Changes in El Niño and La Niña teleconnections over North Pacific–America in the global warming simulations

The change in the teleconnections of both El Niño and La Niña over the North Pacific and American regions due to a future greenhouse warming has been analyzed herein by means of diagnostics of the Intergovernmental Panel on Climate Change-Fourth Assessment Report (IPCC-AR4) coupled general circulation models (CGCMs). Among the IPCC-AR4 CGCM simulations, the composites of the eight-member multimodel ensemble are analyzed. In most CGCMs, the tropical Pacific warming due to the increase of CO₂ concentration in the atmosphere promotes the main convection centers in the equatorial Pacific associated with both El Niño and La Niña to the east. The eastward shift of the convection center causes a systematic eastward shift of not only El Niño but also La Niña teleconnection patterns over the North Pacific and America, which is demonstrated in the composite maps of 500 hPa circulation, surface temperature, and the precipitation against El Niño and La Niña, as observed in a comparison between the pre-industrial and CO₂ doubling experiments. Thus, a systematic eastward migration of convection centers in the tropical Pacific associated with both El Niño and La Niña due to a future global warming commonly causes the eastward shift of the atmospheric teleconnection patterns over the Northern Hemisphere.     

Future changes of El Niño in two global coupled climate models

Idealized forcing experiments with 1% per year CO₂ increase to stabilized doubled and quadrupled CO₂, twenty-first century transient scenario experiments (SRES scenarios A1B and B1), and stabilized twenty-second century A1B and B1 experiments with two global coupled climate models (PCM and CCSM3) are analyzed for possible future changes of El Niño events. With increased CO₂ in the models, there is a reduction of amplitude of El Niño events. This is particularly apparent with larger forcing in the stabilized 4×CO₂ experiment in PCM and the stabilized greenhouse gas A1B experiment in CCSM3, where the reduction of amplitude is outside the range of the inherent multi-century variability of El Niño in the control runs of the models and is statistically significant. With moderately increased forcing (stabilized 2×CO₂ in PCM and the stabilized B1 experiment in CCSM3), the reduction in amplitude is evident, but it is not significant. The change in El Niño behavior with larger forcing is attributed to the change in base state temperature in the equatorial Pacific, which is similar with increased greenhouse gases (GHGs) in both models. Positive temperature anomalies in and below the thermocline, associated with a reduction of the trade winds, and weakened Pacific Ocean subtropical cells, produce a less intense thermocline, and consequently lower amplitude El Niño events. The previously noted intensification of El Niño tropical precipitation anomalies in a warmer mean base state that applied when there was no appreciable change in El Niño amplitude does not hold in the present study where the El Niño events decrease in magnitude in a future warmer climate. North American surface temperature anomalies associated with El Niño are reduced and become less significant in the future events, with the anomalously deepened Aleutian low in the North Pacific weakened and moved eastward with greater radiative forcing. Part of this is attributed to the smaller amplitude events and thus lower amplitude teleconnections as indicated by contrasting composites of medium and high amplitude El Niño events from the control runs. The change in midlatitude base state circulation also contributes to the change in El Niño teleconnections. The effects of this change in base state on the weakened El Niño teleconnections over North America are confirmed in sensitivity experiments with a version of the atmospheric model in which heating anomalies are specified to mimic El Niño events in a base state changed due to increased GHGs.     

Nonlinear precipitation response to El Niño and global warming in the Indo-Pacific

Precipitation changes over the Indo-Pacific during El Niño events are studied using an Atmospheric General Circulation Model forced with sea-surface temperature (SST) anomalies and changes in atmospheric CO₂ concentrations. Linear increases in the amplitude of the El Niño SST anomaly pattern trigger nonlinear changes in precipitation amounts, resulting in shifts in the location and orientation of the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ). In particular, the maximum precipitation anomaly along the ITCZ and SPCZ shifts eastwards, the ITCZ shifts south towards the equator, and the SPCZ becomes more zonal. Precipitation in the equatorial Pacific also increases nonlinearly. The effect of increasing CO₂ levels and warming SSTs is also investigated. Global warming generally enhances the tropical Pacific precipitation response to El Niño. The precipitation response to El Niño is found to be dominated by changes in the atmospheric mean circulation dynamics, whereas the response to global warming is a balance between dynamic and thermodynamic changes. While the dependence of projected climate change impacts on seasonal variability is well-established, this study reveals that the impact of global warming on Pacific precipitation also depends strongly on the magnitude of the El Niño event. The magnitude and structure of the precipitation changes are also sensitive to the spatial structure of the global warming SST pattern.     

