Quasi-isopycnic layer model for large-scale ocean circulation

A hydrothermodynamic model of a multilayer ocean, incorporating the upper mixed layer (UML) is described. The model is based on a system of primitive equations integrated within each layer. All layers are assumed to be horizontally-inhomogeneous, however, the density in each thermocline layer changes within the limits determineda priori by the prescribed basic stratification. It is assumed that the layers may outcrop. Results of the numerical experiment on subduction simulation (downwelling of UML waters toward the main thermocline layers) are given.Translated by V. Puchkin.     

Sea Level Variations Associated with Upwelling/ Downwelling Along the West Coast of India

A three-dimensional numerical model is developed and used to study the coastal upwelling processes and corresponding seasonal changes in the sea level along the west coast of India. The upwelling and associated sea level variations are seen as a response of coastal ocean to pure wind stress forcing. The model is designed to represent coastal ocean physics by resolving surface and bottom Ekman layers as realistically as possible. The prognostic variables are the three components of the velocity field, temperature, salinity and turbulent energy. The governing equations together with their boundary conditions are solved by finite-difference techniques. Experiments are performed to investigate sea level fluctuations associated with the thermal response and alongshore currents of the coastal waters. The model is forced with mean monthly wind stress forcing of January, May, July and September representing northeast monsoon and different phases of the southwest monsoon. It is known from the observational study that the upwelling process reaches to the surface waters by May along the coastal waters of the extreme southwest peninsular region. The process is more intense in July compared to May and September and its strength decreases from south to north. However, during the northeast monsoon season, which is represented by January wind stress forcing in the model, downwelling is simulated along the coast. The model simulations of the coastal response are compared with the observations and are found to be in good agreement. The maximum computed vertical velocity of about 2.0 ×10 -3 cm s -1 is predicted in July in the southern region off the coast.     

Sea Level Variations Associated with Upwelling/ Downwelling Along the West Coast of India

A three-dimensional numerical model is developed and used to study the coastal upwelling processes and corresponding seasonal changes in the sea level along the west coast of India. The upwelling and associated sea level variations are seen as a response of coastal ocean to pure wind stress forcing. The model is designed to represent coastal ocean physics by resolving surface and bottom Ekman layers as realistically as possible. The prognostic variables are the three components of the velocity field, temperature, salinity and turbulent energy. The governing equations together with their boundary conditions are solved by finite-difference techniques. Experiments are performed to investigate sea level fluctuations associated with the thermal response and alongshore currents of the coastal waters. The model is forced with mean monthly wind stress forcing of January, May, July and September representing northeast monsoon and different phases of the southwest monsoon. It is known from the observational study that the upwelling process reaches to the surface waters by May along the coastal waters of the extreme southwest peninsular region. The process is more intense in July compared to May and September and its strength decreases from south to north. However, during the northeast monsoon season, which is represented by January wind stress forcing in the model, downwelling is simulated along the coast. The model simulations of the coastal response are compared with the observations and are found to be in good agreement. The maximum computed vertical velocity of about 2.0 2 10 -3 cm s -1 is predicted in July in the southern region off the coast.     

利用Argo剖面浮标分析上层海洋对台风“布拉万”的响应

In situ observations from Argo profiling floats combined with satellite retrieved SST and rain rate are used to investigate an upper ocean response to Typhoon Bolaven from 20 through 29 August 2012. After the passage of Typhoon Bolaven, the deepening of mixed layer depth(MLD), and the cooling of mixed layer temperature(MLT) were observed. The changes in mixed layer salinity(MLS) showed an equivalent number of increasing and decreasing because the typhoon-induced salinity changes in the mixed layer were influenced by precipitation, evaporation, turbulent mixing and upwelling of thermocline water. The deepening of the MLD and the cooling of the MLT indicated a significant rightward bias, whereas the MLS was freshened to the left side of the typhoon track and increased on the other side. Intensive temperature and salinity profiles observed by Iridium floats make it possible to view response processes in the upper ocean after the passage of a typhoon. The cooling in the near-surface and the warming in the subsurface were observed by two Iridium floats located to the left side of the cyclonic track during the development stage of the storm, beyond the radius of maximum winds relative to the typhoon center. Water salinity increases at the base of the mixed layer and the top of the thermocline were the most obvious change observed by those two floats. On the right side of the track and near the typhoon center when the typhoon was intensified, the significant cooling from sea surface to a depth of 200×104 Pa, with the exception of the water at the top of the thermocline, was observed by the other Iridium float. Owing to the enhanced upwelling near the typhoon center, the water salinity in the near-surface increased noticeably. The heat pumping from the mixed layer into the thermocline induced by downwelling and the upwelling induced by the positive wind stress curl are the main causes for the different temperature and salinity variations on the different sides of the track. It seems that more time is required for the anomalies in the subsurface to be restored to pretyphoon conditions than for the anomalies in the mixed layer.     

