Warm Eddy Research Articles

The Roles of Kuroshio Intrusion and Mesoscale Eddy in Upper Mixing in the Northern South China Sea

Yang, Q.; Zhou, L.; Tian, J., and Zhao, W., 2014. The roles of Kuroshio intrusion and mesoscale eddy in upper mixing in the northern South China Sea. Upper mixing in the northern South China Sea (SCS) is derived from Argo profiles from 2006 to 2012, using the Gregg-Henyey-Polzin parameterization. Temporal and spatial mean estimates reveal clear spatial patterns of mixing over the northern SCS. Enhanced diffusivity on an order of 10−3 m2 s−1 exists in the Luzon Strait and adjacent areas. South of 20° N, diffusivity is one order of magnitude smaller on average than it is near the Luzon Strait. Such observations suggest that other than internal tides, both Kuroshio intrusion and mesoscale eddy might be key factors to the enhanced mixing. Shear instability associated with Kuroshio intrusion plays a major role in strengthening the turbulent mixing near the Luzon Strait. In addition, warm eddies could reinforce the downward-propagating near-inertial waves and thus promote the occurrence of strong mixing, whereas cold eddies could reduce this downward-propagating, inhibiting strong mixing. This work facilitates the contributions of mesoscale eddy and Kuroshio intrusion in the northern SCS.

Comparison between environmental characteristics of larval bluefin tuna Thunnus thynnus habitat in the Gulf of Mexico and western Mediterranean Sea

Despite being well adapted for feeding in cold water on their North Atlantic feeding grounds, Atlantic bluefin tuna undertake long migrations to reach warm, low productivity spawn - ing grounds in the Gulf of Mexico and Mediterranean Sea. Environmental conditions within spawning areas have been presumed to benefit larval survival, through appropriate feeding conditions, and enhanced larval retention and growth rates. However, field collections and studies to explore the potential mechanisms are rare. In this study, a comparison of the environmental characteristics of both spawning sites was completed using standardized environmental data and modeling methods. Predictive models of larval occurrence were constructed using historical larval collections, and environmental variables from both in situ and remotely sensed sources. Results showed that larvae on both spawning grounds were most likely to be found in warm (23 to 28°C), low chlorophyll areas with moderate current velocities and favorable regional retention condi- tions. In the Gulf of Mexico, larvae were located in offshore waters outside of the Loop Current and warm eddies, while in the western Mediterranean, larval occurrences were associated with the confluence of inflowing Atlantic waters and saltier resident surface waters. Although our results suggested common themes within preferred spawning grounds on both sides of the Atlantic Ocean, the ecological processes governing larval survival and eventual recruitment are yet to be fully understood.

Evolution and propagation of a mesoscale eddy in the northern South China Sea during winter

In situ observations, satellite data, and the output from an eddy-resolving ocean circulation model were used to study the generations and propagations of an anticyclonic eddy in the northern South China Sea (NSCS) during the winter of 2009-2010. In the NSCS, the anticyclonic eddy firstly appeared to the west of the Luzon Strait, migrated generally along the continental slope and dissipated around the Xisha Archipelago. The evolution of the warm eddy contains three phases: development, maturation, and decay. The eddy mainly stayed near 119.7 degrees E in December and then gradually moved to 118.7 degrees E until January 15, when its intensity, as indicated by the thermocline temperature and salinity anomalies, increased significantly, reflecting the growth of the eddy. The eddy reached its peak on January 15 and persisted until February 23. During this period, the eddy propagated westward to 116.4 degrees E. After, the warm eddy weakened significantly and dissipated finally near the Xisha Archipelago.

