Eastward propagating MJO during boreal summer and Indian monsoon droughts

Improved understanding of underlying mechanism responsible for Indian summer monsoon (ISM) droughts is important due to their profound socio-economic impact over the region. While some droughts are associated with ‘external forcing’ such as the El-Niño and Southern Oscillation (ENSO), many ISM droughts are not related to any known ‘external forcing’. Here, we unravel a fundamental dynamic process responsible for droughts arising not only from external forcing but also those associated with internal dynamics. We show that most ISM droughts are associated with at least one very long break (VLB; breaks with duration of more than 10 days) and that the processes responsible for VLBs may also be the mechanism responsible for ISM droughts. Our analysis also reveals that all extended monsoon breaks (whether co-occurred with El-Niño or not) are associated with an eastward propagating Madden–Julian Oscillation (MJO) in the equatorial Indian Ocean and western Pacific extending to the dateline and westward propagating Rossby waves between 10° and 25°N. The divergent Rossby wave associated with the dry phase of equatorial convection propagates westward towards Indian land, couple with the northward propagating dry phase and leads to the sustenance of breaks. Thus, the propensity of eastward propagating MJO during boreal summer is largely the cause of monsoon droughts. While short breaks are not accompanied by westerly wind events (WWE) over equatorial western Pacific favorable for initiating air–sea interaction, all VLBs are accompanied by sustained WWE. The WWEs associated with all VLB during 1975–2005 initiate air–sea interaction on intraseasonal time scale, extend the warm pool eastward allowing the convectively coupled MJO to propagate further eastward and thereby sustaining the divergent circulation over India and the monsoon break. The ocean–atmosphere coupling on interannual time scale (such as El-Niño) can also produce VLB, but not necessary.     

Impacts of Two Types of El Ni o on the MJO during Boreal Winter

The features of the MJO during two types of El Ni no events are investigated in this paper using the daily NCEP-2reanalysis data, OLR data from NOAA, and Real-time Multivariate MJO index for the period 1979–2012. The results indicate that the MJO exhibits distinct features during eastern Pacific(EP) El Ni no events, as compared to central Pacific(CP) El Ni no events. First, the intensity of the MJO is weakened during EP El Ni no winters from the tropical eastern Indian Ocean to the western Pacific, but enhanced during CP El Ni no winters. Second, the range of the MJO eastward propagation is different during the two types of El Ni no events. During EP El Ni no winters, the MJO propagates eastwards to 120?W, but only to 180?during CP El Ni no winters. Finally, the frequency in eight phases of the MJO may be affected by the two types of El Ni no. Phases 2 and 3 display a stronger MJO frequency during EP El Ni no winters, but phases 4 and 5 during CP El Ni no winters.     

夏季MJO持续异常的主要特征分析

在MJO传播过程中,其活动中心并不总是规律地沿赤道东传。本文通过资料分析发现,夏季MJO的活动中心会出现东传停滞的情况,表现为MJO在赤道太平洋持续异常活跃或者在印度洋持续异常活跃两种形式。为更好描述MJO这种东传不明显的异常特征,本文定义了一个描述MJO持续异常的指数,并据此对夏季MJO持续异常的主要特征进行分析。通过小波分析的方法,发现夏季MJO持续异常时其振荡周期会出现缩短或变弱。通过对MJO持续异常状况下的大气环流进行合成对比分析后发现,夏季MJO的持续异常会对热带大气环流造成显著的影响。具体表现为:MJO夏季在赤道太平洋（印度洋）持续活跃的时候,赤道沃克环流减弱（增强）,西太平洋哈得来环流增强（减弱）,西太平洋副高位置偏北（偏南）,赤道太平洋（印度洋）高层辐散且对流活跃。     

Modulation of Madden-Julian Oscillation Activity by the Tropical Pacific-Indian Ocean Associated Mode

