Indian Ocean variability in the CMIP5 multi-model ensemble: the zonal dipole mode

The performance of 21 Coupled Model Intercomparison Project Phase 5 (CMIP5) models in the simulation of the Indian Ocean Dipole (IOD) mode is evaluated. Compared to CMIP3, CMIP5 models exhibit a similar spread in IOD intensity. A detailed diagnosis was carried out to understand whether CMIP5 models have shown improvement in their representation of the important dynamical and thermodynamical feedbacks in the tropical Indian Ocean. These include the Bjerknes dynamic air-sea feedback, which includes the equatorial zonal wind response to sea surface temperature (SST) anomaly, the thermocline response to equatorial zonal wind forcing, the ocean subsurface temperature response to the thermocline variations, and the thermodynamic air-sea coupling that includes the wind-evaporation-SST and cloud-radiation-SST feedback. Compared to CMIP3, the CMIP5 ensemble produces a more realistic positive wind-evaporation-SST feedback during the IOD developing phase, while the simulation of Bjerknes dynamic feedback is more unrealistic especially with regard to the wind response to SST forcing and the thermocline response to surface wind forcing. The overall CMIP5 performance in the IOD simulation does not show remarkable improvements compared to CMIP3. It is further noted that the El Niño-Southern Oscillation (ENSO) and IOD amplitudes are closely related, if a model generates a strong ENSO, it is likely that this model also simulates a strong IOD.     

A study of biases in simulation of the Indian Ocean basin mode and its capacitor effect in CMIP3/CMIP5 models

Based on 15 Coupled Model Intercomparison Project (CMIP) phase 3 (CMIP3) and 32 CMIP phase 5 (CMIP5) models, a detailed diagnosis was carried out to understand what compose the biases in simulation of the Indian Ocean basin mode (IOBM) and its capacitor effect. Cloud-radiation-SST (CRS) feedback and wind-evaporation-SST (WES) feedback are the two major atmospheric processes for SST changes. Most CMIP models simulate a stronger CRS feedback and a weaker WES feedback. During boreal fall of the El Niño/Southern Oscillation developing year and the following spring, there are weak biases of suppressed rainfall anomalies over the Maritime Continent and anomalous anticyclone over South Indian Ocean. Most CMIP models simulate reasonable short wave radiation (SWR) and weaker latent heat flux (LHF) anomalies. This leads to a weak bias of atmospheric processes. During winter, however, the rainfall anomalies are stronger due to west bias, and the anomalous anticyclone is comparable to observations. As such, most models simulate stronger SWR and reasonable LHF anomalies, leading to a strong bias of atmospheric processes. The thermocline feedback is stronger in most models. Though there is a deep bias of climatology thermocline, most models capture reasonable sea surface height-induced SST anomalies. Therefore, the effect of oceanic processes offset the weak bias of atmospheric processes in spring, and the tropical Indian Ocean warming persists into summer. However, anomalous northwest Pacific (NWP) anticyclone is weaker due to weak and west bias of the capacitor effect. The unrealistic western Pacific SST anomalies in models favor the westward extension of Rossby wave from the Pacific, weakening the effect of Kelvin wave from the Indian Ocean. Moreover, the western Pacific warming forces the NWP anticyclone move farther north than observations, suggesting a major forcing from the Pacific. Compared to CMIP3, CMIP5 models simulate the feedbacks more realistically and display less diversity. Thus, the overall performance of CMIP5 models is better than that of CMIP3 models.     

