Cut-off Lows in the South Africa region and their contribution to precipitation

The contribution of Cut-off Lows (CoLs) to precipitation and extreme rainfall frequency in South Africa has been quantified from 402 station records over the period 1979–2006. Firstly, 500 hPa CoL trajectories over Southern Africa and surrounding oceans were determined and their features thoroughly analyzed. In a second step, using daily precipitable water, outgoing long wave radiation data and station rainfall records, an area was defined where the occurrence of CoLs is associated with rainfall over South Africa. CoLs transiting in the 2.5°E–32.5°E/20°S–45°S are more likely to produce precipitation over the country. When 500 hPa CoLs are centered just off the west coast of the country (around 15°E/32.5°S) their impact is substantial in term of daily rainfall intensity and spatial coverage. CoL rainy days have been studied and it is shown that they significantly contribute to precipitation in South Africa, more strongly along the south and east coasts as well as inland, over the transition zone between the summer and winter rainfall domains where they contribute between 25 to more than 35 % of annual accumulation. At the country scale, CoL rainfall is more intense and widespread in spring than during other seasons. Over the analyzed period, a significant trend in annual CoLs’ frequency shows an increase of about 25 %. This increase is mainly realized in spring and in a lesser extent in summer. This trend is accompanied by a significant increase in the frequency of CoL rainy days specifically along the south coast and over the East of the country during the spring–summer period. In parallel, it is shown that from late spring until summer CoLs’ frequency varies significantly accordingly with large scale circulation modes of the Southern Hemisphere such as the Pacific South American pattern (PSA). This positive trend in CoLs’ frequency may be related with the positive trend in the PSA during the spring–summer period over the three last decades.     

Pacific and Atlantic influences on Mesoamerican climate over the past millennium

A new tree-ring reconstruction of the Palmer Drought Severity Index (PDSI) for Mesoamerica from AD 771 to 2008 identifies megadroughts more severe and sustained than any witnessed during the twentieth century. Correlation analyses indicate strong forcing of instrumental and reconstructed June PDSI over Mesoamerica from the El Ni?o/Southern Oscillation (ENSO). Spectral analyses of the 1,238-year reconstruction indicate significant concentrations of variance at ENSO, sub-decadal, bi-decadal, and multidecadal timescales. Instrumental and model-based analyses indicate that the Atlantic Multidecadal Oscillation is important to warm season climate variability over Mexico. Ocean-atmospheric variability in the Atlantic is not strongly correlated with the June PDSI reconstruction during the instrumental era, but may be responsible for the strong multidecadal variance detected in the reconstruction episodically over the past millennium. June drought indices in Mesoamerica are negatively correlated with gridded June PDSI over the United States from 1950 to 2005, based on both instrumental and reconstructed data. Interannual variability in this latitudinal moisture gradient is due in part to ENSO forcing, where warm events favor wet June PDSI conditions over the southern US and northern Mexico, but dryness over central and southern Mexico (Mesoamerica). Strong anti-phasing between multidecadal regimes of tree-ring reconstructed June PDSI over Mesoamerica and reconstructed summer (JJA) PDSI over the Southwest has also been detected episodically over the past millennium, including the 1950–1960s when La Ni?a and warm Atlantic SSTs prevailed, and the 1980–1990s when El Ni?o and cold Atlantic SSTs prevailed. Several Mesoamerican megadroughts are reconstructed when wetness prevailed over the Southwest, including the early tenth century Terminal Classic Drought, implicating El Ni?o and Atlantic SSTs in this intense and widespread drought that may have contributed to social changes in ancient Mexico.     

