两类开始型厄尔尼诺事件与次表层海温异常的联系

通过资料分析,探究了东太平洋开始型(EP-onset)和中太平洋开始型(CP-onset)厄尔尼诺发生发展过程中赤道太平洋次表层海温异常(SOTA)的变化情况及其对风场的响应,并将两者进行对比,结果发现:EP-onset型的赤道西太平洋暖池SOTA与赤道中东太平洋的海表温度异常(SSTA)有很好的负相关性,而CP-onset型在爆发前的暖池SOTA与赤道中东太平洋SSTA没有显著相关性.EP-onset型在爆发前,赤道西太平洋暖池区域有较显著的暖水积聚,且其积聚时间较长,强度较强,位置偏西,其发生发展伴随着非常显著的SOTA冷暖循环;而CP-onset型的发生发展没有显著的循环特征,看上去更多是独立事件.分析还表明:EP-onset型和CP-onset型厄尔尼诺在SSTA模态、风应力分布及温跃层结构等方面存在差异.EP-onset型在爆发前,赤道中西太平洋会出现大范围的东风异常,这种东风异常通过密度流、温跃层反馈及海气耦合等机制对其爆发起到关键作用;而CP-onset型在爆发前,赤道地区风异常较小,赤道外尤其是热带中太平洋的西南风异常却较强劲,通过Ekman输送及海气耦合作用等机制对其爆发起到关键作用.

两类El Nio的发生与赤道太平洋次表层海温异常

通过资料分析,对比了东太平洋型El Nio(EP-El Nio)和中太平洋型El Nio(CP-El Nio)的生消演变与赤道太平洋次表层海温异常的关系.其结果表明:两类El Nio与赤道西太平洋次表层海温异常都有明显的负相关,但在不同相位阶段相关性的变化明显不同;两类El Nio的发生与赤道温跃层结构的变化密切相关,但其生消演变过程中温跃层东西向的梯度和显著变化的区域以及温跃层振荡的幅度和时间都有显著差异;两类El Nio生消过程中,赤道次表层海温异常的传播演变均起到了重要作用,但在暖SOTA的聚集时间、强度、持续时间以及位相反转等方面存在较大差异.此外,两类El Nio事件中,次表层海温异常的传播有着明显不同的路径:东太平洋型主要的传播回路在赤道及其北侧,在赤道以南虽有SOTA的传播,但没有形成显著的回路;而中太平洋型主要的传播路径在赤道及其南侧,在赤道以北其传播路径则远不如赤道以南清晰.本文还初步讨论了两类El Nio与赤道纬向风异常关系的异同,结果表明EP-El Nio更多是对赤道中西太平洋大范围异常西风的响应;而CP-El Nio则更多是对赤道西太平洋地区异常西风的响应,并且一定程度上需要赤道东太平洋区域异常东风的配合.

两种形态ENSO对南半球热带外气候变化特征的影响

本研究利用1979—2012年欧洲中期天气预报中心(ECMWF)的ORAP5(Ocean Reanalysis Pilot 5)海洋/海冰再分析资料和ERA-Interim气象再分析资料,采用回归分析方法,分1979—1998年和1999—2012年两个时间段探讨了南半球热带外地区气候变化对两种不同形态ENSO的响应特征。结果表明,南半球热带外区域气候在1999年前后两个时段对ENSO的响应表现出了较大的年代际变化特征。1979—1998年南半球热带外气候变量对Nio3指数在时间上的相关性和空间上的响应强度都普遍大于Nio4指数,说明这一时段东部型ENSO对南半球热带外区域气候变化的影响要更强一些。在1999—2012年,不同形态ENSO与气候变量的相关性大小并无明显的规律,而且空间响应场的差异性并不大。海平面气压、风场和气温对ENSO变化的响应在南半球冬季表现最为强烈,在夏季最弱。三者在1999—2012年秋季对Nio3指数和Nio4指数的响应场中出现了纬向三波数结构。1999—2012年冬季,有异于海平面气压和风场,在罗斯海和阿蒙森海海域海表气温对Nio4变化的正响应明显强于对Nio3的响应,该特征在混合层温度中也有体现,表明海表气温随ENSO的变化受海洋特征变化影响较大。混合层深度和混合层温度的响应场之间存在很大的相关性,混合层温度响应在秋季表现最强,春季最弱,混合层深度响应与之相反。在1979—1998年,海冰密集度对不同Nio指数变化的响应差异主要出现在海冰结冰季节,而海冰厚度对不同Nio指数变化的响应差异在夏季表现较强。海冰密集度和厚度对Nio3变化响应的年代际差异在秋冬季节更加明显,对Nio4变化响应的年代际差异在秋、冬、春季都较明显。

