Day 3 - March 14, 2025 Poster Session (Makana Room)
Title: Modulation of tropical Pacific mean state by Southern Ocean surface warming in transient and stabilized climates
Abstract:
Changes in the tropical Pacific mean state play a key role in modulating the pace of global warming and regional climate impacts. Several studies have highlighted the key role played by Southern Ocean (SO) cooling on the recent enhancement of the zonal sea surface temperature (SST) gradient and strengthening of the Pacific Walker circulation through various physical and dynamical pathways that modulate tropical southeastern Pacific SST. Future projections in climate model simulations suggest that ocean heat uptake is expected to weaken as the ocean subsurface warms, with a delayed, but rapid warming of the SO surface projected scenarios forced by high greenhouse gas (GHG) emissions. Here, we analyze the role played by the delayed SO warming in modulating the tropical southeast Pacific SST and consequently its effect on future tropical Pacific mean state changes under a transient/rapid warming scenario. Our results suggest that delayed SO warming has the potential to create a regime shift in the tropical Pacific with long-term weakening of the zonal SST gradient. Using emission-driven stabilized runs, branching off from the transient projections, at different global warming levels (GWLs), we find that the effects of the delayed SO warming are likely to persist even after we achieve net-zero CO2 emissions, with the effect being larger for stabilization runs at higher GWLs. These results indicate the importance of early stabilization through rapid GHG emissions reductions, to prevent long lasting impacts of tropical Pacific mean state shifts on regional climate variability within and outside the tropics through atmospheric teleconnections.
Title: Future changes in ENSO governed by eastward shift in air-sea coupling
Abstract:
El Niño-Southern Oscillation (ENSO) dynamics have been studied focusing on the Niño3.4 region - the region of strongest air-sea coupling. Studying future ENSO changes focusing on the fixed Niño3.4 region could be problematic because the location of coupling could change, altering the balance of processes driving the onset of El Niño events. For instance, under global warming, climate models project the Pacific cold tongue to warm more than other regions making the atmosphere in this region closer to the threshold of convection and thus more responsive to variations in sea-surface temperature. Here we show that this enhanced equatorial warming produces a robust eastward shift in the coupling region of ENSO across CMIP models. We found that as the air-sea coupling region shifts eastward into the shallower climatological thermocline of the eastern Pacific, thermocline variability is able to positively feedback on the atmosphere. This stronger thermocline feedback favors the growth of El Niño initiated by westerly wind bursts. We also expect El Niño events to become stronger as more mechanisms become involved in the growth of El Niño. Lastly, it would favor the onset of El Niño after La Niña leading to better predictability. Our results show that these changes in ENSO variability, instead of being driven by more responsive winds to SSTs, arise from the coupling shifting towards the eastern Pacific. The shallower climatological thermocline there, makes the thermocline feedback play a bigger role in the onset of El Niño.
Title: Assessing Economic Impacts: The Role of Climate Internal Variability
Abstract:
Over the past decade, understanding the impact of global warming on equitable economic development has become crucial for shaping mitigation and adaptation strategies. Global and regional temperature changes are significantly influenced by internal climate variability, yet socioeconomic climate impact studies often overlook the uncertainties arising from this variability, complicating the attribution of economic damage to climate change.
This study utilizes the Max Planck Institute Grand Ensemble with CMIP6 forcing to incorporate the observed relationship between temperature and economic growth, quantifying the uncertainty in climate-related economic damage due to internal temperature variability. The MPI-GE CMIP6, a 30-member initial-condition large ensemble, effectively quantifies the forced response, internal climate variability, and their evolution under global warming. For each country, observed and simulated population-weighted mean temperatures are calculated, and country-mean temperatures are bias-corrected using the climatology difference between the model and observation from 1991-2014. These corrected temperatures are then used to calculate climate-related economic damage.
The study finds that detecting a clear signal of climate damage in the historical period is challenging due to increased uncertainty in climate-related economic damage. This uncertainty, reflected in the percentage change in global GDP per capita, has risen from the early 1990s to the early 2010s, despite stable global mean surface temperature anomalies. Most countries have experienced a substantial increase in economic damage uncertainty, particularly pronounced in cold high-latitude and warm low-latitude countries, which are more susceptible to economic damage uncertainties driven by climate internal variability.
