热力和动力强迫对热带海洋运动影响的数值试验

本文提出了一个赤道β平面非定常线性模式,其垂直结构在斜温层以上是具有相同密度的两个活动层,斜温层以下是密度较大的均质静止流体。数值试验表明,热强迫与风强迫相比对海洋的直接影响很小,但通过海气相互作用,它可形成风强迫作用于海洋,对海气环流系统有支配作用。     

NUMERICAL EXPERIMENT OF THE INFLUENCES OF HEAT AND WIND STRESS ON THE EQUATORIAL OCEAN CIRCULATION*

A time-dependent linear model on the equatorial beta plane is developed in this paper.Its vertical structure consists of two active layers with equal density and temperature above the thermocline and a quasi-stationary layer with constant density below the thermocline.The results of nurnericat experiments show that the direct influence of heat on the equatorial ocean is much smaller than that of wind stress.Nevertheless,through the tropical ocean-atmosphere interaction,the change of wind stress resulting from the thermal forcing may set a determinant effect on the equilibrium and anomalous development of the ocean-almosphere circulation.     

Relative roles of the equatorial upper ocean zonal current and thermocline in determining the timescale of the tropical climate system

Summary In this study, it is demonstrated that the amplitude of the equatorial upper-ocean zonal current anomaly induced by the fast-varying wind forcing (shorter than a year) is much greater than that induced by the slowly varying wind forcing (longer than 2 year), and the center of maximum zonal current anomaly shifts from the central Pacific to the western Pacific with an increase in the timescale of wind forcing. As a result, the zonal advective feedback (the zonal advection of mean sea surface temperature by anomalous current) in a slowly varying climate system becomes weaker and barely induces a low-frequency mode such as El Niño-Southern Oscillation. On the other hand, both amplitude and zonal location of the maximum thermocline anomaly are little changed by the change in the timescale of wind forcing – confined at the strong equatorial upwelling region of the eastern Pacific. Accordingly, the thermocline feedback (the vertical advection of anomalous subsurface temperature by the mean upwelling) is more favorable to generate a low-frequency mode.The relative roles of these two feedbacks are further explored under the coupled-system context. The eigen analysis of the stripped-down version of an intermediate ocean-atmosphere coupled model shows that by altering the regime space from the weakly coupled to the strongly coupled, the dominant process that leads the leading eigen mode changes from the zonal advective feedback to the thermocline feedback, and at the same time the frequency of the leading mode also changes from the high-frequency to the low-frequency. It implies that each feedback tends to favor the different timescale coupled mode.     

THE ROLE OF MERIDIONAL WIND STRESS IN THE TROPICAL UNSTABLE AIR-SEA INTERACTION

1INTRODUCTIONAsshowninobservationalfacts,theevolutionofElNi駉displaysastheamplitudeexpansionandmigrationofanomalousdisturbancesinair-seacouplingsystems.Theoreticaladdressofthephenomenonhasbeenattemptedfromtheaspectofcouplingdynamics.Theresultsshowthattheair-seainteractionsystemincludesakindofunstablemode,whichispropagatingindirectionsdeterminedbyfactorsgoverningSSTchanges.Withanair-seacouplingmodel,Lau[1]wasamongtheearliestpeoplewhodiscoveredunstablestationarywavemodesintheair-seaintera…     

夏季赤道西印度洋对大气的强迫作用及其机制研究

基于1982—2015年高分辨率海气资料,从海表面温度(Sea Surface Temperature, SST)和海表面风速相关关系的角度研究了年际尺度上赤道印度洋的海气关系。结果表明,印度洋的海气关系具有明显区域性和季节性特征,即整个印度洋除赤道东南印度洋和赤道西印度洋SST与海表风速在夏季(7—9月)为显著正相关关系,主要表现为海洋影响大气;其他地区和月份均为负相关关系,主要表现为大气对海洋的强迫作用。回归分析发现,夏季赤道西印度洋SST异常可能通过海平面气压调整机制影响海表面风场,即海温增温使边界层空气增暖,海表面风场辐合增强;反之则相反。此外,还利用AM2.1模式进行模拟试验,试验结果成功地再现了夏季赤道西印度洋海表面温度与海表风速之间的正相关关系。     

Remote effects of tropical cyclone wind forcing over the western Pacific on the eastern equatorial ocean

