ENSO phase-locking in an ocean-atmosphere coupled model FGCM-1.0

The mean climatology and the basic characteristics of the ENSO cycle simulated by a coupled model FGCM-1.0 are investigated in this study. Although with some common model biases as in other directly coupled models, FGCM-1.0 is capable of producing the interannual variability of the tropical Pacific, such as the ENSO phenomenon. The mechanism of the ENSO events in the coupled model can be explained by “delayed oscillator” and “recharge-discharge” hypotheses. Compared to the observations, the simulated ENSO events show larger amplitude with two distinctive types of phase-locking： one with its peak phase-locked to boreal winter and the other to boreal summer. These two types of events have a similar frequency of occurrence, but since the second type of event is seldom observed, it may be related to the biases of the coupled model. Analysis show that the heat content anomalies originate from the central south Pacific in the type of events peaking in boreal summer, which can be attributed to a different background climatology from the normal events. The mechanisms of their evolutions are also discussed.     

ENSO cycle in a coupled ocean-atmosphere model and its negative feedback mechanism

Summary A coupled ocean-atmosphere anomaly model has been developed for simulating ENSO cycle and its mechanism-study in this paper. After a long model run, the coupled model is successful in demonstrating ENSO-like irregular interannual variability and corresponding horizontal spatial structures. Based on the simulated results, the dynamics and the thermodynamics of the model ENSO cycle have been investigated, and in particular the negative feedback mechanisms that act to oppose instability of air-sea interaction, inducing termination of warm and cold events, have been examined. A detailed analysis of the oceanic wave dynamical properties and heat budget of the SST changes in a representative cycle suggest that the negative feedback mechanism to check the unstable growth of a warm event obviously differs from that of a cold event. The mechanism that induces decay and termination of a cold event is closely related to the negative, delayed feedback effect produced by the oceanic dynamical wave reflection at the western boundary. However, independent of the wave reflection effect, the negative feedback mechanism by which the coupled system returns from a warm event is associated with a slowly eastward-propagating coupling mode. Accompanied with the strong unstable development of the equatorial positive SST anomaly, the anomalous upwelling of cold water generated off the equator and the nonlinear anomalous meridional advection generated in the equator west of instability area jointly restrain the instability and finally plunge the system from a mature warm phase into a weak cold phase. A comparison between the results from the present model and the previous works is also discussed in this paper.With 16 Figures     

Unstable interannual oscillation modes in a linearized coupled ocean-atmosphere model and their associations with ENSO

Summary By using a coupled ocean-atmosphere model with an oceanic surface boundary layer, including linear atmospheric and oceanic dynamics and linearized SST prognostic equation with respect to spatially varying climatological background states, we have investigated the eigenvalue problem of the linearized coupled system in the tropical Pacific, including the characteristic periods, horizontal structures, temporal-spatial evolution and instability of the unstable interannual oscillation characteristic modes and their associations with ENSO. The main results show that the quasi-biennial (QB) oscillation was found to act as the most unstable mode in the tropical Pacific coupled air-sea system. Only the most unstable QB mode displays the ENSO-like structure and temporalspatial evolution, and its existence seems likely to have no essential dependence on the climatological annual cycle (AC). Unfortunately, from the linearized coupled system we have not derived a most unstable mode relevant to the observed principle mode with the preferred 3–4 year lower-frequency (LF) oscillation period in the real world ENSO variability. Therefore, we infer that the LF mode would likely result from certain nonlinear interaction, in which the QB mode that acts as the shortest ENSO cycle could be fundamentally important. Also, we believe that the results in present work could be helpful to fully understand the multiple time scales and the associated mechanism responsible for the real world ENSO variability.With 7 Figures     

Time-dependent greenhouse warming computations with a coupled ocean-atmosphere model

Climate changes during the next 100 years caused by anthropogenic emissions of greenhouse gases have been simulated for the Intergovernmental Panel on Climate Change Scenarios A (business as usual) and D (accelerated policies) using a coupled ocean-atmosphere general circulation model. In the global average, the near-surface temperature rises by 2.6 K in Scenario A and by 0.6 K in Scenario D. The global patterns of climate change for both IPCC scenarios and for a third step-function 2 x CO₂ experiment were found to be very similar. The warming delay over the oceans is larger than found in simulations with atmospheric general circulation models coupled to mixed-layer models, leading to a more pronounced land-sea contrast and a weaker warming (and in some regions even an initial cooling) in the Southern Ocean. During the first forty years, the global warming and sea level rise due to the thermal expansion of the ocean are significantly slower than estimated previously from box-diffusion-upwelling models, but the major part of this delay can be attributed to the previous warming history prior to the start of present coupled ocean-atmosphere model integration (cold start).     