In this study, the impacts of the tropical Pacific–Indian Ocean associated mode (PIOAM) on Madden–Julian Oscillation (MJO) activity were investigated using reanalysis data. In the positive (negative) phase of the PIOAM, the amplitudes of MJO zonal wind and outgoing longwave radiation are significantly weakened (enhanced) over the Indian Ocean, while they are enhanced (weakened) over the central and eastern Pacific. The eastward propagation of the MJO can extend to the central Pacific in the positive phase of the PIOAM, whereas it is mainly confined to west of 160°E in the negative phase. The PIOAM impacts MJO activity by modifying the atmospheric circulation and moisture budget. Anomalous ascending (descending) motion and positive (negative) moisture anomalies occur over the western Indian Ocean and central-eastern Pacific (Maritime Continent and western Pacific) during the positive phase of the PIOAM. The anomalous circulation is almost the opposite in the negative phases of the PIOAM. This anomalous circulation and moisture can modulate the activity of the MJO. The stronger moistening over the Indian Ocean induced by zonal and vertical moisture advection leads to the stronger MJO activity over the Indian Ocean in the negative phase of the PIOAM. During the positive phase of the PIOAM, the MJO propagates farther east over the central Pacific owing to the stronger moistening there, which is mainly attributable to the meridional and vertical moisture advection, especially low-frequency background state moisture advection by the MJO’s meridional and vertical velocities.     

Connections Between Different Types of El Ni~no and Southern/Northern Oscillation

It has long been acknowledged that there are two types of El Ni(n)o events,i.e.,the eastern Pacific El Ni(n)o (EE) and the central Pacific El Ni(n)o (CE),according to the initial position of the anomalous warm water and its propagation direction.In this paper,the oceanic and atmospheric evolutions and the possible mechanisms of the two types of El Ni(n)o events were examined.It is found that all the El Ni(n)o events,CE or EE,could be attributed to the joint impacts of the eastward advection of warm water from the western Pacific warm pool (WPWP) and the local warming in the equatorial eastern Pacific.Before the occurrence of CE events,WPWP had long been in a state of being anomalous warm,so the strength of eastward advection of warm water was much stronger than that of EE,which played a major role in the formation of CE.While for the EE events,most contribution came from the local warming of the equatorial eastern Pacific.It is further identified that the immediate cause leading to the difference of the two types of El Ni(n)o events was the asynchronous variations of the Southern Oscillation (SO) and the Northern Oscillation (NO) as defined by Chen in 1984.When the transition from the positive phase of the NO (NO+) to NO- was prior to that from SO+ to SO-,there would be eastward propagation of westerly anomalies from the tropical western Pacific induced by NO and hence the growth of warm sea surface temperature anomalies in WPWP and its eastward propagation.This was followed by lagged SO-induced weakening of southeast trade winds and local warming in the equatorial eastern Pacific.These were conducive to the occurrence of the CE.On the contrary,the transition from SO+ to SO- leading the transition of NO would favor the occurrence of EE type events.     

Impacts of the MJO on Winter Rainfall and Circulation in China

Impacts of the MJO on winter rainfall and circulation in China are investigated using a real-time multivariate MJO index.Composite results using the daily rainfall anomalies and "rainy day" anomalies according to eight different MJO phases show that the MJO has considerable influence on winter rainfall in China. Rainfall anomalies show systematic and substantial changes(enhanced/suppressed) in the Yangtze River Basin and South China with the eastward propagation of the MJO convective center from the Indian Ocean to the western Pacific.When the MJO is in phase 2 and 3(MJO convective center is located over the Indian Ocean),rainfall probability is significantly enhanced.While in phase 6 and 7(MJO convective center is over the western Pacific),rainfall probability is significantly reduced. MJO in winter influences the rainfall in China mainly through modulating the circulation in the subtropics and mid-high latitudes.For the subtropics,MJO influences the northward moisture transport coming from the Bay of Bengal and the South China Sea by modulating the southern trough of the Bay of Bengal and the western Pacific subtropical high.For the mid-high latitudes,the propagation of the low frequency perturbations associated with the eastward-propagating MJO convection modulate the circulation in the mid-high latitudes,e.g.the East Asian winter monsoon and the low trough over central Asia.     

