北太平洋经向翻转环流和热盐输送研究综述

北太平洋经向翻转环流是北太平洋所有经向翻转环流圈的总称,目前它拥有五个环流圈,即副热带环流圈(the subtropical cell,STC)、热带环流圈(the tropical cell,TC)、副极地环流圈(the subpolar cell,SPC)、深层热带环流圈(the deep tropical cell,DTC)和温跃层环流圈(the thermohaline cell,THC)。这些环流圈是北太平洋经向物质和能量交换的重要通道,它们的变化对海洋上层热盐结构和气候变化皆有重要影响。迄今,人们已对STC、TC和DTC的结构形态、变化特征与机理开展了广泛而深入的研究,并对STC的极向热输送特征也做了一些初步分析。但应指出的是,关于SPC和THC的研究仍较少,迄今尚不清楚这两个环流圈的三维结构和变异机理;而且,对北太平洋经向翻转环流的热盐输送研究尚处于起步阶段,目前对各环流圈的热盐输送特征、变化规律和变异机理仍知之甚少,这些科学问题亟待深入研究。     

北太平洋Hadley 环流纬向结构的季节和年际变化

为了描述北太平洋上空Hadley 环流的纬向结构特征, 利用NCEP 再分析资料(1979～2010 年), 研究了北太平洋上空Hadley 环流纬向结构的季节和年际变化。发现在西太平洋, Hadley 环流季节性上升支呈西北-东南倾斜, 其垂向核心位于对流层中层, 纬向核心在北半球冬季(夏季)位于日界线附近(150°E); 而永久性上升支主要在东太平洋, 其垂向核心位于对流层低层, 且沿经度东移逐渐增强。根据纬向环流结构特征, 北半球冬季环流形态分为3 个区域: 160°E 以西, 主要表现为低层辐合高层辐散;160°E～130°W, 主要表现为高层辐合; 130°W 以东, 表现为低层辐合高层辐散特征。相似地, 北半球夏季环流形态也可沿纬向分为如下3 个区域: 165°E 以西、165°E～165°W 和165°W 以东, 分别对应东亚夏季风主导经圈环流区、过渡区、Hadley 环流主导经圈环流区。在年际变化上, 北太平洋Hadley 环流与ENSO 有很强的相关, 这与前人的研究是一致的。因此北太平洋上空Hadley 环流具有显著的空间性态, 并且对应时间尺度不同, 影响其变化的主要因素也不尽相同。     

引潮力对海洋环流模式的影响

The eight main tidal constituents have been implemented in the global ocean general circulation model with approximate 1° horizontal resolution.Compared with the observation data,the patterns of the tidal amplitudes and phases had been simulated fairly well.The responses of mean circulation,temperature and salinity are further investigated in the global sense.When implementing the tidal forcing,wind-driven circulations are reduced,especially those in coastal regions.It is also found that the upper cell transport of the Atlantic meridional overturning circulation(AMOC) reduces significantly,while its deep cell transport is slightly enhanced from 9×106m3/s to 10×106 m3/s.The changes of circulations are all related to the increase of a bottom friction and a vertical viscosity due to the tidal forcing.The temperature and salinity of the model are also significantly affected by the tidal forcing through the enhanced bottom friction,mixing and the changes in mean circulation.The largest changes occur in the coastal regions,where the water is cooled and freshened.In the open ocean,the changes are divided into three layers:cooled and freshened on the surface and below 3 000 m,and warmed and salted in the middle in the open ocean.In the upper two layers,the changes are mainly caused by the enhanced mixing,as warm and salty water sinks and cold and fresh water rises;whereas in the deep layer,the enhancement of the deep overturning circulation accounts for the cold and fresh changes in the deep ocean.     

北极径流增加对大西洋经向翻转环流影响的模拟研究

观测显示过去几十年间北极入海径流呈现增加趋势,CMIP5耦合模式预测表明21世纪北极入海径流仍会增加,在RCP8.5路径下,21世纪末北极入海径流量将会是1950年的1.4倍。本文利用冰-海耦合数值模式研究了北极径流增加对大西洋经向翻转环流的影响。基于两个数值实验的结果表明,如果北极入海径流按每年0.22%的速度（与RCP8.5路径下的速度相当）增加,大西洋经向翻转环流的强度在100、150和200年后将会分别减弱0.6（3%）、1.2（7%）和1.8（11%） Sv。北极入海径流增加导致大西洋经向翻转环流减弱的主要原因是,北极入海径流增加的淡水被输运到北大西洋后,会抑制北大西洋深层水的生成,这也会导致北大西洋深层水海水年龄的增加。     

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.     