Observed intra-seasonal to interannual variability of the upper ocean thermal structure in the southeastern Arabian Sea during 2002–2008

The southeastern Arabian Sea (SEAS), located in the Indian Ocean warm pool, is a key-region of the regional climate system. It is suspected to play an important role in the dynamics of the Asian summer monsoon system. The present study reports the salient features derived from a newly harvested observational dataset consisting of repeated fortnightly XBT transects in the SEAS over the period 2002–2008. The fortnightly resolution of such a multi-year record duration is unprecedented in this part of the world ocean and provides a unique opportunity to examine the observed variability of the near-surface thermal structure over a wide spectrum, from intra-seasonal to interannual timescales. We find that most of the variability is trapped in the thermocline, taking the form of upwelling and downwelling motions of the thermal stratification. The seasonal variations are consistent with past studies and confirm the role of the monsoonal wind forcing through linear baroclinic waves (coastally-trapped Kelvin and planetary Rossby waves). Sub-seasonal variability takes the form of anomalous events lasting a few weeks to a few months and occurs at two preferred timescales: in the 30–110 day band, within the frequency domain of the Madden–Julian oscillation and in the 120–180 day band. While this sub-seasonal variability appears fairly barotropic in the offshore region, the sign of the anomaly in the upper thermocline is opposite to that in its lower part on many occasions along the coast. Our dataset also reveals relatively large interannual temperature variations of about 1 °C from 50 to 200 m depth that reflect a considerable year-to-year variability of the magnitude of both upwelling and downwelling events. This study clearly demonstrates the necessity for sustained long-term temperature measurements in the SEAS.     

Ocean current and temperature structures in Algoa Bay and beyond in November 1986

Data on ocean temperature, currents, salinity and nutrients were obtained in an area off Algoa Bay on the south-east coast of South Africa during a ship's cruise in early November 1986. Satellite imagery provided information on the position of the Agulhas Current during the cruise period, while wind data were available from weather stations on the eastern and western sides of Algoa Bay. It is surmised that wind-forcing plays a major role in water circulation in the Bay and over the inshore continental shelf remote from the influence of the open ocean. The predominantly barotropic current flow, of the order of 0,5 m·s^?1, was downwind and influenced by topographic features and coastline shape. The Agulhas Current influences the ocean structures by long-term (large episodic meanders) and short-term (upwelling forced by the Current, core upwelling in frontal eddies and warm frontal plumes at the surface) fluctuations. Temperature structures showed well mixed water in Algoa Bay and a strong thermocline over the continental shelf, and were typical of a western boundary current in the Agulhas Current itself. The presence of a thermocline at 30–50 m over the shelf prevented upward mixing of nutrients. The Current exerted a dominant effect on shelf waters north of Algoa Bay.     

