北太平洋西边界流研究综述

综述了近20年来国内外学者在研究北太平洋西边界流的平均结构及NEC分叉动力机制、NM K流系平均输运的分配及变化、NM K流系季节及年际变化规律及其与EN SO之间的关系、NM K流系在热带和亚热带水交换中的作用以及水团的平均分布特征等方面所取得的主要成果。通过分析,发现东亚季风、R ossby波和K e lv in波等是影响北太平洋西边界流的主要因素;而缺乏长期直接的海流观测资料是深入研究北太平洋西边界流遇到的最大障碍。     

ENSO相联系的热带太平洋上层海洋异常环流

本文使用SODA(simple ocean data assimilation)海洋同化资料,系统分析了厄尔尼诺-南方涛动(El Ni?o-Southern Oscillation,ENSO)循环中冷暖位相期间热带太平洋上层海洋环流的演变规律,探讨了形成海洋环流异常的新机制。结果表明,在厄尔尼诺成熟期,热带中东太平洋赤道潜流最弱,赤道两侧出现反气旋性环流异常;西太平洋赤道外热带海域出现气旋性环流异常,该区南、北赤道流、棉兰老流、黑潮、新几内亚沿岸潜流及南赤道逆流增强;北赤道逆流区出现异常气旋性环流串,北赤道逆流接近正常。在厄尔尼诺衰退期和拉尼娜发展期,热带中西太平洋赤道潜流达到极强,赤道两侧出现气旋性环流异常;西太平洋赤道外热带海域异常环流减弱,该处主要流场的强度减弱或处于正常状态;北赤道逆流区反转为异常西向流。结果表明, ENSO循环期间的上层海洋环流异常受到热带太平洋温跃层深度异常产生的压强梯度力异常调控,在赤道外热带海洋温跃层深度异常和科里奥利力共同作用产生大尺度海洋环流异常,而在赤道海域,海洋温跃层深度异常和Gill效应造成赤道潜流异常以及关于赤道对称的气旋或反气旋性环流异常。     

Intraseasonal-to-Interannual Variability of the Upper-Layer Zonal Currents in the Tropical Northwest Pacific Ocean

Ocean Science Journal - The upper-layer circulation in the Tropical Northwest Pacific Ocean (TNWP) is characterized by the alternating zonal currents, such as North Equatorial Current (NEC) and...     

The connection between Labrador Sea buoyancy loss, deep western boundary current strength, and Gulf Stream path in an ocean circulation model

The sensitivity of the North Atlantic gyre circulation to high latitude buoyancy forcing is explored in a global, non-eddy resolving ocean general circulation model. Increased buoyancy forcing strengthens the deep western boundary current, the northern recirculation gyre, and the North Atlantic Current, which leads to a more realistic Gulf Stream path. High latitude density fluxes and surface water mass transformation are strongly dependent on the choice of sea ice and salinity restoring boundary conditions. Coupling the ocean model to a prognostic sea ice model results in much greater buoyancy loss in the Labrador Sea compared to simulations in which the ocean is forced by prescribed sea ice boundary conditions. A comparison of bulk flux forced hindcast simulations which differ only in their sea ice and salinity restoring forcings reveals the effects of a mixed thermohaline boundary condition transport feedback whereby small, positive temperature and salinity anomalies in subpolar regions are amplified when the gyre spins up as a result of increased buoyancy loss and convection. The primary buoyancy flux effects of the sea ice which cause the simulations to diverge are ice melt, which is less physical in the diagnostic sea ice model, and insulation of the ocean, which is less physical with the prognostic sea ice model. Increased salinity restoring ensures a more realistic net winter buoyancy loss in the Labrador Sea, but it is found that improvements in the Gulf Stream simulation can only be achieved with the excessive buoyancy loss associated with weak salinity restoring.     

