Role Of El Nino-Southern Oscillation Research Articles

Synchronous Characteristics of Precipitation Extremes in the Yangtze and Murray-Darling River Basins and the Role of ENSO

The floods caused by the extreme precipitation in the Yangtze River basin (YRB) and Murray–Darling River basin (MDRB), the largest basins in China and Australia, have significant impacts on the society and regional economies. Based on the spatial–temporal analysis of the daily precipitation extremes (DPEs) during 1982–2016, we found that for both basins, the whole-basin-type DPEs have the highest proportion and a synchronous DPE interannual variation characteristic exists in the two basins, with the 3-yr running correlation coefficient of the annual DPE days (DPEDs) reaching almost 0.7 (significant at the 0.01 level). The El Nino–Southern Oscillation (ENSO), which is one of the most significant climate disturbance factors in the world, plays an important role in modulating the variability of the DPEs in the two basins. Singular value decomposition (SVD) analysis revealed that both the YRB and the MDRB’s whole-basin-type DPEs are closely coupled with the procedure that the preceding winter eastern Pacific (EP)-type El Nino faded to a central Pacific (CP)-type La Nina. This means that the DPEs in the YRB and MDRB may synchronously occur more frequently when the above process occurs. Owing to the atmosphere–ocean interaction from the east–west dipole sea surface temperature (SST) anomaly pattern, the atmospheric circulation disturbance exhibits a pattern in which the equatorial eastern Pacific region is a mass source anomaly with a higher pressure, drier air, and weaker convection, while the equatorial western Pacific region is a mass sink anomaly with a lower pressure, wetter air, and stronger convection. Moreover, two wave trains that originated from the tropical western Pacific were found to extend to the YRB and MDRB. The interaction between the wave train’s interphase dynamics and water vapor transport disturbance results in the ascent conditions and enhanced water vapor transport, which leads to the synchronous occurrence of DPEs in the YRB and MDRB on an interannual scale.

Influence of ENSO on the Pacific decadal oscillation in CMIP models

Emerging decadal climate predictions call for an assessment of decadal climate variability in the Coupled Model Intercomparison Project (CMIP) database. In this paper, we evaluate the influence of El Nino Southern Oscillation (ENSO) on Pacific Decadal Oscillation (PDO) in 10 control simulations from the CMIP3 and 22 from the CMIP5 database. All models overestimate the time lag between ENSO forcing and the PDO response. While half of the models exhibit ENSO-PDO correlation which is close to that in observation (>0.5) when the time lag is accounted for, the rest of the models underestimate this relationship. Models with stronger ENSO-PDO correlation tend to exhibit larger PDO-related signals in the equatorial and south Pacific, highlighting the key role of ENSO teleconnection in setting the inter-hemispheric Pacific pattern of the PDO. The strength of the ENSO-PDO relationship is related to both ENSO amplitude and strength of ENSO teleconnection to the North Pacific sea-level pressure variability in the Aleutian Low region. The shape of the PDO spectrum is consistent with that predicted from a combination of direct ENSO forcing, atmospheric stochastic forcing over the North Pacific and the re-emergence process in 27 models out of 32. Given the essential role of ENSO in shaping the Pacific decadal variability, models displaying realistic ENSO amplitude and teleconnections should be preferentially used to perform decadal prediction experiments.

Role of ENSO in the interannual relationship between Tibetan Plateau winter snow cover and Northwest Pacific tropical cyclone genesis frequency

Previous studies have revealed a significantly negative correlation between prior winter snow cover over the Tibetan Plateau (TPSC) and tropical cyclone genesis frequency (TCF) over the western North Pacific (WNP) in the following typhoon season. This study revisited this relationship based on long-term observational data. The results showed that the interannual correlation between TCF over the WNP and TPSC experienced a shift in the early 1990s. This correlation is significant during only 1993–2012 and is considerably weak during 1976–1992. The possible reasons causing the shift were examined further, and the results demonstrated that the central Pacific (CP) El Nino-Southern Oscillation (ENSO) has played a vital role in intensifying the interannual relationship between TCF over the WNP and TPSC since the early 1990s. During 1993–2012, TPSC was negatively related to CP ENSO. When TPSC was higher than (lower than) normal, CP ENSO was often in its cold (warm) phase. Such a combination remarkably enhances the relationship of TPSC with the zonal land-sea thermal difference and thus with the summer monsoon over the WNP. Additionally, it enhances the modulation of TPSC on the dynamical environments controlling TCF. As a result, the linkage between TPSC and TCF was significantly strengthened in this period. In sharp contrast, due to the weak relationship between TPSC and ENSO followed by the weak modulation of TPSC on the summer monsoon over the WNP and the dynamical environment during 1976–1992, the linkage between TPSC and TCF was weak during this time period. The results from additional dynamical diagnostic analyses further showed that during 1993–2012 CP ENSO modulated the barotropic energy conversion of zonal winds over the WNP, contributing to the intensified relationship between TPSC and TCF. These results will improve seasonal forecasting of tropical cyclone activity over the WNP.