东海沿岸海区垂直环流及其温盐结构动力过程研究I．环流的基本特征

利用三维斜压流体动力学模型 ,通过对东海沿岸海区冬、夏季的斜压环流及其温盐结构的数值研究 ,揭示研究海区垂直环流及其温盐结构的动力过程及其成因。垂直环流的模拟结果表明 :冬季 ,沿岸海区的垂直环流以逆时针流动 ,近表层为向岸流 ,沿岸为下降流 ,近表层以下为离岸流 ,其在外海有明显的上升趋势 ,沿岸下降流自表层至底层逐渐由强变弱 ;夏季 ,沿岸海区的垂直环流以顺时针流动 ,近表层以下为向岸流 ,沿岸为上升流 ,近表层为离岸流 ,其在外海有明显的下降趋势 ,沿岸上升流自底层至表层逐渐由弱变强。就整个沿岸海区而论 ,冬季沿岸下降流和夏季沿岸上升流的强度都随着岸界地形坡度、风速及风向与岸线偏角的变化而变化。沿岸下降流形成的主要原因是由于冬季东北风与岸界地形的耦合效应及海区温盐分布不均匀所致 ,而沿岸上升流形成的主要原因则是由于夏季西南风与岸界地形的耦合效应及海区温盐分布不均匀所致。     

Features of coastal upwelling regions that determine net air-sea CO<Subscript>2</Subscript> flux

The influence of the coastal ocean on global net annual air-sea CO₂ fluxes remains uncertain. However, it is well known that air-sea pCO₂ disequilibria can be large (ocean pCO₂ ranging from ∼400 μatm above atmospheric saturation to ∼250 μatm below) in eastern boundary currents, and it has been hypothesized that these regions may be an appreciable net carbon sink. In addition it has been shown that the high productivity in these regions (responsible for the exceptionally low surface pCO₂) can cause nutrients and inorganic carbon to become more concentrated in the lower layer of the water column over the shelf relative to adjacent open ocean waters of the same density. This paper explores the potential role of the winter season in determining the net annual CO₂ flux in temperate zone eastern boundary currents, using the results from a box model. The model is parameterized and forced to represent the northernmost part of the upwelling region on the North American Pacific coast. Model results are compared to the few summer data that exist in that region. The model is also used to determine the effect that upwelling and downwelling strength have on the net annual CO₂ flux. Results show that downwelling may play an important role in limiting the amount of CO₂ outgassing that occurs during winter. Finally data from three distinct regions on the Pacific coast are compared to highlight the importance of upwelling and downwelling strength in determining carbon fluxes in eastern boundary currents and to suggest that other features, such as shelf width, are likely to be important.     

Trace metal (Cd,Cu, Ni and Pb) cycling in the upper water column near the shelf edge of the European continental margin (Celtic Sea)

This study investigates the relative importance of processes that affect trace metal (TM) cycling in the upper water column at the shelf edge of the Celtic Sea on the western European continental margin. The examined processes include external inputs (by atmosphere and river), physical factors (upwelling, winter mixing and water mass advection) and biological processes (in situ uptake, regeneration and export to deep waters). The concentrations of dissolved Cd, Cu, Ni and Pb were measured with this aim in January 1994 and June 1995 at vertical stations across slope, including stations with upwelling, and in the surface waters along the Celtic Sea shelf. Additionally, deep sea (from sediment trap data) and atmospheric fluxes were estimated. The metal profiles over the slope off the Celtic Sea are quite similar to open ocean profiles already described in the northeast Atlantic, and the concentrations in surface waters are only slightly enriched compared to the nearby open ocean (1.2–1.3× for Cd and Ni). The external sources to the system appear to be of weak influence: the fluvial input is locally strong at the coast and then “diluted” along the large continental shelf; the atmospheric deposition is not significant at the annual scale in comparison to the metal content in the upper waters of the shelf edge (at least for Cd, Ni and Cu). In the upwelling zone, a significant increase in concentrations was observed in the summer surface mixed layer (×2 for nitrate and Cd and ×1.5 for Ni) in comparison to the non-upwelling zone. In winter, concentrations of bioreactive metals increased significantly in the surface waters in comparison to the low summer levels (×5 for nitrate and Cd). Our results suggest that upwelling and winter mixing act as regenerated sources that lead to the resupply of the bioreactive elements above the permanent thermocline with a low export to deeper waters. The tracing of the Mediterranean intermediate waters (MIW) from Gibraltar to the studied area shows indeed that its elemental content at the Celtic shelf edge is mainly due to the conservative mixing of the three “end-member” component waters which are thought to make up the MIW. The remineralization of organic matter within this water mass during its transport to the north would contribute only 20% of the nutrients and Cd concentrations recorded at the Celtic Sea shelf edge. According to the correlation found with nutrients in the 10–200-m layer, dissolved Pb would also be subjected to biological uptake and regeneration within the seasonal thermocline. Particulate scavenging removal of Pb would take place below the permanent thermocline throughout the water column.     