Separating freshwater flux effects on ENSO in a hybrid coupled model of the tropical Pacific

Climate Dynamics - Freshwater flux (FWF) at the sea surface, defined as precipitation minus evaporation, is a major atmospheric forcing to the ocean that affects sea surface salinity (SSS) and...     

ENSO simulation in coupled ocean-atmosphere models: are the current models better?

Maintaining a multi-model database over a generation or more of model development provides an important framework for assessing model improvement. Using control integrations, we compare the simulation of the El Niño/Southern Oscillation (ENSO), and its extratropical impact, in models developed for the 2007 Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report with models developed in the late 1990s [the so-called Coupled Model Intercomparison Project-2 (CMIP2) models]. The IPCC models tend to be more realistic in representing the frequency with which ENSO occurs, and they are better at locating enhanced temperature variability over the eastern Pacific Ocean. When compared with reanalyses, the IPCC models have larger pattern correlations of tropical surface air temperature than do the CMIP2 models during the boreal winter peak phase of El Niño. However, for sea-level pressure and precipitation rate anomalies, a clear separation in performance between the two vintages of models is not as apparent. The strongest improvement occurs for the modelling groups whose CMIP2 model tended to have the lowest pattern correlations with observations. This has been checked by subsampling the multi-century IPCC simulations in a manner to be consistent with the single 80-year time segment available from CMIP2. Our results suggest that multi-century integrations may be required to statistically assess model improvement of ENSO. The quality of the El Niño precipitation composite is directly related to the fidelity of the boreal winter precipitation climatology, highlighting the importance of reducing systematic model error. Over North America distinct improvement of El Niño forced boreal winter surface air temperature, sea-level pressure, and precipitation rate anomalies to occur in the IPCC models. This improvement is directly proportional to the skill of the tropical El Niño forced precipitation anomalies.     

Monte Carlo climate change forecasts with a global coupled ocean-atmosphere model

Four time-dependent greenhouse warming experiments were performed with the same global coupled atmosphere-ocean model, but with each simulation using initial conditions from different snapshots of the control run climate. The radiative forcing — the increase in equivalent CO₂ concentrations from 1985–2035 specified in the Intergovernmental Panel on Climate Change (IPCC) scenario A — was identical in all four 50-year integrations. This approach to climate change experiments is called the Monte Carlo technique and is analogous to a similar experimental set-up used in the field of extended range weather forecasting. Despite the limitation of a very small sample size, this approach enables the estimation of both a mean response and the between-experiment variability, information which is not available from a single integration. The use of multiple realizations provides insights into the stability of the response, both spatially, seasonally and in terms of different climate variables. The results indicate that the time evolution of the global mean warming signal is strongly dependent on the initial state of the climate system. While the individual members of the ensemble show considerable variation in the pattern and amplitude of near-surface temperature change after 50 years, the ensemble mean climate change pattern closely resembles that obtained in a 100-year integration performed with the same model. In global mean terms, the climate change signals for near surface temperature, the hydrological cycle and sea level significantly exceed the variability among the members of the ensemble. Due to the high internal variability of the modelled climate system, the estimated detection time of the global mean temperature change signal is uncertain by at least one decade. While the ensemble mean surface temperature and sea level fields show regionally significant responses to greenhouse-gas forcing, it is not possible to identify a significant response in the precipitation and soil moisture fields, variables which are spatially noisy and characterized by large variability between the individual integrations.     