An ocean general circulation model （OGCM） is used to demonstrate remote effects of tropical cyclone wind （TCW） forcing in the tropical Pacific. The signature of TCW forcing is explicitly extracted using a locally weighted quadratic least=squares regression （called as LOESS） method from six-hour satellite surface wind data; the extracted TCW component can then be additionally taken into account or not in ocean modeling, allowing isolation of its effects on the ocean in a clean and clear way. In this paper, seasonally varying TCW fields in year 2008 are extracted from satellite data which are prescribed as a repeated annual cycle over the western Pacific regions off the equator （poleward of 10°N/S）; two long-term OGCM experiments are performed and compared, one with the TCW forcing part included additionally and the other not. Large, persistent thermal perturbations （cooling in the mixed layer （ML） and warming in the thermocline） are induced locally in the western tropical Pacific, which are seen to spread with the mean ocean circulation pathways around the tropical basin. In particular, a remote ocean response emerges in the eastern equatorial Pacific to the prescribed off-equatorial TCW forcing, characterized by a cooling in the mixed layer and a warming in the thermocline. Heat budget analyses indicate that the vertical mixing is a dominant process responsible for the SST cooling in the eastern equatorial Pacific. Further studies are clearly needed to demonstrate the significance of these results in a coupled ocean-atmosphere modeling context.     

Remote Efects of Tropical Cyclone Wind Forcing over the Western Pacific on the Eastern Equatorial Ocean

An ocean general circulation model(OGCM)is used to demonstrate remote efects of tropical cyclone wind(TCW)forcing in the tropical Pacific.The signature of TCW forcing is explicitly extracted using a locally weighted quadratic least-squares regression(called as LOESS)method from six-hour satellite surface wind data;the extracted TCW component can then be additionally taken into account or not in ocean modeling,allowing isolation of its efects on the ocean in a clean and clear way.In this paper,seasonally varying TCW fields in year 2008 are extracted from satellite data which are prescribed as a repeated annual cycle over the western Pacific regions of the equator(poleward of 10 N/S);two long-term OGCM experiments are performed and compared,one with the TCW forcing part included additionally and the other not.Large,persistent thermal perturbations(cooling in the mixed layer(ML)and warming in the thermocline)are induced locally in the western tropical Pacific,which are seen to spread with the mean ocean circulation pathways around the tropical basin.In particular,a remote ocean response emerges in the eastern equatorial Pacific to the prescribed of-equatorial TCW forcing,characterized by a cooling in the mixed layer and a warming in the thermocline.Heat budget analyses indicate that the vertical mixing is a dominant process responsible for the SST cooling in the eastern equatorial Pacific.Further studies are clearly needed to demonstrate the significance of these results in a coupled ocean-atmosphere modeling context.     

经向风应力在热带不稳定海气相互作用中的角色

利用1951-2000年北太平洋海温和副高特征量资料，探讨夏半年副高与前期海温的关系，发现影响副高活动的海温关键区赤道东太平洋前期信息最明显，从前期秋季持续至春季都保持与夏半年各月副高强度、西伸脊点位置的高相关；ENS0事件的形成超前于副高强度的转折约半年时间；El Nino年副高持续偏强偏西，La Nina年则相反，脊线位置也有所反映，但不如强度与西伸显著。在此基础上进一步分析表明，福建雨季降水、副高强度与Nino 3区海温存在着大约相一致的年代际振荡与突变时间；雨季降水分布的异常直接受到副高活动的影响，而副高活动又明显受ENS0循环的制约，因而关注赤道东太平洋海温异常及ENS0信息对福建雨季降水分布趋势的预测旦有重要意义。     

On the mechanisms in a tropical ocean–global atmosphere coupled general circulation model. Part I: mean state and the seasonal cycle

 The mechanisms responsible for the mean state and the seasonal and interannual variations of the coupled tropical Pacific-global atmosphere system are investigated by analyzing a thirty year simulation, where the LMD global atmospheric model and the LODYC tropical Pacific model are coupled using the delocalized physics method. No flux correction is needed over the tropical region. The coupled model reaches its regime state roughly after one year of integration in spite of the fact that the ocean is initialized from rest. Departures from the mean state are characterized by oscillations with dominant periodicites at annual, biennial and quadriennial time scales. In our model, equatorial sea surface temperature and wind stress fluctuations evolved in phase. In the Central Pacific during boreal autumn, the sea surface temperature is cold, the wind stress is strong, and the Inter Tropical Convergence Zone (ITCZ) is shifted northwards. The northward shift of the ITCZ enhances atmospheric and oceanic subsidence between the equator and the latitude of organized convention. In turn, the stronger oceanic subsidence reinforces equatorward convergence of water masses at the thermocline depth which, being not balanced by equatorial upwelling, deepens the equatorial thermocline. An equivalent view is that the deepening of the thermocline proceeds from the weakening of the meridional draining of near-surface equatorial waters. The inverse picture prevails during spring, when the equatorial sea surface temperatures are warm. Thus temperature anomalies tend to appear at the thermocline level, in phase opposition to the surface conditions. These subsurface temperature fluctuations propagate from the Central Pacific eastwards along the thermocline; when reaching the surface in the Eastern Pacific, they trigger the reversal of sea surface temperature anomalies. The whole oscillation is synchronized by the apparent meridional motion of the sun, through the seasonal oscillation of the ITCZ. This possible mechanism is partly supported by the observed seasonal reversal of vorticity between the equator and the ITCZ, and by observational evidence of eastward propagating subsurface temperature anomalies at the thermocline level. Received: 7 April 1997 / Accepted: 15 July 1998     

