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1.
In this study, we use existing observational datasets to evaluate 20th century climate simulations of the tropical Pacific. The emphasis of our work is decadal variability of the shallow meridional overturning circulation, which links the tropical and subtropical Pacific Ocean. In observations, this circulation is characterized by equatorward geostrophic volume transport convergence in the interior ocean pycnocline across 9°N and 9°S. Historical hydrographic data indicate that there has been a decreasing trend in this convergence over the period 1953–2001 of about 11 Sverdrup (1 Sv = 106 m3 s−1), with maximum decade-to-decade variations of 7–11 Sv. The transport time series is highly anti-correlated with sea surface temperature (SST) anomalies in the central and eastern tropical Pacific, implying that variations in meridional overturning circulation are directly linked to decadal variability and trends in tropical SST. These relationships are explored in 18 model simulations of 20th century climate from 14 state-of-the-art coupled climate models. Significant correlation exists between meridional volume transport convergence and tropical SST in the majority of the models over the last half century. However, the magnitude of transport variability on decadal time scales in the models is underestimated while at the same time modeled SST variations are more sensitive to that transport variability than in the observations. The effects of the meridional overturning circulation on SST trends in most the models is less clear. Most models show no trend in meridional transport convergence and underestimate the trend in eastern tropical Pacific SST. The eddy permitting MIROCH model is the only model that reasonably reproduces the observed trends in transport convergence, tropical Pacific SST, and SST gradient along the equator over the last half century. If the observed trends and those simulated in the MIROCH model are ultimately related to greenhouse gas forcing, these results suggest that the Bjerknes feedback, by affecting pycnocline transport convergences, may enhance warming that arises from anthropogenic forcing in the eastern tropical Pacific. 相似文献
2.
Recent global warming caused by humans and the prediction of a reduced Atlantic Ocean meridional overturning circulation in the future has increased interest in the role of the overturning circulation in climate change. A schematic diagram of the overturning circulation called the “Great Ocean Conveyor Belt,” published by Wallace Broecker in 1987, has become a popular image that emphasizes the inter-connected ocean circulation and the northward flux of heat in the Atlantic. This seems a good time to review the development of the conveyor belt concept and summarize the history of overturning circulation schematics.In the 19th century it was thought that symmetric overturning circulation cells were located on either side of the equator in the Atlantic. As new hydrographic measurements were obtained, circulation schematics in the early 20th century began to show the inter-hemispheric overturning circulation in the Atlantic. In the second half of the 20th century schematics showed the global ocean overturning circulation including connections between the Atlantic and the Pacific and Indian Oceans. Some recent schematics of the overturning circulation show its complexities, but as more information is included these schematics have also become complex and not as easy to understand as the simple Broecker 1987 version. However, these complex schematics, especially the quantitative ones, represent valuable syntheses of our developing knowledge of the overturning circulation. 相似文献
3.
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. 相似文献
4.
I.D. Walsh W.D. Gardner M.J. Richardson S.P. Chung C.A. Plattner V.L. Asper 《Deep Sea Research Part II: Topical Studies in Oceanography》1997,44(9-10)
Profiles made during the JGOFS EgPac October 1992 time series cruise to the equator with a Large Aggregate Profiling System (LAPS) recorded the concentration and size distribution of particles in the marine snow size range ( > 0.5 mm diameter). Profiles were made routinely at local midnight during the twenty day occupation of the time series station on the equator. The LAPS data set, when combined with the CTD/transmissometer data set from the EgPac program's intensive profiling operation, reveals a complex dynamic of particle production and aggregation driven by the equatorial flow field.During the cruise, the influence of the passage of a Tropical Instability Wave (TIW) was observed in the transmissometer/aggregate and temperature/salinity data sets. A peak in particle production rate occurred in conjunction with the maximum shoaling of the thermocline and a minimum in the aggregate volume concentration. Subsequently, the particle load in the surface water increased followed by an increase in the aggregate abundance. Quasi -oligotrophic conditions on the equator (subsurface particle and aggregate maxima and decreased particle production rates) toward the end of the time series corresponded to the appearance of stratified low salinity water characteristic of more northerly conditions.The variations in temperature and salinity fields are ascribed to the passage of a TIW; in sequence: the trailing edge, the northwestward-flowing cool cusp water and the convergent front. Current meters at the equator recorded a rotational flow at 80 m, with a shift from southeastward to northeastward flow during this sequence. A simple conceptual model of meridional flow field is presented in which poleward transport of the upwelled water from the equator returns at shallow depths to mix with the EUC. The signature of this return flow is reflected in the decrease in the vertical gradients of nutrients and oxygen at the equator with respect to the poleward gradients, and a subsurface particle maximum below the EUC. The flow field's effect on the distribution of particles is reflected in the meridional pattern of the sediment accumulation rate (Murray and Leinen, 1996). The return flow of particles to the equator is reflected in the maximum in accumulation rate near the equator. The longterm influence of TIW's is reflected in the asymmetry of the accumulation rates across the equator as described by Murray and Leinen (1996), with the maximum accumulation rate found south of the equator, a local minimum in accumulation rate at approximately 2°N, and a local maximum near 4°N. 相似文献
5.
