El Nino-Southern Oscillation Signal Research Articles

The Observed Relationship between Pacific SST Variability and Hadley Cell Extent Trends in Reanalyses

AbstractReanalysis and other observationally based estimates suggest that the tropics have expanded more than simulated by coupled climate models with historical radiative forcing. Previous research has attempted to reconcile this discrepancy by using climate model simulations with constrained tropical Pacific sea surface temperatures (SSTs) to account for the role of internal variability. Here the relationships between Hadley cell extent and internal SST variability and long-term warming are analyzed using purely observational techniques. Using linearly independent components of SST variability with reanalysis datasets, the statistical relationship between Pacific variability and Hadley cell extent is quantified by time scale. There is a strong correlation between North Pacific decadal SST variability and Southern Hemisphere Hadley cell extent. Conversely, there is a weaker observed relation between El Niño–Southern Oscillation (ENSO) and Hadley cell extent when low-frequency variability is filtered out of the ENSO signal. The observed linear sensitivity of Hadley cell width to long-term warming agrees with coupled general circulation model experiments when accounting for uncertainties, and there is a statistically significant relationship between Northern Hemisphere Hadley cell extent and long-term warming during boreal autumn.

Influence of the QBO on tropical convection and its impact on tropical cyclone activity over the western North Pacific

On the basis of 70 hPa zonal wind (U70)-defined quasi-biennial oscillation (QBO) events, after removing the El Nino–Southern Oscillation (ENSO) signal, the present study investigates the process by which the QBO modulates tropospheric circulation and convection during summer (between July and October), when tropical cyclone (TC) activities enter their peak period. Concurrent with the western phase of the QBO (QBOW), significant tripole pattern modulations over the tropical Indo-Pacific Ocean are regressed onto the residual part of U70 after removing the ENSO signal, with enhanced convection observed over its central branch (0°–10°N, 120°E–180°) and the inactive convection branches located to both sides. A ventilation opening-like effect is exerted on the monsoon trough, which is shifted southward under the QBOW phase. According to the QBO-associated changes in the circulations over the tropical western North Pacific (WNP), equatorial environments (with low-level relative vorticity, high-level divergence, tropospheric vertical wind shear (VWS), and midlevel humidity) tend to be favorable for TC genesis. Consequently, the off-equatorial TC tracks show a significantly decreased occurrence frequency in the northern monsoon trough region. The present study provides a summary sketch showing the QBO-tropospheric circulation modulation process. Considering that the westerly or weak easterly is observed over the upper central Pacific (CP), QBOW phase-associated weak VWS can induce anomalous upward motion at equatorial latitudes, together with the upwelling between the equatorial westerly and off-equatorial easterly, leading to an enlarged convective window over the CP. Affected by the prevailing easterly within the tropics, the actual pumping shifts westward over the tropical WNP.

Linking data of ENSO, NDVI-MODIS and crops yield as a base of an early warning system for agriculture in Córdoba, Argentina

