海洋环流对全球增暖趋势的调制：基于FGOALS-s2的数值模拟研究

在工业革命以来全球长期增暖趋势背景下,全球平均表面气温还同时表现出年代际变化特征,二者叠加在一起使得全球平均气温在某些年份增暖相对停滞(如1999～2008年)或者增暖相对较快(如1980～1998年).利用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(LASG)发展的耦合气候模式FGOALS-s2历史气候和典型路径浓度(RCPs)模拟试验结果研究了可能造成全球增暖的年代际停滞及加速现象的原因,特别是海洋环流对全球变暖趋势的调制作用.该模式模拟的全球平均气温与观测类似,即在长期增暖趋势之上,还叠加了显著的年代际变化.对全球平均能量收支分析表明,模拟的气温年代际变化与大气顶净辐射通量无关,意味着年代际表面气温变化可能与能量在气候系统内部的重新分配有关.通过对全球增暖加速和停滞时期大气和海洋环流变化的合成分析及回归分析,发现全球表面气温与大部分海区海表温度(SST)均表现出几乎一致的变化特征.在增暖停滞时期,SST降低,更多热量进入海洋次表层和深层,使其温度增加;而在增暖加速时期,更多热量停留在表层,使得大部分海区SST显著增加,次表层海水和深海相对冷却.进一步分析表明,热带太平洋表层和次表层海温年代际变化主要是由于副热带—热带经圈环流(STC)的年代际变化所致,然后热带太平洋海温异常可以通过风应力和热通量强迫作用引起印度洋、大西洋海温的年代际变化.在此过程中,海洋环流变化起到了重要作用,例如印度尼西亚贯穿流(ITF)年代际异常对南印度洋次表层海温变化起到关键作用,而大西洋经圈翻转环流(AMOC)则能直接影响到北大西洋深层海温变化.     

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.     

基于CMIP5资料的热带太平洋和印度洋海温变化与降水变化的关系及其成因分析

利用第五次耦合模式比较计划(Coupled Model Intercomparison Project Phase 5,简称CMIP5)月平均资料,从季节变化角度,对热带太平洋、印度洋海温变化与降水变化的关系及其成因进行了初步分析。20个模式集合平均结果表明:在全球增暖背景下,热带太平洋年平均的海温变化与降水变化符合"warmer-get-wetter"型特征,而季节平均与年平均存在明显的差异;冬季和春季,海温增暖最大区和降水增加区之间存在东西向和南北向的位置偏差;夏季和秋季,二者只存在明显的南北位置偏差,且与冬季和春季的情况相反。热带印度洋的冬季和春季海温变化与降水变化也存在位置偏差。两个热带大洋季节平均的降水变化均是"warmer-get-wetter"和"wet-get-wetter"两个机制共同作用的结果。     

Relative roles of dynamic and thermodynamic processes in causing positive and negative global mean SST trends during the past 100 years

Global mean surface temperature (GMST) during 1910–2012 experienced four alternated rapid warming and warming hiatus phases. Such a temporal variation is primarily determined by global mean sea surface temperature (SST) component. The relative roles of ocean dynamic and thermodynamic processes in causing such global mean SST variations are investigated, using two methods. The first method is ocean mixed layer heat budget analysis. The budget diagnosis result shows that the thermodynamic processes dominate in the rapid warming phases, while the ocean dynamics dominate during the hiatus phases. The second method relies on the diagnosis of a simple equilibrium state model. This model captures well the horizontal distribution of SST difference between two warmer and cooler equilibrium states during either the rapid warming or hiatus phases. It is found that the SST difference during the rapid warming phases is primarily controlled by the increase of downward longwave radiation as both column integrated water vapor and CO₂ increase during the phases. During the hiatus phases, the water vapor induced greenhouse effect offsets the CO₂ effect, and the SST cooling tendency is primarily determined by the ocean dynamics over the Southern Ocean and tropical Pacific. The SST pattern associated with the Interdecadal Pacific Oscillation (IPO) might be responsible for the remote and local ocean dynamic responses through induced wind change.     