A zonal-average model of the upper branch of the meridional overturning circulation of the southern ocean is constructed and used to discuss the processes – wind, buoyancy, eddy forcing and boundary conditions – that control its strength and sense of circulation. The geometry of the thermocline ‘wedge’, set by the mapping between the vertical spacing of buoyancy surfaces (the stratification) on the equatorial flank of the Antarctic Circumpolar Current and their outcrop at the sea surface, is seen to play a central role by setting the interior large-scale potential vorticity distribution. It is shown that the action of eddies mixing this potential vorticity field induces a residual flow in the meridional plane much as is observed, with upwelling of fluid around Antarctica, northward surface flow and subduction to form intermediate water. Along with this overturning circulation there is a concomitant air-sea buoyancy flux directed in to the ocean. 相似文献
6.
本文采用SODA3.4.2再分析数据和POP2海洋模式研究了季风转换期间（春季和秋季）热带印度洋经向热输运异常（Meridional Heat Transport Anomaly, MHTA）的年际变异特征。春季MHTA存在两个主要模态,即一致模态和辐合辐散模态:一致模态表现为热带印度洋上层一致的向北输运,受热带印度洋海温一致模相关的赤道反对称风场（赤道以北/南为东北风/西北风异常）调控;辐合辐散模态则呈现关于赤道对称的表层辐散次表层辐合特征,受控于赤道以南的热带西南印度洋和副热带东南印度洋海温偶极子。然而,秋季MHTA仅表现为辐合辐散模态,受到印度洋偶极子期间赤道东风和赤道外反气旋式风应力异常影响。此外,POP2敏感性试验也验证了印度洋海温模态影响下异常风场对MHTA的调控作用,即反对称的风引起一致向北的MHTA,赤道东风异常引起MHTA表层辐散、次表层辐合现象。因此,热带印度洋海气耦合模态年际变化对印度洋上层热量再分配有着重要的意义。 相似文献
7.
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。 相似文献
8.
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。 相似文献
9.
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。 相似文献
10.
Spatial and temporal structure of Tropical Pacific interannual variability in 20th century coupled simulations 总被引：1，自引：0，他引：1
Tropical Pacific interannual variability is examined in nine state-of-the-art coupled climate models, and compared with observations and ocean analyses data sets, the primary focus being on the spatial structure and spectral characteristics of El Niño-Southern Oscillation (ENSO). The spatial patterns of interannual sea surface temperature (SST) anomalies from the coupled models are characterized by maximum variations displaced from the coast of South America, and generally extending too far west with respect to observations. Thermocline variability is characterized by dominant modes that are qualitatively similar in all the models, and consistent with the “recharge oscillator” paradigm for ENSO. The meridional scale of the thermocline depth anomalies is generally narrower than observed, a result that can be related to the pattern of zonal wind stress perturbations in the central-western equatorial Pacific. The wind stress response to eastern equatorial Pacific SST anomalies in the models is narrower and displaced further west than observed. The meridional scale of the wind stress can affect the amount of warm water involved in the recharge/discharge of the equatorial thermocline, while the longitudinal location of the wind stress anomalies can influence the advection of the mean zonal temperature gradient by the anomalous zonal currents, a process that may favor the growth and longer duration of ENSO events when the wind stress perturbations are displaced eastwards. Thus, both discrepancies of the wind stress anomaly patterns in the coupled models with respect to observations (narrow meridional extent, and westward displacement along the equator) may be responsible for the ENSO timescale being shorter in the models than in observations. The examination of the leading advective processes in the SST tendency equation indicates that vertical advection of temperature anomalies tends to favor ENSO growth in all the CGCMs, but at a smaller rate than in observations. In some models it can also promote a phase transition. Longer periods tend to be associated with thermocline and advective feedbacks that are in phase with the SST anomalies, while advective tendencies that lead the SST anomalies by a quarter cycle favor ENSO transitions, thus leading to a shorter period. 相似文献
11.
