Steering of upper ocean currents and fronts by the topographically constrained abyssal circulation

A two-layer theory is used to investigate (1) the steering of upper ocean current pathways by topographically constrained abyssal currents that do not impinge on the bottom topography and (2) its application to upper ocean – topographic coupling via flow instabilities where topographically constrained eddy-driven deep mean flows in turn steer the mean pathways of upper ocean currents and associated fronts. In earlier studies the two-layer theory was applied to ocean models with low vertical resolution (2–6 layers). Here we investigate its relevance to complex ocean general circulation models (OGCMs) with high vertical resolution that are designed to simulate a wide range of ocean processes. The theory can be easily applied to models ranging from idealized to complex OGCMs, provided it is valid for the application. It can also be used in understanding some persistent features seen in observed ocean frontal pathways (over deep water) derived from satellite imagery and other data. To facilitate its application, a more thorough explanation of the theory is presented that emphasizes its range of validity. Three regions of the world ocean are used to investigate its application to eddy-resolving ocean models with high vertical resolution, including one where an assumption of the two-layer theory is violated. Results from the OGCMs with high vertical resolution are compared to those from models with low vertical resolution and to observations. In the Kuroshio region upper ocean – topographic coupling via flow instabilities and a modest seamount complex are used to explain the observed northward mean meander east of Japan where the Kuroshio separates from the coast. The Japan/East Sea (JES) is used to demonstrate the impact of upper ocean – topographic coupling in a relatively weak flow regime. East of South Island, New Zealand, the Southland Current is an observed western boundary current that flows in a direction counter to the demands of Sverdrup flow and counter to the direction simulated in nonlinear global flat bottom and reduced gravity models. A model with high vertical resolution (and topography extending through any number of layers) and a model with low vertical resolution (and vertically compressed but otherwise realistic topography confined to the lowest layer) both simulate a Southland Current in the observed direction with dynamics depending on the configuration of the regional seafloor. However, the dynamics of these simulations are very different because the Campbell Plateau and Chatham Rise east and southeast of New Zealand are rare features of the world ocean where the topography intrudes into the stratified water column over a relatively broad area but lies deeper than the nominal 200 m depth of the continental shelf break, violating a limitation of the two-layer theory. Observations confirm the results from the high vertical resolution model. Overall, the model simulations show increasingly widespread upper ocean – topographic coupling via flow instabilities as the horizontal resolution of the ocean models is increased, but fine resolution of mesoscale variability and the associated flow instabilities are required to obtain sufficient coupling. As a result, this type of coupling is critical in distinguishing between eddy-resolving and eddy-permitting ocean models in regions where it occurs.     

Steering of upper ocean currents and fronts by the topographically constrained abyssal circulation

A two-layer theory is used to investigate (1) the steering of upper ocean current pathways by topographically constrained abyssal currents that do not impinge on the bottom topography and (2) its application to upper ocean – topographic coupling via flow instabilities where topographically constrained eddy-driven deep mean flows in turn steer the mean pathways of upper ocean currents and associated fronts. In earlier studies the two-layer theory was applied to ocean models with low vertical resolution (2–6 layers). Here we investigate its relevance to complex ocean general circulation models (OGCMs) with high vertical resolution that are designed to simulate a wide range of ocean processes. The theory can be easily applied to models ranging from idealized to complex OGCMs, provided it is valid for the application. It can also be used in understanding some persistent features seen in observed ocean frontal pathways (over deep water) derived from satellite imagery and other data. To facilitate its application, a more thorough explanation of the theory is presented that emphasizes its range of validity. Three regions of the world ocean are used to investigate its application to eddy-resolving ocean models with high vertical resolution, including one where an assumption of the two-layer theory is violated. Results from the OGCMs with high vertical resolution are compared to those from models with low vertical resolution and to observations. In the Kuroshio region upper ocean – topographic coupling via flow instabilities and a modest seamount complex are used to explain the observed northward mean meander east of Japan where the Kuroshio separates from the coast. The Japan/East Sea (JES) is used to demonstrate the impact of upper ocean – topographic coupling in a relatively weak flow regime. East of South Island, New Zealand, the Southland Current is an observed western boundary current that flows in a direction counter to the demands of Sverdrup flow and counter to the direction simulated in nonlinear global flat bottom and reduced gravity models. A model with high vertical resolution (and topography extending through any number of layers) and a model with low vertical resolution (and vertically compressed but otherwise realistic topography confined to the lowest layer) both simulate a Southland Current in the observed direction with dynamics depending on the configuration of the regional seafloor. However, the dynamics of these simulations are very different because the Campbell Plateau and Chatham Rise east and southeast of New Zealand are rare features of the world ocean where the topography intrudes into the stratified water column over a relatively broad area but lies deeper than the nominal 200 m depth of the continental shelf break, violating a limitation of the two-layer theory. Observations confirm the results from the high vertical resolution model. Overall, the model simulations show increasingly widespread upper ocean – topographic coupling via flow instabilities as the horizontal resolution of the ocean models is increased, but fine resolution of mesoscale variability and the associated flow instabilities are required to obtain sufficient coupling. As a result, this type of coupling is critical in distinguishing between eddy-resolving and eddy-permitting ocean models in regions where it occurs.     