Title: A key role of sea surface temperature patterns to estimate the probability of extreme weather in Japan
Abstract:
We have recently developed a new method that is enable us to rapidly estimate the impact of anthropogenic warming on the occurrence probability of extreme temperature in Japan. Since this method is based on a finding that the temperature variability over Japan is significantly associated with the dominant patterns of sea surface temperature in the tropics to mid-latitude like El Niño Southern Oscillation and Pacific Decadal Oscillation, we are able to evaluate such ocean influence on a heat and cold waves in Japan. I will present our new approach and its applicability.
Title: The role of eddies in equatorial Pacific upwelling: a generalized Eliassen-Sawyer diagnostic approach
Abstract:
Upwelling in the equatorial Pacific cold tongue plays an outsized role in global climate by sustaining a globally significant time-mean regional heat uptake and carbon outgassing and by modulating cold tongue sea-surface temperatures and air-sea interaction in conjunction with ENSO variability.
While the broad-scale picture of the equatorial Pacific upwelling is explained by divergence of the Ekman transport, the mechanisms that determine the detailed meridional and vertical structure of the upwelling are much less understood. For example, the observed climatology of meridional velocity at 15 m from the global drifter program and eddy-resolving numerical simulations show a perplexing meridional asymmetry that is difficult to explain: meridional divergence and upwelling are strong at 1-3 deg N and weak at 1-3 deg S while the meridional Ekman transport divergence is symmetric about the equator.
Here, observations and output from eddy-resolving simulations are used to explore the hypothesis that eddy momentum fluxes play a major role in this perplexing meridional asymmetry of upwelling. We do this by developing and solving a generalized Eliassen-Sawyer equation with inputs from simulations. We show that the Eliassen-Sawyer equation reproduces the upwelling in the simulations reasonably well. Then we use the equation to separate the role of eddy momentum fluxes and show that it is crucial to the tongue of strong north-equatorial upwelling in the Pacific cold tongue. Finally, we will discuss the feasibility of using remote sensing and in-situ observations to test this hypothesis.
Title: Deep ocean warming-induced El Niño changes
Abstract:
The deep ocean, a vast thermal reservoir, absorbs excess heat under greenhouse warming, which ultimately regulates the Earth's surface climate. Even if CO2 emissions are successfully reduced, the stored heat will gradually be released, resulting in a particular pattern of ocean warming. Here, we show that deep ocean warming will lead to El Niño-like ocean warming and resultant increased precipitation in the tropical eastern Pacific with southward shift of the intertropical convergence zone. Consequently, the El Niño-Southern Oscillation shifts eastward, intensifying Eastern Pacific El Niño events. In particular, the deep ocean warming could increase convective extreme El Niño events by 40 to 80% relative to the current climate. Our findings suggest that anthropogenic greenhouse warming will have a prolonged impact on El Niño variability through delayed deep ocean warming, even if CO2 stabilization is achieved.
Title: Predicting the variable atmospheric CO2 growth and fluxes interplay with ENSO using Earth System Models
Abstract:
While fossil fuel CO2 emissions kept increasing and haven’t reached a peak yet, the atmospheric CO2 growth and fluxes vary in interannual to decadal time scales modulated by climate variability, with ENSO playing a major role. We have established an advanced Earth System Model (ESM)-based prediction system with a prognostic computation of atmospheric CO2. Such a prediction system enables us to predict whether, in the presence of climate variability and carbon-climate feedbacks, atmospheric CO2 and surface fluxes will change faster or slower than anticipated from changes in carbon emissions alone. Predictions initialized from the reconstructions through assimilating physical data products demonstrate high confidence in predicting changes in the global carbon cycle for the next years. As several pioneer ESM-based prediction systems were developed, we launch and coordinate CO2 predictions for the Global Carbon Budget annual reports and the WMO Lead Center for Annual-to-Decadal Climate Prediction. In this presentation, I will show reconstructions and predictions from multiple ESMs on the global carbon cycle variations. ESMs-based predictions successfully predicted many events with relatively high or low atmospheric CO2 growth in the past, including the unprecedented record-high rise in atmospheric CO2 levels in 2016. This significant increase in atmospheric CO2 was linked to the 2015/2016 El Niño event with weakened land carbon sink and elevated human-induced emissions. This leads to a greater increase in atmospheric CO2 than in other years. By leveraging the predictions from ESMs, we can further trace the regional imprints of natural and human-induced changes in the carbon cycle.
