Search Results (19)

A comprehensive diagnosis of eddy–mean flow interaction in the Kuroshio Current (KC) region and the associated energy conversion pathway is conducted employing a state-of-the-art high-resolution global ocean–sea ice coupled model. The spatial distributions of the energy reservoirs and their conversions exhibit significant complexity. The cross-stream variation is found in the energy conversion pattern in the along-coast region, whereas a mixed positive–negative conversion pattern is observed in the off-coast region. Considering the area-integrated conversion rates between energy reservoirs, barotropic and baroclinic instabilities dominate the energy transferring from the mean flow to eddy field in the KC region. When the KC separates from the coast, it becomes highly unstable and the energy conversion rates intensify visibly; moreover, the local variations of the energy conversion are significantly influenced by the topography in the KC extension region. The mean available potential energy is the total energetic source to drive the barotropic and baroclinic energy pathway in the whole KC region, while the mean kinetic energy supplies the total energy in the extension region. For the whole KC region, the mean current transfers 84.9 GW of kinetic energy and 37.3 GW of available potential energy to the eddy field. The eddy kinetic energy is generated by mixed barotropic and baroclinic processes, amounting to 84.9 GW and 15.03 GW, respectively, indicating that topography dominates the generation of mesoscale eddy. Mean kinetic energy amounts to 11.08 GW of power from the mean available potential energy and subsequently supplies the barotropic pathway. Full article

The West African atmospheric energy budget is assessed for the first time across three contrasting monsoon seasons (dry, wet, and moderate) using the latest version of the Canadian Regional Climate Model (CRCM6/GEM5). The model is driven by ERA5 reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF). A formalism appropriate for regional climate energetics is employed to quantify the primary physical processes occurring during the West African Monsoon, with the aim of highlighting those that exhibit significant inter-seasonal variability. The atmospheric energy path shows that the time-mean available enthalpy (A_M) reservoir, reflecting high surface temperatures and a lapse rate characteristic of a dry atmosphere, dominates other energy reservoirs. A_M is converted into the time-mean kinetic energy (K_M) and the time-variability available enthalpy (A_E) reservoirs, which are converted into a time-variability kinetic energy reservoir (K_E) through barotropic and baroclinic processes. A_E is the lowest energy reservoir, confirming smaller temperature variations in the tropics compared to higher latitudes. Kinetic energy reservoirs K_M and K_E have the same order of magnitude, suggesting that mean flow is as important as eddy activities during the season. The atmospheric energy cycle computed for three contrasting rainy seasons shows that time-variability energy reservoirs (A_E and K_E) and main terms acting upon them, are proportional to the rainfall activity, being higher (lower) during rainy (dry) years. It also reveals that, while C_A (conversion from A_M to A_E) and the generation term G_E feed wave’s development, the frictional term D_E counteracts the generation of K_E to dampen the creation of transient eddies. These findings suggest that the atmospheric energetic formalism could be applied on West African seasonal forecasts and future climate simulations to implement adaptation strategies. Full article

From a traditional point of view, the growth of a tropical cyclone (TC) requires that the vertical wind shear (VWS) should be weak. However, Typhoon Rammasun (2014) underwent a rapid intensification (RI) even in the presence of a strong VWS background. This study investigates the counterintuive phenomenon, using the multiscale window transform (MWT) and the theory of canonical transfer. For the first time, the diagnostic results show that the strong VWS provided additional available potential energy (APE) to the mid-to-upper troposphere through baroclinic instability. This APE was converted into kinetic energy (KE) via buoyancy conversion and transported to the lower troposphere by pressure gradient, increasing the lower-troposphere wind speed. The strong VWS facilitated the RI in two main ways. First, it was via baroclinic instability. Strong VWS facilitated the transfer of APE from the background flow window to the typhoon scale window, supplying additional APE to the mid-to-upper troposphere, hence enhancing the warm-core structure. Second, the VWS direction shifted from an east-west orientation to a north-south orientation. This directional change put the typhoon’s vertical alignment from a westward tilt back to a straighter one. This effectively suppressed the destructive effects of the asymmetric circulation, and promoted the conversion of APE into KE via buoyancy conversion, hence contributed to the RI. Full article

