共查询到20条相似文献，搜索用时 312 毫秒
1.
ENSO dynamics and seasonal cycle in the tropical Pacific as simulated by the ECHAM4/OPYC3 coupled general circulation model 总被引：3，自引：0，他引：3
The new version of the atmospheric general circulation model (AGCM), ECHAM4, at the Max Planck Institute for Meteorology,
Hamburg, has been coupled to the OPYC3 isopycnic global ocean general circulation and sea ice model in a multi-century present-day
climate simulation. Non-seasonal constant flux adjustment for heat and freshwater was employed to ensure a long-term annual
mean state close to present-day climatology. This study examines the simulated upper ocean seasonal cycle and interannual
variability in the tropical Pacific for the first 100 years. The coupled model’s seasonal cycle of tropical Pacific SSTs is
satisfactory with respect to both the warm pool variation and the Central and Eastern Pacific, with significant errors only
in the cold tongue around April. The cold phase cold tongue extent and strength is as observed, and for this the heat flux
adjustment does not play a decisive role. A well-established South Pacific convergence zone is characteristic for the new
AGCM version. Apart from extending the southeast trades seasonal maximum to midbasin, wind stress pattern and strength are
captured. Overall the subsurface structure is consistent with the observed, with a pronounced thermocline at about 150 m depth
in the west and rising to the surface from 160 °W to 100 °W. The current system is better resolved than in some previous global
models and, on the whole, has the expected shape. The equatorial undercurrent is correctly positioned but the core is only
half as strong as observed. The north equatorial current and counter-current also have reduced maximum speeds but the April
minimum is captured. As with the companion publication from Roeckner et al. this study finds pronounced tropical Eastern and
Central Pacific interannual variability. Simulated and observed NINO3 sea surface temperature (SST) variability is represented
by a single, rather broadband, maximum of power spectral density, centered on about 28 months for the simulation and four
years for the observations. For simulation and observations, SST, windstress, and upper ocean heat content each exhibit a
single dominant large-scale amplitude and phase pattern, suggesting that the model captures the essential dynamics. The amplitude
of the essentially standing oscillation in SST in the NINO3 region attains the observed strength, but is weaker at the eastern
boundary. Anomalies of upper ocean heat content show off-equatorial westward and equatorial eastward propagation, the latter’s
arrival in the east of the basin coinciding with the SST anomalies. Equatorial wind stress anomalies near the date line provide
the appropriate forcing and clearly form a response to the anomalous SST.
Received: 14 June 1996 / Accepted: 11 November 1997 相似文献
2.
This study investigates the effects of air–sea interaction upon simulated tropical climatology, focusing on the boreal summer mean precipitation and the embedded intra-seasonal oscillation (ISO) signal. Both the daily coupling of ocean–atmosphere and the diurnal variation of sea surface temperature (SST) at every time step by accounting for the ocean mixed layer and surface-energy budget at the ocean surface are considered. The ocean–atmosphere coupled model component of the global/regional integrated model system has been utilized. Results from the coupled model show better precipitation climatology than those from the atmosphere-only model, through the inclusion of SST–cloudiness–precipitation feedback in the coupled system. Cooling the ocean surface in the coupled model is mainly responsible for the improved precipitation climatology, whereas neither the coupling itself nor the diurnal variation in the SST influences the simulated climatology. However, the inclusion of the diurnal cycle in the SST shows a distinct improvement of the simulated ISO signal, by either decreasing or increasing the magnitude of spectral powers, as compared to the simulation results that exclude the diurnal variation of the SST in coupled models. 相似文献
3.
文中研究了耦合海-气环流模式中的双热带辐合带(Double ITCZ)现象,并对这一现象的成因从海洋热量收支的角度进行了初步分析。Double ITCZ现象是在热带太平洋赤道南北两侧各出现一条ITCZ的现象,这是耦合海-气环流模式中的较为普遍的一种异常现象,与实际气候中出现的Double ITCZ现象并非指同一问题。文中对比观测和模式结果,指出了Double ITCZ现象的主要特征,针对它的出现过程进行细致分析,再利用模式输出的热量收支各项进行统计,得到了从海洋热收支分析得到的海表温度变化原因。与观测到的正常模态相比,Double ITCZ是一个异常的模态,它的特征突出地表现为西太平洋暖池区的降温和东南太平洋10°S附近的升温。海洋热量收支分析表明,暖池区的降温主要是受到扩散的作用,而表层平流的异常增暖在决定异常辐合带区升温过程中占第一位的作用。 相似文献
4.
