El Nino-Southern Oscillation Variability Research Articles

ENSO Change in Climate Projections: Forced Response or Internal Variability?

AbstractTwo large ensembles are used to quantify the extent to which internal variability can contribute to long‐term changes in El Niño‐Southern Oscillation (ENSO) characteristics. We diagnose changes that are externally forced and distinguish between multi‐model simulation results that differ by chance and those that differ due to different model physics. The range of simulated ENSO amplitude changes in the large ensemble historical simulations encompasses 90% of the Coupled Model Intercomparison Project 5 historical simulations and 80% of moderate (RCP4.5) and strong (RCP8.5) warming scenarios. When considering projected ENSO pattern changes, model differences are also important. We find that ENSO has high internal variability and that single realizations of a model can produce very different results to the ensemble mean response. Due to this variability, 30–40 ensemble members of a single model are needed to robustly compute absolute ENSO variance to a 10% error when 30‐year analysis periods are used.

Coupled Model Biases Breed Spurious Low‐Frequency Variability in the Tropical Pacific Ocean

AbstractCoupled general circulation model (GCM) biases in the tropical Pacific are substantial, including a westward extended cold sea surface temperature (SST) bias linked to El Niño–Southern Oscillation (ENSO). Investigation of internal climate variability at centennial timescales using multicentury control integrations of 27 GCMs suggests that a Pacific Centennial Oscillation emerges in GCMs with too strong ENSO variability in the equatorial Pacific, including westward extended SST variability. Using a stochastic model of climate variability (Hasselmann type), we diagnose such centennial SST variance in the western equatorial Pacific. The consistency of a simple stochastic model with complex GCMs suggests that a previously defined Pacific Centennial Oscillation may be driven by biases in high‐frequency ENSO forcing in the western equatorial Pacific. A cautious evaluation of long‐term trends in the tropical Pacific from GCMs is necessary because significant trends in historical and future simulations are possible consequences of biases in simulated internal variability alone.

Potential ENSO effects on the oxygen isotope composition of modern speleothems: Observations from Jiguan Cave, central China

Despite of the fast development of speleothem records, oxygen isotope (δ18O), the main paleoclimatic proxy, remains complicated in climatic interpretation. Continuous cave monitoring is essential for understanding the response of stalagmite oxygen isotope to East Asian Summer Monsoon moisture transportation. We introduce a 7 years (2010–2016) study on oxygen isotope of atmospheric precipitation, cave drips and modern speleothems at Jiguan Cave, central China, located Chinese north–south divide where is sensitive to Asian Monsoon. The monitoring covered a whole ENSO (El Niño Southern Oscillation) cycle, from El Niño in 2010 to La Niña in 2011 and recovered another El Niño in 2015. The precipitation δ18O shows obvious seasonality (negative in summer and positive in winter), but air temperature and rainfall amount are not primary controlling factors. The interannual δ18O of precipitation corresponds with ENSO variability, which means δ18O value is positive during El Niño event and vice versa. We used HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory) model to simulate the moisture transportation for rainy season in El Niño and La Niña years, and found the Pacific contributed over 50% moisture in El Niño years and the Indian Ocean was the predominant oceanic source in La Niña year. There is no seasonality in drips δ18O value, while the response to ENSO variability is evident on interannual scale. The stable negative δ18O of drips compared with precipitation indicate there is a threshold for infiltration, suggesting cave drips are recharged by summer heavy precipitation with light δ18O value, but it’s the mixture of latest and former rainy precipitation that recharge drips in drought, which has been verified by simple infiltration model. We found the modern speleothems were precipitated under nonequilibrium fractionation during drought years, nevertheless, they can record the El Niño related δ18O positive anomaly. Overall, the modern speleothems can receive the precipitation δ18O signal transferred by drips, and our study offers significance for verification of Asian Summer Monsoon driving force and interpretation of stalagmite δ18O.

Climate variability impacts on rice production in the Philippines.

