The anomalous variation of the lightning activity in southern China during the 1997/98 El Nio event

The El Ni?o Southern Oscillation (ENSO) is a cli- mate anomaly responsible for worldwide weather im- pacts ranging from droughts to floods. It is of scien- tific importance to clarify the influences of the 1997/98 El Ni?o event (for simplicity, named the ENSO) on the regional and global lightning activity. Goodman et al.[1] noticed that during the El Ni?o ma- ture phase from December 1997 to February 1998, the total lightning frequencies recorded by LIS, lightning days and lightning ho…     

Contrast of tropical cyclone frequency in the western North Pacific between two types of La Nia events

According to the different pattern of sea surface temperature anomaly(SSTA) in the previous year of La Nia events,we categorized La Nia events into two types to investigate the different characteristics of tropical cyclone(TC) activity over the western North Pacific(WNP) in TC peak season of two types La Nia events.One type is following the previous El Nio event(La Nia I);the other is following the previous neutral phase or developing La Nia event(La Nia II).Results show that TC genesis frequency in the WNP during TC peak season of La Nia I is less than normal year,whereas it has no differences from normal year during La Nia II.The main reason is attributed to the different amplitude SSTA in the East Indian Ocean(EIO) and the western Pacific Ocean(WPO).Similar to the capacitor effect,strongly positive SSTA in the EIO-WPO during La Nia I triggers an equatorial baroclinic Kelvin wave,which intensifies the easterly in the lower troposphere and weakens the East Asian summer monsoon,and thus the TC frequency decreased during La Nia I.However,the easterly anomaly shows a weak response to the SSTA in the EIO-WPO during La Nia II,and there is no significant change in the environmental pattern over the WNP;so is TC frequency.The modulation of strong EIO-WPO SSTA on large-scale circulation over the WNP reduces the environmental barotropic energy conversion into synoptic-scale disturbances during La Nia I,and also suppresses TC disturbances.The understanding of two different types of La Nia events could help improve the seasonal prediction of TC activity in the WNP during La Nia.     

Role of solar activity in formation of the anomalous El Nin’o current

Monthly indices of Southern Atmospheric Oscillation (SOI) and corresponding Wolf numbers, geoeffective solar flares, magnetic AE indices as well as daily average values of the southward component of the interplanetary magnetic field (IMF B _z) and data on the wind characteristics at Antarctic stations Vostok, Leningradskaya, and Russkaya are analyzed. It is shown that a sharp decrease in the SOI indices, which corresponds to the beginning of El Nin’o (ENSO), is preceded one or two months before by a 20% increase in the monthly average Wolf numbers. In warm years of Southern Atmospheric Oscillation a linear relationship is observed between the SOI indices and the number of geoeffective solar flares with correlation coefficients p < ?0.5. It is shown that in warm years a change in the general direction of the surface wind to anomalous at the above stations is preceded one or two days before by an increase in the daily average values of IMF B _z. An increase in the SOI indices is preceded one or two months before by a considerable increase in the monthly average values of the magnetic AE indices.     

Comparison of the effects of soil moisture and El Niño on summer precipitation in eastern China

Science China Earth Sciences - The effects of spring soil moisture over the vast region from the lower and middle reaches of the Yangtze River valley to North China (YRNC) and El Niño on the...     

The critical role of Indian summer monsoon on the remote forcing between Indian and Northwest Pacific during El Nio decaying year

Recent studies have found a connection between Indian Ocean Basin Warming and the anomalous Northwest Pacific Anticyclone(ANPWA) during El Ni?o decaying year.This study focuses on the necessary condition for this connection by using observation and numerical simulation.The seasonal transition of the Indian Ocean sea surface wind is critical to the climatic effect of Indian Ocean Basin Warming.When the South Asian Summer Monsoon reaches its peak,the background wind becomes desirable for basin warming,which then affects the climate in the Northwest Pacific.Via the Kelvin waves and Ekman divergence,the wind anomalies exist in the lower atmosphere east of the Indian Ocean warm Sea Surface Temperature(SST) anomalies,and intensify and sustain the ANWPA throughout the El Nio decaying summer.This impact plays an important role in the inter-annual variability of the East Asian Summer Monsoon.     

