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1.
The evolution of sea surface temperature (SST) and thermocline (represented by 20 °C isotherm depth, D20) in the east equatorial Indian Ocean (EIO) associated with the Indian Ocean Dipole (IOD) years is studied for the period of 50 years from 1958 to 2007. A new IOD index based on combined anomalies of surface winds, D20 and SST over the equatorial Indian Ocean is defined to identify strong and weak IOD events. It is found that the evolution of strong IOD events is driven by ocean dynamics in the form of thermocline–SST coupling and is strongly interactive with the atmosphere, whereas the weak IOD events are mere response to surface winds without such dynamical coupling. The easterly wind anomalies extend up to the western equatorial Indian Ocean (WIO) during strong IOD years and support enhanced EIO air–sea interactions. On the other hand, the evolution of zonal wind anomalies is weak during the weak IOD years. Thermocline–SST coupling is robust in both EIO and WIO during strong IOD years, which is primarily responsible for the enhanced SST gradient, strong enough to establish anomalous Walker circulation within the Indian Ocean. The strong convection over the WIO associated with the Indian Ocean Walker cell triggers a secondary cell with subsidence over the African landmass. This double cell structure over the equatorial Indian Ocean is not reported before. Such double cell structure is not evident in weak IOD years and instead the convection over WIO extends up to African landmass. These are well supported by the spatial pattern of anomalous precipitable water during strong and weak IOD years. Strengthening of monsoon flow and local Hadley cell associated with strong IOD events enhances precipitation over the Indian subcontinent, whereas weak IOD years have less impact on the Indian summer monsoon circulation and rainfall. Analysis reveals that the EIO thermocline index and combined index could be potential predictors for the central Indian rainfall during summer.  相似文献   

2.
The inverse relationship between the warm phase of the El Ni?o Southern Oscillation(ENSO) and the Indian Summer Monsoon Rainfall(ISMR) is well established. Yet, some El Ni?o events that occur in the early months of the year(boreal spring) transform into a neutral phase before the start of summer, whereas others begin in the boreal summer and persist in a positive phase throughout the summer monsoon season. This study investigates the distinct influences of an exhausted spring El Ni?o(springtime)...  相似文献   

3.
Drought patterns across monsoon and temperate Asia over the period 1877–2005 are linked to Indo-Pacific climate variability associated with the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). Using the Monsoon Asia Drought Atlas (MADA) composed of a high-resolution network of hydroclimatically sensitive tree-ring records with a focus on the June–August months, spatial drought patterns during El Niño and IOD events are assessed as to their agreement with an instrumental drought index and consistency in the drought response amongst ENSO/IOD events. Spatial characteristics in drought patterns are related to regional climate anomalies over the Indo-Pacific basin, using reanalysis products, including changes in the Asian monsoon systems, zonal Walker circulation, moisture fluxes, and precipitation. A weakening of the monsoon circulation over the Indian subcontinent and Southeast Asia during El Niño events, along with anomalous subsidence over monsoon Asia and reduced moisture flux, is reflected in anomalous drought conditions over India, Southeast Asia and Indonesia. When an IOD event co-occurs with an El Niño, severe drought conditions identified in the MADA for Southeast Asia, Indonesia, eastern China and central Asia are associated with a weakened South Asian monsoon, reduced moisture flux over China, and anomalous divergent flow and subsidence over Indonesia. Insights into the relative influences of Pacific and Indian Ocean variability for Asian monsoon climate on interannual to decadal and longer timescales, as recorded in the MADA, provide a useful tool for assessing long-term changes in the characteristics of Asian monsoon droughts in the context of Indo-Pacific climate variability.  相似文献   