A high-resolution tropical Pacific general circulation model (GCM) coupled to a global atmospheric GCM is described in this paper. The atmosphere component is the 5°×4°global general circulation model of the Institute of Atmospheric Physics (IAP) with 9 levels in the vertical direction. The ocean component with a horizontal resolution of 0.5°, is based on a low-resolution model (2°×1°in longitude-latitude).Simulations of the ocean component are first compared with its previous version. Results show that the enhanced ocean horizontal resolution allows an improved ocean state to be simulated; this involves (1) an apparent decrease in errors in the tropical Pacific cold tongue region, which exists in many ocean models,(2) more realistic large-scale flows, and (3) an improved ability to simulate the interannual variability and a reduced root mean square error (RMSE) in a long time integration. In coupling these component models, a monthly "linear-regression" method is employed to correct the model's exchanged flux between the sea and the atmosphere. A 100-year integration conducted with the coupled GCM (CGCM) shows the effectiveness of such a method in reducing climate drift. Results from years 70 to 100 are described.The model produces a reasonably realistic annual cycle of equatorial SST. The large SSTA is confined to the eastern equatorial Pacific with little propagation. Irregular warm and cold events alternate with a broad spectrum of periods between 24 and 50 months, which is very realistic. But the simulated variability is weaker than the observed and is also asymmetric in the sense of the amplitude of the warm and cold events. 相似文献
5.
We report the analysis of two 20-year simulations performed with the low resolution version of the IPSL coupled ocean-atmosphere
model, with no flux correction at the air-sea interface. The simulated climate is characterized by a global sea surface temperature
warming of about 4 °C in 20 years, driven by a net heat gain at the top of the atmosphere. Despite this drift, the circulation
is quite realistic both in the ocean and the atmosphere. Several distinct periods are analyzed. The first corresponds to an
adjustment during which the heat gain weakens both at the top of the atmosphere and at the ocean surface, and the tropical
circulation is slightly modified. Then, the surface warming is enhanced by an increase of the greenhouse feedback. We show
that the mechanisms involved in the model share common features with sensitivity experiments to greenhouse gases or to SST
warming. At the top of the atmosphere, most of the longwave trapping in the atmosphere is driven by the tropical circulation.
At the surface, the reduction of longwave cooling is a direct response to increased temperature and moisture content at low
levels in the atmospheric model. During the last part of the simulation, a regulation occurs from evaporation at the surface
and longwave cooling at TOA. Most of the model drift is attributed to a too large heating by solar radiation in middle and
high latitudes. The reduction of the north–south temperature gradient, and the related changes in the meridional equator-to-pole
ocean heat transport lead to a warming of equatorial and subtropical regions. This is also well demonstrated by the difference
between the two simulations which differ only in the parametrization of sea-ice. When the sea-ice cover is not restored to
climatology the model does not maintain sea-ice at high latitudes. The climate warms more rapidly and the water vapor and
clouds feedback occurs earlier.
Received: 24 May 1996 / Accepted: 29 November 1996 相似文献
6.
J. Venkata Ratnam Filippo Giorgi Akshara Kaginalkar Stefano Cozzini 《Climate Dynamics》2009,33(1):119-139
A regional coupled atmosphere–ocean model was developed to study the role of air–sea interactions in the simulation of the
Indian summer monsoon. The coupled model includes the regional climate model (RegCM3) as atmospheric component and the regional
ocean modeling system (ROMS) as oceanic component. The two-way coupled model system exchanges sea surface temperature (SST)
from the ocean to the atmospheric model and surface wind stress and energy fluxes from the atmosphere to the ocean model.