By influencing interannual rainfall variability in the Province of Córdoba, Argentina, El Niño Southern Oscillation (ENSO) also impact on corn and soybean yields at the main rain-fed agricultural production departments. Spectral NDVI data not only capture the environmental impact on vegetation directly, but also can expose it spread all over a region at the same time. As support of an early warning system and decision-making for agriculture, the impact of ENSO on corn and soybean yields anomaly (YA) was analyzed between 2000 and 2017, by using different indicators: SOI, MEI and ONI over the 12 major production departments of Córdoba. Also, NDVI-MODIS data was assessed all over Río Segundo (RS) territory in the center of Córdoba as a vegetation proxy to depict the agricultural activity in a broad sense. To check signal persistency, the relationship was assessed first using the indicators from July to October prior to the sowing date; and then once the growing season has finished, to show the effective ENSO impact on crops productivity.Persistence of ENSO signal was verified, so that the ENSO influence during the growing season quantified remains or intensifies when is compared to information in advance to the start of crop season. While for SOIGS and MEIGS the correlation with corn yield is significant (p < 0.05) in all Córdoba departments, it is always a little lower for ONIGS. Instead, for soybean only 4 departments show significant relationships when using ONIGS, 5 in the case of MEIGS and it increases to 6 when using SOIGS. The particular response of NDVI anomaly (AND) to ENSO was located and quantified for different indices and periods during the crop season. A consistent relationship was found between ENSO and the NDVI as an indicator of agricultural activity in RS territory but, in any case, the area fraction with a significant condition is quite small.The relationship with NDVI is stronger for SOI particularly when a more extended period is considered that include NDVI data of April and May, at the end of the rainy season. While before the beginning of the growing season SOIA-S and SOIS-O reach a significant relationship with the NDVI accumulated since November to May in 11% and 10% of Río Segundo territory, respectively, the percentage increases to 19% when SOIGS is used. Finally, all this information was integrated in an unsupervised cartographic scheme that recognizes different responses of crops yield and NDVI to ENSO in the RS territory. Multiple regression models for corn and soybean yield prediction were developed from NDVI and ENSO indicators for each one of the identified productive sectors. Crops yield can be estimated before harvest with good accuracy, making use of the maximum NDVI value and SOI, both before (SOIA-S) and during the growing season (SOIA-S-O-N-D). As NDVI data capture the ENSO influence on agriculture as well as over crop productivity, its incorporation into a climate monitoring protocol must conforms the basis of an early warning system for a sustainable agriculture in the region.

ENSO teleconnections in an ensemble of CORDEX-CORE regional simulations

Recent high-spatial-resolution regional simulations from the global program, coordinated regional climate downscaling experiment-coordinated output for regional evaluations (CORDEX-CORE), are examined to evaluate the capability of regional climate models (RCMs) to represent the El Nino–Southern Oscillation (ENSO) precipitation and surface air temperature teleconnections over five regions of the world. We find that the ensemble and individual RCM simulations generally preserve the broad regional scale ENSO signal from the general circulation models (GCMs) over different regions around of the world, reproducing the majority of the observed regional responses to ENSO forcing. Furthermore, in some cases, the RCM ensemble and individual models can improve the spatial pattern of teleconnections and the amplitudes of these patterns compared to the driving global models. Among such cases are the precipitation teleconnections over southern Africa, North America and the Arabian–Asian region. Our study presents the first analysis of ENSO teleconnections in GCM-driven RCMs over multiple regions, and it clearly shows the potential value of using such models non only in a climate change research context, but also in seasonal to annual prediction.

Possible Modulation of the Interannual ENSO‐East Asian Winter Monsoon Relationship by the North American Ice Sheets During the Last 21 ka

AbstractObservations have demonstrated an unstable interannual relationship between El Niño/Southern Oscillation (ENSO) and the East Asian winter monsoon in recent decades due to internal variability. However, it remains unclear whether the interannual ENSO‐East Asian winter monsoon relationship (EER) may change with long‐term external forcings, especially when they vary substantially. The results of a set of transient simulations during the last 21,000 years demonstrate the EER is enhanced during the Last Glacial Maximum relative to the Holocene, characterized by a poleward migration of ENSO's signal over the northwest Pacific. The strengthened EER results from the weakened interannual ENSO‐Aleutian Low teleconnection and the northeastward displacement of the climatological Aleutian Low, ultimately forced by the massive North American ice sheets at that time. Our study suggests a changing EER with various external forcings and implies potential uncertainty in applying the present‐day ENSO teleconnections during glacials.

Diagnosis of ENSO-related precipitation changes during the twentieth and twenty-first centuries using reanalyses and two multi-model clusters