Simulated Relationship between Wintertime ENSO and East Asian Summer Rainfall: From CMIP3 to CMIP6

El Ni?o-Southern Oscillation(ENSO)events have a strong influence on East Asian summer rainfall(EASR).This paper investigates the simulated ENSO-EASR relationship in CMIP6 models and compares the results with those in CMIP3 and CMIP5 models.In general,the CMIP6 models show almost no appreciable progress in representing the ENSO-EASR relationship compared with the CMIP5 models.The correlation coefficients in the CMIP6 models are relatively smaller and exhibit a slightly greater intermodel diversity than those in the CMIP5 models.Three physical processes related to the delayed effect of ENSO on EASR are further analyzed.Results show that,firstly,the relationships between ENSO and the tropical Indian Ocean(TIO)sea surface temperature(SST)in the CMIP6 models are more realistic,stronger,and have less intermodel diversity than those in the CMIP3 and CMIP5 models.Secondly,the teleconnections between the TIO SST and Philippine Sea convection(PSC)in the CMIP6 models are almost the same as those in the CMIP5 models,and stronger than those in the CMIP3 models.Finally,the CMIP3,CMIP5,and CMIP6 models exhibit essentially identical capabilities in representing the PSC-EASR relationship.Almost all the three generations of models underestimate the ENSO-EASR,TIO SST-PSC,and PSC-EASR relationships.Moreover,almost all the CMIP6 models that successfully capture the significant TIO SST-PSC relationship realistically simulate the ENSO-EASR relationship and vice versa,which is,however,not the case in the CMIP5 models.     

Intraseasonal atmospheric variability and its interannual modulation in Central Africa

The spatial and temporal structures of the intraseasonal atmospheric variability over central Africa is investigated using 2.5°?×?2.5° daily outgoing longwave radiation (OLR) and National Centers for Environmental Prediction (NCEP) Reanalysis zonal winds for the period 1980–2010. The study begins with an overview of the Central African rainfall regime, noting in particular the contrast amongst Western and Eastern parts, with different topography and surface conditions features. The annual mean rainfall and OLR over the region revealed a zone of intense convective activity centered on the equator near 30°E, which extends southward and covers almost all the Congo forest. The annual cycle of rainfall reflects the classical bi-annual shift of Inter-Tropical Convergence Zone across the equatorial belt, between 10°S and 10°N. The result of the empirical orthogonal functions (EOFs) analysis has shown that the three leading EOF modes explain about 45?% of total intraseasonal variability. The power spectra of all the three corresponding principal components (PCs) peak around 45–50?days, indicating a Madden–Julian Oscillation (MJO) signal. The first mode exhibits high positive loadings over Northern Congo, the second over Southern Ethiopia and the third over Southwestern Tanzania. The PCs time series revealed less interannual modulation of intraseasonal oscillations for the Congo mode, while Ethiopian and Tanzanian modes exhibit strong interannual variations. H?vmoller plots of OLR, 200 and 850?hPa NCEP zonal winds found the eastward propagating features to be the dominant pattern in all the three times series, but this propagation is less pronounced in the OLR than in the 850 and 200?hpa zonal wind anomalies. An index of MJO strength was built by averaging the 30–50?day power for each day. A plot of MJO indices and El Ni?o Southern Oscillation (ENSO) cycle confirm a strong interannual modulation of MJO over Eastern central Africa partially linked with the ENSO events (El Ni?o and La Ni?a). Strong MJO activity is observed during La Ni?a years or during ENSO-neutral years, while weak or absent MJO activity is typically associated with strong El Ni?o episodes.     

厄尔尼诺/拉尼娜与重庆次年夏季涝/旱的非对称关系

本文详细分析了厄尔尼诺/拉尼娜与重庆夏季典型涝/旱年之间的不对称关系。结果表明:(1)厄尔尼诺和拉尼娜对重庆次年夏季降水有不对称影响。厄尔尼诺年的大气环流异常与重庆夏季典型涝年的特征一致;然而,拉尼娜年的大气环流异常与重庆夏季典型旱年的特征不一致。(2)从冬季到次年夏季,厄尔尼诺对重庆夏季典型涝年的影响主要是通过热带印度洋海温的‘接力效应’维持的。     

Mechanisms controlling warm water volume interannual variations in the equatorial Pacific: diabatic versus adiabatic processes