The anomalous variation of the lightning activity in southern China during the 1997/98 El Niño event

This paper presents the EOF analysis results of the lightning density (LD) anomalies for the different seasons in southeastern China and Indochina Peninsula by using the OTD/LIS database (June 1995 to Feb. 2003) of the global LD with 2.5Ü×2.5× resolution offered by Global Hydrology Resource Center. It is shown that the LD positive anomalies in the region occurred at the same time of NINO3 SSTA steep increase in the spring of 1997 and remained to be a higher level till the next spring, as well the corresponding anomaly percent maximum in different seasons was 89%, 30%, 45%, 498% and 55% successively from the beginning to the end of the 1997/98 El Niño event (ENSO). The centre of the LD positive anomalies for the spring or winter season is located at southeastern China and the adjacent coastal areas, but it for the summer or autumn season is located at the southern Indochina Peninsula and Gulf of Thailand, whose position for each season in the ENSO as contrasted with the normal years has a westward shift, and especially for winter or spring season a northward shift at the same time. In addition, an analysis of the interannual variations in the LD anomaly percent, convective precipitation and H-CAPE days in southern China shows that each among the three anomaly percents is correlative with the other for the positive anomaly zone and Kuroshio area. The relative variation of LD during the El Niño period is the highest among the three rates and is larger than that during the non-El Niño period, meaning that the response of lightning activities to the ENSO is the most sensitive in both areas. But the response of lightning activities and precipitation to the ENSO appears to be more complex and diversified either in Kuroshio area or in the Qinghai-Tibet Plateau and northwestern and northeastern China.     

Conceptual model about the interaction between El Nio/Southern Oscillation and Quasi-Biennial Oscillation in far west equatorial Pacific

In the tropical Pacific region, El Ni?o/Southern Os- (COADS SST from 1945 to 1993) in the eastern cillation (ENSO) and the Quasi-Biennial Oscillation in (150°W-90°W, 5°S-5°N) and the observed SST far west equatorial Pacific (QBOWP) are two most and zonal wind in the far western equatorial Pacific prominent interannual variation phenomena. The for- (120°-140°E, 0°-10°N) (Fig.1), in the eastern Pa- mer is characterized by coupled SST-wind variability cific the period of S…     

Evolution of the 2015/16 El Nio and historical perspective since 1979

The 2015/16 El Nio developed from weak warm conditions in late 2014 and NINO3.4 reached 3℃ in November 2015. We describe the characteristics of the evolution of the 2015/16 El Nio using various data sets including SST, surface winds,outgoing longwave radiation and subsurface temperature from an ensemble operational ocean reanalyses, and place this event in the context of historical ENSO events since 1979. One salient feature about the 2015/16 El Nio was a large number of westerly wind bursts and downwelling oceanic Kelvin waves(DWKVs). Four DWKVs were observed in April-November 2015 that initiated and enhanced the eastern-central Pacific warming. Eastward zonal current anomalies associated with DWKVs advected the warm pool water eastward in spring/summer. An upwelling Kelvin wave(UWKV) emerged in early November 2015 leading to a rapid decline of the event. Another outstanding feature was that NINO4 reached a historical high(1.7℃), which was 1℃(0.8℃) higher than that of the 1982/83(1997/98) El Nio . Although NINO3 was comparable to that of the 1982/83 and 1997/98 El Nio , NINO1+2 was much weaker. Consistently, enhanced convection was displaced 20 degree westward, and the maximum D20 anomaly was about 1/3.1/2 of that in 1997 and 1982 near the west coast of South America.     