The coupled model is run for four years 1997, 1998, 2002 and 2003 and the results are compared with observations and atmosphere-only
model runs employing Reynolds SSTs as lower boundary condition. It is found that the coupled model captures the main features
of the Indian monsoon and simulates a substantially more realistic spatial and temporal distribution of monsoon rainfall compared
to the uncoupled atmosphere-only model. The intraseasonal oscillations are also better simulated in the coupled model compared
to the atmosphere-only model. These improvements are due to a better representation of the feedbacks between the SST and convection
and highlight the importance of air–sea coupling in the simulation of the Indian monsoon. 相似文献
7.
基于耦合器框架，中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室大气环流谱模式 （SAMIL）最近成功地实现了与海洋、海冰等气候分量模式的耦合，形成了“非通量调整”的海－陆－气－冰直接耦合的气候模式系统（FGOALS-s）。在耦合系统中，由于海温、海冰等的分布由预报模式驱动，大气与海洋、海冰之间引入了相互作用过程，这样大气环流的模拟特征与耦合前会有不同。为分析耦合系统的性能，作者对耦合前后的模拟结果进行了分析比较，重点是大气模拟特征的差异。结果表明，耦合前、后大气环流的基本特征相似，都能成功地模拟出主要的环流系统分布及季节变化，但是由于海温和海冰的模拟存在系统性的偏差，使得耦合后的大气环流受到明显影响。例如耦合后热带海温偏冷，南大洋、北太平洋和北大西洋等中纬度地区的海温偏高，导致海温等值线向高纬海域的伸展较弱，海温经向梯度减小。耦合后海冰在北极区域范围偏大，在南极周边地区则偏小。海温、海冰分布模拟的偏差影响到中、高纬低层大气的温度。热带海温偏低，使得赤道地区降水偏弱，凝结潜热减少，热带对流层中高层温度比耦合前要低，大气温度的经向梯度减小。经向温度梯度的改变，直接影响到对平均经圈环流及西风急流强度的模拟。尽管耦合系统中海温、海冰的模拟存在偏差，但在亚洲季风区，耦合后季风环流及降水等的分布都比耦合前单独大气模式的结果合理，表明通过海[CD*2]气相互作用可减少耦合前季风区的模拟误差，改善季风模拟效果。比较发现，海温、海冰模拟的偏差，除与海洋模式中经向热输送偏弱、海冰模式中海冰处理等有关外，也与大气模式中总云量模拟偏低有关。大气模式本身的误差，特别是云、辐射过程带来的误差，对耦合结果具有极为重要的影响。完全耦合后，这些误差通过与海洋、海冰的反馈作用而放大。因此，对于FGOALS-s而言，要提高耦合系统的整体性能，除改进各气候分量模式的模拟性能外，需要重点改进大气模式中的云、辐射过程。 相似文献
8.
The possibility of using a nonlinear empirical atmospheric model for hybrid coupled atmosphere-ocean modelling has been examined
by using a neural network (NN) model for predicting the contemporaneous wind stress field from the upper ocean state. Upper
ocean heat content (HC) from a 6-layer ocean model was a better predictor of the wind stress than the (observed or modelled)
sea surface temperature (SST). Our results showed that the NN model generally had slightly better skills in predicting the
contemporaneous wind stress than the linear regression (LR) model in the off-equatorial tropical Pacific and in the eastern
equatorial Pacific. When the wind stresses from the NN and LR models were used to drive the ocean model, slightly better SST
skills were found in the off-equatorial tropical Pacific and in the eastern equatorial Pacific when the NN winds were used
instead of the LR winds. Better skills for the model HC were found in the western and central equatorial Pacific when the
NN winds were used instead of the LR winds. Why NN failed to show more significant improvement over LR in the equatorial Pacific
for the wind stress and SST is probably because the relationship between the surface ocean and the atmosphere in the equatorial
Pacific over the seasonal time scale is almost linear.
Received: 2 March 1999 / Accepted: 13 July 2000 相似文献
9.