大西洋经向翻转环流对岁差响应的气候背景依赖性

大西洋经向翻转环流（Atlantic Meridional Overturning Circulation,AMOC）是气候系统重要的组成部分,其强度变化可直接影响南北半球的热量分配,厘清其变化机理对全球变暖背景下的未来预估至关重要。海洋沉积物记录发现,在晚更新世,AMOC的变化与地球岁差周期有紧密联系,但其物理机理尚不清楚。本文利用海洋−大气耦合气候模型—COSMOS（ECHAM5/JSBACH/MPIOM）模型,通过敏感试验,分析在冰盛期冷期和间冰期暖期气候背景下,AMOC对地球岁差变化的响应机理。结果表明：岁差降低引起的北半球夏季太阳辐射增强,会导致间冰期暖期背景下的AMOC显著减弱,但对冰盛期AMOC的影响并不明显。通过进一步分析发现,在间冰期暖期,夏季太阳辐射增强,造成高低纬大西洋海表的升温,同时促进北大西洋高纬度地区的局地降水,两者导致北大西洋表层海水密度降低,共同削弱大西洋深层水生成。而在冰盛期冷期,大西洋高低纬度地区的响应对AMOC的影响反向—副热带升温触发的海盆尺度低压异常,通过其南侧的西风异常削弱大西洋向太平洋的水汽输送,导致净降水增多,海表盐度下降;同时,高纬度升温造成的海冰减少,促进了海洋热丧失,海表失热变重,有利于大西洋深层水的生成,最终两者的共同作用导致AMOC对岁差变化的响应偏弱。本文系统揭示了不同气候背景下,岁差尺度AMOC变化的控制机理,对理解晚更新世AMOC重建记录中持续存在的岁差周期具有重要启示意义。     

西北太平洋137°E断面海流的纬向体积输送

利用日本气象厅1967－1989年间沿137°E断面观测到的水文和海流资料，计算了该断面上1°S－34°N的纬向体积输送。纬向体积输送的明显特点是复强冬弱，无论是多年平均的还是个别年份的，不管是东向输送分量还是西向输送分量，该特征都是非常明显的。净输送量有非常大的年际变化，70年代以向西输送为主，80年代则以向东输送为主，峰值出现在ElNio事件前后，二者有一定的关系。     

冬、夏季印度洋越赤道经向翻转环流的年际变化

利用50 a的SODA资料对1月(冬季)和7月(夏季)印度洋越赤道经向翻转环流的年际变化进行研究。通过对2类典型年份的合成分析指出:1月份正异常年对应的经向翻转环流偏强,向北的经向热输送增加;7月份正异常年对应的经向翻转环流则偏弱,向南的经向热输送减少;1月份和7月份的负异常年皆与其正异常年相反;越赤道经向翻转环流有明显的年际变化,平均周期在4 a左右;经向翻转环流的年际变化和海面风场的变化密切相关。提出了反映1月和7月此环流年际变化的几个指数。     

印度洋上层经向翻转环流的冬夏季节对比

上层经向翻转环流(shallow meridional overturning circulation, SMOC)主导热带-副热带上层海洋水体交换,对海洋物质输运和热量交换具有重要意义。基于7套海洋再分析数据产品,本文主要探讨了印度洋SMOC的冬夏季节变化及其差异的原因。结果显示,印度洋SMOC主要由南半球副热带环流圈(southern subtropical cell, SSTC)和跨赤道环流(cross-equatorial cell, CEC)组成,并且具有显著的季节差异。夏季风期间, SSTC和CEC均为表层南向输运,表层以下北向输运的逆时针环流结构。冬季风盛行时, SSTC仍维持逆时针结构,但环流中心南移且深度加深,强度弱于夏季;然而, CEC却转向为表层北向输运,表层以下向南输运的顺时针环流结构,其环流中心位置与夏季接近,环流强度与夏季相当。这种印度洋SMOC冬夏结构差异究其原因主要由风生环流主导, CEC冬夏季节环流方向反转是北印度洋冬夏季风转向的结果,而南印度洋信风的季节性位移和强度变化是SSTC强度和位置季节差异的主要原因。     

The three-dimensional structure and seasonal variation of the North Pacific meridional overturning circulation

The three-dimensional structure and the seasonal variation of the North Pacific meridional overturning circulation (NPMOC) are analyzed based on the Simple Ocean Data Assimilation data and Argo profiling float data.The NPMOC displays a multi-cell structure with four cells in the North Pacific altogether.The TC and the STC are a strong clockwise meridional cell in the low latitude ocean and a weaker clockwise meridional cell between 7°N and 18°N,respectively, while the DTC and the subpolar cell are a weaker ...     