Based on statistical analyses of long-term reanalysis data, we have investigated the interdecadal variations of autumn precipitation in central China (APC-d) and the associated atmospheric teleconnection. It reveals that the increased autumn rainfall in central China during the last decade is a portion of the APC-d, which exhibits a high correlation coefficient of 0.7 with the interdecadal variations of the Europe–China pattern (EC-d pattern) teleconnection. The EC-d pattern teleconnection presents in a “+-+” structure over Eurasia, putting central China into the periphery of a quasi-barotropic anticyclonic high-pressure anomaly. Driven by positive vorticity advection and the inflow of warmer and moist air from the south, central China experiences enhanced ascending motion and abundant water vapor supply, resulting in increased rainfall. Further analysis suggests that the EC-d pattern originates from the exit of the North Atlantic jet and propagates eastward. It is captured by the Asian westerly jet stream and proceeds towards East Asia through the wave–mean flow interaction. The wave train acquires effective potential energy from the mean flow by the baroclinic energy conversion and simultaneously obtains kinetic energy from the basic westerly jet zones across the North Atlantic and the East Asian coasts. The interdecadal variation of the mid-latitude North Atlantic sea surface temperature (MAT-d) exhibits a significant negative relationship with EC-d, serving as a modulating factor for the EC-d pattern teleconnection. Experiments with CMIP6 models predict that the interdecadal variations in APC-d, EC-d, and MAT-d will maintain stable high correlations for the rest of the 21st century. These findings may contribute to forecasting the interdecadal autumn dry–wet conditions in central China. Full article

Previous studies have investigated the characteristics and influencing factors of plume bulge in the Pearl River Estuary (PRE) using observations and numerical simulations. However, the understanding of how river discharge affects plume bulge is not consistent, and the response mechanism of plume bulge to changes in river discharge has not been revealed in detail. In this study, a three-dimensional hydrodynamic Finite-Volume Coastal Ocean Model (FVCOM) is constructed, and five experiments were set to characterize the horizontal and vertical distribution of the plume bulge outside the PRE under different river discharge conditions during spring tide. The physical mechanisms of plume bulge generation and its response mechanisms to river discharge were discussed through standardized analysis and momentum diagnostic analysis. The results indicate that the plume bulge is sensitive to changes in river discharge. When the river discharge is relatively low (e.g., less than 11,720 m³/s observed in the dry season), the bulge cannot be formed. Conversely, when the river discharge is relatively high (e.g., exceeding 23,440 m³/s observed in flood season), the bulge is more significant. The plume bulge is formed by the anticyclonic flow resulting from the action of the Coriolis force on the strongly mixed river plume. The bulge remains stable under the combined effects of barotropic force, baroclinic gradient force, and Coriolis force. The reduction of river discharge weakens the mixing of freshwater and seawater, resulting in the reduction of both the volume and momentum of the river plume, and the balance between advective diffusion and Coriolis forces are altered, resulting in the plume, which is originally flushed out from the Lantau Channel, not being able to maintain the anticyclonic structure and instead floating out along the coast of the western side of the PRE, with the disappearance of the plume bulge. Due to the significant influence of plume bulges on the physical and biogeochemical interactions between estuaries and terrestrial environments, studying the physical mechanisms behind the formation of plume bulges is crucial. Full article