Impact of ocean model resolution on CCSM climate simulations

The current literature provides compelling evidence suggesting that an eddy-resolving (as opposed to eddy-permitting or eddy-parameterized) ocean component model will significantly impact the simulation of the large-scale climate, although this has not been fully tested to date in multi-decadal global coupled climate simulations. The purpose of this paper is to examine how resolved ocean fronts and eddies impact the simulation of large-scale climate. The model used for this study is the NCAR Community Climate System Model version 3.5 (CCSM3.5)—the forerunner to CCSM4. Two experiments are reported here. The control experiment is a 155-year present-day climate simulation using a 0.5° atmosphere component (zonal resolution 0.625 meridional resolution 0.5°; land surface component at the same resolution) coupled to ocean and sea-ice components with zonal resolution of 1.2° and meridional resolution varying from 0.27° at the equator to 0.54° in the mid-latitudes. The second simulation uses the same atmospheric and land-surface models coupled to eddy-resolving 0.1° ocean and sea-ice component models. The simulations are compared in terms of how the representation of smaller scale features in the time mean ocean circulation and ocean eddies impact the mean and variable climate. In terms of the global mean surface temperature, the enhanced ocean resolution leads to a ubiquitous surface warming with a global mean surface temperature increase of about 0.2?°C relative to the control. The warming is largest in the Arctic and regions of strong ocean fronts and ocean eddy activity (i.e., Southern Ocean, western boundary currents). The Arctic warming is associated with significant losses of sea-ice in the high-resolution simulation. The sea surface temperature gradients in the North Atlantic, in particular, are better resolved in the high-resolution model leading to significantly sharper temperature gradients and associated large-scale shifts in the rainfall. In the extra-tropics, the interannual temperature variability is increased with the resolved eddies, and a notable increases in the amplitude of the El Ni?o and the Southern Oscillation is also detected. Changes in global temperature anomaly teleconnections and local air-sea feedbacks are also documented and show large changes in ocean–atmosphere coupling. In particular, local air-sea feedbacks are significantly modified by the increased ocean resolution. In the high-resolution simulation in the extra-tropics there is compelling evidence of stronger forcing of the atmosphere by SST variability arising from ocean dynamics. This coupling is very weak or absent in the low-resolution model.     

全球增暖对ENSO影响的数值模拟研究

利用日本东京大学气候系统研究所、日本环境研究所和日本地球环境研究中心联合开发的海气耦合模式MIROC3.2，研究了全球变暖对ENSO年际变率的影响。该模式较好地模拟了ENSO循环的不同阶段表层和次表层海水温度变化，海表温度最大振幅出现在120°W以东，与观测一致，表明模式可以较好反映热带地区大气、海洋的动力、热力特征。研究还比较了控制试验和CO2浓度年增长1%的瞬时试验，结果表明，在全球变暖的大环境下ENSO事件发生频率没有显著变化，但ENSO事件强度增大，年际变率变大；热带太平洋呈现整体增暖趋势，表层温度尤其是热带中太平洋地区温度升高显著。敏感性分析表明，年际ENSO变率的振幅增大的主要贡献来自于海洋。海水增温导致热带太平洋海温垂直梯度增大，在热带西太平洋海温垂直温度梯度变化最为明显；次表层海温对单位大气风应力变化的响应大于表层海温响应。当这种响应与热带太平洋赤道地区径向温度梯度变化的共同作用导致温室效应下ENSO振幅增大。     

Impact of 1/8° to 1/64° resolution on Gulf Stream model–data comparisons in basin-scale subtropical Atlantic Ocean models