一个高分辨率太平洋-印度洋海盆环流模式的初步结果

利用LASG/IAP发展的一个0.25°×0.25°高分辨率太平洋-印度洋海盆环流模式，初步分析了模式在太平洋区域的模拟结果，并与海洋同化资料以及前人的研究结果作比较，检验此模式对该区域平均气候态、年际变化的模拟能力。分析表明，模式较好地再现了海表温度（SST）分布、赤道温跃层和纬向流结构、赤道流系分布形态、海表高度以及正压流函数空间分布特征；同时，对显著的El Ni?o和La Ni?a事件的模拟等方面与Simple Ocean Data Assimilation（SODA）2.0.2版本结果相近。此外，模式模拟北赤道流（NEC）分叉点位置的季节和年际变化以及吕宋海峡流量的年际变化与已有研究结果基本一致。进一步分析还发现，在年际尺度上，NEC分叉点位置和吕宋海峡流量与ENSO密切相关。     

Seasonal variations of the ocean surface circulation in the vicinity of Palau

The surface circulation in the western equatorial Pacific Ocean is investigated with the aim of describing intra-annual variations near Palau (134°30′ E, 7°30′ N). In situ data and model output from the Ocean Surface Currents Analysis—Real-time, TRIangle Trans-Ocean buoy Network, Naval Research Laboratory Layered Ocean Model and the Joint Archive for Shipboard ADCP are examined and compared. Known major currents and eddies of the western equatorial Pacific are observed and discussed, and previously undocumented features are identified and named (Palau Eddy, Caroline Eddy, Micronesian Eddy). The circulation at Palau follows a seasonal variation aligned with that of the Asian monsoon (December–April; July–October) and is driven by the major circulation features. From December to April, currents around Palau are generally directed northward with speeds of approximately 20 cm/s, influenced by the North Equatorial Counter-Current and the Mindanao Eddy. The current direction turns slightly clockwise through this boreal winter period, due to the northern migration of the Mindanao Eddy. During April–May, the current west of Palau is reduced to 15 cm/s as the Mindanao Eddy weakens. East of Palau, a cyclonic eddy (Palau Eddy) forms producing southward flow of around 25 cm/s. The flow during the period July to September is disordered with no influence from major circulation features. The current is generally northward west of Palau and southward to the east, each with speeds on the order of 5 cm/s. During October, as the Palau Eddy reforms, the southward current to the east of Palau increases to 15 cm/s. During November, the circulation transitions to the north-directed winter regime.     

The mean flow field of the tropical Atlantic Ocean

The mean horizontal flow field of the tropical Atlantic Ocean is described between 20°N and 20°S from observations and literature results for three layers of the upper ocean, Tropical Surface Water, Central Water, and Antarctic Intermediate Water. Compared to the subtropical gyres the tropical circulation shows several zonal current and countercurrent bands of smaller meridional and vertical extent. The wind-driven Ekman layer in the upper tens of meters of the ocean masks at some places the flow structure of the Tropical Surface Water layer as is the case for the Angola Gyre in the eastern tropical South Atlantic. Although there are regions with a strong seasonal cycle of the Tropical Surface Water circulation, such as the North Equatorial Countercurrent, large regions of the tropics do not show a significant seasonal cycle. In the Central Water layer below, the eastward North and South Equatorial undercurrents appear imbedded in the westward-flowing South Equatorial Current. The Antarcic Intermediate Water layer contains several zonal current bands south of 3°N, but only weak flow exists north of 3°N. The sparse available data suggest that the Equatorial Intermediate Current as well as the Southern and Northern Intermediate Countercurrents extend zonally across the entire equatorial basin. Due to the convergence of northern and southern water masses, the western tropical Atlantic north of the equator is an important site for the mixture of water masses, but more work is needed to better understand the role of the various zonal under- and countercurrents in cross-equatorial water mass transfer.     

Recent progress in studies of the South China Sea circulation

The South China Sea (SCS) is a semi-enclosed marginal sea with deep a basin. The SCS is located at low latitudes, where the ocean circulations are driven principally by the Asia-Australia monsoon. Ocean circulation in the SCS is very complex and plays an important role in both the marine environment and climate variability. Due to the monsoon-mountain interactions the seasonal spatial pattern of the sea surface wind stress curl is very specific. These distinct patterns induce different basin-scale circulation and gyre in summer and winter, respectively. The intensified western boundary currents associated with the cyclonic and anticyclonic gyres in the SCS play important roles in the sea surface temperature variability of the basin. The mesoscale eddies in the SCS are rather active and their formation mechanisms have been described in recent studies. The water exchange through the Luzon Strait and other straits could give rise to the relation between the Pacific and the SCS. This paper reviews the research results mentioned above.     

