El Niño Southern Oscillation (ENSO) and Health: An Overview for Climate and Health Researchers <s> The ENSO Phenomenon <s> An attempt is made to obtain a coherent picture of the extent and mode of operation of the ‘southern oscillation.’ This term is used here, following Sir Gilbert Walker, to describe a standing fluctuation of opposed pressure anomalies in both eastern and western hemispheres. The existence of this opposition has been verified, using more recent data, for stations in the Indian and Pacific Ocean regions; results show the oscillation was less marked in recent decades. ::: ::: ::: ::: The representativeness and physical meaning of the index devised by Walker to characterize the state of the oscillation are considered. The geographical extent of the phenomenon is examined using correlation and regression charts of pressures with Walker's index. The temperature and rainfall anomalies associated with it may be derived qualitatively from the pressure anomalies. ::: ::: ::: ::: Recent data are used to verify persistence and lag correlations between station pressures; while there has again been some decline, the lag correlations of elements with previous South America pressures still hold good. The decline in these various quantities is indicative of a minor secular change commencing in the 1920's, which is also evident in a decrease in the variability of pressure. ::: ::: ::: ::: What ‘periodicities’ appear to exist in elements affected by the southern oscillation may well be an outcome of sampling fluctuations in (often persistent) random series. This is suggested by the variety of supposed ‘periods’ reported, and their evanescence in space and time. An example of this evanescence in time is provided from the Darwin pressure record. ::: ::: ::: ::: A mechanism for the oscillation is proposed in terms of variations in a direct toroidal circulation between warmer eastern and cooler western hemispheres. These variations are attributed, (following a model by Palmer of the synoptic climatology of the tropical Pacific) to variations in the south-east trades in the South Pacific and the consequent variations in cyclonic vortex generation in the West Pacific. The persistence of anomalies is then due to the extent of ocean areas in the south-east Pacific where the sea temperature is lower than the air temperature. The lag correlations observed may be due to this persistence and to a transmission of anomalies along the trades through air-sea interaction. <s> BIB001 </s> El Niño Southern Oscillation (ENSO) and Health: An Overview for Climate and Health Researchers <s> The ENSO Phenomenon <s> Abstract The “high index” response of the northeast Pacific westerlies to big positive anomalies of equatorial sea temperature, observed in the winter of 1957–58, has been found to repeat during the major equatorial sea temperature maxima in the winters of 1963–64 and 1965–66. The 1963 positive temperature anomaly started early enough to exert the analogous effect on the atmosphere of the south Indian Ocean during its winter season. The maxima of the sea temperature in the eastern and central equatorial Pacific occur as a result of anomalous weakening of the trade winds of the Southern Hemisphere with inherent weakening of the equatorial upwelling. These anomalies are shown to be closely tied to the “Southern Oscillation” of Sir Gilbert Walker. <s> BIB002 </s> El Niño Southern Oscillation (ENSO) and Health: An Overview for Climate and Health Researchers <s> The ENSO Phenomenon <s> 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 Nino-Southern Oscillation (ENSO). During the warm PDO period, the impact of El Nino (La Nina) 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. <s> BIB003 </s> El Niño Southern Oscillation (ENSO) and Health: An Overview for Climate and Health Researchers <s> The ENSO Phenomenon <s> AbstractThe present study investigates the roles of El Nino–Southern Oscillation (ENSO) and the Pacific decadal oscillation (PDO) in the relationship between the East Asian winter monsoon (EAWM) and the following East Asian summer monsoon (EASM). The variability of the EAWM is divided into an ENSO-related part named EAWMEN and an ENSO-unrelated part named EAWMres. Corresponding to a weak EAWMEN, an anomalous low-level anticyclone forms over the western North Pacific (WNP) and persists from winter to the following summer. This anticyclone enhances southerlies over the coast of East Asia in summer. Hence, a weak EAWMEN tends to be followed by a strong EASM and vice versa. As such, a link is established between the EAWMEN and the EASM. The persistence of this WNP anticyclone may be mainly attributed to the sea surface temperature anomalies associated with the ENSO-related EAWM part in the tropical Indian Ocean and the extratropical North Pacific. In contrast, corresponding to a weak EAWMres, the anomalous WN... <s> BIB004 </s> El Niño Southern Oscillation (ENSO) and Health: An Overview for Climate and Health Researchers <s> The ENSO Phenomenon <s> The most intense El Nino episodes in more than a century occurred after the 1970s climate shift. Previous studies show that the characteristics of the El Nino-Southern Oscillation (ENSO) phenomenon changed synchronously with the shift, but the associated causes are not fully understood. An analysis of the observed tropical Pacific sea surface temperature (SST) anomalies shows that their increase in the eastern part of the basin after the 1970s is not related to the canonical ENSO pattern, but to the tropical Pacific meridional mode (TPMM). We present observational evidence which supports the hypothesis that the change in the TPMM was triggered by the great salinity anomaly (GSA), which manifested in the North Atlantic during the late 1960s. The GSA induced a weak Labrador convection and a SST dipole south of Greenland. The associated atmospheric structure includes a North Pacific