Displacements and transformations of nitrate-rich and nitrate-poor water masses in the tropical Pacific during the 1997 El Niño

A Lagrangian analysis was applied to the outputs of a coupled physical-biogeochemical model to describe the redistribution of nitrate-rich and nitrate-poor surface water masses in the tropical Pacific throughout the major 1997 El Niño. The same tool was used to analyze the causes of nitrate changes along trajectories and to investigate the consequences of the slow nitrate uptake in the high nutrient low chlorophyll (HNLC) region during the growth phase of the event. Three patterns were identified during the drift of water masses. The first mechanism is well known along the equator: oligotrophic waters from the western Pacific are advected eastward and retain their oligotrophic properties along their drift. The second concerns the persistent upwelling in the eastern basin. Water parcels have complex trajectories within this retention zone and remain mesotrophic. This study draws attention to the third process which is very specific to the HNLC region and to the El Niño period. During the 1997 El Niño, horizontal and vertical inputs of nitrate decreased so dramatically that nitrate uptake by phytoplankton became the only mechanism driving nitrate changes along pathways. The study shows that because of the slow nitrate uptake characteristic of the tropical Pacific HNLC system, nitrate in the pre-El Niño photic layer can support biological production for a period of several months. As a consequence, the slow nitrate uptake delays the gradual onset of oligotrophic conditions over nearly all the area usually occupied by upwelled waters. Owing to this process, mesotrophic conditions persist in the tropical Pacific during El Niño events.     

Regional differences in the effects of El Niño and La Niña on low flows and floods

Abstract

The response of monthly 7-day low flow, monthly instantaneous peak flow, and monthly frequency of flood events to El Niño and La Niña episodes is investigated for 18 rivers that represent a diverse range of climatic types throughout New Zealand. A significant positive or negative deviation from the long-term average was observed in over half the possible combinations of river, streamflow index, and type of ENSO episode; significant deviations were most frequent in the case of low flow, especially during La Niña episodes. Patterns of streamflow response differ widely between rivers, and the response of a given river to individual ENSO episodes is very variable. The patterns of streamflow response to ENSO are consistent to some extent with the climatic effects of ENSO already identified by meteorologists. Two core regions can be defined in which streamflow tends to respond in the same way. These are in the northeast of the North Island, and in the axial ranges of the South Island, where there are significant effects of ENSO on the frequency and duration of rain-bearing northeasterly and westerly winds respectively. The patterns of response strongly reflect topography, and the exposure of catchments to predominant air masses.     

New predictors and a new prediction model for the typhoon frequency over western North Pacific

In this paper, the impacts of the atmospheric circulation during boreal winter-spring on the western North Pacific (WNP) typhoon frequency (WNPTF) are studied. Several new factors in winter-spring in- fluencing the typhoon frequency were identified, including the sea ice cover in the North Pacific and the North Pacific oscillation. Based on these results, the multi-linear regression was applied to establishing a new forecast model for the typhoon frequency by using the datasets of 1965―1999. The forecast model shows a high correlation coefficient (0.79) between the model simulated and the actual typhoon frequencies in the period of 1965―1999. The forecast model also exhibits reasonable hindcasts for the typhoon frequencies for the years 2000―2006. Therefore, this work demonstrates that the new pre- dictors are significant for the prediction of the interannual variability of the WNPTF, which could be potentially used in the operational seasonal forecast of the typhoon frequency in the WNP to get a more physically based operational prediction model and higher forecast skill.     