Role of stochastic forcing in ENSO in observations and a coupled GCM

A procedure is presented to estimate the role of atmospheric stochastic forcing (SF) in El Ni?o–Southern Oscillation (ENSO) simulated by a coupled ocean–atmosphere general circulation model (CGCM), in direct comparison to observations represented by a global reanalysis product. SF is extracted from the CGCM and reanalysis as surface wind anomalies linearly independent of the sea-surface temperature anomalies. Madden–Julian Oscillation (MJO) is isolated from SF to quantify its role in ENSO. A coupled ocean–atmosphere model of intermediate complexity is forced with SF, as well as its MJO and non-MJO components, from the reanalysis and CGCM. The role of SF is estimated by comparing the original ENSO in observations and the CGCM with that reproduced by the intermediate model. ENSO statistics in both reanalysis and CGCM are better reproduced when the intermediate model is tuned to be weakly stable than unstable. The intermediate model driven by SF from the reanalysis reproduces most characteristics of observed ENSO, such as its spectrum, seasonal phase-locking, fast decorrelation of ENSO SST during boreal spring, and its lag-correlation with SF. In contrast, not all characteristics of ENSO in the CGCM are reproduced by the intermediate model when SF from the CGCM is used. The seasonal phase-locking of ENSO in the CGCM is not reproduced at all. ENSO, therefore, appears to be driven by SF to a lesser degree in the CGCM than in observations. Characteristics of observed ENSO reproduced by the intermediate model (driven by SF) can be largely attributed to the MJO; which, for instance, is responsible for the fast decorrelation of ENSO SST during boreal spring in both reanalysis and CGCM. The non-MJO component seems to be more responsible than the MJO for erroneous features of ENSO in the CGCM.     

Adjustment and feedbacks in a global coupled ocean-atmosphere model

 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     

青藏高原云对地气系统短波吸收辐射强迫的气候研究

利用地球辐射平衡试验（ＥＲＢＥ）和国际卫星云气候计划（ＥＳＣＣＰ）提供的总云量、得星反射率资料,计算并分析了青藏高原地气系统年、月平均云对太阳短波吸收辐射的强迫,揭示了其与总云量的关系。结果表明：高原的短事射云强迫与总云量有较好的非线性相关,呈幂指数形式,且季节变化明显;短波辐射云强迫的地理分布与高原云的分产好,高原主体和北部是短波辐射云强迫的低值区,高原东南部和西部边缘迎风部位为强迫的高值区。     

云辐射强迫效应对热带气旋发展和结构的影响

利用高分辨率中尺度WRF-ARW模式,进行云辐射强迫效应对热带气旋(Tropical Cyclone,TC)发展和结构影响的敏感性试验研究。结果表明,云辐射强迫效应主要通过改变TC云区的辐射分布影响对流活动,进而影响TC的发展和结构。在TC发展阶段,TC内区对流云区云顶的强烈辐射冷却作用和云层内部的辐射增温过程降低了TC中上层的静力稳定度,中下层的变化相对不明显。总体上考虑云辐射效应的试验更易激发出更多更强的对流活动,有利于TC的发展及TC尺度的增大。鉴于云辐射强迫对TC的影响,在TC数值预报中需要更加重视云辐射强迫效应。     

The Interannual Variability of Climate in a Coupled Ocean-Atmosphere Model

In this paper, the interannual variability simulated by the coupled ocean-atmosphere general circulation model of the Institute of Atmospheric Physics (IAP CGCM) in 40 year integrations is analyzed, and compared with that by the corresponding IAP AGCM which uses the climatic sea surface temperature as the boundary condition in 25 year integrations.The mean climatic states of January and July simulated by IAP CGCM are in good agreement with that by IAP AGCM, i.e., no serious ‘climate drift’ occurs in the CGCM simulation. A comparison of the results from AGCM and CGCM indicates that the standard deviation of the monthly averaged sea level pressure simulated by IAP CGCM is much greater than that by IAP AGCM in tropical region. In addition, both Southern Oscillation (SO) and North Atlantic Oscillation (NAO) can be found in the CGCM simulation for January, but these two oscillations do not exist in the AGCM simulation.The interannual variability of climate may be classified into two types: one is the variation of the annual mean, another is the variation of the annual amplitude. The ocean-atmosphere interaction mainly increases the first type of variability. By means of the rotated EOF, the most important patterns corresponding to the two types of interannual variability are found to have different spatial and temporal characteristics.     

