赤道太平洋近表层上升流的估计

利用1979-2006年间的卫星跟踪漂流浮标资料计算得到多年平均的赤道太平洋近表层流场,并估计赤道中东太平洋上混合层水平流场的散度和上升流流量.主要结论包括:1)赤道中东太平洋上升流区域主要集中在日界线以东以赤道为中心的±2°纬度带内,并且赤道南北约4°处各有一下沉流区域;2)赤道中东太平洋附近(165°E-85°W,2°S-2°N)散度平均约为2.0×10-7s-1,对应30m深处上升流区域整体体积输送约为43Sv,其中大约一半的上升流水体随热带流圈在赤道外4°N/S附近下沉,其余部分向两极输送;3)赤道中东太平洋辐散在春季达到最大值2.1×10-7s-1,而在秋季最弱;4)在El Ni(n)o期间辐散减弱,而在La Ni(n)a期间辐散增强,其中纬向流所致的辐合辐散也起到一定作用.     

南海主要上升流及其与渔场的关系

<正>南海是我国的重要渔业产区,目前我国每年在这一海区的捕捞产量大约3×106 t。无论在南海北部还是在南海中部和南部海域都分布有优良渔场,这些渔场往往与上升流存在着密切关系。上升流是一种海水垂直向上的运动现象,通常因表层水体辐散所致,是海洋环流中的重要组成部分。上升流涌升速度与水平流速相比甚小,一般只有10–4~10–2 cm/s[1]。上升流可以把底层营养盐带到表层,为浮游植物的生长提供物质基础,进而为浮游动物、鱼类、虾类等     

粤东陆架区夏季的上升流

本文根据水温、盐度和溶解氧含量等要素的历史资料分析了南海北部粤东陆架区夏季的上升流现象。结果表明,上升运动的深层高盐、低温冷水使该海区夏季中、下层及近岸水温下降,盐度上升,溶解氧含量相对减少。上升流中心位于广东汕头外海,中心附近上升流明显强化,本海域上升流存在空间上的不一致性和明显的年度间变化。     

ENSO发展期琼东上升流的动力过程响应差异

上升流是海洋中最重要的海洋现象之一,通过ROMS数值模型模拟并研究了2000—2013年间琼东上升流对ENSO信号(2002和2009年作为典型El Nio年; 2008和2010年作为典型La Nia年)的响应。结果表明,琼东上升流对ENSO气候事件有明显的响应。在El Nio信号较强时琼东上升流减弱,近岸海域水温升高;而在La Nia信号较强时琼东上升流加强,沿岸海域水温降低。对海面风场以及琼东海域沿岸流的分析表明,ENSO信号通过局地海面风场以及沿岸流对琼东上升流产生影响,并且风和沿岸流对琼东上升流的影响是协同的,在El Nio期间均不利于上升流的发展,而在La Nia期间二者的变化均有助于上升流的强化。     

流场散度的误差估计

按照误差传递原理,推导出流场散度的误差估计式,并将其应用于赤道中东太平洋上升流区域水平散度误差的估计。应用结果表明:赤道中东太平洋上升流区多年平均流场的散度稳定,平均散度约为3.0×10-7s-1;和通常采用的误差估计公式相比,该估计公式降低了散度误差,从而提高了散度的可信度。     

粤东及闽南近岸上升流对局地风场变化的响应

本文利用2010年6-7月的实测温盐、水位、海流等资料,结合风场数据,讨论了在台风影响较小的情况下,粤东及闽南近岸上升流对局地风场变化的响应特征,主要结论如下:（1）谱分析结果显示,沿岸风、水位、海流、近底层水温均具有3.5～4.0 d、5.0～5.5 d、8.3～9.0 d的波动周期,沿岸风的变化引起上升流强度在3～9 d周期上的波动;（2）上升流对局地风场变化的响应过程如下:利于上升流产生的局地风场发生变化时,沿岸风作用下产生的Ekman输运促使的上升流区水位的下降幅度发生改变,随即向岸方向的压强梯度力也发生变化,进而导致沿岸流及近底层向岸流的增强或减弱,而近底层向岸流强度的改变又会引起近底层水温的变化;（3）相关分析及交叉谱分析的结果表明,沿岸风的变化将在3 d以内影响上升流区近底层水温。以34 m向岸流代表近底层向岸流,则“沿岸风-水位-近底层向岸流-近底层水温”这一过程的响应时间依次为24 h、7 h、27 h左右。     