Change of tropical cyclone heat potential in response to global warming

Tropical cyclone heat potential (TCHP) in the ocean can affect tropical cyclone intensity and intensification. In this paper, TCHP change under global warming is presented based on 35 models from CMIP5 (Coupled Model Intercomparison Project, Phase 5). As the upper ocean warms up, the TCHP of the global ocean is projected to increase by 140.6% in the 21st century under the RCP4.5 (+4.5 W m^-2 Representative Concentration Pathway) scenario. The increase is particularly significant in the western Pacific, northwestern Indian and western tropical Atlantic oceans. The increase of TCHP results from the ocean temperature warming above the depth of the 26°C isotherm (D26), the deepening of D26, and the horizontal area expansion of SST above 26°C. Their contributions are 69.4%, 22.5% and 8.1%, respectively. Further, a suite of numerical experiments with an Ocean General Circulation Model (OGCM) is conducted to investigate the relative importance of wind stress and buoyancy forcing to the TCHP change under global warming. Results show that sea surface warming is the dominant forcing for the TCHP change, while wind stress and sea surface salinity change are secondary.     

BCC_CSM1.1对10年尺度全球及区域温度的预测研究

近期10～30年时间尺度的年代际预测是第五次耦合模式国际比较计划(CMIP5)重要内容之一。按照CMIP5试验要求, 国家气候中心利用气候系统模式BCC_CSM1.1完成并提交了年代际试验结果。本文评估了该模式年代际试验对10年尺度全球及区域地表温度的预测能力, 并通过与20世纪历史气候模拟试验的对比分析, 研究模式模拟对海洋初始观测状态的依赖程度。分析结果表明:(1)在有、无海洋初始化条件下, 模式均能模拟出1960～2005年间全球10年平均实测地表温度的变暖趋势, 但在有海洋初始化条件下, 可以明显减小BCC_CSM1.1模式模拟的全球升温趋势, 使得年代际试验比历史试验的结果更接近观测值。这一特点在观测资料相对丰富的南北纬50°以内地区更为显著。(2)在年代际试验预测前期, 通过Nudging方法, 利用SODA再分析海洋温度资料对模式进行初始化, 经过前期8～12月的协调后, 模式预测的第1年南北纬50°范围海洋、陆面的平均地表气温接近于观测值(CRUTEM3, HadSST2)。由于模式初值SODA再分析SST资料与HadSST2观测值存在明显的全球大洋系统暖偏差以及模式本身系统偏差的影响, 年代际试验模拟的地表气温在2～7年之内, 从观测SST状态逐渐恢复到模式系统本身状态。在同组Decadal试验中, 陆面和海洋恢复调整的时间长度几乎一致。(3) 从10年平均气候异常在区域尺度上的预报技巧来看, 有、无海洋初始同化对预测结果影响不大, 高预测技巧区主要分布在南半球印度洋中高纬度、热带西太平洋以及热带大西洋区域。(4)SST变化与下垫面热通量密切相关, 在热带和副热带海洋区域, 长波辐射和感热通量是影响10年时间尺度SST变化较大的物理量, 在中高纬度海洋, 洋面温度变化主要受潜热通量的影响相对较大。     

North Pacific SST Forcing on the Central United States “Warming Hole” as Simulated in CMIP5 Coupled Historical and Uncoupled AMIP Experiments