4.
Sea surface temperature associations with the late Indian summer monsoon   总被引：1，自引：1，他引：0  
Recent gridded and historical data are used in order to assess the relationships between interannual variability of the Indian summer monsoon (ISM) and sea surface temperature (SST) anomaly patterns over the Indian and Pacific oceans. Interannual variability of ISM rainfall and dynamical indices for the traditional summer monsoon season (June–September) are strongly influenced by rainfall and circulation anomalies observed during August and September, or the late Indian summer monsoon (LISM). Anomalous monsoons are linked to well-defined LISM rainfall and large-scale circulation anomalies. The east-west Walker and local Hadley circulations fluctuate during the LISM of anomalous ISM years. LISM circulation is weakened and shifted eastward during weak ISM years. Therefore, we focus on the predictability of the LISM. Strong (weak) (L)ISMs are preceded by significant positive (negative) SST anomalies in the southeastern subtropical Indian Ocean, off Australia, during boreal winter. These SST anomalies are mainly linked to south Indian Ocean dipole events, studied by Besera and Yamagata (2001) and to the El Niño-Southern Oscillation (ENSO) phenomenon. These SST anomalies are highly persistent and affect the northwestward translation of the Mascarene High from austral to boreal summer. The southeastward (northwestward) shift of this subtropical high associated with cold (warm) SST anomalies off Australia causes a weakening (strengthening) of the whole monsoon circulation through a modulation of the local Hadley cell during the LISM. Furthermore, it is suggested that the Mascarene High interacts with the underlying SST anomalies through a positive dynamical feedback mechanism, maintaining its anomalous position during the LISM. Our results also explain why a strong ISM is preceded by a transition in boreal spring from an El Niño to a La Niña state in the Pacific and vice versa. An El Niño event and the associated warm SST anomalies over the southeastern Indian Ocean during boreal winter may play a key role in the development of a strong ISM by strengthening the local Hadley circulation during the LISM. On the other hand, a developing La Niña event in boreal spring and summer may also enhance the east–west Walker circulation and the monsoon as demonstrated in many previous studies.  相似文献   

5.
This study has investigated the possible relation between the Indian summer monsoon and the Pacific Decadal Oscillation (PDO) observed in the sea surface temperature (SST) of the North Pacific Ocean. Using long records of observations and coupled model (NCAR CCSM4) simulation, this study has found that the warm (cold) phase of the PDO is associated with deficit (excess) rainfall over India. The PDO extends its influence to the tropical Pacific and modifies the relation between the monsoon rainfall and El Niño-Southern Oscillation (ENSO). During the warm PDO period, the impact of El Niño (La Niña) on the monsoon rainfall is enhanced (reduced). A hypothesis put forward for the mechanism by which PDO affects the monsoon starts with the seasonal footprinting of SST from the North Pacific to the subtropical Pacific. This condition affects the trade winds, and either strengthens or weakens the Walker circulation over the Pacific and Indian Oceans depending on the phase of the PDO. The associated Hadley circulation in the monsoon region determines the impact of PDO on the monsoon rainfall. We suggest that knowing the phase of PDO may lead to better long-term prediction of the seasonal monsoon rainfall and the impact of ENSO on monsoon.  相似文献   

6.
Interannual variability of the Indian summer monsoon rainfall has two dominant periodicities, one on the quasi-biennial (2–3 year) time scale corresponding to tropospheric biennial oscillation (TBO) and the other on low frequency (3–7 year) corresponding to El Niño Southern Oscillation (ENSO). In the present study, the spatial and temporal patterns of various atmospheric and oceanic parameters associated with the Indian summer monsoon on the above two periodicities were investigated using NCEP/NCAR reanalysis data sets for the period 1950–2005. Influences of Indian and Pacific Ocean SSTs on the monsoon season rainfall are different for both of the time scales. Seasonal evolution and movement of SST and Walker circulation are also different. SST and velocity potential anomalies are southeast propagating on the TBO scale, while they are stationary on the ENSO scale. Latent heat flux and relative humidity anomalies over the Indian Ocean and local Hadley circulation between the Indian monsoon region and adjacent oceans have interannual variability only on the TBO time scale. Local processes over the Indian Ocean determine the Indian Ocean SST in biennial periodicity, while the effect of equatorial east Pacific SST is significant in the ENSO periodicity. TBO scale variability is dependent on the local factors of the Indian Ocean and the Indian summer monsoon, while the ENSO scale processes are remotely controlled by the Pacific Ocean.  相似文献   