ENSO teleconnections in projections of future climate in ECHAM5/MPI-OM

The teleconnections of the El Niño/Southern Oscillation (ENSO) in future climate projections are investigated using results of the coupled climate model ECHAM5/MPI-OM. For this, the IPCC SRES scenario A1B and a quadrupled CO₂ simulation are considered. It is found that changes of the mean state in the tropical Pacific are likely to condition ENSO teleconnections in the Pacific North America (PNA) region and in the North Atlantic European (NAE) region. With increasing greenhouse gas emissions the changes of the mean states in the tropical and sub-tropical Pacific are El Niño-like in this particular model. Sea surface temperatures in the tropical Pacific are increased predominantly in its eastern part and redistribute the precipitation further eastward. The dynamical response of the atmosphere is such that the equatorial east–west (Walker) circulation and the eastern Pacific inverse Hadley circulation are decreased. Over the subtropical East Pacific and North Atlantic the 200 hPa westerly wind is substantially increased. Composite maps of different climate parameters for positive and negative ENSO events are used to reveal changes of the ENSO teleconnections. Mean sea level pressure and upper tropospheric zonal winds indicate an eastward shift of the well-known teleconnection patterns in the PNA region and an increasing North Atlantic oscillation (NAO) like response over the NAE region. Surface temperature and precipitation underline this effect, particularly over the North Pacific and the central North Atlantic. Moreover, in the NAE region the 200 hPa westerly wind is increasingly related to the stationary wave activity. Here the stationary waves appear NAO-like.     

Impact of in-consistency between the climate model and its initial conditions on climate prediction

We investigated the influence of dynamical in-consistency of initial conditions on the predictive skill of decadal climate predictions. The investigation builds on the fully coupled global model “Coupled GCM for Earth Simulator” (CFES). In two separate experiments, the ocean component of the coupled model is full-field initialized with two different initial fields from either the same coupled model CFES or the GECCO2 Ocean Synthesis while the atmosphere is initialized from CFES in both cases. Differences between both experiments show that higher SST forecast skill is obtained when initializing with coupled data assimilation initial conditions (CIH) instead of those from GECCO2 (GIH), with the most significant difference in skill obtained over the tropical Pacific at lead year one. High predictive skill of SST over the tropical Pacific seen in CIH reflects the good reproduction of El Niño events at lead year one. In contrast, GIH produces additional erroneous El Niño events. The tropical Pacific skill differences between both runs can be rationalized in terms of the zonal momentum balance between the wind stress and pressure gradient force, which characterizes the upper equatorial Pacific. In GIH, the differences between the oceanic and atmospheric state at initial time leads to imbalance between the zonal wind stress and pressure gradient force over the equatorial Pacific, which leads to the additional pseudo El Niño events and explains reduced predictive skill. The balance can be reestablished if anomaly initialization strategy is applied with GECCO2 initial conditions and improved predictive skill in the tropical Pacific is observed at lead year one. However, initializing the coupled model with self-consistent initial conditions leads to the highest skill of climate prediction in the tropical Pacific by preserving the momentum balance between zonal wind stress and pressure gradient force along the equatorial Pacific.

     

Multi-model changes in El Niño teleconnections over North America in a future warmer climate