Evolution of an anticyclonic eddy southwest of Taiwan

Satellite images of sea-surface temperature, surface chlorophyll a concentration, and sea-level anomaly, together with ocean reanalysis data of Asia and Indian–Pacific Ocean (AIPOcean1.0), are utilized to study the three-dimensional characteristics and evolution of an anticyclonic warm eddy adjacent to the southwest coast of Taiwan during October and November 2006. Originated from the Kuroshio intrusion in the Luzon Strait, but unlike previously found westward moving anticyclonic eddies (AE) in the northeastern South China Sea, this AE was so close to the Taiwan coast and stayed where it was formed for over 1 month until it dissipated. Energy analysis is utilized to study the evolution process of the AE, and it shows that the barotropic instability (BTI) and baroclinic instability introduced by the Kuroshio intrusion flow appear to be the main energy sources for the AE. Periodical enhancement/relaxation of local northeasterly monsoon and its associated negative wind stress curl modify the current patterns in this region, reinforce the intraseasonal variability of the Kuroshio intrusion flow, and act together with Kuroshio to form the AE. Eddies detected from AIPOcean1.0 data also show that AEs are most likely to be generated southwest of Taiwan during the transition period of summer monsoon to winter monsoon, and generally, the BTI of Kuroshio intrusion contributes more than the direct wind stress work to the increase of the eddy kinetic energy for the generation and growth of the AEs.

PAHs on a West-to-East Transect Across the Tropical Atlantic Ocean

Surface water and atmospheric samples were collected across the tropical Atlantic Ocean on a transect of the R/V Endeavor in summer 2009 and analyzed for polycyclic aromatic hydrocarbons (PAHs). Across the entire tropical Atlantic Ocean, phenanthrene displayed on average highest dissolved concentrations (170 pg L(-1)), followed by pyrene (70 pg L(-1)) and fluoranthene (30 pg L(-1)). The Amazon plume was characterized by elevated dissolved concentrations of phenanthrene and benzo(g,h,i)fluoranthene. The warm eddy that we accidentally sampled at 66° W displayed highest concentrations of PAHs across the entire cruise, with phenanthrene, pyrene, and fluoranthrene all >1 ng L(-1). After having crossed the warm core, concentrations decreased back to previous levels. Samples taken in the Gulf Stream were below detection limit for all parent PAHs, implying very efficient removal processes. Dissolved dimethylphenanthrenes were frequently detected in the samples from the southern hemisphere, the Amazon plume, and in samples characteristic of the Gulf Stream and the U.S. East Coast. Atmospheric concentrations were dominated by gas-phase fluoranthene, pyrene, phenanthrene, and retene. Air-water gradients indicated that PAHs are mostly undergoing net deposition across the tropical Atlantic Ocean, with conditions closer to equilibrium off the U.S. East Coast and in Rhode Island Sound.

Typhoon Rammasun-Induced Near-Inertial Oscillations Observed in the Tropical Northwestern Pacific Ocean

Wind-induced near-inertial oscillations (NIOs) have been known to propagate their energy downward and equatorward, yet few observations have confirmed this in tropical regions. Using measurements from a moored ADCP in the tropical northwestern Pacific, we report an energetic NIO event associated with Typhoon Rammasun in May 2008, when an anti-cyclonic warm eddy existed around the mooring site. Our analyses reveal that the anti-cyclonic eddy traps the NIO energy at two layers around 120 and 210 m where the buoyancy frequency show high values. The NIO energy continuously decays at layers below its maximum at 210 m, and disappears at depths below the thermocline. During their propagation from 137 to 649 stretchedmeter depths (equivalent to 100-430 m), NIOs shift their frequencies from 0.92f to 1.05f probably due to the effective f, which changes its magnitude from smaller to larger than local inertial frequency f in the anti-cyclonic eddy. In addition, their vertical energy propagation becomes faster from 0.17 to 0.64 mm s^-1. Decomposition of downward and upward NIO energy propagation shows that the typhoon-induced NIOs remain 29% of their energy in the upper layer, and transfer 71% to the subsurface layers. Our results suggest that typhoon-induced NIOs interacting with meso-scale eddies can play an important role in providing the energy source available for ocean mixing in the tropical regions.