The central United States experienced a cooling trend during the twentieth century, called the “warming hole,” most notably in the last quarter of the century when global warming accelerated. The coupled simulations of the models that participated in the Coupled Model Intercomparison Project, Phases 3 and 5 (CMIP3/5), have been unable to reproduce this abnormal cooling phenomenon satisfactorily. An unrealistic representation of the observed phasing of the Pacific Decadal Oscillation (PDO)—one of the proposed forcing mechanisms for the warming hole—in the models is considered to be one of the main causes of this effect. The CMIP5’s uncoupled Atmospheric Model Intercomparison Project (AMIP) experiment, whose duration approximately coincides with the peak warming hole cooling period, provides an opportunity, when compared with the coupled historical experiment, to examine the role of the variation in Pacific Ocean sea surface temperature (SST) in the warming hole’s formation and also to assess the skill of the models in simulating the teleconnection between Pacific SST and the continental climate in North America. Accordingly, this study compared AMIP and historical runs in the CMIP5 suite thereby isolating the role of SST forcing in the formation of the warming hole and its maintenance mechanisms. It was found that, even when SST forcing in the AMIP run was “perfectly” prescribed in the models, the skill of the models in simulating the warming hole cooling in the central United States showed little improvement over the historical run, in which SST is calculated interactively, even though the AMIP run overestimated the anti-correlation between temperature in the central United States and the PDO index. The fact that better simulation of the PDO phasing in the AMIP run did not translate into an improved summer cooling trend in the central United States suggests that the inability of the coupled CMIP5 models to reproduce the warming hole under the historical run is not mainly a result of the mismatch between simulated and observed PDO phasing, as believed.     

Pacific variability reconciles observed and modelled global mean temperature increase since 1950

Global mean temperature change simulated by climate models deviates from the observed temperature increase during decadal-scale periods in the past. In particular, warming during the ‘global warming hiatus’ in the early twenty-first century appears overestimated in CMIP5 and CMIP6 multi-model means. We examine the role of equatorial Pacific variability in these divergences since 1950 by comparing 18 studies that quantify the Pacific contribution to the ‘hiatus’ and earlier periods and by investigating the reasons for differing results. During the ‘global warming hiatus’ from 1992 to 2012, the estimated contributions differ by a factor of five, with multiple linear regression approaches generally indicating a smaller contribution of Pacific variability to global temperature than climate model experiments where the simulated tropical Pacific sea surface temperature (SST) or wind stress anomalies are nudged towards observations. These so-called pacemaker experiments suggest that the ‘hiatus’ is fully explained and possibly over-explained by Pacific variability. Most of the spread across the studies can be attributed to two factors: neglecting the forced signal in tropical Pacific SST, which is often the case in multiple regression studies but not in pacemaker experiments, underestimates the Pacific contribution to global temperature change by a factor of two during the ‘hiatus’; the sensitivity with which the global temperature responds to Pacific variability varies by a factor of two between models on a decadal time scale, questioning the robustness of single model pacemaker experiments. Once we have accounted for these factors, the CMIP5 mean warming adjusted for Pacific variability reproduces the observed annual global mean temperature closely, with a correlation coefficient of 0.985 from 1950 to 2018. The CMIP6 ensemble performs less favourably but improves if the models with the highest transient climate response are omitted from the ensemble mean.

     

Twentieth century Walker Circulation change: data analysis and model experiments

Recent studies indicate a weakening of the Walker Circulation during the twentieth century. Here, we present evidence from an atmospheric general circulation model (AGCM) forced by the history of observed sea surface temperature (SST) that the Walker Circulation may have intensified rather than weakened. Observed Equatorial Indo-Pacific Sector SST since 1870 exhibited a zonally asymmetric evolution: While the eastern part of the Equatorial Pacific showed only a weak warming, or even cooling in one SST dataset, the western part and the Equatorial Indian Ocean exhibited a rather strong warming. This has resulted in an increase of the SST gradient between the Maritime Continent and the eastern part of the Equatorial Pacific, one driving force of the Walker Circulation. The ensemble experiments with the AGCM, with and without time-varying external forcing, suggest that the enhancement of the SST gradient drove an anomalous atmospheric circulation, with an enhancement of both Walker and Hadley Circulation. Anomalously strong precipitation is simulated over the Indian Ocean and anomalously weak precipitation over the western Pacific, with corresponding changes in the surface wind pattern. Some sensitivity to the forcing SST, however, is noticed. The analysis of twentieth century integrations with global climate models driven with observed radiative forcing obtained from the Coupled Model Intercomparison Project (CMIP) database support the link between the SST gradient and Walker Circulation strength. Furthermore, control integrations with the CMIP models indicate the existence of strong internal variability on centennial timescales. The results suggest that a radiatively forced signal in the Walker Circulation during the twentieth century may have been too weak to be detectable.     