The El Niño – La Niña cycle and recent trends in supply and demand of net primary productivity in African drylands

Inter-annual climatic variability over a large portion of sub-Saharan Africa is under the influence of the El Nino-Southern Oscillation (ENSO). Extreme variability in climate is a threat to rural livelihoods in sub-Saharan Africa, yet the role of ENSO in the balance between supply and demand of net primary productivity (NPP) over this region is unclear. Here, we analyze the impact of ENSO on this balance in a spatially explicit framework using gridded population data from the WorldPop project, satellite-derived data on NPP supply, and statistical data from the United Nations. Our analyses demonstrate that between 2000 and 2013 fluctuations in the supply of NPP associated with moderate ENSO events average ± 2.8 g C m−2 yr.−1 across sub-Saharan drylands. The greatest sensitivity is in arid Southern Africa where a + 1 °C change in the Nino-3.4 sea surface temperature index is associated with a mean change in NPP supply of −6.6 g C m−2 yr.−1. Concurrently, the population-driven trend in NPP demand averages 3.5 g C m−2 yr.−1 over the entire region with densely populated urban areas exhibiting the highest mean demand for NPP. Our findings highlight the importance of accounting for the role ENSO plays in modulating the balance between supply and demand of NPP in sub-Saharan drylands. An important implication of these findings is that increase in NPP demand for socio-economic metabolism must be taken into account within the context of climate-modulated supply.

Inter-annual to inter-decadal streamflow variability in Quebec and Ontario in relation to dominant large-scale climate indices

Summary The impacts of large-scale climate oscillations on hydrological systems and their variability have been documented in different parts of the world. Since hydroclimatic data are known to exhibit non-stationary characteristics, spectral analyses such as wavelet transforms are very useful in extracting time–frequency information from such data. As Canadian studies, particularly those of regions east of the Prairies, using wavelet transform-based methods to draw links between relevant climate indices [e.g., the El Nino Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the Pacific Decadal Oscillation (PDO)] and streamflow variability are not common, this study aims to analyze such relationships for the southern regions of Quebec and Ontario. Monthly and annual streamflow data with a record length of 55 years were used to capture streamflow variability at intra-annual, inter-annual and inter-decadal scales. The continuous wavelet transform spectra of monthly streamflow data revealed consistent significant 6- and 12-month periodicities, which are likely associated with strong seasonality factors. Its annual counterparts showed four different significant periodicities: up to 4 years, 4–6 years, 6–8 years, and greater than 8 years – all of which occurred after the late 1960s/early 1970s. Wavelet coherence analyses show that the influence of ENSO and NAO at the inter-annual scale occurs at 2–6 year periodicities, and the influence of PDO occur at periodicities up to 8 years and exceeding 16 years. Correlations between these climate indices and streamflow were computed to determine the time delay of streamflow response to the influence of ENSO, NAO, and PDO. The lag times ranged from 6–48 months (for monthly data) and 1–4 years (for annual data). This research contributes to our understanding of streamflow variability over the southern parts of Quebec and Ontario, and the role of ENSO, NAO, and PDO phenomena on this variability. These relationships can be also used to improve hydrological forecasting and water resources management in Ontario and Quebec.

The role of ENSO and MJO on rapid intensification of tropical cyclones in the Bay of Bengal during October–December

The influence of El Nino/Southern Oscillation (ENSO) and Madden–Julian Oscillation (MJO) and their combined effect on the rapid intensification (RI) of tropical cyclones (TCs) in the Bay of Bengal (BoB) during the primary cyclone season (October–December) is investigated. An empirical index, called genesis potential index (GPI), is used to quantify the relative importance of four environmental parameters responsible for the modulation of TCs characteristics. The analysis shows that TC frequency and RI of TC’s is higher in La Nina than El Nino regime during the primary TC season in the BoB. The combined effect of enhancement (reduction) in mid-tropospheric humidity (primary factor) and relative vorticity (secondary factor) played a major role in the enhancement (reduction) of the TC activity under La Nina (El Nino) regime. In addition, when the MJO is active over the BoB (phases 3–4; characterized by enhanced convective activity in the BoB) under La Nina regime, environmental conditions were more conducive for enhancement of TC activity and RI of TCs compared to corresponding MJO phase under El Nino regime. Increase in mid-tropospheric humidity and reduction in vertical wind shear were identified as the primary and secondary factors enhancing the likelihood of RI of TCs in the BoB during phases 3–4 of MJO under La Nina regime. Further, the role of accumulated tropical cyclone heat potential (ATCHP) on the RI of TC during primary TC season is also investigated. Our analysis demonstrates that ATCHP is large for TCs which undergo RI compared to TCs not undergoing RI.

