Enhanced chlorophyll associated with island-induced cyclonic eddies in the eastern channel of the Tsushima Straits

The relationship between island-induced cyclonic eddies and chlorophyll a (chl-a) was investigated using field data and satellite images in the eastern channel of the Tsushima Straits. The maximum chl-a concentration around the leeward eddy of the Tsushima Islands was two or three times greater than that of outside the eddy. Two different mechanisms of chl-a enhancement associated with island-induced cyclonic eddies were found in the post-bloom periods. In summer, when nutrients were depleted in the surface layer, eddy pumping increased the nutrient supply in the euphotic zone, resulting in enhanced chl-a around the shallow thermocline near the eddy core. In late autumn, when the mixed layer deepened over the euphotic zone, the mixed layer depth became shallow due to the doming effect of the cyclonic eddy, therefore, the improved irradiance condition led to an increase in the chl-a concentration in the surface mixed layer. Nighttime satellite visible images showed a number of fishing vessels in the lee region of the Tsushima Islands, implying that the enhanced phytoplankton biomass may have resulted in good feeding conditions for fishes and squids in the Tsushima Straits.     

Nutrient supply to anticyclonic meso-scale eddies off western Australia estimated with artificial tracers released in a circulation model

The phytoplankton distribution off western Australia in the period from April to October is unique in that high biomass is generally associated with anticyclonic eddies and not with cyclonic eddies. As the western Australian region is oligotrophic this anomalous feature must be related to differing nutrient supply pathways to the surface mixed layer of cyclonic and anticyclonic eddies. A suite of modelled abiotic tracers suggests that cyclonic eddies are predominantly supplied by diapycnal processes that remain relatively weak until June–July, when they rapidly increase because of deepening surface mixed layers, which start to tap into the nutrient-replete waters below the euphotic zone. To the contrary, we find that anticyclonic eddies are predominantly supplied by injection of shelf waters, which carry elevated levels of inorganic nutrients and biomass. These injections start with the formation of the eddies in April–May, continue well into the austral winter and reach as far as several hundred kilometers offshore. The diapycnal supply of nutrients is suppressed in anticyclonic eddies since the injection of warm, low-salinity shelf waters delays the erosion of the density gradient at the base of the mixed layer. Our results are consistent with the observed seasonal cycles of chlorophyll a and observation of particulate organic matter export out of the surface mixed layer of an anticyclonic eddy in the region.     

Mesoscale eddy effects on temporal variability of surface chlorophyll a in the Kuroshio Extension

We investigated the relationship between chlorophyll a (Chl-a) concentrations estimated from satellite observations and the activity of eddies in the Kuroshio Extension region. High (low) area-averaged Chl-a concentrations were frequently observed in the core of cyclonic (anticyclonic) eddies. Such relationships between Chl-a concentrations and eddy cores were not frequently observed in the southern part of the recirculation gyre, and advection of background meridional gradient of Chl-a by eddy-edge currents accounted for Chl-a spatial variability. Decadal-scale changes of Chl-a concentrations around the Kuroshio Extension were strongly affected by eddy activity and transport but not by large-scale near-surface isopycnal heaving. We also found that decadal changes of nutrient concentrations near the main stream could affect Chl-a concentrations in the southern part of the recirculation gyre via southward transport of eddies and mean flow.     

The role of meso-scale eddies in mixed layer deepening and mode water formation in the western North Pacific

Distributions of mixed layer depths around the centers of anti-cyclonic and cyclonic eddies in the North Pacific Ocean were composited by using satellite-derived sea surface height anomaly data and Argo profiling float data. The composite distributions showed that in late winter, deeper mixed layers were more (less) frequently observed inside the cores of the anti-cyclonic (cyclonic) eddies than outside. This relationship was the clearest in the region of 140°E–160°W and 35°N–40°N, where the temperature and salinity of the deep mixed layers were similar to those of the lighter variety of central mode water (L-CMW). A simple one-dimensional bulk mixed layer model showed that both strong sea-surface heat and momentum fluxes and weak preexisting stratification contributed to formation of the deep mixed layer. These conditions were associated with the anti-cyclonic eddies, suggesting that these eddies are important in the formation of mode waters, particularly L-CMW.     