Future change of the Indian Ocean basin-wide and dipole modes in the CMIP5

The Indian Ocean sea surface temperature (SST) variability has been represented with the two dominant variability modes: the Indian Ocean basin-wide (IOBW) and dipole (IOD) modes. Here we investigate future changes of the two modes together with mean state and El Niño and Southern Oscillation (ENSO) relationship under the anthropogenic global warming using 20 coupled models that participated in the phase five of Coupled Model Intercomparison Project by comparing the historical run from 1950 to 2005 and the RCP 4.5 run from 2050 to 2099. The five best models are selected based on the evaluation of the 20 models’ performances in simulating the two modes and Indian Ocean basic state for the latest 56 years. They are capable of capturing the IOBW and IOD modes in their spatial distribution, seasonal cycle, major periodicity, and relationship with ENSO to some extent. The five best models project the significant changes in the Indian Ocean mean state and variability including the two dominant modes in the latter part of twenty-first century under the anthropogenic warming scenario. First, the annual mean climatological SST displays an IOD-like pattern change over the Indian Ocean with enhanced warming in the northwestern Indian Ocean and relatively weaker warming off the Sumatra–Java coast. It is also noted that the monthly SST variance is increased over the eastern and southwestern Indian Ocean. Second, the IOBW variability on a quasi-biennial time scale will be enhanced due to the strengthening of the ENSO–IOBW mode relationship although the total variance of the IOBW mode will be significantly reduced particularly during late summer and fall. The enhanced air-sea coupling over the Indian-western Pacific climate in response to El Nino activity in the future projection makes favorable condition for a positive IOD while it tends to derive relatively cold temperature over the eastern Indian Ocean. This positive IOD-like ENSO response weakens the relationship between the eastern Indian Ocean and El Nino while strengthens the relationship with western Indian Ocean. Third, the IOD mode, intrinsic coupled mode of the Indian Ocean may not be changed appreciably under the anthropogenic global warming.     

Downward surface shortwave radiation over the subtropical Asia-Pacific region simulated by CMIP5 models

本文以两套卫星观测资料为参考,评估了15个CMIP5气候模式对亚太地区太阳总辐射(DSSR)的模拟能力,并将亚太地区按经度位置划分成五个子区域来更好地分析模式误差的成因。许多CMIP5模式在海洋上低估了DSSR,而在陆地上却高估了DSSR。除了地中海-西亚(MWA)和中亚(CA),在年平均或季节平均尺度上,地表短波云辐射强迫(CRF)的误差都能很好地解释CMIP5模式中DSSR的主要误差来源:模式在海洋上高估了CRF的影响,而在陆地上低估了CRF的影响。云是影响亚太地区CMIP5模式中DSSR质量的主要因素,但是其在MWA和CA区域的影响相对较弱,特别是在北方的夏季和秋季。     

The Southwest Indian Ocean thermocline dome in CMIP5 models: Historical simulation and future projection

Using 20 models of the Coupled Model Intercomparison Project Phase 5 (CMIP5), the simulation of the Southwest Indian Ocean (SWIO) thermocline dome is evaluated and its role in shaping the Indian Ocean Basin (IOB) mode following El Niño investigated. In most of the CMIP5 models, due to an easterly wind bias along the equator, the simulated SWIO thermocline is too deep, which could further influence the amplitude of the interannual IOB mode. A model with a shallow (deep) thermocline dome tends to simulate a strong (weak) IOB mode, including key attributes such as the SWIO SST warming, antisymmetric pattern during boreal spring, and second North Indian Ocean warming during boreal summer. Under global warming, the thermocline dome deepens with the easterly wind trend along the equator in most of the models. However, the IOB amplitude does not follow such a change of the SWIO thermocline among the models; rather, it follows future changes in both ENSO forcing and local convection feedback, suggesting a decreasing effect of the deepening SWIO thermocline dome on the change in the IOB mode in the future.     

The Subsurface and Surface Indian Ocean Dipoles and Their Association with ENSO in CMIP6 models

This study assesses the reproducibility of 31 historical simulations from 1850 to 2014 in the Coupled Model Intercomparison Project phase 6(CMIP6) for the subsurface(Sub-IOD) and surface Indian Ocean Dipole(IOD) and their association with El Ni?o-Southern Oscillation(ENSO). Most CMIP6 models can reproduce the leading east-west dipole oscillation mode of heat content anomalies in the tropical Indian Ocean(TIO) but largely overestimate the amplitude and the dominant period of the Sub-IOD. Associat...     