Surface currents and equatorial thermocline in a coupled upper ocean-atmosphere GCM

The Oregon State University coupled upper ocean-atmosphere GCM is evaluated in terms of the simulated winds, ocean currents and thermocline depth variations. Although the zonal wind velocities in the model are underestimated by a factor of about three and the zonal current velocities are underestimated by a factor of about five, the model is seen to qualitatively simulate the major features of the gyral scale currents, and the phases of the seasonal variation of the principal equatorial currents are in reasonable agreement with observations. The simulated tropical currents are dominated by Ekman transport and the eastern boundary currents do not penetrate far enough equatorward, while the western boundary currents do not penetrate far enough poleward. The subtropical trade wind belt and the mid-latitude westerlies are displaced equatorward of observations; hence, the mid-latitude eastward currents, principally the Kuroshio-North Pacific Drift and the Gulf Stream-North Atlantic Current are displaced equatorward. In spite of these shortcomings the surface current simulation of this two-layer upper ocean model is comparable with that of other ocean GCMs of coarse resolution. The coupled model successfully simulates the deepening of the thermocline westward across Pacific as a consequence of the prevailing Walker circulation. The region of most intense simulated surface forcing is located in the western Pacific due to a southwestward displacement of the northeast trade winds relative to observations; hence the equatorial Pacific is dominated by eastward propagation of thermocline depth variations. The excessively strong Ekman divergence and upwelling in the western Pacific cools the local warm pool, while incorrectly simulated westerlies in the eastern Pacific suppress upwelling and inhibit cooling from below. These features reduce the simulated trans-Pacific sea-surface temperature gradient, weakening the Walker circulation and the anomalies associated with the simulated Southern Oscillation. Offprint requests to: KR Sperber     

Fundamental Framework and Experiments of the Third Generation of IAP/LASG World Ocean General Circulation Model

1.IntroductionThefirstbaroclinicoceanicgeneralcirculationmodel(OGCM)developedattheStateKeyLaboratoryofNumericalModelingforAtmosphericSciencesandGeophysicalFluidDynamics(LASG),InstituteofAtmosphericPhysics(IAP)isafour--layermodelwithitshorizontalresol...     

The equatorial Atlantic oscillation and its response to ENSO

An internal equatorial Atlantic oscillation has been identified by analyzing sea surface temperature (SST) observations. The equatorial Atlantic oscillation can be viewed as the Atlantic analogue of the El Niño/Southern Oscillation (ENSO) phenomenon in the equatorial Pacific, but it is much less vigorous. The equatorial Atlantic oscillation is strongly influenced by the Pacific ENSO with the equatorial Atlantic sea surface temperature lagging by about six months. This lag can be explained by the dynamical adjustment time of the equatorial Atlantic to low-frequency wind stress variations and the seasonally varying background state, which favours strongest growth of perturbations in summer. Results of an extended-range simulation with a coupled ocean-atmosphere GCM support this picture.     

东亚夏季对流云顶黑体辐射温度的频率分布及其日变化

利用简单海气耦合模式，分析了经向风应力和纬向风应力对热带太平洋不稳定海气耦合模态的相对贡献。结果表明：在局地热力平衡假设下，海气相互作用导致了海洋Kelvin波的不稳定，该不稳定模态发生在一定波长范围内，只有纬向风应力对其作出贡献；在海温仅由平流过程决定的假设下，海气相互作用导致了海洋Rossby波的不稳定，该不稳定模态在整个波长范围内都有发生，波长越大，不稳定越强，经向风应力与纬向风应力都可以对海洋Rossby波的不稳定作出贡献，但是前者贡献较小；不稳定Rossby波比不稳定Kelvin波的增长率要小；由于无论哪一种不稳定模态，都是纬向风应力在海气耦合中起着绝对的主导作用，所以在研究热带太平洋海气耦合动力学对ENS0不稳定发展的贡献时，采用忽略经向风应力作用的近似是合理可行的。     

