El Nino Southern Oscillation Dynamics Research Articles

El Niño–Southern Oscillation and its impact in the changing climate

AbstractExtensive research has improved our understanding and forecast of the occurrence, evolution and global impacts of the El Niño–Southern Oscillation (ENSO). However, ENSO changes as the global climate warms up and it exhibits different characteristics and climate impacts in the twenty-first century from the twentieth century. Climate models project that ENSO will also change in the warming future and have not reached an agreement about the flavor, as to the intensity and the frequency, of future ENSO conditions. This article presents the conventional view of ENSO properties, dynamics and teleconnections, and reviews the emerging understanding of the diversity and associated climate impacts of ENSO. It also reviews the results from investigations into the possible changes in ENSO under the future global-warming scenarios.

What Controls ENSO‐Amplitude Diversity in Climate Models?

AbstractClimate models depict large diversity in the strength of the El Niño/Southern Oscillation (ENSO) (ENSO amplitude). Here we investigate ENSO‐amplitude diversity in the Coupled Model Intercomparison Project Phase 5 (CMIP5) by means of the linear recharge oscillator model, which reduces ENSO dynamics to a two‐dimensional problem in terms of eastern equatorial Pacific sea surface temperature anomalies (T) and equatorial Pacific upper ocean heat content anomalies (h). We find that a large contribution to ENSO‐amplitude diversity originates from stochastic forcing. Further, significant interactions exist between the stochastic forcing and the growth rates of T and h with competing effects on ENSO amplitude. The joint consideration of stochastic forcing and growth rates explains more than 80% of the ENSO‐amplitude variance within CMIP5. Our results can readily explain the lack of correlation between the Bjerknes Stability index, a measure of the growth rate of T, and ENSO amplitude in a multimodel ensemble.

Dynamic Regimes of El Niño Southern Oscillation and Influenza Pandemic Timing.

El Niño southern oscillation (ENSO) dynamics has been shown to drive seasonal influenza dynamics. Severe seasonal influenza epidemics and the 2009–2010 pandemic were coincident with chaotic regime of ENSO dynamics. ENSO dynamics from 1876 to 2016 were characterized to determine if influenza pandemics are coupled to chaotic regimes. Time-varying spectra of southern oscillation index (SOI) and sea surface temperature (SST) were compared. SOI and SST were decomposed to components using the algorithm of noise-assisted multivariate empirical mode decomposition. The components were Hilbert transformed to generate instantaneous amplitudes and phases. The trajectories and attractors of components were characterized in polar coordinates and state space. Influenza pandemics were mapped to dynamic regimes of SOI and SST joint recurrence of annual components. State space geometry of El Niños lagged by influenza pandemics were characterized and compared with other El Niños. Timescales of SOI and SST components ranged from sub-annual to multidecadal. The trajectories of SOI and SST components and the joint recurrence of annual components were dissipative toward chaotic attractors. Periodic, quasi-periodic, and chaotic regimes were present in the recurrence of trajectories, but chaos–chaos transitions dominated. Influenza pandemics occurred during chaotic regimes of significantly low transitivity dimension (p < 0.0001). El Niños lagged by influenza pandemics had distinct state space geometry (p < 0.0001). Chaotic dynamics explains the aperiodic timing, and varying duration and strength of El Niños. Coupling of all influenza pandemics of the past 140 years to chaotic regimes of low transitivity indicate that ENSO dynamics drives influenza pandemic dynamics. Forecasts models from ENSO dynamics should compliment surveillance for novel influenza viruses.