南海上层经向翻转环流的低频变化

The low-frequency variability of the shallow meridional overturning circulation(MOC) in the South China Sea(SCS) is investigated using a Simple Ocean Data Assimilation(SODA) product for the period of 1900–2010. A dynamical decomposition method is used in which the MOC is decomposed into the Ekman, external mode, and vertical shear components. Results show that all the three dynamical components contribute to the formation of the seasonal and annual mean shallow MOC in the SCS. The shallow MOC in the SCS consists of two cells: a clockwise cell in the south and an anticlockwise cell in the north; the former is controlled by the Ekman flow and the latter is dominated by the external barotropic flow, with the contribution of the vertical shear being to reduce the magnitude of both cells. In addition, the strength of the MOC in the south is found to have a falling trend over the past century, due mainly to a weakening of the Luzon Strait transport(LST) that reduces the transport of the external component. Further analysis suggests that the weakening of the LST is closely related to a weakening of the westerly wind anomalies over the equatorial Pacific, which leads to a southward shift of the North Equatorial Current(NEC) bifurcation and thus a stronger transport of the Kuroshio east of Luzon.     

Numerical study of the meridional overturning circulation with “mixing hotspots” in the Pacific Ocean

Using an idealized ocean general circulation model, we examine the effect of “mixing hotspots” (localized regions of intense diapycnal mixing) predicted based on internal wave-wave interaction theory (Hibiya et al., 2006) on the meridional overturning circulation of the Pacific Ocean. Although the assumed diapycnal diffusivity in the mixing hotspots is a little larger than the predicted value, the upwelling in the mixing hotspots is not sufficient to balance the deep-water production; out of 17 Sv of the downwelled water along the southern boundary, only 9.2 Sv is found to upwell in the mixing hotspots. The imbalance as much as 7.8 Sv is compensated by entrainment into the surface mixed layer in the vicinity of the downwelling region. As a result, the northward transport of the deep water crossing the equator is limited to 5.5 Sv, much less than estimated from previous current meter moorings and hydrographic surveys. One plausible explanation for this is that the magnitude of the meridional overturning circulation of the Pacific Ocean has been overestimated by these observations. We raise doubts about the validity of the previous ocean general circulation models where diapycnal diffusivity is assigned ad hoc to attain the current magnitude suggested from current meter moorings and hydrographic surveys.     

Recent changes in the North Atlantic circulation simulated with eddy-permitting and eddy-resolving ocean models

Three eddy-permitting (1/4°) versions and one eddy-resolving (1/12°) version of the OCCAM ocean model are used to simulate the World Ocean circulation since 1985. The first eddy-permitting simulation has been used extensively in previous studies, and provides a point of reference. A second, improved, eddy-permitting simulation is forced in the same manner as the eddy-resolving simulation, with a dataset based on a blend of NCEP re-analysis and satellite data. The third eddy-permitting simulation is forced with a different dataset, based on the ERA-40 re-analysis data. Inter-comparison of these simulations in the North Atlantic clarifies the relative importance of resolution and choice of forcing dataset, for simulating the mean state and recent variability of the basin-scale circulation in that region. Differences between the first and second eddy-permitting simulations additionally reveal an erroneous influence of sea ice on surface salinity, dense water formation, and the meridional overturning circulation. Simulations are further evaluated in terms of long-term mean ocean heat transport at selected latitudes (for which hydrographic estimates are available) and sea surface temperature errors (relative to observations). By these criteria, closest agreement with observations is obtained for the eddy-resolving simulation. In this simulation, there is also a weak decadal variation in mid-latitudes, with heat transport strongest, by around 0.2 PW, in the mid-1990s. In two of the eddy-permitting simulations, by contrast, heat transport weakens through the study period, by up to 0.4 PW in mid-latitudes. The most notable changes of heat transport in all simulations are linked to a weakening of the subpolar gyre, rather than changes in the meridional overturning circulation. It is concluded that recent changes in the structure of mid-latitude heat transport in the North Atlantic are more accurately represented if eddies are explicitly resolved.     

The seasonal variability of an air-sea heat flux in the northern South China Sea

The seasonal variabilities of a latent-heat flux (LHF), a sensible-heat flux (SHF) and net surface heat flux are examined in the northern South China Sea (NSCS), including their spatial characteristics, using the in situ data collected by ship from 2006 to 2007. The spatial distribution of LHF in the NSCS is mostly controlled by wind in summer and autumn owing to the lower vertical gradient of air humidity, but is influenced by both wind and near-surface air humidity vertical gradient in spring and winter. The largest area-averaged LHF is in autumn, with the value of 197.25 W/m 2 , followed by that in winter; the third and the forth are in summer and spring, respectively. The net heat flux is positive in spring and summer, so the NSCS absorbs heat; and the solar shortwave radiation plays the most important role in the surface heat budget. In autumn and winter, the net heat flux is negative in most of the observation region, so the NSCS loses heat; and the LHF plays the most important role in the surface heat budget. The net heating is mainly a result of the offsetting between heating due to the shortwave radiation and cooling due to the LHF and the upward (outgoing) long wave radiation, since the role of SHF is negligible. The ratio of the magnitudes of the three terms (shortwave radiation to LHF to long-wave radiation) averaged over the entire year is roughly 3:2:1, and the role of SHF is the smallest.     

