Analytical expression development for eddy field and the beam dynamics effect on the CEPC booster

An analytical magnetic field expression for a circular vacuum chamber with induced eddy on it is obtained. Two boundary conditions are discussed: with and without the iron poles; the former implies the use of the image current methods. After the current angular distribution in the vacuum chamber is calculated, a contour integration is applied to obtain the field distribution in the aperture. The application to the CEPC booster is also presented, which is one of the most important issues in the design stage.

The impact of lee waves on the Southern Ocean circulation

Lee waves play an important role in transferring energy from the geostrophic eddy field to turbulent mixing in the Southern Ocean. As such, lee waves can impact the Southern Ocean circulation and modulate its response to changing climate through their regulation on the eddy field and turbulent mixing. The drag effect of lee waves on the eddy field and the mixing effect of lee waves on the tracer field have been studied separately to show their importance. However, it remains unclear how the drag and mixing effects act together to modify the Southern Ocean circulation. In this study, a lee wave parameterization that includes both lee wave drag and its associated lee-wave-driven mixing is developed and implemented in an eddy-resolving idealized model of the Southern Ocean to simulate and quantify the impacts of lee waves on the Southern Ocean circulation. The results show that lee waves enhance the baroclinic transport of the Antarctic Circumpolar Current (ACC) and strengthen the lower overturning circulation. The impact of lee waves on the large-scale circulation are explained by the control of lee wave drag on isopycnal slopes through their effect on eddies, and by the control of lee-wave-driven mixing on deep stratification and water mass transformation. The results also show that the drag and mixing effects are coupled such that they act to weaken one another. The implication is that the future parameterization of lee waves in global ocean and climate models should take both drag and mixing effects into consideration for a more accurate representation of their impact on the ocean circulation.

State estimates and forecasts of the eddy field in the Subtropical Countercurrent in the Northern Philippine Sea

A strongly nonlinear eddy field is present in and around the Subtropical Countercurrent in the Northern Philippine Sea (NPS). A regional implementation of the Massachusetts Institute of Technology general circulation model–Estimating the Circulation and Climate of the Ocean four-dimensional variational (MITgcm-ECCO 4DVAR) assimilation system is found to be able to produce a series of two-month-long dynamically-consistent optimized state estimates between April 2010 and April 2011 for the eddy-rich NPS region. The assimilation provides a stringent dynamical test of the model, showing that a free run of the model forced using adjusted controls remains consistent with the observations for two months. The 4DVAR iterative optimization reduced the total cost function for the observations and controls by 40–50% from the reference solution, initialized using the Hybrid Coordinate Ocean Model 1/12° global daily analysis, achieving residuals approximately equal to the assumed uncertainties for the assimilated observations. The state estimates are assessed by comparing with assimilated and withheld observations and also by comparing one-month model forecasts with future data. The state estimates and forecasts were more skillful than model persistence and the reference solutions. Finally, the continuous state estimates were used to detect and track the eddies, analyze their structure, and quantify their vertically-integrated meridional heat and salt transports.

Does the interior of the ocean hide a major part of the eddy field?

<p>In ocean research, mesoscale eddies typically are detected through surface signatures based on satellite data. The assumption is that most eddies are surface intensified and have a vertical structure consistent with a surface intensified mode. However, in-situ eddy observations, especially in the tropical oceans, showed that the vertical eddy structure is often more complex than previously assumed (higher baroclinic modes), and a diverse subsurface eddy field is present, which does not show any surface signatures at all. Our objective here is a first step towards a quantification of the occurrence of subsurface relative to surface eddies. To do this, we use an actively eddying model to compare the subsurface eddy field to its surface signatures in order to be able to estimate which vertical eddy structures prevail and how much of the eddy field is hidden in the subsurface. In addition, the model results are compared against an unprecedented assemblage of observations of subsurface eddies in the tropical oceans. In a first step we focus on eddies in the model that are detectable at the surface for more than 120 days. We found that around 60 % of the detected eddies have a vertical structure associated with a surface intensified mode as previously assumed which are characterized by a strong surface signature. Around 40 % of the eddy field have a vertical structure associated to a higher baroclinic mode. They are often called “intrathermocline” eddies and are characterized by a rather weak surface signature. In a second step we track subsurface eddies (lifetime > 120 days) in the model by identifying density layer thickness anomalies and connect them with possible surface signatures. Around 30 % of the total eddy field of the model, are hidden in the subsurface with no detectable surface signature. In conclusion, our results show that subsurface eddies form a substantial contribution to the total eddy field. Consequently it is difficult to estimate the impact of the eddy field on the ocean when only working with surface based satellite data.</p>

Classification of wintertime atmospheric teleconnection patterns in the Northern Hemisphere