A further study on an extended nonlinear ocean-atmosphere coupled hydrodynamic characteristic system and the abrupt feature of ENSO events

An extended ocean-atmosphere coupled characteristic system including thermodynamic physical processes in ocean mixed layer is formulated in order to describe SST explicitly and remove possible limitation of ocean-atmos-phere coupling assumption in hydrodynamic ENSO models. It is revealed that there is a kind of abrupt nonlinear characteristic behaviour, which relates to rapid onset and intermittency of El Nino events, on the second order slow time scale due to the nonlinear interaction between a linear unstable low-frequency primary eigen component of ocean-atmosphere coupled Kelvin wave and its higher order harmonic components under a strong ocean-atmos-phere coupling background. And, on the other hand, there is a kind of finite amplitude nonlinear characteristic behaviour on the second order slow time scale due to the nonlinear interaction between the linear unstable primary eigen component and its higher order harmonic components under a weak ocean-atmosphere coupling background in this model system.     

The extratropical sensitivity to the meridional extent of tropical ENSO forcing

The extratropical response to tropical remote forcing has been examined with so-called tropical ocean-global atmosphere experiments, which use prescribed sea surface temperature (SST) in the tropical Pacific and a slab mixed-layer ocean model elsewhere. In this study we have revisited this experimental design and found that the extratropical response is quite sensitive to the meridional extent of tropical prescribed SST domain. Even in the case of a prescribed annual cycle only (i.e., no ENSO), the differences in the prescribed SST regions lead to different atmospheric motions in the adjacent extratropics. When the tropical forcing includes ENSO, the sensitivity to the meridional domain is more prominent, especially during La Niña events. In La Niña, the prescribed SST is warmer than the simulated SST in the northern subtropics, and the warmer SST differences continue to 30°N. This broad SST differences accompany enhanced atmospheric meridional circulation that directly connects the tropics and extratropics within the Pacific basin. Moreover, the Rossby wave excitation also increases, so the effect of prescribed region difference is felt beyond the Pacific basin. On the other hand, the effect of ENSO sea surface temperature anomalie (i.e., ENSO experiment composite minus control experiment annual cycle, both of which have the same prescribed SST domain) is stronger in the broad tropical forcing experiment. However, the ENSO anomaly composite from own annual cycle is similar regardless of the meridional extent of forcing region, and commonly mimics the Northern Hemisphere El Niño composite of nature in the boreal winter season.     

On the attractors of an intermediate coupled ocean-atmosphere model

Techniques of numerical bifurcation theory are used to study stationary and periodic solutions of an intermediate coupled model for tropical ocean-atmosphere interaction. The qualitative dynamical behavior is determined for a volume in parameter space spanned by the atmospheric damping length, the coupling parameter, the surface layer feedback strength and the relative adjustment time coefficient. Time integration methods have previously shown much interesting dynamics, including multiple steady states, eastward- or westward-propagating orbits and relaxation oscillations. The present study shows how this dynamics arises in parameter space through the interaction of the different branches of equilibrium solutions and the singularities on these branches. For example, we show that westward-propagating periodic orbits arise through an interaction of two unstable stationary modes and that relaxation oscillations occur through a limit cycle-saddle node interaction. There are several dynamical regimes in the coupled model which are determined by the primary bifurcation structure; this structure depends strongly on the parameters in the model. Although much of the dynamics may be studied in the fast-wave limit, it is shown that ocean wave dynamics introduces additional oscillatory instabilities and how these relate to propagating oscillations.     

The annual cycle and the predictability of the tropical coupled ocean-atmosphere system

