Day 2 - March 13, 2025 Poster Session (Makana Room)
Title: Dynamics and Predictability of 2020-23 La Niña Initiation through tropical inter-basin interaction
Abstract:
The recent triple-dip La Niña event from 2020 to 2023 in the equatorial Pacific, which occurred without a preceding strong El Niño, challenges the traditional understanding of multi-year La Niña dynamics governed solely by the recharge oscillator theory. This study investigates the mechanisms behind the initiation and persistence of the 2020–2023 La Niña and evaluates the predictive capabilities of a fully coupled climate model. Utilizing a decadal climate prediction system based on CESM2 with ocean data assimilation, we assess the contributions of various climate factors, including inter-basin forcings such as the Indian Ocean Dipole (IOD) and tropical Atlantic warming (TAW). Our findings reveal that, while the model underestimates the role of the IOD, the combination of IOD and TAW substantially influenced La Niña initiation through the eastward shift of the atmospheric surface pressure system. Partial ocean data assimilation experiments corroborate these results and suggest that the Pacific Meridional Mode (PMM) may have acted as a disruptor in the initiation process. The persistence of the event appears to involve distinct mechanisms, indicating separate forcings for the second and third phases of cooling. These insights underscore the importance of inter-basin interactions in extending ENSO predictability beyond the spring predictability barrier, thereby enhancing long-term climate forecasting capabilities.
Title: The role of subsurface turbulent mixing in the Tropical Pacific: enhanced vertical heat fluxes, modulation of primary productivity and control of penetrative heating
Abstract:
Sea surface temperature (SST) variability in the Equatorial Pacific has profound impacts on atmospheric circulation, controlling the El Niño-Southern Oscillation (ENSO) and affecting weather patterns all across the globe. Tropical Instability Waves (TIWs), which happen more frequently during La Niña and neutral phases, play a key role in controlling the strength of the equatorial cold tongue. Observations have shown that TIWs can: 1) drive meridional and zonal heat transport that contribute to increase SST (acting as a negative feedback on La Niña), and 2) drive SST cooling through mixing at the TIW-Equatorial Undercurrent interface (producing a positive feedback on La Niña). Numerical simulations suggest that other processes associated with TIW fronts, such as primary productivity and the modulation of penetrative heating, can significantly contribute to changes in SST and, thus, impact ENSO variability. In November 2024, as part of the MOTIVE (Mixing belOw Tropical Instability waVEs) project, we deployed three drifting Wirewalkers (from 0-750 m) on different parts of a TIW with the objective of getting high-resolution measurements (down to 0.25m in the vertical and hourly in time) of turbulent mixing and its impacts on the heat budget and primary productivity. With our Wirewalker array, we observed enhanced subsurface mixing on TIW cold cusps off the Equator. In the upper 100 m, mixing is triggered by the diurnal descent of shear layers, while below 100 m, strong mixing is observed due to enhanced shear at the bottom of the EUC. Turbulent vertical heat fluxes are downward and can exceed 500 W m-2. Furthermore, we observe that subsurface mixing supports primary productivity in the cold cusp. As a consequence, penetrative heating is restricted to shallower depths compared to observations outside the cold cusp. The misrepresentation of subsurface mixing and its impacts on heat and primary productivity might contribute to model SST biases and misrepresentation of ENSO variability. Further process studies are needed in other TIWs and seasons to get a comprehensive view of TIW-turbulent scale interactions and their biogeochemical feedbacks.
Title: A high-resolution air-sea coupled configuration of the tropical Pacific to quantify the impact of mesoscale activity on ENSO
Abstract:
Recent observational and modeling studies have demonstrated that coupled atmospheric and oceanic mesoscale processes exert a significant influence on ENSO dynamics. However, the spatial resolution of the majority of CGCMs within the IPCC climate models remains insufficient to fully resolve mesoscale air-sea interactions. Furthermore, CGCMs exhibit notable biases in the simulation of the tropical Pacific mean state which can influence the model's capacity to accurately reproduce ENSO variability. Here, we present a novel high-resolution oceanic (1/12°) and atmospheric (1/4°) coupled simulation of the tropical Pacific basin, over the 1980-2020 period, to quantify the impact of mesoscale activity on ENSO. The coupled simulation effectively reproduces the mean state and seasonal cycle of the tropical climate, specially the SST pattern with an accurate representation of the warm pool and cold tongue extension. Additionally, the model properly simulates the equatorial current system, the equatorial thermocline, and key atmospheric characteristics of the tropics (e.g., ITCZ-SPCZ). These features enable the model to successfully represent the ENSO seasonal phase, including the onset, development, and termination of the most intense El Niño events (1982/83, 1997/98, 2015/16). The rectifying role of mesoscale air-sea coupling processes (i.e., the atmospheric response to anomalous SST and surface currents) on ENSO dynamics is assessed through various perturbation experiments that manipulate the fields transferred between the ocean and the atmosphere by Gaussian smoothing. Such experiments, designed to isolate specific aspects of the air-sea interaction, allows us to properly disentangle and quantify the oceanic versus coupled effects of Tropical Instability Waves on ENSO asymmetries.