Jun Wei Paola Malanotte-Rizzoli Elfatih A. B. Eltahir Pengfei Xue Danya Xu 《Climate Dynamics》2014,43(5-6):1575-1594
Climatological high resolution coupled climate model simulations for the maritime continent have been carried out using the regional climate model (RegCM) version 3 and the finite volume coastal ocean model (FVCOM) specifically designed to resolve regions characterized by complex geometry and bathymetry. The RegCM3 boundary forcing is provided by the EMCWF-ERA40 re-analysis. FVCOM is embedded in the Global MITgcm which provides boundary forcing. The domain of the coupled regional model covers the entire South China Sea with its through-flow, the entire Indonesian archipelago with the Indonesian through-flow (ITF) and includes a large region in the western Pacific and eastern Indian oceans. The coupled model is able to provide stable and realistic climatological simulations for a specific decade of atmospheric–oceanic variables without flux correction. The major focus of this work is on oceanic properties. First, the coupled simulation is assessed against ocean-only simulations carried out under two different sets of air–sea heat fluxes. The first set, provided by the MITgcm, is proved to be grossly deficient as the heat fluxes are evaluated by a two-dimensional, zonally averaged atmosphere and the simulated SST have anomalous cold biases. Hence the MITgcm fluxes are discarded. The second set, the NCEP re-analysis heat fluxes, produces a climatological evolution of the SST with an average cold bias of ~?0.8 °C. The coupling eliminates the cold bias and the coupled SST evolution is in excellent agreement with the analogous evolution in the SODA re-analysis data. The detailed comparison of oceanic circulation properties with the International Nusantara Stratification and Transport observations shows that the coupled simulation produces the best estimate of the total ITF transport through the Makassar strait while the transports of three ocean-only simulations are all underestimated. The annual cycle of the transport is also very well reproduced. The coupling also considerably improves the vertical thermal structure of the Makassar cross section in the upper layer affected by the heat fluxes. On the other hand, the coupling is relatively ineffective in improving the precipitation fields even though the coupled simulation captures reasonably well the precipitation annual cycle at three land stations in different latitudes. 相似文献
10.
Based on the surface energy budget, the sea surface temperature (SST) variance is related to the product of three factors: the sum of the variances of surface radiative and turbulent energy fluxes and of ocean heat transport, an efficiency factor depending on the covariances among them, and a transfer factor involving the persistence of surface temperature via its lagged autocorrelation. These quantities are analyzed for current climate conditions based on results from the NCEP/NCAR reanalyses and a simulation with the CCCma coupled climate model. Potential changes with climate change are considered based on two quasi-equilibrium climate change integrations for which the forcing has been stabilized at years 2050 and 2100 values of the IS92a forcing scenario. The surface energy fluxes, which contribute to the variance of SST, are similar in the modelled and reanalyzed atmosphere but modelled temperature variance is conditioned on the thickness of the upper ocean model layer. Changes of SST variance with global warming show broad scale patterns with decreases in the tropical central-eastern Pacific and the northern extra-tropical Pacific, and increases in both the sub-tropical Pacific and mid-latitudes of the North Atlantic. The changes in SST variance are not associated only with changes in the variances of surface energy fluxes/transports but also with changes in the covariances among them and by changes in the temperature autocorrelation structure. 相似文献
11.