Variations of the volume of warm water above the thermocline in the equatorial Pacific are a good predictor of ENSO (El Ni?o/Southern Oscillation) and are thought to be critical for its preconditioning and development. In this study, the Warm Water Volume (WWV) interannual variability is analysed using forced general circulation model experiments and an original method for diagnosing processes responsible for WWV variations. The meridional recharge/discharge to higher latitudes drives 60% of the ENSO-related equatorial WWV variations, while diabatic processes in the eastern equatorial Pacific account for the remaining 40%. Interior meridional transport is partially compensated by western boundary transports, especially in the southern hemisphere. Diabatic equatorial WWV formation (depletions) during La Ni?a (El Ni?o) are explained by enhanced (reduced) diathermal transport through enhanced (reduced) vertical mixing and penetrating solar forcing at the 20°C isotherm depth. The respective contribution of diabatic and adiabatic processes during build-ups/depletions strongly varies from event-to-event. The WWV build-up during neutral ENSO phases (e.g. 1980–1982) is almost entirely controlled by meridional recharge, providing a text-book example for the recharge/discharge oscillator’s theory. On the other hand, diabatic processes are particularly active during the strongest La Ni?a events (1984, 1988, 1999), contributing to more than 70% of the WWV build-up, with heating by penetrative solar fluxes explaining as much as 30% of the total build-up due to a very shallow thermocline in the eastern Pacific. This study does not invalidate the recharge/discharge oscillator theory but rather emphasizes the importance of equatorial diabatic processes and western boundary transports in controlling WWV changes.     

Observational study of the seasonality of the submonthly and intraseasonal signal over the tropics

Summary Atmospheric variability in outgoing long-wave radiation (OLR) and tropospheric relative vorticity (VOR) over the South American region was studied from 1979 to 1996 using the complex Morlet wavelet function. The analyses focus on spatial variation in intraseasonal and submonthly scales. Scalograms were used to measure submonthly intraseasonal oscillations in convection, which were found to be predominant in the tropical regions. However, 7-day and 15-day oscillations were observed at tropical and extratropical latitudes in spring and winter, indicating that transient disturbances play a more prominent role. Regarding VOR, tropical energy intensities were highest in the spring and summer, whereas subtropical and extratropical energy intensities were highest in the autumn and winter. The dynamics of the 25-day and 45-day VOR oscillations indicates a possible correlation with Rossby waves over the eastern tropical Pacific Ocean, mainly during the summer. During winter, the 7-day and 15-day VOR oscillations are more frequent at higher latitudes and are enhanced along storm tracks. It was also observed that convection amplitudes in the regions of maximum intensity change appreciably from year to year and from season to season, showing that the behavior of the submonthly and intraseasonal oscillations is nonperiodic and correlates strongly with El Ni?o/Southern Oscillation years. These results confirm the efficiency of wavelet analysis for time-scale studies of atmospheric variability.     

Robustness of SST teleconnections and precursory patterns associated with the Indian summer monsoon

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.     

Intraseasonal and interannual zonal circulations over the Equatorial Indian Ocean

El Ni?o Southern Oscillation (ENSO) and given phases of the Madden?CJulian Oscillation (MJO) show similar regional signatures over the Equatorial Indian Ocean, consisting in an enhancement or reversing of the convective and dynamic zonal gradients between East Africa and the Maritime Continent of Indonesia. This study analyses how these two modes of variability add or cancel their effects at their respective timescales, through an investigation of the equatorial cellular circulations over the central Indian Ocean. Results show that (1) the wind shear between the lower and upper troposphere is related to marked regional rainfall anomalies and is embedded in larger-scale atmospheric configurations, involving the Southern Oscillation; (2) the intraseasonal (30?C60?days) and interannual (4?C5?years) timescales are the most energetic frequencies that modulate these circulations, confirming the implication of the MJO and ENSO; (3) extreme values of the Indian Ocean wind shear result from the combination of El Ni?o and the MJO phase enhancing atmospheric convection over Africa, or La Ni?a and the MJO phase associated with convective activity over the Maritime Continent. Consequences for regional rainfall anomalies over East Africa and Indonesia are then discussed.     