夏季南亚季风系统中期变动过程中的200hPa辐散环流

本文用合成资料讨论了夏季南亚季风系统中期变动过程中200hPa辐散环流的变化,揭示了一些有意义的新事实:(1)南亚季风区到太平洋信风区存在两个主要的辐散中心,它们分别位于菲律宾以东海面和西太平洋的加罗林群岛。前者存在明显的东西振荡过程,它和南亚季风环流的中期变动有密切的联系,后者位置稳定少变,但强度变化明显。(2)对澳大利亚高压起着主要作用的是源自加罗林群岛辐散中心的南支辐散气流,而不是源自南海的辐散流;在源自上述两个辐散中心的南支辐散气流的共同作用下,在澳大利亚以西海面上还维持一个独立的辐合下沉实体,其强弱变化有明显的准双周振荡过程。(3)在黄淮地区存在一个与东亚大陆季风雨带相对应的次级辐散中心,其向南的辐散气流与源自南海到西太平洋向北的辐散气流在东亚大陆的副热带地区汇合下沉,引导西太平洋副高伸入大陆;一旦这一次级中心消失,副高随即退出大陆。(4)当菲律宾以东海面的辐散中心移到中南半岛,则与加罗林群岛辐散中心之间的辐散气流在南海地区辐合下沉,这时该地区赤道反气旋活跃,表明其动力性质明显。     

夏季南亚季风系统中期变动过程中的200hPa辐散环流

本文用合成资料讨论了夏季南亚季风系统中期变动过程中200hPa辐散环流的变化,揭示了一些有意义的新事实:(1)南亚季风区到太平洋信风区存在两个主要的辐散中心,它们分别位于菲律宾以东海面和西太平洋的加罗林群岛.前者存在明显的东西振荡过程,它和南亚季风环流的中期变动有密切的联系,后者位置稳定少变,但强度变化明显.(2)对澳大利亚高压起着主要作用的是源自加罗林群岛辐散中心的南支辐散气流,而不是源自南海的辐散流;在源自上述两个辐散中心的南支辐散气流的共同作用下,在澳大利亚以西海面上还维持一个独立的辐合下沉实体,其强弱变化有明显的准双周振荡过程.(3)在黄淮地区存在一个与东亚大陆季风雨带相对应的次级辐散中心,其向南的辐散气流与源自南海到西太平洋向北的辐散气流在东亚大陆的副热带地区汇合下沉,引导西太平洋副高伸入大陆;一旦这一次级中心消失,副高随即退出大陆.(4)当菲律宾以东海面的辐散中心移到中南半岛,则与加罗林群岛辐散中心之间的辐散气流在南海地区辐合下沉,这时该地区赤道反气旋活跃,表明其动力性质明显.     

2006年夏季琼东、粤西沿岸上升流研究

利用2006年夏季广东、海南、广西近海的海洋水文调查资料和卫星遥感QuikSCAT风场资料分析琼东、粤西沿岸上升流的空间结构特征, 探讨风场、风应力旋度对上升流的影响以及上升流区水温、海流、海平面对上升流的响应。结果表明:琼东、粤西沿岸上升流区并非相互独立, 从10 m层以下已经连成一片。琼东沿岸上升流主要由夏季西南季风驱动而产生, 风应力旋度也有一定贡献。琼东沿岸上升流的强度比粤西强。琼东沿岸海域的上层海水(18 m以浅)以离岸运动为主, 中下层海水以向岸运动为主。上层的离岸流速大于中下层的向岸流速。琼东沿岸的上升流现象是间歇性的, 与沿岸风速强弱有关。琼东沿岸海域海平面的升降与上升流的强弱有良好的关系, 上升流的强弱滞后于海平面的升降约1～2 d。     

粤东至闽南沿岸海域夏季上升流的调查研究

利用2001、2002、2006和2009年夏季6个航次的温盐深(conductivity-temperature-depth,CTD)调查资料,揭示粤东至闽南沿岸海域上升流空间结构和强度的年际差异;并利用卫星遥感风场资料、海床基的海流和底层水温资料、广东南澳海洋站表层水温资料探讨海面风场、热带气旋对上升流的影响,上升流强度的时空特征,上升流的短期变化、生消过程及上升流的水体来源。结果表明,粤东至闽南沿岸海域上升流的范围和强度存在年际差异。上升流涌升至表层只出现在2009年7~8月航次。闽粤交界区沿岸海域,2006年7~8月航次,上升流范围较窄,且偏东北;2009年7~8月航次,范围较宽;2001年7~8月航次,范围偏西南。粤东沿岸海域,2006年7~8月航次上升流涌升高度较低,其他航次较高。研究海域2002年7~8月航次上升流强度较强,其他航次较弱。热带气旋使得上升流区海水垂向混合强烈,中下层冷水与表层暖水混合后迅速升温,上升流被破坏。粤东沿岸海域上升流强度强于闽南沿岸,出现时间也早于闽南沿岸。7月初至7月中旬,上升流开始形成,但不稳定;7月中旬至8月上旬,上升流处于强盛阶段;8月中旬至9月上旬,上升流减弱;9月上旬至9月中旬,上升流迅速消亡。粤东至闽南沿岸海域上升流的水体来源于粤东沿岸外海深层冷水。     

