El Nino Southern Oscillation Frequency Research Articles

The Hybrid Recharge Delayed Oscillator: A More Realistic El Niño Conceptual Model

Abstract El Niño–Southern Oscillation (ENSO) is the leading mode of climate interannual variability, with large socioeconomical and environmental impacts, potentially increasing with climate change. Improving its understanding may shed further light on its predictability. Here we revisit the two main conceptual models for explaining ENSO cyclic nature, namely, the recharge oscillator (RO) and the advective–reflective delayed oscillator (DO). Some previous studies have argued that these two models capture similar physical processes. Yet, we show here that they actually capture two distinct roles of ocean wave dynamics in ENSO’s temperature tendency equation, using observations, reanalyses, and Climate Model Intercomparison Project (CMIP) models. The slow recharge/discharge process mostly influences central-eastern Pacific by favoring warmer equatorial undercurrent and equatorial upwelling, while the 6-month delayed advective–reflective feedback process dominates in the western-central Pacific. We thus propose a hybrid recharge delayed oscillator (RDO) that combines these two distinct processes into one conceptual model, more realistic than the RO or DO alone. The RDO eigenvalues (frequency and growth rate) are highly sensitive to the relative strengths of the recharge/discharge and delayed negative feedbacks, which have distinct dependencies to mean state. Combining these two feedbacks explains most of ENSO frequency diversity among models. Thanks to the two different spatial patterns involved, the RDO can even capture ENSO spatiotemporal diversity and complexity. We also develop a fully nonlinear and seasonal RDO, even more robust and realistic, investigating each nonlinear term. The great RDO sensitivity may explain the observed and simulated richness in ENSO’s characteristics and predictability. Significance Statement El Niño and La Niña events, and the related Southern Oscillation, cause the largest year-to-year variations of Earth’s climate. Yet the theories behind them are still debated, with two main conceptual models being the recharge oscillator and the delayed oscillator. Our purpose here is to address this debate by developing a more realistic theory, a hybrid recharge delayed oscillator. We show how simple yet realistic it is, with equivalent contributions from the slow recharge process and from the faster delayed feedback. It even captures the observed El Niño and La Niña diversity in space and in frequency. Future studies could use the simple theoretical framework provided here to investigate El Niño–Southern Oscillation (ENSO) in observations, theories, climate models diagnostics and forecasts, and global warming projections.

Biomass Burning Emissions of Black Carbon over the Maritime Continent and ENSO Variability

Abstract Fire emissions from the Maritime Continent (MC) over the western tropical Pacific are strongly influenced by El Niño–Southern Oscillation (ENSO), posing various climate effects to the Earth system. In this study, we show that the historical biomass burning emissions of black carbon (BCbb) aerosol in the dry season from the MC are strengthened in El Niño years due to the dry conditions. The eastern Pacific type of El Niño exerts a stronger modulation in BCbb emissions over the MC region than the central Pacific type of El Niño. Based on simulations using the fully coupled Community Earth System Model (CESM), the impacts of increased BCbb emissions on ENSO variability and frequency are also investigated in this study. With BCbb emissions from the MC scaled up by a factor of 10, which enables the identification of climate response from the internal variability, the increased BCbb heats the local atmosphere and changes land–sea thermal contrast, which suppresses the westward transport of the eastern Pacific surface water. It leads to an increase in sea surface temperature in the eastern tropical Pacific, which further enhances ENSO variability and increases the frequency of extreme El Niño and La Niña events. This study highlights the potential role of BCbb emissions on extreme ENSO frequency, and this role may be increasingly important in the warming future with higher wildfire risks.