Quantifications of the Two “Flavours” of El Niño using Upper-Ocean Heat Content

The sea surface temperature (SST) or sea level pressure (SLP) has usually been used to measure the strength of El Niño–Southern Oscillation (ENSO) events. In this study, two new indices, based on the upper-ocean heat content (HC), are proposed to quantify the two “flavours” of El Niño (i.e., the Cold Tongue El Niño (CTE) and Warm Pool El Niño (WPE)). Compared with traditional SST or SLP indices, the new HC-based indices can distinguish CTE and WPE events much better and also represent the two leading modes of the interannual variability of the atmosphere–ocean coupled system in the tropical Indo-Pacific region. The two leading modes are obtained by performing multivariate Empirical Orthogonal Function analysis on two oceanic variables (SST and HC) over the tropical Pacific (30°S–30°N, 120°E–80°W) and six atmospheric variables (outgoing longwave radiation, SLP, streamfunction, and velocity potential at 850?hPa and 200?hPa) over the tropical Indo-Pacific region (30°S–30°N, 60°E–80°W) for the period 1980–2010. Because the two new HC-based indices are capable of better depicting coherent variations between the ocean and atmosphere, they can provide a supplementary tool for ENSO monitoring of and climate research into the two flavours of El Niño.     

Factors controlling January�CApril rainfall over southern India and Sri Lanka

Most of the annual rainfall over India occurs during the Southwest (June?CSeptember) and Northeast (October?CDecember) monsoon periods. In March 2008, however, Southern peninsular India and Sri Lanka received the largest rainfall anomaly on record since 1979, with amplitude comparable to summer-monsoon interannual anomalies. This anomalous rainfall appeared to be modulated at intraseasonal timescale by the Madden Julian Oscillation, and was synchronous with a decaying La Ni?a event in the Pacific Ocean. Was this a coincidence or indicative of a teleconnection pattern? In this paper, we explore factors controlling rainfall over southern India and Sri Lanka between January and April, i.e. outside of the southwest and northeast monsoons. This period accounts for 20% of annual precipitation over Sri Lanka and 10% over the southern Indian states of Kerala and Tamil Nadu. Interannual variability is strong (about 40% of the January?CApril climatology). Intraseasonal rainfall anomalies over southern India and Sri Lanka are significantly associated with equatorial eastward propagation, characteristic of the Madden Julian Oscillation. At the interannual timescale, we find a clear connection with El Ni?o-Southern Oscillation (ENSO); with El Ni?os being associated with decreased rainfall (correlation of ?0.46 significant at the 98% level). There is also a significant link with local SST anomalies over the Indian Ocean, and in particular with the inter-hemispheric sea surface temperature (SST) gradient over the Indian Ocean (with colder SST south of the equator being conducive to more rainfall, correlation of 0.55 significant at the 99% level). La Ni?as/cold SSTs south of the equator tend to have a larger impact than El Ni?os. We discuss two possible mechanisms that could explain these statistical relationships: (1) subsidence over southern India remotely forced by Pacific SST anomalies; (2) impact of ENSO-forced regional Indian Ocean SST anomalies on convection. However, the length of the observational record does not allow distinguishing between these two mechanisms in a statistically significant manner.     