Title: STC Variability and its Role in Cold Tongue Biases
Abstract:
CMIP6 models struggle to simulate the observed cooling in the eastern tropical Pacific Ocean and tend to show a weakening of the zonal sea surface temperature (SST) gradient, which intensifies in future projections. Few climate models reproduce the historical strengthening of this gradient observed over the last 50 years. These biases can affect the representation of the cold tongue, a region of cooler SSTs in the eastern equatorial Pacific, which is important for accurately captured ENSO dynamics, as it plays a key role in modulating the amplitude and frequency of ENSO events. Previous literature points to a cold bias in the models’ eastern equatorial Pacific Ocean mean state, which may be a consequence of errors in the oceanic Pacific Subtropical Cells (STCs). We investigate the STCs, which are shallow meridional overturning circulation structures that act as an “ocean tunnel” connecting the subtropical and tropical oceans. STCs transport subtropical water into the equatorial thermocline, are a consequence of equatorial upwelling connected to surface Ekman return flow from the tropics to the subtropics. Using both the historical and future simulations from the 100 ensemble members of the CESM2 Large Ensemble, we quantify the range of STC variability and its connection to the Tropical SST. In addition, we explore whether the processes that control this connection change under warming. More specifically, we conduct a heat budget analysis to elucidate the mechanisms underlying changes in STC strength.
Title: Tropical Pacific impacts on atmospheric blocking and midlatitude weather extremes: insights from observations, models, and paleoclimate studies.
Abstract:
Projected changes in atmospheric blocking and associated extreme weather events are marked by significant uncertainties due to climate model biases, the complex nature of the phenomenon, and its strong natural variability. This presentation will focus on the role of the tropical Pacific in shaping the frequency of atmospheric blocking across multiple timescales, from interannual to multidecadal and centennial. First, I will use a deep-learning-based reconstruction of summertime atmospheric blocking over the Last Millennium to show that a weakened tropical Pacific zonal temperature gradient during the Little Ice Age correlates with a hemispherically reduced blocking frequency and altered regional patterns. Interannual variability in N. Hemisphere blocking is enhanced post-1600, coinciding with both the reduction in tropical Pacific zonal gradient and an apparent increase in Niño3.4 variance. However, changes in ENSO diversity, and not just Niño3.4 variance, are important to consider in order to explain multiscale variability in N. Hemisphere blocking, as Eastern vs. Central Pacific ENSO events can have opposite-sign impacts on the main centers of blocking activity. To further investigate the impact of ENSO diversity on blocking, I will present an energetics-based analysis of idealized Pacific-Ocean-Global-Atmosphere (POGA) experiments. Finally, I will show that any model biases in simulating the tropical Pacific-blocking relationships, or erroneous projections in tropical Pacific mean state and variability change can be carried over to their projections of blocking with implications for impact projections.
Title: Understanding Tropical Mixing Processes and Their Parameterization Using an Ocean State Estimate
Abstract:
The cold tongue of sea surface temperatures (SSTs) in the central Equatorial Pacific is maintained by upwelling and turbulent mixing. This region serves an important role in the Bjerknes feedback that drives the El Niño-Southern Oscillation, however modeling and prediction of Cold Tongue SST has proven difficult even for state-of-the-art climate models. While vertical fluxes of momentum and heat happen at small scales, variability of the shear and stratification in this region is modulated by much larger phenomena (e.g., Kelvin waves and Tropical Instability Waves (TIWs)). A comprehensive understanding of the processes that drive vertical fluxes in the Cold Tongue is critical to modeling ocean-atmosphere energy exchange in the tropics, which directly impacts modeling of the global climate.