Assessing the future influence of the North Pacific trade wind precursors on ENSO in the CMIP6 HighResMIP multimodel ensemble

The El Niño Southern Oscillation (ENSO), as one of the largest coupled climate modes, influences the livelihoods of millions of people and ecosystems survival. Thus, how ENSO is expected to behave under the influence of anthropogenic climate change is a substantial question to investigate. In this paper, we analyze future predictions of specific traits of ENSO, in combination with a subset of well-established precursors—the Trade Wind Charging and North Pacific Meridional Mode (TWC/NPMM). We study it across three sets of experiments from a protocol-driven ensemble from CMIP6—the High Resolution Model Intercomparison Project (HighResMIP). Namely, (1) experiments at constant 1950’s radiative forcings, and (2) experiments of present (1950–2014) and (3) future (2015–2050) climate with prescribed increasing radiative forcings. We first investigate the current and predicted spatial characteristics of ENSO events, by calculating area, amplitude and longitude of the Center of Heat Index (CHI). We see that TWC/NPMM-charged events are consistently stronger, in both the presence and absence of external forcings; however, as anthropogenic forcings increase, the area of all ENSO events increases. Since the TWC/NPMM-ENSO relationship has been shown to affect the oscillatory behavior of ENSO, we analyze ENSO frequency by calculating CHI-analogous indicators on the Continuous Wavelet Transform (CWT) of its signal. With this new methodology, we show that across the ensemble, ENSO oscillates at different frequencies, and its oscillatory behavior shows different degrees of stochasticity, over time and across models. However, we see no consistent indication of future trends in the oscillatory behavior of ENSO and the TWC/NPMM-ENSO relationship.

Marine ecological effect of typhoon influenced by the frequency-phases of ENSO and anthropogenic activity: the East China Sea example

In coastal regions, typhoons are one of the most frequent natural disasters, and instrumental data indicate that they can greatly boost marine primary productivity. Typhoons' long-term ecological repercussions, however, are still little understood. To investigate the ecological effects of typhoons since the mid-Holocene, we focused on the sediment cores in the East China Sea, with key parameters of organic CN and Bio-Si to reveal the paleo-productivity variation. Over a millennium, the occurrence of El Niño-Southern Oscillation (ENSO) has had a substantial impact on primary productivity in the southern East China Sea. The primary productivity in the southern East China Sea was risen by humid climate conditions in ∼3.0–2.0 ka and frequent typhoons in 1.5–0 ka. With a millennial period, ENSO frequency has been significantly influencing primary productivity in the southern East China Sea since the mid-Holocene. During 1.5–0 ka, frequent ENSO events have led to more frequent typhoons, which caused terrestrial materials input and water vertically mix in the southern East China Sea, ultimately increasing primary productivity. Particularly on a centennial time scale, ENSO phases determined how much primary productivity was increased, and this magnitude was bigger during the La Niña-like periods than it was during the El Niño-like periods since La Niña is when typhoons occur more frequently. During the Little Ice Age, the cold climate hindered the reproduction of phytoplankton, but increased human activities in Taiwan resulted in more soil erosion and the supply of terrestrial-derived nutrients into the sea, then amplified the typhoon-induced increase in primary productivity. As a result, this stage showed the highest primary productivity since the mid-Holocene. This study reveals that frequent typhoons promote marine primary productivity on centennial-millennial timescales and emphasizes that human activities amplify the ecological effect of typhoons in recent centuries.

Changes in ENSO Characteristics in Model Simulations with Considerably Altered Background Climate States

Abstract Changes in the background climate are known to affect El Niño–Southern Oscillation (ENSO) by altering feedbacks that control ENSO’s characteristics. Here, the sensitivity of ENSO variability to the background climate is investigated by utilizing two Community Earth System Model, version 1 (CESM1), simulations in which the solar constant is altered by ±25 W m−2. The resulting stable warm and cold climate mean state simulations differ in terms of ENSO amplitude, frequency, diversity, asymmetry, and seasonality. In the warm run, ENSO reveals a larger amplitude and occurs at higher frequencies relative to the cold and control runs as well as observations. The warm run also features more eastern Pacific El Niños, an increased asymmetry, and a stronger seasonal phase locking. These changes are linked to changes in the mean state via the amplifying and damping feedbacks. In the warm run, a shallower mean thermocline results in a stronger subsurface–surface coupling, whereas the cold run reveals reduced ENSO variability due to a reduced Bjerknes feedback in accordance with a deeper mean thermocline and enhanced surface wind stress. A strong zonal advective and upwelling feedback further contribute to the large ENSO amplitude in the run with a warmer mean state. In the cold run, ENSO events are partly forced by anomalous shortwave radiation. However, in light of the large temperature contrast between the simulations of up to 6 K in the tropical Pacific, the relatively small changes in ENSO variability highlight the robustness of ENSO dynamics under vastly different climate mean states.