Fundamental framework and experiments of the third generation of IAP / LASG world ocean general circulation model

A new generation of the IAP / LASG world ocean general circulation model is designed and presented based on the previous 20-layer model, with enhanced spatial resolutions and improved parameterizations. The model uses a triangular-truncated spectral horizontal grid system with its zonal wave number of 63 (T63) to match its atmospheric counterpart of a T63 spectral atmosphere general circulation model in a planned coupled ocean-atmosphere system. There are 30 layers in vertical direction, of which 20 layers are located above 1000 m for better depicting the permanent thermocline. As previous ocean models developed in IAP / LASG, a free surface (rather than “rigid-lid” approximation) is included in this model. Compared with the 20-layer model, some more detailed physical parameterizations are considered, including the along / cross isopycnal mixing scheme adapted from the Gent-MacWilliams scheme. The model is spun up from a motionless state. Initial conditions for temperature and salinity are taken from the three-dimensional distributions of Levitus’ annual mean observation. A preliminary analysis of the first 1000-year integration of a control experiment shows some encouraging improvements compared with the twenty-layer model, particularly in the simulations of permanent thermocline, thermohaline circu?lation, meridional heat transport, etc. resulted mainly from using the isopycnal mixing scheme. However, the use of isopycnal mixing scheme does not significantly improve the simulated equatorial thermocline. A series of numerical experiments show that the most important contribution to the improvement of equatori?al thermocline and the associated equatorial under current comes from reducing horizontal viscosity in the equatorial regions. It is found that reducing the horizontal viscosity in the equatorial Atlantic Ocean may slightly weaken the overturning rate of North Atlantic Deep Water.     

ENSO amplitude change in observation and coupled models

Observations show that the tropical E1 Nifio-Southern Oscillation （ENSO） variability, after removing both the long term trend and decadal change of the background climate, has been enhanced by as much as 60% during the past 50 years. This shift in ENSO amplitude can be related to mean state changes in global climate. Past global warming has caused a weakening of the Walker circulation over the equatorial Indo-Pacific oceans, as well as a weakening of the trade winds and a reduction in the equatorial upwelling. These changes in tropical climatology play as stabilizing factors of the tropical coupling system. However, the shallower and strengthening thermocline in the equatorial Pacific increases the SST sensitivity to thermocline and wind stress variabilities and tend to destabilize the tropical coupling system. Observations suggest that the destabilizing factors, such as the strengthening thermocline, may have overwhelmed the stabilizing effects of the atmosphere, and played a deterministic role in the enhanced ENSO variability, at least during the past half century. This is different from the recent assessment of IPCC-AR4 coupled models.     

赤道太平洋高低频纬向环流差异及其物理机制

利用1982—2017年欧洲中期天气预报中心再分析资料和美国国家大气海洋管理局的观测资料,分析了赤道太平洋高、低频海平面气压距平场的特征及差异,并通过诊断赤道太平洋海-气耦合的时间尺度,探讨了导致赤道太平洋高、低频纬向环流差异的原因。结果表明,南方涛动海平面气压东西跷跷板耦合现象只是在低频场中才存在,在高频场中并不存在。低频场上,主要受到热带最明显的年际信号(ENSO)的调控,海平面气压场和海表温度场呈现出东西振荡型。相比而言,在高频场上由于时间短,海、气异常还没有发生较好的耦合,气压场和风场呈现出全海域一致型。高频纬向环流与热带季节内振荡（MJO）紧密联系,具有明显的东传特征,传播速度大约为5 m/s,其变率方差的34%可以由MJO线性解释。相干谱分析表明海-气耦合具有时间依赖性,南方涛动通常只在20候以上的时间尺度才能存在,这与赤道海洋开尔文波横穿太平洋时间相当。      

The influence of thermocline slope on equatorial thermocline displacement

The influence of a zonal slope in the thermocline on both wave- and wind-induced equatorial upwelling is studied. It is shown that the vertical displacement of the thermocline induced by a long Kelvin or planetary wave varies as the thermocline depth to the power $\text{?3}\text{8}$, as the wave propagates into a region of changed thermocline depth The zonal and meridional velocities induced by a wave vary as the thermocline depth to the power $\text{?7}\text{8}$. The wind-induced upwelling associated with a zonal slope of the thermocline is shown to be strongly confined to the equator. The importance of thermocline slope on the Ekman divergence is found to increase with time from the initial application of the wind stress perturbation. In the Pacific Ocean the meridional velocity may be altered by up to 60% by the thermocline slope before the influence of the boundaries becomes important.     