The Tropical Pacific Ocean State Estimate (TPOSE) is a dynamically consistent, data-assimilating model that covers the Pacific basin from 20N to 20S. In this presentation we will use TPOSE to understand processes that drive upwelling and mixing in the Cold Tongue. We will detail the performance of the model at a range of temporal and spatial scales, including those relevant to Kelvin wave and TIW activity. We will then use the validated model to investigate the evolution of the zonal momentum and heat budgets. Finally, we will compare the parameterization of turbulent fluxes to observations at 0N, 140W. Where TPOSE and the data are inconsistent, we can begin to identify avenues for improvement through further observations and assimilation experiments.
Title: Dynamics and impacts of Coastal El Niño events in global climate models
Abstract:
Coastal El Niño events, characterized by anomalous warming confined to the coastal regions of Peru and Ecuador, have a significant impact on regional hydroclimate, often mirroring or surpassing the effects of basin-scale El Niño events. The recent extreme events in boreal spring of 2017 and 2023 have renewed attention to the dynamics of coastal El Niño. However, key uncertainties persist regarding the relative roles of remote versus local wind forcing —through equatorial Kelvin waves and coastal upwelling—or local surface heat flux anomalies in the onset and development of these events. Here, we develop a suite of metrics to identify coastal El Niño events and diagnose their underlying mechanisms in CMIP6 model simulations. More than half of the models either strongly underestimate or overestimate the number of coastal events per century, while the simulated SST and precipitation response patterns also exhibit biases. These biases are associated with basin-scale model discrepancies in simulating tropical climate - such as the zonal temperature gradient and the Intertropical Convergence Zone (ITCZ) - as well as the strength of local feedback processes in the far eastern Pacific. These in turn manifest into considerable model spread in projections of the frequency of coastal events under high-emission scenarios and the resulting precipitation extremes in the affected regions.
Title: ENSO Onset Timing Distribution and Its Biases in Climate Models
Abstract:
The El Niño-Southern Oscillation (ENSO) is the most prominent interannual climate variability in the tropical Pacific, characterized by distinct seasonal variations in sea surface temperature (SST). While its seasonal evolution is well-recognized, the mechanisms governing the distribution of onset timing, which marks the transition from neutral conditions to El Niño or La Niña, are not fully understood. This study employs the recharge-discharge oscillator (RO) framework to investigate ENSO onset timing distribution characteristics and their controlling mechanisms. Results demonstrate that the RO framework effectively explains ENSO onset timing distribution in both observations and climate models. In observations, the seasonal variation of SST growth rate is the main factor contributing to ENSO's tendency to onset in boreal spring, corresponding to when anomalous SST growth rate transitions from negative to positive. The seasonal variation of the SST phase transition rate plays a secondary role. In climate models, however, the ENSO onset timing distribution is more dispersed with a weaker seasonal preference. This dispersion is primarily attributed to the weaker seasonal cycle amplitude of the SST growth rate. By elucidating the mechanisms behind ENSO onset timing distribution, this study can provide new insights for improving climate models and enhancing ENSO prediction skill.
Title: Untangling sources of remote ENSO forcing using model-analogs
Abstract:
The El Niño-Southern Oscillation (ENSO) emerges primarily from equatorial Pacific Ocean dynamics. Other ocean basins also contain processes that influence ENSO evolution, however, there is no unifying framework to compare the relative impacts of these remote processes. Neither is there a consensus on which remote regions are most important. We adapt analog forecasting to identify the improvement in ENSO forecast skill when information in regions outside the equatorial Pacific is included. Here, we consider the influence of each of the tropical Atlantic, tropical Indian, north tropical Pacific and south tropical Pacific Oceans and investigate similarities and differences across 12 coupled general circulation models.
In most models, we find that ENSO forecast skill can be improved by including information from ocean regions external to the equatorial Pacific. This implies that these regions contain independent variability that impacts ENSO evolution. Overall, the largest skill increases come from considering information in the Atlantic Ocean, followed by the south Pacific, Indian, then north Pacific Ocean. However, the magnitude of skill increase depends on the specific model, lead time and forecast initialization month. We find no skill increase from including information in any region poleward of 30°. Finally, we note that the GFDL-CM2.1 model used for many pacemaker studies exhibits the weakest remote forcing of any of the models we consider. Our results, and this method which may be applied to other models, lay the groundwork for contextualising and comparing studies on ENSO remote forcing in individual climate models.