Intensification of ENSO frequency drives forest disturbance in the andes during the holocene

The biodiverse montane forests of the tropical Andes are today frequently disturbed by rainfall-driven mass movements which occur mostly during extreme El Niño events. Over the coming decades these events are projected to double under the 1.5 °C global warming scenario. The consequent increased rainfall and mass movement events likely present an elevated risk to millions of people living in the Andes. However, the impact of more frequent rainfall extremes remains unclear due to a lack of studies that directly link past changes in El Niño-Southern Oscillation (ENSO) frequency to forest and landscape disturbance patterns. Here, we present the first Holocene palaeoecological record from Laguna Pallcacocha, southern Ecuador, a key site for El Niño reconstructions. We demonstrate that for the past 10,000 years plant taxa indicative of recolonization – such as Alnus acuminata – covary with El Niño-induced flood layers in the lake. An amplified forest disturbance pattern is observed in the late Holocene, suggesting enhanced slope instability following deforestation. The temporal pattern is not explained by tree line fluctuations or human impact, while the latter does amplify the impact of ENSO on landscape disturbance. Spatial correlations between modern ENSO and precipitation are consistent with a regional comparison of Holocene records of landscape disturbance. Our results indicate that climate extremes, such as those associated with future intensification of El Niño, combined with ongoing land use change will increase the frequency of mass movements elevating risks for millions of people in the Andes.

Relationships Between Air‐Sea CO2 Flux and New Production in the Equatorial Pacific

AbstractThe equatorial Pacific is the largest oceanic source of carbon dioxide to the atmosphere. This outgassing varies depending on the El Niño‐Southern Oscillation (ENSO) and decadal climate variability. New production, the amount of phytoplankton net primary production driven by upwelled nitrate, plays a significant role in modulating air‐sea CO2 fluxes as the biological carbon pump removes carbon from the surface ocean. We aim to understand how the physical drivers of sea surface temperature and wind speed influence interannual and decadal variability of the equatorial Pacific carbon cycle. In the equatorial Pacific, there are three biogeochemical regimes: the upwelling cold tongue east of 140°W and south of the equator (3°N–15°S); the eastern Pacific warm pool north of the equator (3°–15°N); and the 28.5°C western Pacific warm pool, west of 140°W. We find that between 2000 and 2020, air‐sea CO2 flux and ΔpCO2 increased in the cold tongue (45 mmolC m−2 yr−2, 1.5 μatm yr−1, respectively) but decreased elsewhere, while new production decreased everywhere. The western Pacific occasionally became a weak carbon sink, depending on ENSO and this sink was strongest at 165°E during central Pacific “Modoki” El Niño events. We find that changes in wind speed, temperature and ENSO frequency have altered the surface carbon budget. The mean basin‐wide (150°E−90°W and 15°N–15°S) new production for 2000–2020 was 1.2 ± 0.1 PgC yr−1 and air‐sea CO2 flux was 0.5 ± 0.1 PgC yr−1. New production decreased at −7.7 ± 1.6 TgC yr−2, compared to the CO2 flux trend of −1.7 ± 1.4 TgC yr−2.

Unraveling forced responses of extreme El Niño variability over the Holocene.

Uncertainty surrounding the future response of El Niño–Southern Oscillation (ENSO) variability to anthropogenic warming necessitates the study of past ENSO sensitivity to substantial climate forcings over geological history. Here, we focus on the Holocene epoch and show that ENSO amplitude and frequency intensified over this period, driven by an increase in extreme El Niño events. Our study combines new climate model simulations, advances in coral proxy system modeling, and coral proxy data from the central tropical Pacific. Although the model diverges from the observed coral data regarding the exact magnitude of change, both indicate that modern ENSO variance eclipsed paleo-estimates over the Holocene, albeit against the backdrop of wide-ranging natural variability. Toward further constraining paleo-ENSO, our work underscores the need for multimodel investigations of additional Holocene intervals alongside more coral data from periods with larger climate forcing. Our findings implicate extreme El Niño events as an important rectifier of mean ENSO intensity.