NCEP/CFSR再分析耦合资料中MJO对ENSO的影响研究

基于1979—2008年NCEP/CFSR再分析耦合数据集,研究了冬季MJO对ENSO事件的影响。结果表明,在年际时间尺度以及长期的年代际时间尺度上,热带印度洋MJO活动的强弱性都可以影响热带中东太平洋ENSO事件的发生和发展。在年际时间尺度上,ENSO发生前期征兆的赤道中东太平洋的西风爆发事件(Westerly Wind Burst,WWB),作为MJO影响ENSO的主要途径,存在着显著的次季节时间尺度的变化。相对于气候平均的赤道太平洋西部暖池区上升而东部下沉的Walker环流,MJO正位相东传后的西风异常,减弱了低层东风和赤道东太平洋海水上翻。这一上升海流的减弱导致了中东赤道太平洋的海温升高,从而有利于ENSO暖海温事件的发生。而在年代际时间尺度上,MJO范围和强度在1998年前后出现了明显的转变,1998年之前MJO的东移范围更东,强度更强,从而导致了西太平洋西风爆发区的次季节西风异常事件更加显著,在Bjeknes正反馈机制下对应了年代际时间尺度下的强尼诺事件出现,1998年之后则与之相反。冬季MJO对ENSO影响的这一年代际特征主要体现在晚冬季节,而在早冬伴随着印度洋的增暖,MJO强度一直在逐年增加。     

Simulations of the Madden–Julian oscillation in four pairs of coupled and uncoupled global models

The status of the numerical reproduction of the Madden–Julian Oscillation (MJO) by current global models was assessed through diagnoses of four pairs of coupled and uncoupled simulations. Slow eastward propagation of the MJO, especially in low-level zonal wind, is realistic in all these simulations. However, the simulated MJO suffers from several common problems. The MJO signal in precipitation is generally too weak and often eroded by an unrealistic split of an equatorial maximum of precipitation into a double ITCZ structure over the western Pacific. The MJO signal in low-level zonal wind, on the other hand, is sometimes too strong over the eastern Pacific but too weak over the Indian Ocean. The observed phase relationship between precipitation and low-level zonal wind associated with the MJO in the western Pacific and their coherence in general are not reproduced by the models. The seasonal migration in latitude of MJO activity is missing in most simulations. Air–sea coupling generally strengthens the simulated eastward propagating signal, but its effects on the phase relationship and coherence between precipitation and low-level zonal wind, and on their geographic distributions, seasonal cycles, and interannual variability are inconsistent among the simulations. Such inconsistency cautions generalization of results from MJO simulations using a single model. In comparison to observations, biases in the simulated MJO appear to be related to biases in the background state of mean precipitation, low-level zonal wind, and boundary-layer moisture convergence. This study concludes that, while the realistic simulations of the eastward propagation of the MJO are encouraging, reproducing other fundamental features of the MJO by current global models remains an unmet challenge.

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Simulation of the intraseasonal variability over the Eastern Pacific ITCZ in climate models

During boreal summer, convective activity over the eastern Pacific (EPAC) inter-tropical convergence zone (ITCZ) exhibits vigorous intraseasonal variability (ISV). Previous observational studies identified two dominant ISV modes over the EPAC, i.e., a 40-day mode and a quasi-biweekly mode (QBM). The 40-day ISV mode is generally considered a local expression of the Madden-Julian Oscillation. However, in addition to the eastward propagation, northward propagation of the 40-day mode is also evident. The QBM mode bears a smaller spatial scale than the 40-day mode, and is largely characterized by northward propagation. While the ISV over the EPAC exerts significant influences on regional climate/weather systems, investigation of contemporary model capabilities in representing these ISV modes over the EPAC is limited. In this study, the model fidelity in representing these two dominant ISV modes over the EPAC is assessed by analyzing six atmospheric and three coupled general circulation models (GCMs), including one super-parameterized GCM (SPCAM) and one recently developed high-resolution GCM (GFDL HIRAM) with horizontal resolution of about 50?km. While it remains challenging for GCMs to faithfully represent these two ISV modes including their amplitude, evolution patterns, and periodicities, encouraging simulations are also noted. In general, SPCAM and HIRAM exhibit relatively superior skill in representing the two ISV modes over the EPAC. While the advantage of SPCAM is achieved through explicit representation of the cumulus process by the embedded 2-D cloud resolving models, the improved representation in HIRAM could be ascribed to the employment of a strongly entraining plume cumulus scheme, which inhibits the deep convection, and thus effectively enhances the stratiform rainfall. The sensitivity tests based on HIRAM also suggest that fine horizontal resolution could also be conducive to realistically capture the ISV over the EPAC, particularly for the QBM mode. Further analysis illustrates that the observed 40-day ISV mode over the EPAC is closely linked to the eastward propagating ISV signals from the Indian Ocean/Western Pacific, which is in agreement with the general impression that the 40-day ISV mode over the EPAC could be a local expression of the global Madden-Julian Oscillation (MJO). In contrast, the convective signals associated with the 40-day mode over the EPAC in most of the GCM simulations tend to originate between 150°E and 150°W, suggesting the 40-day ISV mode over the EPAC might be sustained without the forcing by the eastward propagating MJO. Further investigation is warranted towards improved understanding of the origin of the ISV over the EPAC.     