7.
Indian summer monsoon (ISM) variability is forced from external factors (like the El Niño Southern Oscillation, ENSO) but it contains also an internal component that tends to reduce its potential for predictability. Large-scale and local monsoon indices based on precipitation and atmospheric circulation parameters are used as a measure of ISM variability. In a 9-members ensemble of AMIP-type experiments (with same boundary SST forcing and different initial conditions) their potential predictability is comparable using both local and large-scale monsoon indices. In the sample analyzed, about half of more predictable monsoon years coincide with El Niño and/or positive Indian Ocean Dipole (IOD) events. Summer monsoon characteristics during ENSO and IOD years are analyzed through composites computed over a three years period (i.e. one year before and one year after the event peak) to investigate the mutual relationship between the events lagged in time. The connection between ISM and IOD is mostly confined in the summer and autumn, while that with ENSO is stronger and extends more in time. In the coupled model results the IOD influence on the monsoon is large, even because in the model IOD events are intense and easily reproduced due to a strong air-sea feedback in the eastern side of the basin. Monsoon seasons preceding or following an El Niño or a La Niña event are not exactly symmetric, even in terms of their biennial character. In most of the cases, both in reanalysis and model, El Niño and positive IOD events tend to co-occur with larger anomalies either in the Indo-Pacific ocean sector or over India, while La Niña and negative IOD do not. From the observed record, the ENSO-IOD correlation is positive strong and significant since mid-60s and it may correspond with either strong or weak ENSO-monsoon relationship and with strong or weak IOD-monsoon relationship. A main difference between those periods is the relationship between Indian monsoon rainfall and SST in other ocean basins rather than the Indo-Pacific sector alone.  相似文献   

8.
This study investigates the El Niño Southern Oscillation (ENSO) teleconnections to tropical Indian Ocean (TIO) and their relationship with the Indian summer monsoon in the coupled general circulation model climate forecast system (CFS). The model shows good skill in simulating the impact of El Niño over the Indian Oceanic rim during its decay phase (the summer following peak phase of El Niño). Summer surface circulation patterns during the developing phase of El Niño are more influenced by local Sea Surface Temperature (SST) anomalies in the model unlike in observations. Eastern TIO cooling similar to that of Indian Ocean Dipole (IOD) is a dominant model feature in summer. This anomalous SST pattern therefore is attributed to the tendency of the model to simulate more frequent IOD events. On the other hand, in the model baroclinic response to the diabatic heating anomalies induced by the El Niño related warm SSTs is weak, resulting in reduced zonal extension of the Rossby wave response. This is mostly due to weak eastern Pacific summer time SST anomalies in the model during the developing phase of El Niño as compared to observations. Both eastern TIO cooling and weak SST warming in El Niño region combined together undermine the ENSO teleconnections to the TIO and south Asia regions. The model is able to capture the spatial patterns of SST, circulation and precipitation well during the decay phase of El Niño over the Indo-western Pacific including the typical spring asymmetric mode and summer basin-wide warming in TIO. The model simulated El Niño decay one or two seasons later, resulting long persistent warm SST and circulation anomalies mainly over the southwest TIO. In response to the late decay of El Niño, Ekman pumping shows two maxima over the southern TIO. In conjunction with this unrealistic Ekman pumping, westward propagating Rossby waves display two peaks, which play key role in the long-persistence of the TIO warming in the model (for more than a season after summer). This study strongly supports the need of simulating the correct onset and decay phases of El Niño/La Niña for capturing the realistic ENSO teleconnections. These results have strong implications for the forecasting of Indian summer monsoon as this model is currently being adopted as an operational model in India.  相似文献   