Oscillation sea level pressure dipole in the Pacific sector. This excites the TPMM which contributes to the intense El Nino events and to the enhanced ENSO’s asymmetry, observed after the shift. Our results imply that, if the GSA has not an anthropic origin, as was suggested, then the tropical Pacific climate shift has a natural origin. This is supported by the end of the North Atlantic regime in the 1990s and by the rebound of the tropical Pacific after 1998. <s> BIB005 </s> El Niño Southern Oscillation (ENSO) and Health: An Overview for Climate and Health Researchers <s> The ENSO Phenomenon <s> The relationship between ENSO and tropical cyclones (TCs) activity in the Bay of Bengal (BoB) during October–December under cold (1950–1974) and warm (1975–2006) phase of Pacific Decadal Oscillation (PDO) is investigated. A statistically significant difference in the formation of total number of TCs and intense TCs (Category-1 and above) between El Nino and La Nina years is observed when the PDO was in warm phase. Our analysis shows that, there is a tendency to form more number of TCs during La Nina years (2.62 TCs per season) than during El Nino years (1.6 TCs per season) under warm phase of PDO. Moreover, the difference is quite high for intense TCs cases, such as, relatively more number of intense TCs forms in the BoB during La Nina years (1.4 TCs per season) compared to El Nino years (0.10 TCs per season) under warm phase of PDO. However, the difference in the formation of total number of TCs and intense TCs between La Nina and El Nino years is not significant under cold phase of PDO. Significant enhancement in low level cyclonic vorticity and mid-troposphere humidity during La Nina years compared to El Nino years when the PDO was in warm phase, rather than the PDO was in cold phase leads to this difference. Our analysis further shows that how the ENSO related teleconnection to the Indian Ocean region differ under warm and cold phase of PDO. <s> BIB006 </s> El Niño Southern Oscillation (ENSO) and Health: An Overview for Climate and Health Researchers <s> The ENSO Phenomenon <s> This study investigates how the Pacific Decadal Oscillation (PDO) modulates the effect of El Nino/Southern Oscillation (ENSO) on tropical cyclone rapid intensification (RI) in the western North Pacific. The analysis shows that the interannual relationship between ENSO and annual RI number in warm PDO phases is strong and statistically significant. In cold PDO phases, however, there is no significant correlation between ENSO and RI on the interannual timescale. The enhancement of the interannual ENSO–RI relationship in warm PDO phases is mainly attributable to the change of the environmental vertical wind shear. The PDO in warm (cold) phases can strengthen (weaken) an El Nino event to increase (reduce) the effects of the warm pool of water over the equatorial Pacific in typhoon season by local diabatic heating. El Nino events are accompanied by the stronger Walker circulation in the equatorial Pacific in the warm PDO phase than in the cold PDO phase. In contrast, the Walker circulation pattern and amplitude associated with La Nina events is less affected by the alternate PDO phase. This tends to make the atmospheric response to ENSO stronger (weaker) in warm (cold) PDO phase, and so is the atmospheric teleconnection of ENSO. Our results indicate that the stratification of ENSO-based statistical RI forecast by the PDO can greatly improve the accuracy of statistical RI predictions. <s> BIB007 </s> El Niño Southern Oscillation (ENSO) and Health: An Overview for Climate and Health Researchers <s> The ENSO Phenomenon <s> AbstractThe interaction between the El Nino–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) has been noted in many previous studies. However, the interaction is often presumed to exist in a robust manner. This paper questions this assumption and looks for statistical evidence for the robust existence of the ENSO–IOD interaction rather than chance co-occurrences. Consistent definitions of ENSO and IOD events were developed so that ENSO and IOD events identified are comparable in nature, strength, persistence, and time of occurrence during the year, relative to their natural variability. We applied a combinatorial probability test to the probability tables of ENSO and IOD events to test for robustness of our results and those computed from two published works. Our findings show that there is an asymmetry in the interaction between ENSO and IOD. The interaction between El Nino (EN) and the positive and negative phases of the IOD (pIOD and nIOD, respectively) is more robust, enhancing the EN–pIO... <s> BIB008 </s> El Niño Southern Oscillation (ENSO) and Health: An Overview for Climate and Health Researchers <s> The ENSO Phenomenon <s> The aim of this study was to characterize occurrence of extreme temperature and depth conditions affecting fish community in a shallow lake as result of local climate variability, in turn influenced by the ENSO and PDO phenomena. Extreme depth and water temperature events (modeled from local weather conditions) were characterized from 1966 to 2012 to estimate changes in Chascomus lake fish communities. The ENSO and PDO influences on the occurrence probability of these ecosystem changes were investigated. Four significant changes in Chascomus Lake fish assemblage were identified during period assessed, as response to extreme temperature and depth events. Extreme high depth conditions would have changed fish community during 1987 and 2002, leading to a configuration characterized by the absence of the most emblematic fish species in Chascomus Lake, the pejerrey (Odontesthes bonariensis). On the other hand, extreme low water temperatures would have promoted a fish community characterized by the dominance of this last species during 1966–1986, 1997–2001 and 2008–2013 periods. Furthermore, extreme shortening in pejerrey spawning season was significantly related with decrease of its relative abundance. The occurrence probability of the extreme physical conditions modifying Chascomus Lake fish communities was significantly explained by ENSO (by depth influences) and by PDO (by water temperature influences). Thus, this study showed strong correlations between the ENSO and PDO influences and the occurrence probability of the extreme physical conditions changing fish community in Chascomus Lake. <s> BIB009