热带大气季节内振荡与2008年初中国南方雪灾的关系

利用热带大气季节内振荡(MJO)指数、向外长波辐射(OLR)资料和NCEP/NCAR全球逐日再分析资料以及中国南方地区降水资料,应用合成分析等探讨了MJO与2008年初南方雪灾的关系,进而从MJO角度研究雪灾过程成因。结果表明：雪灾过程中,MJO存在明显的东传现象,即由西太平洋向东传送至印度洋附近。伴随着MJO的向东传播,中国南方降水强度及集中区也在随之变化,MJO在一定程度上对雪灾过程有调制作用。通过对OLR的分析,表明2008年初中国南方4次异常降水(雪)过程受到了MJO两次东传的影响,第一次导致西太平洋副热带高压持续偏强,第二次导致南支槽活跃;副热带高压西侧偏南风带来的暖湿气流,以及南支槽带来的印度洋和孟加拉湾的暖湿气流不断向中国输送,为中国南方强降水过程提供了水汽条件。     

Connections Between Different Types of El Nio and Southern/Northern Oscillation

It has long been acknowledged that there are two types of El Nio events, i.e., the eastern Pacific El Nio (EE) and the central Pacific El Nio (CE), according to the initial position of the anomalous warm water and its propagation direction. In this paper, the oceanic and atmospheric evolutions and the possible mechanisms of the two types of El Nio events were examined. It is found that all the El Nio events, CE or EE, could be attributed to the joint impacts of the eastward advection of warm water from the western Pacific warm pool (WPWP) and the local warming in the equatorial eastern Pacific. Before the occurrence of CE events, WPWP had long been in a state of being anomalous warm, so the strength of eastward advection of warm water was much stronger than that of EE, which played a major role in the formation of CE. While for the EE events, most contribution came from the local warming of the equatorial eastern Pacific. It is further identified that the immediate cause leading to the difference of the two types of El Nio events was the asynchronous variations of the Southern Oscillation (SO) and the Northern Oscillation (NO) as defined by Chen in 1984. When the transition from the positive phase of the NO (NO+) to NO- was prior to that from SO+ to SO-, there would be eastward propagation of westerly anomalies from the tropical western Pacific induced by NO and hence the growth of warm sea surface temperature anomalies in WPWP and its eastward propagation. This was followed by lagged SO-induced weakening of southeast trade winds and local warming in the equatorial eastern Pacific. These were conducive to the occurrence of the CE. On the contrary, the transition from SO+ to SO- leading the transition of NO would favor the occurrence of EE type events.     

Impact of MJO on the intraseasonal variation of summer monsoon rainfall over India