Changes in crop yield and production over time are driven by a combination of genetics, agronomics, and climate. Disentangling the role of these various influences helps us understand the capacity of agriculture to adapt to change. Here we explore the impact of climate variability on rice yield and production in the Philippines from 1987–2016 in both irrigated and rainfed production systems at various scales. Over this period, rice production is affected by variations in soil moisture, which are largely driven by the El Niño–Southern Oscillation (ENSO). We found that the climate impacts on rice production are strongly seasonally modulated and differ considerably by region. As expected, rainfed upland rice production systems are more sensitive to soil moisture variability than irrigated paddy rice. About 10% of the variance in rice production anomalies on the national level co-varies with soil moisture changes, which in turn are strongly negatively correlated with an index capturing ENSO variability. Our results show that while temperature variability is of limited importance in the Philippines today, future climate projections suggest that by the end of the century, temperatures might regularly exceed known limits to rice production if warming continues unabated. Therefore, skillful seasonal prediction will likely become increasingly crucial to provide the necessary information to guide agriculture management to mitigate the compounding impacts of soil moisture variability and temperature stress. Detailed case studies like this complement global yield studies and provide important local perspectives that can help in food policy decisions.

A high-resolution pollen record from East China reveals large climate variability near the Northgrippian-Meghalayan boundary (around 4200 years ago) exerted societal influence

The large and rapid climate change event that occurred around 4200 years ago (hereafter the 4.2-kyr event that marks the onset of the Meghalayan stage) is one of the most significant climatic anomalies in the Holocene. It may have exerted profound societal impact globally; however, its triggering mechanism remains uncertain. Here we present a high-resolution, well-dated pollen record covering this interval from the Tianluoshan archaeological site in East China. Our results show that the hydroclimatic conditions in the study area across this event strikingly resembled the regional expression of modern El Niño-Southern Oscillation (ENSO) and exhibited interdecadal ENSO-like variability from ca. 4.50 to 4.00 cal kyr BP. Also, our pollen data indicate a marked shift toward an overall increased intensity and frequency of ENSO-like climate variability from ca. 4.50 to 4.30 cal kyr BP, which was immediately followed by a decline of rice-based agriculture, providing direct evidence for the causal link between climate deterioration and the collapse of Neolithic civilization in East China some 4200 years ago. This observation is in accord with the enhanced ENSO variability reconstructed from proxy records in the tropical Pacific Ocean. As ENSO is intimately coupled with the Asian summer monsoon and the Intertropical Convergent Zone (ITCZ), we suggest that the enhanced ENSO-like climate variability may have triggered the 4.2-kyr event.

Inter‐annual variability of global monsoon precipitation in present‐day and future warming scenarios based on 33 Coupled Model Intercomparison Project Phase 5 models

Simulations and future projections of year‐to‐year variation in global monsoon precipitation (GMP), defined as the summer precipitation amount per unit area within the global monsoon domain, are investigated using the 33 models that participated in Phase 5 of the Coupled Model Intercomparison Project (CMIP5). The inter‐annual standard deviation of GMP, which presents the amplitude of year‐to‐year variability in monsoon rainfall, is simulated well by the multi‐model ensemble (MME) mean of the 33 models and of the best five (B5) models. The B5 models show superior skills in reproducing the climatological monsoon precipitation and its inter‐annual variability. The inter‐annual standard deviations of 25‐year GMP in the present day (1979–2005) derived from the MME average of the 33 models and of the B5 models are 0.17 and 0.15 mm/day per unit area, respectively, both being close to the observation (0.16 mm/day per unit area). Consistent with the observed El Niño–Southern Oscillation (ENSO)–GMP relationship, the simulated variability of GMP is negatively correlated with the sea surface temperature in the Niño areas at the inter‐annual timescale.Projections by the 33 CMIP5 models under the RCP4.5 scenario indicate that wet and dry monsoon years would occur more intensively in a warmer climate. Around 61% (20 out of the 33) of the models show enhanced inter‐annual variability of GMP by the end of the 21st century (2072–2098) compared to the period 1979–2005. The MME of the B5 models suggests that the amplitude of projected GMP variability would increase by ~20% from present day to the late 21st century. However, the increasing rate of GMP variability detected by the 33 models’ MME is relatively small (~6%), which is even smaller than the inter‐model standard deviation, suggesting the amplitude changes in GMP variability feature non‐negligible uncertainty among the 33 models. Under global warming, the inter‐annual variation of GMP would still be tightly connected with ENSO, although the projected amplitude changes in ENSO variability are noticeably diverse across the 33 CMIP5 models. The enhanced amplitude of inter‐annual GMP variability may be attributable to the increase in mean‐state moisture associated with global warming. The dynamic effect related to changes in the variability of monsoon circulation is small and less robust among the 33 CMIP5 models.