Heartbeat of the Southern Oscillation explains ENSO climatic resonances

AbstractThe El Niño‐Southern Oscillation (ENSO) nonlinear oscillator phenomenon has a far reaching influence on the climate and human activities. The up to 10 year quasi‐period cycle of the El Niño and subsequent La Niña is known to be dominated in the tropics by nonlinear physical interaction of wind with the equatorial waveguide in the Pacific. Long‐term cyclic phenomena do not feature in the current theory of the ENSO process. We update the theory by assessing low (&gt;10 years) and high (&lt;10 years) frequency coupling using evidence across tropical, extratropical, and Pacific basin scales. We analyze observations and model simulations with a highly accurate method called Dominant Frequency State Analysis (DFSA) to provide evidence of stable ENSO features. The observational data sets of the Southern Oscillation Index (SOI), North Pacific Index Anomaly, and ENSO Sea Surface Temperature Anomaly, as well as a theoretical model all confirm the existence of long‐term and short‐term climatic cycles of the ENSO process with resonance frequencies of {2.5, 3.8, 5, 12–14, 61–75, 180} years. This fundamental result shows long‐term and short‐term signal coupling with mode locking across the dominant ENSO dynamics. These dominant oscillation frequency dynamics, defined as ENSO frequency states, contain a stable attractor with three frequencies in resonance allowing us to coin the term Heartbeat of the Southern Oscillation due to its characteristic shape. We predict future ENSO states based on a stable hysteresis scenario of short‐term and long‐term ENSO oscillations over the next century.

Asymmetric Modulation of El Niño and La Niña and the Linkage to Tropical Pacific Decadal Variability

El Niño–Southern Oscillation (ENSO) in a 1300-yr preindustrial control simulation of the Community Climate System Model, version 4 (CCSM4), exhibits distinct modulation in association with tropical Pacific decadal variability (TPDV). The frequency and duration of El Niño events modulate with changes in the interbasin sea surface temperature (SST) gradient related to the leading mode of TPDV, which resembles the interdecadal Pacific oscillation (IPO). La Niña shows similar changes with the IPO but is also controlled by changes in El Niño that often precedes La Niña, and these effects tend to cancel each other. The amplitude of ENSO, on the other hand, is closely related to the second leading mode of TPDV that affects the zonal and meridional contrast of tropical Pacific climate. Significant changes in the pattern and seasonal evolution related to this TPDV mode are found mainly for El Niño because of the nonlinear relation between the atmospheric deep convection and SSTs. The resultant changes in the amplitude of El Niño, in turn, affect the amplitude and duration of the following La Niña, as well as the asymmetry in their patterns and duration. The decadal ENSO modulation associated with both TPDV modes is not symmetrical between El Niño and La Niña and thus is not likely to occur solely as a result of random variability. The patterns of TPDV in CCSM4 have resemblance to those simulated by its atmospheric component coupled to a slab ocean model, suggesting that TPDV induced by stochastic atmospheric variability interacts with the ENSO dynamics.

Impact of Mountains on Tropical Circulation in Two Earth System Models

Abstract Two state-of-the-art Earth system models (ESMs) were used in an idealized experiment to explore the role of mountains in shaping Earth’s climate system. Similar to previous studies, removing mountains from both ESMs results in the winds becoming more zonal and weaker Indian and Asian monsoon circulations. However, there are also broad changes to the Walker circulation and El Niño–Southern Oscillation (ENSO). Without orography, convection moves across the entire equatorial Indo-Pacific basin on interannual time scales. ENSO has a stronger amplitude, lower frequency, and increased regularity. A wider equatorial wind zone and changes to equatorial wind stress curl result in a colder cold tongue and a steeper equatorial thermocline across the Pacific basin during La Niña years. Anomalies associated with ENSO warm events are larger without mountains and have greater impact on the mean tropical climate than when mountains are present. Without mountains, the centennial-mean Pacific Walker circulation weakens in both models by approximately 45%, but the strength of the mean Hadley circulation changes by less than 2%. Changes in the Walker circulation in these experiments can be explained by the large spatial excursions of atmospheric deep convection on interannual time scales. These results suggest that mountains are an important control on the large-scale tropical circulation, impacting ENSO dynamics and the Walker circulation, but have little impact on the strength of the Hadley circulation.

Deterministic Chaos, El Niño Southern Oscillation, and Seasonal Influenza Epidemics