Contribution of Ekman Transport to the ENSO Periodicity Estimated With an Extended Wyrtki Index

AbstractOff‐equatorial wind stress curl anomalies that mainly drive geostrophic transport are essential for the phase transition of the El Niño‐Southern Oscillation (ENSO) induced by a recharge‐discharge process. The ENSO‐induced zonal wind stress anomaly also drives surface Ekman currents, which may counteract the geostrophic transport, but its effect on ENSO periodicity remains unclear in the recharge oscillator theory. Here, we extended an ENSO diagnostic framework, called the Bjerknes‐Wyrtki‐Jin index, to evaluate Ekman transport. The extended method applied to reanalysis data sets shows that the ENSO period lengthens by approximately 30%, reconciling the discrepancy between the observed ENSO frequency and estimates from the conventional method. This result demonstrates that the self‐inhibitory process by Ekman transport is an inherent feature of the observed ENSO periodicity. In addition, Ekman transport efficiently works with a wide meridional structure of ENSO, suggesting that the meridional width of individual ENSO events explains the difference in their duration.

Late Holocene ENSO-related fire impact on vegetation, nutrient status and carbon accumulation of peatlands in Jambi, Sumatra, Indonesia

Peat fires in Indonesia are predominantly anthropogenic and intensified by El Niño Southern Oscillation (ENSO)-related droughts. In recent decades, peat fires, which typically occurred in degraded areas (partially/entirely deforested peatlands), released massive amounts of carbon by burning the peat swamp vegetation and peat substrate. However, it could also influence atmospheric nutrient deposition, have a fertilizing effect on peatlands, and enhance their productivity. Little is known about how ENSO-related peat fires would affect the vegetation in times of little or no human impact. It is also unclear whether in the past such an ash-fertilization effect also occurred in the long term and how fires affected peatland carbon sequestration, which is one of their important ecosystem services. In order to understand this, we examine the fire regime, vegetation composition, nutrient status and carbon (C) accumulation rates of the Air Hitam and Sungai Buluh peatlands in Jambi, Sumatra, Indonesia during the Holocene. The results suggest that increased ENSO frequency and magnitude in the Late Holocene increased the fire occurrence and severity in both sites. During times of little human influence, those fires had little effect on peatland vegetation composition. The coincidence of more frequent fires, a nutrient enrichment and an increased carbon accumulation rate in the Late Holocene suggest that ash-fertilization might have also played a role in the recent past. Thus, low-severity fires could potentially enhance the peatlands' capacity to accumulate carbon if the water table is sufficiently high.

Spurious North Tropical Atlantic precursors to El Ni\xf1o

The El Niño-Southern Oscillation (ENSO), the primary driver of year-to-year global climate variability, is known to influence the North Tropical Atlantic (NTA) sea surface temperature (SST), especially during boreal spring season. Focusing on statistical lead-lag relationships, previous studies have proposed that interannual NTA SST variability can also feed back on ENSO in a predictable manner. However, these studies did not properly account for ENSO’s autocorrelation and the fact that the SST in the Atlantic and Pacific, as well as their interaction are seasonally modulated. This can lead to misinterpretations of causality and the spurious identification of Atlantic precursors for ENSO. Revisiting this issue under consideration of seasonality, time-varying ENSO frequency, and greenhouse warming, we demonstrate that the cross-correlation characteristics between NTA SST and ENSO, are consistent with a one-way Pacific to Atlantic forcing, even though the interpretation of lead-lag relationships may suggest otherwise.

Stalagmite flooding frequency record since the middle Little Ice Age from Central China

Comparison of the climate, especially extreme climate changes between the Little Ice Age (LIA) and the present, could provide important information for the predication of extreme climate change in the future. Based on a laminated stalagmite from the middle reaches of the Yangtze River, Central China, an annually resolved stalagmite record of growth rate has been produced dating from 1640 AD. An 11-year running variance sequence of the growth rate shows a positive correlation with a local historical flooding frequency record from 1640 to 1920 AD, suggesting that the stalagmite growth rate variance could be used as a local flooding frequency indicator. In the middle Yangtze catchment, the flooding frequency during the late LIA (1750–1900 AD) is much higher than during the middle LIA (1640–1750 AD) and the early twentieth century (1900–1920 AD). The discrepancy between the stalagmite growth rate variance and the flooding frequency after 1920s AD suggests that the scale of modern flooding is possibly larger than during the earlier periods. Spectral power analysis on growth rate variance of the stalagmite suggests that the flooding cycles around the middle Yangtze River catchment may relate to El Nino/Southern Oscillation (ENSO). The comparison between the stalagmite growth rate variance and reconstructed ENSO records further suggests a link between strong ENSO and high flooding frequency in the middle Yangtze catchment.