Upwelling front and two-cell circulation

A two-cell circulation associated with a front observed in coastal upwelling regions is studied numerically in a three-dimensional level model. An ocean with a flat bottom is forced by the wind stress with a longshore variation. Upwelling is induced in the region next to the coast. In association with the upwelling, the pycnocline slopes up toward the coast and intersects the sea surface forming a front. After that, downwelling is induced just inshore-side of the front and upwelling offshore-side. The transverse circulation in the present model seems to reproduce the observed two-cell circulation. It is found that the generation of the two-cell circulation is due to deviations of the longshore flow from the thermal-wind relation (geostrophy). The deviations are caused by the onshore-offshore movements of the front. Although no vorticity input through the wind stress is assumed, several barotropic vortices are induced by the effect of the inclination of the pycnocline and grow as long as the winds continue to blow. The observed poleward undercurrent may be interpreted as a combination of motions of the internal mode associated with the front and a barotropic flow associated with a cyclonic vortex.     

琼东上升流的年际变化及长期变化趋势

全球变暖背景下沿岸上升流的年际变化是近年来的研究热点。本文基于1982—2012年的海表面温度和风场资料,分析了琼东上升流的强度和中心位置的年际变化规律以及沿岸风应力及其旋度的作用。结果显示,近30年来,琼东上升流强度总体减弱,相比于沿岸风应力,其变化与减弱的局地风应力旋度相关性更高;琼东上升流强中心位置最大概率发生在19.2°—19.3°N,与最大风应力旋度位置接近,且存在北移趋势。琼东上升流强度和位置的年际变化还存在周期约3年、5年和10年的本征模态,以3年周期变化为主。局地风应力旋度在琼东上升流的年际变化中发挥了重要作用。     

Spatio‐temporal variability of zooplankton biomass and environmental control in the Northern Benguela Upwelling System: field investigations and model simulation

Spatial and temporal distribution patterns of zooplankton are highly variable in the Northern Benguela Upwelling System. We studied the distribution of zooplankton (size class ≥ 0.33 mm) and used field data from four cruises that took place between March 2008 and February 2011, as well as simulation results of a regional ecosystem model. Remotely sensed sea surface temperatures (SST) and surface chlorophyll concentrations were analysed to investigate environmental influences on zooplankton biomass. The Intense Benguela Upwelling Index showed a distinct seasonal signal throughout the years and the highest upwelling peaks in August/September. Even though surface chlorophyll concentrations were very variable throughout the year, the highest concentrations were always detected in September, following the upwelling of nutrient‐rich water. In field catches, zooplankton biomass concentration in the upper 200 m was highest above the outer shelf and shelf‐break in December 2010 and February 2011, i.e. 6 months after the upwelling peaks. In contrast, zooplankton biomass simulated by the model in the surface water was highest in September. In March/April, biomass maxima were typically measured in the field at intermediate water depths, but the vertical distribution was also affected by extensive oxygen minimum zones. The ecosystem model reproduced this vertical pattern. Although general trends were similar, simulation data of zooplankton standing stocks overestimated the field data by a factor of 3. In upwelling systems, food webs are generally considered to be short and dominated by large cells. However, our field data indicate more small‐sized zooplankton organisms above the shelf than offshore.     

Factors responsible for the differences in satellite-based chlorophyll a concentration between the major global upwelling areas

The aim of this study was to identify the factors responsible for the differences in chlorophyll a concentration (Chl-a) observed between the California, Canary, Humboldt and Benguela upwelling areas. Monthly climatologic values of Chl-a obtained from satellite images, covering the years 1998–2004, revealed that this pigment was higher in the Benguela system than in the other areas. Upwelling intensity, as derived from offshore Ekman transport computations, was higher in the Benguela and Humboldt regions and, for the same upwelling intensity, Chl-a was higher in Benguela than in the other regions. Upwelling intensity appears to be able to drive Chl-a densities through nutrient supply, as nutrients are correlated to offshore Ekman transport. A linear regression model including the fraction of sea surface over the shelf in each 1° × 1° box, nitrate, silicate, turbulence and variability of offshore Ekman transport explained the 84.8% of the variance in Chl-a among the areas. Differences in offshore Ekman transport explained the lower Chl-a observed in Canary and California and the higher Chl-a observed in Benguela and Peru-Humboldt. A narrow continental shelf and low water column stability also contribute to reducing phytoplankton pigment biomass in the Canary and California areas. The higher Chl-a values observed in Benguela compared to Humboldt-Peru are due to a wider extension of the continental shelf in the Benguela region.     