동해에서 저주파 음파전파에 미치는 난수성 소용돌이의 영향

It is well known that sound waves in the sea propagates under the influence of sea surface and bottom roughness, the sound speed profile, the water depth, and the density of sea floor sediment. In particular, an abrupt change of sound speed with depth can greatly affect sound propagation through an eddy. Eddies are frequently generated in the East Sea near the Korean Peninsula. A warm eddy with diameter of about 150 km is often observed, and the sound speed profile is greatly changed within about 400 m of water depth at the center by the eddy around the Ulleung Basin in the East Sea. The characteristics of low-frequency sound propagation across a warm eddy are investigated by a sound propagation model in order to understand the influence of warm eddies. The acoustic rays and propagation losses are calculated by a range-dependent acoustic model in conditions where the eddy is both present and absent. We found that low-frequency sound propagation is affected by the warm eddy, and that the phenomena dominate the upper ocean within 800 m of water depth. The propagation losses of a 100 Hz frequency are variable within <TEX>${\pm}15$</TEX> dB with depth and range by the warm eddy. Such variations are more pronounced at the deep source near the sound channel axis than the shallow source. Furthermore, low-frequency sound propagation from the eddy center to the eddy edge is more affected by the warm eddy than sound propagation from the eddy edge to the eddy center.

Enhanced biological activity by an anticyclonic warm eddy during early spring in the East Sea (Japan Sea) detected by the geostationary ocean color satellite

The high primary production enhanced by anticyclonic eddies and hourly variation pattern in the productivity during the spring season in the East Sea were first investigated using the first Korean Geostationary Ocean Color Imager (GOCI). Even though the stratification for a seasonal spring bloom is not well developed in the water column in early April in the East Sea, a physical upward water flux movement at the periphery of the anticyclonic eddies could remain the phytoplankton in euphotic zone to sustain high chlorophyll-a concentration conditions in the Ulleung Basin. At this time, nutrients were no major controlling factor for phytoplankton growth since concentrations of major nutrients (nitrate, silicate, and phosphate) were relatively high in the observed eddy sites based on the observation data from the Korean Oceanographic Data Center (KODC). The estimated mixed layer depth (MLD) significantly shallower at the periphery supports for this mechanism. The hourly primary productivity estimation based on a Carbon-based Productivity Model (CbPM) provides a bimodal pattern along the time especially in L1 with an approximately one order magnitude difference between the lowest and highest values of productivities on 5 April, 2011. Potential possibilities for this large discrepancy in the hourly productivity and some thoughts on a short time in situ incubation method were discussed.

Western Arctic primary productivity regulated by shelf-break warm eddies

The response of phytoplankton to the Beaufort shelf-break eddies in the western Arctic Ocean is examined using the eddy-resolving coupled sea ice–ocean model including a lower-trophic marine ecosystem formulation. The regional model driven by the reanalysis 2003 atmospheric forcing from March to November captures the major spatial and temporal features of phytoplankton bloom following summertime sea ice retreat in the shallow Chukchi shelf and Barrow Canyon. The shelf-break warm eddies spawned north of the Barrow Canyon initially transport the Chukchi shelf water with high primary productivity toward the Canada Basin interior. In the eddy-developing period, the anti-cyclonic rotational flow along the outer edge of each eddy moving offshore occasionally traps the shelf water. The primary production inside the warm eddies is maintained by internal dynamics in the eddy-maturity period. In particular, the surface central area of an anti-cyclonic eddy acquires adequate light, nutrient, and warm environment for photosynthetic activity partly attributed to turbulent mixing with underlying nutrient-rich water. The simulated biogeochemical properties with the dominance of small-size phytoplankton inside the warm eddies are consistent with the observational findings in the western Arctic Ocean. It is also suggested that the light limitation before autumn sea ice freezing shuts down the primary production in the shelf-break eddies in spite of nutrient recovery. These results indicate that the time lag between the phytoplankton bloom in the shelf region following the summertime sea ice retreat and the eddy generation along the Beaufort shelf break is an important index to determine biological regimes in the Canada Basin.