We investigate the impact of 1/8°, 1/16°, 1/32°, and 1/64° ocean model resolution on model–data comparisons for the Gulf Stream system mainly between the Florida Straits and the Grand Banks. This includes mean flow and variability, the Gulf Stream pathway, the associated nonlinear recirculation gyres, the large-scale C-shape of the subtropical gyre and the abyssal circulation. A nonlinear isopycnal, free surface model covering the Atlantic from 9°N to 47°N or 51°N, including the Caribbean and Gulf of Mexico, and a similar 1/16° global model are used. The models are forced by winds and by a global thermohaline component via ports in the model boundaries. When calculated using realistic wind forcing and Atlantic model boundaries, linear simulations with Munk western boundary layers and a Sverdrup interior show two unrealistic mean Gulf Stream pathways between Cape Hatteras and the Grand Banks, one proceeding due east from Cape Hatteras and a second one continuing northward along the western boundary until forced eastward by the regional northern boundary. The northern pathway is augmented when a linear version of the upper ocean global thermohaline contribution to the Gulf Stream is added as a Munk western boundary layer. A major change is required to obtain a realistic pathway in nonlinear models. Resolution of 1/8° is eddy-resolving but mainly gives a wiggly version of the linear model Gulf Stream pathway and weak abyssal flows except for the deep western boundary current (DWBC) forced by ports in the model boundaries. All of the higher resolution simulations show major improvement over the linear and 1/8° nonlinear simulations. Additional major improvement is seen with the increase from 1/16° to 1/32° resolution and modest improvement with a further increase to 1/64°. The improvements include (1) realistic separation of the Gulf Stream from the coast at Cape Hatteras and a realistic Gulf Stream pathway between Cape Hatteras and the Grand Banks based on comparisons with Gulf Stream pathways from satellite IR and from GEOSAT and TOPEX/Poseidon altimetry (but 1/32° resolution was required for robust results), (2) realistic eastern and western nonlinear recirculation gyres (which contribute to the large-scale C-shape of the subtropical gyre) based on comparisons with mean surface dynamic height from the generalized digital environmental model (GDEM) oceanic climatology and from the pattern and amplitude of sea surface height (SSH) variability surrounding the eastern gyre as seen in TOPEX/Poseidon altimetry, (3) realistic upper ocean and DWBC transports based on several types of measurements, (4) patterns and amplitude of SSH variability which are generally realistic compared to TOPEX/Poseidon altimetry, but which vary from simulation to simulation for specific features and which are most realistic overall in the 1/64° simulation, (5) a basin wide explosion in the number and strength of mesoscale eddies (with warm core rings (WCRs) north of the Gulf Stream, the regional eddy features best observed by satellite IR), (6) realistic statistics for WCRs north of the Gulf Stream based on comparison to IR analyses (low at 1/16° resolution and most realistic at 1/64° resolution for mean population and rings generated/year; realistic ring diameters at all resolutions), and (7) realistic patterns and amplitude of abyssal eddy kinetic energy (EKE) in comparison to historical measurements from current meters.     

Sensitivity studies with the North Atlantic sigma coordinate Princeton Ocean Model

The sigma coordinate, Princeton Ocean Model (POM) has been configured for the North Atlantic Ocean between 5°N and 50°N as part of data assimilation, model predictability and intercomparison studies. The model uses a curvilinear orthogonal grid with higher resolution in the western North Atlantic and lower resolution in the eastern North Atlantic. A series of experiments, each one of a 10-year duration, are performed to evaluate the sensitivity of the ocean mean state and variability to model parameters and model configuration; these experiments include open vs. closed boundary conditions, low vs. high resolution grids, and different choices of diffusion and viscosity. The results show that the use of closed boundaries together with near-boundary buffer zones where temperature and salinity are relaxed towards the observed values give less realistic flows, weaker recirculation gyres and less realistic Gulf Stream separation than do open boundary conditions. The experiments show that the sensitivity of the ocean variability in the model to the choice of the Smagorinsky diffusion and viscosity coefficients significantly differs from one region to another and largely depends on other attributes such as the mean position of the Gulf Stream in each simulation. A 50% change in model resolution in the Gulf Stream region has a larger effect on ocean variability than a change of diffusivity by a factor of 10. In areas where either the high or the low resolution models have sufficient resolution, as in the Gulf of Mexico, they are able to produce variability comparable to that observed from altimeter data; elsewhere, model variability is underestimated.     