Title: Improvement of ENSO Simulation by the Conditional Multi-model Ensemble Method
Abstract:
The El Niño Southern Oscillation (ENSO) is an important interannual atmosphere-ocean interaction phenomenon in the tropical Pacific. To study ENSO, the atmosphere-ocean coupled models are frequently used to simulate ENSO behaviors and mechanisms. The capability of a single model is limited to simulate the temporal-spatial variations of ENSO; therefore, the simulations are required to be improved through ensemble methods. Using the coupled models of CMIP, this study applied the conditional multi-model ensemble (CMME) method, compared with the MME method, to investigate the ensemble results of ENSO simulation and physical reasons of improvement. The CMME can reproduce ENSO behaviors very well, including ENSO’s interannual variability, temporal-spatial evolution, seasonal phase-locking, as well as asymmetry of El Niño and La Niña. The excellent reproductions of the CMME for ENSO initiation, positive and negative feedback and nonlinear heating mechanism explain the reason why it can perform ENSO characteristics. Our results demonstrate that this CMME method is an effective, stable and reliable ensemble approach which can be widely applied in the ensemble research of numerical models.
Title: Insights of Dynamic Forcing Effects of MJO on ENSO from a Shallow Water Model
Abstract:
Madden-Julian Oscillation (MJO) is believed to play a significant role in triggering El Niño-Southern Oscillation (ENSO) events and affect the dynamics of ENSO. In this study, the dynamic forcing effects of MJO on the equatorial oceanic dynamic fields and the onsets of different types of ENSO events are investigated through sensitive experiments using spatiotemporally filtered forcing based on an anomalous shallow water model. The comparisons between observations and model responses provide meaningful insights into the extent of MJO's impacts on sea surface dynamics relative to other atmospheric variabilities. The following conclusions are made. Firstly, the MJO-forced perturbations on zonal currents are stronger and more significant than those on sea surface heights. Secondly, MJO is essential for improving zonal current simulation in the western-central Pacific and generating activity centers of zonal currents in the eastern Pacific in the model. Thirdly, MJO tends to contribute to the onset of El Niño events rather than La Niña events. Strong intraseasonal oceanic Kelvin waves forced by MJO are confirmed in simulations during the onset stages of the 1997/98 and 2004/05 events. The 120-day running standard deviations of zonal current and sea surface height anomaly series forced by MJO exhibit positive skewness similar to those of the 20-100-day band-passed observational series. Yet not all the onsets of historical ENSO events are in company with strong MJO-related perturbations. Additionally, the wind stress formula can amplify the responses of zonal current and sea surface height anomalies to synoptic forcings with periods shorter than 20 days through entraining lower-frequency variabilities.
Title: Diverse Response of Western North Pacific Anticyclone to Fast-decay El Niño during Decaying Summer
Abstract:
Previous studies suggested that fast-decay El Niño events are more favorable in generating the western North Pacific anticyclone (WNPAC) in the decaying summer. However, we found that this is not the case for all fast-decay El Niño events. By comparing two groups of fast-decay El Niño events with significant and insignificant WNPAC in the following summer, we found that the westward extension of the equatorial Pacific cold sea surface temperature anomalies (SSTA) and the subtropical central-north Pacific cold SSTA play important roles in the generation and intensification of the WNPAC during decaying summer. Further analyses indicated that the internal atmospheric mode - North Pacific Oscillation (NPO) during boreal spring can affect the formation of the cold SSTA over the subtropical central-north Pacific and the westward extension of the equatorial Pacific cold SSTA during summer. Additional effects of tropical Indian and Atlantic forcing on the maintenance of the WNPAC are also shown.