Previous studies with single models have suggested that El Niño teleconnections over North America could be different in a future warmer climate due to factors involving changes of El Niño event amplitude and/or changes in the midlatitude base state circulation. Here we analyze a six-member multi-model ensemble, three models with increasing future El Niño amplitude, and three models with decreasing future El Niño amplitude, to determine characteristics and possible changes to El Niño teleconnections during northern winter over the North Pacific and North America in a future warmer climate. Compared to observed El Niño events, all the models qualitatively produce general features of the observed teleconnection pattern over the North Pacific and North America, with an anomalously deepened Aleutian Low, a ridge over western North America, and anomalous low pressure over the southeastern United States. However, associated with systematic errors in the location of sea surface temperature and convective heating anomalies in the central and western equatorial Pacific (the models’ anomaly patterns are shifted to the west), the anomalous low pressure center in the North Pacific is weaker and shifted somewhat south compared to the observations. For future El Niño events, two different stabilization experiments are analyzed, one with CO₂ held constant at year 2100 concentrations in the SRES A1B scenario (roughly doubled present-day CO₂), and another with CO₂ concentrations held constant at 4XCO₂. Consistent with the earlier single model results, the future El Niño teleconnections are changed in the models, with a weakened as well as an eastward- and northward-shifted anomalous low in the North Pacific. This is associated with weakened anomalous warming over northern North America, strengthened cooling over southern North America, and precipitation increases in the Pacific Northwest in future events compared to present-day El Niño event teleconnections. These changes are consistent with the altered base state upper tropospheric circulation with a wave-5 pattern noted in previous studies that is shown here to be consistent across all the models whether there are projected future increases or decreases in El Niño amplitude. The future teleconnection changes are most consistent with this anomalous wave-5 pattern in the models with future increases of El Niño amplitude, but less so for the models with future decreases of El Niño amplitude.     

El Niño–mean state–seasonal cycle interactions in a multi-model ensemble

The modelled El Niño–mean state–seasonal cycle interactions in 23 coupled ocean–atmosphere GCMs, including the recent IPCC AR4 models, are assessed and compared to observations and theory. The models show a clear improvement over previous generations in simulating the tropical Pacific climatology. Systematic biases still include too strong mean and seasonal cycle of trade winds. El Niño amplitude is shown to be an inverse function of the mean trade winds in agreement with the observed shift of 1976 and with theoretical studies. El Niño amplitude is further shown to be an inverse function of the relative strength of the seasonal cycle. When most of the energy is within the seasonal cycle, little is left for inter-annual signals and vice versa. An interannual coupling strength (ICS) is defined and its relation with the modelled El Niño frequency is compared to that predicted by theoretical models. An assessment of the modelled El Niño in term of SST mode (S-mode) or thermocline mode (T-mode) shows that most models are locked into a S-mode and that only a few models exhibit a hybrid mode, like in observations. It is concluded that several basic El Niño–mean state–seasonal cycle relationships proposed by either theory or analysis of observations seem to be reproduced by CGCMs. This is especially true for the amplitude of El Niño and is less clear for its frequency. Most of these relationships, first established for the pre-industrial control simulations, hold for the double and quadruple CO₂ stabilized scenarios. The models that exhibit the largest El Niño amplitude change in these greenhouse gas (GHG) increase scenarios are those that exhibit a mode change towards a T-mode (either from S-mode to hybrid or hybrid to T-mode). This follows the observed 1976 climate shift in the tropical Pacific, and supports the—still debated—finding of studies that associated this shift to increased GHGs. In many respects, these models are also among those that best simulate the tropical Pacific climatology (ECHAM5/MPI-OM, GFDL-CM2.0, GFDL-CM2.1, MRI-CGM2.3.2, UKMO-HadCM3). Results from this large subset of models suggest the likelihood of increased El Niño amplitude in a warmer climate, though there is considerable spread of El Niño behaviour among the models and the changes in the subsurface thermocline properties that may be important for El Niño change could not be assessed. There are no clear indications of an El Niño frequency change with increased GHG.     

The role of mean state on changes in El Niño’s flavor

Recently, many studies have argued for the existence of two types of El Niño phenomena based on different spatial distributions: the conventional El Niño [or Eastern Pacific (EP) El Niño], and the Central Pacific (CP) El Niño. Here, we investigate the decadal modulation of CP El Niño occurrences using a long-term coupled general circulation model simulation, focusing, in particular, on the role of climate state in the regime change between more and fewer CP El Niño events. The higher occurrence regime of the CP El Niño coincides with the lower occurrence regime of EP El Niño, and vice versa. The climate states associated with these two opposite regimes resemble the leading principal component analysis (PCA) modes of tropical Pacific decadal variability, indicating that decadal change in climate state may lead to regime change in terms of two different types of El Niño. In particular, the higher occurrence regime of CP El Niño is associated with a strong zonal gradient of mean surface temperature in the equatorial Pacific, along with a strong equatorial Trade wind over the area east of the dateline. In addition, the oceanic variables—the mixed layer depth and the thermocline depth—show values indicating increased depth over the western-to-central Pacific. The aforementioned climate states obviously intensify zonal advective feedback, which promotes increased generation of the CP El Niño. Frequent CP El Niño occurrences are not fully described by oceanic subsurface dynamics, and dynamical or thermodynamical processes in the ocean mixed layer and air–sea interaction are important contributors to the generation of the CP El Niño. Furthermore, the atmospheric response with respect to the SSTA tends to move toward the west, which leads to a weak air–sea coupling over the eastern Pacific. These features could be regarded as evidence that the climate state can provide a selection mechanism of the El Niño type.     