Comparison of past and future simulations of ENSO in CMIP5/PMIP3 and CMIP6/PMIP4 models

Abstract. El Niño–Southern Oscillation (ENSO) is the strongest mode of interannual climate variability in the current climate, influencing ecosystems, agriculture, and weather systems across the globe, but future projections of ENSO frequency and amplitude remain highly uncertain. A comparison of changes in ENSO in a range of past and future climate simulations can provide insights into the sensitivity of ENSO to changes in the mean state, including changes in the seasonality of incoming solar radiation, global average temperatures, and spatial patterns of sea surface temperatures. As a comprehensive set of coupled model simulations is now available for both palaeoclimate time slices (the Last Glacial Maximum, mid-Holocene, and last interglacial) and idealised future warming scenarios (1 % per year CO2 increase, abrupt four-time CO2 increase), this allows a detailed evaluation of ENSO changes in this wide range of climates. Such a comparison can assist in constraining uncertainty in future projections, providing insights into model agreement and the sensitivity of ENSO to a range of factors. The majority of models simulate a consistent weakening of ENSO activity in the last interglacial and mid-Holocene experiments, and there is an ensemble mean reduction of variability in the western equatorial Pacific in the Last Glacial Maximum experiments. Changes in global temperature produce a weaker precipitation response to ENSO in the cold Last Glacial Maximum experiments and an enhanced precipitation response to ENSO in the warm increased CO2 experiments. No consistent relationship between changes in ENSO amplitude and annual cycle was identified across experiments.

The adoption of agropastoralism and increased ENSO frequency in the Andes

Humans have been present in the Andes since about 12,000 calibrated years before present (cal yr BP), transitioning from hunter-gatherers to agropastoralist societies in the mid-Holocene. Yet, the timing and effects of this change in behavior on the ecosystem are largely unknown. Using titanium from XRF analysis, loss-on-ignition (LOI), fossil pollen, Sporormiella, and charcoal, we present a high-resolution 12,000-yr paleoecological history of Lake Llaviucu, a mid-elevation lake in the Ecuadorian Andes. For the entire period of study, humans were active in this landscape, with signs of agropastoralism being present since c. 6100 cal yr BP. Apparent ENSO activity, inferred from titanium inputs to the sediment is also heightened at c. 6100 cal yr BP. A trajectory of increasing landuse seems to have peaked in terms of maize production between c. 2900 and 800 cal yr BP. Thereafter, larger abundances of Sporormiella spores suggest that pastoralism was of increasing importance; a pattern disrupted by European arrival. Apparent peaks in El Niño Southern Oscillation (ENSO) appears to be linked to fire and maize frequency. We investigated the relationship between the Lake Llaviucu record and that of nearby Lake Pallcacocha, which is often cited as providing a regional ENSO history.

The Interdecadal Shift of ENSO Properties in 1999/2000: A Review

AbstractFollowing the interdecadal shift of El Niño–Southern Oscillation (ENSO) properties that occurred in 1976/77, another regime shift happened in 1999/2000 that featured a decrease of variability and an increase in ENSO frequency. Specifically, the frequency spectrum of Niño-3.4 sea surface temperature shifted from dominant variations at quasi-quadrennial (~4 yr) periods during 1979–99 to weaker fluctuations at quasi-biennial (~2 yr) periods during 2000–18. Also, the spectrum of warm water volume (WWV) index had almost no peak in 2000–18, implying a nearly white noise process. The regime shift was associated with an enhanced zonal gradient of the mean state, a westward shift in the atmosphere–ocean coupling in the tropical Pacific, and an increase in the static stability of the troposphere. This shift had several important implications. The whitening of the subsurface ocean temperature led to a breakdown of the relationship between WWV and ENSO, reducing the efficacy of WWV as a key predictor for ENSO and thus leading to a decrease in ENSO prediction skill. Another consequence of the higher ENSO frequency after 1999/2000 was that the forecasted peak of sea surface temperature anomaly often lagged that observed by several months, and the lag increased with the lead time. The ENSO regime shift may have altered ENSO influences on extratropical climate. Thus, the regime shift of ENSO in 1999/2000 as well as the model default may account for the higher false alarm and lower skill in predicting ENSO since 1999/2000.