Summary Using large-scale circulation statistics from the Pacific Ocean basin, predictability of the coupled ocean-atmosphere system on interannual time scales is found both to be limited in extent and to possess a strong annual cycle. Irrespective of when lagged correlations are commenced, correlations decrease rapidly through the boreal spring, indicating an inherent predictability limitation for large scale coupled oceanicatmospheric processes such as El Niño. Long term prediction experiments using numerical coupled-models show that the models are excellent facsimiles of the real system. They, too, encounter the predictability barrier and exhibit a substantial decrease in observation-prediction correlation across the boreal spring. Thus, a predictive system based solely on the interactive physics of the Pacific Basin appears limited to a maximum of less than one year and a minimum of only one or two months.Two hypotheses are made to explain the existence of the predictability barrier. First, it is argued that the tropical coupled system is at its frailest state during the boreal spring and that the signal-to-noise ratio is weakest. In such a system, maximum random error growth may occur as the atmosphere and the ocean become temporally detached and wander onto different climate trajectories. A series of 144 preliminary Monte Carlo experiments were conducted with a coupled ocean-atmosphere model to test the hypothesis. Irrespective of when the experiments were commenced, error growth was maximized at the same time of the year. The second hypothesis suggests that the near-equatorial circulation is perturbed at the time of its weakest state by external influences such as the monsoon and that the climate wanderings are nudged deterministically. There is observational and theoretical evidence to support the hypothesis. Observations suggest that anomalous monsoons impart basin-wide coherent alterations of the wind stress field in the Pacific Ocean. Experiments with a coupled ocean-atmosphere model show that the period of an ENSO event is altered substantially by an anomalous monsoon. Given that there appear to be precursors to anomalous monsoons, it is suggested that there may be ways to avoid the predictability barrier and thus extend prediction of the entire system.Finally, noting that the two hypotheses are not mutually exclusive, they are combined to form a unified theory. As the asymmetric monsoonal and the symmetric near-equatorial heating are in approximate quadrature, it is argued that the monsoons influence the Walker circulation during the boreal spring. However, during the boreal fall and early winter the near-equatorial heating variability dominates the winter monsoon.With 18 Figures     

Extratropical large-scale air-sea interaction in a coupled and uncoupled ocean-atmosphere model

Spatial patterns of mid-latitude large-scale ocean-atmosphere interaction on monthly to seasonal time scales have been observed to exhibit a similar structure in both the North Pacific and North Atlantic basins. These patterns have been interpreted as a generic oceanic response to surface wind anomalies, whereby the anomalous winds give rise to corresponding anomalous regions of surface heat flux and consequent oceanic cooling. This mechanistic concept is investigated in this study using numerical models of a global atmosphere and a mid-latitude ocean basin (nominally the Atlantic). The models were run in both coupled and uncoupled mode. Model output was used to generate multi-year time series of monthly mean fields. Empirical orthogonal function (EOF) and singular value decomposition (SVD) analyses were then used to obtain the principal patterns of variability in heat flux, air temperature, wind speed, and sea surface temperature (SST), and to determine the relationships among these variables. SVD analysis indicates that the turbulent heat flux from the ocean to the atmosphere is primarily controlled by the surface scalar wind speed, and to a lesser extent by air temperature and SST. The principal patterns of air-sea interaction are closely analogous to those found in observational data. In the atmosphere, the pattern consists of a simultaneous strengthening (or weakening) of the mid-latitude westerlies and the easterly trades. In the ocean there is cooling (warming) under the anomalously strong (weak) westerlies and trade winds, with a weaker warming (cooling) in the region separating the westerly and easterly wind regimes. These patterns occur in both coupled and uncoupled models and the primary influence of the coupling is in localizing the interaction patterns. The oceanic patterns can be explained by the principal patterns of surface heat flux and the attendant warming or cooling of the ocean mixed layer.     

Mean climate state simulated by a coupled ocean-atmosphere general circulation model

Summary The result of a 100-year integration of a coupled ocean-atmosphere general circulation model (CGCM) is analyzed, and compared with that of a 25-year integration of the corresponding uncoupled atmospheric general circulation model (AGCM) and observed data. The large-scale circulation patterns of mean climate state simulated by the CGCM are in good agreement with the observed ones, although differences exit in the positions and intensities between the simulated and the observed patterns. Having compared the standard deviations of monthly mean sea level pressure simulated by the CGCM to those by the AGCM, we found that the interaction between ocean and atmosphere mainly increases the interannual variability in the tropics especially in summer. The CGCM can also produce El Niño and Southern Oscillation (ENSO) events, whereas the AGCM cannot reproduce the main features of the Southern Oscillation. This implies that the air-sea interaction may be a principal mechanism for the occurrence of ENSO phenomena. The fundamental features of simulated regional climates are also analyzed. The CGCM can reproduce principal characteristics of surface air temperature and precipitation at five selected typical regions (desert region, plain region, monsoon region etc.). The distributions of annual mean surface ait temperature and precipitation in East Asia can also be reasonably simulated.With 9 Figures