The Relationship between the El Ni ?no/La Ni ?na Cycle and the Transition Chains of Four Atmospheric Oscillations. Part II: The Relationship and a New Approach to the Prediction of El Ni ?no

ABSTRACT The authors explored the connection and transition chains of the Northern Oscillation （NO） and the North Pacific Oscilla tion （NPO）, the Southern Oscillation （SO）, and the Antarctic Oscillation （AAO） on the interannual timescale in a companion paper. In this study, the connection between the transition chains of the four oscillations （the NO and NPO, the SO and AAO） and the El Nifio/La Nifia cycle were examined. It was found that during the transitions of the four oscillations, alternate anticyclonic/cyclonic correlation centers propagated from the Western Pacific to the Eastern Pacific along both sides of the equator. Between the anticyclonic/cyclonic correlation centers, the zonal wind anomalies also moved eastwardly, favoring the advection of sea surface temperature anomalies from the tropical Western Pacific to the Eastern Pacific. When the anti cyclonic anomalies arrived in the Eastern Pacific, the positive phase of NO/SO and La Nifia were established and vice versa. Thus, in 4-6 years, with an entire transition chain of the four oscillations, an E1 Nifio/La Nifia cycle completed. The eastward propagation of the covarying anomalies of the sea level pressure, zonal wind, and sea surface temperature was critical to the transition chains of the four oscillations and the cycle of E1 Nifio/La Nifia. Based on their close link, a new empirical prediction method of the timing of E1 Nifio by the transition chains of the four oscillations was proposed. The assessment provided confidence in the ability of the new method to supply information regarding the long-term variations of the ocean and atmosphere in the tropical Pacific.     

ENSO evolution asymmetry: EP versus CP El Niño

Through an oceanic mixed-layer heat budget analysis, the dominant processes contributing to the largest decay rate (− 0.37 °C/mon) in EP El Nino, the moderate delay rate (− 0.22 °C/mon) in CP El Nino and the smallest decay rate (0.13 °C/mon) in La Nina, are identified. The result shows that both dynamic (wind induced equatorial ocean waves and thermocline changes) and thermodynamic (net surface solar radiation and latent heat flux changes) processes contribute to a fast decay and thus phase transition in EP El Niño composite, whereas the thermodynamic process has less effect on the decay rate for both CP El Niño and La Niña due to the westward shift of sea surface temperature anomaly (SSTA) centers. Thus, the difference in surface wind stress forcing is critical in contributing to evolution asymmetry between CP El Niño and La Niña, while the difference in both the wind stress and heat flux anomalies contribute to evolution asymmetry between EP El Niño and La Niña. It is interesting to note that El Nino induced anomalous anticyclone over the western North Pacific is stronger and shifts more toward the east during EP El Niño than during CP El Niño, while compared to CP El Niño, the center of an anomalous cyclone during La Niña shifts further to the west. As a consequence, both EP and CP El Niño decay fast and transform into a La Niña episode in the subsequent year, whereas La Niña has a much slower decay rate and re-develops in the second year.