A recipe for simulating the interannual variability of the Asian summer monsoon and its relation with ENSO

This study investigates how accurately the interannual variability over the Indian Ocean basin and the relationship between the Indian summer monsoon and the El Niño Southern Oscillation (ENSO) can be simulated by different modelling strategies. With a hierarchy of models, from an atmospherical general circulation model (AGCM) forced by observed SST, to a coupled model with the ocean component limited to the tropical Pacific and Indian Oceans, the role of heat fluxes and of interactive coupling is analyzed. Whenever sea surface temperature anomalies in the Indian basin are created by the coupled model, the inverse relationship between the ENSO index and the Indian summer monsoon rainfall is recovered, and it is preserved if the atmospherical model is forced by the SSTs created by the coupled model. If the ocean model domain is limited to the Indian Ocean, changes in the Walker circulation over the Pacific during El-Niño years induce a decrease of rainfall over the Indian subcontinent. However, the observed correlation between ENSO and the Indian Ocean zonal mode (IOZM) is not properly modelled and the two indices are not significantly correlated, independently on season. Whenever the ocean domain extends to the Pacific, and ENSO can impact both the atmospheric circulation and the ocean subsurface in the equatorial Eastern Indian Ocean, modelled precipitation patterns associated both to ENSO and to the IOZM closely resemble the observations.     

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.     

Asian summer monsoon onset in simulations and CMIP5 projections using four Chinese climate models

The reproducibility and future changes of the onset of the Asian summer monsoon were analyzed based on the simulations and projections under the Representative Concentration Pathways(RCP) scenario in which anthropogenic emissions continue to rise throughout the 21 st century(i.e. RCP8.5) by all realizations from four Chinese models that participated in the Coupled Model Intercomparison Project Phase 5(CMIP5). Delayed onset of the monsoon over the Arabian Sea was evident in all simulations for present-day climate, which was associated with a too weak simulation of the low-level Somali jet in May.A consistent advanced onset of the monsoon was found only over the Arabian Sea in the projections, where the advanced onset of the monsoon was accompanied by an increase of rainfall and an anomalous anticyclone over the northern Indian Ocean. In all the models except FGOALS-g2, the enhanced low-level Somali jet transported more water vapor to the Arabian Sea, whereas in FGOALS-g2 the enhanced rainfall was determined more by the increased wind convergence. Furthermore,and again in all models except FGOALS-g2, the equatorial SST warming, with maximum increase over the eastern Pacific,enhanced convection in the central West Pacific and reduced convection over the eastern Indian Ocean and Maritime Continent region, which drove the anomalous anticyclonic circulation over the western Indian Ocean. In contrast, in FGOALS-g2, there was minimal(near-zero) warming of projected SST in the central equatorial Pacific, with decreased convection in the central West Pacific and enhanced convection over the Maritime Continent. The broader-scale differences among the models across the Pacific were related to both the differences in the projected SST pattern and in the present-day simulations.     

Investigating the Initial Errors that Cause Predictability Barriers for Indian Ocean Dipole Events Using CMIP5 Model Outputs

By analyzing the outputs of the pre-industrial control runs of four models within phase 5 of the Coupled Model Intercomparison Project, the effects of initial sea temperature errors on the predictability of Indian Ocean Dipole events were identified. The initial errors cause a significant winter predictability barrier(WPB) or summer predictability barrier(SPB).The WPB is closely related with the initial errors in the tropical Indian Ocean, where two types of WPB-related initial errors display opposite patterns and a west–east dipole. In contrast, the occurrence of the SPB is mainly caused by initial errors in the tropical Pacific Ocean, where two types of SPB-related initial errors exhibit opposite patterns, with one pole in the subsurface western Pacific Ocean and the other in the upper eastern Pacific Ocean. Both of the WPB-related initial errors grow the fastest in winter, because the coupled system is at its weakest, and finally cause a significant WPB. The SPB-related initial errors develop into a La Ni ?na–like mode in the Pacific Ocean. The negative SST errors in the Pacific Ocean induce westerly wind anomalies in the Indian Ocean by modulating the Walker circulation in the tropical oceans. The westerly wind anomalies first cool the sea surface water in the eastern Indian Ocean. When the climatological wind direction reverses in summer, the wind anomalies in turn warm the sea surface water, finally causing a significant SPB. Therefore, in addition to the spatial patterns of the initial errors, the climatological conditions also play an important role in causing a significant predictability barrier.     