Title: New mechanism for North Pacific Oscillation influence on ENSO
Abstract:
The North Pacific Oscillation (NPO), a representative midlatitude atmospheric variability, plays an important role in the development of the El Niño-Southern Oscillation (ENSO). To explain this extratropical–tropical linkage, previous studies have focused on the atmospheric boundary layer processes coupled with the mixed-layer ocean. Different from the existing hypothesis, in this study, we propose a new mechanism to link the NPO to ENSO via upper-tropospheric teleconnections. Analyses of the wave activity flux show that wave energy associated with the NPO directly propagates from midlatitude to the tropics, modulating the tropical circulation. During the NPO event, this equatorward energy flux becomes pronounced after the NPO peak phase and persists for more than two weeks. As a result, when a positive NPO grows (here, north anticyclonic–south cyclonic circulation), upper-level easterly wind anomalies are situated along the equatorial Pacific. Accordingly, anomalous lower-level westerly winds simultaneously occur in the equatorial Pacific, contributing to the development of El Niño events. To demonstrate the wave energy propagation via the upper-level troposphere, a stationary wave model experiment was performed with an NPO-like barotropic vorticity forcing. The results show equatorward wave propagation consistent with the observation.
Title: Discrepant effects of atmospheric adjustments in shaping the spatial pattern of SST anomalies between extreme and moderate El Niños
Abstract:
The surface heat flux anomalies during El Niño events have always been treated as an atmospheric response to sea surface temperature anomalies (SSTAs). However, whether they play roles in the formation of SSTAs remain unclear. In this study, we find that the surface net heat flux anomalies in different El Niño types have different effects on the development of the spatial pattern of SSTAs. By applying the fuzzy clustering method, El Niño events during 1982–2018 are classified into two types: extreme (moderate) El Niños with strong (moderate) positive SSTAs, with the largest SSTAs in the eastern (central) equatorial Pacific. The surface net heat flux anomalies in extreme El Niños generally display a “larger warming gets more damping” zonal paradigm, and essentially do not impact the formation of the spatial pattern of SSTAs. Those in moderate El Niños, however, can impact the formation of the spatial pattern of SSTA, by producing more damping effects in the eastern than in the central equatorial Pacific, thus favoring the largest SSTAs being confined to the central equatorial Pacific. The more damping effects of net heat flux anomalies in the eastern equatorial Pacific in moderate El Niños are contributed by the surface latent heat flux anomalies, which are mainly regulated by the negative relative humidity–SST feedback and the positive wind–evaporation–SST feedback. Therefore, we highlight that these two atmospheric adjustments should be considered during the development of moderate El Niños in order to obtain a comprehensive understanding of the formation of El Niño diversity
Title: Understanding deep learning predictions of strong eastern Pacific El Niño
Abstract:
El Niño can produce severe climatic impacts in coastal Peru and Ecuador, particularly associated with strong eastern Pacific warming and heavy precipitation in the coastal desert, such as in 1983 and 1998. However, global climate models have relatively low seasonal forecast skill in this region. Artificial intelligence (AI), particularly deep learning, offers the possibility of improving this skill at a low computational cost, but has the limitation that it is not based on physics and, therefore, may not perform adequately under new climate conditions.
In this study, we present the IGP-UHM AI model, based on a convolutional neural network, that predicts monthly eastern and central equatorial Pacific sea surface temperature (SST) indices for up to 12 months in advance, taking the monthly SST, sea surface height (SSH), and surface zonal and meridional wind for the most recent three months. The model was trained with long climate model simulations and fine tuned with observational reanalysis data. The independent testing with observational data for 1985-2022 indicates good skill in the eastern Pacific.
Using an explainability tool for AI, we can find that the predictions of strong El Niño are reasonably supported by documented precursors, i.e. the model appears to have learned adequately. However, in contrast to physics-based models, when we subject the AI model to artificial scenarios in which the favorable precursor values are even reversed, the AI model predictions in some cases do not present the expected sensitivity. Although this does not invalidate the usefulness of the AI model, it reminds us that machine learning does not establish causality and does not produce necessarily an adequate conceptual model of the world.