Changes in El Nino-Southern Oscillation (ENSO)-related precipitation anomalies during the twentieth century are poorly known. The ENSO-related precipitation anomalies over the tropical Pacific Ocean are projected to shift eastward during the twenty-first century. However, intermodel diversity in simulating and projecting these changes has not been fully studied. Understanding such diversity is vital for providing robust projections of future changes in the ENSO-related precipitation. Here, we use an approach that can cleanly separate ENSO signals from long-term variations to study multi-decadal and centennial changes of ENSO-related precipitation anomalies during the twentieth and twenty-first centuries in two precipitation reanalyses and two groups of the Coupled Model Intercomparison Project Phase 5 (CMIP5) models. The two precipitation reanalyses disagree in the multi-decadal changes during the twentieth century: precipitation anomalies in the twentieth century global circulation reanalysis indicates zonal shifts of the positive and negative ENSO-related precipitation anomalies, whereas reconstructed precipitation derived from observations suggests an intensification of the anomalies. Most members of the ensemble of 20 CMIP5 models project significant strengthening or zonal shift in ENSO-related precipitation during the twenty-first century, which is almost entirely influenced by changes in the ENSO-related circulation from a moisture budget analysis. The ‘good-ENSO’ group with stronger ENSO atmospheric feedbacks projects an intensification of ENSO-related precipitation anomalies over the eastern-central equatorial Pacific, as a result of strengthened deep convection anomalies in the middle to upper troposphere above. This increased variability of convection is driven by a warmer SST mean state of the central-eastern Pacific Ocean. The underperforming group that simulates colder equatorial SSTs, weaker atmospheric feedbacks, and more westward extended ENSO-related precipitation anomalies, exhibits both strong intensification and eastward shift of the entire ENSO-related air–sea interactive system and the related precipitation over the western-central Pacific during the twenty-first century, in correspondence with a mean-state SST warming in the entire equatorial Pacific. Our clustered results suggest that the changes of ENSO-related precipitation are closely tied with the historical simulation of structure of the ENSO-related air–sea system and the projection of changes in the mean-state SST under global warming.

Tree-ring δ18O from Southeast China reveals monsoon precipitation and ENSO variability

Considerable advances have been made in use of tree-ring δ18O for climate reconstructions in Southeast China, the East Asian summer monsoon region. However, the relationships among tree-ring cellulose δ18O (δ18Ocell), local hydroclimate, precipitation δ18O (δ18Opre) and El Niño–Southern Oscillation (ENSO) have not yet been fully resolved. The usual interpretation has been that local hydroclimate influences δ18Ocell through both the “amount effect” on δ18Opre and evaporative enrichment, but it cannot fully explain the high inter-site correlation of δ18Ocell chronologies or their response pattern to ENSO. In this study, we use a newly-developed δ18Ocell chronology of Pinus massoniana from a water-stressed site in Zhejiang province, in combination with another three δ18Ocell chronologies in Southeast China, to investigate their climatic implications from a regional perspective. The results show that besides local hydroclimate, δ18Opre is also significantly correlated with δ18Ocell, but with different effects. Spatially homogeneous δ18Opre variation causes high spatial correlations among δ18Ocell from different sites. Analyses show that ENSO variations are responsible for the large-scale common signals in δ18Ocell by modulating δ18Opre. Therefore, combining δ18Ocell chronologies from different locations can enhance ENSO signals, which provides us new opportunities to reconstruct paleo-ENSO activities. Furthermore, eliminating the part of ENSO signal that is not related to the growing season hydroclimate from δ18Ocell can increase the explained variance of observed values. These findings provide a guide for future optimized ENSO and local paleoclimate reconstructions using δ18Ocell.

Quantifying the impacts of pre-occurred ENSO signals on wheat yield variation using machine learning in Australia

Australia is one of the top wheat exporting countries in the world and the reliable prediction of wheat production plays a key role in ensuring regional and global food security. However, wheat yield in Australia is highly exposed to the impacts of climate variability, especially seasonal rainfall, as wheat is mostly grown in the drylands. Previous studies showed that El Niño Southern Oscillation (ENSO) has a strong influence on Australia's climate and found the ENSO-related phenomena have prognostic features for future climatic conditions. Therefore, we examined the predictability of state-scale variation in Australian wheat yields based on ENSO-related large-scale climate precursors using machine learning techniques. Here, we firstly established a set of random forest (RF, a machine learning method) models based on pre-occurred climate indices to forecast spring rainfall for the four major wheat producing states of Australia, the forecasted rainfall was then combined with selected precedent climate drivers to predict yield variations using another set of RF models for each state. We explored the most influential variables in determining spring rainfall and yield variation. We found that the first set of RF models accounted for 43-59% of the change in spring rainfall across the four states. By incorporating forecasted spring rainfall with selected ENSO climate indices, the RF model accounted for 33-66% of the variation in yield which was greater than the 22-50% of yield variations explained by ENSO-related indices alone. The results suggest that wheat yield variation at a state level could be reliably forecasted at lead-times of three months prior to the commencement of harvest. We also found that forecasted spring rainfall and precedent Southern Oscillation Index (SOI) in July were the most important factors in estimation of crop yield in the winter dominant rainfall states. ENSO climate indices are easy to obtain and can be rapidly used to drive the forecasting model. Therefore, we believe the proposed models for predicting wheat yield variations at three-month lead time would be helpful for state governments and policy makers to develop effective planning to reduce monetary loss and ensure food security.