An efficient climate model with a 3D ocean and statistical–dynamical atmosphere*

We describe a coupled climate model of intermediate complexity designed for use in global warming experiments. The atmospheric component is a two-dimensional (zonally averaged) statistical-dynamical model based on the Goddard Institute for Space Study's atmospheric general circulation model (GCM). In contrast with energy-balance models used in some climate models of intermediate complexity, this model includes full representation of the hydrological and momentum cycles. It also has parameterizations of the main physical processes, including a sophisticated radiation code. The ocean component is a coarse resolution ocean GCM with simplified global geometry based on the Geophysical Fluid Dynamics Laboratory modular ocean model. Because of the simplified geometry the resolution in the western boundary layers can be readily increased compared to conventional coarse resolution models, without increasing the model's computational requirements in a significant way. The ocean model's efficiency is also greatly increased by using a moderate degree of asynchronous coupling between the oceanic momentum and tracer fields. We demonstrate that this still allows an accurate simulation of transient behavior, including the seasonal cycle. A 100 years simulation with the model requires less than 8 hours on a state-of the art workstation. The main novelty of the model is therefore a combination of computational efficiency, statistical-dynamical atmosphere and 3D ocean. Long-term present-day climate simulations are carried out using the coupled model with and without flux adjustments, and with either the Gent-McWilliams (GM) parametrization scheme or horizontal diffusion (HD) in the ocean. Deep ocean temperatures systematically decrease in the runs without flux adjustment. We demonstrate that the mismatch between heat transports in the uncoupled states of two models is the main cause for the systematic drift. In addition, changes in the circulation and sea-ice formation also contribute to the drift. Flux adjustments in the freshwater fluxes are shown to have a stabilizing effect on the thermohaline circulation in the model, whereas the adjustments in the heat fluxes tend to weaken the global "conveyor". To evaluate the model's response to transient external forcing global warming simulations are also carried out with the flux-adjusted version of the coupled model. The coupled model reproduces reasonably well the behavior of more sophisticated coupled GCMs for both current climate and for the global warming scenarios.     

区域海气耦合模式FROALS的发展及其应用

区域海气耦合模式是研究局地海气相互作用过程影响气候变率的重要平台,也是对全球气候模式进行"动力降尺度"的重要工具.本文介绍了LASG(State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics)/IAP(Institute of Atmospheric Physics)发展的区域海气耦合模式FROALS(Flexible Regional Ocean-Atmosphere-Land System model),并总结了过去五年围绕该区域海气耦合模式开展的研究工 作.FROALS的特点之一是有两个完全不同的大气模式分量和海洋模式分量选项,可以适应不同的模拟研究需 求.针对区域海气耦合模式在西北太平洋地区的模拟偏差,通过分步骤考察不同大气模式分量和不同海洋模式分量对模式模拟性能的影响,指出大气模式是导致区域海气耦合偏差的主要分量.通过改进对流触发的相对湿度阈值标准,有效地改善了此前区域海气耦合模式在亚洲季风区普遍出现的"模拟海温冷偏差".改进的FROALS对西北太平洋地区的大气和海洋环境有较好的模拟能力,合理地再现了西北太平洋地区表层洋流气候态和年际变率.较之非耦合模式,考虑区域海气耦合过程后,改进了东亚和南亚地区的降水和热带气旋潜势年际变率的模拟.最后,针对东亚—西北太平洋地区,利用FROALS对IAP/LASG全球气候模式模拟和预估的结果进行了动力降尺 度,得到了东亚区域50 km高分辨率区域气候变化信息.分析显示,FROALS模拟得到的东亚区域气候较之全球气候模式和非耦合区域气候模式结果具有明显的"增值",显示出区域海气耦合模式在该区域良好的应用前景.     

Argo大洋观测资料的同化及其在短期气候预测和海洋分析中的应用

国际Argo(Array for Real-time Geostrophic Oceanography)计划的实施,提供了前所未有的全球深海大洋0～2000 m水深范围内的海水温度和盐度观测资料,在大气和海洋科研业务中应用这一全新的资料,是深入认识大气和海洋变异、提高我国气候预测、海洋监测分析和预报能力的一个关键所在.通过开发非线性温—盐协调同化方案和利用同化高度计资料来调整模式的温度和盐度场,建立了可同化包括Argo等多种海洋观测资料的全球海洋资料变分同化系统,提高了对全球海洋的监测分析能力.实现了海洋资料同化系统与全球海气耦合模式的耦合,显著提高了短期气候预测水平.利用Argo资料改进了海洋动力模式中的物理过程参数化方案,有效提高了海洋模式对真实大洋的模拟能力和对厄尔尼诺/拉尼娜的预测能力.开发了利用Argo浮标漂流轨迹推算全球海洋表层和中层流的方法,提高了推算的全球表层流、中层流资料质量,有效弥补了洋流观测的匮乏.     