Circulation patterns in the Delagoa Bight,Mozambique, and the influence of deep ocean eddies

An investigation of the circulation patterns and thermohaline structures in the Delagoa Bight, Mozambique, was undertaken during May 2004, August 2004, April 2005, and April 2006, using hydrographic surveys, surface drifters and satellite imagery. Hydrographic and satellite data during May 2004 illustrated a cyclonic eddy centred at 26° S, 34.25° E in the Bight. A surface drifter remained trapped in this eddy for six weeks between 8 May and 20 June 2004 before moving southward in the Agulhas Current. During August 2004, the core of a cyclonic eddy was located south of the Bight, while no cyclonic eddy was observed during April 2005 or in April 2006. The Delagoa Bight eddy appeared to be more transient than previously thought. Important observations were the recurrent northward current (25–30 cm s^?1) occurring subsurface on the shelf, and the prominence of cooler upwelled water at various locations due to the interaction of passing eddies with the bottom topography of the Bight.     

Evidence for current-controlled sedimentation along the southern Mozambique continental margin since Early Miocene times

Major plastered drift sequences were imaged using high-resolution multichannel seismics during R/V Meteor cruises M63/1 and M75/3 south of the Mozambique Channel along the continental margin of Mozambique off the Limpopo River. Detailed seismic-stratigraphic analyses enabled the reconstruction of the onset and development of the modern, discontinuous, eddy-dominated Mozambique Current. Major drift sequences can first be identified during the Early Miocene. Consistent with earlier findings, a progressive northward shift of the depocenter indicates that, on a geological timescale, a steady but variable Mozambique Current existed from this time onward. It can furthermore be shown that, during the Early/Middle Miocene, a coast-parallel current was established off the Limpopo River as part of a lee eddy system driven by the Mozambique Current. Modern sedimentation is controlled by the interplay between slope morphology and the lee eddy system, resulting in upwelling of Antarctic Intermediate Water. Drift accumulations at larger depths are related to the reworking of sediment by deep-reaching eddies that migrate southward, forming the Mozambique Current and eventually merging with the Agulhas Current.     

New seismic data in the Mozambique Channel

This paper presents seismic reflection and refraction data from the Mozambique Channel, collected between 1971 and 1973. A deep sedimentary basin (up to 5 km of sediments) opens southwards to the Mozambique Basin, and is bounded to the east by the Davie Ridge and beyond by the marginal plateau of Malagasy. A continuous reflector (C), possibly of Cretaceous age, is identified between layers having seismic interval velocities of 2.4–2.8 km/s and 3.1–3.4 km/s. The deepest sediments have velocities of 4.5–4.9 km/s and overlie a layer with velocity 5.5 km/s, which may be volcanic in the north-east of the Channel.The crust occupying most of the Channel is probably pre-Cretaceous in age, and may be largely continental in nature. This is supported by subdued magnetic anomalies and the possibility of a continuous Karroo sedimentary section across the northern Channel. The oceanic crust of the Mozambique Basin may extend as far north as 24°S, into the western Channel only. The problem of the origin of the Mozambique Channel remains unresolved, although a long sedimentary history indicates that Malagasy may have separated from Mainland Africa prior to Karroo times. The Davie Ridge may possibly represent a relict strike-slip fault, which permitted movement along a north-south line.     

Effects of deep-sea eddies on the northern KwaZulu-Natal shelf,South Africa

This paper describes the westward movement of a cyclonic eddy across the Mozambique Channel and the subsequent south-westward propagation of the eddy along the east coast of South Africa and its interaction with the shelf. A hydrographic survey on 13 September 2006 off Nine-mile Reef (NMR), Sodwana Bay, showed a well-developed Agulhas Current along the continental shelf inshore of a cyclonic eddy flanked by two anti-cyclonic eddies, further offshore. A satellite-tracked drifter and complementary altimetry data confirmed the dimensions of the eddy and tracked its movement towards the coast. Shelf-edge upwelling was measured at NMR by an underwater temperature recorder (UTR) when the cyclonic eddy first came into contact with the shelf and again when the cyclonic eddy interacted with the leading edge of the anti-cyclonic eddy moving onto the shelf. Further shelf–eddy interactions off Aliwal Shoal, south of Durban, and consequent upwelling were similarly caused by the same cyclonic eddy as it progressed south-westward along the east coast. Analysis of UTR data between 2004 and 2006 indicated that between two and five cyclonic eddies impact the shelf off NMR per year.     