Attribution of SST variability in global oceans and the role of ENSO

Based on a novel design of coupled model simulations where sea surface temperature (SST) variability in the equatorial tropical Pacific was constrained to follow the observed El Nino—Southern Oscillation (ENSO) variability, while rest of the global oceans were free to evolve, the ENSO response in SSTs over the other ocean basins was analyzed. Conceptually the experimental setup was similar to discerning the contribution of ENSO variability to interannual variations in atmospheric anomalies. A unique feature of the analysis was that it was not constrained by a priori assumptions on the nature of the teleconnected response in SSTs. The analysis demonstrated that the time lag between ENSO SST and SSTs in other ocean basins was about 6 months. A signal-to-noise analysis indicated that between 25 and 50 % of monthly mean SST variance over certain ocean basins can be attributed to SST variability over the equatorial tropical Pacific. The experimental setup provides a basis for (a) attribution of SST variability in global oceans to ENSO variability, (b) a method for separating the ENSO influence in SST variations, and (c) understanding the contribution from other external factors responsible for variations in SSTs, for example, changes in atmospheric composition, volcanic aerosols, etc.

Analysis of ENSO-based climate variability in modulating drought risks over western Rajasthan in India

This paper investigates the role of El Nino-Southern Oscillation (ENSO)-based climate variability in modulating multivariate drought risks in the drought-prone region of Western Rajasthan in India. Droughts are multivariate phenomenon, often characterized by severity, duration and peak. By using multivariate ENSO index, annual drought events are partitioned into three climatic states – El Nino, La Nina and neutral phases. For multivariate probabilistic representation of drought characteristics, trivariate copulas are employed, which have the ability to preserve the dependence structure of drought variables under uncertain environment. The first copula model is developed without accounting the climate state information to obtain joint and conditional return periods of drought characteristics. Then, copula-based models are developed for each climate state to estimate the joint and conditional probabilities of drought characteristics under each ENSO state. Results of the study suggest that the inclusion of ENSO-based climate variability is helpful in knowing the associated drought risks, and useful for management of water resources in the region.

Spatiotemporal influences of ENSO on precipitation and flood pulse in the Mekong River Basin

Summary The Mekong River Basin in South East Asia has experienced in recent decades large inter-annual variations between large floods and severe droughts. The general perception is that climate variability has increased in the region. In this paper our aim was to examine the role of El Nino-Southern Oscillation (ENSO) on these recent climate variations. To achieve our aim, we analysed the influence of ENSO on precipitation (over period 1981–2005) and discharge (1910–2008) by using spatial GIS analyses and statistical methods, such as linear correlations, spectral analysis and stochastic regression models. We found that the Mekong’s hydrology was significantly influenced by ENSO, particularly in the decay year of ENSO events. The influence was strongest in the southern parts of the basin. The precipitation and discharge decreased (increased) during El Nino (La Nina) and the annual flood period was shorter (longer) during El Nino (La Nina). We found also decadal variations in the ENSO-discharge relationship. Our findings indicate high potential for developing prediction methods for ENSO induced hydrological anomalies. These findings may significantly improve the possibility of preparing for extreme hydrological events, either drought or exceptional floods, and thus potentially improve water resources management in the basin.

Seasonal variability of Indonesian rainfall in ECHAM4 simulations and in the reanalyses: The role of ENSO

A study of the skill of the ECHAM version 4 atmospheric general circulation model and two reanalyses in simulating Indonesian rainfall is presented with comparisons to 30 years of rain gauge data. The reanalyses are those performed by the European Centre for Medium-Range Weather Forecasts and of the National Centers for Environmental Prediction jointly with National Center for Atmospheric Research. This study investigates the skill of the reanalyses and ECHAM4 with regard to three climate regions of Indonesia, the annual and interannual variability of rainfall and its responses to El Nino-Southern Oscillation (ENSO) events. The study is conducted at two spectral resolutions, T42 and T106.