Title: ENSO Spatiotemporal Complexity in a Hierarchical Extended Recharge-Oscillator Model
Abstract:
The El Niño-Southern Oscillation (ENSO) features significant spatiotemporal complexity and is characterized by irregular occurrences of multiple ENSO flavors. In our previous work, sensitivity experiments with an intermediate coupled model suggest the essential effect of scale interactions for such complexity. Specifically, the nonlinear interaction between the generic ENSO mode(s) is found to efficiently result in the coexistence of multiple ENSO types. Moreover, ENSO’s interactions with the annual cycle and other climate modes bring in more irregularity. In this talk, we will discuss whether and how this mechanism can be reproduced in a low-order conceptual model. Various model complexities are introduced into the Recharge-Oscillator (RO) model in a hierarchical setting to investigate the validity of the above-mentioned mechanism. We will begin by presenting a minimalistic extended RO model in which the central Pacific SSTA is considered an additional independent variable. The model simulation forced by stochastic forcing exhibits observation-like ENSO spatiotemporal complexity. Next, we will further extend the model by replacing the equatorial basin-averaged thermocline fluctuation, which is the single variable in the ocean dynamics component of the classic RO model, with thermocline fluctuations averaged in multiple zonal boxes along various latitudinal strips. This allows both the two generic ENSO modes (i.e., recharge-oscillator mode and wave-oscillator mode) to be resolved. The fundamental mechanism for ENSO spatiotemporal complexity will be revisited by comparing ENSO behavior in simulations where these additional independent model variables, along with the stochastic forcing, are individually switched on and off in the RO model.
Title: The future in the past: Risk of more frequent extreme El Niño supported by paleoclimate data and models
Abstract:
Are extreme El Niño events becoming more frequent? We approached this question focusing on the Last Glacial Maximum, a past interval with robust paleoclimatic evidence of much weaker ENSO variability. Using model simulations, we show that past and future ENSO changes are connected to warming and cooling via a single mechanism governed by changes in the strength of the Walker Circulation. This mechanism is consistent with the dynamics of observed extreme El Niño events, which develop when warm pool waters expand rapidly eastward due to strongly coupled ocean currents and winds. These coupled interactions weaken under glacial conditions because a deep mixed layer makes zonal currents less responsive to wind variability during the onset of El Niño -- hindering the occurrence of extreme events. Conversely, a shallow mixed layer under greenhouse warming makes currents more responsive to winds favoring the onset of extreme El Niño events. This consistent mechanism linking past with future along with a validation against past changes increases our confidence in model predictions of more frequent extreme El Niño in the future. The outstanding source of uncertainty is the response of the mean state – cementing this question’s importance.
Title: A new tool to attribute ENSO’s impact
Abstract:
ENSO teleconnections can have a large impact on the climate of many regions around the globe. These teleconnections are complex, particularly in Australia, where large differences are seen seasonally, spatially and even between La Niña and El Niño events. In this study, we use a new approach to explore and quantify these differences, and provide a clear communication tool for community outreach. We use Fraction of Attributable Risk and Risk Ratio, two methods that have been used in the medical and climate change communities for the attribution of certain risk factors to a particular event. In our case, we used the observed monthly rainfall distributions as our event with the risk factors being either a La Niña or El Niño event, compared to neutral conditions. This method provides the public with an historically accurate quantification of the impact of ENSO. For instance, the contribution of ENSO to a past event, or provide the ENSO-induced probability change of a hypothetical rainfall monthly total.
The benefits of this analysis include: no assumption of linearity, so ENSO teleconnection asymmetry is well captured; furthermore, as the analysis is performed on the full monthly distribution, the influence of ENSO on either above or below mean rainfall or extreme rainfall events are easily identified. In addition, we use this technique to explore the unique seasonality of ENSO teleconnections and identify regional hotspots that show persistent influence during a La Niña or El Niño event.