The circulation of the eastern tropical Pacific: A review

During the 1950s and 1960s, an extensive field study and interpretive effort was made by researchers, primarily at the Scripps Institution of Oceanography, to sample and understand the physical oceanography of the eastern tropical Pacific. That work was inspired by the valuable fisheries of the region, the recent discovery of the equatorial undercurrent, and the growing realization of the importance of the El Niño phenomenon. Here we review what was learned in that effort, and integrate those findings with work published since then as well as additional diagnoses based on modern data sets.Unlike the central Pacific, where the winds are nearly zonal and the ocean properties and circulation are nearly independent of longitude, the eastern tropical Pacific is distinguished by wind forcing that is strongly influenced by the topography of the American continent. Its circulation is characterized by short zonal scales, permanent eddies and significant off-equatorial upwelling. Notably, the Costa Rica Dome and a thermocline bowl to its northwest are due to winds blowing through gaps in the Central American cordillera, which imprint their signatures on the ocean through linear Sverdrup dynamics. Strong annual modulation of the gap winds and the meridional oscillation of the Intertropical Convergence Zone generates a Rossby wave, superimposed on the direct forcing, that results in a southwestward-propagating annual thermocline signal accounting for major features of observed thermocline depth variations, including that of the Costa Rica Dome, the Tehuantepec bowl, and the ridge–trough system of the North Equatorial Countercurrent (NECC). Interannual variability of sea surface temperature (SST) and altimetric sea surface height signals suggests that the strengthening of the NECC observed in the central Pacific during El Niño events continues all the way to the coast, warming SST (by zonal advection) in a wider meridional band than the equatorially trapped thermocline anomalies, and pumping equatorial water poleward along the coast.The South Equatorial Current originates as a combination of equatorial upwelling, mixing and advection from the NECC, and Peru coastal upwelling, but its sources and their variability remain unresolved. Similarly, while much of the Equatorial Undercurrent flows southeast into the Peru Undercurrent and supplies the coastal upwelling, a quantitative assessment is lacking. We are still unable to put together the eastern interconnections among the long zonal currents of the central Pacific.     

Research on the Propagation Acting of the Equatorial Planetary Waves on the Western Equatorial Pacific Warm Pool Heat

1 IntroductionIn the Equatorial Pacific, due to the difference between the atmospheric circulation and air-sea interaction, the near-surface seawater heat structure in the eastern and western Pacific presents two ℃obviously different characteristics: warm pool ( > 28 ) in the western equatorial Pacific and cold ℃tongue ( < 24 ) in the eastern equatorial Pacific. The water bodies of these two heat structures would give rise to change in spatial distribution under the action of the equato…     

赤道大洋定常风生环流的理论模型

为了模拟以南、北赤道流、赤道逆流和赤道潜流为特征的赤道流系,从长时间平均的线性化的海水运动方程出发,考虑海水层化、保留经向摩擦作用,利用赤道太平洋中部海区经向流通量不随纬度变化的量阶估计,得到海水密度跃层对赤道风场的响应,即跃层深度的分布规律,然后应用我们已构建的改进Fourier方法求解(由于地转口效应所引入的)变系数的海水运动方程,得到适用于中部赤道太平洋的级数形式的三维流场解。     