Towards a parametrization of river discharges into ocean general circulation models: a closure through energy conservation

Diagnostic methods are defined in order to compare two numerical simulations of ocean dynamics in a region of freshwater influence. The first one is a river plume simulation based on a high resolution numerical configuration of the POM coastal ocean model in which mixing parametrizations have been previously defined. The second one is a simulation based on the NEMO Global Ocean Model used for climate simulations in its half-a-degree configuration in which a river inflow is represented as precipitation on two coastal grid cells. Both simulations are forced with the same freshwater inflows and wind stresses. The divergence of volumetric fluxes above and below the halocline are compared. Results show that when an upwelling wind blows, the two models display similar behavior although the impact of lack of precision can be observed in the NEMO configuration. When a downwelling wind blows, the NEMO Global Ocean configuration can not reproduce the coastally trapped baroclinic dynamics because its grid resolution is too coarse. To find a parametrization to help represent these dynamics in ocean general circulation models, a method based on energy conservation is investigated. This method shows that it is possible to link the energy fluxes provided by river inflows to the divergence of energy fluxes integrated over the grid cells of ocean general circulation models. A parametrization of the dynamics created by freshwater inflows is deduced from this method. This enabled creation of a box model that proved to have the same behavior as the fluxes previously computed from the high resolution configuration.     

A numerical world ocean general circulation model: Part II. A baroclinic experiment

A new six-layer world ocean general circulation model based on the primitive system of equations is described in detail and its performance in the case of a homogeneous ocean is described. These test integrations show that the model is capable of reproducing the observed mean barotropic or vertically-integrated transport, as well as the seasonal variability of the major ocean gyres. The surface currents, however, are dominated by the Ekman transport, and such non-linear features as the western boundary currents and the equatorial countercurrents are poorly represented. The abyssal boundary countercurrents are also absent due to the lack of thermohaline forcing. The most conspicuous effect of the bottom topography on a homogeneous ocean is seen in the Southern ocean where the calculated Antarctic circumpolar transport through the Drake passage ( ≈ 10 Sv, with bathymetry included) greatly underestimates the observed transport (≈ 100 Sv).     

A numerical world ocean general circulation model Part I. Basic design and barotropic experiment

A new six-layer world ocean general circulation model based on the primitive system of equations is described in detail and its performance in the case of a homogeneous ocean is described. These test integrations show that the model is capable of reproducing the observed mean barotropic or vertically-integrated transport, as well as the seasonal variability of the major ocean gyres. The surface currents, however, are dominated by the Ekman transport, and such non-linear features as the western boundary currents and the equatorial countercurrents are poorly represented. The abyssal boundary countercurrents are also absent due to the lack of thermohaline forcing. The most conspicuous effect of the bottom topography on a homogeneous ocean is seen in the Southern ocean where the calculated Antarctic circumpolar transport through the Drake passage ( ≈ 10 Sv, with bathymetry included) greatly underestimates the observed transport (≈ 100 Sv).     