Title: Redefined background state in the tropical Pacific resolves the entanglement between the background and ENSO
Abstract:
Understanding the co-variability between the El Niño–Southern Oscillation (ENSO) and the background state in the tropical Pacific is critical for projecting future ENSO. The difficulty is rooted in a circular logic that the background state routinely defined by multi-decadal mean modulates, and is modulated by, ENSO. This circularity arises due to the asymmetry between El Niño and La Niña, resulting in a non-zero mean, referred to as the ENSO rectification effect. Here, we develop a method based on Box–Cox normalization to define the tropical Pacific background state and its associated anomalies, which removes the ENSO rectification effect and is referred to as the normalized mean state. The normalized mean state accurately quantifies ENSO-related anomalies, ENSO asymmetry, and the ENSO rectification effect. It is evident in both observations and model simulations that the normalized mean state has a clear asymmetric impact on the amplitude of ENSO. A warm background state weakens El Niño but strengthens La Niña through two key processes: the nonlinear response of precipitation to SST and oceanic zonal advection feedback. The normalized mean state successfully solves the circular reasoning fallacy resulting from ENSO asymmetry and offers a framework to study ENSO and tropical climate dynamics with far-reaching impacts on global climate.
Title: Subsurface chlorophyll-a concentration in the tropical Pacific
Abstract:
Chlorophyll-a (Chl-a) concentration exhibits subsurface maxima in the tropical Pacific at 40–80 m depth, which is under a mixed layer but in a photosynthetically active layer. During El Niño, subsurface Chl-a concentrations are higher in the middle and eastern equatorial Pacific but lower to the west in comparison with La Niña, a pattern which is opposite to that on the surface. The spatiotemporal variability of the Chl-a concentrations has implications to not only for the biogeochemical cycling in the ocean but also for understanding the thermal structure and dynamics of the ocean via absorption of shortwave radiation. We plan to deploy a BGC-Argo float with a Chl-a sensor and collect Chl-a profiles twice a day. The diurnal cycle of Chl-a will be observed and the impact of Chl-a profiles on ENSO prediction will be explored.
Title: Data-driven approach in investigating the differences in wind measurement in the Tropical Pacific between buoys and QuikSCAT.
Abstract:
Accurate wind measurements in the Tropical Pacific are essential for atmospheric and oceanic studies. We investigate the discrepancies between wind measurements from the TAO buoy array and the QuikSCAT satellite. While buoys provide direct and localized data, they are limited in spatial coverage and subject to calibration errors and mechanical issues. QuikSCAT, on the other hand, offers comprehensive spatial coverage but relies on indirect measurements of sea surface roughness, which can be influenced by atmospheric conditions and rain, introducing potential biases. We employ a data-driven approach to identify conditions associated with good and poor agreement between QuikSCAT and buoy wind measurements. We find that the discrepancies increase under higher relative humidity and SST, low winds, and slightly lower air temperature. Most of these discrepancies come from the ITCZ and the western Equatorial Pacific region. This suggests poor rain flagging in QuikSCAT as a likely contributor. However, using a random forest model for independent rain flagging revealed a significant fraction of discrepancies from non-rainy conditions, indicating other underlying sources. These findings can help improve the understanding of the biases in situ and/or satellite wind measurements and correct them.
Title: Mid-Holocene ENSO Variability reduced by northern African vegetation changes: a model intercomparison study
Abstract:
The relationship between the mean state of the Pacific Ocean and El Niño Southern Oscillation (ENSO) and its variability through time is inadequately understood, especially on longer timescales. Several studies have indicated that the mid-Holocene (6,000 years before present) was characterized by stronger east-west temperature contrast and lower ENSO variability relative to the present day. While climate models show a reduction in ENSO variability, they underestimate this reduction compared to many paleoclimate reconstructions. Further, the drivers behind these changes remain unclear. In this work, we use five global climate models to show that incorporating vegetation changes over northern Africa during the mid-Holocene are vital to capturing global circulation changes. Greening the Sahara alters the Walker Circulation, enhancing zonal temperature and pressure gradients in the equatorial Pacific and driving it to a La Niña-like state. Incorporating Green Sahara boundary conditions leads to reductions in interannual variability in all Niño index regions relative to orbital and GHG changes, with reductions of up to 18% in the Niño3.4 region. Our work highlights the importance of the Atlantic influence on ENSO and provides paleoclimatic evidence for this synergistic teleconnection.