Planetary albedo in strongly forced climate, as simulated by the CMIP3 models

In an ensemble of general circulation models, the global mean albedo significantly decreases in response to strong CO₂ forcing. In some of the models, the magnitude of this positive feedback is as large as the CO₂ forcing itself. The models agree well on the surface contribution to the trend, due to retreating snow and ice cover, but display large differences when it comes to the contribution from shortwave radiative effects of clouds. The ??cloud contribution?? defined as the difference between clear-sky and all-sky albedo anomalies and denoted as ??CC is correlated with equilibrium climate sensitivity in the models (correlation coefficient 0.76), indicating that in high sensitivity models the clouds to a greater extent act to enhance the negative clear-sky albedo trend, whereas in low sensitivity models the clouds rather counteract this trend. As a consequence, the total albedo trend is more negative in more sensitive models (correlation coefficient 0.73). This illustrates in a new way the importance of cloud response to global warming in determining climate sensitivity in models. The cloud contribution to the albedo trend can primarily be ascribed to changes in total cloud fraction, but changes in cloud albedo may also be of importance.     

Seasonality in the relationship between El Nino and Indian Ocean dipole

The seasonal change in the relationship between El Nino and Indian Ocean dipole (IOD) is examined using the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40), and the twentieth century simulations (20c3m) from the Geophysical Fluid Dynamics Laboratory Coupled Model, version 2.1. It is found that, both in ERA-40 and the model simulations, the correlation between El Nino (Nino3 index) and the eastern part of the IOD (90?C110°E; 10°S-equator) is predominantly positive from January to June, and then changes to negative from July to December. Correlation maps of atmospheric and oceanic variables with respect to the Nino3 index are constructed for each season in order to examine the spatial structure of their seasonal response to El Nino. The occurrence of El Nino conditions during January to March induces low-level anti-cyclonic circulation anomalies over the southeastern Indian Ocean, which counteracts the climatological cyclonic circulation in that region. As a result, evaporation decreases and the southeastern Indian Ocean warms up as the El Nino proceeds, and weaken the development of a positive phase of an IOD. This warming of the southeastern Indian Ocean associated with the El Nino does not exist past June because the climatological winds there develop into the monsoon-type flow, enhancing the anomalous circulation over the region. Furthermore, the development of El Nino from July to September induces upwelling in the southeastern Indian Ocean, thereby contributing to further cooling of the region during the summer season. This results in the enhancement of a positive phase of an IOD. Once the climatological circulation shifts from the boreal summer to winter mode, the negative correlation between El Nino and SST of the southeastern Indian Ocean changes back to a positive one.     

Current and future atmospheric circulation at 500 hPa over Greenland simulated by the CMIP3 and CMIP5 global models

The Greenland ice sheet is projected to be strongly affected by global warming. These projections are either issued from downscaling methods (such as Regional Climate Models) or they come directly from General Circulation Models (GCMs). In this context, it is necessary to evaluate the accuracy of the daily atmospheric circulation simulated by the GCMs, since it is used as forcing for downscaling methods. Thus, we use an automatic circulation type classification based on two indices (Euclidean distance and Spearman rank correlation using the daily 500 hPa geopotential height) to evaluate the ability of the GCMs from both CMIP3 and CMIP5 databases to simulate the main circulation types over Greenland during summer. For each circulation type, the GCMs are compared to three reanalysis datasets on the basis of their frequency and persistence differences. For the current climate (1961–1990), we show that most of the GCMs do not reproduce the expected frequency and the persistence of the circulation types and that they simulate poorly the observed daily variability of the general circulation. Only a few GCMs can be used as reliable forcings for downscaling methods over Greenland. Finally, when applying the same approach to the future projections of the GCMs, no significant change in the atmospheric circulation over Greenland is detected, besides a generalised increase of the geopotential height due to a uniform warming of the atmosphere.     