Evolution of IOD-ENSO relationship at multiple time scales

Both El Nino-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are important indicators of the potential impact of climate at the global scale. The IOD-ENSO relationship at different timescales is temporally non-uniform, which is important to understand in order to evaluate its socioeconomic impacts. Here, we developed and applied a wavelet cross-correlation analysis approach for investigating the evolution of the IOD-ENSO relationship over a 150-year period. We found that the IOD-ENSO relationship was statistically significant at the dominant timescales of 1.5, 3, and 24 year, with the correlation degree varying with time. The hysteresis effect between IOD and ENSO and the evolution of their relationship obviously differ at the three dominant timescales. The positive (negative) IOD and El Nino (La Nina) events were mainly determined by the variability of IOD and ENSO at the timescale < 3 year. Strong positive IOD events in the September-October-November period would more likely be related to El Nino events 2 months later and La Nina events 14 months later, but negative IOD events would rarely co-occur with El Nino or La Nina events. The result achieved in this study can serve as a useful guide for the long-term forecast of IOD and ENSO.

Relationship between El Niño–Southern Oscillation and the Symmetry of the Hadley Circulation: Role of the Sea Surface Temperature Annual Cycle

El Niño–Southern Oscillation (ENSO), which features an equatorial quasi-symmetric sea surface temperature anomaly (SSTA), is related to both the symmetric and asymmetric components of the Hadley circulation (HC) variability. However, the mechanisms for such a nonlinear HC–ENSO relationship are still unclear. Using 36-yr monthly reanalysis datasets, this study shows that the month-to-month HC variability is dominated by two principal modes, the asymmetric mode (AM) and symmetric mode (SM), both of which are highly correlated with ENSO variability. Furthermore, the relationship between the HC principal modes and the ENSO SSTA is modulated by the western Pacific SST annual cycle. When the zonal mean western Pacific SST peaks off (on) the equator, the ENSO SSTA leads to the AM (SM) of HC variability. This is because the zonal mean western Pacific SST peak provides a warmer background favorable for the SSTA to stimulate convection, indicating the important role of the combined effect of the SST annual cycle and the ENSO SSTA in affecting the HC variability. Importantly, the western Pacific SST annual cycle has no such modulation effect during central Pacific El Niño or La Niña events. The results have important implications for simulating and predicting the climatic impacts of ENSO and HC variability.

Remote forcing of the northern tropical Atlantic SST anomalies on the western North Pacific anomalous anticyclone

This study uses both observations and numerical modeling experiments to investigate the lead-lag relationship and the associated physical mechanism of the western North Pacific anomalous anticyclone (WNPAC) with sea surface temperature (SST) anomalies over the northern tropical Atlantic (NTA). The results show that the WNPAC from late spring to the middle of autumn has a significant in-phase relationship with the NTA SST anomalies up to two seasons ahead. This relationship reaches a peak when the NTA SST leads by approximately 1–2 months and is nearly independent of the El Nino-Southern Oscillation variability. Diagnosis based on observations and numerical experiments using the Community Atmospheric Model version 5.3 reveals that the NTA warming favors intensified local convection activity during the spring–autumn seasons, causing enhanced low-level convergence and upper-level divergence (i.e., ascending motion) over the NTA and opposite flow anomalies over the central tropical Pacific. The enhanced subsidence over the central tropical Pacific, in turn, triggers an anomalous low-level anticyclone over the western North Pacific. Moreover, the intensity of the anomalous local convection activity that is associated with the NTA SST is closely related to the seasonal migration of the Atlantic intertropical convergence zone (ITCZ). As the Atlantic ITCZ migrates northward, the NTA SST-induced local convection activity extends northward from a narrow band near the equator during early spring to nearly the entire NTA region during the middle of summer, leading to the strongest remote effect of the NTA SST anomalies on the WNPAC during late summer and early autumn.