印度洋对ENSO事件的响应:观测与模拟

观测事实显示,在El Ni(n～)o期间,伴随着赤道中东太平洋表层海温(SST)的升高,热带印度洋SST出现正距平.作者利用海气耦合模式模拟了印度洋对ENSO事件的上述响应,并进而讨论了其物理机制.所用模式为法国国家科研中心Pierre-Simon-Laplace 全球环境科学联合实验室(IPSL)发展的全球海气耦合模式.该模式成功地控制了气候漂移,能够合理再现印度洋的基本气候态.观测中与ENSO相关的热带印度洋SST变化,表现为全海盆一致的正距平,并且这种变化要滞后赤道中东太平洋SST变化大约一个季度,意味着它主要是对东太平洋SST强迫的一种遥响应,模式结果也支持这一机制,尽管模式中的南方涛动现象被夸大了,使得模拟的与ENSO相关联的SST正距平的位置南移,阿拉伯海和孟加拉湾被负距平(而不是正距平)所控制.研究表明,东太平洋主要通过大气桥影响潜热释放来影响印度洋SST变化.赤道东太平洋El Ni(n～)o事件的发展,导致印度洋上空风场异常自东而西传播;伴随着风场的变化,潜热发生相应变化,并最终导致SST异常的发生.非洲东海岸受索马里急流控制的海域,其SST的变化不能简单地利用热通量的变化来解释.证据显示,印度洋的增暖是ENSO事件发生的结果而不是其前期信号.     

全球变暖趋缓研究进展

近十几年来,全球年平均表面温度上升趋势显示出停滞状态,即全球变暖趋缓,这引起了国际社会的广泛关注,同时也引发了对全球变暖的质疑,各国气候学家正努力就全球变暖趋缓的事实、原因及其可能影响展开研究。本文综述了目前国内外对全球变暖趋缓的研究结果。多数科学家认可近十几年来全球变暖停滞的事实,并认为太阳活动处于低位相、大气气溶胶（自然和人为）增加以及海洋吸收热量是变暖停滞的可能影响因子,其中海洋（尤其是700米以下的深海）对热量的储存可能是变暖停滞的关键。国际耦合模式比较计划第5阶段中的模式并未精确地描述各种有利降温影响因子的近期位相演变,因而其模拟的近期增暖趋势较观测偏强。由此推断,变暖停滞主要是自然因素造成的,并且预测变暖趋缓将在近几年或几十年内结束（依赖于太平洋年代际振荡的位相转变）,未来气温将仍主要受到温室气体增加的影响而表现出明显的上升趋势。因此,目前的全球变暖趋缓不大可能改变到本世纪末全球大幅度变暖带来的风险。本综述展望未来的研究热点包括:精确估算全球气温和海洋热含量的变率及其不确定性,海洋年代际信号（太平洋以及大西洋的年代际振荡）的转型机制,存储在深海的热量将在何时返回海洋表面及其对区域气候的潜在影响。     

Diagnosing southeast tropical Atlantic SST and ocean circulation biases in the CMIP5 ensemble

