Cold Phase Of El Nino Southern Oscillation Research Articles

Impacts of El Nino and La Nina on Palm Oil Production: Evidence from Selected Palm Oil Producing Countries

Palm oil is one of the most important edible oils commercialized in world oils and fats market. However, agriculture is highly vulnerable to climate change. For instance, El Nino is the warm phase of El Nino Southern Oscillation (ENSO) while La Nina is the opposite of El Nino, the cold phase of ENSO. Strong El Nino and La Nina events cause global changes especially in temperature and rainfall, subsequently affect agricultural areas and causing considerable economic losses. Therefore, the objective of this study is to explore the impacts of El Nino and La Nina on palm oil production in selected palm oil producing countries namely Indonesia, Malaysia and Thailand from 1980 to 2019 using the Ordinary Least Square (OLS) multiple regression analysis. The results revealed that area harvested and palm oil price affect palm oil production significantly in all palm oil producing countries. Nevertheless, the ENSO effects are not apparent. Only the El Nino dummy coefficients are negative and statistically significant in Thailand, suggesting that the occurrence of El Nino events is more pronounced than the La Nina events on palm oil production in Thailand. Hence, this study suggests that a good water management system is needed to allow the palm oil to achieve optimum production.

ENSO signatures observed in tropical tropopause layer parameters using long-term COSMIC RO data

The El Nino Southern Oscillation (ENSO) signatures interpreted in different tropical tropopause layer (TTL) parameters are investigated by using long-term Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC)-1 radio occultation temperature data from July 2006 to March 2019. The TTL parameters include the cold point tropopause height (CPT-H) and corresponding temperature (CPT-T), lapse rate tropopause height (LRT-H) and corresponding temperature (LRT-T), convective outflow level height (COH), and TTL thickness. Results indicate that the warm phase of ENSO is associated with prominent negative CPT-T anomalies, whereas the cold phase of ENSO is associated with positive CPT-T anomalies over the tropical central and eastern Pacific Ocean. Further, correlation analysis is carried out between TTL parameters obtained over the Nino 3.4 region (5°N–5°S, 170°W–120°W) and sea surface temperature Nino 3.4 Index. We found weak positive correlations in CPT-H (0.37) and LRT-H (0.49), and moderate negative correlations in CPT-T (− 0.62), LRT-T (− 0.64), and TTL thickness (− 0.5) with Nino 3.4 Index. Interestingly, a higher correlation of 0.8 is noticed between COH and the Nino 3.4 Index. Furthermore, lag correlation analysis reveals that a 4-month lag is noticed in tropopause heights (CPT-H and LRT-H) and a 2-month lag is evident in tropopause temperatures (CPT-T and LRT-T) with the Nino 3.4 Index. In addition, only 1-month lag is noticed between COH and Nino 3.4 index. With a high correlation and 1-month lag with the Nino 3.4 index, it is concluded that the COH is the most suitable TTL parameter to detect the ENSO signatures among the TTL parameters.

Interannual variability of wave climate in the Caribbean Sea

Low-frequency interannual climatic oscillations have been reported to influence the precipitation regimes, river discharges, winds, and sea level in the Caribbean Sea. Here, we analyzed the presence and relevance of low-frequency oscillations in the waves’ climate of the Caribbean Sea using data from WAVEWATCH III (NOAA) reanalysis and applying frequency domain wavelet transform technique. Results show a clear sectorization of the Caribbean Sea according to the presence and relevance of interannual oscillations in wave climate, where the southwestern region (Colombian basin) exhibits a larger number of statistically significant oscillations (at 95% confidence levels) with recurrence periods of 2–4, 4–8, and 8–12 years. Only some of these frequencies are observed in the rest of Caribbean basin. Two climatic events occurring in the periods 1997–1998 and 2010–2011 stand out in the study zone for their high energy in wavelet analysis. These events have been associated with the occurrence of the warm and cold phases of the El Nino Southern Oscillation (ENSO), respectively, and are well known due to their strong global effects on hydroclimatology. Thus, an additional analysis was performed in the time domain to investigate the effect of the two events on Caribbean Sea waves. The results revealed an influence of ENSO on significant wave height and peak period comparing with multi-year average conditions, producing highly variable spatial and temporal wave patterns, with more evident effects during the cold phase of ENSO (La Nina). These results confirm the sectorization of the Caribbean Sea regarding the energy and interannual variability of waves.