CMIP5模式对西太平洋副热带高压的模拟和预估

利用国际耦合模式比较计划第五阶段（CMIP5）26个模式的模拟结果,从空间分布和振幅变化、年际周期及年代际趋势等方面,初步评估了CMIP5模式对西太平洋副热带高压（副高）的模拟能力。在此基础上,还对未来不同典型浓度路径（RCPs）情景下副高的可能变化给出了定性的预估。CMIP5模式历史试验结果显示,大多数模式对500 hPa位势高度气候平均值的模拟有明显误差,这主要是由于模式对热带印度洋和西太平洋地区海表温度（SST）的模拟普遍较观测值低,从而导致模式对副高的模拟能力有限。但大多数模式对高度场和纬向风场变化的空间形态与振幅都有较强的模拟能力。因此,通过用NCEP/NCAR再分析资料的气候平均值替代CMIP5模式气候平均值的简单方法,对CMIP5模拟结果进行了订正。经订正后的模式结果均有能力刻画副高指数的历史时间序列,且能够反映出20世纪70年代末期之后,副高面积增大、强度增强和显著西伸的变化趋势。此外,通过对副高指数的长期趋势、年际周期及标准差等的定量评估,注意到CNRM-CM5、FGOALS-g2、FIO-ESM、MIROC-ESM和MPI-ESM-P这5个模式对副高的模拟能力较强。未来气候预估试验中,副高面积和强度均增大,且显著西伸;其线性增长趋势在RCP8.5情景下最高,RCP4.5情景下次之,RCP2.6情景下最弱。有趣的是副高脊线指数在3种排放情景下都没有明显的长期变化趋势。这些结果为选取和利用CMIP5模式进行东亚地区气候变化的归因分析和未来预估提供了一定的科学依据。     

Indian Ocean dipole interpreted in terms of recharge oscillator theory

In this paper we use sea surface height (SSH) derived from satellite altimetry and an analytical linear equatorial wave model to interpret the evolution of the Indian Ocean Dipole (IOD) in the framework of recharge oscillator theory. The specific question we address is whether heat content in the equatorial band, for which SSH is a proxy, is a predictor of IOD development as it is for El Niño and the Southern Oscillation (ENSO) in the Pacific. We find that, as in the Pacific, there are zonally coherent changes in heat content along the equator prior to the onset of IOD events. These changes in heat content are modulated by wind-forced westward propagating Rossby waves in the latitude band 5°–10°S, which at the western boundary reflect into Kelvin waves trapped to the equator. The biennial character of the IOD is affected by this cycling of wave energy between 5° and 10°S and the equator. Heat content changes are a weaker leading indicator of IOD sea surface temperature anomaly development than is the case for ENSO in the Pacific though because other factors are at work in generating IOD variability, one of which is ENSO forcing itself through changes in the Walker Circulation.     

The interannual variability in the tropical Indian Ocean as simulated by a CGCM

The interannual variability in the tropical Indian Ocean, and in particular the Indian Ocean dipole mode (IODM), is investigated using both observations and a multi-decadal simulations performed by the coupled atmosphere-ocean general circulation model SINTEX. Overall, the characteristics of the simulated IODM are close to the features of the observed mode. Evidence of significant correlations between sea level pressure anomalies in the southeastern Indian Ocean and sea surface temperature anomalies in the tropical Indian and Pacific Oceans have been found both in observations and a multi-decadal simulation. In particular, a positive SLP anomaly in the southeastern part of the basin seems to produce favorable conditions for the development of an IODM event. The role played by the ocean dynamics both in the developing and closing phases of the IODM events is also investigated. Our results suggest that, during the developing phase, the heat content and SST variability associated with the IODM are influenced by a local response of the ocean to the winds, and a remote response with the excitation of Kelvin and Rossby waves. Ocean wave dynamics appear to be important also during the dying phase of the IODM, when equatorial downwelling Kelvin waves transport positive heat content anomalies from the western to the eastern part of the basin, suppressing the zonal heat content anomaly gradient. The results obtained from the model suggest a mechanism for the IODM. This mechanism is generally consistent with the characteristics of the observed IODM. Furthermore, it might give some clue in understanding the correlation between IODM and ENSO activity found both in the model and in the observations.     

