How does the Red Sea outflow water interact with Gulf of Aden Eddies?

As the Red Sea overflow water (RSOW) enters the Gulf of Aden (GOA), it interacts with a sequence of nearly barotropic, mesoscale eddies originating in the Indian Ocean. To investigate how these eddies impact the dispersal and eastward transport of the RSOW toward the Indian Ocean, a high resolution 3D regional model is employed to explore systematically the interaction between the RSOW and mesoscale eddies. Two types of experiments are conducted. In the first set, we simulate the behavior of RSOW in the presence of an idealized cyclone and an idealized anticyclone. The second type of simulation involves nesting of the regional model (ROMS) within a data-assimilating global model (HYCOM), in which a sequence of mesoscale eddies entering the Gulf of Aden is realistically captured. This simulation is integrated for one year, and includes a simple representation of the seasonality of the RSOW.Bower et al. (2002) suggest that the Red Sea overflow might be a western boundary undercurrent. Consistent with these expectations, the idealized simulations show that the preferred pathway of the RSOW in the absence of eddies is along the coast of Somalia (southern continental shelf) as a western boundary undercurrent. Simultaneously, a cyclonic circulation is generated in the far western GOA due to vortex stretching by the descending outflow. The presence of a cyclone in the western GOA increases the peak RSOW transport, but the cyclone itself rapidly loses its coherence after interacting with the rough topography in the western GOA. The presence of an anticyclone tends to block the preferred boundary pathway and inhibits the eastward transport of the RSOW. The eddies also result in substantially increased mixing of the RSOW in the western GOA.On the basis of the more realistic ROMS experiment, it is found that the modeled RSOW leaves the western part of the Gulf of Aden in short episodic bursts with transports that are an order of magnitude greater than that associated with the quasi-steady RSOW inflow into GOA. Such enhancement in RSOW transport is shown to be induced by cyclonic eddies that cause a rapid discharge of RSOW from the western part of the GOA. We conclude that mesoscale eddies play a key role in the transport and mixing of the RSOW within GOA.     

基于拉格朗日分析法的中尺度涡精细三维结构研究及其误差估计

在前人的工作中,拉格朗日分析法被用来演示大尺度环流,同时拉格朗日拟序结构可以较好的演示中尺度涡两维结构的发展过程。然而,很少研究关注怎么利用拉格朗日分析法针对中尺度涡三维结构进行演示。与以往利用欧拉方法研究中尺度涡三维结构的工作不同,我们利用拉格朗日分析法,从另一个视角来研究涡旋结构。我们在海山上方模拟出一个理想的气旋涡,涡旋内的下沉流和涡旋旁的上升流形成一个闭合的环流。这种结构很难从欧拉角度来演示。然而,粒子的运动轨迹很好地展示了整个循环:流体在涡旋中旋转下沉,汇聚到底层的上升流区,并通过上升流返回到海表面。我们也将拉格朗日分析法应用于真实的模拟结果中。作为中国南海的一个重要现象,靠近越南中部的海域中的偶极子（反气旋涡/气旋涡）,关于其结构的研究已经比较成熟了,但这些研究主要关注的是海面过程。通过拉格朗日分析,我们很好的演示了偶极子的三维结构:流体在反气旋涡(气旋涡)内部旋转上升(下沉)。更重要的是,粒子的轨迹表明,这两个涡旋之间不存在水团交换,因为强边界急流将它们彼此分开。以上结论均得到了计算误差估计的可信度支持。尽管在强辐散流和强垂直扩散流中,计算误差逐渐增大,但是在一定的时间步长和积分周期内,计算误差始终保持在一个较小的值。     

A synoptic survey of young mesoscale eddies in the Eastern Gulf of Alaska

Eddies in the Gulf of Alaska are important sources of coastal water and associated nutrients, iron, and biota to the high-nutrient, low-chlorophyll central Gulf of Alaska. Three primary eddy formation regions along the eastern boundary of the gulf have been identified, (from south to north, Haida, Sitka, and Yakutat). In the spring of 2005, three eddies (one of each type) were sampled soon after their formation. The subsurface eddy core water in all three eddies was defined by high iron concentrations and low dissolved oxygen compared with surrounding basin water. The Sitka and Yakutat core waters also exhibited a subsurface temperature maximum (mesothermal water) coincident in depth with the iron maximum, suggesting that eddies may play a role in the formation of temperature inversions observed throughout the Gulf of Alaska. The data suggest different formation regions, with the Yakutat eddy forming in shallow shelf water with riverine input, while the Sitka and Haida eddies appear to form in deeper water.     