T. Furevik M. Bentsen H. Drange I. K. T. Kindem N. G. Kvamstø A. Sorteberg 《Climate Dynamics》2003,21(1):27-51
A new coupled atmosphere–ocean–sea ice model has been developed, named the Bergen Climate Model (BCM). It consists of the atmospheric model ARPEGE/IFS, together with a global version of the ocean model MICOM including a dynamic–thermodynamic sea ice model. The coupling between the two models uses the OASIS software package. The new model concept is described, and results from a 300-year control integration is evaluated against observational data. In BCM, both the atmosphere and the ocean components use grids which can be irregular and have non-matching coastlines. Much effort has been put into the development of optimal interpolation schemes between the models, in particular the non-trivial problem of flux conservation in the coastal areas. A flux adjustment technique has been applied to the heat and fresh-water fluxes. There is, however, a weak drift in global mean sea-surface temperature (SST) and sea-surface salinity (SSS) of respectively 0.1 °C and 0.02 psu per century. The model gives a realistic simulation of the radiation balance at the top-of-the-atmosphere, and the net surface fluxes of longwave, shortwave, and turbulent heat fluxes are within observed values. Both global and total zonal means of cloud cover and precipitation are fairly close to observations, and errors are mainly related to the strength and positioning of the Hadley cell. The mean sea-level pressure (SLP) is well simulated, and both the mean state and the interannual standard deviation show realistic features. The SST field is several degrees too cold in the equatorial upwelling area in the Pacific, and about 1 °C too warm along the eastern margins of the oceans, and in the polar regions. The deviation from Levitus salinity is typically 0.1 psu – 0.4 psu, with a tendency for positive anomalies in the Northern Hemisphere, and negative in the Southern Hemisphere. The sea-ice distribution is realistic, but with too thin ice in the Arctic Ocean and too small ice coverage in the Southern Ocean. These model deficiencies have a strong influence on the surface air temperatures in these regions. Horizontal oceanic mass transports are in the lower range of those observed. The strength of the meridional overturning in the Atlantic is 18 Sv. An analysis of the large-scale variability in the model climate reveals realistic El Niño – Southern Oscillation (ENSO) and North Atlantic–Arctic Oscillation (NAO/AO) characteristics in the SLP and surface temperatures, including spatial patterns, frequencies, and strength. While the NAO/AO spectrum is white in SLP and red in temperature, the ENSO spectrum shows an energy maximum near 3 years. 相似文献
12.
The climatology of the OPA/ARPEGE-T21 coupled general circulation model (GCM) is presented. The atmosphere GCM has a T21
spectral truncation and the ocean GCM has a 2°×1.5° average resolution. A 50-year climatic simulation is performed using the OASIS coupler, without flux correction techniques.
The mean state and seasonal cycle for the last 10 years of the experiment are described and compared to the corresponding
uncoupled experiments and to climatology when available. The model reasonably simulates most of the basic features of the
observed climate. Energy budgets and transports in the coupled system, of importance for climate studies, are assessed and
prove to be within available estimates. After an adjustment phase of a few years, the model stabilizes around a mean state
where the tropics are warm and resemble a permanent ENSO, the Southern Ocean warms and almost no sea-ice is left in the Southern
Hemisphere. The atmospheric circulation becomes more zonal and symmetric with respect to the equator. Once those systematic
errors are established, the model shows little secular drift, the small remaining trends being mainly associated to horizontal
physics in the ocean GCM. The stability of the model is shown to be related to qualities already present in the uncoupled
GCMs used, namely a balanced radiation budget at the top-of-the-atmosphere and a tight ocean thermocline.
Received: 1 February 1996 / Accepted: 1 August 1996 相似文献
13.
Observations indicated that for the El Niño/Southern Oscillation (ENSO) there have been eastward displacements of the zonal wind stress (WS) anomalies and surface heat flux (short wave heat flux and latent heat flux) anomalies during El Niño episodes in the 1981–1995 regime relative to the earlier regime of 1961–1975 (without corresponding displacements during La Niña episodes). Our numerical experiments with the Zebiak–Cane coupled model generally reproduced such displacements when the model climatological fields were replaced by the observed climatologies [of sea surface temperature (SST), surface WS and surface wind atmospheric divergence] and simulated climatologies (of oceanic surface layer currents and associated upwelling) for the 1981–1995 regime. Sensitivity tests indicated that the background atmospheric state played a much more important role than the background ocean state in producing the displacements, which enhanced the asymmetry between El Niño and La Niña in the later regime. The later regime climatology state resulted in the eastward shifts in the ENSO system (WS and SST) only during El Niño, through the eastward shift of the atmosphere convergence heating rate in the coupled model. The ENSO period and ENSO predictability were also enhanced in the coupled model under the later regime climatology. That the change in the mean state of the tropical Pacific atmosphere and ocean after the mid 1970s could have produced the observed changes in ENSO properties is consistent with our findings. 相似文献
14.
Impact of ocean model resolution on CCSM climate simulations 总被引：1，自引：1，他引：0
Ben P. Kirtman Cecilia Bitz Frank Bryan William Collins John Dennis Nathan Hearn James L. Kinter III Richard Loft Clement Rousset Leo Siqueira Cristiana Stan Robert Tomas Mariana Vertenstein 《Climate Dynamics》2012,39(6):1303-1328
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. 相似文献
15.
