Effect Of ENSO Research Articles

Possible effect of ENSO on annual sediment discharge of debris flows in the Jiangjia Ravine based on Morlet wavelet transforms

The multi-time-scale structures of an annual sediment discharge series of debris flow in the Jiangjia Ravine and the Southern Oscillation index are analysed using the method of Morlet wavelet transformations. The possible effects of El Niño episodes on the annual sediment discharge are discussed by comparing the period variations of ENSO and the discharge. The results show that the annual sediment discharge series of debris flow is related to El Niño episodes. Generally, the annual sediment discharge of debris flow is less than usual during an El Niño episode and debris flow is less active. On the contrary, the annual sediment discharge of debris flows is greater than usual during a La Niña episode and debris flows are more frequent. There is a relationship between the annual sediment discharges of debris flow in the Jiangjia Ravine and the summer Southern Oscillation index, with both having quasi-periodic variations of 2 and 5–6 years.

The influence of moderate ENSO on summer rainfall in eastern China and its comparison with strong ENSO

The 6 major ENSO events since 1979 are classified into the strong and moderate ENSO based on intensity. The composite analysis is performed to reveal the influence of ENSO on East Asian summer monsoon (EASM) and summer rainfall in eastern China. It is shown that the influence is changed with the seasonal cycle in summer, with a weaker influence in June and a stronger influence in August, indicating a long lagged effect of ENSO on EASM. Besides, the circulation and rainfall anomalies caused by the strong ENSO are also stronger with an earlier starting time, while the influence of the moderate ENSO is evident in August. The composite summer rainfall in eastern China for the moderate ENSO exhibits a northern rainfall pattern, which is totally different from the classical ENSO-type rainfall pattern. Based on the composite analysis, two moderate ENSO years with a similar intensity (i.e., 1995 and 2003) are compared. The result shows that, the response of EASM to the moderate ENSO during June and July is, to a certain degree, modulated by the circulation systems in mid-high latitudes of Eurasia and in the Southern Hemisphere, thereby inducing a different rainfall distribution in eastern China. In comparison with the strong ENSO in 1983, it is further revealed that, the strong ENSO plays a dominant role in summer rainfall anomalies in eastern China as well as in controlling the influence of the other factors on EASM. The strong ENSO is therefore different with the moderate ENSO.

Reconstruction of the starting time series of rainy season in Yunnan and the evolvement of summer monsoon during 1711–1982

According to the textual research into the historical documents dominated by archives yearly, as well as the verification with several other kinds of data, the later or earlier starting time of the rainy seasons in Yunnan during 1711–1982 has been reconstructed. The analysis indicates that there are obvious fluctuations in the starting date of the rainy seasons in Yunnan in a year or years, and long fluctuation on the decadal scale. The rainy season comes earlier in the early 18th century, later in the 19th century and earlier again in the 20th century. This reflects to a certain degree the gradual change of the summer monsoon in Yunnan. There exists an obvious quasi-3 years cycle, which is related to El-Nino’s quasi-3 years cycle, and a 11.3-year cycle which is notably related to the 11-year cycle of the solar activity of starting date of the rainy seasons in Yunnan. Meanwhile, the dissertation finds that the El-Nino is very important to the starting date of the rainy seasons in Yunnan. The starting date of the rainy seasons in Yunnan often comes later or normally in the year of El-Nino. However, there is an obvious imperfect period in such influence, which in turn may mean that there is a certain fluctuation in the effect of ENSO on Asian summer monsoon.

Effect of ENSO and volcanic events on the Sun–cloud link

Results of correlation studies between solar proxies and clouds suggest that there is a solar effect on the occurrence of clouds. However, there is a possibility that terrestrial quasi-periodic and sporadic phenomena, such as ENSO and/or major volcanic eruptions, which have an effect on the cloud formation, may influence the results of statistical studies of the Sun–cloud relation. We show that removing ENSO and volcanic years from the full-set analysis does not alter the results. Moreover, the correlation between clouds of different type and two solar proxies, UV irradiance and cosmic ray induced ionisation, is partly improved. This supports the idea that the solar signal affects clouds directly. An interesting result relates to an area in the eastern Pacific where the full-set analysis showed that the relationship between clouds and cosmic ray induced ionization is opposite to the global one. When ENSO and volcanic years are removed this odd correlation disappears, suggesting that in this particular area, the ENSO effect prevails over solar effects.