Interannual linkage between Arctic/North Atlantic Oscillation and tropical Indian Ocean precipitation during boreal winter

In the study authors analyzed the interannual relationship between the Arctic Oscillation (AO)/North Atlantic Oscillation (NAO) and the tropical Indian Ocean (TIO) precipitation in boreal winter for the period 1979–2009. A significant simultaneous teleconnection between them is found. After removing the El Niño/Southern Oscillation and Indian Ocean dipole signals, the AO/NAO and the TIO precipitation (0°–10°S, 60°–80°E) yield a correlation of +0.56, which is also consistent with the AO/NAO-outgoing longwave radiation correlation of ?0.61. The atmospheric and oceanic features in association with the AO/NAO-precipitation links are investigated. During positive AO/NAO winter, the Rossby wave guided by westerlies tends to trigger persistent positive geopotential heights in upper troposphere over about 20°–30°N and 55°–70°E, which is accompanied by a stronger Middle East jet stream. Meanwhile, there are anomalous downward air motions, strengthening the air pressure in mid-lower troposphere. The enhanced Arabian High brings anomalous northern winds over the northern Indian Ocean. As a result the anomalous crossing-equator air-flow enhances the intertropical convergence zone (ITCZ). On the other hand, the anomalous Ekman transport convergence by the wind stress curl over the central TIO deepens the thermocline. Both the enhanced ITCZ and the anomalous upper ocean heat content favor in situ precipitation in the central TIO. The AO/NAO-TIO precipitation co-variations in the IPCC AR4 historical climate simulation (1850–1999) of Bergen Climate Model version 2 were investigated. The Indian Ocean precipitation anomalies (particularly the convective precipitation along the ITCZ), in conjunction with the corresponding surface winds and 200 hPa anticyclonic atmospheric circulation and upper ocean heat contents were well reproduced in simulation. The similarity between the observation and simulation support the physical robustness of the AO/NAO-TIO precipitation links.     

Interannual variability of the South Pacific Convergence Zone and implications for tropical cyclone genesis

The interannual variability of the South Pacific Convergence Zone (SPCZ) and its influence on tropical cyclone (TC) genesis in the South Pacific are investigated using observations and ERA40 reanalysis over the 1979?C2002 period. In austral summer, the SPCZ displays four typical structures at interannual timescales. The first three are characterized by a diagonal orientation of the SPCZ and account for 85% of the summer seasons. One is close to climatology and the other two exhibit a 3° northward or southward departure from the SPCZ climatological position. In contrast, the fourth one, that only encompasses three austral summer seasons (the extreme 1982/1983 and 1997/1998 El Ni?o events and the moderate 1991/1992 El Ni?o event), displays very peculiar behaviour where the SPCZ largely departs from its climatological position and is zonally oriented. Variability of the western/central Pacific equatorial sea surface temperature (SST) is shown to modulate moisture transport south of the equator, thereby strongly constraining the location of the SPCZ. The SPCZ location is also shown to strongly modulate the atmospheric circulation variability in the South Pacific with specific patterns for each class. However, independently of its wide year-to-year excursions, the SPCZ is always collocated with the zero relative vorticity at low levels while the maximum vorticity axis lies 6° to the south of the SPCZ position. This coherent atmospheric organisation in the SPCZ region is shown to constrain tropical cyclogenesis to occur preferentially within 10° south of the SPCZ location as this region combines all the large-scale atmospheric conditions that favour the breeding of TCs. This analysis also reveals that cyclogenesis in the central Pacific (in the vicinity of French Polynesia) only occurs when the SPCZ displays a zonal orientation while this observation was previously attributed to El Ni?o years in general. Different characteristics of El Ni?o Southern Oscillation (ENSO)-related Pacific equatorial warming are shown to impact differently on the SPCZ position, suggesting that for regional scales, such as the South Pacific, the SPCZ classification is more appropriate than a simple ENSO index to characterize TC interannual variability. These findings suggest that forecasting the strength of El Ni?o through SST variations in the eastern Pacific may not be sufficient to accurately predict cyclogenesis in the South Pacific, especially east of the dateline.     

近65年ENSO事件强度变化及时频特征研究

利用海洋尼诺指数(ONI)、南方涛动指数(SOI)和多变量ENSO指数(MEI)等ENSO特征值分析了1951年1月—2016年5月近65年ENSO事件的强度与时频特征,并将其强度划分为5个等级。结果表明:近65年共发生22次暖事件（El Ni?o）和13次冷事件（La Ni?a）;对ENSO特征值进行频次分析发现,强El Ni?o月份所占比例比强La Ni?a多;使用连续小波、交叉小波和小波相干分析得出,ENSO循环主要具有2~7 a的周期,还具有10~16 a的年代际变化。      

Why Does Extreme Rainfall Occur in Central China during the Summer of 2020 after a Weak El Ni?o?