Effect of winter-to-summer El Niño transitions on tropical cyclone activity in the North Atlantic

El Nino-Southern Oscillation (ENSO) is known to significantly modulate Atlantic tropical cyclone (TC) activity. In this paper, we separate El Nino events into El Nino resurgence (ELR) and La Nina-transition-El Nino (LAT) events according to the warm or cold ENSO phase in the preceding winter. Different influences of the two types of El Nino events on North Atlantic TC activity are explored during the hurricane season of June–November. The average number of TCs is more, with shorter average duration and slightly larger intensity during ELR events than during LAT events. There are more TCs passing through and making landfall at the Gulf of Mexico, Caribbean Sea during ELR events, indicating that TCs may have a greater impact on coastal cities of Gulf of Mexico and Caribbean Sea during ELR events than LAT events. Composite analysis reveals that thermodynamic factors such as relative humidity (RH), sea surface temperature and tropical cyclone heat potential are mainly responsible for the TC activity difference between ELR and LAT events. Genesis potential index diagnosis shows that the RH and potential intensity are the dominant factors that influence TC genesis in the MDR during ELR events. During LAT events, the relative vorticity, vertical wind shear and RH over Cape Verde are favorable for TC genesis. In summary, the preceding winter ENSO phases associated with ELR and LAT events lead to opposite changes of environmental factors, and thus induce different TC activities in ELR and LAT events.

ENSO influence on summer temperature over Arabian Peninsula: role of mid-latitude circulation

This study discusses the interannual variability of the summer (June–August) season surface air temperature (SAT) over the Arabian Peninsula and its relationship to the global circulation patterns under the influence of El Nino Southern Oscillation (ENSO). A composite analysis based on the above (below) normal summer SAT over the Arabian Peninsula reveals a wavelike pattern resembling the mid-latitude Circumglobal Wave Train (CGT). The positive (negative) phase of the CGT is associated with above (below) normal SAT anomalies over the Arabian Peninsula. Further, the Arabian Peninsula summer SAT is also found to be associated with ENSO and this association has strengthened in recent decades as compared to before 1980s. The ENSO induced upper level circulation patterns resemble the CGT pattern. The ENSO warm phase over the central and eastern Pacific is associated with anomalous low pressure anomalies over the Eurasian region while the cold phase associated with anomalous high pressure. This favours the above normal SAT anomalies over the Eurasian region including the northern parts of the Peninsula during the cold phase of ENSO and below normal temperature during the warm phase. The weakening of upper level low-pressure anomalies over central Asia during the positive phase of CGT also favours the extension of the Eurasian high over the Arabian Peninsula. Additionally, the climate model simulations experiments performed with newly developed Saudi-KAU Atmospheric Global Climate Model (AGCM) also support the observed findings.

Monitoring Water Resources over the Kotmale Reservoir in Sri Lanka Using ENSO Phases

In this study, the El Nino Southern Oscillation (ENSO) phase index is used for water management over the Kotmale reservoir in Sri Lanka. Daily rainfall data of 9 stations over the Kotmale catchment during 1960–2005 June-September (JJAS) season is investigated over the Kotmale catchment. The ENSO phases are identified based on the 0.5°C sea surface temperature (SST) anomaly over Nino 3 region. The study has brought out few stations showing increasing and a few decreasing seasonal rainfall trends for JJAS season, while there is no change in the annual rainfall for the catchment. Monthly and seasonal rainfall of all the selected stations showed negative correlation with the sea surface temperature (SST) over the Nino-3 region index during JJAS season with varying magnitudes. During the warm phase of ENSO, below average rainfall is prominent for JJAS season over many stations. The rainfall especially during early September showed a significant below average rainfall during the warm ENSO phase. The seasonal rainfall during neutral and cold ENSO phases does not experience similar significant changes as seen during warm ENSO phase. Inflow of the Kotmale reservoir shows decreasing trend for the period of 1960–2005 in the observation from all stations collectively.