The linkage between two ENSO types/modes and the interdecadal changes of ENSO around the year 2000

This study focuses on the interdecadal changes in ENSO properties emerging around the year 2000. Compared to 1980-1999, after 2000, the ENSO amplitude weakened, the occurrence of the central Pacific （CP） Et Nino increased, and the eastern Pacific （EP） El Nino became suppressed. Meanwhile, the dominant period of ENSO shortened from quasi-quadrennial （QQ） to quasi-biennial （QB）. The authors show that these changes in ENSO properties are evidently consistent with the change in the stability of the ENSO mode through connecting the two ENSO types with the two coupled ENSO modes, i.e. the QQ and QB modes. It is suggested that the relative activity or stability of the two ENSO modes changed after the year 2000. The intensity of both the QQ and QB mode weakened. The QQ mode, which is linked to EP ENSO and was significantly strong during 1980-1999, became much weaker after 2000 in terms of the EP type almost disappearing. Compared with the weakness of the QQ mode, the QB mode, as manifested by the CP type, remained active and became dominant in the tropical Pacific after 2000. Analysis shows that the changes in mean states in the tropical Pacific were likely responsible for the interdecadal ENSO changes around the year 2000.

Energetic processes regulating the strength of MJO circulation over the Maritime Continent during two types of El Ni?o

The zonal shift of SST warming patterns associated with the eastern Pacific （EP） and central Pacific （CP） El Ni（n）o leads to a significant contrast in MJO strength over the Maritime Continent.The MJO circulation tends to be stronger over the Maritime Continent during the mature phase （autumnwinter） of CP El Ni（n）o than of EP El Ni（n）o.Based on a new MJO kinetic energy （KE） budget equation,in which the effects of mean flow and high-frequency disturbances on the MJO are separated,we found that the low-level MJO gains more KE from the background mean flow during CP El Ni（n）o events,although at the same time the enhanced MJO transfers more KE to high-frequency eddies.Among the three-dimensional circulation anomalies,the low-level convergence and cyclonic anomalies associated with upward anomalies of the Walker circulation over the Maritime Continent play leading roles in inducing the enhanced barotropic energy conversion from mean flow to MJO during CP El Ni（n）o events relative to EP El Ni（n）o events.The more vigorous MJO with strengthened vertical motion and heating anomalies at the upper troposphere can maintain its amplitude through the baroclinic energy conversion from the MJO available potential energy to KE.Both the low-tomid tropospheric barotropic energy conversion from mean flow to MJO and upper-level baroclinic energy conversion contribute positively to the enhanced MJO over the Maritime Continent during CP El Ni（n）o years compared to during EP El Ni（n）o years.