Title: Role of synoptic transient eddy feedback in shaping ENSO-related extratropical climate anomalies
Abstract:
Over the past decades, substantial processes in understanding the ENSO’s impact on extratropical climate emphasize the role of the Rossby wave propagation (e.g., the wintertime ENSO-forced PNA-like pattern), and the relaying effect arising from tropical air-sea interaction (e.g., the delayed response of East Asian summer monsoon to ENSO events). However, the role of the midlatitude air-sea interaction and associated atmospheric dynamics in shaping the ENSO-related extratropical climate anomalies remain certainly unclear. Here we present recent progress in exploring the ENSO’s extratropical impact, taking the early-summer (June-July) North Pacific climate's delayed response as a main example. During the developing winter, El Niño events induce basin-scale cold SST anomalies in the central North Pacific, which can persist into the following summer. These anomalies significantly influence early-summer atmospheric circulation by enhancing atmospheric baroclinicity and transient eddy activities. Primarily driven by synoptical transient eddy vorticity forcing, an equivalent-barotropic low pressure anomaly emerges over the North Pacific. Enhanced by the southwesterly winds of the atmospheric low, tropical moisture is transported farther northeastward in the early summer, resulting in increased rainfall in the Pacific Northwest region. Similarly, by elucidating the role of synoptical transient eddy feedback, we also investigate the maintenance mechanism of Tasman Sea high pressure anomaly during La Niña events, which is a key circulation system embedded in PSA pattern and have important influences on the eastern Australian rainfall.
Title: Distinct North American teleconnection of the strong El Niños as modulated by the ENSO-annual cycle combination mode
Abstract:
It has been documented that the nonlinear interaction between the strong El Niño and tropical annual cycle can derive a new atmospheric mode named the ENSO-annual cycle combination mode (C-mode), and our study further reveals a modulation of this mode on the teleconnection pattern of strong El Niños over the Pacific-North American (PNA) sector. Results show that the teleconnection response to strong El Niños features a distinct meridional pattern over North America, but the response to moderate El Niños tends to be zonally oriented like the classic PNA pattern. The zonally-oriented pattern mainly results from the mid-latitude eastward wave activity fluxes, as usually seen in all El Niños. In contrast, for strong El Niños, additional northeastward wave fluxes emanate from the central-eastern subtropical North Pacific and contribute to the meridional teleconnection pattern in North America. This meridional wave activity can be attributed to the C-mode that peaks in the late boreal winter, which generates an equator-asymmetric circulation response over the central-eastern Pacific and thus the subtropical upper-level Rossby wave source exciting the distinct teleconnection pattern over North America. The effect of annual cycle is further validated with an atmospheric general circulation model, as the meridional teleconnection pattern of strong El Niños will switch to zonal if excluding the annual cycle in the tropical sea surface temperature boundary. The analyses herein highlight the key role of the annual cycle in shaping the strong El Niño’s teleconnection pattern.
Title: Roles of tropical-Pacific interannual–interdecadal variability in forming the super long La Niña events
Abstract:
The super long La Niña phenomenon, which has an extremely long duration, like the recent 2020–2023 La Niña event, is less concerned than the super El Niño. In this study, we identify five super long La Niña events after 1950 and investigate roles of the 2–3-year quasi-biennial (QB) and 3–7-year low-frequency (LF) ocean–atmosphere coupled processes of El Niño–Southern Oscillation (ENSO), and the interdecadal background in forming the large-scale prolonged negative sea surface temperature anomalies (SSTAs) in the central to eastern equatorial Pacific during these events. We group the five events into the thermocline-driven type (the 1983–1986 and 1998–2002 events) and the wind-driven type (the 1954–1957, 1973–1976, and 2020–2023 events). The former inherited a sufficiently discharged state of equatorial upper-ocean heat content from the preceding super El Niño and dominated by the thermocline feedback, leading to a LF oceanic dynamical adjustment to support the maintenance of negative ENSO SSTAs. The latter were promoted by the relatively more important zonal advective feedback and Ekman pumping feedback and deeply affected by a strongly negative equatorial zonal wind stress background state that sourced from the strong negative phase of the Interdecadal Pacific Oscillation. Besides, the QB ENSO variability with casual contributions during these events is less important. Results show that both the LF ENSO variability and the interdecadal Pacific background could assist in generating prolonged La Niñas.