Low precipitation enhances transmission of influenza viruses, which cause seasonal epidemics during the winter in northern and southern hemispheres. El Nino southern oscillation (ENSO) which modulates global precipitation is a multicomponent signal that is composed of sub-annual to multi-decadal oscillations. The dynamics of oscillatory components of ENSO and influenza are characterized, and causal relationship of annual oscillatory components is determined. Seasonality of influenza was determined in five geographical areas of north and south hemispheres. Monthly influenza time series of these regions and of ENSO were decomposed to oscillatory components. The oscillatory components were characterized in time-frequency and phase space domains. Periodicities of the oscillatory components of ENSO and influenza range from sub-annual to multidecadal. Time-dependent intrinsic correlations of instantaneous amplitude and frequencies of annual oscillatory components of ENSO and influenza are > 0.9. The dynamics of ENSO and influenza, which are dissipative with multifractal chaotic attractors, transit from quasi-periodic to chaotic regimes. Five most severe peaks of epidemic, which include 2009–2010 pandemic, occurred during chaos. ENSO and influenza dynamics are phase coherent, but there is unidirectional causal effect of ENSO on influenza. Amplitude and frequency modulations of annual oscillatory components of ENSO and influenza are strongly coupled. Chaotic dynamics of ENSO determines the timing and severity of influenza epidemics. Monitoring of ENSO dynamics will aid public health surveillance of influenza epidemics.

Evidence of reduced mid‐Holocene ENSO variance on the Great Barrier Reef, Australia

AbstractGlobally, coral reefs are under increasing pressure both through direct anthropogenic influence and increases in climate extremes. Understanding past climate dynamics that negatively affected coral reef growth is imperative for both improving management strategies and for modeling coral reef responses to a changing climate. The El Niño–Southern Oscillation (ENSO) is the primary source of climate variability at interannual timescales on the Great Barrier Reef (GBR), northeastern Australia. Applying continuous wavelet transforms to visually assessed coral luminescence intensity in massive Porites corals from the central GBR we demonstrate that these records reliably reproduce ENSO variance patterns for the period 1880–1985. We then applied this method to three subfossil corals from the same reef to reconstruct ENSO variance from ~5200 to 4300 years before present (yBP). We show that ENSO events were less extreme and less frequent after ~5200 yBP on the GBR compared to modern records. Growth characteristics of the corals are consistent with cooler sea surface temperatures (SSTs) between 5200 and 4300 yBP compared to both the millennia prior (~6000 yBP) and modern records. Understanding ENSO dynamics in response to SST variability at geological timescales will be important for improving predictions of future ENSO response to a rapidly warming climate.

A 2000-year leaf wax-based hydrogen isotope record from Southeast Asia suggests low frequency ENSO-like teleconnections on a centennial timescale

Limited understanding of the complex dynamics of the tropical monsoon exists, partly due to inadequate paleo (hydro)-climate proxy data from monsoonal regions. This study presents a 2000-year long record of hydrogen isotope values of leaf wax (δDwax) from a sedimentary sequence recovered from Lake Pa Kho, Northern Thailand. Evaluation of present day rainfall patterns and water isotope data indicates that δDwax reflects the amount of rainfall and is also influenced by El Nino-Southern Oscillation (ENSO) dynamics. Over the last 2000 years, wettest conditions occurred between ca. 700 AD and ca. 1000 AD, whereas the driest intervals lasted from ca. 50 BCE to ca. 700 AD and from ca. 1300 AD to ca. 1500 AD. Further investigations to establish the spatiotemporal variability of ENSO within the wider tropical Asian-Pacific realm over centennial timescales revealed a low-frequency-tripole pattern between mainland SE Asia (MSEA), the tropical West Pacific, and the central-eastern Pacific, with a wetter than normal MSEA during El Nino-like climate conditions. This pattern stands in contrast to the annual event where El Nino cause drier conditions in MSEA. We hypothesize that on centennial timescales the land-sea contrast, which drives monsoon intensity in MSEA, is modulated by the latitudinal shift of the Walker circulation and associated ENSO dynamics.

ENSO dynamics and diversity resulting from the recharge oscillator interacting with the slab ocean

El Nino-Southern Oscillation (ENSO) is the leading mode of interannual global climate variability, which in its essence is often described by the equatorial dynamics of the recharge oscillator with a fixed pattern. Here we explore the idea that ENSO can be simulated in a model with a fixed pattern of sea surface temperature variability following the recharge oscillator mechanism, which interacts with the thermodynamic red noise of a slab ocean. This model is capable of simulating the leading modes of sea surface temperature variability in the tropical Pacific in good agreement with the observations and most coupled general circulation models. ENSO dynamics, amplitude, seasonality, the structure of the leading patterns, its meridional extension, its variations in an eastern and central Pacific pattern and associated positive feedbacks are all influenced and simulated well by including the interaction of recharge oscillator and the thermodynamic coupling to the slab ocean model. We further point out that much of the ENSO diversity in the spatial structure is a reflection of this interaction. However, it also has to be noted that some equatorial dynamics are missing in this model and in coupled general circulation models that are important for the ENSO diversity.