A remarkable intraseasonal reversal of temperature anomaly is witnessed in eastern China in early 2022, characterized by a warm January and a cold February. ERA5 daily reanalysis data, multiple regression and the Linear Baroclinic Model (LBM) are employed to investigate the characteristics and causes of this abnormal temperature Pattern. The findings indicate that: (1) The two Rossby wave trains along the south and north westerly jets over Eurasia have synergistic impacts on middle and high latitudes. In January, the south branch Rossby wave train exhibited a positive phase, coinciding with a negative phase in the north branch wave train. As a result, the south trough strengthens, while the north trough weakens, leading to anomalous warm advection that warms eastern China. In February, the phases of these two Rossby waves are reversed, causing anomalous cold advection as the southern trough diminishes and the northern trough intensifies, resulting in colder conditions in eastern China. (2) Tropical convection activity weakens in January, whereas it intensifies in February in the northeast Indian Ocean. The weakening of the East Asian trough as a result of the convective latent heat anomalies caused an anticyclonic circulation over the Korean Peninsula in January through the Pacific-Japan teleconnection-like pattern, which is necessary for the maintenance of warm anomalies. Conversely, increased convective activity in February induces cyclonic circulation, deepening the East Asian trough over the Korean Peninsula and contributing to the persistence of cold anomalies. (3) The Rossby wave trains along the two westerly jets and the tropical convective activity in the northeastern Indian Ocean work in tandem, simultaneously strengthening or weakening the East Asian trough. Consequently, the East Asian trough weakens in January and strengthens in February. Full article

Cavitation is one of the most important aspects of the stable and safe operation of a centrifugal pump. To examine the dynamics of cavitation vortex in a centrifugal pump, the cavitating flow is investigated by using the modified shear stress transport (SST) k-ω turbulence model with the Zwart cavitation model. The numerical results are confirmed by comparing them with those obtained from experimental tests. The results show that there is a critical cavitation number of σ_c at each flow rate condition. As the cavitation number σ exceeds σ_c, the pump head remains stable. Conversely, the head rapidly decreases when the σ falls below σ_c. As the σ decreases, the pump experiences successively incipient cavitation, slight cavitation, and severe cavitation. At the stage of severe cavitation conditions, the vortex structures are generated at the tail of cavitation in the flow passage. The vorticity transport method is employed to analyze the vortex dynamics, and it is found that the vortex area contains high vorticity. The dominant contribution to the generation of vorticity comes from the vortex stretching and dilation terms acting in different directions. The contribution of the baroclinic torque to vorticity generation at the vapor-liquid interface is significant. The Coriolis force has a negligible impact on vorticity transport. Full article

Polar lows (PLs) are maritime mesoscale cyclones associated with severe weather. They develop during marine cold air outbreaks near coastlines and the sea ice edge. Unfortunately, our knowledge about the mechanisms leading to PL development is still incomplete. This study aims to provide a detailed analysis of the development mechanisms of a PL that formed over the Norwegian Sea on 25 March 2019 using the output of a simulation with the sixth version of the Canadian Regional Climate Model (CRCM6/GEM4), a convection-permitting model. First, the life cycle of the PL is described and the vertical wind shear environment is analysed. Then, the horizontal wind divergence and the baroclinic conversion term are computed, and a surface pressure tendency equation is developed. In addition, the roles of atmospheric static stability, latent heat release, and surface heat and moisture fluxes are explored. The results show that the PL developed in a forward-shear environment and that moist baroclinic instability played a major role in its genesis and intensification. Baroclinic instability was initially only present at low levels of the atmosphere, but later extended upward until it reached the mid-troposphere. Whereas the latent heat of condensation and the surface heat fluxes also contributed to the development of the PL, convective available potential energy and barotropic conversion do not seem to have played a major role in its intensification. In conclusion, this study shows that a convection-permitting model simulation is a powerful tool to study the details of the structure of PLs, as well as their development mechanisms. Full article

Two-dimensional (2D) and three-dimensional (3D) cloud-resolving model (CRM) results from the Tropical Rainfall Measuring Mission Kwajalein Experiment (KWAJEX) were applied to analyze the diurnal cycle of cloud development in the tropics. Cloud development is intimately associated with the growth of secondary circulation, which can be analyzed in the budget of perturbation kinetic energy (PKE). The ice and liquid water path (IWP+LWP) is a fundamental parameter for estimating clouds, with the analyzed results suggesting that (1) the ice and liquid water path (IWP+LWP) and PKE values attained in convective regions were higher during the nighttime than during the daytime and that the maxima of IWP+LWP and PKE occurred at midnight in the lower troposphere in the 3D model run, and that (2) the IWP+LWP and PKE values in stratiform regions were much higher in the afternoon than in the morning, while the maxima of IWP+LWP and PKE occurred in the afternoon in the middle troposphere in the 2D model run. Further analysis demonstrated that both the high IWP+LWP and PKE values in the lower troposphere at midnight were mainly associated with the warm–humid lower troposphere in convective regions. However, those in the middle troposphere in the afternoon were primarily linked to the dry–cold upper troposphere and moist–warm lower troposphere in stratiform regions. The results further revealed that (1) both IWP+LWP and PKE exhibited shorter time scales in the 2D model runs than in the 3D model runs and that (2) the maximum IWP+LWP values occurred in the afternoon in the 2D model runs and at midnight in the 3D model runs. Full article