Hydroclimate variations in the Northern China Plain and their possible socio-cultural influences

ABSTRACT The Northern China Plain is China’s traditional and important agricultural production base, where climate change in the past has significantly affected China in many aspects such as national agricultural policies, ecological construction and foreign trade exchanges. High-resolution paleoclimatology research reveals that the precipitation, relative humidity, and drought index in this area have a significantly variability in the past few centuries, and spectral analysis shows that each time series has an obviously short period coinciding with the El Niño-Southern Oscillation (ENSO) cycle. In this study, we summarized that ENSO signals in hydroclimate variations of northern China are probably from the Indian water vapor partly. In addition, the possible impact of hydroclimate variations in northern China on social and cultural events are briefly discussed considering historical records.

A proxy modelling approach to assess the potential of extracting ENSO signal from tropical Pacific planktonic foraminifera

Abstract. A complete understanding of past El Niño–Southern Oscillation (ENSO) fluctuations is important for the future predictions of regional climate using climate models. One approach to reconstructing past ENSO dynamics uses planktonic foraminifera as recorders of past climate to assess past spatio-temporal changes in upper ocean conditions. In this paper, we utilise a model of planktonic foraminifera populations, Foraminifera as Modelled Entities (FAME), to forward model the potential monthly average δ18Oc and temperature signal proxy values for Globigerinoides ruber, Globigerinoides sacculifer, and Neogloboquadrina dutertrei from input variables covering the period of the instrumental record. We test whether the modelled foraminifera population δ18Oc and Tc associated with El Niño events statistically differ from the values associated with other climate states. Provided the assumptions of the model are correct, our results indicate that the values of El Niño events can be differentiated from other climate states using these species. Our model computes the proxy values of foraminifera in the water, suggesting that, in theory, water locations for a large portion of the tropical Pacific should be suitable for differentiating El Niño events from other climate states. However, in practice it may not be possible to differentiate climate states in the sediment record. Specifically, comparison of our model results with the sedimentological features of the Pacific Ocean shows that a large portion of the hydrographically/ecologically suitable water regions coincide with low sediment accumulation rate at the sea floor and/or of sea floor that lie below threshold water depths for calcite preservation.

Generalized Synchronization Between ENSO and Hydrological Variables in Colombia: A Recurrence Quantification Approach