Spatial and temporal variability of major ocean currents and mesoscale eddies

A global picture of ocean current variability may be obtained by analyzing surface drift currents in terms of their mean and eddy kinetic energies. High values for both quantities are found in the western boundary currents and in the equatorial current system; low values are found in the interior of major gyres. However, nowhere are eddy energies less than 200 cm² s^–2, indicating that, even in the least energetic parts of the oceans, surface speeds of ~20 cm s^–1 prevail. Recent experimental studies also support the widespread occurrence of mesoscale mid-oceanic eddies. Another type of eddy is abundant in the vicinity of boundary currents: examples include Gulf Stream Rings, the Great Whirl of the Somali Current, and disturbances of the predominantly zonal equatorial flow manifested by large-scale meandering about the equator. Recent numerical models using low-viscosity and high-resolution computational grids also reveal the ubiquitous existence of mesoscale structures. The importance of eddies is that they seem to be energetic enough and sufficiently widespread so as to play some part — not yet understood — in the circulation of the world ocean. Speculative analogies to the atmosphere suggest that the mesoscale ocean eddies are the storms and weather systems of the sea. We need global statistics on their distribution, their occurrence in various oceanic regions, their dimensions, and their lifetimes. The prospect of even a single global oceanic weather map, comparable to those obtained daily for the atmosphere, is hopeless in terms ofin situ oceanographic observations. Remote sensing may provide a partial solution.In the past, sea-surface temperature observations by satellites have revealed cold and warm eddies shed by western boundary currents. Satellite observations, moreover, have shown the thermal effects of continental shelf waves and areas with pronounced upwelling. These phenomena are characterized by strong temperature gradients and relatively large differences in surface elevation. Mid-ocean eddies are far more subtle and difficult to observe since they possess smaller differences of temperature and sea-surface topography. Three representative examples, including recent experimental results in western boundary currents, the equatorial region, and a typical mid-ocean region, are discussed in detail. Typical signals of temperature and sea-level topography, as well as typical temporal and spatial scales of the observed phenomena, are given.     

Wind driven general circulation of the Mediterranean Sea simulated with a Spectral Element Ocean Model

This work is an attempt to simulate the Mediterranean Sea general circulation with a Spectral Finite Element Model. This numerical technique associates the geometrical flexibility of the finite elements for the proper coastline definition with the precision offered by spectral methods. The model is reduced gravity and we study the wind-driven ocean response in order to explain the large scale sub-basin gyres and their variability. The study period goes from January 1987 to December 1993 and two forcing data sets are used. The effect of wind variability in space and time is analyzed and the relationship between wind stress curl and ocean response is stressed. Some of the main permanent structures of the general circulation (Gulf of Lions cyclonic gyre, Rhodes gyre, Gulf of Syrte anticylone) are shown to be induced by permanent wind stress curl structures. The magnitude and spatial variability of the wind is important in determining the appearance or disappearance of some gyres (Tyrrhenian anticyclonic gyre, Balearic anticyclonic gyre, Ionian cyclonic gyre). An EOF analysis of the seasonal variability indicates that the weakening and strengthening of the Levantine basin boundary currents is a major component of the seasonal cycle in the basin.The important discovery is that seasonal and interannual variability peak at the same spatial scales in the ocean response and that the interannual variability includes the change in amplitude and phase of the seasonal cycle in the sub-basin scale gyres and boundary currents. The Coriolis term in the vorticity balance seems to be responsible for the weakening of anticyclonic structures and their total disappearance when they are close to a boundary.The process of adjustment to winds produces a train of coastally trapped gravity waves which travel around the eastern and western basins, respectively in approximately 6 months. This corresponds to a phase velocity for the wave of about 1 m/s, comparable to an average velocity of an internal Kelvin wave in the area.     

Effects of Spectral Nudging on Oceanic States in a Coarse-Resolution Model

A 1-degree global model is used to investigate the skill of spectral nudging at coarse resolution by performing two numerical experiments, one with spectral nudging and the other without. In the spectral nudging experiment, the model temperature and salinity are nudged to an observed climatological monthly-mean field. The study compares the model mean state, as well as the interannual and decadal variability of oceanic quantities with observations, (e.g., sea surface height (SSH) and sea surface temperature (SST)). Spectral nudging is found to be effective in constraining model drift from the observed mean state of temperature and salinity in the global ocean, which has been reported in previous studies. The present study further shows that spectral nudging significantly improves the model skill of topostrophy (a measure of currents flowing along the topography) in water depth below 2000?m with no clear improvement elsewhere. Despite its known ability to damp oceanic variability at various time scales, spectral nudging can still represent the interannual and decadal variability of SSH and SST well, to a degree comparable to the other experiment.     

全球热带海气耦合距平模式及其数值模拟

用观测海温距平对一个中等分辨率的细网格大气模式和观测 FSU假风应力对全球热带三大洋距平海洋模式分别进行强迫试验。结果表明 :无论是大气模式还是海洋模式均在一定程度上较好地模拟了观测事实 ,热带大西洋的模拟效果好于热带印度洋。对大气模式而言 ,经向风距平的模拟要好于纬向风距平 ,热带大西洋西部好于东部 ,热带印度洋东部好于西部 ,赤道南侧好于北侧。对海洋模式而言 ,热带大西洋模拟最好的区域是赤道中东部 ,对赤道东印度洋的暖事件及偶极子事件年际变率模拟也较好。在此基础上 ,用和海气耦合模式同样的耦合方式将两者耦合起来 ,构成了一个中等复杂程度的全球热带海气耦合模式 ,这是进一步研究全球热带海气相互作用的基础     