9.
The leading modes of daily variability of the Indian summer monsoon in the climate forecast system (CFS), a coupled general circulation model, of the National Centers for Environmental Predictions (NCEP) are examined. The space?Ctime structures of the daily modes are obtained by applying multi-channel singular spectrum analysis (MSSA) on the daily anomalies of rainfall. Relations of the daily modes to intraseasonal and interannual variability of the monsoon are investigated. The CFS has three intraseasonal oscillations with periods around 106, 57 and 30?days with a combined variance of 7%. The 106-day mode has spatial structure and propagation features similar to the northeastward propagating 45-day mode in the observations except for its longer period. The 57-day mode, despite being in the same time scale as of the observations has poor eastward propagation. The 30-day mode is northwestward propagating and is similar to its observational counterpart. The 106-day mode is specific to the model and should not be mistaken for a new scale of variability in observations. The dominant interannual signal is related to El Ni?o-Southern Oscillation (ENSO), and, unlike in the observations, has maximum variance in the eastern equatorial Indian Ocean. Although the Indian Ocean Dipole (IOD) mode was not obtained as a separate mode in the rainfall, the ENSO signal has good correlations with the dipole variability, which, therefore, indicates the dominance of ENSO in the model. The interannual variability is largely determined by the ENSO signal over the regions where it has maximum variance. The interannual variability of the intraseasonal oscillations is smaller in comparison.  相似文献   

10.
The El Niño–Southern Oscillation (ENSO) is a main driving force of the northern hemisphere summer monsoon rainfall, including the Indian Summer Rainfall (ISR). The impacts of typical ENSO and atypical ENSO events on the ISR remain unclear during their developing summers. This study examines the different linkages between a typical ENSO and the ISR and between an a typical ENSO and the ISR. During the developing summer of a typical El Niño, negative rainfall anomalies are seen over the northeastern Indian subcontinent, while the anomalous rainfall pattern is almost the opposite for a typical La Niña; as for an atypical ENSO, the approximate “linear opposite” phenomenon vanishes. Furthermore, an anomalous global zonal wave train is found at mid-latitudes, with a local tripole circulation pattern over central–eastern Eurasia during the developing summer of a typical ENSO, which might explain the corresponding rainfall response over the Indian Peninsula. By contrast, such features are not obvious during the developing summer of an atypical ENSO. Among 106-year historical runs (1900–2005) of nine state-of-the-art models from the Coupled Model Intercomparison Project, Phase 5 (CMIP5), HadGEM2-ES exhibits promising skill in simulating the anomalous circulation pattern over mid-latitudes and central–eastern Eurasia. Probably, it is the model’s ability to capture the linkage between a typical ENSO and the ISR and the characteristics of a typical ENSO that makes the difference.  相似文献   

11.
This work attempts to reconcile in a common and comprehensive framework the various conflicting results found in the literature regarding Indian Summer Monsoon (ISM) rainfall-Sea Surface Temperature (SST) relationships, especially the links with El-Ni?o Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). To do so, we first examine the linear relationships between ISM rainfall and global SST anomalies during 1950–1976 and 1979–2006 periods. Our results highlight the existence of significant modulations in SST teleconnections and precursory patterns between the first (June–July, JJ) and second part (August–September, AS) of the monsoon. This JJ–AS rainfall dichotomy is more pronounced after the 1976–1977 climate regime shift and tends to blur the global ISM-ENSO signal during the recent period, leading to an apparent weakening of this relationship at the seasonal time scale. Although ISM rainfall in JJ and AS is still strongly linked to ENSO over both periods, the lead-lag relationships between ENSO and AS Indian rainfall have changed during recent decades. Indeed, ENSO variability in the preceding boreal winter has now a significant impact on rainfall variability during the second half of ISM. To evaluate in more details the impact of this JJ-AS dichotomy on the ISM-ENSO-IOD relationships, ISM correlations are also examined separately during El Ni?o and La Ni?a years. Results indicate that the early onset of El Ni?o during boreal spring causes deficient monsoon rainfall in JJ. In response to weaker monsoon winds, warm SST anomalies appear in the west equatorial IO, generating favorable conditions for the development of a positive IOD in AS. Local air-sea processes triggered by the SST anomalies in the eastern node of IOD seem, in turn, to have a more active role on AS rainfall variability, as they may counteract the negative effect of El Ni?o on ISM rainfall via a modulation of the local Hadley circulation in the eastern IO. The JJ–AS rainfall dichotomy and its recent amplification may then result from an enhancement of these IO feedbacks during recent El Ni?o years. This explains why, although El Ni?o events are stronger, a weakening of the ISM-ENSO relationship is observed at the seasonal scale after 1979. Results during La Ni?a years are consistent with this hypothesis although local processes in the southeast IO now play a more prominent role and act to further modulate ISM rainfall in AS. Finally, our results highlight the existence of a biennal rhythm of the IOD-ENSO-ISM system during the recent period, according to which co-occurring El Ni?o and positive IOD events tend to be followed by a warming of the IO, a wet ISM during summer and, finally, a La Ni?a event during the following boreal winter.  相似文献   