Decadal modulation of East China winter precipitation by ENSO

The decadal modulation of East China winter precipitation by the El Nino-Southern Oscillation (ENSO) is examined using both observational data and coupled global climate model simulations. The co-variability between 68-year (1948–2015) observed East China precipitation and tropical Pacific sea surface temperature (SST) is quantified by the singular value decomposition (SVD) method. The first SVD mode relates Southeast China winter pluvial (drought) to the tropical Pacific El Nino (La Nina) SST. A comparison between two 480-year model simulations with and without ENSO suggests that ENSO can modulate both the intensity and frequency of East China winter precipitation. In the presence of ENSO, maximum precipitation anomalies over Southeast China can be increased by 50% and largely on the interannual timescale (3–6 years). It is also demonstrated that there is an asymmetry in the precipitation and circulation responses to warm and cold phases of ENSO. The responses are sensitive to the intensity of SST anomalies during El Nino, but less sensitive to SSTs during La Nina. This sensitivity, together with the decadal variations of ENSO, helps understand the observed decadal changes in the strength of the association between wintertime tropical Pacific SST and East China precipitation. The association is relatively weak during 1948–1977 when La Nina occurred more frequently, but strong during 1978–1999 when El Nino occurred more frequently. In the last 16 years (2000–2015) the association is weakest, likely due to the weakened variability of tropical Pacific SST since 2000.

Variability of the Caribbean low-level jet during boreal winter: large-scale forcings

An index capturing the anomalies of the zonal wind at 925 hPa from 1950 to 2010 was defined to explore the relationship between the fluctuations of the Caribbean low-level jet (CLLJ) and the main climate variability modes affecting the Intra-Americas Sea Region. El Nino Southern Oscillation (ENSO) events, here defined using the Nino 3.4 index, are found to be the most important variability modes for the jet anomalies, in agreement with previous studies. However, the Pacific Decadal Oscillation (PDO) and the Pacific/North American (PNA) teleconnection pattern also show significant correlations with the CLLJ anomaly index during February. The North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) reveal a possible interaction with the jet anomalies that could be connected with the cold fronts and cold air surges arriving to the Caribbean basin from the Northern Hemisphere during winter. A composite technique is used to explain the correlations with the Pacific indexes. We found that ENSO events are connected to CLLJ anomalies by modulating the sea-level pressure (SLP) near the east coast of the United States and the Aleutian Low. The pattern displayed by the SLP anomalies (SLPa) is also associated with the PNA. During warm (cold) ENSO phases, negative (positive) anomalies in the SLP field over the east coast of North America produce cyclonic (anticyclonic) circulations at low levels. However, the ENSO signal in the SLPa and the PNA pattern are modulated by the phases of the PDO. Results indicate that when the ENSO and PDO are in phase (out of phase), the SLPa signal is enhanced (weakened or cancelled), affecting the CLLJ anomalies in both direction and intensity, also changing the spatial distribution of precipitation.

Climate regimes, El Niño-Southern oscillation and konzo epidemics

Epidemics of konzo have occurred during severe droughts in parts of east, central, and southern Africa since the 1920s. Occurrence is attributed to exposure to cyanide from poorly processed cassava foods, the sole source of calories when other food crops fail. El Nino, the warm phase of the El Nino-Southern Oscillation (ENSO), induces severe droughts in the geographical areas of Africa where epidemics of konzo occur. Climate regimes are determined by modes of Pacific Decadal Oscillation, which modulates the ENSO. Study was done to determine the relationship of konzo epidemics to climate regimes and phases of ENSO, and to propose a model to explain while konzo epidemics do not occur in all drought affected areas. Data of all konzo epidemics in the past century and in DR Congo from 1974–1996, and indices of ENSO and PDO from 1915 to 2014 were obtained. Konzo epidemics were mapped to phases of ENSO and PDO. Wavelet spectral and wavelet spectral coherence analysis of climate indices and konzo epidemics were done. All konzo epidemics of the past century occurred during warm climate regimes. Of 19 warm phases of ENSO from 1974–1996 in DR Congo, 17 were coupled to konzo epidemics, while of 4 cold phases of ENSO, 1 was coupled to konzo epidemic, odds ratio 26 (95 % CI, 2–378). Global spectral of ENSO and konzo showed dominant periodicity of 5 years, while spectograms showed significant periodicities and coherence between 3–6 years. Spatial distribution of konzo is restricted to the area of maximal impact of El Nino on precipitation in Africa. El Nino is the underlying cause of konzo epidemics. Control of konzo epidemics requires management of the impact of El Nino on agriculture in areas where the population depends on cassava as sole source of calories during droughts. There is the need to develop forecast models of changes in cassava production to predict likely periods of konzo epidemics.