A multi-model study of the restratification phase in an idealized convection basin

The representation of baroclinic instability in numerical models depends strongly upon the model physics and significant differences may be found depending on the vertical discretization of the governing dynamical equations. This dependency is explored in the context of the restratification of an idealized convective basin with no external forcing. A comparison is made between an isopycnic model including a mixed layer (the Miami Isopycnic Coordinate Ocean Model, MICOM), its adiabatic version (MICOM-ADIAB) in which the mixed layer physics are removed and the convective layer is described by a deep adiabatic layer outcropping at the surface instead of a thick dense mixed layer, and a z-coordinate model (OPA model).In the absence of a buoyancy source at the surface, the mixed layer geometry in MICOM prevents almost any retreat of this layer. As a result, lateral heat exchanges in the upper layers are limited while mass transfers across the outer boundary of the deep convective mixed layer result in an unrealistic outward spreading of this layer. Such a widespread deep mixed layer maintains a low level of baroclinic instability, and therefore limits lateral heat exchanges in the upper layers over most of the model domain. The behavior of the adiabatic isopycnic model and z-coordinate model is by far more satisfactory although contrasted features can be observed between the two simulations. In MICOM-ADIAB, the more baroclinic dynamics introduce a stronger contrast between the surface and the dense waters in the eddy kinetic energy and heat flux distributions. Better preservation of the density contrasts around the dense water patch maintains more persistent baroclinic instability, essentially associated with the process of dense water spreading. The OPA simulation shows a faster growth of the eddy kinetic energy in the early stages of the restratification which is attributed to more efficient baroclinic instability and leads to the most rapid buoyancy restoring in the convective area among the three simulations. Dense water spreading and warm surface capping occur on fairly similar time scales in MICOM-ADIAB although the former is more persistent that the latter. In this model, heat is mainly transported by anticyclonic eddies in the dense layer while both cyclonic and anticyclonic eddies are involved in the upper layers. In OPA, heat is mainly brought into the convective zone through the export of cold water trapped in cyclonic eddies with a strong barotropic structure. Probably the most interesting difference between the z-coordinate and the adiabatic isopycnic model is found in the temperature distribution ultimately produced by the restratification process. OPA generates a spurious volume of intermediate water which is not seen in MICOM-ADIAB where the volume of the dense water is preserved.     

Hawaii Cyclonic Eddies and Blue Marlin Catches: The Case Study of the 1995 Hawaiian International Billfish Tournament

The combination of prevailing northeasterly tradewinds and island topography results in the formation of vigorous, westward propagating cyclonic eddies in the lee of the Hawaiian Islands on time scales of 50–70 days. These mesoscale (∼10² km) features are nowhere more conspicuous or spin up more frequently than in the Alenuihaha Channel between the Island of Maui and the Big Island of Hawaii. Cyclonic eddies in subtropical waters such as those around Hawaii vertically displace the underlying nutricline into the overlying, nutrient-depleted euphotic zone creating localized biologically enhanced patches. Insight into how these eddies may directly influence pelagic fish distribution is provided by examination of recreational fish catch data coinciding with the presence of eddies on the fishing grounds. We highlight the 1995 Hawaii International Billfish Tournament in which a cyclonic eddy dominated the ocean conditions during the weeklong event and the fish catch distribution differed significantly from the average historical tournament catch patterns. On the tournament fishing grounds, well-mixed surface layers and strong current flows induced by the eddy's presence characterized the inshore waters where the highest catches of the prized Pacific blue marlin (Makaira mazara) occurred, suggesting possible direct (e.g., physiological limitations) or indirect (e.g., prey availability) biological responses of blue marlin to the prevailing environment. This revised version was published online in August 2006 with corrections to the Cover Date.     