Influence of the interaction between the Atlantic and the Arctic Ocean on the Gulf Stream

Numerical experiments with the circulation model of the North Atlantic based on the splitting algorithms in the σ-coordinate system with a spatial resolution allowing for reproducing synoptic eddies were performed in two versions: with the Arctic Ocean and without it (boundary along 78°N). They showed that the account for the water exchange with the Arctic is fundamentally important for reproducing jet dynamics at the western boundary of the Atlantic down to the subtropical zone. The influence of the conditions at the liquid boundary that separates the Atlantic and the Arctic extends not only over the subarctic area [29] but is also “transferred” by the Labrador Current and the Slope Water Current (SWC) to the area of the Gulf Stream proper. One cannot properly describe the detachment of the Gulf Stream from the coast without adequate reproducing of the Labrador Current and SWC. An hypothesis is posed that the location of the detachment region at 35°N is caused by strong vertical motions at the interface between the SWC and the Gulf Stream jet with horizontal velocities that are almost equal to those at the exit from the Florida Strait. A comparison of the model circulation with that retrieved from the hydrological data and the drift of neutral buoyancy floats [14, 22] showed both qualitative and quantitative coincidences of the features of the northward warm water transfer such as the streamline around the so-called northwestern “corner” (motion “along the topography”) and the jet-wise transport of these waters from Labrador to the northeast inside a kind of “pipeline,” which is limited in the upper baroclinic layer 1 km thick by mean velocity contour lines of about 10 cm/s. A comparison between the experimental [19] and model fields of the ocean level showed that, at the absence of direct representation of the water (mass) exchange between the Atlantic and the Arctic Ocean, the decrease of the gradient velocities in the Gulf Stream may reach 30%.     

Intrinsic and forced interannual variability of the Gulf of Alaska mesoscale circulation

The response of the Gulf of Alaska (GOA) circulation to large-scale North Pacific climate variability is explored using three high resolution (15 km) regional ocean model ensembles over the period 1950-2004. On interannual and decadal timescales the mean circulation is strongly modulated by changes in the large scale climate forcing associated with PDO and ENSO. Intensification of the model gyre scale circulation occurs after the 1976-1977 climate shift, as well as during 1965-1970 and 1993-1995. From the model dynamical budgets we find that when the GOA experiences stronger southeasterly winds, typical during the positive phase of the PDO and ENSO, there is net large-scale Ekman convergence in the central and eastern coastal boundary. The geostrophic adjustment to higher sea surface height (SSH) and lower isopycnals lead to stronger cyclonic gyre scale circulation. The opposite situation occurs during stronger northwesterly winds (negative phase of the PDO).Along the eastern side of the GOA basin, interannual changes in the surface winds also modulate the seasonal development of high amplitude anticyclonic eddies (e.g. Haïda and Sitka eddies). Large interannual eddy events during winter-spring, are phase-locked with the seasonal cycle. The initial eddy dynamics are consistent with a quasi-linear Rossby wave response to positive SSH anomalies forced by stronger downwelling favorable winds (e.g. southwesterly during El Niño). However, because of the fast growth rate of baroclinic instability and the geographical focusing associated with the coastal geometry, most of the perturbation energy in the Rossby wave is locally trapped until converted into large scale nonlinear coherent eddies. Coastally trapped waves of tropical origin may also contribute to positive SSH anomalies that lead to higher amplitude eddies. However, their presence does not appear essential. The model ensembles, which do not include the effects of equatorial coastally trapped waves, capture the large Haïda and Sitka eddy events observed during 1982 and 1997 and explain between 40% and 70% of the tidal gauges variance along the GOA coast.In the western side of the GOA basin, interannual eddy variability located south of the Alaskan Stream is not correlated with large scale forcing and appears to be intrinsic. A comparison of the three model ensembles forced by NCEP winds and a multi-century-long integration forced only with the seasonal cycle, shows that the internal variability alone explains most of the eddy variance. The asymmetry between the eddy forced regime in the eastern basin, and the intrinsic regime in the western basin, has important implications for predicting the GOA response to climate change. If future climate change results in stronger wintertime winds and increased downwelling in the eastern basin, then increased mesoscale activity (perhaps more or larger eddies) might occur in this region. Conversely, the changes in the western basin are not predictable based on environmental forcing. Eastern eddies transport important biogeochemical quantities such as iron, oxygen and chlorophyll-a into the gyre interior, therefore having potential upscale effects on the GOA high-nutrient-low-chlorophyll region.     