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

Subtropical hydroclimate during Termination V (∼430-422 ka): Annual records of extreme precipitation, drought, and interannual variability from Santa Barbara Basin

Hydroclimate extremes are expected to become more frequent and intense with anthropogenic climate forcing, but future El Niño-Southern Oscillation (ENSO) behavior is unclear. Understanding of extreme hydroclimate variability and magnitude prior to human-influences is limited by short instrumental records, however, continuous sedimentary archives of past hydroclimate variability can complement and extend these records. Laminated Santa Barbara Basin (SBB) sediment cores MV0508-33JPC, −21JPC, and −29JPC preserve annually-resolved, multi-centennial-scale precipitation records from Termination V (TV), the transition from glacial Marine Isotope Stage (MIS) 12 to interglacial MIS 11 at 424 ka. These records provide insights into the subtropical hydroclimate response to warming. Foraminiferal δ18O variations indicate rapid warming and/or salinity changes, and were used to verify the TV age assignment. A paleoprecipitation proxy was developed using the first principal component of scanning XRF elemental counts (PC1), which has high loadings for siliciclastic sediment-associated elements K, Ti, and Si. Sedimentary laminae couplets identified in PC1 were annually-tuned to investigate TV paleoprecipitation variability, as modern SBB laminae represent annual deposits. Extreme flooding and decadal-to-centennial droughts were identified, with magnitudes exceeding modern observations. ENSO-like (2–7 year) paleoprecipitation periodicities coincide with wetter intervals, but ENSO variability is reduced during droughts. A 1500-year arid interval may be related to poleward shifting of general atmospheric circulation as ice sheets melted, such that the subtropical dry zone intersected California. Southern California paleoprecipitation reconstructions from past warm climates provide insight into precipitation variability and magnitude on interannual timescales, and can reduce uncertainties in predictions of interannual climate, including ENSO.

Drought severity based on the SPI index and its relation to the ENSO and PDO climatic variability modes in the regions North and Northwest of the State of Rio de Janeiro - Brazil

Drought severity in regions North and Northwest of Rio de Janeiro state (SRJ) was evaluated using the Standardized Precipitation Index (SPI) for the monthly (SPI-1) and annual (SPI-12) scales, as well as its relationship with El Niño - Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO). Data of 14 rainfall stations from a time series considering the period of 1967–2013 was used. The SPI calculation was based on the SCI package available at R software version 3.4.2 library. SPI categories used are Very Dry, Moderately Dry and Extremely Dry associated with the hot and cold phases of the PDO, followed by ENSO events according to the Oceanic Niño index (ONI) in El Niño region 3.4 using an interaction matrix. The matrix was evaluated by the coefficients of Kendall (τ), Sperman (ρ), Pearson (r) and R2 determination, followed by Willmott's concordance index (d), Standard Error of Estimation (SEE) and Student's t-test. The intense activity of La Niña in the cold 1st phase of the PDO increased rainfall with greater intensity in the Northwest region compared to the North. In the 2nd cold phase of the PDO, the events of El Niño, La Niña and Neutral years of the ENSO mutually contribute to the changes in rainfall regime of both regions. The most frequent SPI categories are Moderately Dry and Extremely Dry. SPI-1 was efficient in perceiving extreme droughts, while SPI-12 results were not significant in the revealing of extreme droughts. In addition, there was no relational pattern through statistical indexes between droughts detected by SPI, ENSO and PDO. Nevertheless, when ENSO cycles were persistent within both phases of the PDO, droughts occurred, mainly on the monthly scale.