A Numerical Modeling of the Upper and the Intermediate Layer Circulation in the East Sea

Circulation in the upper and the intermediate layer of the East Sea is investigated by using a fine resolution, ocean general circulation model. Proper separation of the East Korean Warm Current from the coast is achieved by adopting the isopycnal mixing, and using the observed heat flux (Hirose et al., 1996) and the realistic wind stress (Na et al., 1992). The simulated surface circulation exhibits a remarkable seasonal variation in the flow patterns of the Nearshore Branch, the East Korean Warm Current and the Cold Currents. East of the Oki Bank, the Nearshore Branch follows the isobath of shelf topography from late winter to spring, while in summer and autumn it meanders offshore. The Nearshore Branch is accompanied by cyclonic and anticyclonic eddies in a fully developed meandering phase. The meandering and the eddy formation of the Nearshore Branch control the interior circulation in the Tsushima Current area. A recirculation gyre is developed in the region of the East Korean Warm Current in spring and grown up to an Ulleung Basin scale in summer. A subsurface water is mixed with the fresh surface water by winter convection in the northeastern coastal region of Korea. The well-mixed low salinity water is transported to the south by the Cold Currents, forming the salinity minimum layer (Intermediate Water) beneath the East Korean Warm Current water. The recirculation gyre redistributes the core water of the salinity minimum layer in the Ulleung Basin. This revised version was published online in August 2006 with corrections to the Cover Date.     

中尺度涡影响下的南海西部活性铝分布

To understand the distribution of aluminum(Al) under the influence of mesocale eddies in the western South China Sea(SCS), sea level anomaly, geostrophic current, environmental parameters and reactive Al were investigated in the western SCS in August 2013. The highest reactive Al concentration((180±64) nmol/L) was observed in the surface waters, indicating a substantial atmospheric input. Vertically, the reactive Al decreased from the surface high concentration to the subsurface minima at the depth of chlorophyll a(Chl a) maxima and then increased again with depth at most of the stations. The average concentration of reactive Al in the upper 100 m water column was significantly lower in the cyclonic eddy((137±6) nmol/L) as compared with that in the noneddy waters((180±21) nmol/L). By contrast, the average concentrations of Chl a and silicate in the upper 100 m water column were higher in the cyclonic eddy and lower in the anticyclonic eddy. There was a significant negative correlation between the average concentrations of reactive Al and Chl a in the upper 100 m water column. The vertical distribution of reactive Al and the negative correlation between reactive Al and Chl a both suggest that the reactive Al in the upper water column was significantly influenced by biological removal processes. Our results indicate that mesoscale eddies could regulate the distribution of reactive Al by influencing the primary production and phytoplankton community structure in the western SCS.     

Impact of a cyclonic eddy on phytoplankton community structure and photosynthetic competency in the subtropical North Pacific Ocean

A synoptic spatial examination of the eddy Haulani (17–20 November 2000) revealed a structure typical of Hawaiian cyclonic eddies with divergent surface flow forcing the upward displacement of deep waters. Hydrographic surveys revealed that surface water in the eddy center was ca. 3.5°C cooler, 0.5 saltier, and 1.4 kg m⁻³ denser than surface waters outside the eddy. Vertically integrated concentrations of nitrate+nitrite, phosphate and silicate were enhanced over out-eddy values by about 2-fold, and nitrate+nitrite concentrations were ca. 8× greater within the euphotic zone inside the eddy than outside. Si:N ratios were lower within the upper mixed layer of the eddy, indicating an enhanced Si uptake relative to nitrate+nitrite. Chlorophyll a concentrations were higher within the eddy compared to control stations outside, when integrated over the upper 150 m, but were not significantly different when integrated over the depth of the euphotic zone. Photosynthetic competency, assessed using fast repetition-rate fluorometry, varied with the doming of the isopycnals and the supply of macro-nutrients to the euphotic zone. The physical and chemical environment of the eddy selected for the accumulation of larger phytoplankton species. Photosynthetic bacteria (Prochlorococcus and Synechococcus) and small (<3 μm diameter) photosynthetic eukaryotes were 3.6-fold more numerically abundant outside the eddy as compared to inside. Large photosynthetic eukaryotes (>3 μm diameter) were more abundant inside the eddy than outside. Diatoms of the genera Rhizosolenia and Hemiaulus outside the eddy contained diazotrophic endosymbiontic cyanobacteria, but these endosymbionts were absent from the cells of these species inside the eddy. The increase in cell numbers of large photosynthetic eukaryotes with hard silica or calcite cell walls is likely to have a profound impact on the proportion of the organic carbon production that is exported to deep water by sinking of senescent cells and cells grazed by herbivorous zooplankton and repackaged as large fecal pellets.     

