atmosphere. Sea ice production along West Spitsbergen has been reduced, or even nonexistent, in some fjords since 2006. Here, the authors argue that this is a consequence of the strong southerly wind periods along the WSS during winter.

A recently developed tool, the multiscale window transform, along with the theory of canonical energy transfer is used to investigate the roles of multiscale interactions and instabilities in the Gulf of Mexico Loop Current (LC) eddy shedding. A three-scale energetics framework is employed, in which the LC system is reconstructed onto a background flow window, a mesoscale eddy window, and a high-frequency eddy window. The canonical energy transfer between the background flow and the mesoscale windows plays an important role in LC eddy shedding. Barotropic instability contributes to the generation/intensification of the mesoscale eddies over the eastern continental slope of the Campeche Bank. Baroclinic instability favors the growth of the mesoscale eddies that propagate downstream to the northeastern portion of the well-extended LC, eventually causing the shedding by cutting through the neck of the LC. These upper-layer mesoscale eddies lose their kinetic energy back to the background LC through inverse cascade processes in the neck region. The deep eddies obtain energy primarily from the upper layer through vertical pressure work and secondarily from baroclinic instability in the deep layer. In contrast, the canonical energy transfer between the mesoscale and the high-frequency frontal eddy windows accounts for only a small fraction in the mesoscale eddy energy balance, and this generally acts as a damping mechanism for the mesoscale eddies. A budget analysis reveals that the mesoscale eddy energy gained through the instabilities is balanced by horizontal advection, pressure work, and dissipation.

The submesoscale energy budget is complex and remains understood only in region-by-region analyses. Based on a series of nested numerical simulations, this study investigated the submesoscale energy budget and flux in the upper ocean of the Kuroshio Extension, including some innovations for examining submesoscale energy budgets in general. The highest-resolution simulation on a ~500 m grid resolves a variety of submesoscale instabilities allowing an energetic analysis in the submesoscale range. The frequency–wavenumber spectra of vertical vorticity variance (i.e., enstrophy) and horizontal divergence variance were used to identify the scales of submesoscale flows as distinct from those of inertia-gravity waves but dominating horizontal divergence variance. Next, the energy transfers between the background scales and the submesoscale were examined. The submesoscale kinetic and potential energy (SMKE and SMPE) were mainly contained in the mixed layer and energized through both barotropic (shear production) and baroclinic (buoyancy production) routes. Averaged over the upper 50 m of ROMS2, the baroclinic transfers amounted to approximately 75% of the sources for the SMKE (3.42 × 10 −9 W/kg) versus the remaining 25% (1.12 × 10 −9 W/kg) via barotropic downscale KE transfers. The KE field was greatly strengthened by energy sources through the boundary—this flux is larger than the mesoscale-to-submesoscale transfers in this region. Spectral energy production, importantly, reveals upscale KE transfers at larger submesoscales and downscale KE transfers at smaller submesoscales (i.e., a transition from inverse to forward KE cascade). This study seeks to extend our understanding of the energy cycle to the submesoscale and highlight the forward KE cascade induced by upper-ocean submesoscale activities in the research domain.

An idealized framework of steady barotropic flow past an isolated seamount in a background of constant stratification (with frequency N ) and rotation (with Coriolis parameter f ) is used to examine the formation, separation, instability of the turbulent bottom boundary layers (BBLs), and ultimately, the genesis of submesoscale coherent vortices (SCVs) in the ocean interior. The BBLs generate vertical vorticity ζ and potential vorticity q on slopes; the flow separates and spawns shear layers; barotropic and centrifugal shear instabilities form submesoscale vortical filaments and induce a high rate of local energy dissipation; the filaments organize into vortices that then horizontally merge and vertically align to form SCVs. These SCVs have O (1) Rossby numbers ( ) and horizontal and vertical scales that are much larger than those of the separated shear layers and associated vortical filaments. Although the upstream flow is barotropic, downstream baroclinicity manifests in the wake, depending on the value of the nondimensional height , which is the ratio of the seamount height to that of the Taylor height , where L is the seamount half-width. When , SCVs span the vertical extent of the seamount itself. However, for , there is greater range of variation in the sizes of the SCVs in the wake, reflecting the wake baroclinicity caused by the topographic interaction. The aspect ratio of the wake SCVs has the scaling , instead of the quasigeostrophic scaling .