Association of Kawasaki disease with tropospheric winds in Central Chile: Is wind-borne desert dust a risk factor?

It has been found that Kawasaki disease (KD) cases diagnosed in Japan, Hawaii and San Diego, USA increase when tropospheric wind patterns arrive from central Asia, suggesting a common, wind-borne causal agent. We analyzed KD cases hospitalized in Santiago, Chile to look for associations with local, regional and large scale meteorological variables. We compiled monthly data of KD incidence rates, local meteorological variables, large scale wind patterns and several El Niño Southern Oscillation (ENSO) indices for 2001–2010; we considered standardized anomalies in all analyses and used linear time series models to account for data autocorrelation. We found that meteorological variables explain 38% of variance in KD rates. A unit increase in northerly wind at 3 lagged months, temperature at 1 and 3 lagged months and monthly change of ENSO 4 index are associated with changes in KD rates of 0.203 (95% CI 0.049–0.358), 0.181 (95% CI 0.014–0.347), 0.192 (95% CI 0.030–0.353) and −0.307 (95% CI −0.458–0.156), respectively. These results are robust when northerly wind level is changed or when a shorter period (2005–2010) is used to estimate model parameters. We found a statistical association of KD at Santiago, Chile with tropospheric, northerly wind patterns suggesting that dust transported from the Atacama Desert could include a causative agent. A novel result is that ENSO dynamics also explain part of KD variability with a decrease in KD when La Niña is dissipating or El Niño is on the rise; hence climate scale dynamics might be taken into account in future studies worldwide — at least as a potential explanatory variable that may confound KD seasonality on a global scale.

Empirical evidence of El Niño–Southern Oscillation influence on land surface phenology and productivity in the western United States

Timing of plant life cycle events (phenology) and annual plant productivity represent key interactions between the climate system and the biosphere, with implications and feedbacks for climate and ecosystem functions. The El Niño–Southern Oscillation (ENSO) system is a dominant source of interannual climate variability in the western United States, with important effects on temperature, precipitation, and drought. In this study, we examine the connection between ENSO and terrestrial vegetation dynamics using MOD17 annual net primary productivity (NPP) and land surface phenology derived from MODIS and AVHRR vegetation indices. Three phenology metrics—start of growing season (SOS), end of growing season (EOS), and length of growing season (LOS)—were estimated separately for El Niño and La Niña years by fitting difference logistic functions to split-sample time series of the MODIS enhanced vegetation index (for the period 2000–2011) and AVHRR-derived 3rd generation GIMMS NDVI (NDVI3g) for two temporal periods (2000–2011 and 1982–2011). El Niño events were associated with a significantly earlier SOS than La Niña events in most regions of the western U.S. ENSO-related differences in timing of the end of growing season were generally smaller and more heterogeneous. ENSO impacts on total length of the growing season tended to be spatially heterogeneous but mostly positive in El Niño years. Relative to La Niña events, El Niño events were generally associated with significantly higher NPP in each ecoregion (with mean differences ranging from 2 to 45gCm−2year−1). The correlation between total annual production and the Southern Oscillation Index was highest in mid- to late-winter prior to the growing season, suggesting some predictive power in advance of the growing season. These results suggest that future intensification of the ENSO system could have serious consequences for terrestrial ecosystems in the western U.S., especially in regions where ENSO already has a strong impact on interannual climate variability and vegetation dynamics.