A metric for quantifying El Niño pattern diversity with implications for ENSO–mean state interaction

Recent research on the El Nino–Southern Oscillation (ENSO) phenomenon increasingly reveals the highly complex and diverse nature of ENSO variability. A method of quantifying ENSO spatial pattern uniqueness and diversity is presented, which enables (1) formally distinguishing between unique and “canonical” El Nino events, (2) testing whether historical model simulations aptly capture ENSO diversity by comparing with instrumental observations, (3) projecting future ENSO diversity using future model simulations, (4) understanding the dynamics that give rise to ENSO diversity, and (5) analyzing the associated diversity of ENSO-related atmospheric teleconnection patterns. Here we develop a framework for measuring El Nino spatial SST pattern uniqueness and diversity for a given set of El Nino events using two indices, the El Nino Pattern Uniqueness (EPU) index and El Nino Pattern Diversity (EPD) index, respectively. By applying this framework to instrumental records, we independently confirm a recent regime shift in El Nino pattern diversity with an increase in unique El Nino event sea surface temperature patterns. However, the same regime shift is not observed in historical CMIP5 model simulations; moreover, a comparison between historical and future CMIP5 model scenarios shows no robust change in future ENSO diversity. Finally, we support recent work that asserts a link between the background cooling of the eastern tropical Pacific and changes in ENSO diversity. This robust link between an eastern Pacific cooling mode and ENSO diversity is observed not only in instrumental reconstructions and reanalysis, but also in historical and future CMIP5 model simulations.

A Review of Paleo El Niño-Southern Oscillation

The Earth has seen El Niño-Southern Oscillation (ENSO)—the leading mode of interannual climate variability—for at least millennia and likely over millions of years. This paper reviews previous studies from perspectives of both paleoclimate proxy data (from traditional sediment records to the latest high-resolution oxygen isotope records) and model simulations (including earlier intermediate models to the latest isotope-enabled coupled models). It summarizes current understanding of ENSO’s past evolution during both interglacial and glacial periods and its response to external climatic forcings such as volcanic, orbital, ice-sheet and greenhouse gas forcings. Due to the intrinsic irregularity of ENSO and its complicated relationship with other climate phenomena, reconstructions and model simulations of ENSO variability are subject to inherent difficulties in interpretations and biases. Resolving these challenges through new data syntheses, new statistical methods, more complex climate model simulations as well as direct model-data comparisons can potentially better constrain uncertainty regarding ENSO’s response to future global warming.

Response of O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and pH to ENSO in the California Current System in a high-resolution global climate model

Abstract. Coastal upwelling systems, such as the California Current System (CalCS), naturally experience a wide range of O2 concentrations and pH values due to the seasonality of upwelling. Nonetheless, changes in the El Niño–Southern Oscillation (ENSO) have been shown to measurably affect the biogeochemical and physical properties of coastal upwelling regions. In this study, we use a novel, high-resolution global climate model (GFDL-ESM2.6) to investigate the influence of warm and cold ENSO events on variations in the O2 concentration and the pH of the CalCS coastal waters. An assessment of the CalCS response to six El Niño and seven La Niña events in ESM2.6 reveals significant variations in the response between events. However, these variations overlay a consistent physical and biogeochemical (O2 and pH) response in the composite mean. Focusing on the mean response, our results demonstrate that O2 and pH are affected rather differently in the euphotic zone above ∼ 100 m. The strongest O2 response reaches up to several hundreds of kilometers offshore, whereas the pH signal occurs only within a ∼ 100 km wide band along the coast. By splitting the changes in O2 and pH into individual physical and biogeochemical components that are affected by ENSO variability, we found that O2 variability in the surface ocean is primarily driven by changes in surface temperature that affect the O2 solubility. In contrast, surface pH changes are predominantly driven by changes in dissolved inorganic carbon (DIC), which in turn is affected by upwelling, explaining the confined nature of the pH signal close to the coast. Below ∼ 100 m, we find conditions with anomalously low O2 and pH, and by extension also anomalously low aragonite saturation, during La Niña. This result is consistent with findings from previous studies and highlights the stress that the CalCS ecosystem could periodically undergo in addition to impacts due to climate change.