The climatic response of baldcypress (Taxodium mucronatum Ten.) in San Luis Potosi, Mexico

Baldcypress, a long-lived species of riparian habitats, is sensitive to interannual climate variability. The annual radial growth is associated with the Standardized Precipitation Evaporation Index (SPEI) and Palmer Drought Severity Index (PDSI), both depending on a water balance. The baldcypress (Taxodium mucronatum Ten.) is a long-lived species and Mexico’s national tree. The objective of this study was to analyze the dendroclimatic response of baldcypress at sites located on the transitional zone between the semiarid and subtropical regions of Mexico, known as the Middle Zone of the state of San Luis Potosi, Mexico. The first site, Los Peroles (LPO), is a wetland where the oldest known baldcypress specimens in Mexico are found. The second site, Rio Verde (RVDE), is a riparian site located along a tributary of the Moctezuma River that drains into the Gulf of Mexico. Two ring-width chronologies were analyzed. The first one from LPO with a length of 633 years and the second one from RVDE with a length of 423 years. The species responded positively to the seasonal January–July precipitation (r = 0.52, p = 0.001) and negatively to the maximum monthly temperature of January, May, and June (r = − 0.42, −0.46, and −0.34; respectively). The average chronology was significantly associated with the mean January–August Standardized Precipitation Evaporation Index (SPEI) (r = 0.62, p = 0.000). An SPEI reconstruction was developed for the period 1575–1996 where the most prolonged and severe droughts were found at 70-year peaks in the El Nino Southern Oscillation frequency band. The annual radial growth of the species is a suitable indicator of the January–August water availability, and is important information that can be used to establish water management strategies for this semiarid environment and to preserve millennial baldcypress specimens.

Contributing Factors to Spatiotemporal Variations of Outgoing Longwave Radiation (OLR) in the Tropics

Abstract Factors governing spatiotemporal variations of the daily outgoing longwave radiation (OLR) are studied using 35-yr (1979–2013) data records by employing multiple linear regression, wavelet transforms, and bandpass filtering methods. From the regression coefficients of nine predictors and the explained variances, we found that the largest contributions to OLR variability are associated with the Madden–Julian oscillation and El Niño–Southern Oscillation (ENSO). The ENSO signatures on OLR show dipole patterns over the Maritime Continent (MC) and Pacific regions with an extension to the Atlantic. Subsequently, the third significant contribution of the Indian Ocean dipole is confined to the Indian Ocean and Africa. Then, the solar cycle and stratospheric aerosols show mainly negative correlations, while a positive linear trend is observed mainly in the Northern Hemisphere. Lastly, factors associated with the stratospheric quasi-biennial oscillation (QBO) are the least significant contributor to OLR. In terms of oscillatory signals, time–longitude variations of the annual cycle (AC) show pairs of contrasting phases that characterize monsoon systems, in which the MC and Pacific regions are found to be in the same phase group. The most consistent AC signals are found to correspond with North and South American monsoons that respectively exhibit weakening and strengthening trends. Wavelet spectra and filtered OLR signals in intraseasonal oscillation, QBO, and ENSO frequency bands show an interdependent relationship that largely varies with time scale and longitudes.