Convective velocity perturbations (CVPs) are known to play an important role in the response of flames to acoustic perturbations and in thermoacoustic combustion instabilities. In order to elucidate the flow-physical origin of CVPs, the present study models the response of laminar premixed slit flames to low amplitude perturbations of the upstream flow velocity with a reduced order flow decomposition approach: A linearized G-equation represents the shape and heat release rate of the perturbed flame, while the velocity perturbation field is decomposed into irrotational and solenoidal contributions. The former are determined with a conformal mapping from geometry and boundary conditions, whereas the latter are governed by flame front curvature and flow expansion across the flame, which generates baroclinic vorticity. High-resolution CFD analysis provides values of model parameters and confirms the plausibility of model results. This flow decomposition approach makes it possible to explicitly evaluate and analyze the respective contributions of irrotational and solenoidal flows to the flame response, and conversely the effect of flame perturbations on the flow. The use of the popular ad hoc hypothesis of convected velocity perturbation is avoided. It is found that convected velocity perturbations do not result from immediate acoustic-to-hydrodynamic mode conversion, but are generated by flame-flow feedback. In this sense, models for flame dynamics that rely on ad-hoc models for CVPs do not respect causality. Furthermore, analysis of the flame impulse response reveals that for the configuration investigated, flame-flow feedback is also responsible for “excess gain” of the flame response, that is, the magnitude of the flame frequency response above unity. Full article

This paper presents an assessment of the seasonal variability of the velocity fields, mean and eddy kinetics, and available potential energies, and the energy conversion rates for the eddy and basin-scale circulation regimes. The data were obtained through the numerical modeling of the Black Sea circulation for 2011 and 2016. It revealed significant differences in the current structure in the southern and central sea parts for 2011 and 2016. In 2011, the Rim Current was an almost continuous cyclonic basin-scale gyre, while in 2016 a system of mesoscale anticyclones was observed in the southern part. The variability of the mean kinetic energy depends more on the circulation regime than on the season of the year, while the distribution of the mean available potential energy is predominantly seasonal. The eddy kinetic energy depends on both the circulation regime and the season. In winter, the energy transport from the mean current via a barotropic instability mechanism sustains the mesoscale eddy generation. In summer, the mesoscale variability in the basin-scale regime is provided by commensurate contributions of barotropic and baroclinic instability, and, in the eddy regime, mainly by the energy transport from the available potential energy through the baroclinic instability. Full article

The heavy precipitation in Northern California—brought about by a landfalling atmospheric river (AR) on 25–27 February 2019—is investigated for an understanding of the underlying dynamical processes. By the peaks in hourly accumulation, this rainstorm can be divided into two stages (Stage I and Stage II). Using a recently developed multiscale analysis methodology, i.e., multiscale window transform (MWT), and the MWT-based theory of canonical transfer, the original fields are reconstructed onto three scale windows, namely, the background flow, synoptic-scale and mesoscale windows, and the interactions among them are henceforth investigated. In both stages, the development of the precipitation is attributed to a vigorous buoyancy conversion and latent heating, and besides, the instability of the background flow. In Stage I, the instability is baroclinic, while in Stage II, it is barotropic. Interestingly, in Stage I, the mesoscale kinetic energy is transferred to the background flow where it is stored, and is released back in Stage II to the mesoscale window again, triggering intense precipitation. Full article