Title: El Niño and the Exceptional Climate of 2023-24: A Crossroad of Variability and Change
Abstract:
This presentation examined the processes that make the global and regional climate of 2023-24 exceptional. For the global perspective, we conducted heat budget analysis. It is found that surface air temperature (SAT), sea surface temperature (SST), atmospheric heat content (AHC), and ocean heat content (OHC) at 0-100 m are highly correlated. Therefore, it is examined how the combination of AHC and 0-100 m OHC is forced by net radiation from the top of the atmosphere (EEI) and heat transfer from the deeper ocean, which is closely related to El Niños, is evaluated. In 2023, the contributions of these two drivers were comparable. However, the EEI was stronger compared to previous El Niño events, while the oceanic heat transfer was similar. Thus, the enhanced EEI is suggested to be what makes the 2023-24 climate exceptional, but El Niño triggered the exceptional condition. From a regional perspective, we examined the southeastern North Atlantic and the Southern Ocean. We found that cloud reduction and increased shortwave radiation were key in the North Atlantic. On the other hand, an unprecedented wave number 3 pattern, likely associated with tropical variations, significantly impacted SAT, SST, and sea ice patterns in the Southern Ocean.
These results suggest that even if an El Niño event is not as strong as past super El Niños, it can still have a remarkable impact on the Earth's climate as global warming continues. In the future, the influence of El Niño is likely to be even more surprising.
Title: The response of extreme La Niña events to global warming: theory, mechanisms, and CMIP model projections
Abstract:
This study aims to offer a comprehensive overview of how extreme La Niña events may change in a warming climate. In the first part, we introduce a newly developed theoretical framework that analytically constrains the maximum intensity of La Niña events based on the tropical Pacific mean state. In the second part, we assess changes in extreme La Niña events in CMIP model future projections. Specifically, we document changes in the intensity and frequency of extreme La Niña events, test the newly developed theory, and diagnose the underlying physical mechanisms driving these changes. The insights gained from these results provide a deeper understanding of extreme La Niña dynamics and their potential impacts in a warmer world.
Title: Decreased ENSO post-2100 in response to formation of a permanent El Niño-like state under greenhouse warming
Abstract:
During strong El Niño, substantial SST warm anomalies establish atmospheric convection in the usually non-convective equatorial central and eastern Pacific, and erase much of the climatological west-minus-east and meridional SST gradients of the equatorial Pacific, which determine the potential intensity of an El Niño. Under transient greenhouse warming, ENSO is projected to increase pre-2100, due to several conducive processes, including an easier movement of the Pacific convergence zones to the equator and easier establishment of atmospheric convection in the equatorial eastern Pacific, where SST warms faster than surrounding regions leading to an El Niño-like warming pattern. Post-2100, how ENSO SST variability may change remains unknown. Here, using available CMIP models forced under high emission scenarios, we find that ENSO variability post-2100 reverses in majority of models from the initial increase to an amplitude far smaller than that of the 20th century. The fast eastern warming persists into 2300, reducing the potential intensity and shrinking the non-convective area of the equatorial Pacific, such that establishing convection in the non-convective area, as during an El Niño, requires smaller convective and SST anomalies. The reduction in ENSO variability is thus a symptom of the persistent El Niño-like warming pattern, reminiscent of a permanent El Niño condition suggested to have occurred during the earlier Pliocene warm period. Thus, the oscillatory ENSO impact could be replaced by that from a permanent El Niño with a cumulative influence on affected regions. Such dramatic change would be avoided if target of the Paris Agreement could be achieved.
Title: Change of El Niño onset location around 1970
Abstract:
An Extended Empirical Orthogonal Function (EEOF) analysis of sea surface temperature anomalies (SSTAs) with a 41-year moving window during the past 100+ year period shows a marked interdecadal shift of El Niño onset location around 1970, that is, El Niño started its warming in eastern Pacific (EP) before 1970 but in western Pacific (WP) after 1970. Accompanied to the distinctive onset location shift is opposite zonal propagation of the maximum SSTA.
Physical mechanisms responsible for this interdecadal shift were investigated. A simple coupled atmosphere-ocean model suitable for air-sea interaction in WP was constructed. An eigenvalue analysis of this model demonstrates that the post-1970 mean state favors the growth of an SSTA in WP whereas the pre-1970 mean state does not. The dominant process responsible for this instability is the wind-evaporation-SST feedback, while the Ekman feedback and the zonal advective feedback also play a role. Thus, an SSTA perturbation can be easily triggered and maintained in WP under the post-1970 mean condition.