The summer monsoon rainfall over India exhibits strong intraseasonal variability. Earlier studies have identified Madden Julian Oscillation (MJO) as one of the most influencing factors of the intraseasonal variability of the monsoon rainfall. In this study, using India Meteorological Department (IMD) high resolution daily gridded rainfall data and Wheeler?CHendon MJO indices, the intra-seasonal variation of daily rainfall distribution over India associated with various Phases of eastward propagating MJO life cycle was examined to understand the mechanism linking the MJO to the intraseasonal variability. During MJO Phases of 1 and 2, formation of MJO associated positive convective anomaly over the equatorial Indian Ocean activated the oceanic tropical convergence zone (OTCZ) and the resultant changes in the monsoon circulation caused break monsoon type rainfall distribution. Associated with this, negative convective anomalies over monsoon trough zone region extended eastwards to date line indicating weaker than normal northern hemisphere inter tropical convergence zone (ITCZ). The positive convective anomalies over OTCZ and negative convective anomalies over ITCZ formed a dipole like pattern. Subsequently, as the MJO propagated eastwards to west equatorial Pacific through the maritime continent, a gradual northward shift of the OTCZ was observed and negative convective anomalies started appearing over equatorial Indian Ocean. During Phase 4, while the eastwards propagating MJO linked positive convective anomalies activated the eastern part of the ITCZ, the northward propagating OTCZ merged with monsoon trough (western part of the ITCZ) and induced positive convective anomalies over the region. During Phases 5 and 6, the dipole pattern in convective anomalies was reversed compared to that during Phases 1 and 2. This resulted active monsoon type rainfall distribution over India. During the subsequent Phases (7 and 8), the convective and lower tropospheric anomaly patterns were very similar to that during Phase 1 and 2 except for above normal convective anomalies over equatorial Indian Ocean. A general decrease in the rainfall was also observed over most parts of the country. The associated dry conditions extended up to northwest Pacific. Thus the impact of the MJO on the monsoon was not limited to the Indian region. The impact was rather felt over larger spatial scale extending up to Pacific. This study also revealed that the onset of break and active events over India and the duration of these events are strongly related to the Phase and strength of the MJO. The break events were relatively better associated with the strong MJO Phases than the active events. About 83% of the break events were found to be set in during the Phases 7, 8, 1 and 2 of MJO with maximum during Phase 1 (40%). On the other hand, about 70% of the active events were set in during the MJO Phases of 3 to 6 with maximum during Phase 4 (21%). The results of this study indicate an opportunity for using the real time information and skillful prediction of MJO Phases for the prediction of break and active conditions which are very crucial for agriculture decisions.     

Modulation of Twin Tropical Cyclogenesis by the MJO Westerly Wind Burst during the Onset Period of 1997／98 ENSO

The modulation of twin tropical cyclogenesis in the Indian-western Pacific Oceans by the Madden-Julian Oscillation (MJO) during the onset period of 1997/98 ENSO is explored for the period of September 1996 to June 1997 based on daily OLR, NCEP/NCAR wind vector, and JTWC best track datasets. The MJO westerly wind burst associated with its eastward propagation can result in a series of tropical cyclogeneses in a multi-day interval. Only in the transition seasons are pairs of tropical cyclones observed in both the tropical sectors of the Indian-western Pacific Oceans. Two remarkable twin tropical cyclogeneses probably modulated by the MJO westerly wind burst are found: one is observed in the Indian Ocean in the middle of October 1996, and the other is observed in the Western Pacific Ocean in late May 1997. The twin tropical cyclogenesis in mid-October 1996 is observed when the super cloud cluster separates into two isolated clusters by the enhanced westerly wind, which is accompanied by two independent vortices in the equatorial tropical sectors. The other one, in late-May 1997, however, is characterized by one cyclonic flow that later results in another cyclonic cell in its opposite equatorial sector. Thus, there are two very important conditions for twin cyclogenesis: one is the MJO westerly wind straddling the equator, and the other is the integral super cloud cluster, which later splits into two cloud convective clusters with independent vortices.     

MJO特征的多尺度子空间重构

通过构造两种不同类型的理想序列,将多尺度子空间变换(Multiscale Window Transform,MWT)与传统使用最多的带通滤波方法(Butterw orth)进行比较发现:MWT滤波的结果在整体上与原序列几乎完全吻合,而Butterworth的滤波结果在振幅上总是偏小;同时MWT对边界的处理也比Butterworth要好。为此使用这两种方法分别对MJO进行重构,并比较两种方法下MJO特性的差异。结果表明:在经向(meridional)传播上,两种方法得出的MJO基本类似;但在纬向(zonal)传播及其季节变化上,两种方法的结果无论在强度还是分布上都存在明显的差异,并且这种差异主要存在于西太平洋地区。本文发现:MJO波列从年初到夏季由南向北传播,从夏季到岁末传播方向却逆转成由北向南(尤其是在西太平洋地区),这一点与前人的结果迥异。     