Rong-Hua Zhang 《Climate Dynamics》2014,42(1-2):467-485
Tropical instability waves (TIWs) arise from oceanic instability in the eastern tropical Pacific and Atlantic Oceans, having a clear atmospheric signature that results in coupled atmosphere–ocean interactions at TIW scales. In this study, the extent to which TIW-induced surface wind feedback influences the ocean is examined using an ocean general circulation model (OGCM). The TIW-induced wind stress (τTIW) part is diagnostically determined using an empirical τTIW model from sea surface temperature (SST) fields simulated in the OGCM. The interactively represented TIW wind tends to reduce TIW activity in the ocean and influence the mean state, with largest impacts during TIW active periods in fall and winter. In December, the interactive τTIW forcing induces a surface cooling (an order of ?0.1 to ?0.3 °C), an increased heat flux into the ocean, a shallower mixed layer and a weakening of the South Equatorial Current in the eastern equatorial Pacific. Additionally, the TIW wind effect yields a pronounced latitudinal asymmetry of sea level field across the equator, and a change to upper thermal structure, characterized by a surface cooling and a warming below in the thermocline, leading to a decreased temperature gradient between the mixed layer and the thermocline. Processes responsible for the τTIW–induced cooling effects are analyzed. Vertical mixing and meridional advection are the two terms in the SST budget that are dominantly affected by the TIW wind feedback: the cooling effect from the vertical mixing on SST is enhanced, with the maximum induced cooling in winter; the warming effect from the meridional advection is reduced in July–October, but enhanced in November–December. Additional experiments are performed to separate the relative roles the affected surface momentum and heat fluxes play in the cooling effect on SST. This ocean-only modeling work indicates that the effect of TIW-induced wind feedback is small but not negligible, and may need to be adequately taken into account in large-scale climate modeling. 相似文献
16.
The Kuroshio Extension region is characterized by energetic oceanic mesoscale and frontal variability that alters the air–sea fluxes that can influence large-scale climate variability in the North Pacific. We investigate this mesoscale air-sea coupling using a regional eddy-resolving coupled ocean–atmosphere (OA) model that downscales the observed large-scale climate variability from 2001 to 2007. The model simulates many aspects of the observed seasonal cycle of OA coupling strength for both momentum and turbulent heat fluxes. We introduce a new modeling approach to study the scale-dependence of two well-known mechanisms for the surface wind response to mesoscale sea surface temperatures (SSTs), namely, the ‘vertical mixing mechanism’ (VMM) and the ‘pressure adjustment mechanism’ (PAM). We compare the fully coupled model to the same model with an online, 2-D spatial smoother applied to remove the mesoscale SST field felt by the atmosphere. Both VMM and PAM are found to be active during the strong wintertime peak seen in the coupling strength in both the model and observations. For VMM, large-scale SST gradients surprisingly generate coupling between downwind SST gradient and wind stress divergence that is often stronger than the coupling on the mesoscale, indicating their joint importance in OA interaction in this region. In contrast, VMM coupling between crosswind SST gradient and wind stress curl occurs only on the mesoscale, and not over large-scale SST gradients, indicating the essential role of the ocean mesocale. For PAM, the model results indicate that coupling between the Laplacian of sea level pressure and surface wind convergence occurs for both mesoscale and large-scale processes, but inclusion of the mesoscale roughly doubles the coupling strength. Coupling between latent heat flux and SST is found to be significant throughout the entire seasonal cycle in both fully coupled mode and large-scale coupled mode, with peak coupling during winter months. The atmospheric response to the oceanic mesoscale SST is also studied by comparing the fully coupled run to an uncoupled atmospheric model forced with smoothed SST prescribed from the coupled run. Precipitation anomalies are found to be forced by surface wind convergence patterns that are driven by mesoscale SST gradients, indicating the importance of the ocean forcing the atmosphere at this scale. 相似文献
17.