Distinctive preceding SSTA patterns occurred prior to El Niño onset between the pre- and post- 1970 periods. A single-pole La Nina-like SSTA pattern appeared in the preceding winter prior to 1970, whereas a dipole SSTA pattern with a positive (negative) SSTA in WP (EP) occurred after 1970. The different SSTA patterns led to distinctive zonal wind responses at the equator. Prior to 1970, westerly anomalies appeared in EP, which worked together with ocean dynamics and triggered El Niño onset in EP. After 1970, easterly anomalies appeared in EP, which suppress initial warming in situ through enhanced upwelling and evaporation.
While the post-1970 mean state favors the growth and maintenance of an SSTA perturbation in WP, two specific processes triggered El Niño onset in WP after 1970. One is anomalous downward solar radiation forcing associated with local atmospheric meridional overturning circulation. Another is anomalous zonal advection by the thermocline-induced geostrophic currents.
Title: Abrupt shift of El Niño periodicity under CO2 mitigation
Abstract:
Removing CO2 from the atmosphere is emerging as a viable strategy to mitigate global warming, yet the responses of the climate system to CO2 reduction remain uncertain. One of the most uncertain aspects of El Niño behavior is the change in periodicity in response to CO2 forcing. In this study, we show that an abrupt shortening of El Niño periodicity is projected once CO2 reduction begins in the ramp-up and –down CO2 experiments. Besides the contribution of slow mean state changes, this phenomenon is shown to be driven by a southward shift of the Intertropical Convergence Zone (ITCZ) and the consequent narrowing of El Niño's spatial pattern, which enhances the effectiveness of ocean heat recharge/discharge processes, thereby shortening its periodicity. This suggests that the abrupt shift in El Niño periodicity results from a cascading reaction involving ITCZ dynamics and El Niño's spatial configuration. These findings highlight the critical role of the global energy balance in shaping El Niño characteristics.
Title: Episodic slowdown of global warming by a multi-year La Niña and its subsequent temperature jump
Abstract:
Global-mean surface temperature (GMST), which has continued to rise due to anthropogenic greenhouse gas forcing, is closely related to the sea surface temperature variability in the tropical Pacific. In particular, GMST is known to warm during a strong and short-lived El Niño. By contrast, the global cooling effect of a weak and long-lived La Niña, an opposite but asymmetric phenomenon of an El Niño, has largely been unexplored. Here we show that multi-year La Niña events tend to have a stronger cooling effect on GMST based on observations and climate model simulations. The persistent La Niña cools the pantropical climate, causing a temporary GMST warming stagnancy, whereas the cooling effect of a single-year La Niña is weaker due to the lagged response of other tropical basins and its short-livedness. Once the multi-year La Niña ends, the cooling effect terminates, resulting in a larger GMST increase than after a single-year La Niña. Abrupt global warming observed in 2023 is qualitatively explained as an effect of a combination of the episodic slowdown of global warming due to the triple-dip La Niña in the early 2020s and subsequent strong El Niño.
Title: Trends and Pathways of the Contributions of Southeastern Subtropical Pacific Water to the Equatorial Undercurrent
Abstract:
By implementing an offline particle tracer scheme using high-resolution ocean reanalysis datasets, GLORYS and ORAS5, a comprehensive investigation of long-term trends in the water sources and pathways of the Equatorial Undercurrent (EUC) is performed. In agreement with previous findings, we find that the majority of the off-equatorial EUC water originates from the southeastern subtropical Pacific (SESP) (14% of total). We identify an increasing trend in the contribution of this source to the western-central Pacific portion of the EUC, and a decreasing trend in its contribution to the eastern Pacific portion. Furthermore, we observe a deepening trend in the water pathway from the SESP into the EUC, along with a southwestward shift in the flow towards the eastern Pacific portion of the EUC. We attribute these trends to the increased mixed layer depth and Ekman pumping, and accelerated northwestward currents in the SESP, resulting from increased southwest trades over the region. The identified trends in the SESP water sources and pathways may contribute to the strengthening of the equatorial Pacific zonal SST gradient by allowing for enhanced water mixing along the preferred pathway towards the EUC and cooling of the EUC core. Thus, this study provides insights into the non-local ocean contributions to the tropical Pacific SST response to greenhouse gas forcing, which is characterized by significant disagreements between climate model simulations and observations in the past decades, casting doubt into the projected decrease of the zonal SST gradient under strong anthropogenic forcing in the future.