Warm sea-surface temperature (SST) biases in the southeastern tropical Atlantic (SETA), which is defined by a region from 5°E to the west coast of southern Africa and from 10°S to 30°S, are a common problem in many current and previous generation climate models. The Coupled Model Intercomparison Project Phase 5 (CMIP5) ensemble provides a useful framework to tackle the complex issues concerning causes of the SST bias. In this study, we tested a number of previously proposed mechanisms responsible for the SETA SST bias and found the following results. First, the multi-model ensemble mean shows a positive shortwave radiation bias of ~20 W m^?2, consistent with models’ deficiency in simulating low-level clouds. This shortwave radiation error, however, is overwhelmed by larger errors in the simulated surface turbulent heat and longwave radiation fluxes, resulting in excessive heat loss from the ocean. The result holds for atmosphere-only model simulations from the same multi-model ensemble, where the effect of SST biases on surface heat fluxes is removed, and is not sensitive to whether the analysis region is chosen to coincide with the maximum warm SST bias along the coast or with the main SETA stratocumulus deck away from the coast. This combined with the fact that there is no statistically significant relationship between simulated SST biases and surface heat flux biases among CMIP5 models suggests that the shortwave radiation bias caused by poorly simulated low-level clouds is not the leading cause of the warm SST bias. Second, the majority of CMIP5 models underestimate upwelling strength along the Benguela coast, which is linked to the unrealistically weak alongshore wind stress simulated by the models. However, a correlation analysis between the model simulated vertical velocities and SST biases does not reveal a statistically significant relationship between the two, suggesting that the deficient coastal upwelling in the models is not simply related to the warm SST bias via vertical heat advection. Third, SETA SST biases in CMIP5 models are correlated with surface and subsurface ocean temperature biases in the equatorial region, suggesting that the equatorial temperature bias remotely contributes to the SETA SST bias. Finally, we found that all CMIP5 models simulate a southward displaced Angola–Benguela front (ABF), which in many models is more than 10° south of its observed location. Furthermore, SETA SST biases are most significantly correlated with ABF latitude, which suggests that the inability of CMIP5 models to accurately simulate the ABF is a leading cause of the SETA SST bias. This is supported by simulations with the oceanic component of one of the CMIP5 models, which is forced with observationally derived surface fluxes. The results show that even with the observationally derived surface atmospheric forcing, the ocean model generates a significant warm SST bias near the ABF, underlining the important role of ocean dynamics in SETA SST bias problem. Further model simulations were conducted to address the impact of the SETA SST biases. The results indicate a significant remote influence of the SETA SST bias on global model simulations of tropical climate, underscoring the importance and urgency to reduce the SETA SST bias in global climate models.     

Relationship Between Upper-Ocean Heat Content in the Tropical Indian Ocean and Summer Precipitation in China

An analysis of the Ishii ocean heat content(OHC) in the tropical Indian Ocean from the surface to 700-m depth shows that the OHC changes dramatically on the interannual timescale in the Indian Ocean.The first mode of empirical orthogonal function(EOF1) of the OHC shows that there is a strong air-sea interaction pattern in the Indian Ocean with a positive(negative) loading in the east and a negative(positive) loading in the west.This seesaw oscillation pattern influences the summer precipitation in China with a North-South reversed distribution.Composite analysis shows that during a positive(negative) OHC episode,an anomalous cyclonic(anticyclonic) circulation over the western Pacific and South China weakens(enhances) the monsoonal northward flow in the lower troposphere;meanwhile,anomalous meridional circulation connects the descending(ascending) branch over the Southeast Indian Ocean and the ascending(descending) branch in South China as well as a descending(ascending) branch over North China.Analysis of the mechanism behind these features suggests that(1) the accumulation of OHC-induced vorticity is related to the wave activity over the mid-latitudes and that(2) the meridional teleconnection induced by the Indo-Pacific air-OHC interaction appears over East Asia and the western Pacific.Both of these patterns can cause summer precipitation anomalies in China.     