The Philippines–Taiwan Oscillation: Monsoonlike Interannual Oscillation of the Subtropical–Tropical Western North Pacific Wind System and Its Impact on the Ocean

Tide gauge and satellite data reveal an interannual oscillation of the ocean’s thermoclines east of the Philippines and Taiwan, forced by a corresponding oscillation in the wind stress curl. This so-called Philippines–Taiwan Oscillation (PTO) is shown to control the interannual variability of the circulation of the subtropical and tropical western North Pacific. The PTO shares some characteristics of known Pacific indices, for example, Niño-3.4. However, unlike PTO, these other indices explain only portions of the western North Pacific circulation. The reason is because of the nonlinear nature of the forcing in which mesoscale (ocean) eddies play a crucial role. In years of positive PTO, the thermocline east of the Philippines rises while east of Taiwan it deepens. This results in a northward shift of the North Equatorial Current (NEC), increased vertical shear of the Subtropical Countercurrent (STCC)/NEC system, increased eddy activity dominated by warm eddies in the STCC, increased Kuroshio transport off the northeastern coast of Taiwan into the East China Sea, increased westward inflow through Luzon Strait into the South China Sea, and cyclonic circulation and low sea surface height anomalies in the South China Sea. The reverse applies in years of negative PTO.

Validation of remote sensing and weather model forecasts in the Agulhas ocean area to 57°S by ship observations

The region south of South Africa, encompassing the Agulhas Current and Retroflection, and part of the Southern Ocean, is known for its severe meteorological conditions. Because of these conditions, in-situ observations are rare. Consequently, remote-sensing satellite observations and high-resolution regional weather forecasts at the ocean surface are difficult to assess. However, atmospheric data collected in the southern hemisphere summer of 2008 during the International Polar Year-BONUS-GoodHope campaign were used to validate two satellite data sets: the twice daily QuikSCAT winds and the daily OAflux data set of latent and sensible heat fluxes. The surface winds and heat fluxes forecasts produced by a regional atmospheric model were also assessed along the ship track. In this study, we have shown that the two data sets exhibited a very good accordance with daily in-situ observations. During the campaign, the correlation coefficients for wind speed and direction were 0.97 and 0.91, respectively, and those for latent and sensible heat fluxes were 0.92 and 0.90, respectively. The QuikSCAT wind speed was underestimated by 1.37 m/s relative to in-situ data, south of the Subtropical Front. Large differences in heat fluxes in both OAflux and the atmospheric model were observed when crossing the Subtropical Front and a warm eddy, as well as during a storm, when gale force winds reached more than 20 m/s. The two data sets were then used to assess the regional model forecasts over a larger area south of South Africa, not limited to the ship track. Most of the model errors were located in a region north of the Subtropical Front, where the sea surface temperature used by the model was not accurate enough to reproduce the relevant mesoscale oceanic features driving the spatial variability of the surface winds and heat fluxes. Finally, compared to in-situ and remote sensing observations, the numerical modelling weather forecast produced realistic atmospheric conditions over the sea south of the Subtropical Front.