Time-Dependent Nonlinear Forcing Singular Vector-Type Tendency Error of the Zebiak-Cane Model

Based on the Zebiak-Cane model, the timedependent nonlinear forcing singular vector （NFSV）-type tendency errors with components of 4 and 12 （denoted by NFSV-4 and NFSV-12） are calculated for predetermined El Nifio events and compared with the constant NFSV （denoted by NFSV-1） from their patterns and resultant prediction errors. Specifically, NFSV-1 has a zonal dipolar sea surface temperature anomaly （SSTA） pattern with negative anomalies in the equatorial eastern Pacific and positive anomalies in the equatorial central-western Pa- cific. Although the first few components in NFSV-4 and NFSV-12 present patterns similar to NFSV-1, they tend to extend their dipoles farther westward; meanwhile, the positive anomalies gradually cover much smaller regions with the lag times. In addition, the authors calculate the predic- tion errors caused by the three kinds of NFSVs, and the results indicate that the prediction error induced by NFSV-12 is the largest, followed by the NFSV-4. However, when compared with the prediction errors caused by random tendency errors, the NFSVs generate significantly larger prediction errors. It is therefore shown that the spatial structure of tendency errors is important for producing large prediction errors. Furthermore, in exploring the tendency errors that cause the largest prediction error for E1 Nifio events, the timedependent NFSV should be evaluated.

Impact of two types of La Nia on boreal autumn rainfall around Southeast Asia and Australia

The distinct influences of eastern Pacific （EP） and central Pacific （CP） La Nina on rainfall anomalies over Southeast Asia and Australia in boreal autumn （September to November） are explored in this study. Composite results reveal that CP La Nina gives rise to significant and severe flooding over Southeast Asia and Australia, whereas EP La Nina fails to exert any evident impacts on rainfall over this region. This difference can be attributed to the distinct features of cooling sea surface temperature anomalies （SSTAs） between EP and CP La Nina. With a more westward location and stronger intensity of the negative SSTAs during CP La Nina autumn, the highest and lowest SLP anomalies are substantially enhanced and shift westwards too, further causing intense easterly winds over the western Pacific and westerly anomalies over the Indian Ocean driven by this SLP gradient. Subsequently, robust low-level convergence and high-level divergence is observed over the Maritime Continent and Australia, resulting in significant above-normal rainfall anomalies in those regions. In contrast, weak and eastern Pacific-confined cooling SSTAs during EP La Nina produce correspondingly weak low-level convergence over the Maritime Continent conditions that make it hard for significant rainfall anomalies to arise.

Climate variability and predictability associated with the Indo-Pacific Oceanic Channel Dynamics in the CCSM4 Coupled System Model

An experiment using the Community Climate System Model(CCSM4), a participant of the Coupled Model Intercomparison Project phase-5(CMIP5), is analyzed to assess the skills of this model in simulating and predicting the climate variabilities associated with the oceanic channel dynamics across the Indo-Pacific Oceans. The results of these analyses suggest that the model is able to reproduce the observed lag correlation between the oceanic anomalies in the southeastern tropical Indian Ocean and those in the cold tongue in the eastern equatorial Pacific Ocean at a time lag of 1 year. This success may be largely attributed to the successful simulation of the interannual variations of the Indonesian Throughflow, which carries the anomalies of the Indian Ocean Dipole(IOD) into the western equatorial Pacific Ocean to produce subsurface temperature anomalies, which in turn propagate to the eastern equatorial Pacific to generate ENSO. This connection is termed the "oceanic channel dynamics" and is shown to be consistent with the observational analyses. However, the model simulates a weaker connection between the IOD and the interannual variability of the Indonesian Throughflow transport than found in the observations. In addition, the model overestimates the westerly wind anomalies in the western-central equatorial Pacific in the year following the IOD, which forces unrealistic upwelling Rossby waves in the western equatorial Pacific and downwelling Kelvin waves in the east. This assessment suggests that the CCSM4 coupled climate system has underestimated the oceanic channel dynamics and overestimated the atmospheric bridge processes.