Change of Indonesian Throughflow outflow in response to East Asian monsoon and ENSO activities since the Last Glacial

The Indonesian Throughflow (ITF) links upper ocean waters of the west Pacific and Indian Ocean, modulates heat and fresh water budgets between these oceans, and in turn plays an important role in global climate change. The climatic phenomena such as the East Asian monsoon and El Nino-Southern Oscillation (ENSO) exert a strong influence on flux, water properties and vertical stratification of the ITF. This work studied sediments of Core SO18462 that was retrieved from the outflow side of the ITF in the Timor Sea in order to investigate response of the ITF to monsoon and ENSO activities since the last glacial. Based on Mg/Ca ratios and oxygen isotopes in shells of planktonic foraminiferal surface and thermocline species, seawater temperatures and salinity of both surface and thermocline waters and vertical thermal gradient of the ITF outflow were reconstructed. Records of Core SO18462 were then compared with those from Core 3cBX that was recovered from the western Pacific warm pool (WPWP). The results displayed that similar surface waters occurred in the Timor Sea and the WPWP during the last glacial. Since ∼16 ka, an apparent difference in surface waters between these two regions exists in salinity, indicated by much fresher waters in the Timor Sea than in the WPWP. In contrast, there is little change in difference of sea surface temperatures (SSTs). With regard to thermocline temperature (TT), it increased until ∼11.5 ka since the last glacial, and then remained an overall unchanged trend in the WPWP but continuously decreased in the Timor Sea towards the late Holocene. Since ∼6 ka, thermocline waters have tended to be close to each other in between the Timor Sea and the WPWP. It is indicated that intensified precipitation due to East Asian monsoon and possible ENSO cold phase significantly freshened surface waters over the Indonesian Seas, impeding the ITF surface flow and in turn having enhanced thermocline flow during the Holocene. Consequently, thermocline water of the ITF outflow was cooling and thermocline was shoaling towards the late Holocene. It is speculated that, in addition to strengthening of East Asian winter monsoon, increasing ENSO events during the late Holocene likely played an important role in influencing thermocline depth of the ITF outflow.

La Niña diversity and Northwest Indian Ocean Rim teleconnections

The differences in tropical Pacific sea surface temperature (SST) expressions of El Nino-Southern Oscillation (ENSO) events of the same phase have been linked with different global atmospheric circulation patterns. This study examines the dynamical forcing of precipitation during October–December (OND) and March–May (MAM) over East Africa and during December–March (DJFM) over Central-Southwest Asia for 1950–2010 associated with four tropical Pacific SST patterns characteristic of La Nina events, the cold phase of ENSO. The self-organizing map method along with a statistical distinguishability test was used to isolate La Nina events, and seasonal precipitation forcing was investigated in terms of the tropical overturning circulation and thermodynamic and moisture budgets. Recent La Nina events with strong opposing SST anomalies between the central and western Pacific Ocean (phases 3 and 4), force the strongest global circulation modifications and drought over the Northwest Indian Ocean Rim. Over East Africa during MAM and OND, subsidence is forced by an enhanced tropical overturning circulation and precipitation reductions are exacerbated by increases in moisture flux divergence. Over Central-Southwest Asia during DJFM, the thermodynamic forcing of subsidence is primarily responsible for precipitation reductions, with moisture flux divergence acting as a secondary mechanism to reduce precipitation. Eastern Pacific La Nina events in the absence of west Pacific SST anomalies (phases 1 and 2), are associated with weaker global teleconnections, particularly over the Indian Ocean Rim. The weak regional teleconnections result in statistically insignificant precipitation modifications over East Africa and Central-Southwest Asia.