典型低纬度海区(南海、孟加拉湾)初级生产力比较

PDF HTML阅读 XML下载 导出引用 引用提醒 典型低纬度海区(南海、孟加拉湾)初级生产力比较 DOI: 10.5846/stxb201108221228 作者: 作者单位: 广东省渔业生态环境重点实验室,中国水产科学研究院南海水产研究所;中国科学院南海海洋研究所,中国科学院南海海洋研究所,中国科学院南海海洋研究所,中国科学院南海海洋研究所 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金资助项目(41130855, 40906057);中央级公益性科研院所基本科研业务费(中国水产科学院南海水产研究所)资助项目(2011TS06);农业部南海渔业资源开发利用重点实验室开放基金(LSF2011-05);中国科学院海洋生物资源可持续利用重点实验室开放基金(LMB111010) A comparison study on primary production in typical low-latitude seas (South China Sea and Bay of Bengal) Author: Affiliation: South China Sea Fisheries Institute, China Academy of Fishery Sciences,,, Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:南海和孟加拉湾有着相似的纬度范围,均处于低纬度季风区,但环境的开放性和水体交换特征有所不同。将南海和孟加拉湾的初级生产力进行对比,有助于加深人们对低纬度海区生物生产过程的认识。南海有着复杂的物理过程,存在涡旋、上升流、黑潮、台风和冲淡水等多种现象,显著地影响着初级生产力大小和时空分布。南海初级生产力有以下几个特点:(1)受冲淡水和沿岸上升流的影响,沿岸海域常常高于开阔海区;(2)初级生产力的高值通常不出现在表层,大都出现在次表层;(3)在开阔海区受水体交换(黑潮等)和中尺度现象影响(涡旋)显著。此外,南海初级生产力的季节变化也比较明显,但季度变化规律有较强的区域性。孟加拉湾物理环境与南海差别明显,初级生产力主要受淡水输入、涡旋和光照的影响,受台风和上升流影响不如南海明显。在沿岸区,高温低盐水覆盖了沿岸上层水体,使得混合层较稳定,抑制了深层富含营养盐水体的涌升补充,导致初级生产力下降;光强(悬浮物多、多云天气多)也限制了初级生产力。孟加拉湾开阔海区常常有涡旋形成,也对初级生产力有一定影响。沿岸区初级生产力南海高于孟加拉湾,而在开阔海区两者差别不大,因此整体上南海初级生产力水平高于孟加拉湾。 Abstract:The Bay of Bengal (BOB), the largest bay in the world, forms the northeastern part of the Indian Ocean. Both the BOB and the South China Sea (SCS) were influenced by monsoon seasonally with similar latitude range. Primary productions (PP) of the two seas and their influencing factors were reviewed in this paper. The spatial distributions of PP and their environmental regulation mechanisms showed obvious differences between the two seas. Complicated physical phenomena could be found in the SCS, such as mesoscale eddies, coastal upwelling, Kuroshio, typhoon and river discharges. River discharges (especially those from the Pearl River and the Mekong river) and coastal upwelling (e.g. the Eastern Guangdong upwelling) bring abundant nutrients to the coastal waters in the southwest monsoon period, and result in the stimulation of phytoplankton growth. Mesoscale eddies (cold eddy or warm eddy) have different physical structures from adjacent waters, and can influence the nutrients availability. Kuroshio intrusion (characterized as warm and oligotrophic waters) and typhoon can also regulate PP in the SCS. Various physical processes could potentially control nutrient supply in the euphotic layer, and consequently influence the spatial and temporal variation of PP in the SCS. In the BOB, copious rainfall and river discharge often decreased salinity in the upper layers during summer, and sea surface temperature was warmer than in the SCS, leading to strong stratification in the upper layer. Weak wind over the Bay was unable to erode the strongly stratified surface waters; thereby the wind-driven vertical mixing was restricted within a shallow depth. This inhibits any possible introduction of nutrients from deeper water below the mixed layer into the upper layer. Meanwhile, suspended solids from the discharges and cloudy weather decreased light availability and therefore limited phytoplankton growth. In the near-shore area, river discharge and coastal upwelling can enhance PP in the SCS, while stratification in the BOB can inhibit algal growth in the euphotic zone, so PP in the coastal waters of SCS was higher than that in the BOB. In the open sea area, PP in the SCS is supposed to be close to that in the BOB. As a whole, PP in the SCS is evaluated to be higher than in the BOB. 参考文献 相似文献 引证文献