Diagnoses of vertical transport in a three-dimensional finite element model of the tidal circulation around an island

A three-dimensional finite element model is used to investigate the formation of shallow-water eddies in the wake of Rattray Island (Great Barrier Reef, Australia). Field measurements and visual observations show that stable eddies develop in the lee of the island at rising and falling tides. The water turbidity downstream of the island suggests the existence of strong upwelling that would be responsible for carrying bed sediments up to the sea surface. We first propose to look at the upwelling velocity and then use the theory of the age to diagnose vertical transport. The water age is defined as the time elapsed since particles of water left the sea bottom, where the age is prescribed to be zero. Two versions of this diagnosis are considered. Although the model predicts upwelling within the eddies, it is not sufficiently intense to account for vertical transport throughout the water column during the life span of the eddies. As mesh resolution increases, this upwelling does not intensify. However, strong upwelling is then resolved off the island's tips, which is confirmed by the results obtained with the age. This study also shows that the finite element method, together with unstructured meshes, performs well for representing three-dimensional flow past an island.     

Three-dimensional circulation in the NW Africa coastal transition zone

High-resolution data collected southeast of the Canary Islands during late winter 2006 are analyzed to describe the hydrography and three-dimensional circulation in the coastal transition zone off NW Africa. The data are optimally interpolated over a regular grid, the geostrophic velocity field is calculated and the Q-vector formulation of the omega equation is used to compute the quasi-geostrophic (QG) mesoscale vertical velocity. The coastal transition zone is divided into upwelling, frontal and offshore regions with distinct physical and dynamic characteristics. The upwelling region is characterized by cold and weakly stratified waters flowing towards the equator, with a poleward undercurrent of approximately 0.05 m s⁻¹ over the continental slope. The frontal region exhibits a southwestward baroclinic jet associated with cross-shore raising isopycnals; the jet transport is close to 1 Sv, with maximum velocities of 0.18 m s⁻¹ at surface decreasing to 0.05 m s⁻¹ at 300 db. Vertical sections across the frontal region show the presence of deep eddies probably generated by the topographic blocking of the islands to the southward current, as well as much shallower eddies that likely have arisen as instabilities of the baroclinic upwelling jet. The QG mesoscale vertical velocity field is patchy, estimated to range from −18 to 12 m day⁻¹, with the largest absolute values corresponding to an anticyclonic eddy located south of Fuerteventura Island. These values are significantly larger than estimates for other vertical velocities: diapycnal vertical velocities associated with mixing in the frontal region (a few meters per day), and wind-induced vertical velocities (non-linear Ekman pumping arising from the interaction between the wind stress and the background vorticity, maximum values of a few meters per day; linear Ekman pumping due to the divergence of Ekman transport, a fraction of a meter per day; or the coastal constraint in the upwelling region, about 0.7 m day⁻¹). However, the patchiness in both the QG mesoscale vertical velocity and the non-linear Ekman pumping velocity cause their integrated vertical transports to be one order of magnitude smaller than either coastal Ekman transport (0.08 Sv), integrated linear Ekman pumping (−0.05 Sv) or diapycnal transfer (about 0.1–0.2 Sv). The pattern of the near-surface fluorescence field is a good indicator of these different contributions, with large homogeneous values in the coastal upwelling region and a patchy structure associated with the offshore mesoscale structures.     

Northern Gulf of Alaska eddies and associated anomalies

In recent years, large anticyclonic eddies have been observed quasi-annually in the region seaward of Kodiak Island, Alaska. In situ sampling in 3 of these eddies was undertaken in 2002, 2003, and 2004. Satellite altimetry data showed that these 3 eddies had 3 different formation regions but their translation pathways were similar near Kodiak Island. Eddies in this region can persist for several years, moving southwestward along the Alaskan Peninsula to the Aleutian Archipelago. Water properties in the cores of the 2003 and 2004 eddies were significantly different from each other, probably because the 2003 eddy formed on the shelf near Yakutat while the 2004 eddy formed farther out in the basin in the northern Gulf of Alaska. Calculation of heat, salinity, and nutrient anomalies associated with the eddies showed that, in their subsurface core waters, the eddies carry excess heat, salt, nitrate and silicic acid seaward from the eddy formation regions.     