In summer 2020, extreme rainfall occurred throughout the Yangtze River basin, Huaihe River basin, and southern Yellow River basin, which are defined here as the central China (CC) region. However, only a weak central Pacific (CP) El Ni?o happened during winter 2019/20, so the correlations between the El Ni?o–Southern Oscillation (ENSO) indices and ENSO-induced circulation anomalies were insufficient to explain this extreme precipitation event. In this study, reanalysis data and numerical experiments are employed to identify and verify the primary ENSO-related factors that cause this extreme rainfall event. During summer 2020, unusually strong anomalous southwesterlies on the northwest side of an extremely strong Northwest Pacific anticyclone anomaly (NWPAC) contributed excess moisture and convective instability to the CC region, and thus, triggered extreme precipitation in this area. The tropical Indian Ocean (TIO) has warmed in recent decades, and consequently, intensified TIO basinwide warming appears after a weak El Ni?o, which excites an extremely strong NWPAC via the pathway of the Indo-western Pacific Ocean capacitor (IPOC) effect. Additionally, the ENSO event of 2019/20 should be treated as a fast-decaying CP El Ni?o rather than a general CP El Ni?o, so that the circulation and precipitation anomalies in summer 2020 can be better understood. Last, the increasing trend of tropospheric temperature and moisture content in the CC region after 2000 is also conducive to producing heavy precipitation.     

Effects of El Ni?o Modoki on winter precipitation in Korea

This study compares the impacts of El Ni?o Modoki and El Ni?o on precipitation over Korea during the boreal winters from 1954 to 2009. Precipitation in Korea tends to be equal to or greater than the normal level during an El Ni?o Modoki winter, whereas there is no significant change during an El Ni?o winter. Greater than normal precipitation during El Ni?o Modoki was also found over the lower reaches of the Yangtze River, China and much of southern Japan. The latitudes of these regions are 5–10° further north than in southern China, where precipitation increases during El Ni?o. The following two anomalous atmospheric circulations were found to be causes that led to different precipitation distributions over East Asia. First, an atmospheric wave train in the lower troposphere, which propagated from the central tropical Pacific (cyclonic) through the southern Philippine Sea (anticyclonic) to East Asia (cyclonic), reached the southern China and northern Philippine Sea during El Ni?o, whereas it reached Korea and southern Japan during El Ni?o Modoki. Second, an anomalous local meridional circulation, which consists of air sinking in the tropics, flowing poleward in the lower troposphere, and rising in the subtropics, developed between the southern Philippine Sea and northern Philippine Sea during El Ni?o. During El Ni?o Modoki, however, this circulation expanded further to the north and was formed between the southern Philippine Sea and regions of Korea and southern Japan.     

Understanding the SAM influence on the South Pacific ENSO teleconnection

The relationship between the El Niño Southern Oscillation (ENSO) and the Southern Hemisphere Annular Mode (SAM) is examined, with the goal of understanding how various strong SAM events modulate the ENSO teleconnection to the South Pacific (45°–70°S, 150°–70°W). The focus is on multi-month, multi-event variations during the last 50 years. A significant (p < 0.10) relationship is observed, most marked during the austral summer and in the 1970s and 1990s. In most cases, the significant relationship is brought about by La Niña (El Niño) events occurring with positive (negative) phases of the SAM more often than expected by chance. The South Pacific teleconnection magnitude is found to be strongly dependent on the SAM phase. Only when ENSO events occur with a weak SAM or when a La Niña (El Niño) occurs with a positive (negative) SAM phase are significant South Pacific teleconnections found. This modulation in the South Pacific ENSO teleconnection is directly tied to the interaction of the anomalous ENSO and SAM transient eddy momentum fluxes. During La Niña/SAM+ and El Niño/SAM? combinations, the anomalous transient momentum fluxes in the Pacific act to reinforce the circulation anomalies in the midlatitudes, altering the circulation in such a way to maintain the ENSO teleconnections. In La Niña/SAM? and El Niño/SAM+ cases, the anomalous transient eddies oppose each other in the midlatitudes, overall acting to reduce the magnitude of the high latitude ENSO teleconnection.     

