Features of eddy kinetic energy and variations of upper circulation in the South China Sea

The features of eddy kinetic energy (EKE) and the variations of upper circulation in theSouth China Sea (SCS) are discussed in this paper using geostrophic currents estimated from Maps of Sea Level Anomalies of the TOPEX/Poseidon altimetry data. A high EKE center is identified in the southeast of Vietnam coast with the highest energy level 1 400 cm2 ·s^(-2) in both summer and autumn. This high EKE center is caused by the instability of the current axis leaving the coast of Vietnam in summer and the transition of seasonal circulation patterns in autumn. There exists another high EKE region in the northeastern SCS, southwest to Taiwan Island in winter. This high EKE region is generated from the eddy activities caused by the Kuroshio intrusion and accumulates more than one third of the annual EKE, which confirms that the eddies are most active in winter. The transition of upper circulation patterns is also evidenced by the directions of the major axises of velocity variance ellipses between 10?and 14.5°N, which supports the model results reported before.

Seasonal variation of eddy kinetic energy in the South China Sea

Mesoscale eddy activity and its modulation mechanism in the South China Sea （SCS） are inves- tigated with newly reprocessed satellite altimetry observations and hydrographic data. The eddy kinetic energy （EKE） level of basin-wide averages show a distinct seasonal cycle with the maximum in August-December and the minimum in February-May. Furthermore, the seasonal pattern of EKE in the basin is dominated by region offshore of central Vietnam （OCV）, southwest of Taiwan Island （SWT）, and southwest of Luzon （SWL）, which are also the breeding grounds of mesoscale eddies in the SCS. Instability theory analysis suggests that the seasonal cycle of EKE is modulated by the baroclinic instability of the mean flow. High eddy growth rate （EGR） is found in the active eddy regions. Vertical velocity shear in the upper 50-500 m is crucial for the growth of baroclinic instability, leading to seasonal EKE evolution in the SCS.

Eddy Kinetic Energy Study of the Snowstorm over Southern China in January 2008

The energetics of the third stage of a snowstorm over China was analyzed using ECWMF data.The analysis of the energy budget for the Middle East trough and the western Pacific trough that developed toward China on 25-28 January 2008 showed the advection of the geopotential by the ageostrophic wind to be both a crucial source and the primary sink of the eddy kinetic energy centers associated with the troughs.The magnitudes of the energy conversion terms,interaction kinetic energy conversion and baroclinic conversion,were too small to explain the development of the energy centers and the jet streaks.The energy centers gained energy at their entrance regions via the convergence of the ageostrophic geopotential fluxes,and then lost energy at their exit regions by the same fluxes.At the entrance regions,the fluxes converged,increasing the geopotential gradient,which generated a stronger geostrophic wind and higher kinetic energy,resulting in an ascending motion in this area.When the troughs moved to China,the ascending motion caused by the convergence of the fluxes at entrance region intensified the snowstorms over central and southern China.

The variability of eddy ki/netic energy in the South China Sea deduced from satellite altimeter data

We used fifteen years （1993-2007） of altimetric data, combined from different missions （ERS-1/2, TOPEX/Poseidon, Jason-l, and Envisat）, to analyze the variability of the eddy kinetic energy （EKE） in the South China Sea （SCS）. We found that the EKE ranged from 64 cm2/s2 to 1 390 cm2/s2 with a mean value of 314 cm2/s2. The highest EKE center was observed to the east of Vietnam （with a mean value of 509 cm2/s2） and the second highest EKE region Was located to the southwest of Taiwan Island （with a mean value of 319 cm2/s2）. We also found that the EKE structure is the consequence of the superposition of different variability components. First, interannual variability is important in the SCS. Spectral analysis of the EKE interannual signal （IA-EKE） shows that the main periodicities of the IA-EKE to the east of Vietuam, to the southwest of Taiwan Island, and in the SCS are 3.75, 1.87, and 3.75 years, respectively. It is to the south of Taiwan Island that the IA-EKE signal has the most obvious impact on EKE variability. In addition, the IA-EKE exhibit different trends in different regions. An obvious positive trend is observed along the east coast of Vietnam, while a negative trend is found to the southwest of Taiwan Island and in the east basin of Vietnam. Correlation analysis shows that the IA-EKE has an obvious negative correlation with the SSTA in Nifio3 （5°S-5°N, 90°W-150°W）. E1 Nifio-Southem Oscillation （ENSO） affects the 1A-EKE variability in the SCS through an atmospheric bridge--wind stress curl over the SCS. Second, the seasonal cycle is the most obvious timescale affecting EKE variability. The locations of the most remarkable EKE seasonal variabilities in the SCS are to the east of Vietnam, to the southwest of Taiwan, and to the west of Philippines. To the east of Vietnam, the seasonal cycle is the dominant mechanism controlling EKE variability, which is attributed primarily to the annual cycle there of wind stress curl. In this area, the maximum EKE is observed in autumn. To the southwest of Taiwan Island, the EKE is enlarged by the stronger SCS circulation, which is caused by the intrusion branch from the Kuroshio in winter. Finally, intra-annual and mesoscale variability, although less important than the former, cannot be neglected. The most obvious intra-annual and mesoscale variability, which may be the result of baroclinic instability of the background flow, are observed to the southwest of Taiwan Island. Sporadic events can have an important effect on EKE variability.