Sea level response to wind field fluctuation around the tip of the Izu Peninsula

The mechanism of a characteristic sea level response (barotropic coastal ocean response) to wind field fluctuation around the tip of the Izu Peninsula observed during the middle of December 2000 to the middle of January 2001 was investigated based on three types of numerical experiments using the Princeton Ocean Model with various parameters. The response was characterized by the relaxation of sea level falling (rising) during eastward upwelling (westward downwelling) favorable wind regime. Analyses of quasi-realistic numerical model results in terms of the vertically integrated momentum balances and vorticity balance for the barotropic mode revealed that: 1) development/abatement of two anomalous circulations generated around the tip of the Izu Peninsula controls the sea level response through the acceleration/deceleration of a quasi-geostrophic barotropic coastal current between the circulations; 2) nonlinear vorticity advection by the Kuroshio Current and by the coastal current, coupled with vorticity diffusion, decelerates the quasi-geostrophic coastal current in the latter half of the wind regimes, which induces the relaxation of sea level rise/fall. The results of the quasi-realistic numerical experiment suggest that an analysis of the vorticity balance for the barotropic mode contributes to a better understanding of sea level responses to wind in coastal regions with strong currents and complex topography. In addition, a numerical experiment with idealized spatially uniform density stratification and a quasi-realistic wind field shows that if the Kuroshio Current had been shifted far offshore from the Izu Peninsula during the observation period, westward propagating continental shelf waves would have controlled the coastal sea level response.     

Seasonal cooling and blooming in tropical oceans

The relative importance of tropical pelagic algal blooms in not yet fully appreciated and the way they are induced not well understood. The tropical Atlantic supports pelagic blooms together equivalent to the North Atlantic spring bloom. These blooms are driven by thermocline tilting, curl of wind stress and eddy upwelling as the ocean responds to intensified basin-scale winds in boreal summer. The dimensions of the Pacific Ocean are such that seasonal thermocline tilting does not occur, and nutrient conditions are such that tilting might not induce bloom, in any case. Divergence at the equator is a separate process that strengthens the Atlantic bloom, is more prominent in the eastern Pacific, and in the Indian Ocean induces a bloom only in the western part of the ocean. Where western jet currents are retroflected from the coast off Somalia and Brazil, eddy upwelling induces prominent blooms. In the eastward flow of the northern equatorial countercurrents, positive wind curl stress induces Ekman pumping and the induction of algal blooms aligned with the currents. Some apparent algal bloom, such as that seen frequently in CZCS images westwards from Senegal, must be due to interference from airborne dust.     

A quasi-isopycnic model of the eastern boundary layer in the ocean

This paper considers the response of the eastern boundary layer and restructuring of the upwelling and downwelling regimes due to the periodic changing of the alongshore wind direction using the data from a numerical experiment conducted in the frame of a multi-layer non-linear model of the ocean, based on hydrothermodynamic primitive equations.Translated by Vladimir A. Puchkin.     

A Holistic Approach to Upwelling and Downwelling along the South-West Coast of India

An attempt has been made to develop a holistic understanding of upwelling and downwelling along the south-west coast of India. The main objective was to elucidate the roles of different forcings involved in the vertical motion along this coast. The south-west coast of India was characterized by upwelling during the south-west monsoon (May to September) and by downwelling during the north-east monsoon and winter (November to February). The average vertical velocity calculated along the south-west coast from the vertical shift of the 26?°C isotherm is 0.57?m/day during upwelling and 0.698?m/day during downwelling. It was concluded that upwelling along the south-west coast of India is driven by offshore Ekman transport due to the alongshore wind, Ekman pumping, horizontal divergence of currents and by the propagation of coastally trapped waves. Whereas downwelling along the coast is driven only by convergence of currents and the propagation of coastally trapped Kelvin waves. Along the west coast of India, the downwelling-favorable Kelvin waves come from the equator and upwelling-favorable waves come from the Gulf of Mannar region.     