Decadal Changes in Interannual Dependence of the Bay of Bengal Summer Monsoon Onset on ENSO Modulated by the Pacific Decadal Oscillation

Interannual variations of the Bay of Bengal summer monsoon (BOBSM) onset in association with El Ni?o?Southern Oscillation (ENSO) are reexamined using NCEP1, JRA-55 and ERA20C atmospheric and Hadley sea surface temperature (SST) reanalysis datasets over the period 1900?2017. Decadal changes exist in the dependence of the BOBSM onset on ENSO, varying with the Pacific Decadal Oscillation (PDO). A higher correlation between the BOBSM onset and ENSO arises during the warm PDO epochs, with distinct late (early) onsets following El Ni?o (La Ni?a) events. In contrast, less significant correlations occur during the cold PDO epochs. The mechanism for the PDO modulating the ENSO?BOBSM onset relationship is through the variations in SST anomaly (SSTA) patterns. During the warm PDO epochs, the superimpositions of the PDO-related and ENSO-related SSTAs lead to the SSTA distribution of an El Ni?o (La Ni?a) event exhibiting significant positive (negative) SSTAs over the tropical central?eastern Pacific and Indian Ocean along with negative (positive) SSTAs, especially over the tropical western Pacific (TWP), forming a strong zonal interoceanic SSTA gradient between the TWP and tropical Indian Ocean. Significant anomalous lower tropospheric easterlies (westerlies) together with upper-tropospheric westerlies (easterlies) are thus induced over the BOB, favoring an abnormally late (early) BOBSM onset. During the cold PDO epochs, however, the superimpositions of PDO-related SSTAs with El Ni?o-related (La Ni?a-related) SSTAs lead to insignificant SSTAs over the TWP and a weak zonal SSTA gradient, without distinct circulation anomalies over the BOB favoring early or late BOBSM onsets.     

Interannual variability and predictability of sea level along the Indian Coast

Summary The Indian coast stretching more than 7,500 km constitutes the major portion of the South Asian coastline in the North Indian Ocean region. The South Asian region is significantly influenced by meteorological/oceanographic phenomena like monsoons, El Ni?o/Southern Oscillation (ENSO) and tropical cyclones. Direct/indirect impacts of these phenomena, which exhibit large interannual variabilities, on sea level changes in this region are considerable. Our results show that the mean sea level along the eastern coast of India, which is highly vulnerable to the incidence of severe tropical cyclones, is considerably higher than normal during the intense cyclonic period of a year falling in the positive phase of the Southern Oscillation (La Ni?a epoch), thereby enhancing the hazardous potential of tropical cyclones. Further, in the closing phase of the La Ni?a southwest monsoon, higher sea level anomalies prevail along the Indian coast raising the flooding potential of such monsoons. Over the west coast of India significant simultaneous correlations have been found between the amount of southwest monsoon rainfall and the mean sea level during the period from June to September. Over the east coast of India at Visakhapatnam, mean sea level is predictable with a fair degree of confidence one month in advance, by using the Sea Surface Temperature (SST) and the Southern Oscillation Index (SOI) as predictors. These results will be useful in the annual preparedness programmes aimed at mitigating the impacts of natural disasters like tropical cyclones and floods in the South Asian region. Received November 9, 2001     

Contrasting Regional Responses of Indian Summer Monsoon Rainfall to Exhausted Spring and Concurrently Emerging Summer El Ni?o Events

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)...