Influences of Indian Ocean interannual variability on different stages of El Nio: A FOAM1.5 model approach

Both the tropical Indian and tropical Pacific Oceans are active atmosphere-ocean interactive regions with robust interannual variability, which also constitutes a linkage between the two basins in the mode of variability. Using a global atmosphereocean coupled model, we conducted two experiments(CTRL and PC) to explore the contributions of Indian Ocean interannual sea surface temperature(SST) modes to the occurrence of El Ni?o events. The results show that interannual variability of the SST in the Indian Ocean induces a rapid growth of El Ni?o events during the boreal autumn in an El Ni?o developing year. However, it weakens El Ni?o events or even promotes cold phase conversions in an El Ni?o decaying year. Therefore, the entire period of the El Ni?o is shortened by the interannual variations of the Indian Ocean SST. Specifically, during the El Ni?o developing years, the positive Indian Ocean Dipole(IOD) events force an anomalous Walker circulation, which then enhances the existing westerly wind anomalies over the west Pacific. This will cause a warmer El Ni?o event, with some modulations by ocean advection and oceanic Rossby and Kelvin waves. However, with the onset of the South Asian monsoon, the Indian Ocean Basin(IOB) warming SST anomalies excite low level easterly wind anomalies over the west tropical Pacific during the El Ni?o decaying years. As a result, the El Ni?o event is prompted to change from a warm phase to a cold phase. At the same time, an associated atmospheric anticyclone anomaly appears and leads to a decreasing precipitation anomaly over the northwest Pacific. In summary, with remote forcing in the atmospheric circulation, the IOD mode usually affects the El Ni?o during the developing years, whereas the IOB mode affects the El Ni?o during the decaying years.     

Sea surface salinity-derived indexes for distinguishing two types of El Ni?o events in the tropical Pacific

In this study, sea surface salinity(SSS) indexes are derived from reanalysis and observational datasets to distinguish the two types of(Central Pacific(CP) and Eastern Pacific(EP)) El Ni?o events in the tropical Pacific. Based on the SSS anomalous spatial and temporal pointwise correlations with sea surface temperature(SST) indexes of two types of El Ni?o events, the key areas with SSS variations for EP and CP El Ni?o events are identified. For EP El Ni?o events, the key areas are located over an arcuate area centered at(0°, 130°E) and in the central equatorial Pacific covering(5°S–5°N, 175°W–158°W). For CP El Ni?o events, the key areas are located in the northeastern western Pacific covering(2°N, 142°E–170°E) and in the southeastern Pacific covering(20°S–10°S, 135°W–95°W). The key areas for EP and CP El Ni?o events in this study are not located near the dateline in the equatorial Pacific and differ from those obtained from the regression or composite methods.Accordingly, these key areas are used to construct SSS indexes, termed as the CP/EP El Ni?o SSS index(CSI/ESI), to distinguish EP and CP El Ni?o events independently. The SSS indexes are verified by different datasets over varying time periods and they can be adequately used to identify the two types of El Ni?o events and serve as another useful tool for monitoring ENSO. These analyses offer novel insight into how to represent the diversity of El Ni?o events.     

Processes involved in the second-year warming of the 2015 El Nio event as derived from an intermediate ocean model

The tropical Pacific experienced a sustained warm sea surface condition that started in 2014 and a very strong El Nio event in 2015. One striking feature of this event was the horseshoe-like pattern of positive subsurface thermal anomalies that was sustained in the western-central equatorial Pacific throughout 2014–2015. Observational data and an intermediate ocean model are used to describe the sea surface temperature(SST) evolution during 2014–2015. Emphasis is placed on the processes involved in the 2015 El Nio event and their relationships with SST anomalies, including remote effects associated with the propagation and reflection of oceanic equatorial waves(as indicated in sea level(SL) signals) at the boundaries and local effects of the positive subsurface thermal anomalies. It is demonstrated that the positive subsurface thermal anomaly pattern that was sustained throughout 2014–2015 played an important role in maintaining warm SST anomalies in the equatorial Pacific. Further analyses of the SST budget revealed the dominant processes contributing to SST anomalies during 2014–2015. These analyses provide an improved understanding of the extent to which processes associated with the 2015 El Nio event are consistent with current El Nio and Southern Oscillation theories.     