南海浅层经向翻转环流及相关的内部水体流动

The structure of the annual-mean shallow meridional overturning circulation(SMOC) in the South China Sea(SCS) and the related water movement are investigated,using simple ocean data assimilation(SODA) outputs.The distinct clockwise SMOC is present above 400 m in the SCS on the climatologically annual-mean scale,which consists of downwelling in the northern SCS,a southward subsurface branch supplying upwelling at around 10°N and a northward surface flow,with a strength of about 1×10~6 m~3/s.The formation mechanisms of its branches are studied separately.The zonal component of the annual-mean wind stress is predominantly westward and causes northward Ekman transport above 50 m.The annual-mean Ekman transport across 18°N is about 1.2×10~6 m~3/s.An annual-mean subduction rate is calculated by estimating the net volume flux entering the thermocline from the mixed layer in a Lagrangian framework.An annual subduction rate of about 0.66×10~6m~3/s is obtained between 17° and 20°N,of which 87% is due to vertical pumping and 13% is due to lateral induction.The subduction rate implies that the subdution contributes significantly to the downwelling branch.The pathways of traced parcels released at the base of the February mixed layer show that after subduction water moves southward to as far as 11°N within the western boundary current before returning northward.The velocity field at the base of mixed layer and a meridional velocity section in winter also confirm that the southward flow in the subsurface layer is mainly by strong western boundary currents.Significant upwelling mainly occurs off the Vietnam coast in the southern SCS.An upper bound for the annual-mean net upwelling rate between 10° and 15°N is 0.7×10~6m~3/s,of which a large portion is contributed by summer upwelling,with both the alongshore component of the southwest wind and its offshore increase causing great upwelling.     

A hydrothermodynamic quasi-isopycnic model of the ocean thermocline

The results of a numerical experiment on the formation of the large-scale circulation and the main thermocline in the temperate latitudes of an ocean at steady external factors are discussed. A multilayer model [1] is used which involves the upper mixed layer (UML). The boundaries of the areas where the internal layers reach the UML are calculated. The zones where seawater is entrapped from the thermocline to the UML are revealed, as well as the subduction zones where seawater is brought from the UML to the internal layers and moves there, ventilating the thermocline.Translated by Mikhail M. Trufanov. UDK 551.465.     

Settlement patterns,size and growth of puerulus and juvenile rock lobster Jasus lalandii at an oyster farm in Saldanha Bay,South Africa

Trends in the abundance, size and growth of newly settled pueruli and early juveniles of the rock lobster Jasus lalandii were determined from samples obtained from mesh bags used for oyster mariculture in Saldanha Bay, South Africa. Totals of 3 842 lobster pueruli and 10 158 juveniles were captured between August 2005 and June 2006, and distinct puerulus settlement events were identified in November and January. Juveniles also peaked during these months as well as in March. The size of newly settled pueruli increased significantly between October and March. The carapace length (CL) of pueruli and juveniles overlapped between 8.5 mm (smallest juvenile) and 10.4 mm (largest puerulus), and 50% of pueruli moulted into juveniles by 9.5–9.6 mm CL. There was a size progression in juveniles, with larger individuals in February and March reflecting an accumulation of juveniles from the earlier settlement events. The estimated growth increment from puerulus to first juvenile instar was 0.5–1.5 mm (4.9–16.1%), increasing to 1.4–2.5 mm (14.4–23.3%) for the second instar. Recruitment of J. lalandii pueruli occurred during periods when strong southerly winds prevailed, which results in a general offshore movement of surface waters along the West Coast and upwelling of cold nutrient-rich bottom waters. However, specific settlement events appeared to coincide with a change in wind direction, reduction in upwelling strength and increase in surface water temperatures, implying that larvae may take advantage of landward movements of surface waters during downwelling, or reversal of upwelling, to return them to shallow coastal waters.     