     

Theories on formation of an anomalous anticyclone in western North Pacific during El Niño: A review

The western North Pacific anomalous anticyclone (WNPAC) is an important atmospheric circulation system that conveys El Niño impact on East Asian climate. In this review paper, various theories on the formation and maintenance of the WNPAC, including warm pool atmosphere–ocean interaction, Indian Ocean capacitor, a combination mode that emphasizes nonlinear interaction between ENSO and annual cycle, moist enthalpy advection/Rossby wave modulation, and central Pacific SST forcing, are discussed. It is concluded that local atmosphere–ocean interaction and moist enthalpy advection/Rossby wave modulation mechanisms are essential for the initial development and maintenance of the WNPAC during El Niño mature winter and subsequent spring. The Indian Ocean capacitor mechanism does not contribute to the earlier development but helps maintain the WNPAC in El Niño decaying summer. The cold SST anomaly in the western North Pacific, although damped in the summer, also plays a role. An interbasin atmosphere–ocean interaction across the Indo-Pacific warm pool emerges as a new mechanism in summer. In addition, the central Pacific cold SST anomaly may induce the WNPAC during rapid El Niño decaying/La Niña developing or La Niña persisting summer. The near-annual periods predicted by the combination mode theory are hardly detected from observations and thus do not contribute to the formation of the WNPAC. The tropical Atlantic may have a capacitor effect similar to the tropical Indian Ocean.     

The importance of the eastward zonal current for generating extreme El Niño

Extreme El Niño (e.g., 1983/1983 and 1997/1998) causes severe weather and climate impacts globally, but the associated dynamics is not fully understood. The present study shows that advection of mean temperature by anomalous eastward zonal current plays an important role in producing such extreme events especially during the early part of the developing period. While the climatological direction of the upper oceanic current in the equatorial Pacific is westward, at times the direction reverses. These eastward current events are well distinguished from the normal, westward conditions. The upper-layer zonal current in the equatorial Pacific is basically in geostrophic balance and forced by wind stress. However, in the case of the eastward zonal current events, persistent westerly winds are observed in the Western Pacific, and the current becomes synchronized with the westerly wind stress above. The advection of the mean temperature by the anomalous zonal current in the early developing period always precedes strong El Niño, though it does not significantly contribute to the growth of La Niña, neutral, and moderate El Niño; and is the major contributor of asymmetry in the early developing phase.     

Distinctive Precursory Air–Sea Signals between Regular and Super El Ni os

Statistically different precursory air–sea signals between a super and a regular El Ni no group are investigated, using observed SST and rainfall data, and oceanic and atmospheric reanalysis data. The El Ni no events during 1958–2008 are first separated into two groups: a super El Ni no group(S-group) and a regular El Ni no group(R-group). Composite analysis shows that a significantly larger SST anomaly(SSTA) tendency appears in S-group than in R-group during the onset phase[April–May(0)], when the positive SSTA is very small. A mixed-layer heat budget analysis indicates that the tendency difference arises primarily from the difference in zonal advective feedback and the associated zonal current anomaly(u).This is attributed to the difference in the thermocline depth anomaly(D) over the off-equatorial western Pacific prior to the onset phase, as revealed by three ocean assimilation products. Such a difference in D is caused by the difference in the wind stress curl anomaly in situ, which is mainly regulated by the anomalous SST and precipitation over the Maritime Continent and equatorial Pacific.     

Effects of interannual salinity variability on the barrier layer in the western-central equatorial Pacific: A diagnostic analysis from Argo

ABSTRACT In this paper, interannual variations in the barrier layer thickness （BLT） are analyzed using Argo three-dimensional temperature and salinity data, with a locus on the effects of interannually varying salinity on the evolution of the El Nifio Southern Oscillation （ENSO）. The interannually varying BLT exhibits a zonal seesaw pattern across the equatorial Pacific during ENSO cycles. This phenomenon has been attributed to two different physical processes. During E1 Nifio （La Nifia）, the barrier layer （BL） is anomalously thin （thick） west of about 160°E, and thick （thin） to the east. In the western equatorial Pacific （the western part： 130°-160°E）, interannual variations of the BLT indicate a lead of one year relative to those of the ENSO onset. The interannual variations of the BLT can be largely attributed to the interannual temperature variability, through its dominant effect on the isothermal layer depth （ILD）. However, in the central equatorial Pacific （the eastern part： 160~E- 170~W）, interannual variations of the BL almost synchronously vary with ENSO, with a lead of about two months relative to those of the local SST. In this region, the interannual variations of the BL are significantly affected by the interannually varying salinity, mainly through its modulation effect on the mixed layer depth （MLD）. As evaluated by a onedimensional boundary layer ocean model, the BL around the dateline induced by interannual salinity anomalies can significantly affect the temperature fields in the upper ocean, indicating a positive feedback that acts to enhance ENSO.     