Structure of Haida Eddies and Their Transport of Nutrient from Coastal Margins into the NE Pacific Ocean

Anticyclonic mesoscale eddies form near shore each winter in the Gulf of Alaska. One site near the Queen Charlotte Islands is shown to produce eddies that transport from 3000 to 6000 km³ of coastal water up to 1000 km westward. Eddies carry shelf nutrients either into the high nutrient, low chlorophyll waters of the NE Pacific, or in a more southerly direction into seasonally nitrate depleted waters. A large eddy sampled in summer 1998 was found to have elevated particulate levels on its perimeter. Nitrate supplied to the euphotic zone by this eddy during its natal summer is estimated to be three times greater than the usual seasonal nutrient transport in the Gulf of Alaska. This revised version was published online in August 2006 with corrections to the Cover Date.     

基于2005~2019年卫星遥感观测的南大洋印度洋扇区中部涡旋特征分布研究

在南大洋印度洋扇区中部海域,除了地形控制(凯尔盖朗高台),南极绕极流和厄加勒斯回流的汇合流进一步加强了下游的斜压剪切强度,导致涡旋能量显著增强,因此,对该海域涡旋的研究有助于了解该海域的涡旋特征以及地形与涡旋的分布关系。基于2005~2019年卫星遥感数据,对该海域涡旋特征进行统计,并对涡旋产生地分布、跨锋面涡旋的移动状况进行分析,同时结合Argo剖面数据,进一步剖析涡旋内部水文分布特征。结果表明:该海域涡旋生命周期多在20 d以内(64.25%),平均半径多在30~100 km(96.13%);平均半径与平均振幅呈正相关关系(相关系数R=0.55);生命周期越大的涡旋平均传播距离也越大。2014年开始涡旋数量明显增加,主要由短寿命涡旋(<30 d)数量增加所贡献。反之, 21世纪10年代后期年平均涡动能异常呈减小趋势。涡旋产生地随着寿命增长,逐渐从亚南极锋与南极绕极流南部边界之间的锋面区域向亚南极锋以北移动。跨锋面涡旋中,暖涡向高纬,冷涡向低纬移动,大部分具有携带水团移动的能力。由涡旋内部水文特征分析结果可知,不同极性的涡旋能够实现完全不同来源水团的远距离输送,对同一来源水团,气旋涡具有抬升作用,而反气旋涡具有压沉作用。该研究工作有助于提升对南大洋涡旋特征及变动的认识,为进一步的涡旋动力研究提供支撑。     

Characterization of carbon and nitrogen stable isotope gradients in the northern Gulf of Alaska using terminal feed stage copepodite-V Neocalanus cristatus

Individual terminal-feeding copepodite-V stage Neocalanus cristatus were collected systematically in the northern Gulf of Alaska (GOA) near 60°N from 1998 to 2004 from which the natural abundance of stable carbon and nitrogen isotopes was measured. The data confirmed the existence of an isotopic cross-shelf gradient such that values low in ¹³C content are diagnostic of oceanic production from the GOA when measured in organisms taken from Prince William Sound (PWS). The mean ¹³C/¹²C cross-shelf gradient of 3–4 delta units was relatively strong, with generally good separation between GOA and Prince William Sound observations, whereas the mean ¹⁵N/¹⁴N gradient of ∼2 delta units was relatively weak, with frequent overlap between GOA and PWS observations. There was a seasonal ¹⁵N/¹⁴N increase in the GOA. The ¹³C/¹²C values observed in PWS were more consistent over time than those observed in the GOA. Distinctively high ¹³C/¹²C values that were similar to those typical of PWS were observed at the continental slope during three of the Mays in the 1998–2004 period. The circulation pattern associated with mesoscale eddies, when they occurred just south of the sampling line based on satellite sea-surface height anomaly data, suggested that cross-shelf flow in the offshore direction drove high slope ¹³C/¹²C values. These observations led to positing that high ¹³C/¹²C values reflect coastal carbon isotope signatures and diatom blooms. Based on samples from May 1996, the three GOA Neocalanus congeners had concordant isotopic patterns with relatively small systematic species differences confirming that the isotopic patterns observed for N. cristatus apply to other zooplankton.     