Roles of SST Anomalies on the Wintertime Turbulent Heat Fluxes in the Kuroshio–Oyashio Confluence Region: Influences of Warm Eddies Detached from the Kuroshio Extension

Abstract Variations of turbulent heat fluxes (sum of sensible and latent heat fluxes) in the North Pacific during 16 winters from December 1992/February 1993 to December 2007/February 2008 are investigated because the months from December to February correspond to the period having peak winter conditions in the atmosphere field. Turbulent heat fluxes are calculated from the bulk formula using daily variables [surface wind speed, surface air specific humidity, surface air temperature, and sea surface temperature (SST)] of the objectively analyzed air–sea flux (OAFlux) dataset and bulk coefficients based on the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) bulk flux algorithm 3.0. The winter turbulent heat fluxes over the Kuroshio–Oyashio Confluence Region (KOCR; 142°–150°E, 35°–40°N) have the largest temporal variances in the North Pacific. The relative contributions among observed variables in SST, surface air temperature, and surface wind speed causing turbulent heat flux variations in the KOCR are assessed quantitatively by performing simple experiments using combinations of two types of variables: raw daily data and daily climatological data. Results show that SST is primarily responsible for the turbulent heat flux variations—a huge amount of heat is released in the state of the positive SST anomaly. Using the datasets of satellite-derived SST and sea surface height with high spatial and temporal resolutions, it is found that the SST anomalies in the KOCR are formed through activities of the anticyclonic (warm) eddies detached northward from the Kuroshio Extension; SSTs take positive (negative) anomalies when more (less) anticyclonic eddies are distributed there, associated with a more convoluted (straight) Kuroshio Extension path.

Extraction of spatial-temporal rules from mesoscale eddies in the South China Sea based on rough set theory

Abstract. In this paper, a rough set theory is introduced to represent spatial-temporal relationships and extract the corresponding rules from typical mesoscale-eddy states in the South China Sea (SCS). Three decision attributes are adopted in this study, which make the approach flexible in retrieving spatial-temporal rules with different features. The results demonstrate that this approach is effective, and therefore provides a powerful approach to forecasts in the future studies. Spatial-temporal rules in the SCS indicate that warm eddies following the rules are generally in the southeastern and central SCS around 2000 m isobaths in winter. Their intensity and vorticity are weaker than those of cold eddies. They usually move a shorter distance. By contrast, cold eddies are in 2000 m and deeper regions of the southwestern and northeastern SCS in spring and fall. Their intensity and vorticity are strong. Usually they move a long distance. In winter, a few rules are followed by cold eddies in the northern tip of the basin and southwest of Taiwan Island rather than warm eddies, indicating cold eddies may be well-regulated in the region. Several warm-eddy rules are achieved west of Luzon Island, indicating warm eddies may be well-regulated in the region as well. Otherwise, warm and cold eddies are distributed not only in the jet flow off southern Vietnam induced by intraseasonal wind stress in summer-fall, but also in the northern shallow water, which should be a focus of a future study.

Influence of mesoscale eddies on sound propagation in the northwest Pacific Ocean

The influence of a warm anticyclonic eddy occurring in the northwestern part of the Pacific Ocean near the Kuroshio flow on the sound field structure under radiation of acoustic signals with the frequencies of 232, 348, and 696 Hz is studied. The results of field measurements are compared to the acoustic field calculations in parabolic approximation and by the normal mode method. Calculations are in good compliance with experimental data, which makes it possible to estimate the influence of the eddy on transmission losses and the interference structure of the sound field.