Increased influence of ENSO on Antarctic temperature since the Industrial Era

Under the influence of recent global warming, modulation of frequencies and amplitude of El Niño-Southern Oscillation (ENSO) and its impacts on global climate have become great concerns to the global community. Antarctic climate is sensitive to these changes owing to tropical and Southern Hemispheric (SH) teleconnections. Antarctic surface air temperature (SAT) reconstructed approximately for the past five centuries (~1533 to 1993 CE) based on multiple oxygen isotope (δ18O) records of ice cores from East and West Antarctica show dominant oscillations in ENSO and Pacific Decadal Oscillation (PDO) frequency bands. Further, variance of the East Antarctica (EA) temperature record shows significant increasing trend at ENSO band and decreasing trend at PDO band since the industrial era (~1850 CE). This observation is consistent with the earlier report of increasing ENSO activity, reconstructed based on tropical-subtropical tree ring records. ENSO influence in the SH high-latitude is known to be characterized by Pacific South American (PSA) pattern reflected in the atmospheric pressure fields. Our investigation of greenhouse gas (GHG) forced model simulation results show an increasing trend in PSA activity since the industrial era. Thus, we suggest ENSO activity and its influence on Antarctic temperature are increasing in response to increasing radiative GHG forcing since the industrial era.

Dynamical and Thermodynamical Influences of the Maritime Continent on ENSO Evolution

El Niño-Southern Oscillation (ENSO) exerts tremendous influences on the global climate. Through dynamic lifting and thermal forcing, the Maritime Continent (MC) plays an important role in affecting global atmospheric circulation. In spite of the extensive studies on ENSO mechanisms, the influence of MC on the characteristics of ENSO life cycle remains unclear. Our coupled model experiments reveal that the absence of the MC land contributes to a strong ENSO asymmetry and a weakened nonlinear atmospheric response to the combined seasonal and interannual SST variations (i.e. the combination mode) that prolongs the warm events, resulting in a reduction of ENSO frequency. On the other hand, our experiments suggest that the global climate model applied (NCAR CESM) overestimates the MC topographic uplifting effect on ENSO simulation. Overall, this study provides a new physical insight into the nature of the MC influence on ENSO evolution.

Paleo-ENSO revisited: Ecuadorian Lake Pallcacocha does not reveal a conclusive El Niño signal

Information about decadal to millennial variability of El Nino Southern Oscillation (ENSO) is fundamental for the assessment of ENSO responses to natural and anthropogenic forcings. Despite a growing number of ENSO reconstructions, the overall picture of Holocene ENSO variability is inconsistent. Here, we revisit the iconic Holocene ENSO sediment record of Lake Pallcacocha, Ecuador (Rodbell et al., 1999). We asked: (i) How coherent are the records of clastic layers (flood layers) in the sediments of Lake Pallcacocha and adjacent Lake Fondococha? (ii) What are the synoptic-scale atmospheric conditions that lead to intense precipitation and, potentially, to alluvial activity promoting the deposition of clastic layers in these lakes? (iii) Is intense precipitation in this area associated with El Nino, or not? We analyzed clastic layers in Late-Holocene sediments from multiple cores in Lakes Pallcacocha and Fondococha from Cajas National Park, southern Ecuadorian Andes. Additionally, we investigated precipitation data from 13 nearby meteorological stations to test if intense precipitation (percentiles P0.95, P0.99, P0.995) is predominantly related to El Nino conditions or not (based on 15 different ENSO indices). Our results show that the absolute flood frequencies (clastic layers per 100 years) differ substantially from lake to lake. This indicates that the frequency of clastic layers reflects different sensitivities (thresholds of precipitation) of the catchments to alluvial activity. 210Pb ages suggest that neither the 1982/83 nor the 1997/98 very strong El Ninos produced clastic layers comparable to those found in the late Holocene. Daily precipitation records from meteorological stations close to Lake Pallcacocha including a high-altitude station from the western slope of the Andes did not show unusually high precipitation during the super El Nino 2015/16. We further find that intense precipitation in this area occurs at roughly equal probability under El Nino, La Nina and neutral conditions. Although the spectral properties of the late Holocene clastic layers in Lake Pallcacocha fall into the typical ENSO frequency band, we do not find evidence in the recent sediments and the meteorological data that would support a diagnostic link between alluvial activity in Lake Pallcacocha and strong El Nino events. Our data do not support the idea that the (late) Holocene flood record of Lake Pallcacocha is a conclusive paleo-El Nino record