Regional climate model simulation of winter climate over Central–Southwest Asia, with emphasis on NAO and ENSO effects

AbstractWe present a regional climate modeling study of the winter climate of Central–Southwest Asia (CSWA), focusing on the mean model climatology of temperature and precipitation, the simulated storm characteristics and the effects of El‐Nino Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) on storm activity and precipitation. Forty‐three winter seasons are simulated with the model RegCM3 driven by ERA40 reanalysis fields over a region encompassing CSWA and the Mediterranean sector. The model shows a good performance in reproducing the observed mean surface climate of the region as well as the characteristics of the western disturbances affecting CSWA. Observations show that both the NAO and ENSO have a substantial influence on the CSWA climate, particularly in the region of Northern Pakistan, Afghanistan and Tajikistan, where precipitation increases (decreases) during the positive (negative) NAO phase and warm (cold) ENSO phase. The model captures well the observed signals and a detailed storm analysis shows that they are mostly associated with an intensification of western disturbances originating in the Eastern Mediterranean and Middle East regions and moving eastward across a 500 hPa trough situated over the CSWA during the positive NAO and warm ENSO phases. The NAO and ENSO signals are reinforced by enhanced moisture sources from the Mediterranean, Caspian and Arabian Seas. Our study provides encouraging indications towards the use of the model RegCM3 for climate simulations over CSWA. Copyright © 2009 Royal Meteorological Society

Predictability of stream flow and rainfall based on ENSO for water resources management in Sri Lanka

Summary We investigate the viability of using El Nino-Southern Oscillation (ENSO) and sea surface temperature (SST) data to predict seasonal streamflow for one of the major rivers in Sri Lanka, the Kelani, using correlation analysis, contingency tables, and principal component analysis. The agricultural seasons in Sri Lanka are Yala (April-September) and Maha (October-March). The correlation between the Kelani River streamflow during Yala and ENSO indices (r = � 0.41) is significant at 99% level. In addition, the Kelani streamflow during Yala has a correlation with the Central Indian Ocean SST (r = � 0.40) that is also sig- nificant at the 99% level. The first principal component of the Indo-Pacific Ocean SST is reminiscent of the SST associated with the ENSO mode. A prediction scheme based on this mode for the streamflow during Yala has a skill characterized by a correlation of 0.5 in a cross-validated mode. The prediction of streamflow during Maha is best carried out separately for the two halves of the season. During the El Nino phase, the rainfall during Maha is enhanced during the first half of the season (October-December) and diminished in the second half (Jan- uary-February). Rainfall rather than streamflow has a better relationship with ENSO from October to December. During the second half of the Maha season, rainfall declines with both warm and cold ENSO phases and any prediction scheme has to take into account this non-linear relationship. Overall, useful skill for seasonal streamflow predictions has been demonstrated for the Yala season and skill for seasonal rainfall predictions for the first and second half of the Maha season has been elucidated. a 2006 Elsevier B.V. All rights reserved.

Effect of QBO and ENSO on the Solar Cycle Modulation of Winter North Atlantic Oscillation

The effect of the Quasi-Biennial Oscillation (QBO) and the El Nino Southern Oscillation (ENSO) on the 11-year solar cycle modulation of the winter-mean North Atlantic Oscillation (NAO) is examined through analysis of observational data from 1958 to 2000. It is found that the solar cycle modulation of the NAO is more strongly enhanced in the westerly phase of the 50-hPa QBO wind and the cold phase of ENSO, although separation of these effects is statistically difficult. On these phases, the signal of the winter-mean NAO extends more to the upper stratosphere and summer-AO reappears more strongly in high solar years, whereas the signal is weaker throughout in low solar years.