Some characteristics of Yakutat Eddies propagating along the continental slope of the northern Gulf of Alaska

Data from array for real-time geostrophic oceanography (ARGO) profiling floats, oil tanker thermosalinographs, shipboard ADCP and towed-CTD surveys, and satellite altimetry are used to examine properties of two ∼200 km diameter, anticyclonic Yakutat Eddies that propagated westward at ∼1.5 km day⁻¹ along the continental slope of the northern Gulf of Alaska (GOA) in 2001 and 2003. The eddies had lifetimes of up to 5 years, remained close to the shelfbreak, and had relatively constant size and strength until they encountered the Alaskan Stream where they appeared to spawn smaller, shorter-lived anticyclones. The azimuthal velocity field was vigorous (25–40 cm s⁻¹) and in gradient wind balance with Rossby numbers of ∼0.05. Conservation of salt and azimuthal mass transports (between 20 m and the depth of the 32.2 isohaline) from shipboard surveys in May and August 2003 suggest little mass exchange occurred between the surface layers of the eddy and ambient waters. Chlorophyll concentrations were greater in the eddy than in ambient waters in both May and August. In May, the chlorophyll was patchily distributed, while in August dense chlorophyll concentrations occurred in and beneath the seasonal thermocline within 50 km of the eddy center. The high August chlorophyll concentrations might have been fostered by a broad and shallow (<∼150 m) upwelling of the eddy center between May and August.It appears likely that as Yakutat Eddies approach the shelfbreak non-linear processes will modify the slope flow field (and the stability and structure of the shelfbreak front), leading to cross-slope flows and flow reversals. This interaction may induce ∼30 km-wide streamers of shelf water to flow around the trailing edge of the anticyclone. The role of streamers in the freshwater and nutrient budgets of the GOA shelf and basin is unknown, but their contribution to these budgets will depend on the trajectory of a Yakutat Eddy, especially its proximity to the shelfbreak as the eddy propagates along the GOA continental slope.     

Transport through Unimak Pass, Alaska

We examined inflow through Unimak Pass (<200 m deep), which is the only major connection between the shelves of the North Pacific Ocean and the eastern Bering Sea. Geostrophic transport was generally northward from the Gulf of Alaska into the Bering Sea. The flow through the pass appeared to be modulated by the seasonal cycle of freshwater discharge. On shorter time scales, transport also was affected by semi-daily variations in tidal mixing. This effect was significant and not anticipated. Near-bottom currents, measured from moorings, were maximum during winter, and significantly correlated (r=0.7) with the alongshore winds. Although the flow through Unimak Pass transported some nutrients from the North Pacific Ocean, the Gulf of Alaska shelf is not the major source of nutrients to the Bering Sea shelf.     

台湾岛西南部一个暖涡的跨海盆粒子输运

台湾岛西南部是暖涡产生的源地之一, 以往的研究集中于讨论暖涡本身的运动特征和规律, 如暖涡的半径、产生位置、寿命、移动路径等, 暖涡生成机制也引发了深入的研究和探讨, 但关于该地区暖涡导致的物质输运, 仅有少量的观测且结果较为离散。文章通过AVISO(Archiving Validation and Interpretation of Satellite Oceanographic Data)卫星高度计的地转流场、海表高度异常数据以及区域海洋环流模式(ROMS)结果, 得到一个暖涡从2003年秋季至2004年春季的生消过程, 并在涡旋内10m、50m、100m层各放入10000个示踪粒子, 追踪它们的运动轨迹, 从而讨论暖涡的跨海盆粒子输运特征。该暖涡的寿命为121d, 平均半径50.4km, 移动路程1437.5km, 平均移动速度 13.5cm·s^-1。研究表明, 该暖涡具有较好的物质保守性, 可以将大部分的粒子裹挟在涡旋内部并携带着沿1000~2000m等深线向西南方向运动, 在该暖涡的消亡阶段, 由于地形的阻隔以及与流的相互作用, 涡的不稳定性和形变增加, 大量的粒子被甩出涡旋, 最后仅有18.6%~35.6%粒子随暖涡运动至海南岛南部。尽管这个比例并不大, 但涡致输运的积累作用不容小觑。示踪粒子的数目与位涡具有较好的相关性, 相关系数为0.63, 暖涡内的物质被位涡线裹挟着前行。示踪粒子在暖涡内呈螺旋状下沉, 大部分粒子在释放后的5d内基本在起始的深度上下浮沉, 仅有3.9%的粒子沉降至15m以下。在10m、50m和100m层释放的粒子平均深度最大值分别为48.7m、88.7m和130.6m。     