With the advent of submesoscale $O(1\mathrm{km})$ permitting basin-scale ocean simulations, the seasonality of mesoscale $O(50\mathrm{km})$ eddies with kinetic energies peaking in summer has been commonly attributed to submesoscale eddies feeding back onto the mesoscale via an inverse energy cascade under the constraint of stratification and Earth’s rotation. In contrast, by running a 101-member, seasonally forced, three-layer quasi-geostrophic (QG) ensemble configured to represent an idealized double-gyre system of the subtropical and subpolar basin, we find that the mesoscale kinetic energy shows a seasonality consistent with the summer peak without resolving the submesoscales; by definition, a QG model only resolves small Rossby and Froude number dynamics ($O(Ro)\ll 1,O(Fr)\ll 1$) while submesoscale dynamics are associated with $O(Ro)\sim 1,O(Fr)\gtrsim 1$. Here, by quantifying the Lorenz cycle of the mean and eddy energy, defined as the ensemble mean and fluctuations about the mean, respectively, we propose a different mechanism from the inverse energy cascade. During summer, when the Western Boundary Current is stabilized and strengthened due to increased stratification, stronger mesoscale eddies are shed from the separated jet. Conversely, the opposite occurs during the winter; the separated jet destablizes and results in overall lower mean and eddy kinetic energies despite the domain being more susceptible to baroclinic instability from weaker stratification. Full article

In this study, M₂ tidal energy and tide-induced mixing in the Mariana double ridges are investigated with a high-resolution three-dimensional non-hydrostatic numerical model and baroclinic energy budget analysis. The interference effect of the double ridges on the internal tide in the Mariana is examined by omitting either the eastern or the western ridge. Our results show that the baroclinic velocity on the sides of the interior facing slopes of the double ridges is larger than that on the other sides. In the double ridges, high values of dissipation reaching O (10⁻⁶ W kg⁻¹) are accompanied by diapycnal diffusivity reaching O (10⁻¹ m² s⁻¹), which is several orders of magnitude higher than the mixing of the open ocean. The bottom diapycnal mixing in the inner region between the two ridges is one order of magnitude larger than the mixing outside the ridges, indicating the important role of the interference of the double-ridge topography on the mixing in the Mariana Arc. Omitting either the eastern or the western ridge would have a significant impact on tide current, baroclinic energy flux and dissipation, and diapycnal mixing. The internal tide conversion, dissipation, and flux divergence are amplified by the double ridge topography, especially in the central part of the double ridges. Through energy budgets analysis, we conclude that the eastern ridge is the main source of the baroclinic tide in the Mariana double ridges. Full article

We have investigated the surface and subsurface submesoscale dynamics in the Gulf of Aden and the Gulf of Oman. Our results are based on the analyses of regional numerical simulations performed with a primitive equation model (HYCOM) at submesoscale permitting horizontal resolution. A model zoom for each gulf was embedded in a regional mesoscale-resolving simulation. In the Gulf of Aden and the Gulf of Oman, the interactions of mesoscale structures and fronts instabilities form submesoscale eddies and filaments. Rotational energy spectra show that the Gulf of Aden has a higher ratio of submesoscale to mesocale energy than the Gulf of Oman. Fast waves (internal gravity waves, tidal waves, Kelvin waves) and slow waves (Rossby waves) were characterized via energy spectra of the divergent velocity. Local upwelling systems which shed cold filaments, coastal current instabilities at the surface, and baroclinic instability at capes in subsurface were identified as generators of submesocale structures. In particular, the Ras al Hamra and Ras al Hadd capes in the Gulf of Oman, and the Cape of Berbera in the Gulf of Aden, are loci of submesoscale eddy generation. To determine the instability mechanisms involved in these generations, we diagnosed the Ertel potential vorticity and the energy conversion terms: the horizontal and vertical Reynolds stresses and the vertical buoyancy flux. Finally, the impacts of the subsurface submesoscale eddy production at capes on the diffusion and fate of the Red Sea Water (in the Gulf of Aden) and the Persian Gulf Water (in the Gulf of Oman) are highlighted. Full article