The response of precipitation and surface hydrology to tropical macro‐climate forcing in Colombia

AbstractIn tropical South America, weather and climate are affected by the meridional oscillation of the inter‐tropical convergence zone, the Pacific and Atlantic oceans, the Amazon basin, and the Andes Mountains. The dominant mode of the inter‐annual climate variability in the tropical Pacific is the El Niño/Southern Oscillation (ENSO) phenomenon. Colombia is among the South American countries that experiences large climate anomalies because of ENSO. However, the hydrologic response in Colombia to a particular ENSO event is highly complex and non‐linear.To describe this complex, non‐linear hydro‐climatic response to large‐scale atmospheric forcing in Colombia, a one‐way coupled atmospheric‐land surface hydrologic modelling approach is implemented whose main components are Colorado State University's Regional Atmospheric Modelling System (RAMS) and the Swedish Hydrologiska Byråns Vattenbalansavdelning‐Hydrologic Modelling System (HBV‐HMS). RAMS is implemented in a two‐nested grid domain for seasonal simulations and a four‐nested grid domain for daily simulations. Seasonal simulations emphasize the annual cycle at regional scales, while daily simulations focus on describing the diurnal cycle for specific watersheds. The HBV‐HMS is implemented for a specific watershed and used to examine its response at daily time scales.Results of the seasonal simulations reproduce quite well the observed spatial and temporal patterns of the hydro‐climatic variables: simulated temperature, pressure, wind velocity, and precipitation distributions show good agreement with observations. Results also show that the 1992 warm phase of ENSO led to a generalized decrease of precipitation along the Andes ranges and that the low lands of Colombia received more precipitation than during the 1989 cold ENSO phase. Similarly, daily‐simulated fields show good agreement with observed values. However, the simulated diurnal cycle of precipitation exhibits a temporal phase lag of several hours and simulated precipitation tends to underestimate both the total amount of precipitation and the duration of precipitation events. Despite these discrepancies, the results of the modelling effort clearly demonstrate the ability of this approach to capture the non‐linear and complex interactions of large‐scale atmospheric forcing and local characteristics as indicated by the satisfactory simulation of stream flow responses for a set of small watersheds. Copyright © 2006 John Wiley & Sons, Ltd.

North Atlantic Oscillation sensitivity to the El Niño/Southern Oscillation polarity in a large-ensemble simulation

A large ensemble modeling experiment with the Melbourne University General Circulation Model is presented. Thirty 17-year-long independent simulations were performed. All integrations were forced by the same observed sea surface temperatures, obtained from the Atmospheric Model Intercomparison Project II. The simulations were analyzed to assess the sensitivity of the North Atlantic Oscillation (NAO) to the El Nino/Southern Oscillation (ENSO) polarity. The results show signals of the ENSO phases both in the mean strength of the NAO as well as in its internal variability. During the cold ENSO phase, the probability density function of the NAO index presents a small but positive mean value, whereas it is negative during the warm ENSO phase. Also, the NAO variability associated with each ENSO phase shows a different behavior: during the warm phase the probability density function of the NAO index presents a larger variance and suggests a bimodality, whereas no bimodality is suggested in the cold phase.

Quantifying the ENSO-Related Shifts in the Intensity and Probability of Drought and Wet Periods in Iran

Abstract This paper quantifies the impact of the El Nino–Southern Oscillation (ENSO) on the intensity and occurrence probability of dry and wet periods in Iran during boreal autumn and winter. Three phases (warm, cold, and neutral) were defined based on the Southern Oscillation (SO) status, and precipitation composites were constructed for each phase. The 30th and 70th percentiles of neutral phases were used as the thresholds for distinguishing normal conditions from dry and wet anomalies, respectively. The shifts in the amount and occurrence probability of these thresholds associated with warm and cold ENSO phases were then quantified. It has been found that, compared to the neutral period, warm events substantially reduce (increase) the intensity and occurrence probability of autumnal drought (wet) periods, particularly for southern districts. On the other hand, when a vigorous La Nina prevails, the chance of wet (dry) conditions is low (high) and the probability of severe autumnal drought is intensifie...