We use Recurrence Quantification Analysis (RQA) to study features of Generalized Synchronization (GS) between El Niño-Southern Oscillation (ENSO) and monthly hydrological anomalies (HyAns) of rainfall and streamflows in Colombia. To that end, we check the sensitivity of the RQA concerning diverse HyAns estimation methods, which constitutes a fundamental procedure for any climatological analysis at inter-annual timescales. In general, the GS and its sensitivity to HyAns methods are quantified by means of time-lagged joint recurrence analysis. Then, we link the GS results with the dynamics of major physical mechanisms that modulate Colombia's hydroclimatology, including the Caribbean, the CHOCO and the Orinoco Low-Level Jets (LLJs), and the Cross-Equatorial Flow (CEF) over northwestern Amazonia (southern Colombia). Our findings show that RQA exhibits significant differences depending on the HyAns methods. GS results are similar for the HyAns methods with variable annual cycle but the time-lags seem to be sensitive. On the other hand, our results make evident that HyAns in the Pacific, Caribbean and Andean regions of Colombia exhibit strong (weak) GS with the ENSO signal during La Niña (El Niño), when hydrological anomalies are positive (negative). Results from the GS analysis allow us to identify spatial patterns of non-linear dependence between ENSO and the Colombian's climatology. The mentioned moisture transport sources constitute the interdependence mechanism and contribute to explain hydrological anomalies in Colombia during the phases of ENSO. During La Niña (El Niño), GS is strong (weak) for the Caribbean and the CHOCO LLJs. Moreover, moisture advection by the Caribbean and CHOCO LLJs exhibit synchrony with HyAns at 0 to 2 (2 to 4) month-lags over north-western Colombia. Furthermore, our results suggest a strong (weak) GS between negative (positive) Sea Surface Temperatures (SST) anomalies in the Eastern Pacific and rainfall anomalies in Colombia. In contrast, GS is strong (weak) for positive (negative) SST anomalies in the Central Pacific. Our GS results contribute to advance our understanding on the regional effects of ENSO in Colombia, whose socio-economical, environmental and ecological impacts cannot be overstated. This work provides a novel approach that reveals new insights into the impact of ENSO on northern South America.

Climatology and Interannual Variability of Floods during the TRMM Era (1998–2013)

AbstractSpatial and temporal variations of global floods during the TRMM period (1998–2013) are explored by means of the outputs of the Dominant River Routing Integrated with VIC Environment model (DRIVE) driven by the precipitation rates from the TRMM Multisatellite Precipitation Analysis (TMPA). Climatological and seasonal mean features of floods including frequency (FF), duration (FD), and mean and total intensity (FI and FTI) are examined and further compared to those for a variety of precipitation indices derived from the daily TMPA rain rates. In general, floods and precipitation manifest similar spatial distributions, confirming that more precipitation (both amount and frequency) often indicates higher probability of floods. However, different flood indices can be associated with different precipitation characteristics with a highly region-dependent distribution. FF and FD tend to be more related to daily precipitation frequency globally, especially the mid- to high-end precipitation frequencies (F10, F25, F50). However, FI and FTI tend to be more associated with the mean volume/magnitude of those (extreme) daily precipitation events (Pr10 and Pr25). Nonetheless, daily precipitation intensity except the very high end one (R50) generally has a relatively weak effect on floods. The precipitation–flood relations at the 10 large regions are further examined, providing an improved understanding of precipitation-related flood-generating mechanisms in different locations. On the interannual time scale, El Niño–Southern Oscillation (ENSO) can significantly affect floods in many flood-prone zones. However, it is noted that even though the ENSO effect on floods is mostly through modulating various aspects of precipitation events, significant ENSO signals in precipitation cannot always translate to an effective, simultaneous impact on floods.

An alternative approach for quantitatively estimating climate variability over China under the effects of ENSO events

Abstract Previous studies generally use the traditional composite analysis to diagnose the physical interrelationships between the El Nino-Southern Oscillation (ENSO) and climate variables such as temperature and precipitation. This study presents a simple probabilistic tool for quantifying changes in precipitation and temperature in the wet season over China during either developing or decaying phases of El Nino and La Nina events with a particular focus on the extreme conditions. We first construct the joint dependence structure between each climate variable (e.g. precipitation and temperature) and ENSO using a variety of bivariate copulas. We then examine variations of climate variables related to ENSO conditions through the conditioning sets of bivariate copulas. This approach allows a quantitative estimation of precipitation and temperature anomalies and a delineation of their spatial pattern across the country under individual effects of the developing and decaying phases of ENSO events. Comparison of results produced by the conditional probabilistic approach with those by conventional composite analysis reveals large similarity, highlighting the robustness of the presented approach in examining the response of climate variations to ENSO phases and its potential for a broader application in other regional/global diagnoses. Of particular importance is that this approach offers a way to yield probabilistic predictive information on extreme climate anomalies conditioned by ENSO signals. Despite only ENSO's effect considered in the current study, the presented approach could also be used to detect the effect of other large-scale climate signals on regional or global climate variations.