ENSO and Sea Surface Temperature Anomalies in Association with Canadian Wheat Yield Variability

ABSTRACTInterannual variations of spring wheat yields in Canadian agricultural regions are analyzed, together with the associated sea surface temperature (SST) anomalies in the northern hemisphere tropics and extratropics, from 1961 to 2015. The cubic trend is calculated and used to represent the trend related to advances in agricultural technology over this time period. The correlations between Canadian wheat yields at regional scales and the tropical El Niño–Southern Oscillation (ENSO) variability are not robust at any stage of the evolution of ENSO. Based on the power spectrum and cross-spectrum analysis, the most prominent yield variance is found in the Canadian Prairies, with a significant power peak of 4.5 years but does not co-vary significantly with interannual ENSO variability. ENSO weakly affects temperature and precipitation anomalies in the Canadian Prairie Region in summer—two important agroclimatic conditions for crop growth—and hence insignificantly impacts wheat yields. This indicates that there would be little benefit to including tropical ENSO indices in the operational wheat yield forecasting system. For Canadian wheat yield forecasting, attention should be paid to the preceding winter and spring SST anomalies in the northern extratropics. The SST anomalies associated with yields in the Canadian Prairie region and Central Region are generally stronger than those associated with yields in the Canadian Pacific Coast Region and eastern Maritime Region. In association with the Prairie Region and Central Region yields, SST shows pronounced anomalies in the mid-high latitudes of the North Pacific from winter to summer. The non-linearity of the SST anomalies associated with the Canadian yields is also clearly evident. Stronger (weaker) SST anomalies in the extratropical North Pacific correspond to low wheat yields in the Prairie (Central) Region, while weaker (stronger) SST anomalies correspond to high yields in the Prairie (Central) Region.

Changes in global vegetation activity and its driving factors during 1982–2013

Vegetation activity plays a crucial role in the global carbon cycle and climate. Many studies have examined recent changes in vegetation growth and the associated local climatic drivers. They revealed a global greening trend during the recent decades. However, few studies have analyzed how remote oceanic conditions affect land vegetation growth through atmospheric teleconnection, and the causes of the recent greening needs further investigation. In this study, we investigate the spatio-temporal variations (including trends) of vegetation activity using satellite data of growing-season normalized difference vegetation index (NDVIgs), and examine their relationship to local and remote climate oscillations and external anthropogenic forcing by statistical means. As expected, there is an increasing trend in global-mean NDVIgs from 1982–2013, with significant greening over Europe and many other land areas. NDVIgs is temperature-limited at northern high-latitudes, but water-limited in arid and semi-arid regions, and radiation-limited in the Amazon and eastern and southern Asia. Globally, El Niño-Southern Oscillation (ENSO) is the leading climatic driver of interannual variability of NDVIgs, especially over southern and eastern Africa, eastern Australia, northeastern Asia, and northern South America. Consistent with previous modeling studies, a regression-based attribution analysis suggests that historical anthropogenic forcing (mainly increases in greenhouse gases) explains about two thirds of the NDVIgs trend from 1982 to 2013, with the rest coming mainly from the Atlantic Multi-decadal Oscillation (AMO). Contributions to the recent NDVIgs trend from ENSO and Pacific decadal variability and Artctic Oscillation appear to be small.