Multiple late Quaternary episodes of exceptional diatom production in the Gulf of Alaska

Hole 887B of the Ocean Drilling Program (ODP) comprises a 44 m (750 kyr) long continuous section recovered from the Patton–Murray Rise, an elevated plateau that is largely isolated from turbidite deposition. The Patton–Murray area is centered under the Alaska Gyre, a region characterized by the domal upwelling of nutrient-rich waters. Marked increases in productivity and rapid settling of biogenic matter are suggested throughout the section by the episodic accumulation of diatomaceous oozes up to ∼1 m thick that are accompanied by barium enrichments. Significant δ¹³C_org maxima in the major diatomaceous bands suggest that mixed-layer [CO_2(aq)] may have been drawn down significantly during some of the productivity events. The episodes of enhanced productivity at Site 887 occur synchronously with short-lived minima in planktonic foram δ¹⁸O, suggesting a direct link to low salinity, or less likely, warming, events in the Gulf of Alaska. There is no obvious explanation for the events, but they may be related to seasonal incursions of meltwater from Alaska. We speculate that episodic input of meltwater- or dust-borne iron from Asian or Alaskan sources may have promoted the extraordinary diatom production events recorded in the sedimentary record.     

Pacific Basin climate variability and patterns of Northeast Pacific marine fish production

A review of oceanographic and climate data from the North Pacific and Bering Sea has revealed climate events that occur on two principal time scales: a) 2–7 years (i.e. El Niño Southern Oscillation, ENSO), and b) inter-decadal (i.e. Pacific Decadal Oscillation, PDO). The timing of ENSO events and of related oceanic changes at higher latitudes were examined. The frequency of ENSO was high in the 1980s. Evidence of ENSO forcing on ocean conditions in the North Pacific (Niño North conditions) showed ENSO events were more frequently observed along the West Coast than in the western Gulf of Alaska (GOA) and Eastern Bering Sea (EBS). Time series of catches for 30 region/species groups of salmon, and recruitment data for 29 groundfish and 5 non-salmonid pelagic species, were examined for evidence of a statistical relationship with any of the time scales associated with Niño North conditions or the PDO. Some flatfish stocks exhibited high autocorrelation in recruitment coupled with a significant step in recruitment in 1977 suggesting a relationship between PDO forcing and recruitment success. Five of the dominant gadid stocks (EBS and GOA Pacific cod, Pacific hake and EBS and GOA walleye pollock) exhibited low autocorrelation in recruitment. Of these, Pacific hake, GOA walleye pollock and GOA Pacific cod exhibited significantly higher incidence of strong year classes in years associated with Niño North conditions. These findings suggest that the PDO and ENSO may play an important role in governing year-class strength of several Northeast Pacific marine fish stocks.     

Numerical simulation of upwelling currents in pockmarks,and data from the Inner Oslofjord,Norway

The deflection of oceanic or tidal currents into pockmarks has been studied by both general three-dimensional computational fluid dynamics simulations and acoustic measurements in a number of pockmarks in the Inner Oslofjord, Norway. The modeling demonstrates upstream convergence of flow lines, followed by upwelling over the pockmark. This upwelling is an effect of deflected regional currents, not of expulsion of fluids or gas from the seafloor, and is sufficiently strong to prevent the settling of fine particles. The field measurements, although noisy at low vertical velocities, are consistent with the hypothesis of upwelling. The reduction in sedimentation rate inferred over the pockmarks (relative to that of the flat surrounding seabed) can explain the maintenance, or even deepening of pockmarks in the absence of fluid or gas seepage. The current pattern may also have consequences for the marine biology of pockmarks.     