ENSO Impacts on Peak Wind Gusts in the United States

Abstract Changes in the peak wind gust magnitude in association with the warm and cold phases of the El Nino–Southern Oscillation (ENSO) are identified over the contiguous United States. All calculations of the peak wind gust are differences in the extreme phases of ENSO (warm and cold) relative to neutral for all stations in the study that pass the completeness criteria. Monthly composites were created for all years in the study (1 January 1948 through 31 August 1998). The differences in the mean peak wind gust are calculated for each month. A nonparametric statistical test was invoked to determine significant shifts in the extreme phase distributions. Differences in the frequency of gale-force wind gusts were also calculated. The results show a dominant, ENSO cold phase wintertime signal. Regions most greatly affected are the Pacific Northwest, Southwest, the Great Plains, and the region extending from the Great Lakes through the Ohio River valley, and southwest into Texas. During the cold phase months ...

Variations of Teleconnection of ENSO and Interannual Variation in Summer Rainfall in the Central United States

Summer rainfall in the central United States has singular interannual variations of a 3-6-yr period. Identifying the causes of these variations assures improvement in predictions of summer rainfall in the region. A review of previous studies revealed a puzzling situation: the outstanding interannual variations of the summer rainfall in the central United States showed no persistent correlations with known influential interannual variations in the Northern Hemisphere and the El Nino-Southern Oscillation (ENSO). This study was undertaken to identify the cause of this situation and ultimately explain the causes of the observed interannual summer rainfall variations. Its results showed a teleconnection of the ENSO with the summer rainfall in the central United States. The intensity of which has varied over the last 125 years. The teleconnection was active in two epochs, 1871-1916 and 1948-78, and absent in the two epochs 1917-47 and 1979-present. This variation was associated with a multidecadal variation in both sea surface temperature and sea level pressure in the mid- and high-latitude North Pacific. In the epochs of active teleconnection, the circulation in the warm phase of ENSO favored a deformation field in the lower troposphere in the central United States causing wet summers and a reversed circulation in cold phase of ENSO yielding dry summers, a process that partially explains the interannual summer rainfall variations. The result also showed that the variations of the teleconnection were ''in phase'' with the variation in the average surface temperature of the Northern Hemisphere. When the ''abrupt warming'' of the surface temperature developed in 1917-47 and the most recent two decades, the teleconnection broke down. Because of the limitation in data record length, this observed relationship and the persistence of the variation in the teleconnection need further investigations when additional data are available.

Western Pacific interannual variability associated with the El Niño‐Southern Oscillation

Observations of sea surface temperature (SST), sea level pressure (SLP), surface wind, and outgoing longwave radiation (OLR) show that the El Niño‐Southern Oscillation (ENSO) displays western Pacific anomaly patterns in addition to eastern Pacific anomaly patterns. During the warm phase of ENSO, warm SST and low SLP anomalies in the equatorial eastern Pacific and low OLR anomalies in the equatorial central Pacific are accompanied by cold SST and high SLP anomalies in the off‐equatorial western Pacific and high OLR anomalies in the off‐equatorial far western Pacific. Also, while the zonal wind anomalies over the equatorial central Pacific are westerly, those over the equatorial far western Pacific are easterly. The nearly out‐of‐phase behavior between the eastern and western tropical Pacific is also observed during the cold phase of ENSO, but with anomalies of opposite sign. These western Pacific interannual anomaly patterns are robust features of ENSO, independent of data sets. It is argued that equatorial easterly (westerly) wind anomalies over the far western Pacific during the warm (cold) phase of ENSO are initiated by off‐equatorial western Pacific cold (warm) SST anomalies, and that these winds are important for the evolution of ENSO. An atmosphere model is employed to demonstrate that small off‐equatorial western Pacific cold (warm) SST anomalies (compared to those in the east) are sufficient to produce equatorial easterly (westerly) wind anomalies as observed over the far western Pacific. The coupled ocean‐atmosphere model of Zebiak and Cane is then modified to investigate the evolution of the western Pacific interannual anomaly patterns in a coupled ocean‐atmosphere system, by including a meridional structure to the subsurface temperature parameterization in the western Pacific. The modified model produces both western and eastern Pacific interannual anomaly patterns.