A regulatory effect of ENSO on the time‐mean thermal stratification of the equatorial upper ocean

To investigate the role of ENSO in regulating the time‐mean thermal stratification of the equatorial Pacific, perturbation experiments are conducted in pairs with a coupled model. In one experiment, ENSO is turned off while in the other experiment ENSO is kept on. Perturbations are introduced through either enhancing tropical heating or increasing subtropical cooling. In the absence of ENSO, the time‐mean difference between the warm‐pool SST (Tw) and the characteristic temperature of the equatorial thermocline (Tc) responds sensitively to either enhanced tropical heating or enhanced subtropical cooling. In the presence of ENSO, such a sensitivity to destabilizing forcing disappears. The lack of sensitivity in the response of Tw‐Tc is linked to a stronger ENSO in response to the destabilizing forcing. ENSO in the model acts as a basin‐scale heat “mixer” that enables surface heat to be transported to the depths of the equatorial thermocline. The study raises the question whether models with poor simulations of ENSO can give reliable predictions of the response of the time‐mean climate to global warming.

ENSO and sandy beach macrobenthos of the tropical East Pacific: some speculations

Abstract. The influence of the ENSO cycle on marine fauna and flora has only recently been given the attention it deserves. The very strong 1997–1998 El Niño and its obvious effects on marine biota was a key point in ENSO research, but unfortunately few quantitative data about the 1997–1998 El Niño itself are available. To gather information about the effect of ENSO on the macrobenthos, we performed a bi-weekly transect monitoring on an Ecuadorian sandy beach in 2000–2001, during the strong La Niña following the 1997–1998 El Niño, and in the normal period of 2002–2004. In this paper, intertidal macrofaunal densities at higher taxonomic level are used to compare a La Niña phase with the 'normal' situation. The few existing documents about El Niño and sandy beach macrobenthos, and scattered data from previous and current research, were used to complete the picture. Total macrobenthos densities were 300% lower during the La Niña phase compared with equal months in the normal phase. Especially Crustacea and Mollusca showed a marked increase in densities towards the normal situation (94% and 341% respectively). Polychaeta and Echinodermata, however, showed higher densities during the La Niña phase (22% and 73% respectively). Two possible explanations are proposed. (1) Low densities during the La Niña could be due to the very strong preceding El Niño, suggesting the populations were still recovering. This hypothesis is supported by previous work done in the south of Peru. This is, however, a cold water system, compared to the Ecuadorian warm water system. (2) The second hypothesis states that a La Niña will have a very severe impact on the intertidal macrofauna of a warm water system like the Ecuadorian coast.

The Effect of ENSO on Tibetan Plateau Snow Depth: A Stationary Wave Teleconnection Mechanism and Implications for the South Asian Monsoons

Abstract An atmospheric stationary wave teleconnection mechanism is proposed to explain how ENSO may affect the Tibetan Plateau snow depth and thereby the south Asian monsoons. Using statistical analysis, the short available record of satellite estimates of snow depth, and ray tracing, it is shown that wintertime ENSO conditions in the central Pacific may produce stationary barotropic Rossby waves in the troposphere with a northeastward group velocity. These waves reflect off the North American jet, turning equatorward, and enter the North African–Asian jet over the eastern Atlantic Ocean. Once there, the waves move with the jet across North Africa, South Asia, the Himalayas, and China. Anomalous increases in upper-tropospheric potential vorticity and increased wintertime snowfall over the Tibetan Plateau are speculated to be associated with these Rossby waves. The increased snowfall produces a larger Tibetan Plateau snowpack, which persists through the spring and summer, and weakens the intensity of the south Asian summer monsoons.

Climatic effect of ENSO associated with landslide occurrence in the Central Andes, Mendoza Province, Argentina

Temporal distribution of landslides can be verified by means of climatic anomalies linked to the ENSO phenomenon. An increasing number of landslides triggered by rainfall have been recorded during warm episodes (El Nino) in the Cordillera Frontal, and a decreasing number during cold episodes (La Nina), concluding that this geological province is mainly influenced by the Pacific Anticyclone. However, slope instability in the Precordillera, located east of the Cordillera Frontal, seems to be mainly influenced by the Atlantic Anticyclone. Analysis of variance shows that there is no significant difference between landslide records and cold-warm episodes, and a higher number of landslides were recorded in years linked to wet periods than during dry periods. Furthermore, the precipitation threshold value associated with landslide occurrence and antecedent precipitation are analysed.