ENSO循环相联系的北太平洋低纬度异常西边界流

用SODA海洋同化和NCEP大气再分析资料,分析了热带太平洋次表层海温异常主要模态与北太平洋低纬度西边界流海域上层海洋环流和亚洲-北太平洋地区大气垂直和水平流场变化之间的关系,得到以下结果:（1） 在热带太平洋海洋次表层ENSO事件具有两种模态,二者组合构成ENSO循环。第一模态为ENSO成熟期,主要出现在冬季,第二模态为ENSO过渡期,主要出现夏季。（2） ENSO循环对北太平洋低纬度西边界流区上层海洋环流有重要影响。在El Niño发展期或La Niña 衰退期,该区出现气旋性异常环流,北赤道流（NEC）加强,NEC分叉位置北移,棉兰老海流（MC）加大,菲律宾以东黑潮（KC）减小,北赤道逆流（NECC）最强。在El Niño（La Niña）成熟期,该区气旋性（反气旋性）异常环流达最强,NEC最强（最弱）,NEC分叉位置最北（最南）,MC最大（最小）,KC最小（最大）,NECC减弱（加强）。在El Niño衰退期或La Niña发展期与El Niño发展期相反,该区出现反气旋性异常环流,由此导致相应流系异常发生反位相变化。（3） ENSO循环对北太平洋低纬度西边界流海域上层海洋环流的影响是通过ENSO事件期间热带太平洋热力状况异常改变上空大气环流来实现的。ENSO事件首先造成热带太平洋海洋热力状况异常,导致其上空对流活动异常,后者直接或间接通过“大气桥”能量传输引起相关地区大气环流场的变化,致使海面风应力场异常,进而强迫上层海洋环流场的相应变化。文章最后还分析了ENSO事件期间菲律宾附近异常反气旋或异常气旋性风场的产生和持续原因,讨论了北太平洋低纬度西边界流海域海气相互作用在ENSO循环中的贡献。     

Interactions between the Indonesian Throughflow and circulations in the Indian and Pacific Oceans

Circulations associated with the Indonesian Throughflow (IT), particularly concerning subsurface currents in the Pacific Ocean, are studied using three types of models: a linear, continuously stratified (LCS) model and a nonlinear, -layer model (LOM), both confined to the Indo-Pacific basin; and a global, ocean general circulation model (COCO). Solutions are wind forced, and obtained with both open and closed Indonesian passages. Layers 1-4 of LOM correspond to near-surface, thermocline, subthermocline (thermostad), and upper-intermediate (AAIW) water, respectively, and analogous layers are defined for COCO.The three models share a common dynamics. When the Indonesian passages are abruptly opened, barotropic and baroclinic waves radiate into the interiors of both oceans. The steady-state, barotropic flow field from the difference (open − closed) solution is an anticlockwise circulation around the perimeter of the southern Indian Ocean, with its meridional branches confined to the western boundaries of both oceans. In contrast, steady-state, baroclinic flows extend into the interiors of both basins, a consequence of damping of baroclinic waves by diapycnal processes (internal diffusion, upwelling and subduction, and convective overturning). Deep IT-associated currents are the subsurface parts of these baroclinic flows. In the Pacific, they tend to be directed eastward and poleward, extend throughout the basin, and are closed by upwelling in the eastern ocean and Subpolar Gyre. Smaller-scale aspects of their structure vary significantly among the models, depending on the nature of their diapycnal mixing.At the exit to the Indonesian Seas, the IT is highly surface trapped in all the models, with a prominent, deep core in the LCS model and in LOM. The separation into two cores is due to near-equatorial, eastward-flowing, subsurface currents in the Pacific Ocean, which drain layer 2 and layer 3 waters from the western ocean to supply water for the upwelling regions in the eastern ocean; indeed, depending on the strength and parameterization of vertical diffusion in the Pacific interior, the draining can be strong enough that layer 3 water flows from the Indian to Pacific Ocean. The IT in COCO lacks a significant deep core, likely because the model’s coarse bottom topography has no throughflow passage below 1000 m. Consistent with observations, water in the near-surface (deep) core comes mostly from the northern (southern) hemisphere, a consequence of the wind-driven circulation in the tropical North Pacific being mostly confined to the upper ocean; as a result, it causes the near-surface current along the New Guinea coast to retroflect eastward, but has little impact on the deeper New Guinea undercurrent.In the South Pacific, the IT-associated flow into the basin is spread roughly uniformly throughout all four layers, a consequence of downwelling processes in the Indian Ocean. The inflow first circulates around the Subtropical Gyre, and then bends northward at the Australian coast to flow to the equator within the western boundary currents. To allow for this additional, northward transport, the bifurcation latitude of the South Equatorial Current shifts southward when the Indonesian passages are open. The shift is greater at depth (layers 3 and 4), changing from about 14°S when the passages are closed to 19°S when they are open and, hence, accounting for the northward-flowing Great Barrier Reef Undercurrent in that latitude range.After flowing along the New Guinea coast, most waters in layers 1-3 bend offshore to join the North Equatorial Countercurrent, Equatorial Undercurrent, and southern Tsuchiya Jet, respectively, thereby ensuring that northern hemisphere waters contribute significantly to the IT. In contrast, much of the layer 4 water directly exits the basin via the IT, but some also flows into the subpolar North Pacific. Except for the direct layer 4 outflow, all other IT-associated waters circulate about the North Pacific before they finally enter the Indonesian Seas via the Mindanao Current.     