We analyze the space-and-time variability of the meridional heat transport in the North Atlantic. The contribution of various
mechanisms to the integral meridional heat transport (MHT) is estimated. The key role played by the drift transport of the
Tropical Atlantic in the formation of the meridional oceanic heat transport is confirmed. On the basis of the general analysis
of estimations obtained by various authors according to the data accumulated for 1870–2008 and the results of numerical analyses
based on the data of NCEP/NCAR reanalysis, we show that the long-term average meridional drift heat (mass) transport attains its maximum values equal to (1.6 ± 0.1)
PW [(17.4 ± 1.5) Sv] in the vicinity of 12.5°N in the Tropical Atlantic. The contribution of the heat transport caused by
the horizontal Sverdrup circulation to the integral meridional heat transport is maximum in the vicinity of 30° N. On the
average, it is equal to ∼ 40%. In the Subtropical Atlantic, the meridional heat transport varies with a period of ∼ 50–70 yr.
The minimum value of the integral meridional heat transport was attained in the mid-1960s and its maximum value was at attained
at the beginning of the 1990s. The location of the center of Azores pressure maximum makes it possible to conclude that the
intensification of the total meridional heat transport in the Subtropical Atlantic on these time scales is accompanied by
the displacement of the center of the North Subtropical anticyclonic gyre in the southwest direction. 相似文献
12.
The problem of understanding linear predictability of elements of the ocean circulation is explored in the Atlantic Ocean for two disparate elements: (1) sea surface temperature (SST) under the storm track in a small region east of the Grand Banks and, (2) the meridional overturning circulation north of 30.5°S. To be worthwhile, any nonlinear method would need to exhibit greater skill, and so a rough baseline from which to judge more complex methods is the goal. A 16-year ocean state estimate is used, under the assumption that internal oceanic variability is dominating externally imposed changes. No evidence exists of significant nonlinearity in the bulk of the system over this time span. Linear predictability is the story of time and space correlations, and some predictive skill exists for a few months in SST, with some minor capability extending to a few years. Sixteen years is, however, far too short for an evaluation for interannual, much less decadal, variability, although orders of magnitude are likely stably estimated. The meridional structure of the meridional overturning circulation (MOC), defined as the time-varying vertical integral to the maximum meridional volume transport at each latitude, shows nearly complete decorrelation in the variability across about 35°N—the Gulf Stream system. If a time-scale exists displaying coherence of the MOC between subpolar and subtropical gyres, it lies beyond the existing observation duration, and that has consequences for observing system strategies and the more general problem of detectability of change. 相似文献
13.
The seasonal variation of undercurrent and temperature in the equatorial Pacific jointly derived from buoy measurement and assimilation analysis 总被引：3，自引：2，他引：1
Based on the TOGA-TAO buoy chain observed data in the equatorial Pacific and the assimilation analysis results from SODA(simple ocean data assimilation analysis), the role of the meridional cells in the subsurface of the tropical Pacific was discussed. It was found that, the seasonal varying direction of EUC (the quatorial Undercurrent)in the Peacific is westwards beginning from the eastern equatorial Pacific in the boreal spring. The meridional cell south of the equator plays important role on this seasonal change of EUC.On the other hand, although the varying direction is westwards,the seasonal variation of temperature in the same region gets its minimum values in the boreal autumn beginning from the eastern equatorial Pacific.The meridional cell north of the equator is most responsible for the seasonal temperature variation in the eastern equatorial Pacific while the meridional cell south of the equator mainly controls the seasonal temperature change in the central Pacific. It is probably true that the asymmetry by the equator is an important factor influencing the seasonal cycle of EUC and temperature in the tropical Pacific. 相似文献
14.