Anomalous summer climate in China influenced by the tropical Indo-Pacific Oceans

Possible influences of three coupled ocean–atmosphere phenomena in the Indo-Pacific Oceans, El Niño, El Niño Modoki and the Indian Ocean Dipole (IOD), on summer climate in China are studied based on data analysis for the summers of 1951–2007. Partial correlation/regression analysis is used to find the influence paths through the related anomalous mid- and low-level tropospheric circulations over the oceanic region and East Eurasia, including the western North Pacific summer monsoon (WNPSM). Among the three phenomena, El Niño Modoki has the strongest relationship with the WNPSM. When two or three phenomena coexist with either positive or negative phase, the influences exerted by one phenomenon on summer climate in different regions of China may be enhanced or weakened by other phenomena. In 1994 when both El Niño Modoki and IOD are prominent without El Niño, a strong WNPSM is associated with severe flooding in southern China and severe drought in the Yangtze River Valley (YRV). The 500 hPa high systems over China are responsible for heat waves in most parts of China. In 1983 when a strong negative phase of El Niño Modoki is accompanied by moderate El Niño and IOD, a weak WNPSM is associated with severe flooding in the YRV and severe drought in southern China. The 500 hPa low systems over China are responsible for the cold summer in the YRV and northeastern China. For rainfall, the influence path seems largely through the low-level tropospheric circulations including the WNPSM. For temperature, the influence path seems largely through the mid-level tropospheric circulations over East Eurasia/western North Pacific Ocean.     

Proposed scheme for modeling of ocean equatorial currents in the phase of El Niño and La Niña: Implementation of the mesoscale turbulence theory

El Niño is a phenomenon of the catastrophic increase of surface temperature in the eastern part of the Pacific Ocean. It has a significant impact to weather of the American continent and western regions of the tropical Pacific, as well as on the weather and climate of entirely the Earth. Most important factors influencing El Niño are the wind, ocean currents and slope of the water surface (and temperature resulting from these factors) at the equator in the Pacific Ocean. The paper considers results of mathematical modeling of the equatorial Pacific Ocean currents in the El Niño and La Niña phases using the theory of mesoscale turbulence. This theory has been successfully tested in modeling of global circulation of atmosphere and ocean (Arsen’yev et al., 2010) and it has been able to calculate the ocean current changes at equator under changing external conditions. It is shown that the water currents at the equator have a four-tier vertical structure. The surface trade-wind current is located above the subsurface undercurrent, below which we observe the intermediate current, turning into the equatorial deep counter flow. When El Niño begins, the currents are rearranged, change signs and sometimes merge with each other. In the phase of maximum development of the phenomenon there is a two-tier structure: (1) surface current heading the American coast is underlain (below the depth of 440 m) by (2) deep equatorial current directed to the Indonesian coast. The theoretical calculations are compared with the physical observations of ocean currents in the El Niño and La Niña phases. The obtained results indicate that the proposed mathematical apparatus makes it possible to explain the set of physical observations in the Pacific Ocean.     

Tropical forcing of Australian extreme low minimum temperatures in September 2019

We explore the causes and predictability of extreme low minimum temperatures (T_min) that occurred across northern and eastern Australia in September 2019. Historically, reduced T_min is related to the occurrence of a positive Indian Ocean Dipole (IOD) and central Pacific El Niño. Positive IOD events tend to locate an anomalous anticyclone over the Great Australian Bight, therefore inducing cold advection across eastern Australia. Positive IOD and central Pacific El Niño also reduce cloud cover over northern and eastern Australia, thus enhancing radiative cooling at night-time. During September 2019, the IOD and central Pacific El Niño were strongly positive, and so the observed T_min anomalies are well reconstructed based on their historical relationships with the IOD and central Pacific El Niño. This implies that September 2019 T_min anomalies should have been predictable at least 1–2 months in advance. However, even at zero lead time the Bureau of Metereorolgy ACCESS-S1 seasonal prediction model failed to predict the anomalous anticyclone in the Bight and the cold anomalies in the east. Analysis of hindcasts for 1990–2012 indicates that the model's teleconnections from the IOD are systematically weaker than the observed, which likely stems from mean state biases in sea surface temperature and rainfall in the tropical Indian and western Pacific Oceans. Together with this weak IOD teleconnection, forecasts for earlier-than-observed onset of the negative Southern Annular Mode following the strong polar stratospheric warming that occurred in late August 2019 may have contributed to the T_min forecast bust over Australia for September 2019.