Title: The Influence of a More La Niña-like Tropical Pacific Mean State on Global Tropical Cyclones in High-Resolution CAM6 simulations
Abstract:
Recent studies found a discrepancy in tropical Pacific mean state trends in recent decades: while observations show a La Niña-like trend pattern, most climate models project an El Niño-like trend pattern. One hypothesis attributes this to incorrect model simulation of the response to anthropogenic radiative forcing, resulting from common tropical Pacific cold tongue model bias. Zhuo et al. (2024) tested this hypothesis by conducting 2° Community Earth System Model 2 (CESM2) simulations with and without using flux adjustment to ameliorate the cold tongue bias in the tropical Pacific. They found reducing bias leads to a more La Niña-like near-term trend, aligning better with observations. The flux-adjusted simulations provide an alternative storyline to unadjusted simulations. Here we ask: How do global TCs respond to these two differing trends in the tropical Pacific?
We address the question by conducting two sets of historical High-resolution (~0.25°) Community Atmospheric Model 6 (HiCAM6) simulations. The first simulation, HiCAM6-ADJ, is forced by the flux-adjusted CESM2 lower boundary conditions with their more La Niña-like Pacific SST trends. While the control simulation, HiCAM6-CTL, adopts lower boundary forcings from coupled CESM2 simulations with their El Niño-like trend. TCs will be tracked using the TempestExtremes tracking algorithm and will be analyzed through standard diagnostics including genesis, tracks, and intensity, along with relevant ambient environmental conditions. The characteristics of the TC patterns will be further compared with those from the HighResMIP historical simulations: coupled and SST-forced runs, representing El Niño- and La Niña-like trend patterns in the tropical Pacific, respectively.
Title: Understanding the Driving Mechanisms behind Triple-Dip La Niñas: Insights from the Prediction Perspective
Abstract:
This study investigates the mechanisms and predictability of consecutive La Niña events, with a focus on the triple-dip events of 1998-2001 and 2020-2023, using a physically-based statistical ENSO prediction model (EPM). The results highlight distinct driving mechanisms behind these two events. The 1998-2001 event was primarily initiated by substantial negative heat content anomalies in the equatorial Pacific, which stemmed from the preceding strong El Niño and sustained the cold sea surface temperature anomalies (SSTA) into the third year. In contrast, the 2020-2023 La Niña event, lacking significant negative heat content, was characterized by persistent equatorial easterly wind anomalies induced by the extratropical forcing from the Southern Hemisphere. The EPM successfully captures these distinct processes, with tropical ocean-atmosphere coupling dominating the 1998-2001 prediction skill, while extratropical forcing significantly improves the forecast for 2020-2023. Incorporating extratropical influences is crucial for enhancing the prediction of multi-year La Niña events.
Title: Seasonal Varying Optimal Precursors and Predictability for Marine Heatwaves off Western Australia
Abstract:
Marine heatwaves (MHWs) off Western Australia (WA), especially the extreme 2011 event, have caused significant ecological and socio-economic impacts. Previous studies have identified links between austral summer WA MHWs and the El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) climate modes, but MHWs occur year-round, and their seasonal predictability remains unclear. To address this, we develop a cyclostationary linear inverse model (CS-LIM) using vertically averaged temperature (VAT) to ~300 m depth from an ocean model (ACCESS-OM2) simulation. We examine seasonal variations in WA MHW predictability by evaluating the CS-LIM’s forecast skill, optimal initial conditions, and the roles of ENSO and IOD. Our findings show that the CS-LIM effectively forecasts WA VAT up to 36 months in advance, demonstrating statistically significant correlations at the 95% confidence level with ACCESS-OM2 data. The optimal initial condition and growth pattern for WA MHW onset in different seasons show strong seasonal variations. La Niña of the 4-year Eastern Pacific (EP) ENSO cycle and weak positive IOD significantly contribute to the growth (thus predictability) of WA MHWs onset in austral autumn and early winter, with a lead time of approximately 12 months. Central Pacific (CP) La Niña with weak positive IOD contributes most to the predictability of WA MHWs onset in austral autumn and winter at 5- and 20-month lead times. La Niña of the 2.5-year EP ENSO cycle and strong positive IOD primarily contribute to the predictability of WA MHWs onset in late austral winter to early austral summer, especially at 5- and 20-month lead times.