Dynamics of the ENSO Teleconnection and NAO Variability in the North Atlantic–European Late Winter

AbstractThe winter extratropical teleconnection of El Niño–Southern Oscillation (ENSO) in the North Atlantic–European (NAE) sector remains controversial, concerning both the amplitude of its impacts and the underlying dynamics. However, a well-established response is a late-winter (January–March) signal in sea level pressure (SLP) consisting of a dipolar pattern that resembles the North Atlantic Oscillation (NAO). Clarifying the relationship between this “NAO-like” ENSO signal and the actual NAO is the focus of this study. The ENSO–NAE teleconnection and NAO signature are diagnosed by means of linear regression onto the sea surface temperature (SST) Niño-3.4 index and an EOF-based NAO index, respectively, using long-term reanalysis data (NOAA-20CR, ERA-20CR). While the similarity in SLP is evident, the analysis of anomalous upper-tropospheric geopotential height, zonal wind, and transient-eddy momentum flux, as well as precipitation and meridional eddy heat flux, suggests that there is no dynamical link between the phenomena. The observational results are further confirmed by analyzing two 10-member ensembles of atmosphere-only simulations (using an intermediate-complexity and a state-of-the-art model) with prescribed SSTs over the twentieth century. The SST-forced variability in the Northern Hemisphere is dominated by the extratropical ENSO teleconnection, which provides modest but significant SLP skill in the NAE midlatitudes. The regional internally generated variability, estimated from residuals around the ensemble mean, corresponds to the NAO pattern. It is concluded that distinct dynamics are at play in the ENSO–NAE teleconnection and NAO variability, and caution is advised when interpreting the former in terms of the latter.

Towards Probabilistic Multivariate ENSO Monitoring

AbstractA probabilistic approach to describing El Niño–Southern Oscillation (ENSO), based on consideration of the signal‐to‐noise ratio for the coupled ocean‐atmosphere ENSO state, is presented. The ENSO signal is estimated using an ensemble of historical atmospheric model simulations forced by observed sea surface temperatures and sea ice during 1980–2016. The noise is estimated from departures of individual model realizations from their ensemble average when subjected to identical forcing. It is found that this atmospheric noise effect is substantial and yields considerable uncertainty in detecting the true coupled ENSO mode. This uncertainty exceeds analysis errors by an order of magnitude. Greater atmospheric noise is found to prevail during El Niño than La Niña, suggesting that the intensity of the monitored ENSO state during El Niño is prone to greater misattribution. Our results demonstrate that a deterministic state estimate of ENSO conditions may not be representative of the true real‐time coupled ENSO mode.

Dominant modes of winter precipitation variability over Central Southwest Asia and inter-decadal change in the ENSO teleconnection

This study contributes to an improved understanding of Central Southwest Asia (CSWA) wintertime (November–April) precipitation by analyzing the dominant spatial–temporal modes of the regional winter precipitation and examining their relationship with global sea surface temperature (SST) and large-scale atmospheric circulation fields, for 1950/51–2014/15. Empirical orthogonal function (EOF) analysis results show that the first mode (EOF-1) of winter precipitation is characterized by a mono-sign pattern, with significant links to El Nino-Southern Oscillation (ENSO). EOF-2 displays a north–south dipole related to the latitudinal shift in the jet stream position, while the west–east dipole in EOF-3 appears to be influenced by the thermal contrast between the equatorial regions and higher latitudes. Further, we focus on the interdecadal change of connection between wintertime ENSO and EOF-1 observed around the early 1980s. The relationship is weak during 1950/51–1983/84 (P1), but strong and statistically significant in 1984/85–2014/15 (P2). In P1, without the ENSO signal, EOF-1 related atmospheric circulation anomalies are confined mainly over the mid- to high-latitudes of the Northern Hemisphere, associated with the extratropical East Atlantic–Western Russia (EA–WR) teleconnection. In P2, however, a close connection to the tropical Pacific is observed that includes a pronounced SST expression similar to that of ENSO. As a result of the increased relationship between ENSO and EOF-1 in P2, the regional impact of EA–WR observed over CSWA in P1 is linearly superimposed by the anomalous hemispheric–wide atmospheric response forced by the ENSO conditions in the Pacific.