The effect of water transport on nitrogen flow through a kelp-bed community

A simulation model is used to investigate possible ecological effects of up- and downwelling on nitrogen flows through a kelp-bed food web off the Cape Peninsula, South Africa. The model depicts the flow of nitrogen, which is often limiting in marine ecosystems, from kelps, other macrophytes and phytoplankton, through filter-feeders to carnivores, with a feedback loop via faeces and bacteria to detritus and filter-feeders. When modelled as a closed system, bacteria associated with detritus and animal faeces form a large component of the particulate nitrogen available to filter-feeders, and the faeces feedback loop dominates nitrogen flow. When measured rates of water transport are incorporated into the model, bacteria have little opportunity to accumulate before being transported out of the system. Animal faeces and kelp detritus are the dominant filter-feeder food components under upwelling conditions, whereas phytoplankton is the major contributor to particulate organic nitrogen under downwelling conditions. When realistic pulses of upwelling/downwelling derived from wind indices are used as model input, filter-feeders are shown to decline during the summer upwelling season when much potential food is advected out of the system, and they increase during the winter when downwelling conditions are more prevalent, bringing in nitrogen-rich phytoplankton from the blooms developing offshore.     

台风“布拉万”过境黄海时的异常增温事件研究

海洋对台风的响应通常表现为海表温度的降低,然而,出现在2012年8月的台风"布拉万"在经过黄海时却引起朝鲜半岛木浦沿岸海域海表的增温(而非降温),且增温幅度达4.2°C。本研究详细分析了此次异常事件的时空特征,并探讨了其可能的成因。结果发现,此次事件的产生和黄海表层冷水斑块(Surface Cold Patch,SCP)存在密切关系,并恰好出现在木浦SCP所在位置。上升流和潮混合是木浦SCP的两大形成机制,此次增温事件主要是台风"布拉万"通过抑制其生成机制之一的上升流而导致降温不足之故。具体而言,台风"布拉万"过境时位于木浦SCP的左侧,其上的北向风应力带来向岸的Ekman输运,造成外围暖水在木浦SCP地区堆积,从而抑制了该地区原本的上升流(甚至变为下降流)。     

2009/2010年El Ni(n)o事件变化特征及其机理

应用TAO (Tropical Atmosphere Ocean project)热带太平洋实测海温和风场资料,分析研究了发生在2009/2010年的El Ni(n)o事件的变化特征,讨论了此次El Ni(n)o事件发生过程中,赤道东、西太平洋次表层异常海温的变化特征及其传播过程,特别是对赤道太平洋次表层异常海温变化的...     

Response of subsurface waters in the eastern Arabian Sea to tropical cyclones

Thermister chain data at different depths for June 1998 cyclone in the Arabian Sea at a location (69.2 E,15.5 N) which is about 60 km to the left of the cyclone track indicates subsurface warming below 60 m and inertial oscillations of temperature with a periodicity of about 2 days. The oscillations continued for ∼15 days even after the cyclone crossed the coast. The analysis of the buoy, DS1 located at the same position also suggests a stabilized southward flow after about two weeks of the cyclone crossed the coast. Analysis of the buoy data for May 1999 cyclone in the same region also indicates similar pattern. In order to investigate the effect of cyclone–ocean interaction and primarily to understand the process for the subsurface warming, 3-dimensional Princeton Ocean Model is configured for the eastern part of the Arabian Sea. The model uses high horizontal resolution of about 6 km near the coast and a terrain following sigma coordinate in the vertical with 26 levels. The study focuses on surface cooling and temperature rise in the underlying waters and explains its mechanism through upwelling and downwelling respectively. The simulations in concurrence with the observations suggest that the occurrence of subsurface warming precedes the surface cooling with a lag of ˜a day as the cyclone advances DS1. The simulations also demonstrate local temperature stratification plays an important role for cooling of the upper ocean and warming of the subsurface waters and extent of warming is directly related to the depth of the thermocline.