On the Energy Exchange between Tropical Ocean Basins Related to ENSO*

Abstract Vast amounts of energy are exchanged between the ocean, atmosphere, and space in association with El Niño–Southern Oscillation (ENSO). This study examines energy budgets of all tropical (30°S–30°N) ocean basins and the atmosphere separately using different, largely independent oceanic and atmospheric reanalyses to depict anomalous energy flows associated with ENSO in a consistent framework. It is found that variability of area-averaged ocean heat content (OHC) in the tropical Pacific to a large extent is modulated by energy flow through the ocean surface. While redistribution of OHC within the tropical Pacific is an integral part of ENSO dynamics, variability of ocean heat transport out of the tropical Pacific region is found to be mostly small. Noteworthy contributions arise from the Indonesian Throughflow (ITF), which is anticorrelated with ENSO at a few months lag, and from anomalous oceanic poleward heat export during the La Niña events in 1999 and 2008. Regression analysis reveals that atmospheric energy transport and radiation at the top of the atmosphere (RadTOA) almost perfectly balance the OHC changes and ITF variability associated with ENSO. Only a small fraction of El Niño–related heat lost by the Pacific Ocean through anomalous air–sea fluxes is radiated to space immediately, whereas the major part of the energy is transported away by the atmosphere. Ample changes in tropical atmospheric circulation lead to enhanced surface fluxes and, consequently, to an increase of OHC in the tropical Atlantic and Indian Ocean that almost fully compensates for tropical Pacific OHC loss. This signature of energy redistribution is robust across the employed datasets for all three tropical ocean basins and explains the small ENSO signal in global mean RadTOA.

Process-based analysis of climate model ENSO simulations: Intermodel consistency and compensating errors

Systematic and compensating errors can lead to degraded predictive skill in climate models. Such errors may be identified by comparing different models in an analysis of individual physical processes. We examine model simulations of El Nino–Southern Oscillation (ENSO) in five Coupled Model Intercomparison Project (CMIP) models, using transfer functions to analyze nine processes critical to ENSO's dynamics. The input and output of these processes are identified and analyzed, some of which are motivated by the recharge oscillator theory. Several errors and compensating errors are identified. The east-west slope of the equatorial thermocline is found to respond to the central equatorial Pacific zonal wind stress as a damped driven harmonic oscillator in all models. This result is shown to be inconsistent with two different formulations of the recharge oscillator. East Pacific sea surface temperature (SST) responds consistently to changes in the thermocline depth in the eastern Pacific in the five CMIP models examined here. However, at time scales greater than 2 years, this consistent model response disagrees with observations, showing that the SST leads thermocline depth at long time scales. Compensating errors are present in the response of meridional transport of water away from the equator to SST: two different models show different response of the transport to off-equatorial wind curl and wind curl response to East Pacific SST. However, these two models show the same response of meridional transport to East Pacific SST. Identification of errors in specific physical processes can hopefully lead to model improvement by focusing model development efforts on these processes.

Tropical Pacific internal atmospheric dynamics and resolution in a coupled GCM

A noise reduction technique, namely the interactive ensemble (IE) approach is adopted to reduce noise at the air–sea interface due to internal atmospheric dynamics in a state-of-the-art coupled general circulation model (CGCM). The IE technique uses multiple realization of atmospheric general circulation models coupled to a single ocean general circulation model. The ensembles mean fluxes from the atmospheric simulations are communicated to the ocean component. Each atmospheric simulation receives the same SST coming from the ocean component. The only difference among the atmospheric simulations comes from perturbed initial conditions, thus the atmospheric states are, in principle synoptically independent. The IE technique can be used to better understand the importance of weather noise forcing of natural variability such as El Nino Southern Oscillation (ENSO). To study the impact of weather noise and resolution in the context of a CGCM, two IE experiments are performed at different resolutions. Atmospheric resolution is an important issue since the noise statistics will depend on the spatial scales resolved. A simple formulation to extract atmospheric internal variability is presented. The results are compared to their respective control cases where internal atmospheric variability is left unchanged. The noise reduction has a major impact on the coupled simulation and the magnitude of this effect strongly depends on the horizontal resolution of the atmospheric component model. Specifically, applying the noise reduction technique reduces the overall climate variability more effectively at higher resolution. This suggests that “weather noise” is more important in sustaining climate variability as resolution increases. ENSO statistics, dynamics, and phase asymmetry are all modified by the noise reduction, in particular ENSO becomes more regular with less phase asymmetry when noise is reduced. All these effects are more marked for the higher resolution case. In contrast, ENSO frequency is unchanged by the reduction in the weather noise, but its phase-locking to the annual cycle is strongly dependent on noise and resolution. At low resolution the noise structure is similar to the signal, whereas the spatial structure of the noise deviates from the spatial structure of the signal as resolution increases. It is also suggested that event-to-event differences are largely driven by atmospheric noise as opposed to chaotic dynamics within the context of the large-scale coupled system, suggesting that there is a well-defined “canonical” event.