Interannual variations in energy conversion and interaction between the mesoscale eddy field and mean flow in the Kuroshio south of Japan

Using 19-year satellite altimetric data, variations in the eddy kinetic energy, energy exchanges and interaction between the eddy fi eld and mean fl ow are discussed for the Kuroshio south of Japan. In the seasonal cycle, the eddy kinetic energy level is a minimum in December/January and a maximum in April/May. In addition to seasonal variations, the eddy kinetic energy undergoes interannual changes. The energy transfers mainly from the mean fl ow to the eddy fi eld in the Kuroshio south of Japan, and dominant energy exchanges mainly occur along the Kuroshio path south of Japan in each year from 1993 to 2011. In addition, there is often barotropic instability south of Honshu. Regarding interactions between the eddy fi eld and mean fl ow, cyclonic and anticyclonic accelerations are also found along the Kuroshio path and they fl ank each other. There is cyclonic acceleration always imposed on southeast of Kyushu, and anticyclonic acceleration dominates south of Honshu from 2001 to mid-2005. Reynolds stress is used to explain the dynamic process of energy exchange. Furthermore, lag-correlation and linear regression analysis show that variability of the energy conversion rate and Reynolds stress involve responses to eddy acceleration at two time scales. The enhanced eddy acceleration induces large Reynolds stress, and enhanced Reynolds stress or barotropic instability further enforces energy transfer from the mean fl ow to the eddy fi eld.

Eddy properties in the Pacific sector of the Southern Ocean from satellite altimetry data

Mesoscale eddies play a key role in the ocean dynamics of the Southern Ocean, and eddy response to the climate changes has also been widely noted. Both eddy kinetic energy（EKE） and eddy detection algorithm are used to study the eddy properties in the Pacific sector of the Southern Ocean. Consistent with previous works,the maps of the EKE illustrate that higher energy confines to the Antarctic Polar Frontal Zone（APFZ） and decreases progressively from west to east. It also shows that the most significant increase in the EKE occurs in the western and central parts of the Pacific sector, where the baroclinicity of the Antarctic Circumpolar Current（ACC） is much stronger. Statistical eddy properties reveal that both of the spatial pattern and interannual variation of the EKE are primarily due to the eddy amplitude and the eddy rotational speed, rather than the eddy number or the eddy radius. In general, these results furtherly confirm that anomalous westerly wind forcing associated with the positive Southern Annular Mode（SAM） index enhances the Southern Ocean eddy activity by strengthening the eddy properties.

Surface available gravitational potential energy in the world oceans

Satellite altimetry observations,including the upcoming Surface Water and Ocean Topography mission,provide snapshots of the global sea surface high anomaly field.The common practice in analyzing these surface elevation data is to convert them into surface velocity based on the geostrophic approximation.With increasing horizontal resolution in satellite observations,sea surface elevation data will contain many dynamical signals other than the geostrophic velocity.A new physical quantity,the available surface potential energy,is conceptually introduced in this study defined as the density multiplied by half of the squared deviation from the local mean reference surface elevation.This gravitational potential energy is an intrinsic property of the sea surface height field and it is an important component of ocean circulation energetics,especially near the sea surface.In connection with other energetic terms,this new variable may help us better understand the dynamics of oceanic circulation,in particular the processes in connection with the free surface data collected through satellite altimetry.The preliminary application of this concept to the numerically generated monthly mean Global Ocean Data Assimilation System data and Archiving,Validation,and Interpretation of Satellite Oceanographic altimeter data shows that the available surface potential energy is potentially linked to other dynamic variables,such as the total kinetic energy,eddy kinetic energy and available potential energy.