THE IMPACT OF INITIAL FORCED WIND ON THE PREDICTABILITY OF THE ZEBIAK-CANE COUPLED OCEAN-ATMOSPHERE MODEL

With simultaneous observed sea surface temperature anomaly (SSTA), the difference between NCEP/NCAR 925hPa reanalysis wind stress anomaly (NCEPWSA) and FSU wind stress anomaly (FSUWSA) is analyzed, and the prediction abilities of Zebiak-Cane coupled ocean-atmosphere model (ZC coupled model) with NCEPWSA and FSUWSA serving respectively as initialization wind are compared. The results are as follows. The distribution feature of NCEPWSA matches better with that of the observed SSTA than counterpart of FSUWSA both in 1980s and in 1990s; The ZC ocean model has a better skill under the forcing of NCEPWSA than that of FSUWSA, especially in 1990s. Meanwhile, the forecast abilities of the ZC coupled model in 1990s as well as in 1980s have been improved employing NCEPWSA as initialization wind instead of FSUWSA. Particularly, it succeeded in predicting 1997/1998 El Ni?o 6 to 8 months ahead; further analysis shows that on the antecedent and onset stages of the 1997/1998 El Ni?o event, the horizontal cold and warm distribution characteristics of the simulated SSTA from ZC ocean model, with NCEPWSA forcing compared to FSUWSA forcing, match better with counterparts of the corresponding observed SSTA, whereby providing better predication initialization conditions for ZC coupled model, which, in turn, is favorable to improve the forecast ability of the coupled model.     

Lagrangian sources of frontogenesis in the equatorial Atlantic front

Estimating the processes that control the north equatorial sea surface temperature (SST)-front on the northern edge of the cold tongue in the tropical Atlantic is a key issue for understanding the dynamics of the oceanic equatorial Atlantic and the West African Monsoon. Diagnosis of the frontogenetic forcings on a realistic high-resolution simulation was used to identify the processes involved in the formation and evolution of the equatorial SST-front. The turbulent forcing associated with the mixed-layer turbulent heat flux was found to be systematically frontolytic while the dynamic forcing associated with currents was found to be frontogenetic for the equatorial SST-front. Nevertheless, the low-frequency component of the turbulent forcing was frontogenetic and initiated the SST-front which was then amplified and maintained by the leading dynamic forcing. This forcing was mainly driven by the meridional convergence of the northern South Equatorial Current (nSEC) and the Guinea Current, which points out the essential role played by the circulation in the equatorial SST-front evolution. The quasi-biweekly variability of the equatorial SST-front and its forcings were found to be more strongly coupled to the wind energy flux (WEF) than to the surface wind stress. In fact the WEF controlled the convergence/divergence of the nSEC and Guinea Current and thus the meridional component of the leading dynamic forcing. The WEF explains the equatorial SST-front development better than the wind does because it is a coupled ocean-atmosphere process.     

Reduction of the thermocline feedback associated with mean SST bias in ENSO simulation

Associated with the double Inter-tropical convergence zone problem, a dipole SST bias pattern (cold in the equatorial central Pacific and warm in the southeast tropical Pacific) remains a common problem inherent in many contemporary coupled models. Based on a newly-developed coupled model, we performed a control run and two sensitivity runs, one is a coupled run with annual mean SST correction and the other is an ocean forced run. By comparison of these three runs, we demonstrated that a serious consequence of this SST bias is to severely suppress the thermocline feedback in a realistic simulation of the El Ni?o/Southern Oscillation. Firstly, the excessive cold tongue extension pushes the anomalous convection far westward from the equatorial central Pacific, prominently diminishing the convection-low level wind feedback and thus the air-sea coupling strength. Secondly, the equatorial surface wind anomaly exhibits a relatively uniform meridional structure with weak gradient, contributing to a weakened wind-thermocline feedback. Thirdly, the equatorial cold SST bias induces a weakened upper-ocean stratification and thus yields the underestimation of the thermocline-subsurface temperature feedback. Finally, the dipole SST bias underestimates the mean upwelling through (a) undermining equatorial mean easterly wind stress, and (b) enhancing convective mixing and thus reducing the upper ocean stratification, which weakens vertical shear of meridional currents and near-surface Ekman-divergence.