Energetics of the major atmospheric teleconnection patterns of the Northern Hemisphere winter are examined to investigate the role of baroclinic and barotropic energy conversions in their growth. Based on characteristics of the energetics and the horizontal structures, the patterns are classified into three general types: meridional dipole (D-type), wave (W-type), and hybrid (H-type). The primary energy conversion term that differentiates these patterns is the baroclinic energy conversion of the available potential energy from the climatology to the eddy field associated with the teleconnections. For this conversion term, D-type patterns exhibit the comparable conversion of potential energy via the eddy heat flux across the climatological thermal gradient in both the zonal and meridional directions. In contrast, baroclinic conversion for W-type patterns occurs primarily in the meridional direction, while H-type patterns exhibit a structure that combines the characteristics of the other two pattern types. An important secondary factor is barotropic conversion from the climatology to the eddy field, which takes place mainly in the regions where the climatological shear is strong. For the D-type patterns, conversion occurs on the flank of the climatological jet exit, while it occurs at the center of the jet exit for the W-type patterns. Lastly, for all the patterns, synoptic time-scale eddies make a negative contribution via the baroclinic process, but a positive contribution via the barotropic process. Damping by diabatic heating weakens the temperature anomalies associated with the patterns.

The mesoscale eddy field in the Lofoten Basin from high-resolution Lagrangian simulations

Warm Atlantic-origin waters are modified in the Lofoten Basin in the Nordic Seas on their way toward the Arctic. An energetic eddy field redistributes these waters in the basin. Retained for extended periods, the warm waters result in large surface heat losses to the atmosphere and an impact on fisheries and regional climate. Here, we describe the eddy field in the Lofoten Basin by analysing Lagrangian simulations forced by a high-resolution numerical model. We obtain trajectories of particles seeded at three levels: near the surface, at 200 m and 500 m depth, using 2D and 3D velocity fields. About 200,000 particle trajectories are analyzed from each level and each simulation. Using multivariate wavelet ridge analysis, we identify coherent cyclonic and anticyclonic vortices in the trajectories and describe their characteristics. We then compare the evolution of water properties inside cyclones and anticyclones as well as in the ambient flow outside vortices. As measured from Lagrangian particles, anticyclones have longer lifetimes than cyclones (16–24 days compared to 13–19 days), larger radius (20–22 km compared to 17–19 km) and a more circular shape (ellipse linearity of 0.45–0.50 compared to 0.51–0.57). The angular frequencies for cyclones and anticyclones have similar magnitudes (absolute values of about 0.05f). The anticyclones are characterized by warm temperature anomalies whereas cyclones are colder than the background state. Along their path, water parcels in anticyclones cool at a rate of 0.02–0.04 °C/day while those in cyclones warm at a rate of 0.01–0.02 °C/day. Water parcels experience a net downward motion in anticyclones and upward motion in cyclones, often found to be related to changes in temperature and density. The along-path changes of temperature, density and depth are smaller for particles in the ambient flow. An analysis of the net temperature and vorticity fluxes into the Lofoten Basin shows that while vortices contribute significantly to the heat and vorticity budgets, they only cover a small fraction of the domain area (about 6 %). The ambient flow, including filaments and other non-coherent variability undetected by the ridge analysis, hence plays a major role in closing the budgets of the basin.

Kinetic Energy Conversion in A Wind-forced Submesoscale Flow

<p>Despite recent progress in measuring the ocean eddy field with satellite missions at the mesoscale (order of 100 km), containing the major fraction of ocean kinetic energy, many questions still remain regarding the generation, conversion and dissipation mechanisms of eddy kinetic energy (K<sub>e</sub>). In this work, we use the output from an idealized 500-m resolution ocean numerical simulation to study the conversion of K<sub>e</sub> in the absence and presence of wind stress forcing. In contrast to the result of the unforced run, K<sub>e</sub> increased approximately nine times in the mixed layer and considerably in the pycnocline in the forced run. Eddies and filaments were seen to re-stratify the mixed layer and wind-induced turbulence at the base of the mixed layer promoted its deepening and therefore dramatically enhanced the exchange between K<sub>e</sub> and eddy available potential energy (P<sub>e</sub>). The wind stress forcing additionally affected the conversion processes between P<sub>e</sub> and mean kinetic energy (K<sub>m</sub>). The wind also excited inertial and superinertial motions throughout almost the whole water column. Although those motions played a major role in the conversion between P<sub>e</sub> and K<sub>e</sub>, the net effect by inertial and superinertial flows was almost null. In addition, we found an asymmetric character in kinetic energy conversion in eddies. Cyclonic and anti-cyclonic eddies showed different behaviour regarding conversion from P<sub>e</sub> and K<sub>e</sub>, which was positive on the high K<sub>e</sub> part in the anti-cyclonic eddy but negative in the cyclonic eddy.</p>