Double-averaging analysis of turbulent kinetic energy fluxes and budget based on large-eddy simulation

The turbulent flow over a channel bed roughened by three layers of closely packed spheres with a Reynolds number of Re= 15 000 is investigated using the large eddy simulation（LES） and the double-averaging（DA） method. The DA velocity is compared with the results of the corresponding laboratory experiments to validate the LES results. The existence of the types of vortex structures is demonstrated by the Q-criterion above the permeable bed. The turbulent kinetic energy（TKE） fluxes and budget are quantified and discussed. The results show that the TKE fluxes are directed downward and downstream near the virtual bed level. In the TKE budget, the form-induced diffusion rate is significant in the vicinity of the crest bed level, and the TKE production rate and the dissipation rate attain their peaks at the crest bed level and decrease sharply below it.

Variability of the Kuroshio extension system in 1992-2013 from satellite altimetry data

The Kuroshio Extension （KE） plays an important role in climate and environmental change in the North Pacific. In this paper, more than 20 years of merged absolute dynamic topography and merged sea level anomaly products from satellite altimetry are used to analyze the stability of the KE system. By analyzing the annually averaged sea surface topography, the variations of inter-annual path and annually averaged eddy kinetic energy at the KE region, the KE＇s two dynamic states are given as： the relatively stable state during 1993 1995, 2002-2005, and 2010-2012, and the unstable dynamic state among 1996-2001 and 2006-2009. During the stable state, the KE spindle had a shorter path length and smaller time-varying amplitude, as well as a trend to move northward. While during the unstable state, the KE spindle had a longer path length and an integral southward transport trend, and was observed to oscillate significantly over time. The analysis on the KE＇s upstream and downstream region gives the same variations, indi- cating that they are significantly affected by the El Nino events. The power spectrum of the mean latitudinal position variation of the KE＇s upstream and downstream shows significant quasi-decadal oscillation characteristics and strong annual signals. Furthermore, the correlation of the strength vari- ation between the southern RG and the KE＇s upstream is calculated to be 0.50 after low-pass filtering, and that of the mean latitudinal position variation between the southern RG and the KE＇s upstream/ downstream are 0.75/0.69 after low-pass filtering, respectively. The strong correlations demonstrated that the southern RG and the KE are closely linked.

Mesoscale surface circulation and variability of Southern Indian Ocean derived by combining satellite altimetry and drifter observations

High resoultion Eulerian mean velocity field has been derived by combining the satellite tracked surface drifter data with satellite altimetry and ocean surface winds. The drifter data used in this study includes Argos and surface drifter data from Global Drifter Program. Maps of Sea Level Anomaly （MSLA） weekly files with a resolution of （1/3）° in both Latitude and Longitude for the period 1993-2012 have been used. The Ekman current is computed using ocean surface mean wind fields from scatterometers onboard ERS 1/2, Quikscat and ASCAT. The derived mean velocity field exhibits the broad flow of Antarctic Circumpolar Current with speeds up to 0.6 m/s. Anomalous field is quite significant in the western part between 20~ and 40~E and in the eastern part between 80~E and 100~E with velocity anomaly up to 0.3 m/s. The estimated mean flow pattern well agrees with the dynamic topography derived from in-situ observations. Also, the derived velocity field is consistent with the in-situ ADCP current measurements. Eddy kinetic energy illustrates an increasing trend during 1993-2008 and is in phase coherence with the Southern Annular Mode by three month lag. Periodic modulations are found in the eddy kinetic energy due the low frequency Antarctic Circumpolar Wave propagation.

Regional Characteristics of Typhoon-Induced Ocean Eddies in the East China Sea

The asymmetrical structure of typhoon-induced ocean eddies（TIOEs） in the East China Sea（including the Yellow Sea）and the accompanying air–sea interaction are studied using reanalysis products. Thirteen TIOEs are analyzed and divided into three groups with the k-prototype method： Group A with typhoons passing through the central Yellow Sea; Group B with typhoons re-entering the sea from the western Yellow Sea after landing on continental China; and Group C with typhoons occurring across the eastern Yellow Sea near to the Korean Peninsula. The study region is divided into three zones（Zones Ⅰ, Ⅱ and Ⅲ） according to water depth and the Kuroshio position. The TIOEs in Group A are the strongest and could reverse part of the Kuroshio stream, while TIOEs in the other two groups are easily deformed by topography. The strong currents of the TIOEs impact on the latent heat flux distribution and upward transport, which facilitates the typhoon development. The strong divergence within the TIOEs favors an upwelling-induced cooling. A typical TIOE analysis shows that the intensity of the upwelling of TIOEs is proportional to the water depth, but its magnitude is weaker than the upwelling induced by the topography. In Zones Ⅰ and Ⅱ, the vertical dimensions of TIOEs and their strong currents are much less than the water depths.In shallow water Zone Ⅲ, a reversed circulation appears in the lower layer. The strong currents can lead to a greater, faster,and deeper energy transfer downwards than at the center of TIOEs.