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 ... 相似文献
15.
《Deep Sea Research Part I: Oceanographic Research Papers》2006,53(3):528-546
As a component of the meridional overturning variability experiment in the tropical North Atlantic, a four-year-long time series of meridional transport of North Atlantic deep water has been obtained from moored end point measurements of density and bottom pressure. This study presents a quality assessment of the measurement elements. Rigorous pre- and post- deployment in situ calibration of the density sensors and subsequent data processing establish an accuracy of O(1.5 Sv) in internal transport in the 1200–5000 dbar range at subinertial time scales. A similar accuracy is reached in the bottom pressure-derived external transport fluctuations. However, for pressure, variability with periods longer than a deployment's duration (presently about one year) is not measurable. This effect is demonstrated using numerical simulations and a possible solution for detecting long-term external transport changes is presented. 相似文献
16.
Quantitative estimates of the sensitivity of the number and size of extratropical cyclones in the Northern Hemisphere to changes in the surface temperature are obtained with the use of NCEP/NCAR reanalysis data over a 60-year period and are compared with estimates on the basis of a relatively simple model of the cyclonic and anticyclonic activities in the atmosphere of extratropical latitudes associated with characteristics of atmospheric temperature stratification (MMPKh model). The model estimates are also obtained for a dry and moist atmosphere. With the use of the reanalysis data, extratropical latitudes are, on the whole, characterized by a general decrease in the number of cyclones and the density of their packing in extratropical latitudes as the surface temperature increases. However, in the MMPKh model for moist atmosphere, estimates of the parameter of sensitivity of the number of cyclones at midlatitudes and at extratropical latitudes in the Northern Hemisphere as a whole are close to those based on the reanalysis data. The influences of the meridional gradient of the surface temperature and the vertical temperature gradient in the troposphere on changes in the number and size of extratropical cyclones are estimated from the reanalysis data and model calculations. It is noted that the most significant changes in annual mean variations in the number and size of extratropical cyclones are associated with the vertical temperature gradient in the troposphere. In this case, an increase in the vertical temperature gradient in the troposphere decreases the size of cyclones. The relative influences of the vertical and meridional temperature gradients are different for different latitudinal zones. 相似文献
17.
关键海区潜沉率对全球变暖停滞的可能影响 总被引：1，自引：0，他引：1
本文从潜沉率入手，探究了潜沉率在全球变暖停滞过程中可能发挥的作用。本文利用SODA资料首先分析了全球潜沉率的时空分布特征，然后基于EOF分解明确了北大西洋翻转流区域和南极绕极流区域是潜沉率变率较大的两个海区，在此基础上选出了4个关键海区研究了局地潜沉率变化与全球海表温度异常之间的相关关系，最后对关键区潜沉率变化的原因进行了初步探索。结果表明，北大西洋翻转流和南极绕极流范围内的关键区域与全球变暖停滞之间存在超前10年的相关关系，潜沉过程可能是北大西洋翻转流和南极绕极流对全球变暖停滞产生作用的一种机制。平流项在这些关键区域的潜沉率变化中起主导作用。在南极绕极流地区，海面风应力的大小与该区域的潜沉变化密切相关。 相似文献
18.