Impact of an anticyclonic eddy on the summer nutrient and chlorophyll a distributions in the Ulleung Basin, East Sea (Japan Sea)

Abstract Kim, D., Yang, E. J., Kim, K. H., Shin, C-W., Park, J., Yoo, S., and Hyun, J-H. 2012. Impact of an anticyclonic eddy on the summer nutrient and chlorophyll a distributions in the Ulleung Basin, East Sea (Japan Sea). – ICES Journal of Marine Science, 69: 23–29. The impact of the anticyclonic Ulleung Warm Eddy (UWE) on the vertical distributions of nutrient and chlorophyll a (Chl a) concentrations in the Ulleung Basin (UB) was investigated during the contrasting summers of 2005 and 2007. The physical structure of the water column was characterized by an intrathermocline eddy (ITE) in 2005, whereas the UWE remained distant from the sampling transect in 2007. Water column structures appeared to be highly stratified, and nutrients in the surface waters were totally depleted at all stations. In 2005, an exceptionally high concentration of Chl a (5.5 mg m−3) was measured below the surface mixed layer in the eddy core (station D3), and values of ∼2.5 mg m–3 were observed at the eddy edge (stations D2 and D4). Formation of an ITE efficiently mixed surface and deep-ocean waters, the latter supplying sufficient nutrients to generate an extremely high concentration of Chl a at the base of the subsurface layer. Overall, the results indicated that the anticyclonic UWE plays a key ecological role in supporting substantial phytoplankton biomass in the nutrient-depleted surface waters in summer and maintaining high benthic mineralization in the deep-sea sediments of the UB.

Beaufort shelf break eddies and shelf-basin exchange of Pacific summer water in the western Arctic Ocean detected by satellite and modeling analyses

[1] Mesoscale eddies and shelf-basin exchange of Pacific summer water in the western Arctic Ocean are examined using satellite data sets and an eddy-resolving coupled sea ice–ocean model. Several surface eddy-like features along the Beaufort shelf break are detected by the Moderate-Resolution Imaging Spectroradiometer (MODIS) sea surface temperature and the Global Imager (GLI) radiance image. The QuikSCAT sea wind vectors indicate that summertime shelf-wide wind is an important factor for interannual variations in the eddy properties. A realistic numerical experiment reveals that the origin of shelf break warm eddies and timing of their generation can be classified into three types. Type I eddies are frequently produced in the vicinity of the Barrow Canyon throughout the summer season by a combination of outflow of Pacific summer water with a low potential vorticity from the Barrow Canyon and lateral velocity shear of the canyon jet. Surface wind variability modulates the timing of generation. Type II and Type III eddies originate from an eastward current along the Beaufort shelf break, although origin depth and background hydrographic structure differ between them. Type II eddies are spawned from the shelf slope at middepth during early summer. Type III eddies are generated from the surface isopycnal front over the shelf break during late summer. The shift from Type II eddies to Type III eddies is caused by surface buoyancy input in the upstream region. A sensitivity experiment using different atmospheric forcing data suggest that the mechanism controlling Pacific water transport from the Chukchi shelf to the Canada Basin differs depending on surface wind fields in the shelf region. The mesoscale eddies primarily induce the shelf-to-basin transport under weak or westerly wind conditions during summer, while wind-driven Ekman transport is a major driver in the easterly wind regime. Year 2003 (2007) corresponds to the former (latter) case.

Observed three-dimensional structure of a cold eddy in the southwestern South China Sea

[1] The dynamic structure of an ocean eddy in the eddy-abundant South China Sea has rarely been captured by measurements and has seldom been discussed in the literature. In the present study, in situ current and hydrographic measurements from a weeklong cruise and concurrent satellite altimeter observations were utilized to examine the three-dimensional structure and physical properties of a cold eddy in the southwestern South China Sea. The underlying forcing mechanism for the formation of this cyclonic cold eddy was found to be tightly associated with the recirculation in a coastal baroclinic jet that had separated off the Vietnamese coast. The eddy was significantly influenced by a coexisting, anticyclonic warm eddy in the separated jet. With relatively steady intensity and radius, the cold eddy endured for two weeks after its swift formation in late August and prior to its quick dissipation in mid-September. This cold eddy was horizontally and vertically heterogeneous. Asymmetric currents with much stronger magnitude were found on its southeastern flank, next to the warm eddy, where a front in the pycnocline was responsible for the sharp decrease in the cold eddy's intensity in the water below. The distributions of temperature, vorticity, and vertical velocity in the cold eddy were spatially asymmetric and not overlapping. The intensity of the cold eddy gradually decreased with the depth and the eddy extended downward for more than 250 m with a vertically tilted central axis. The upward velocities around the center of the eddy and the downward velocities to the southwest and to the east of the center jointly formed the upward domes of isotherms and isohalines in the central part of the cold eddy.