Abstract A new coupled atmosphere‐ocean model has been developed for climate predictions at decade to century scales. The atmospheric model is similar to that of Hansen et al. (1983) except that the atmospheric dynamic equations for mass and momentum are solved using Arakawa and Lamb's (1977) C grid scheme and the advection of potential enthalpy and water vapour uses the linear upstream scheme (Russell and Lerner, 1981). The new global ocean model conserves mass, allows for divergent flow, has a free surface and uses the linear upstream scheme for the advection of potential enthalpy and salt. Both models run at 4° × 5° resolution, with 9 vertical layers for the atmosphere and 13 layers for the ocean. Twelve straits are included, allowing for subgrid‐scale water flow. Runoff from land is routed into appropriate ocean basins. Atmospheric and oceanic surface fluxes are of opposite sign and are applied synchronously. Flux adjustments are not used. Except for partial strength alternating binomial filters (Shapiro, 1970), which are applied to the momentum components in the atmosphere and oceans, there is no explicit horizontal diffusion. A 120‐year simulation of the coupled model starting from the oceanic initial conditions of Levitus (1982) is discussed. The model dynamics stabilize after several decades. The maximum northward ocean heat flux is 1.4 × 1015 W at 16°N. The model appears to maintain the vertical gradients characterizing the separation between the upper and deep ocean spheres. Inadequacies in the coupled model simulation lead to decreasing temperature and salinity in the high latitude North Atlantic and to a poor simulation of the northern North Atlantic thermohaline circulation. The mass transport of the Gulf Stream is about half of observed values, while the transports of the Kuroshio and Antarctic Circumpolar Currents are similar to observations. Additional deficiencies include a climate drift in the surface air temperature of 0.006°C year‐1 due to a radiation imbalance of 7.4 Wm‐2 at the top of the atmosphere and too warm temperatures in the eastern portions of tropical oceans. The coupled model should be useful for delineating modelling capabilities without the use of flux adjustments and should serve as a benchmark for future model improvements. 相似文献
18.
The sea surface temperature (SST) or sea level pressure (SLP) has usually been used to measure the strength of El Niño–Southern Oscillation (ENSO) events. In this study, two new indices, based on the upper-ocean heat content (HC), are proposed to quantify the two “flavours” of El Niño (i.e., the Cold Tongue El Niño (CTE) and Warm Pool El Niño (WPE)). Compared with traditional SST or SLP indices, the new HC-based indices can distinguish CTE and WPE events much better and also represent the two leading modes of the interannual variability of the atmosphere–ocean coupled system in the tropical Indo-Pacific region. The two leading modes are obtained by performing multivariate Empirical Orthogonal Function analysis on two oceanic variables (SST and HC) over the tropical Pacific (30°S–30°N, 120°E–80°W) and six atmospheric variables (outgoing longwave radiation, SLP, streamfunction, and velocity potential at 850?hPa and 200?hPa) over the tropical Indo-Pacific region (30°S–30°N, 60°E–80°W) for the period 1980–2010. Because the two new HC-based indices are capable of better depicting coherent variations between the ocean and atmosphere, they can provide a supplementary tool for ENSO monitoring of and climate research into the two flavours of El Niño. 相似文献
19.
Jennifer A. Graham David P. Stevens Karen J. Heywood Zhaomin Wang 《Climate Dynamics》2011,37(1-2):297-311
Recent observations suggest Antarctic Intermediate Water (AAIW) properties are changing. The impact of such variations is explored using idealised perturbation experiments with a coupled climate model, HadCM3. AAIW properties are altered between 10 and 20°S in the South Atlantic, maintaining constant potential density. The perturbed AAIW remains subsurface in the South Atlantic, but as it moves northwards, it surfaces and interacts with the atmosphere leading to density anomalies due to heat exchanges. For a cooler, fresher AAIW, there is a significant decrease in the mean North Atlantic sea surface temperature (SST), of up to 1°C, during years 51?C100. In the North Atlantic Current region there are persistent cold anomalies from 2,000?m depth to the surface, and in the overlying atmosphere. Atmospheric surface pressure increases over the mid-latitude Atlantic, and precipitation decreases over northwest Africa and southwest Europe. Surface heat flux anomalies show that these impacts are caused by changes in the ocean rather than atmospheric forcing. The SST response is associated with significant changes in the Atlantic meridional overturning circulation (MOC). After 50?years there is a decrease in the MOC that persists for the remainder of the simulation, resulting from changes in the column-averaged density difference between 30°S and 60°N. Rather than showing a linear response, a warmer, saltier AAIW also leads to a decreased MOC strength for years 51?C100 and resulting cooling in the North Atlantic. The non-linearity can be attributed to opposing density responses as the perturbed water masses interact with the atmosphere. 相似文献
20.