The Madden–Julian oscillation in ECHAM4 coupled and uncoupled general circulation models

The Madden-Julian oscillation (MJO) dominates tropical variability on timescales of 30–70 days. During the boreal winter/spring, it is manifested as an eastward propagating disturbance, with a strong convective signature over the eastern hemisphere. The space–time structure of the MJO is analyzed using simulations with the ECHAM4 atmospheric general circulation model run with observed monthly mean sea-surface temperatures (SSTs), and coupled to three different ocean models. The coherence of the eastward propagation of MJO convection is sensitive to the ocean model to which ECHAM4 is coupled. For ECHAM4/OPYC and ECHO-G, models for which ~100 years of daily data is available, Monte Carlo sampling indicates that their metrics of eastward propagation are different at the 1% significance level. The flux-adjusted coupled simulations, ECHAM4/OPYC and ECHO-G, maintain a more realistic mean-state, and have a more realistic MJO simulation than the nonadjusted scale interaction experiment (SINTEX) coupled runs. The SINTEX model exhibits a cold bias in Indian Ocean and tropical West Pacific Ocean sea-surface temperature of ~0.5°C. This cold bias affects the distribution of time-mean convection over the tropical eastern hemisphere. Furthermore, the eastward propagation of MJO convection in this model is not as coherent as in the two models that used flux adjustment or when compared to an integration of ECHAM4 with prescribed observed SST. This result suggests that simulating a realistic basic state is at least as important as air–sea interaction for organizing the MJO. While all of the coupled models simulate the warm (cold) SST anomalies that precede (succeed) the MJO convection, the interaction of the components of the net surface heat flux that lead to these anomalies are different over the Indian Ocean. The ECHAM4/OPYC model in which the atmospheric model is run at a horizontal resolution of T42, has eastward propagating zonal wind anomalies and latent heat flux anomalies. However, the integrations with ECHO-G and SINTEX, which used T30 atmospheres, produce westward propagation of the latent heat flux anomalies, contrary to reanalysis. It is suggested that the differing ability of the models to represent the near-surface westerlies over the Indian Ocean is related to the different horizontal resolutions of the atmospheric model employed.     

Application of UTCI in China from tourism perspective

This paper investigates the processes and mechanisms by which the East Asian winter monsoon (EAWM) affects the Madden-Julian oscillation (MJO) over the equatorial western Pacific in boreal winter (November–April). The results show that both the EAWM and MJO over the equatorial western Pacific have prominent interannual and interdecadal variabilities, and they are closely related, especially on the interannual timescales. The EAWM influences MJO via the feedback effect of convective heating, because the strong northerlies of EAWM can enhance the ascending motion and lead the convection to be strengthened over the equatorial western Pacific by reinforcing the convergence in the lower troposphere. Daily composite analysis in the phase 4 of MJO (i.e., strong MJO convection over the Maritime Continent and equatorial western Pacific) shows that the kinetic energy, outgoing longwave radiation (OLR), moisture flux, vertical velocity, zonal wind, moist static energy, and atmospheric stability differ greatly between strong and weak EAWM processes over the western Pacific. The strong EAWM causes the intensity of MJO to increase, and the eastward propagation of MJO to become more persistent. MJO activities over the equatorial western Pacific have different modes. Furthermore, these modes have differing relationships with the EAWM, and other factors can also affect the activities of MJO; consequently, the relationship between the MJO and EAWM shows both interannual and interdecadal variabilities.     

Triggering of El Niño by westerly wind events in a coupled general circulation model