The ENSO phenomenon refers to the variations in atmospheric and ocean conditions, or in the climate conditions, arising from variations in sea surface temperatures and atmospheric pressure across the Pacific Ocean Basin. ENSO is comprised of two major components that reflect its complex coupled nature, the El Niño or ocean and the Southern Oscillation or atmospheric component. Human society has chronicled the impacts of El Niño for far longer than its atmospheric counterpart. Peruvian fisherman in the 1500s, who originally used the term "El Niño de Navidad", understood well the impact on fisheries of unusually warm waters that occasionally occurred off the coast of Peru around Christmas time. Because El Niño events are associated with anomalously high sea surface temperatures they are also referred to as 'warm events'. The 'cool event' counterpart carries the name of La Niña. H. Hidebrandsson, working in the late 1890s, is often credited with unearthing the rudiments of what we now know as the Southern Oscillation or the atmospheric pressure "seesaw" (barometric seesaw) between the eastern and western Pacific. Building on this work and that of Norman and Lockyer in 1902, and extensive research of his own and that of his collaborator E W Bliss, Gilbert Walker in 1928 named and presented the first coherent ideas about the Southern Oscillation (SO) and extensively described the implications of the SO for inter-annual climate variability across the tropics, including how a Southern Oscillation Index (SOI) could be applied to climate forecasting a season ahead  . While Troup BIB001 reconfirmed and refined many of the earlier long distance associations (now referred to as teleconnections) discovered by Walker, Bjerknes BIB002 conceptualised the association between El Niño and the Southern Oscillation as an outcome of air-sea or oceanatmosphere interaction that led to the coining of the term ENSO. El Niño and La Niña are part of the ENSO cycle that lasts from 12 -18 months, over periods of 2-7 years, associated with alterations of the SO; although some El Niño and La Niña events can last beyond 24 months. The ENSO cycle refers to the alteration of climate fields associated with the development, peak and decay of sea surface temperature anomalies in the eastern and central Pacific along with alterations to the atmospheric circulation and weather pattern across vast areas. El Niño (La Niña) or warm (cool) event conditions first begin to manifest as positive (negative) SST anomalies in the central and eastern Pacific around July. These continue to develop as the ENSO cycle progresses, reaching a peak in the Northern Hemisphere around January to February of the following year, trailed by a decay or lessening of SST anomalies in the subsequent months of March to July/August and cessation of the El Niño (La Niña) event by the end of summer. The swing between El Niño and La Niña phases is not immediate and successive. Rather El Niño and La Niña events can be punctuated by 'neutral' conditions when SST conditions in the eastern and central Pacific are in and around 'normal'. ENSO events can be viewed as a self-sustained and naturally oscillatory mode of the coupled ocean-atmosphere system  , or a stable mode triggered by stochastic forcing, positive ocean-atmosphere feedback processes, with negative feedbacks required to terminate events (Bjerknes 1996) . No two ENSO events are alike. Two broad types of ENSO emerged from exhaustive analyses of ocean and atmosphere climate fields: Eastern Pacific (EP) and Central Pacific (CP). The two types were identified in relation to where maximum SST anomalies tend to occur, with the CP type also referred to by a variety of other names particularly the widely used 'El Niño Modoki'  . While in the context of ENSO and health the different types or flavours of ENSO event might appear inconsequential, the nuanced differences in their climate impacts may hold implications for the temporal and spatial dynamics of ENSO-related health responses. A further characteristic of ENSO that holds possible implications for climate and health associations is the multi-decadal changes observed for ENSO's amplitude, period, propagation characteristics, asymmetry, onset and predictability  BIB005 . The variance of the 2-7 year periodicity of ENSO was relatively high during the periods 1875-1920 and 1960-1990 , but relatively low from 1920 to 1960 BIB005 . A clear shift in the amplitude of SST anomalies in the EP occurred in and around the mid-1970s. Such a shift and the variation in the variance of the 2-7 year periodicity appear to be related to the background climate state of the Pacific Ocean, or the phase of the Pacific Decadal Oscillation (PDO). As the Pacific Ocean transitions from a cool (warm) phase with lower (higher) than normal SSTs to a warm (cool) phase with positive (negative) SST anomalies over a period of 3-4 decades, ENSO characteristics and their link to climate impacts are affected  BIB009 BIB006 BIB003 BIB007 BIB004 . The implication is that ENSO-related health impacts may be non-stationary at the multi-decadal scale. That there are non-symmetric relationships between ENSO and the Indian Ocean Dipole (IOD), another form of ocean climate variability BIB008 , also raises the question as to whether the strength and direction of ENSO health links in the broad region of IOD influence might be IOP phase dependent.