Twentieth-century Pacific Decadal Oscillation simulated by CMIP5 coupled models

太平洋年代际振荡(PDO)是影响亚洲—太平洋气候,乃至全球气候变化非常重要的海洋模态之一。目前为止,有关PDO形成的动力根源及机制仍不清楚。本文聚焦20世纪PDO的空间模态和时间序列演变,系统分析了CMIP5框架下25个耦合模式开展的109个历史时期模拟试验的结果。结果显示,大多数耦合模式可以成功模拟出20世纪PDO"马蹄形"海表温度距平,其中BNUESM,CanESM2,CCSM4,CESM1-FASTCHEM,GFDL CM3,MIROC5,NorESM1-M等模式和中国科学院大气物理研究所的FGOALS-g2模拟的PDO太平洋海表温度遥相关型空间分布与观测相似度较高;耦合模式模拟的PDO时间序列与观测相差较大,表明现有的耦合模式几乎不能模拟出20世纪PDO的位相演变特征。     

ENSO modes of the equatorial Pacific Ocean in observations and CMIP5 models

El Niño/Southern Oscillation (ENSO) is the predominant interannual variability of the global climate system. How might ENSO change in a warmer world? The dominant two Combined Empirical Orthogonal Functions (CEOF) of the equatorial ocean temperature and zonal and vertical motion identify two modes that shown a transition in the eastern Pacific from a warming eastward/downward motion to a cooling westward/upward flow. These results also suggest consistent changes to the west and at depths down to 300 m. These dominate CEOFs provide a compact tool for assessing Coupled Model Intercomparison Project Phase 5 ocean model output for both the recent historical period and for the latter part of the twenty first century. Most of the analyzed models replicate well the spatial patterns of the dominant observational CEOF modes, but nearly always underestimate the magnitudes. Comparing model output for the twentieth and twenty first centuries there is very little change between the spatial patterns of the ENSO modes of the two periods. This lack of response to climate change is shown to be partly related to competing influences of climatic changes in the mean ocean circulation.     

Projection of subtropical gyre circulation and associated sea level changes in the Pacific based on CMIP3 climate models

For all of the IPCC Special Report on Emission Scenarios (SRESs), sea level is projected to rise globally. However, sea level changes are not expected to be geographically uniform, with many regions departing significantly from the global average. Some of regional distributions of sea level changes can be explained by projected changes of ocean density and dynamics. In this study, with 11 available Coupled Model Intercomparison Project Phase 3 climate models under the SRES A1B, we identify an asymmetric feature (not recognised in previous studies) of projected subtropical gyre circulation changes and associated sea level changes between the North and South Pacific, through analysing projected changes of ocean dynamic height (with reference to 2,000 db), depth integrated steric height, Sverdrup stream function, surface wind stress and its curl. Poleward expansion of the subtropical gyres is projected in the upper ocean for both North and South Pacific. Contrastingly, the subtropical gyre circulation is projected to spin down by about 20 % in the subsurface North Pacific from the main thermocline around 400 m to at least 2,000 m, while the South Pacific subtropical gyre is projected to strengthen by about 25 % and expand poleward in the subsurface to at least 2,000 m. This asymmetrical distribution of the projected subtropical gyre circulation changes is directly related to differences in projected changes of temperature and salinity between the North and South Pacific, forced by surface heat and freshwater fluxes, and surface wind stress changes.     

The Role of the Aerosol Indirect Effect in the Northern Indian Ocean Warming Simulated by CMIP5 Models

The northern Indian Ocean （NIO） experienced a decadal-scale persistent warming from 1950 to 2000, which has influenced both regional and global climate. Because the NIO is a region susceptible to aerosols emis- sion changes, and there are still large uncertainties in the representation of the aerosol indirect effect （ALE） in CMIP5 （Coupled Model Intercomparison Project Phase 5） models, it is necessary to investigate the role of the AIE in the NIO warming simulated by these models. In this study, the authors select seven CMIP5 models with both the aerosol direct and indirect effects to investigate their performance in simulating the basin-wide decadal-scale NIO warming. The results show that the decreasing trend of the downwelling shortwave flux （FSDS） at the surface has the major damping effect on the SST increasing trend, which counteracts the warming effect of greenhouse gases （GHGs）. The FSDS decreasing trend is mostly contrib- uted by the decreasing trend of cloudy-sky surface downwelling shortwave flux （FSDSCL）, a metric used to measure the strength of the AIE, and partly by the clear-sky surface downwelling shortwave flux （FSDSC）. Models with a relatively weaker AIE can simulate well the SST increasing trend, as compared to observation. In contrast, models with a relatively stronger AIE produce a much smaller magnitude of the increasing trend, indicat- ing that the strength of the AIE in these models may be overestimated in the NIO.