热带西南印度洋上升流区季节和年际变化研究及与印太系统海气相互作用

用59年Ishii再分析温度资料,讨论了热带西南印度洋(SWTIO)上升流区的季节和年际变化以及与上升流区有关的温度距平的变化,同时分析了其与热带印太海气系统的关系,结果显示SWTIO 上升流在南半球冬、夏季比较强,春季最弱。它的范围在5°~1°S,在东西向从50°E可以伸展到90°E。该上升流区的变化与温跃层的温度距平有密切的关系,并存在明显的5 a振荡周期。SWTIO上升流区温度距平的5 a周期振荡是由热带东印度洋温度距平在最大垂直温度距平曲面(MTAL)上向西沿着11.5°~6.5°S传播过来的,它与热带太平洋的温度距平传播方式不同。SWTIO上升流是热带印太海气系统的一个重要组成部分,印度洋偶极子 超前SWTIO上升流区温度变化5个月,最大相关系数达到0.57,NINO3区指数超前SWTIO上升流区指数2个月达到0.49。当热带印太区域的大气风场改变,影响热带太平洋和印度洋表层SSTA,出现ENSO和DIPOLE,进一步向西传播到SWTIO次表层,导致SWTIO上升流区出现改变。     

The transition zone of the Canary Current upwelling region

Like all the major upwelling regions, the Canary Current is characterised by intense mesoscale structure in the transition zone between the cool, nutrient-rich waters of the coastal upwelling regime and the warmer, oligotrophic waters of the open ocean. The Canary Island archipelago, which straddles the transition, introduces a second source of variability by perturbing the general southwestward flow of both ocean currents and Trade winds. The combined effects of the flow disturbance and the eddying and meandering of the boundary between upwelled and oceanic waters produce a complex pattern of regional variability. On the basis of historical data and a series of interdisciplinary field studies, the principal features of the region are described. These include a prominent upwelling filament originating near 28°N off the African coast, cyclonic and anti-cyclonic eddies downstream of the archipelago, and warm wake regions protected from the Trade winds by the high volcanic peaks of the islands. The filament is shown to be a recurrent feature, apparently arising from the interaction of a topographically trapped cyclonic eddy with the outer edge of the coastal upwelling zone. Its role in the transport and exchange of biogenic material, including fish larvae, is considered. Strong cyclonic eddies, observed throughout the year, drift slowly southwestward from Gran Canaria. One sampled in late summer was characterised by large vertical isopycnal displacements, apparent surface divergence and strong upwelling, producing a fourfold increase in chlorophyll concentrations over background values. Such intense eddies can be responsible for a major contribution to the vertical flux of nitrogen. The lee region of Gran Canaria is shown to be a location of strong pycnocline deformation resulting from Ekman pumping on the wind shear boundaries, which may contribute to the eddy formation process.     

Statistical characteristics and thermohaline properties of mesoscale eddies in the Bay of Bengal

The statistical characteristics and vertical thermohaline properties of mesoscale eddies in the Bay of Bengal are studied from the view of satellite altimetry data and Argo profiles. Eddy propagation preferences in different lifetimes, eddy evolution process, and geographical distribution of eddy kinetic properties are analyzed in this area. Eddies exist principally in the western Bay of Bengal, and most of them propagate westward. There is a clear southward(equatorward) preference for eddies with long lifetimes, especially for cyclones. Moreover, the eddies in different areas of the bay show different north-southward preferences. Evolution of eddy kinetic properties with lifetime shows that eddies have the significant three-stage feature: the growth period in the former one-fifth lifetime, the stable period in the middle two-fifth to four-fifth lifetime, and the dying period in the last one-fifth lifetime. Large-amplitude and high-intensity eddies occur only in the relatively confined regions of highly unstable currents, such as the East Indian Coastal Current and eastern Sri Lanka. Based on Argo profile data and climatology data, the eddy synthesis method was used to construct three-dimensional temperature and salt structures of eddies in this area. The mean temperature anomaly is negative/positive to the cyclonic/anticyclonic eddies in the upper 300×10~4 Pa, and below this depth, the anomaly becomes weak. The salinity structures of positive anomalies inside cyclonic eddies and negative anomalies inside anticyclonic eddies in the Bay of Bengal are not consistent with other regions. Due to the special characteristics of the water mass in the bay, especially under the control of the low-salinity Bay of Bengal water at the surface and the Indian equatorial water in the deep ocean, the salinity of seawater shows a monotonic increase with depth. For regional varieties of temperature and salinity structures, as the eddies move westward, the temperature anomaly induced by the eddies increases, the effecting depth of the eddies deepens, and the salinity structures are more affected by inflows. In the north-south direction, the salinity structures of the eddies are associated with the local water masses, which comprise lowsalinity water in the northern bay due to the inflow of freshwater from rivers and salty water in the southern bay due to the invasion of Arabian Sea high-salinity water from the north Indian Ocean.