Analysis of summer Antarctic sea ice anomalies associated with the spring Indian Ocean dipole

Abstract In this study, we applied the empirical orthogonal function (EOF) to analyze the anomalous characteristics of the summer Antarctic sea ice. Through correlation analysis, it is found that two strong signals existed in the spring Indian Ocean. These two signals constitute the Indian Ocean dipole (IOD). Although the IOD often occurs with El Nino-Southern Oscillation (ENSO), the first mode of the EOF is mainly resultant from the IOD. The IOD can cause sea ice anomalies in the Atlantic and Pacific Oceans, as well as the Weddell and Ross Seas. When the ENSO signal in the IOD was removed, the sea-ice anomalies in the Pacific Ocean and the eastern Ross Sea disappear, and the sea ice in the Atlantic Ocean decreases and moves westward. However, the sea ice anomalies in the Weddell Sea remain almost unchanged. These results demonstrate that the IOD excites geopotential height anomalies, causing anomalous atmospheric circulation, resulting in an increase (decrease) in sea ice caused by cold (warm) meridional flow.

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

ABSTRACTThe 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.

Multidecadal Modulation of the ENSO Teleconnection to Precipitation and Tree Growth Over Subtropical North America

AbstractThe teleconnection of the El Niño/Southern Oscillation (ENSO) to instrumental precipitation and temperature during the cool season over North America is strongest and most temporally stable in the TexMex sector of northern Mexico and the borderlands of southwestern United States. The ENSO impact on North American hydroclimate expands and contracts out of this region on multidecadal timescales, possibly associated with the positive and negative phases of the Atlantic Multidecadal Oscillation. A subset of tree‐ring chronologies from the TexMex sector also has the strongest and most stable ENSO signal detected in the North American network, similar to the strong ENSO signal measured in instrumental climate data from the same region. This subset of chronologies is used to reconstruct the multivariate ENSO index (MEI) as a measure of ENSO impact on North American hydroclimate during the instrumental and preinstrumental eras. The reconstruction exhibits improved fidelity in the frequency domain and better registration of spatial changes in ENSO signal over North America when compared to an MEI reconstruction based on all ENSO‐correlated tree‐ring chronologies irrespective of temporal stability of correlation. When correlated with gridded instrumental and tree‐ring reconstructed Palmer drought indices across North America, the stable MEI estimate reproduces the changes in spatial impact of ENSO signal measured with instrumental data, and it reveals similar multidecadal changes in prehistory, potentially linked to the Atlantic Multidecadal Oscillation. The Great Plains drought of the 1850s and 1860s may have been an example of this Pacific‐Atlantic configuration.

High‐Resolution Modeling of ENSO‐Induced Precipitation in the Tropical Andes: Implications for Proxy Interpretation

AbstractSediment records from Lake Pallcacocha, Ecuador, have been interpreted as proxies of El Niño–Southern Oscillation (ENSO) variability, owing to increased precipitation in the area during El Niño events. However, the lake's watershed receives precipitation from processes arising from both the eastern and western Andes, where ENSO has different impacts; this has led to ambiguity in observed regional ENSO signals and has consequently challenged the suitability of the lake's records as ENSO proxies. Here, a mesoscale weather prediction model is used to investigate the regional circulation dynamics and precipitation response during different ENSO events, namely, Eastern Pacific (EP), Central Pacific (CP), coastal El Niño (COA), and La Niña (LN). The region receives more accumulated precipitation during COA and LN compared to EP and CP events. However, during EP and COA events, the region is prone to extreme precipitation associated with convective bursts originating from the Pacific. During CP and LN, moisture originates from the Atlantic and may reach the area as broader‐scale less‐intense precipitation. Statistical analysis of modeled precipitation reveals consistency between the number of threshold‐exceeding precipitation events in the high Andean elevations and the number of events identified in the late Holocene Pallcacocha record. These results illustrate the importance of considering ENSO flavors when interpreting paleoclimate proxies, highlight the role of COA events in understanding eastern Pacific proxy records, and support the hypothesis that Holocene changes in the number of events recorded in the lake sediment may indicate a change in the relative frequency of ENSO flavors.