The Interannual Variability in the Genesis Location of Tropical Cyclones in the Northwest Pacific

Abstract Variations in the seasonal mean (July–October) genesis positions of tropical cyclones (TCs) in the western North Pacific associated with variations in the large-scale atmospheric circulation are investigated. Analysis shows considerable interannual variability in the seasonal TC mean genesis positions (MGPs) during the 1979–99 period. The variability is shown to be related to the 200–850-hPa vertical wind shear, the west Pacific sea surface temperature (SST), the position and strength of the monsoon trough, and the position and strength of the western Pacific subtropical high (WPSH). Each of these circulation features as well as the SST is, in turn, related to the El Nino–Southern Oscillation (ENSO). However, while this study suggests that ENSO is a major factor in determining seasonal MGP, the relatively short satellite observational period also suggests that ENSO is not the sole determinant, the La Nina year of 1988 being one example. The study further suggests that the role of ENSO is complica...

Reversal of three global atmospheric fields linking changes in SST anomalies in the Pacific, Atlantic and Indian oceans at tropical latitudes and midlatitudes

Analysis of upper tropospheric relative humidity (UTH) variations from 1979 through 1992 shows the existence of a non-linear reversal in the tropical atmospheric anomalous circulation patterns characteristic of an El Nino (EN) phase. In addition to the tropics, both the original structures and their reversal affect midlatitudes. These changes begin a few months after the decaying of a strong EN episode and end one to two years after the peak in sea surface temperatures (SST) in the Pacific. This anomalous atmospheric circulation is matched by parallel reversals in cloud cover anomalies and absorbed solar radiation. At present, the limited time span covered by accurate satellite records prevents any conclusion regarding this reversal. An extension of coherent El Nino-Southern Oscillation (ENSO) signals in these correlation fields to higher latitudes is also reported. (By approximately between one to two years after the onset, the anomalous warming in the eastern equatorial Pacific had almost vanished, and the anomalies associated with the local Hadley cells and the Walker circulation revert towards a pre-El Nino stage). The coherence of these bimodal patterns might be responsible for certain climatic phenomena associated with ENSO in midlatitude regions. Though a relation between SST anomalies in the Pacific and the Indian and Atlantic oceans has already been postulated, this work shows how the return to near-normal conditions might yield a reversal in atmospheric patterns even in the absence of La Nina (LN) episodes. This in turn, might also produce climatic responses and feedbacks altering the net heat flux entering these remote regions, leading to droughts and floods. The results obtained strongly reinforce, for the most recent record, the hypothesis of a tropical atmospheric bridge connecting with the extratropics. In addition, it has been possible to isolate the ENSO signal from western Mediterranean (WM) SSTA, by means of advanced statistical techniques. The existence of anomalous atmospheric structures, that might link W Mediterranean SSTA and SSTA in the tropical North Atlantic (TNA), remains an open question, worthy of further investigation. Similarly, the role of ENSO in forcing this TNA region, and the subsequent atmospheric changes, will have to be further analysed with the aid of regional coupled models.

The role of ENSO in determining climate and maize yield variability in the U.S. cornbelt

Recent advances in understanding the role of the El Nino-Southern Oscillation (ENSO) in climate variability present opportunities for improving efficiency in agricultural production. We investigated the relationships between ENSO, climate and maize yields in the U.S. cornbelt, using both observed data and crop simulations. Using a time-series of sea-surface temperature anomalies (SSTA) from the NINO3 region of the Pacific Ocean and historical records of temperature and precipitation spatially averaged across 51 mid-western climate divisions from 1950 to 1995, we ran linear correlation tests at three different lags. Northern hemisphere wintertime SSTAs were significantly correlated with air temperature at the 95% level of confidence in both the previous (r 0.32) and following (r 0.41) summer, but had opposite signs. Correlations with precipitation were significant only in the summer preceding the ENSO event (r0.31). Detrended maize yield for the same area and time period was also significantly related to SSTAs in the winter after harvest, with a correlation coefficient of 0.39, indicating that ENSO accounts for : 15% of interannual maize yield variability in the cornbelt. Crop growth simulations at seven sites across the region suggest that water stress in July and August is the primary cause of lowered corn yield in La Nina years, but shortened grainfill period due to higher temperatures is also important. The benefits of El Nino-related rainfall and cooler temperatures are less pronounced than the negative impacts of warmer and dryer La Ninas. However, advance warning of both ENSO phases may present opportunities for improved crop management in the cornbelt. Copyright © 1999 Royal Meteorological Society.