Two ten-members ensemble experiments using a coupled ocean-atmosphere general circulation model are performed to study the dynamical response to a strong westerly wind event (WWE) when the tropical Pacific has initial conditions favourable to the development of a warm event. In the reference ensemble (CREF), no wind perturbation is introduced, whereas a strong westerly wind event anomaly is introduced in boreal winter over the western Pacific in the perturbed ensemble (CWWE). Our results demonstrate that an intense WWE is capable of establishing the conditions under which a strong El Niño event can occur. First, it generates a strong downwelling Kelvin wave that generates a positive sea surface temperature (SST) anomaly in the central-eastern Pacific amplified through a coupled ocean-atmosphere interaction. This anomaly can be as large as 2.5°C 60 days after the WWE. Secondly, this WWE also initiates an eastward displacement of the warm-pool that promotes the occurrence of subsequent WWEs in the following months. These events reinforce the initial warming through the generation of additional Kelvin waves and generate intense surface jets at the eastern edge of the warm-pool that act to further shift warm waters eastward. The use of a ten-members ensemble however reveals substantial differences in the coupled response to a WWE. Whereas four members of CWWE ensemble develop into intense El Niño warming as described above, four others display a moderate warming and two remains in neutral conditions. This diversity between the members appears to be due to the internal atmospheric variability during and following the inserted WWE. In the four moderate warm cases, the warm-pool is initially shifted eastward following the inserted WWE, but the subsequent weak WWE activity (when compared to the strong warming cases) prevents to further shift the warm-pool eastwards. The seasonal strengthening of trade winds in June–July can therefore act to shift warm waters back into the western Pacific, reducing the central-eastern Pacific warming. This strong sensitivity of the coupled response to WWEs may therefore limit the predictability of El Niño events, as the high frequency wind variability over the warm pool region remains largely unpredictable even at short time lead.     

Low and high frequency Madden–Julian oscillations in austral summer: interannual variations

The Madden–Julian oscillation (MJO) is the main component of intraseasonal variability of the tropical convection, with clear climatic impacts at an almost-global scale. Based on satellite observations, it is shown that there are two types of austral-summer MJO events (broadly defined as 30–120 days convective variability with eastward propagation of about 5 m/s). Equatorial MJO events have a period of 30–50 days and tend to be symmetric about the equator, whereas MJO events centered near 8°S tend to have a longer period of 55–100 days. The lower-frequency variability is associated with a strong upper-ocean response, having a clear signature in both sea surface temperature and its diurnal cycle. These two MJO types have different interannual variations, and are modulated by the Indian Ocean Dipole (IOD). Following a negative IOD event, the lower-frequency southern MJO variability increases, while the higher-frequency equatorial MJO strongly diminishes. We propose two possible explanations for this change in properties of the MJO. One possibility is that changes in the background atmospheric circulation after an IOD favour the development of the low-frequency MJO. The other possibility is that the shallower thermocline ridge and mixed layer depth, by enhancing SST intraseasonal variability and thus ocean–atmosphere coupling in the southwest Indian Ocean (the breeding ground of southern MJO onset), favour the lower-frequency southern MJO variability.     

热带大气季节内振荡对西北太平洋台风的调制作用

利用澳大利亚气象局的RMM-MJO (Real-time Multivariate MJO) 指数, 分析研究了热带大气季节内振荡 (简称MJO) 对西北太平洋台风的调制作用及其机理, 结果表明MJO活动对西北太平洋台风的生成有比较明显调制作用。在MJO活跃期, 对流中心位于赤道东印度洋 (即MJO第2、3位相) 和对流中心越过海洋性大陆来到西太平洋地区 (即MJO 第5、6位相) 时台风生成的个数比例为2∶1。本文对西太平洋地区的大气环流场进行了多种气象要素的合成分析, 在MJO的不同位相, 西太平洋地区的动力因子分布形势有很明显不同。在第2、3位相, 各种因子均呈现出抑制西太平洋地区对流及台风发展的态势; 而在第5、6位相则明显有促进对流发生发展, 为台风生成和发展创造了有利条件的大尺度环流动力场。这说明MJO 在不断东移的过程中, 将改变大气环流形势, 最终影响了台风的生成和发展。接着, 我们从积云对流这个联系台风和MJO的重要因子出发, 研究了不同MJO位相时凝结加热的水平和垂直分布, 以及与台风环流、 水汽通量的配置情况。结果表明在MJO不同位相, 热源分布明显不同, 而这种水平和垂直方向的不同分布特征必然反映潜热释放和有效位能向有效动能转换的差异, 再与水汽的辐合辐散相配合, 就从台风获得的能量角度揭示了大气MJO调节台风的生成和发展, 造成不同位相时台风生成有根本差别的原因。