12.
Based on observational and reanalysis data,the relationships between the eastern Pacific(EP)and central Pacific(CP)types of El Ni?o?Southern Oscillation(ENSO)during the developing summer and the South Asian summer monsoon(SASM)are examined.The roles of these two types of ENSO on the SASM experienced notable multidecadal modulation in the late 1970s.While the inverse relationship between the EP type of ENSO and the SASM has weakened dramatically,the CP type of ENSO plays a far more prominent role in producing anomalous Indian monsoon rainfall after the late 1970s.The drought-producing El Ni?o warming of both the EP and CP types can excite anomalous rising motion of the Walker circulation concentrated in the equatorial central Pacific around 160°W to the date line.Accordingly,compensatory subsidence anomalies are evident from the Maritime Continent to the Indian subcontinent,leading to suppressed convection and decreased precipitation over these regions.Moreover,anomalously less moisture flux into South Asia associated with developing EP El Ni?o and significant northwesterly anomalies dominating over southern India accompanied by developing CP El Ni?o,may also have been responsible for the Indian monsoon droughts during the pre-1979 and post-1979 sub-periods,respectively.El Ni?o events with the same“flavor”may not necessarily produce consistent Indian monsoon rainfall anomalies,while similar Indian monsoon droughts may be induced by different types of El Ni?o,implying high sensitivity of monsoonal precipitation to the detailed configuration of ENSO forcing imposed on the tropical Pacific.  相似文献   

13.
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14.
Based on the data of 1950 – 1999 monthly global SST from Hadley Center, NCAR/NCEP reanalysis data and rainfall over 160 weather stations in China, investigation is conducted into the difference of summer rainfall in China (hereafter referred to as the “CS rainfall”) between the years with the Indian Ocean Dipole (IOD) occurring independently and those with IOD occurring along with ENSO so as to study the effects of El Ni?o - Southern Oscillation (ENSO) on the relationship between IOD and the CS rainfall. It is shown that CS rainfall will be more than normal in South China (centered in Hunan province) in the years of positive IOD occurring independently; the CS rainfall will be less (more) than normal in North China (Southeast China) in the years of positive IOD occurring together with ENSO. The effect of ENSO is offsetting (enhancing) the relationship between IOD and summer rainfall in Southwest China, the region joining the Yangtze River basin with the Huaihe River basin (hereafter referred to as the “Yangtze-Huaihe basin”) and North China (Southeast China). The circulation field is also examined for preliminary causes of such an influence.  相似文献   

15.
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.  相似文献   

16.
陈隽  孙淑清 《大气科学》1999,23(1):101-111
利用ECMWF资料挑选出一个强冬季风年（1986年）和一个弱冬季风年（1980年）,通过个例分析对各种气象要素场及中高纬度大尺度环流在强弱冬季风年的差异特征进行比较。分析结果表明：东亚冬季风是全球大气环流的一个重要组成部分,冬季风异常关联着全球环流的异常;这种异常不仅在中高纬度环流中表现出来,而且在热带地区大尺度流场上也尤为显著。强弱冬季风所对应的长波槽脊分布、低纬对流特征、三维流场结构都是截然不同的。冬季风异常不但造成了同期环流形势的差异,而且对后期环流和天气状况也有影响。  相似文献   