STUDY OF A COMPREHENSIVE MONITORING INDEX FOR TWO TYPES OF ENSO EVENTS

Some recent studies presented two existing types of ENSO events, one is the Eastern-Pacific(EP) type and the other the Central-Pacific(CP) type. This study examined the monitoring ability of several current operational ENSO indices. The results indicated that a single index could not distinguish the EP and CP in the historical ENSO events during 1950-2009. The Nio 3 index may only be suitable for monitoring the EP-type ENSO, while the Nio 4 index works only for the CP-type ENSO. In order to capture the occurrence of ENSO events and distinguish the type, we considered a new monitoring index group using Nio 3 and Nio 4 indices. Further analysis confirmed that this index group can monitor different types of historical ENSO events with different spatial distribution of sea surface temperature. It has a good performance in determining the characteristics of the ENSO events, including peak intensity, onset,decay, and mature phase.

Analysis of Decadal Climate Variability in the Tropical Pacific by Coupled GCM

This study presents the spatial and temporal structures of the decadal variability of the Pacific from an extended control run of a coupled global climate model (GCM).The GCM used was version-g2.0 of the Flexible Global Ocean Atmosphere Land System (FGOALS-g2.0) developed at LASG/IAP.The GCM FGOALS-g2.0 re-produces similar spatial-temporal structures of sea surface temperature (SST) as observed in the Pacific decadal os-cillation (PDO) with a significant period of approximately 14 years.Correspondingly,the PDO signals were closely related to the decadal change both in the upper-ocean temperature anomalies and in the atmospheric circulation.The present results suggest that warm SST anomalies along the equator relax the trade winds,causing the SSTs to warm even more in the eastern equatorial Pacific,which is a positive feedback.Meanwhile,warm SST anomalies along the equator force characteristic off-equa-torial wind stress curl anomalies,inducing much more poleward transport of heat,which is a negative feedback.The upper-ocean meridional heat transport,which is asso-ciated with the PDO phase transition,links the equatorial to the off-equatorial Pacific Ocean,acting as a major mechanism responsible for the tropical Pacific decadal variations.Therefore,the positive and negative feedbacks working together eventually result in the decadal oscilla-tion in the Pacific.

The projected temporal evolution in the interannual variability of East Asian summer rainfall by CMIP3 coupled models

The projected temporal evolution in the interannual variability of East Asian summer rainfall in the 21st century is investigated here,by analyzing the simulated results of 18 coupled models under the 20th century climate experiment and scenario A1B.The multi-model ensemble(MME)mean projects two prominent changes in the interannual variability of East Asian summer rainfall in the 21st century under scenario A1B.The first change occurs around the 2030s,with a small change before and a large increase afterward.The intensity of the interannual variability increases up to approximately 0.53 mm/d in the 2070s,representing an increase of approximately 30% relative to the early 21st century.The second change happens around the 2070s,with a decrease afterward.By the end of the 21st century,the increase is approximately 12% relative to the early 21st century.The interannual variability of two circulation factors,the western North Pacific subtropical high(WNPSH)and the East Asian upper-tropospheric jet(EAJ),are also projected to exhibit two prominent changes around the 2030s and 2070 under scenario A1B,with consistent increases and decreases afterward,respectively.The MME result also projects two prominent changes in the interannual variability of water vapor transported to East Asia at 850 hPa,which occurs separately around the 2040s and 2070s,with a persistent increase and decrease afterward.Meanwhile,the precipitable water interannual variability over East Asia and the western North Pacific is projected to exhibit two prominent enhancements around the 2030s and 2060s and an increase from 0.1 kg/m2 in the early 21st century to 0.5 kg/m2 at the end of the 21st century,implying a continuous intensification in the interannual variability of the potential precipitation.Otherwise,the intensities of the three factors'(except EAJ)interannual variability are all projected to be stronger at the end of the 21st century than that in the early period.These studies indicate that the change of interannual variability of the East Asian summer rainfall is caused by the variability of both the dynamic and thermodynamic variables under scenario A1B.In the early and middle 21st century,both factors lead to an intensified interannual variability of rainfall,whereas the dynamic factors weaken the interannual variability,and the thermodynamic factor intensifies the interannual variability in the late period.