Why are rings regularly shed in the western equatorial Atlantic but not in the western Pacific?

The western equatorial Atlantic is characterized by the formation and shedding of 3–4 large anticyclonic rings per year. These rings originate from the North Brazil Current which, in response to the vanishing wind stress curl (over the ocean interior), retroflects and turns eastward at around 4°N. After their formation and shedding the rings propagate toward the northwest along the South American coast carrying an annual average of about 4Sv. As such, the rings constitute an important part of the meridional heat flux in the Atlantic.The same cannot be said, however, of the western equatorial Pacific. Here, the situation is entirely different even though the South Equatorial Current retroflects at roughly the same latitude as its Atlantic counterpart, the North Brazil Current. Although the South Equatorial Current retroflection is flanked by two quasi-permanent eddies (the so-called Halmahera and the Mindanao eddies), these eddies are an integral part of the current itself and are not shed. Consequently, they are not associated with any meridional heat flux. An important question is, then, why the two oceans behave in such a fundamentally different way even though the source of the rings, the retroflected currents, are very similar in the two oceans.To answer this question, the two oceans are compared using recently developed analytical and numerical models for the western equatorial oceans. It is first pointed out that, according to recent developments in the modelling of the western equatorial Atlantic, the North Brazil Current retroflection rings are formed, shed and drift to the west because, in the Atlantic, this is the only way by which the momentum flux of the approaching and retroflecting current can be balanced. In this scenario, the northwestward flow force exerted by the approaching and retroflecting North Brazil Current (analogous to the force created by a rocket) is balanced by the southwestward force exerted by the rings as they are formed (analogous in some sense to the kickback associated with a firing gun).On the other hand, in the western equatorial Pacific, the formation and shedding of rings is unnecessary because the southward flowing Mindanao Current provides an alternative mechanism for balancing the northward momentum flux of the South Equatorial Current. This implies that it is the absence of a counter current (such as the Mindanao) in the western Atlantic that causes the formation and shedding of North Brazil Current rings. A remaining difficulty with the above scenario is that most colliding and retroflecting currents (i.e. the Mindanao and South Equatorial currents) are not “balanced” in the sense that they cannot be stationary but rather must drift along the coast. It is shown that, in the case of the western Pacific, the long-shore migration is arrested by the Indonesian Throughflow which allows the “unbalanced” fraction of the approaching currents to leak out into the Indian Ocean. This resolves the above difficulty and allows the retroflection to be approximately steady.     

Set-up of deep circulation in multi-level numerical models

The present study investigates the way an ocean filled with homogeneous warm water is cooled by prescribing cold water formation inside the ocean in the southern part of the southern hemisphere using multi-level numerical models. Cooling of the whole ocean starts with introduction of the cold water from the formation region into the deepest part of the ocean in the equatorial and eastern boundary regions by Kelvin wave-type density currents. The cold water along the eastern boundary extends westward as a Rossby wave-type density current setting up an interior poleward flow, and hits the western boundary to form a northward flowing boundary current in the northern hemisphere. Only then does the western boundary current cross the equator. Cooling of the rest of the ocean basin is accomplished by upwellings in the interior and also along the coasts. During this introduction the cold water is mixed with surrounding warm waters, and the thermocline, rather than forming just below the top level where heating is imposed, tends to spread down to deeper depths. Consequently the circulation at a steady state has a significant vertical structure such that the maximum upwelling in the interior occurs in the mid-depths, and only the deeper part of the deep ocean yields the Stommel and Arons circulation pattern. In the equatorial region higher vertical mode motions dominate, and a set of alternating zonal jets forms along the equator.     