On the low-frequency component of the ENSO–Indian monsoon relationship: a paired proxy perspective

Abstract. There are a number of clear examples in the instrumental period where positive El Niño–Southern Oscillation (ENSO) events were coincident with a severely weakened Indian summer monsoon (ISM). ENSO's influence on ISM precipitation has therefore remained the centerpiece of various predictive schemes of ISM rainfall for over a century. The teleconnection between ISM precipitation and ENSO has undergone a protracted weakening since the late 1980s, suggesting the strength of ENSO's influence on ISM precipitation may vary on multidecadal timescales. The recent weakening has occurred despite the fact that the ENSO system has experienced variance levels during the latter part of the 20th century that are as high as any period in the past millennium. The recent change in the ENSO–ISM coupling has prompted questions as to whether this shift represents a natural mode of climate variability or a fundamental change in ENSO and/or ISM dynamics due to anthropogenic warming or aerosol impacts on the ISM. Here we place the 20th century ENSO–ISM relationship in a millennial context by assessing the phase relationship between the two systems across the time spectrum using a a series of high-resolution reconstructions of ENSO and the ISM from tree rings, speleothems and corals. The results from all the proxies suggest that in the high-frequency domain (5–15 yr), warm (cool) sea surface temperatures in the eastern tropical Pacific lead to a weakened (strengthened) monsoon. This finding is consistent with the observed relationship between the two systems during the instrumental period. However, in the multidecadal domain (30–90 yr) the phasing between the systems is reversed such that periods of strong monsoons were, in general, coincident with periods of enhanced ENSO variability. This result is counterintuitive to the expectation that enhanced ENSO variance favors an asymmetric increase in the frequency of El Niño events and therefore a weakened monsoon system. The finding implies that the prominent multidecadal variability that characterizes the last 1000 yr of the ISM is not likely attributable to multidecadal shifts in ENSO. If there is a continued trend towards enhanced ENSO variance in the coming decades, the results presented here do not suggest this will force a reduction in ISM precipitation.

Non-stationary seabird responses reveal shifting ENSO dynamics in the northeast Pacific

The impacts of the El Nino Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) on the ecology of the northeast Pacific are well known. However, recently there has been a shift in the dominance of El Nino events from the eastern Pacific (canonical) El Nino, to the central Pacific (Modoki) El Nino, concurrent with a strengthening of the North Pacific Gyre Oscillation (NPGO). Our examination of ocean conditions and seabird reproductive success in central California shows that the way these physical factors affect the pelagic food web is also changing. Reproduction of Cassin's auklet Ptycoramphus aleuticus and Brandt's cormorant Pha- lacrocorax penicillatus, species that forage at different trophic levels, responded primarily to ENSO variability from the 1970s to the 1990s. By 1995, however, NPGO had become the dominant variable determining Cassin's auklet reproductive success. Eventually, NPGO also became corre- lated with Brandt's cormorant success but in the opposite direction to Cassin's auklet. Thus, during the mid-1990s, the correlation between the reproduction of these 2 species weakened and even- tually became inversely correlated. This shift from coherent reproduction, presumably bottom-up driven, to an inverse relationship between the 2 species suggests that the structure of the local marine food web changed as the equatorial forcing changed. This non-stationary response of sea- birds to physical forcing is cause for concern since predictions of future ecosystem productivity and effects of climate change rely on the assumption that a species' response to environmental conditions is consistent over time.

Analysis of the Slab Ocean El Nino atmospheric feedbacks in observed and simulated ENSO dynamics