     

ENSO modes of the equatorial Pacific Ocean in observations and CMIP5 models

El Niño/Southern Oscillation (ENSO) is the predominant interannual variability of the global climate system. How might ENSO change in a warmer world? The dominant two Combined Empirical Orthogonal Functions (CEOF) of the equatorial ocean temperature and zonal and vertical motion identify two modes that shown a transition in the eastern Pacific from a warming eastward/downward motion to a cooling westward/upward flow. These results also suggest consistent changes to the west and at depths down to 300 m. These dominate CEOFs provide a compact tool for assessing Coupled Model Intercomparison Project Phase 5 ocean model output for both the recent historical period and for the latter part of the twenty first century. Most of the analyzed models replicate well the spatial patterns of the dominant observational CEOF modes, but nearly always underestimate the magnitudes. Comparing model output for the twentieth and twenty first centuries there is very little change between the spatial patterns of the ENSO modes of the two periods. This lack of response to climate change is shown to be partly related to competing influences of climatic changes in the mean ocean circulation.     

Mixed layer heat budget of the El Niño in NCEP climate forecast system

The mechanisms controlling the El Niño have been studied by analyzing mixed layer heat budget of daily outputs from a free coupled simulation with the Climate Forecast System (CFS). The CFS is operational at National Centers for Environmental Prediction, and is used by Climate Prediction Center for seasonal-to-interannual prediction, particularly for the prediction of the El Niño and Southern Oscillation (ENSO) in the tropical Pacific. Our analysis shows that the development and decay of El Niño can be attributed to ocean advection in which all three components contribute. Temperature advection associated with anomalous zonal current and mean vertical upwelling contributes to the El Niño during its entire evolutionary cycle in accordance with many observational, theoretical, and modeling studies. The impact of anomalous vertical current is found to be comparable to that of mean upwelling. Temperature advection associated with mean (anomalous) meridional current in the CFS also contributes to the El Niño cycle due to strong meridional gradient of anomalous (mean) temperature. The surface heat flux, non-linearity of temperature advection, and eddies associated with tropical instabilities waves (TIW) have the tendency to damp the El Niño. Possible degradation in the analysis and closure of the heat budget based on the monthly mean (instead of daily) data is also quantified.     

ENSO nonlinearity in a warming climate

The El Niño Southern Oscillation (ENSO) is known as the strongest natural inter-annual climate signal, having widespread consequences on the global weather, climate, ecology and even on societies. Understanding ENSO variations in a changing climate is therefore of primordial interest to both the climate community and policy makers. In this study, we focus on the change in ENSO nonlinearity due to climate change. We first analysed high statistical moments of observed Sea Surface Temperatures (SST) timeseries of the tropical Pacific based on the measurement of the tails of their Probability Density Function (PDF). This allows defining relevant metrics for the change in nonlinearity observed over the last century. Based on these metrics, a zonal “see-saw” (oscillation) in nonlinearity patterns is highlighted that is associated with the change in El Niño characteristics observed in recent years. Taking advantage of the IPCC database and the different projection scenarios, it is showed that changes in El Niño statistics (or “flavour”) from a present-day climate to a warmer climate are associated with a significant change in nonlinearity patterns. In particular, in the twentieth century climate, the “conventional” eastern Pacific El Niño relates more to changes in nonlinearity than to changes in mean state whereas the central Pacific El Niño (or Modoki El Niño) is more sensitive to changes in mean state than to changes in nonlinearity. An opposite behaviour is found in a warmer climate, namely the decreasing nonlinearity in the eastern Pacific tends to make El Niño less frequent but more sensitive to mean state, whereas the increasing nonlinearity in the west tends to trigger Central Pacific El Niño more frequently. This suggests that the change in ENSO statistics due to climate change might result from changes in the zonal contrast of nonlinearity characteristics across the tropical Pacific.