Tropical paleoclimates at the Last Glacial Maximum: comparison of Paleoclimate Modeling Intercomparison Project (PMIP) simulations and paleodata

 Seventeen simulations of the Last Glacial Maximum (LGM) climate have been performed using atmospheric general circulation models (AGCM) in the framework of the Paleoclimate Modeling Intercomparison Project (PMIP). These simulations use the boundary conditions for CO₂, insolation and ice-sheets; surface temperatures (SSTs) are either (a) prescribed using CLIMAP data set (eight models) or (b) computed by coupling the AGCM with a slab ocean (nine models). The present-day (PD) tropical climate is correctly depicted by all the models, except the coarser resolution models, and the simulated geographical distribution of annual mean temperature is in good agreement with climatology. Tropical cooling at the LGM is less than at middle and high latitudes, but greatly exceeds the PD temperature variability. The LGM simulations with prescribed SSTs underestimate the observed temperature changes except over equatorial Africa where the models produce a temperature decrease consistent with the data. Our results confirm previous analyses showing that CLIMAP (1981) SSTs only produce a weak terrestrial cooling. When SSTs are computed, the models depict a cooling over the Pacific and Indian oceans in contrast with CLIMAP and most models produce cooler temperatures over land. Moreover four of the nine simulations, produce a cooling in good agreement with terrestrial data. Two of these model results over ocean are consistent with new SST reconstructions whereas two models simulate a homogeneous cooling. Finally, the LGM aridity inferred for most of the tropics from the data, is globally reproduced by the models with a strong underestimation for models using computed SSTs. Received: 9 September 1998 / Accepted: 18 March 1999     

Sensitivities of numerical model forecasts of extreme cyclone events

A global forecast model is used to examine various sensitivities of numerical predictions of three extreme winter storms that occurred near the eastern continental margin of North America: the Ohio Valley blizzard of January 1978, the New England blizzard of February 1978, and the Mid-Atlantic cyclone of February 1979. While medium-resolution simulations capture much of the intensification, the forecasts of the precise timing and intensity levels suffer from various degrees of error. The coastal cyclones show a 5-10 hPa dependence on the western North Atlantic sea surface temperature, which is varied within a range (± 2.5℃) compatible with interannual fluctuations. The associated vertical velocities and precipitation rates show proportionately stronger dependences on the ocean temperature perturbations. The Ohio Valley blizzard, which intensified along a track 700-800 km from the coast, shows little sensitivity to ocean temperature. The effect of a shift of - 10?latitude in the position of the snow boundary is negligible in each case. The forecasts depend strongly on the model resolution, and the coarse-resolution forecasts are consistently inferior to the medium-resolution forecasts. Studies of the corresponding sensitivities of extreme cyclonic events over eastern Asia are encouraged in order to identify characteristics that are common to numerical forecasts for the two regions.     

ENSIP: the El Niño simulation intercomparison project

An ensemble of twenty four coupled ocean-atmosphere models has been compared with respect to their performance in the tropical Pacific. The coupled models span a large portion of the parameter space and differ in many respects. The intercomparison includes TOGA (Tropical Ocean Global Atmosphere)-type models consisting of high-resolution tropical ocean models and coarse-resolution global atmosphere models, coarse-resolution global coupled models, and a few global coupled models with high resolution in the equatorial region in their ocean components. The performance of the annual mean state, the seasonal cycle and the interannual variability are investigated. The primary quantity analysed is sea surface temperature (SST). Additionally, the evolution of interannual heat content variations in the tropical Pacific and the relationship between the interannual SST variations in the equatorial Pacific to fluctuations in the strength of the Indian summer monsoon are investigated. The results can be summarised as follows: almost all models (even those employing flux corrections) still have problems in simulating the SST climatology, although some improvements are found relative to earlier intercomparison studies. Only a few of the coupled models simulate the El Niño/Southern Oscillation (ENSO) in terms of gross equatorial SST anomalies realistically. In particular, many models overestimate the variability in the western equatorial Pacific and underestimate the SST variability in the east. The evolution of interannual heat content variations is similar to that observed in almost all models. Finally, the majority of the models show a strong connection between ENSO and the strength of the Indian summer monsoon.     

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