The Pacific–Japan (PJ) pattern, also known as the East Asia–Pacific pattern, is a teleconnection that significantly influences the East Asian summer climate on various time scales. Based on several reanalysis and observational datasets, this study suggests that the PJ pattern has experienced a distinct three-dimensional structural change in the late 1990s. Compared with those during 1979–98, the PJ pattern shifts eastward by approximately 20° during 1999–2015, and the intensity of its barotropic structure in the extratropics weakens significantly. As a result, its influences on the summer rainfall along the mei-yu band are weakened after the late 1990s. These observed changes can be attributed to three reasons. First, the location where the PJ pattern is excited shifts eastward. Second, the easterly shear of the background wind is very weak around the source region of the PJ pattern after the late 1990s, which prevents the convection-induced baroclinic mode from converting into barotropic mode and thereby from propagating into the extratropics. Third, the PJ pattern–induced rainfall anomalies are weak along the mei-yu band after the late 1990s. As a result, their feedbacks to the PJ pattern become weak and play a considerably reduced role in maintaining the structure of the PJ pattern in the midlatitudes. In contrast, the eddy energy conversion from the basic flow efficiently maintains the PJ pattern before and after the late 1990s and thereby contributes little to the observed change.

The waveguidability of an upper-tropospheric zonal jet quantifies its propensity to duct Rossby waves in the zonal direction. This property has played a central role in previous attempts to explain large wave amplitudes and the subsequent occurrence of extreme weather. In these studies, waveguidability was diagnosed with the help of ray tracing arguments using the zonal average of the observed flow as the relevant background state. Here, it is argued that this method is problematic both conceptually and mathematically. The issue is investigated in the framework of the nondivergent barotropic model. This model allows the straightforward computation of an alternative “zonalized” background state, which is obtained through conservative symmetrization of potential vorticity contours and that is argued to be superior to the zonal average. Using an idealized prototypical flow configuration with large-amplitude eddies, it is shown that the two different choices for the background state yield very different results; in particular, the zonal-mean background state diagnoses a zonal waveguide, while the zonalized background state does not. This result suggests that the existence of a waveguide in the zonal-mean background state is a consequence of, rather than a precondition for, large wave amplitudes, and it would mean that the direction of causality is opposite to the usual argument. The analysis is applied to two heatwave episodes from summer 2003 and 2010, yielding essentially the same result. It is concluded that previous arguments about the role of waveguidability for extreme weather need to be carefully reevaluated to prevent misinterpretation in the future.

The British–Baikal Corridor (BBC) pattern, a new teleconnection along the summertime upper-tropospheric polar front jet (PFJ), is investigated based on observational and reanalysis datasets. The BBC pattern consists of four geographically fixed centers over the west of the British Isles, the Baltic Sea, western Siberia, and Lake Baikal, respectively. It features a zonally oriented and meridionally confined wavelike structure with a zonal wavenumber 5, and it influences the climate along its route significantly. The BBC pattern forms from the trapped effect of the PFJ waveguide that is characterized by a strong meridional gradient of stratification.As a preferred dynamical mode inherent in the PFJ, it is maintained through the baroclinic energy conversion from the basic flow and the feedback forcing of high-frequency transient eddies. Meanwhile, its geographical location is determined by the barotropic energy conversion, which is sensitive to the configuration of the basic flow. The interannual variability of the BBC pattern is dominated by atmospheric internal dynamics considering its loose relation with immediate atmospheric external forcing. Further analyses suggest that the BBC pattern is excited by the active multiscale interactions among the climatological mean flow, the low-frequency flow, and the synoptic-scale transient eddies in the exit region of the North Atlantic jet, which may also determine the preferential upstream forcing region and anchor the BBC pattern geographically. Budget analyses on vorticity, temperature, and water vapor are performed to interpret the physical nature of the BBC pattern. The possible linkage to the North Atlantic Oscillation is also discussed.