The ‘suitcase hypothesis’: Can entrainment of meroplankton by eddies provide a pathway for gene flow between Madagascar and KwaZulu-Natal,South Africa?

Similarities in the marine fauna found off the coasts of southern Madagascar and KwaZulu-Natal Province (KZN), South Africa, led to the development of the ‘suitcase project,’ with the aim of establishing whether eddies that form off southern Madagascar may package and transport biological material across the Mozambique Channel, facilitating connectivity and gene flow. Meroplankton (larval stages of fishes and benthic invertebrates) were collected on the Madagascan shelf and along a transect through a cyclonic eddy in the Mozambique Channel. The samples were analysed using microscopy and DNA barcoding, seeking to identify species known to be common to both the southeast coast of Madagascar and the east coast of South Africa and thereby to reveal potential indicators of connectivity between these regions. The greatest zooplankton biovolume in the upper 200 m occurred on the shelf, followed by in the western part of the eddy and in the region outside the eddy to the west, and was lowest in the region outside the eddy to the east. The meroplankton were dominated by taxa of coastal origin and these were also most abundant on the shelf and in the western part of the eddy, with the lowest abundances in the region outside the eddy to the east. The findings show greater zooplankton biovolumes and larval abundances and the presence of reef-associated larval assemblages on the Madagascan shelf and along the transect through the cyclonic eddy, providing support for the suitcase hypothesis that planktonic organisms are entrained within eddies as they propagate south-westwards of the Madagascan shelf.     

南海东北部叶绿素a浓度对台风“风泵”和黑潮共同作用的响应

南海东北部是寡营养海域，夏季浮游植物叶绿素浓度较低，热带气旋“风泵”效应带来的上层海洋扰动可能引起表层浮游植物的显著增长。以往的研究通常关注热带气旋风应力和海洋中尺度涡对上层海洋浮游植物的影响，本文利用航次CTD、实测叶绿素a浓度、Argo温盐剖面和遥感数据，探讨了台风“风泵”和黑潮共同作用下真光层内浮游植物的变化特征及其成因。结果表明，2015年台风“莲花”过境1周后产生向吕宋海峡西北侧南海海域(A区)入侵的黑潮流套，该入侵的黑潮流套使台风前原有的气旋涡消失，抑制了台风产生的上升流对表层(0～40 m)营养盐供给，使次表层(60～90 m)营养盐富集，进而抑制了表层的叶绿素a增长，促进了次表层叶绿素a的增长；吕宋海峡西侧南海海域(B区)表层的浮游植物叶绿素a浓度增加不仅是源于叶绿素最大层浮游植物的向上输运，更是由于浮游植物的繁殖增长；A区台风引起的流套式的黑潮入侵，促进了B区台风后气旋式流场的形成，产生的持续增强的气旋涡为B区表层叶绿素持续增长提供了充足的营养盐供给。     

南海东北部中尺度涡对海表叶绿素a浓度的影响规律研究

基于南海东北部1998~2019年的多源卫星遥感数据和风场再分析数据, 较系统地分析了南海东北部涡旋内部叶绿素a浓度的分布特征, 通过量化统计和涡心坐标系参数合成等方法探究了中尺度涡对叶绿素a浓度变化的影响规律及潜在机制。结果表明: (1)南海东北部约有60%的中尺度涡旋内部存在叶绿素a浓度增加和减少的现象。(2)南海东北部中尺度涡内部叶绿素a扰动受到涡旋抽吸和涡致Ekman抽吸机制的共同调控, 其中约有38% (39%)的暖(冷)涡内涡旋抽吸的贡献更大, 21% (24%)的暖(冷)涡内涡致Ekman抽吸的贡献更大。(3)南海东北部中尺度涡生命周期内的海表叶绿素a浓度变化存在显著的阶段性差异, 在冷暖涡的生成期, 涡旋抽吸的作用更为显著, 而在冷暖涡的顶峰和消亡期, 涡致Ekman抽吸的作用更为明显。上述研究结果有助于理解南海东北部初级生产力对中尺度涡的响应过程与机理, 对认识海洋物理-生物耦合过程具有一定的参考价值和研究意义。     