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。     

台湾东北海域夏季冷穹及上升流的数值模拟

夏季,黑潮在台湾东北向东海陆架的入侵表现为黑潮次表层水的强烈涌升,并在陆架上形成明显的冷穹。本研究利用ROMS(Regional Ocean Modeling System)模式,模拟了夏季黑潮入侵所形成的冷穹及上升流的三维结构,并讨论了上升流形成的动力机制。结果表明,冷穹中心在50 m以上的深度位于25.5°N,122.5°E附近,最大降温5 ℃以上;在50 m以下的深度,冷穹的中心位于台湾岛北缘。表层黑潮在台湾北缘不存在明显入侵,在陆坡东向转向附近则以气旋式环流入侵至陆架以上。此外,上升流主要位于陆坡坡度最大的区域,且黑潮次表层水的涌升存在两个较为明显的路径,分别位于台湾岛以北的100 m与200 m等深线之间以及东向转向的陆坡区域。在上层,平流作用是上升流产生的主要机制;而在近底层,平流作用与底摩擦都对上升流有贡献。     

Air–sea carbon dioxide fluxes in the coastal southeastern tropical Pacific

Comprehensive sea surface surveys of the partial pressure of carbon dioxide (pCO₂) have been made in the upwelling system of the coastal (0–200 km from shore) southeastern tropical Pacific since 2004. The shipboard data have been supplemented by mooring and drifter based observations. Air–sea flux estimates were made by combining satellite derived wind fields with the direct sea surface pCO₂ measurements. While there was considerable spatial heterogeneity, there was a significant flux of CO₂ from the ocean to the atmosphere during all survey periods in the region between 4° and 20° south latitude. During periods of strong upwelling the average flux out of the ocean exceeded 10 moles of CO₂ per square meter per year. During periods of weaker upwelling and high productivity the CO₂ evasion rate was near 2.5 mol/m²/yr. The average annual fluxes exceed 5 mol/m²/yr. These findings are in sharp contrast to results obtained in mid-latitude upwelling systems along the west coast of North America where the average air–sea CO₂ flux is low and can often be from the atmosphere into the ocean. In the Peruvian upwelling system there are several likely factors that contribute to sea surface pCO₂ levels that are well above those of the atmosphere in spite of elevated primary productivity: (1) the upwelling source waters contain little pre-formed nitrate and are affected by denitrification, (2) iron limitation of primary production enhanced by offshore upwelling driven by the curl of the wind stress and (3) rapid sea surface warming. The combined carbon, nutrient and oxygen dynamics of this region make it a candidate site for studies of global change.     

Wind-driven upwelling and surface chlorophyll blooms in Greater Cook Strait

We present the results of a combined observational and numerical study to investigate cool plumes of nutrient-rich upwelled water that emanate near the Kahurangi Shoals and extend into Greater Cook Strait. Surface temperature and chlorophyll are mapped using satellite observations to produce surface climatologies, to validate a numerical simulation and to show the utility of using spatial temperature differences as a measure of upwelling. We find upwelling near the Kahurangi Shoals is strongly wind-driven in the weather band. Upwelling occurs at all times of the year, but its surface signature is only visible in summer months. The upwelled nutrient-rich water supports increased primary production compared to surrounding waters, particularly in summer when the water column is more stratified and surrounding surface waters are presumably nutrient depleted.     

A study of cross-shore maximum upwelling intensity along the Northwest Africa coast

Satellite images of sea surface temperature (SST) show that the location of cross-shore SST minimum (LCSM) stretches along the isobaths in the Northwest Africa Upwelling System. To understand and interpret these observations better, we set up a two-dimensional analytical model that takes into account the surface and bottom Ekman transport and the alongshore geostrophic current, as well as bottom friction and variations in bottom topography. The structure of vertical velocity with a realistic topography clearly illustrates the variations of SST drop in a sample cross-shore section. Some idealized theoretical model experiments are carried out to examine the effects of eddy viscosity, Coriolis force, and cross-shore wind on the location of the cross-shore maximum upwelling intensity. The results show that the cross-shore wind largely impacts on the location where the coldest water outcrops to the surface through an adjustment of the cross-shore pressure gradient. This is also verified by the remotely sensed data, which indicate that the maximum correlation coefficient between cross-shore wind stress and the depth of LCSM is ?0.65 with a lag of approximately 1 day.