春季青藏高原感热通量对不同海区海温强迫的响应及其对我国东部降水的影响

本文利用在青藏高原适用性较好的ERA-interim地表感热通量资料,研究了1981-2010年青藏高原春季地表感热通量的年际变率与前期不同海区海温强迫的联系,以及这种联系对我国东部降水可能造成的影响。结果表明,春季青藏高原地表感热通量的年际变化有两个主要的模态,分别与前期太平洋以及印度洋海温有密切联系。与冬季ENSO事件相应的赤道中东太平洋海温强迫可以激发一个向极向西的波列,通过改变青藏高原南侧的环流和降水异常,形成一个纬向偶极型分布的高原感热第一模态,其对应的时间序列主要表现为准5 a的振荡,与ENSO事件的周期较为吻合;而春季印度洋的三极型海温分布可以强迫出一个跨越南北半球的波列,使青藏高原主体表现为东风异常,减弱背景西风,从而形成一个青藏高原主体与周围反相关的回字形感热第二模态,其主要呈现5~7 a的振荡周期。ENSO事件以及印度洋海温分布分别与青藏高原春季感热两个主模态相联系,并且冬春季海温与高原春季感热主模态对我国东部春季降水有协同影响,对于我国北方降水异常而言,高原的贡献相对海洋更重要。     

北冰洋上层双扩散阶梯热通量的分析

北冰洋中,低温低盐的上层水与高温高盐的大西洋水之间,广泛存在着稳定的双扩散阶梯。基于锚定剖面仪、冰基剖面仪和微结构剖面仪的数据,对温盐廓线中的阶梯进行研究,分析阶梯的热通量。固定跟踪了锚定剖面仪的3个阶梯,研究阶梯参数随时间的变化。发现由经验公式得出的上下两界面的热通量差,与混合层内热量的变化有较好的相关性。利用微结构剖面仪数据,计算阶梯界面通过分子热传导输送的热通量。当选取最大位温梯度时,算出的传导热通量与经验公式算出的热通量接近。因此,实验室研究得到的热通量经验公式,可以用于计算北冰洋双扩散阶梯的热通量。     

Modeling the tide and wind-induced circulation in Buzzards Bay

Hydrodynamic model application to Buzzards Bay is performed using a three-dimensional Boundary-fitted Hydrodynamic model in this study. The model is forced with observed tidal harmonic constants along the open boundaries and winds on the surface. The main focus of the present study is to model the detailed wind and tide-induced circulation in Buzzards Bay. The observed surface elevations and currents given in [Butman, B., Signell, R., Shoukimas, P., Beardsley, R.C., 1988. Current Observations in Buzzards Bay, 1982–1986. Open File Report 88-5. United States Geological Survey] and the tide and current harmonics given in [Signell, R.P., 1987. Tide- and Wind-forced Currents in Buzzards Bay, Massachusetts. Technical Report WH-87-15. Woods Hole Oceanographic Institution, Woods Hole, Massachusetts] are used to validate the model predictions. The calibrated model is then used to study the relative contributions of tidal and wind forcing on the instantaneous and residual circulation in Buzzards Bay. The amplitudes and phases of the principal tidal constituents at 10 tidal stations in Buzzards Bay obtained from a harmonic analysis of a 60-day simulation compare well with the observed data. The predicted amplitude and phase of the M₂ tidal constituent of surface elevations at these stations are, respectively, within 4 cm and 5° of the observed data. The errors in the model-predicted M₂ harmonic principal current speeds are less than 6 cm/s, and the principal current directions and phases are within 14° of the observations. The observed surface elevations and currents given in [Butman, B., Signell, R., Shoukimas, P., Beardsley, R.C., 1988. Current Observations in Buzzards Bay, 1982–1986. Open File Report 88-5. United States Geological Survey] are used to validate the model-predicted low-frequency surface elevations and currents. The model predictions in low-frequency surface elevations at Woods Hole closely follow the trends seen in the observations with a correlation coefficient of 0.735, but fail to capture some of the peak surges seen in the observations. The model-predicted low-frequency currents in the east–west direction at stations in Buzzards Bay compare well with the observations with the correlation coefficient exceeding 0.811 and the model capturing the trends seen in the observations, for the most part. However, the model-predicted north–south velocities does not compare well with the observations. The model-predictions agree with the observations that the tidal currents in Vineyard Sound lagged the currents in Buzzards Bay by more than 3 h. The interaction of wind stress with large bathymetric gradients was shown to cause many vortices in Buzzards Bay, as seen from the model predictions. Model simulations show that the winds play a more dominant role than the tides in the generation of the barotropic residual currents in Buzzards Bay, while the model-predicted tide-induced residual current was seen to be small.