Coupled ocean-atmosphere surface variability and its climate impacts in the tropical Atlantic region
This study examines time evolution and statistical relationships involving the two leading ocean-atmosphere coupled modes
of variability in the tropical Atlantic and some climate anomalies over the tropical 120 °W–60 °W region using selected historical
files (75-y near global SSTs and precipitation over land), more recent observed data (30-y SST and pseudo wind stress in the
tropical Atlantic) and reanalyses from the US National Centers for Environmental Prediction (NCEP/NCAR) reanalysis System
on the period 1968–1997: surface air temperature, sea level pressure, moist static energy content at 850 hPa, precipitable
water and precipitation. The first coupled mode detected through singular value decomposition of the SST and pseudo wind-stress
data over the tropical Atlantic (30 °N–20 °S) expresses a modulation in the thermal transequatorial gradient of SST anomalies
conducted by one month leading wind-stress anomalies mainly in the tropical north Atlantic during northern winter and fall.
It features a slight dipole structure in the meridional plane. Its time variability is dominated by a quasi-decadal signal
well observed in the last 20–30 ys and, when projected over longer-term SST data, in the 1920s and 1930s but with shorter
periods. The second coupled mode is more confined to the south-equatorial tropical Atlantic in the northern summer and explains
considerably less wind-stress/SST cross-covariance. Its time series features an interannual variability dominated by shorter
frequencies with increased variance in the 1960s and 1970s before 1977. Correlations between these modes and the ENSO-like
Nino3 index lead to decreasing amplitude of thermal anomalies in the tropical Atlantic during warm episodes in the Pacific.
This could explain the nonstationarity of meridional anomaly gradients on seasonal and interannual time scales. Overall the
relationships between the oceanic component of the coupled modes and the climate anomaly patterns denote thermodynamical processes
at the ocean/atmosphere interface that create anomaly gradients in the meridional plane in a way which tends to alter the
north–south movement of the seasonal cycle. This appears to be consistent with the intrinsic non-dipole character of the tropical
Atlantic surface variability at the interannual time step and over the recent period, but produces abnormal amplitude and/or
delayed excursions of the intertropical convergence zone (ITCZ). Connections with continental rainfall are approached through
three (NCEP/NCAR and observed) rainfall indexes over the Nordeste region in Brazil, and the Guinea and Sahel zones in West
Africa. These indices appear to be significantly linked to the SST component of the coupled modes only when the two Atlantic
modes+the ENSO-like Nino3 index are taken into account in the regressions. This suggests that thermal forcing of continental
rainfall is particularly sensitive to the linear combinations of some basic SST patterns, in particular to those that create
meridional thermal gradients. The first mode in the Atlantic is associated with transequatorial pressure, moist static energy
and precipitable water anomaly patterns which can explain abnormal location of the ITCZ particularly in northern winter, and
hence rainfall variations in Nordeste. The second mode is more associated with in-phase variations of the same variables near
the southern edge of the ITCZ, particularly in the Gulf of Guinea during the northern spring and winter. It is primarily linked
to the amplitude and annual phase of the ITCZ excursions and thus to rainfall variations in Guinea. Connections with Sahel
rainfall are less clear due to the difficulty for the model to correctly capture interannual variability over that region
but the second Atlantic mode and the ENSO-like Pacific variability are clearly involved in the Sahel climate interannual fluctuations:
anomalous dry (wet) situations tend to occur when warmer (cooler) waters are present in the eastern Pacific and the gulf of
Guinea in northern summer which contribute to create a northward (southward) transequatorial anomaly gradient in sea level
pressure over West Africa.
Received: 14 April 1998 / Accepted: 24 December 1998 相似文献