Title: Stationarity of tropical Pacific teleconnections to the North Pacific and North America
Abstract:
ENSO causes pronounced precipitation and associated diabatic heating anomalies in the tropics, which in turn result in upper-level divergence and Rossby wave propagation to the extratropics. It is essential that these atmospheric teleconnections are realistically represented by climate models. However, coupled model intercomparison projects (CMIPs) exhibit a wide spread in the characteristics of ENSO teleconnections across models—so-called structural uncertainty. Determining which models are realistic is complicated by the temporal limitations of the observations used for validation and the fact that ENSO teleconnections can vary in space and time—so-called nonstationarity (time uncertainty). We systematically characterize structural uncertainty and nonstationarity in ENSO teleconnections to the North Pacific and North America using statistical climate models, reanalyses, and climate models, with simulations from CMIP5 and 6. We find that CMIP-class models largely simulate ENSO teleconnections that are consistent with the statistics of reanalyses and that there are no systematic differences between the current (CMIP6) and previous generation of models. However, in some models nonstationarity in ENSO teleconnections is so large that the consistency with reanalyses likely resulted from a common response to external forcing. Interestingly, teleconnection nonstationarity also varies between models, suggesting that nonstationarity itself has a large degree of structural uncertainty. Determining if models with nonstationarity are realistic is critical, as the presence of nonstationarity would undermine seasonal forecasting efforts and the paleoclimate reconstruction problem. As such, we use mean state and variance metrics to relate structural uncertainty and nonstationarity in ENSO teleconnections to the representation of climate in each model.
Title: Fingerprints of atmospheric rivers in marine heatwaves
Abstract:
Marine heatwaves (MHWs) are intensifying worldwide, threatening marine ecosystems and regional climate stability. Yet their physical mechanisms remain poorly understood. How atmospheric rivers (ARs) influence marine heatwaves are largely unknown. Here, we analyze satellite observations and reanalysis products to show that ARs can have strong impacts on MHWs across the global oceans. Further, heat budget analysis is performed to reveal the relative contributions from AR-induced changes in turbulent heat fluxes and radiative fluxes. The magnitude and sign of the ARs’ impacts on MHWs vary in space and with season, depending on the delicate balance among different mechanisms. These findings highlight a previously unrecognized role of ARs in driving oceanic heat extremes and offer new perspectives on MHW predictability in a warming climate.
Title: Importance of the Western Boundary Component of the Subtropical Cells on Ocean Heat Content Variability associated with Tropical Pacific Decadal Variability
Abstract:
The Subtropical Cells (STCs) in the Pacific Ocean are recognized as important elements of Tropical Pacific Decadal Variability (TPDV). Convergence/divergence of meridional heat transport by the STCs across the tropical-subtropical boundaries modifies the sea surface temperature in the equatorial Pacific through the modulation of the equatorial upwelling. Although most of the studies focus on the transports within the interior ocean (IN) far from the lateral boundaries, the contribution of the western boundary currents (BC) could have significant impacts on the transport.
In this study, the role of the STCs in TPDV is revisited in detail using outputs from a high-resolution OGCM, with a particular focus on the contribution of the BC in terms of the meridional heat transports across 10N/S. The decadal variations of the meridional heat transport convergence by the STCs largely explain those of the ocean heat content in the tropical Pacific, which is closely related to TPDV. Comparison of the heat transport variations among branches of the STCs, i.e. the Ekman transport (EKM), the IN, and the BC, show that a significant part of the IN is compensated by the BC in the Northern Hemisphere and about half of the IN in the Southern Hemisphere. The rate of compensation greatly varies over the study period depending on the compensation among the overturning components, composed of the EKM and the IN, particularly in the Northern Hemisphere. These results clearly demonstrate the importance of the BC contribution for the quantitative assessment of the role of the STCs in TPDV.
Title: Noise or nonlinearity? Avoiding tricky questions about ENSO asymmetry using Koopman theory
Abstract:
The recharge-oscillator model (ROM) captures the essential dynamics of ENSO and can explain several of ENSO’s most prominent characteristics. However, the ROM cannot represent the societally-impactful asymmetry between the warm and cold phases of ENSO (e.g., La Niñas – and their impact on North American rainfall – tend to last longer than El Niños). While it’s possible to make the ROM asymmetric by forcing it with state-dependent noise or including non-linear terms, these extensions require an artificial distinction between noise and non-linearity, limiting insight into which physical processes are most important for observed asymmetry.