The Impact of a Warm Ocean Eddy on Typhoon Morakot (2009): A Preliminary Study from Satellite Observations and Numerical Modelling

On 6 August 2009, typhoon Morakot encountered a giant warm ocean eddy approximately 700 km by 500 km in the southern eddy rich zone of the western North Pacific Ocean. Soon after passing over the warm ocean eddy, Morakot reached its peak intensity at category 2. Results based on multiple satellite observations and numerical modelling suggest very favourable ocean conditions provided by the warm ocean eddy during this earlier developmental stage of Morakot. It is found that in the presence of the observed warm ocean eddy, the upper ocean heat content increased significantly by ~100%, from ~60 to 120 KJ cm-2. This very deep and warm subsurface temperature effectively reduced the negative feedback of typhoon-induced ocean cooling. As a result, the during-storm sea surface temperature remained high at ~29 - 30°C. This very warm duringtyphoon sea surface temperature (SST) provided an increase in air-sea enthalpy flux supply by ~200% (i.e., ~500 W m(superscript -2) under the warm eddy situation .vs. the ~170 W m(superscript -2) under the without eddy situation). Also, since the during-typhoon SST remained high, the moisture supply was increased to enhance convective activities. Numerical experiments using the Weather Research and Forecasting (WRF) model suggest that the presence of the warm ocean eddy does not change the overall structure orcharacteristics of Morakot. Rather, it contributes to a ~10% increase in Morakot’s precipitation. This research shows that in addition to the favourable atmospheric conditions such as the Intra-Seasonal Oscillation or the southwestern monsoon flow, there also exist favourable ocean conditions provided by the presence of a warm ocean eddy during the early developmental stage of Morakot. Further studies based on a full-physics typhoon-ocean coupled model are needed to quantify the role of the upper ocean features in affecting the evolution of Morakot, including its rainfall over Taiwan.

Mooring observations and numerical modeling of thermal structures in the South China Sea

Three sets of Autonomous Temperature Line Acquisition Systems were deployed in the South China Sea. Gaps aside, the data covered nearly 3 years at the northern station and about 2 years farther south. Fluctuations ranged from episodic to interannual. Internal tides, more diurnal than semidiurnal, were active basinwide. Twelve typhoons passed during measurement periods. The most severe one, typhoon Babs in 1998, caused a temperature drop of over 7°C at 50 m. Despite strong monsoons, only near‐surface temperature showed clear seasonal variations. Intraseasonal variations induced by mesoscale eddy stood out much better at subsurface depths. Propagating eddies aside, some eddies were seasonal and nearly stationary. From daily archives of an eddy‐resolving, data‐assimilating ocean model (East Asian Seas Nowcast/Forecast System), we identified two paradigms leading to the generation of a persistent spring‐summer warm eddy in the central‐western basin. In normal years, a complete cyclonic gyre was driven by a strong winter northeast monsoon. Water piled up along the periphery of the South China Sea. Afterward, a warm eddy could be generated from west of Luzon Island and propagated westward while intensifying. Under a weak northeast monsoon, such as in El Niño years, piled‐up water tended to stay in the southern basin. When the wind relaxed in spring, warm water returned northward to form a warm eddy in the central‐western basin. Transition from SW to NE monsoon also often led to a warm eddy generation in southern latitudes, when the summer eastward jet departing from central Vietnam broke up.