17.
This paper examines an issue concerning the simulation of anomalously wet Indian summer monsoons like 1994 which co-occurred with strong positive Indian Ocean Dipole (IOD) conditions in the tropical Indian Ocean. Contrary to observations it has been noticed that standalone atmospheric general circulation models (AGCM) forced with observed SST boundary condition, consistently depicted a decrease of the summer monsoon rainfall during 1994 over the Indian region. Given the ocean?Catmosphere coupling during IOD events, we have examined whether the failure of standalone AGCM simulations in capturing wet Indian monsoons like 1994 can be remedied by including a simple form of coupling that allows the monsoon circulation to dynamically interact with the IOD anomalies. With this view, we have performed a suite of simulations by coupling an AGCM to a slab-ocean model with spatially varying mixed-layer-depth (MLD) specified from observations for the 1994 IOD; as well as four other cases (1983, 1997, 2006, 2007). The specification of spatially varying MLD from observations allows us to constrain the model to observed IOD conditions. It is seen that the inclusion of coupling significantly improves the large-scale circulation response by strengthening the monsoon cross-equatorial flow; leading to precipitation enhancement over the subcontinent and rainfall decrease over south-eastern tropical Indian Ocean??in a manner broadly consistent with observations. A plausible physical mechanism is suggested to explain the monsoonal response in the coupled frame-work. These results warrant the need for improved monsoon simulations with fully coupled models to be able to better capture the observed monsoon interannual variability.  相似文献   

18.
基于1951—2012年逐月海洋和大气多种要素的再分析资料,分析了与两类El Nino相伴的IOD(Indian Ocean Dipole,印度洋偶极子)事件盛期的海洋和大气异常特征,并进一步对比了与不同类型El Nino相伴的IOD事件的季节演变及对应的海气耦合过程。结果表明:两类IOD事件盛期时,暖海温强度和位置有显著差异。发生在东部型El Nino期间的IOD事件(简称EP-IOD)盛期,正(负)SSTA中心出现在热带西北(赤道东南)印度洋,强度相当,对应的热带印度洋—海洋大陆异常Walker环流强度较强、范围较大;与中部型CP El Nino相伴的IOD事件(简称CP-IOD)的正SSTA相对较弱,且偏于南印度洋,异常Walker环流较弱、较窄。在季节演变中,两类IOD事件期间的局地海气过程差异显著,伴随着西印度洋西南季风减弱和东印度洋异常东风加强,EP-IOD事件的发展以西正东负的偶极型异常海温的出现及加强为主要特征;而CP-IOD事件的发生发展则与西北印度洋异常冷海温的生消及南印度洋暖水的堆积相伴,表现为"-+-"三极型SSTA的出现并转为西正东负偶极型的过程,夏季时出现在东印度洋的异常东风以及赤道中印度洋低层负涡度异常水平环流对其发展具有重要作用。  相似文献   

19.
亚洲夏季风环流结构与热带印度洋偶极型海温异常   总被引：1，自引：0，他引：1  
使用T42L28大气环流模式就夏季风时期大气对印度洋海温偶极子型异常的响应进行了数值试验研究,结果表明,印度洋偶极子型海温异常可以引起感热和潜热加热异常并进而形成异常辐合辐散,导致热带印度洋及其邻近地区夏季降水异常。同时此热带扰动可激发或造成中纬度异常波列。通过改变季风区温度场分布,偶极子型海温强迫可以影响大气的正／斜压环流结构和斜压性强弱。强的纬向风垂直切变趋向于靠近海洋异常偏暖的地区。不论是正偶极子型强迫或负偶极子型强迫,西太平洋暖池和东亚地区的大气环流均出现异常并激发出中纬度的异常波列,但异常类型并未显著反相。  相似文献   

20.
Climate Dynamics - Indian Summer Monsoon (ISM) rainfall and El Niño-Southern Oscillation (ENSO) exhibit an inverse relationship during boreal summer, which is one of the roots of ISM...  相似文献   

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