Equatorially trapped Rossby waves in the presence of meridionally sheared baroclinic flow in the Pacific Ocean 总被引：1，自引：0，他引：1
TOPEX/POSEIDON altimeter data are analyzed for the 8.5-year period November 1992 to May 2001 to investigate the sea surface height (SSH) and geostrophic velocity signatures of quasi-annual equatorially trapped Rossby waves in the Pacific. The latitudinal structures of SSH and both components of geostrophic velocity are found to be asymmetric about the equator across the entire Pacific with larger amplitude north of the equator. The westward phase speeds are estimated by several different methods to be in the range 0.5-0.6 m s−1. These observed characteristics are inconsistent with the classical theory for first vertical, first meridional mode equatorially trapped Rossby waves, which predicts a phase speed of about 0.9 m s−1 with latitudinally symmetric structures of SSH and zonal velocity and antisymmetric structure of meridional velocity. The observations are even less consistent with the latitudinal structures of SSH and geostrophic velocity components for other modes of the classical theory.The latitudinal asymmetries deduced here have also been consistently observed in past analyses of subsurface thermal data and altimeter data and have been variously attributed to sampling errors in the observational data, a superposition of multiple meridional Rossby wave modes, asymmetric forcing by the wind, and forcing by cross-equatorial southerly winds in the eastern Pacific. We propose a different mechanism to account for the observed asymmetric latitudinal structure of low-frequency equatorial Rossby waves. From the free-wave solutions of a simple 1.5-layer model, it is shown that meridional shears in the mean equatorial current system significantly alter the potential vorticity gradient in the central and eastern tropical Pacific. The observed asymmetric structures of sea surface height and geostrophic velocity components are found to be a natural consequence of the shear modification of the potential vorticity gradient. The mean currents also reduce the predicted westward phase speed of first meridional mode Rossby waves, improving consistency with the observations. 相似文献
19.
D. Nof 《Progress in Oceanography》2003,56(2):223-247
A ‘quasi-island’ approach for examining the meridional flux of warm and intermediate water from the Southern Ocean into the South Atlantic, the South Pacific and the Indian Ocean has recently been proposed (
[Nof, 2000a] and [Nof, 2002]). This approach considers the continents to be ‘pseudo islands’ in the sense that they are entirely surrounded by water, but have no circulation around them. The method employs an integration of the linearized momentum equations along a closed contour containing the continents. This allows the meridional transport into these oceans to be computed without having to find the detailed solution to the complete wind-thermohaline problem.The solution gives two results; one expected, the other unexpected. It shows that, as expected, about 9±5 Sv of upper and intermediate water enter the South Atlantic from the Southern Ocean. The unexpected result is that the Pacific-Indian Ocean system should contain a ‘shallow’ meridional overturning cell carrying 18±5 Sv. What is meant by shallow here is that the cell does not extend all the way to the bottom (as it does in the Atlantic) but is terminated at mid-depth. (This reflects the fact that there is no bottom water formation in the Pacific.) Both of these calculations rely on the observation that there is almost no flow through the Bering Strait and on the assumption that there is a negligible pressure torque on the Bering Strait’s sill.Here, we present a new and different approach, which does not rely on either of the above two conditions regarding the Bering Strait and yet gives essentially the same result. The approach does not involve any quasi-island calculation but rather employs an integration of the linearized zonal momentum equation along a closed open-water latitudinal belt connecting the tips of South Africa and South America. The integration relies on the existence of a belt (corridor) where the linearized general circulation equations are valid. It allows for a net northward mass flux through either the Sverdrup interior or the western boundary currents. It is found that the belt-corridor approach gives 29±5 Sv for the total meridional flux of surface and intermediate water from the Southern Ocean. This agrees very well with the quasi-island calculations, which give a total northward flux of 27±5 Sv. Given the spacing between the continents and the small variability of the southern winds with longitude, one may assume that 9 Sv of the total 29 Sv enters the Atlantic and the other 20 Sv enters the combined Pacific-Indian Ocean system, which is also in agreement with the quasi-island calculation. These agreements indicate that the assumptions made in the earlier studies regarding the Bering Strait are probably valid. 相似文献
20.
Antarctic Bottom Water（AABW） plays an important role in the meridional overturning circulation and contributes significantly to global heat transport and sea level rise（SLR）. Based on the Global Ocean（1/12）°Physical Reanalysis（GLORYS12V1） products and conductivity-temperature-depth instrument data from the World Ocean Circulation Experiment hydrographic program, we analyzed the trends in the thickness, volume,temperature, salinity, and neutral density of the AABW in the Amundsen Sea from 1993 to... 相似文献