Impact of Lateral Mixing in the Ocean on El Nino in a Suite of Fully Coupled Climate Models

AbstractWe examine the dependence of the amplitude of the El Nino‐Southern Oscillation (ENSO) on the mixing coefficient parameterizing the lateral mixing of tracers (ARedi) The value of this coefficient is very uncertain, ranging in Earth System Models between a few hundred and a few thousand m2 s−1, with some observational estimates showing even higher values. A suite of simulations is made with two spatially varying distributions of ARedi derived from satellite observations as well as four simulations where a spatially constant ARedi is varied over a factor of 6. Surprisingly, larger values of ARedi result in stronger ENSO variability despite the higher mixing coefficients producing more efficient lateral diffusive damping of anomalies. This is because lateral mixing also warms the cold tongue, increasing vertical temperature gradients and decreasing horizontal temperature gradients. Larger vertical temperature gradients make sea surface temperatures more responsive to atmospheric forcing, while smaller horizontal temperature gradients shift the location of convection and make the atmosphere more responsive to sea surface temperature anomalies. The last effect holds across simulations as well as within individual simulations and thus also helps to explain interdecadal variability in ENSO amplitude. By contrast, a previously proposed anticorrelation between the amplitude interannual and annual variability does not hold across simulations, although it does hold within simulations. The propagation of thermocline depth anomalies is relatively insensitive to the value of ARedi. Properly specifying the lateral mixing along the equator (including distinguishing the impacts of subgridscale turbulence and tropical instability waves) appears essential to simulating ENSO.

Joint Modulation of Intraseasonal Rainfall in Tropical Australia by the Madden‐Julian Oscillation and El Niño‐Southern Oscillation

AbstractRainfall in tropical Australia is a critical resource for the agricultural sector. However, its high variability implores improvements in our understanding of its variability. Australian tropical rainfall is influenced by both the Madden‐Julian Oscillation (MJO) on intraseasonal time scales and El Niño‐Southern Oscillation (ENSO) on interannual time scales. This study examines the joint relationship between the MJO, ENSO, and tropical Australian rainfall variability. We analyze daily precipitation data from stations across tropical Australia during the wet season (November to April). The wet season rainfall response to the MJO is found to be greater during El Niño than La Niña. We demonstrate that this relationship is not due to the statistical relationship between the MJO and ENSO indices but instead due to differences in how the MJO modulates the large‐scale circulation during El Niño versus during La Niña.

Contributions of the North Pacific Meridional Mode to Ensemble Spread of ENSO Prediction

Abstract Seasonal prediction of El Niño–Southern Oscillation (ENSO) employs the ensemble method, which samples the uncertainty in initial conditions. While much attention has been given to the ensemble mean, the ensemble spread limits the reliability of the forecast. Spatiotemporal coevolution of intermember anomalies of sea surface temperature (SST) and low-level winds over the Pacific is examined in ensemble hindcasts. Two types of evolution of intermember SST anomalies in the equatorial Pacific are identified. The first features an apparent southwestward propagation of the SST spread from the subtropical northeastern Pacific southeast of Hawaii to the central equatorial Pacific in boreal winter–spring, indicative of the precursor effect of the North Pacific meridional mode (NPMM) on ENSO variability. Extratropical atmospheric variability generates ensemble spread in ENSO through wind–evaporation–SST (WES) in the subtropical northeastern Pacific and then Bjerknes feedback on the equator. In the second type, ensemble spread grows in the equatorial Pacific with a weak contribution from the subtropical southeastern Pacific in summer. Thus, the extratropical influence on ENSO evolution is much stronger in the Northern Hemisphere than in the Southern Hemisphere. The growth of Niño-4 SST ensemble spread shows a strong seasonality. In hindcasts initialized in September–March, the Niño-4 SST spread grows rapidly in January–April, stabilizes in May–June, and grows again in July–September. The rapid growth of the Niño-4 SST spread in January–April is due to the arrival of NPMM, while the slowdown in May–June and rapid growth in July–September are attributable primarily to the seasonality of equatorial ocean–atmosphere interaction. NPMM contributes to the ensemble spread in equatorial Pacific SST, limiting the reliability of ENSO prediction.