两个西边界流延伸体区域中尺度涡统计特征分析

黑潮和湾流是世界大洋中最典型的两支西边界流，黑潮延伸体（Kuroshio Extention，KE）和湾流延伸体（Gulf Stream Extention，GSE）区域中尺度涡活动十分活跃。本文综合利用卫星高度计资料和Argo浮标资料，对KE和GSE区域中尺度涡的表层特征及其对温盐影响进行了统计研究和对比分析。结果表明:黑潮和湾流主轴附近为涡旋频率的高值区，主轴南北两侧分别以气旋涡和反气旋涡数量占多，主轴附近的涡旋强度明显大于其他区域；两个区域的涡旋以西向移动为主，气旋涡和反气旋涡都具有向南（赤道）偏离的趋势；两个区域的涡旋数量都以夏、秋季较多，涡旋强度都在春、夏季较大，且GSE区域涡旋强度明显大于KE区域；气旋涡（反气旋涡）引起内部明显的温度负（正）异常，KE区域气旋涡（反气旋涡）内部呈"负-正"（"正-负"）上下层相反的盐度异常分布，GSE区域气旋涡（反气旋涡）在各层呈现较为一致的盐度负（正）异常；两个区域中尺度涡对温盐场的平均影响深度可达1 000×104 Pa以上。     

Temporal variations in lower trophic level biological environments in the northwestern North Pacific Subtropical Gyre from 1950 to 1997

An examination of large archives (1950–1997) of the oceanographic and atmospheric data from the northwestern North Pacific Subtropical Gyre has revealed clear linkages between atmospheric forcing factors, physical processes and biological events. Large changes in the winter and spring biomass of phytoplankton and macroplankton observed over annual, decadal and inter-decadal time scales could clearly be attributed to climate-related changes in oceanographic processes. Interannual changes in the intensity of the winter-time East Asian Monsoon had a significant impact on the extent of convective overturning, on nitrate inputs into the euphotic zone and the concentrations of chlorophyll a in winter and during the following spring. A prolonged period of deeper winter mixed layers observed from the mid-1970s to the mid-1980s led to a sizeable increase in winter mixed-layer nitrate concentrations. This change resulted in a decrease in winter-time phytoplankton biomass. Spring-time chlorophyll a, in contrast, showed a steady increase during this period. The decline in winter phytoplankton biomass could be attributed to the depths of mixed layer. A deeper mixed layer prevents phytoplankton from remaining in the euphotic zone for long enough to photosynthesize and grow, leaving substantial amounts of nutrients unutilised. However, as a result of stratification of the water column in spring following each of these winters, phytoplankton could take advantage of the enhanced ambient concentrations of nutrients and increase its biomass. Another noteworthy observation for the period from the mid-1970s to the early 1980s is that the western subtropical gyre progressively became phosphate limited. The period of diminishing mixed-layer phosphate concentrations was observed in our study area from the early 1990s onwards was consistent with recent observations at Station ALOHA in the eastern subtropical gyre.     

Topographic effect of a marine ridge on the spin-down of a cyclonic eddy

The topographic effect of a meridional marine ridge on the spin-down of a cyclonic eddy is discussed by use of a two layer model. It is shown that cyclonic eddies on the eastern side of a marine ridge decay in a shorter time in comparison with those on the western side. The fast decay on the eastern side is mainly due to the planetary effect, which carries the eddy westward to a shallower region on the ridge, where remarkable frictional spin-down is carried out. Conversely, westward shift of the similar cyclonic eddy yields a weak bottom effect in the western side. It is inferred that the resulting remarkable spin-down on the eastern side of a marine ridge is one of the causes of the observed fast decay of the cyclonic cold water mass accompanied by the large meander of the Kuroshio on the eastern side of the Izu Ridge south of Japan.