In this work, we model ENSO with an “embedded” version of the ROM, inspired by Koopman theory, which permits warm/cold asymmetry while avoiding a priori assumptions about the causes of observed asymmetry. As in the original ROM, the model state evolves linearly – and symmetrically – as a single pair of eigenmodes. Unlike in the ROM, this evolution occurs in an embedded state. Because the embedding function is nonlinear, the “observed state” (e.g., the Niño 3.4 index) can evolve asymmetrically.
We find that the spring predictability barrier is reduced in the embedded model compared to the ROM, which suffers from a strong bias in the spring (strong El Niños tend to decay faster – and strong La Niñas slower – than predicted by the ROM). The embedded model also captures the prominent phase asymmetry in the growth stage of ENSO, which allows it to better identify developing La Niñas with the capacity to persist for multiple years.
Title: Evaluation of the tropical ocean observing system through SynObs international multi-system OSEs
Abstract:
“Synergistic Observing Network for Ocean Prediction (SynObs)” is a project under the United Nations Decade of Ocean Science for Sustainable Development. SynObs aims to find the way to extract maximum benefits from the combination among various ocean observation platforms, including satellite and in situ observations. A major ongoing effort led by SynObs is the interntional multi-system OSEs/OSSEs. In this activity, multiple operational centers and research institutes will conduct Observing System Experiments (OSEs) and Observing System Simulation Experiments (OSSEs) using a variety of ocean or coupled ocean-atmosphere prediction systems based on the standardized guideline of the experimental setting. Through this multi-system activity, we aim to evaluate ocean observation impacts which are robust for most ocean prediction systems. In this activity, we will evaluate the impacts of various observation data, including tropical mooring, Argo floats, and satellite altimetry data.
The analysis of the OSE results will be conducted under the collaboration with ocean observational communities. We plan to analyze the difference in sea surface temperature and salinity, 0-50m and 0-300m vertically averaged temperature, tropical cyclone heat potential, mixed layer depth, sea surface height, 15m current velocity, and other variables between each OSE and the control run. We will also investigate the dependency of the multi-system ensemble spread on the assimilated observation data. We will present an overview of our OSE/OSSEs and their preliminary results, focusing on the impact of the Tropical Pacific Observing System (TPOS) on the ocean state monitoring and predictions related to ENSO.
Title: Decadal variability of the Kuroshio Extension and its non-stationary relationship with the tropical Pacific in an eddy-resolving CESM simulation
Abstract:
The Kuroshio Extension (KE) exhibits significant decadal variations, particularly following the 1976/77 Pacific climate regime shift. Due to robust ocean-atmosphere interactions over the KE, imply a potential key role in basin-scale climate variability. Recent studies suggest that North Pacific Oscillation (NPO)-like atmospheric teleconnections from the central tropical Pacific dominantly influence KE decadal variability through westward propagating oceanic Rossby waves. However, this relationship varies on interdecadal timescales and only achieves statistical significance post the regime shift. This study utilizes outputs from an unprecedented 500-year pre-industrial control simulation conducted with an eddy-resolving coupled general circulation model to explore the potential and mechanisms of natural variability-induced interdecadal modulation. The entire simulation period is segmented into five segments, and subsequent analyses are conducted for each period. The results reveal a relationship between the KE and the central tropical Pacific, resembling that of the post-regime shift period, during the segment marked by the strongest decadal variability in the central tropical Pacific. The strong tropical variability may be reinforced by the high- frequency NPO and partly triggered by the high-frequency NPO. In contrast, the relationship is disrupted in other periods, likely due to weaker and less persistent tropical variability, differences in the location of atmospheric teleconnections, and/or influences of the Kuroshio large meander. These findings suggest that interdecadal modulation of the KE may occur independently of anthropogenic forcing. Furthermore, the amplitude and persistence of central tropical Pacific variability emerge as critical factors influencing its tight relationship with the KE through atmospheric teleconnections and oceanic Rossby waves.