Interannual variations of temperature and vertical motion at the tropical tropopause associated with ENSO

Interannual variations of tropopause temperature and vertical p‐velocity around the tropical tropopause are investigated. Observations show the tropopause temperature (altitude) is higher (lower) over the east side of maritime continent and lower (higher) over the equatorial eastern Pacific during El Niño compared with La Niña. As for the QBO, the tropopause temperature (altitude) is higher (lower) in all longitudes in the westerly phase. A multiple regression analysis shows the combined effect of ENSO and QBO can explain 25–50 % of the interannual variation. An AGCM simulation agrees with the observation for the ENSO signal. The simulation supports the existence of the stratospheric drain over the maritime continent in the tropopause region and indicates a zonal shift of the upward (downward) motion below (above) the tropopause with ENSO.

The Effect of Local Sea Surface Temperatures on Atmospheric Circulation over the Tropical Atlantic Sector

The effects of tropical Atlantic sea surface temperature (SST) anomalies on atmospheric circulation are examined by analyzing several ensembles of integrations of an atmospheric general circulation model (AGCM) forced with differently configured SSTs. An attempt is made to separate the atmospheric response to local SST forcing from internal atmospheric variability, using various statistical analyses. The analyses reveal a robust pattern of atmospheric response to SST forcing. The dominant response is largely confined within the tropical Atlantic sector and may be associated with the variation in location and intensity of the intertropical convergence zone (ITCZ) in response to changes in SST gradient near the equator. Within the deep Tropics, particularly in the western tropical Atlantic warm pool region, there is an indication of a positive feedback between surface heat flux and SST anomalies. In this warm SST region, the latent heat flux tends to dominate surface heat flux variability, and the positive feedback takes place between the wind-induced flux and SST. Outside the deep Tropics, surface heat flux generally tends to dampen the SST, indicating a negative feedback. A strong negative feedback is found off the coast of west Saharan Africa, where the mean SST is cold and the surface heat flux variability is largely induced by the air‐sea temperature difference. The forced response exhibits a seasonal dependence. The structure throughout the year bears a close resemblance to the winter pattern, suggesting that the anomalies during the boreal winter contribute to much of the year-to-year variability in the tropical Atlantic sector. An ENSO-like response in the tropical Atlantic is also identified. It appears as the second dominant forced response and has its strongest manifestation during the boreal fall. By comparing the responses of different sets of ensemble experiments, it is found that the dominant near-surface atmospheric response in the tropical Atlantic sector primarily comes from the local SST forcing. A significant remote influence of the Pacific ENSO on the tropical Atlantic variability is also noted. The strongest remote influence from the tropical Pacific occurs during the boreal spring. The companion paper by Saravanan and Chang further explores the effect of ENSO using an index-based regression analysis.

The effect of ENSO / Anti ENSO on northeast monsoon rainfall

Northeast monsoon precipitation data of 5 meteorological sub-divisions in India, spanning the period 1901-97, were analysed to identify the effect of ENSO/Anti ENSO events on the rainfall over southern peninsular India. ENSO/Anti ENSO years were selected on the basis of seasonal Southern Oscillation Index (SOI). The analysis revealed that ENSO years were generally associated with enhanced northeast monsoon precipitation while there was reduced precipitation during Anti ENSO years, the reduction in Anti ENSO years being significant for Tamil Nadu (at 0.1% level), for Kerala (at 1% level) and for South Peninsular India (at 1% level). Of 22 ENSO years, 18 years were found to be either flood or wet years, while 11 years out of 15 Anti ENSO years were found to be either drought or dry years.
 
 During ENSO years, the Sea Surface Temperature (SST) anomalies both over the Arabian Sea and the Bay of Bengal were positive during the months October to December, while the reverse was the case during Anti ENSO years. A concurrent significant positive correlation was noted between SST over east central Arabian Sea and the north central Bay regions and northeast monsoon rainfall.
 
 The cyclonic systems were observed to form relatively at lower latitudes during ENSO years as compared to those during Anti ENSO years. These systems were also found to move in a more westerly direction, hit Tamil Nadu and south Andhra coast, thus giving more rain over peninsula during ENSO years. The ridge line at 200 hPa level during ENSO years was located 3° south as compared to its location during Anti ENSO years.

North Pacific midlatitude cyclone characteristics and their effect upon winter precipitation during selected El Niño/Southern Oscillation events

North Pacific cyclone frequency, intensity and proximity to the receiving stations (track) in the two ENSO winters that contribute most strongly to the variability of precipitation are examined along with the regional scale circulation and sea surface temperatures (SSTs) prevailing during those winters. During winter 1982/83, the number of cyclones was not significantly different from average, but they were much more intense and moved closer to the receiving stations than average. In 1976/77 there were far fewer cyclones than average. In both winters, the effect of ENSO on precipitation was moderated by the cyclone characteristics and regional atmospheric circulation and SST patterns that prevailed.