Regional characteristics of the effects of the El Niño-Southern Oscillation on the sea level in the China Sea

Based on coastal tide level, satellite altimetry, and sea surface temperature (SST) data of offshore areas of China’s coast and the equatorial Pacific Ocean, the regional characteristics of the effects of the El Niño-Southern Oscillation (ENSO) on the sea level in the China Sea were investigated. Singular value decomposition results show a significant teleconnection between the sea level in the China Sea and the SST of the tropical Pacific Ocean; the correlation coefficient decreases from south to north. Data from tide gauges along China’s coast show that the seasonal sea-level variations are significantly correlated with the ENSO. In addition, China’s coast was divided into three regions based on distinctive regional characteristics. Results obtained show that the annual amplitude of sea level was low during El Niño developing years, and especially so during the El Niño year. The ENSO intensity determined the response intensity of the annual amplitude of the sea level. The response region (amplitude) was relatively large for strong ENSO intensities. Significant oscillation periods at a timescale of 4–7 years existed in the sea level of the three regions. The largest amplitude of oscillation was 1.5 cm, which was the fluctuation with the 7-year period in the South China Sea. The largest amplitude of oscillation in the East China Sea was about 1.3 cm. The amplitude of oscillation with the 6-year period in the Bohai Sea and Yellow Sea was the smallest (less than 1 cm).     

El Niño and La Niña influence on mean river flows of southern South America in the 20th century

The influence of the El Niño Southern Oscillation (ENSO) phenomenon on monthly mean river flows of 12 rivers in the extreme south of South America in the 20th century is analysed. The original dataset of each river is divided into two subsets, i.e. warm ENSO events or El Niño, and cold ENSO events or La Niña. The elements of the subsets are composites of 24 consecutive months, from January of the year when the ENSO event begins to December of the following year. The ENSO signal is analysed by comparing the monthly mean value of each subset to the long-term monthly mean. The results reveal that, in general, monthly mean El Niño (La Niña) river flows are predominantly larger (smaller) than the long-term monthly mean in the rivers studied. The anomalies are more evident during the second half of the year in which the event starts and the first months of the following year.     

Some characteristics and precipitation effects of the El Niño of 1997–1998

The 1997–1998 El Niño was the strongest in known history. However, its effects on rainfall in different parts of the globe were not all as expected (floods were expected in some regions and droughts in others). The characteristics of this El Niño, and the expected and observed precipitation effects are described; the reasons for the expectations not coming true in some regions are discussed. This paper attempts to review the important scientific issues involved in El Niño phenomena for the general reader.     

Impacts of El Niño–southern oscillation on global runoff: Characteristic signatures and potential mechanisms

Runoff signatures, including low flow, high flow, mean flow and flow variability, have important implications on the environment and society, predominantly through drought, flooding and water resources. Yet, the response of runoff signatures has not been previously investigated at the global scale, and the influencing mechanisms are largely unclear. Hence, this study makes a global assessment of runoff signature responses to the El Niño and La Niña phases using daily streamflow observations from 8217 gauging stations during 1960–2015. Based on the Granger causality test, we found that ~15% of the hydrological stations of multiple runoff signatures show a significant causal relationship with El Niño–southern oscillation (ENSO). The quantiles of all runoff signatures were larger during the El Niño phase than during the La Niña phase, implying that the entire flow distribution tends to shift upward during El Niño and downward during La Niña. In addition, El Niño has different effects on low and high flows: it tends to increase the low and mean flow signatures but reduces the high flow and flow variability signatures. In contrast, La Niña generally reduces all runoff signatures. We highlight that the impacts of ENSO on streamflow signatures are manifested by its effects on precipitation (P), potential evaporation (PET) and leaf area index (LAI) through ENSO-induced atmospheric circulation changes. Overall, our study provides a comprehensive picture of runoff signature responses to ENSO, with valuable insights for water resources management and flood and drought disaster mitigation.