Ocean Model Simulation of Southern Indian Ocean Surface Currents

The dynamic importance of the Southern Indian Ocean (SIO) lies in the fact that it connects the three major world oceans: the Pacific, Atlantic, and Indian Oceans. Modeling study has been used to understand the circulation pattern of this very important region. Simulation of SIO (10°N–60°S and 30°E–120°E) is performed with z-coordinate Ocean General Circulation Model (OGCM) viz; MOM3.0 and the results have been compared with observed ship drift data. It is found that except near coastal boundaries and in equatorial region, the simulated current reproduce most well known current pattern such as Antarctic Circumpolar Current (ACC), South Equatorial Current (SEC) etc. and bears a resemblance to that of the observed data; however the magnitude of the surface current is weaker in model than the observed data, which may be due to deficiency in the forcing field and boundary condition and problem with observed data. The annual mean wind stress curl computed over the oceanic domain reveals about ACC and its similar importance. The way in which the ocean responds to the windstress and vertically integrated transport using model output is fascinating and rather good.     

Ocean Model Simulation of Southern Indian Ocean Surface Currents

The dynamic importance of the Southern Indian Ocean (SIO) lies in the fact that it connects the three major world oceans: the Pacific, Atlantic, and Indian Oceans. Modeling study has been used to understand the circulation pattern of this very important region. Simulation of SIO (10°N-60°S and 30°E-120°E) is performed with z-coordinate Ocean General Circulation Model (OGCM) viz; MOM3.0 and the results have been compared with observed ship drift data. It is found that except near coastal boundaries and in equatorial region, the simulated current reproduce most well known current pattern such as Antarctic Circumpolar Current (ACC), South Equatorial Current (SEC) etc. and bears a resemblance to that of the observed data; however the magnitude of the surface current is weaker in model than the observed data, which may be due to deficiency in the forcing field and boundary condition and problem with observed data. The annual mean wind stress curl computed over the oceanic domain reveals about ACC and its similar importance. The way in which the ocean responds to the windstress and vertically integrated transport using model output is fascinating and rather good.     

A modification to the Munk wind-driven ocean circulation theory

In order to fulfill the no-slip condition at the western and eastern boundaries of the ocean basin, introduced ＂effective wind stress＂, which has much larger spatial variations towards the boundaries than in the ocean interior. The effective wind stress can thus be decomposed into spatially slow-varying and fast varying components. Careful scale analysis on the classical Munk winddriven ocean circulation theory, which consists of the interior Sverdrup flow and the western boundary current but of no eastern boundary current, shows that the wind stress curl appearing in the Sverdrup equation must have negligible spatial variations. In the present model the spatially slow-varying component of the wind stress appears in the Sverdrup equation, and the spatially fastvarying component becomes the forcing term of the boundary equations. As a result, in addition to the classical Munk solution the present model has an extra term at the western boundary which （Northern Hemisphere） increases the northward transport as well as the southward return transport, and has a term at the eastern boundary corresponding to the eastern boundary current.     

Numerical simulation of the upper ocean currents in South China Sea

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The effect of topography-enhanced diapycnal mixing on ocean and atmospheric circulation and marine biogeochemistry

The impact of topographically catalysed diapycnal mixing on ocean and atmospheric circulation as well as marine biogeochemistry is studied using an earth system model of intermediate complexity. The results of a model run in which diapycnal mixing depends on seafloor roughness are compared to a control run that uses a simple depth-dependent parametrization for vertical background diffusivity. A third model run is conducted that uses the horizontal mean of the topographically catalysed mixing as vertical profile in order to distinguish between the overall effect of larger diffusivities and the spatial heterogeneity of the novel mixing parametrization.The new mixing scheme results in a strengthening of the deep overturning cell and enhances equatorial upwelling. Surface temperatures in the Southern Ocean increase by about 1 K (in the overall effect) whereas cooling of a similar magnitude in low latitudes is generated by the spatial heterogeneity of the mixing. The corresponding changes in the atmospheric circulation involve a weakening of the southern hemispheric Westerlies and a strengthening of the Walker circulation. Biogeochemical changes are dominated by an improved ventilation of the deep ocean from the south. Water mass ages decline significantly in the deep Indian Ocean and the deep North Pacific whereas oxygen increases in the two ocean basins. The representation of the global volume of water with an oxygen concentration lower than 90 μmol/kg in the model is improved using the topography catalysed mixing. Furthermore, primary production is stimulated in equatorial regions through increased upwelling of nutrients and reduced in the oligotrophic gyres.