In a recent study it was illustrated that the El Nino Southern Oscillation (ENSO) mode can exist in the absence of any ocean dynamics. This oscillating mode exists just due to the interaction between atmospheric heat fluxes and ocean heat capacity. The primary purpose of this study is to further explore these atmospheric Slab Ocean ENSO dynamics and therefore the role of positive atmospheric feedbacks in model simulations and observations. The positive solar radiation feedback to sea surface temperature (SST), due to reduced cloud cover for anomalous warm SSTs, is the main positive feedback in the Slab Ocean El Nino dynamics. The strength of this positive cloud feedback is strongly related to the strength of the equatorial cold tongue. The combination of positive latent and sensible heat fluxes to the west and negative ones to the east of positive anomalies leads to the westward propagation of the SST anomalies, which allows for oscillating behavior with a preferred period of 6-7 years. Several indications are found that parts of these dynamics are indeed observed and simulated in other atmospheric or coupled general circulation models (AGCMs or CGCMs). The CMIP3 AGCM-slab ensemble of 13 different AGCM simulations shows unstable ocean-atmosphere interactions along the equatorial Pacific related to stronger cold tongues. In observations and in the CMIP3 and CMIP5 CGCM model ensemble the strength and sign of the cloud feedback is a function of the strength of the cold tongue. In summary, this indicates that the Slab Ocean El Nino dynamics are indeed a characteristic of the equatorial Pacific climate that is only dominant or significantly contributing to the ENSO dynamics if the SST cold tongue is sufficiently strong. In the observations this is only the case during strong La Nina conditions. The presence of the Slab Ocean ENSO atmospheric feedbacks in observations and CGCM model simulations implies that the family of physical ENSO modes does have another member, which is entirely driven by atmospheric processes and does not need to have the same spatial pattern nor the same time scales as the main ENSO dynamics.

High-resolution productivity record and reconstruction of ENSO dynamics during the Holocene in the Eastern Equatorial Pacific using coccolithophores

We provide high-resolution paleoproductivity data for the Holocene in the Eastern Equatorial Pacific (EEP). We describe the coccolithophore assemblages at Ocean Drilling Program (ODP) Site 1240 located in the Panama basin. Coccolithophore assemblages are proposed as a productivity proxy and nutricline position for the tropical Pacific Ocean. Our proxies can be used as a tool to reconstruct the El Niño-Southern Oscillation (ENSO), the main factor controlling the climate variability in the EEP. The equatorial upwelling intensity and the influence of waters with a Sub-Antarctic origin have controlled the productivity and the phytoplankton composition during the Holocene, and this has been strongly controlled by ENSO dynamics and, as our data suggest, by Southern Hemisphere ocean dynamics. Our results reveal a clear prevalence of dominant La Niña-like conditions during the early Holocene, with an intense upwelling and high primary productivity conditions in the EEP. La Niña-like conditions prevailed during the middle Holocene, although important fluctuations were observed in paleoproductivity, and some periods with a low primary productivity were recognizable (between 8.2 and 8 kyr and around 7 and 6.5 kyr), indicating a weakened upwelling stage, as occurs during El Niño events. A strong decrease in paleoproductivity occurred between 5 and 4.3 kyr, suggesting a relevant shift toward dominant El Niño-like conditions, with an increase in the stratification of the water column. An alternation between El Niño-like and La Niña-like dominant conditions occurred during the late Holocene, characterized by a clear trend toward prevailing El Niño-like conditions, with a low primary productivity.

Quantifying errors in coral-based ENSO estimates: Toward improved forward modeling ofδ18O

[1] The oxygen isotopic ratio (δ18O) in tropical Pacific coral skeletons reflects past El Nino–Southern Oscillation (ENSO) variability, but the δ18O-ENSO relationship is poorly quantified. Uncertainties arise when constructing δ18O data sets, combining records from different sites, and converting between δ18O and sea surface temperature (SST) and salinity (SSS). Here we use seasonally resolved δ18O from 1958 to 1985 at 15 tropical Pacific sites to estimate these errors and evaluate possible improvements. Observational uncertainties from Kiritimati, New Caledonia, and Rarotonga are 0.12–0.14‰, leading to errors of 8–25% on the typical δ18O variance. Multicoral syntheses using five to seven sites capture the principal components (PCs) well, but site selection dramatically influences ENSO spatial structure: Using sites in the eastern Pacific, western Pacific warm pool, and South Pacific Convergence Zone (SPCZ) captures “eastern Pacific-type” variability, while “Central Pacific-type” events are best observed by combining sites in the warm pool and SPCZ. The major obstacle to quantitative ENSO estimation is the δ18O/climate conversion, demonstrated by the large errors on both δ18O variance and the amplitude of the first principal component resulting from the use of commonly employed bivariate formulae to relate SST and SSS to δ18O. Errors likely arise from either the instrumental data used for pseudoproxy calibration or influences from other processes (δ18O advection/atmospheric fractionation, etc.). At some sites, modeling seasonal changes to these influences reduces conversion errors by up to 20%. This indicates that understanding of past ENSO dynamics using coral δ18O could be greatly advanced by improving δ18O forward models.