Decadal and interannual variability of the Indian Ocean Dipole

This study investigates the decadal and interannual variability of the Indian Ocean Dipole （IOD）. It is found that the long-term IOD index displays a decadal phase variation. Prior to 1920 negative phase dominates but after 1960 positive phase prevails. Under the warming background of the tropical ocean, a larger warming trend in the western Indian Ocean is responsible for the decadal phase variation of the IOD mode. Due to reduced latent heat loss from the local ocean, the western Indian Ocean warming may be caused by the weakened Indian Ocean westerly summer monsoon. The interannual air-sea coupled IOD mode varies on the background of its decadal variability. During the earlier period （1948-1969）, IOD events are characterized by opposing SST anomaly （SSTA） in the western and eastern Indian Ocean, with a single vertical circulation above the equatorial Indian Ocean. But in the later period （1980-2003）, with positive IOD dominating, most IOD events have a zonal gradient perturbation on a uniform positive SSTA. However, there are three exceptionally strong positive IOD events （1982, 1994, and 1997）, with opposite SSTA in the western and eastern Indian Ocean, accompanied by an El Nifio event. Consequently, two anomalous reversed Walker cells are located separately over the Indian Ocean and western-eastern Pacific; the one over the Indian Ocean is much stronger than that during other positive IOD events.     

THE WARMING MECHANISM IN THE SOUTHERN ARABIAN SEA DURING THE DEVELOPMENT OF INDIAN OCEAN DIPOLE EVENTS

This study aims to explore the relative role of oceanic dynamics and surface heat fluxes in the warming of southern Arabian Sea and southwest Indian Ocean during the development of Indian Ocean Dipole (IOD) events by using National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) daily reanalysis data and Global Ocean Data Assimilation System (GODAS) monthly mean ocean reanalysis data from 1982 to 2013, based on regression analysis, Empirical Orthogonal Function (EOF) analysis and combined with a 2? layer dynamic upper-ocean model. The results show that during the initial stage of IOD events, warm downwelling Rossby waves excited by an anomalous anticyclone over the west Indian Peninsula, southwest Indian Ocean and southeast Indian Ocean lead to the warming of the mixed layer by reducing entrainment cooling. An anomalous anticyclone over the west Indian Peninsula weakens the wind over the Arabian Sea and Somali coast, which helps decrease the sea surface heat loss and shallow the surface mixed layer, and also contributes to the sea surface temperature (SST) warming in the southern Arabian Sea by inhibiting entrainment. The weakened winds increase the SST along the Somali coast by inhibiting upwelling and zonal advection. The wind and net sea surface heat flux anomalies are not significant over the southwest Indian Ocean. During the antecedent stage of IOD events, the warming of the southern Arabian Sea is closely connected with the reduction of entrainment cooling caused by the Rossby waves and the weakened wind. With the appearance of an equatorial easterly wind anomaly, the warming of the southwest Indian Ocean is not only driven by weaker entrainment cooling caused by the Rossby waves, but also by the meridional heat transport carried by Ekman flow. The anomalous sea surface heat flux plays a key role to damp the warming of the west pole of the IOD.     

Impact of improved assimilation of temperature and salinity for coupled model seasonal forecasts

We assess the impact of improved ocean initial conditions for predicting El Niño-Southern Oscillation (ENSO) and Indian Ocean dipole (IOD) using the Bureau of Meteorology’s Predictive Ocean Atmosphere Model for Australia (POAMA) coupled seasonal prediction model for the period 1982–2006. The new ocean initial conditions are provided by an ensemble-based analysis system that assimilates subsurface temperatures and salinity and which is a clear improvement over the previous optimal interpolation system which used static error covariances and was univariate (temperature only). Hindcasts using the new ocean initial conditions have better skill at predicting sea surface temperature (SST) variations associated with ENSO than do the hindcasts initialized with the old ocean analyses. The improvement derives from better prediction of subsurface temperatures and the largest improvements come during ENSO–IOD neutral years. We show that improved prediction of the Niño3.4 SST index derives from improved initial depiction of the thermocline and halocline in the equatorial Pacific but as lead time increases the improved depiction of the initial salinity field in the western Pacific become more important. Improved ocean initial conditions do not translate into improved skill for predicting the IOD but we do see an improvement in the prediction of subsurface temperatures in the Indian Ocean (IO). This result reflects that the coupling between subsurface and surface temperature variations is weaker in the IO than in the Pacific, but coupled model errors may also be limiting predictive skill in the IO.