Chile Niño is a seasonally modulated coastal warm mode that affects northern–central Chile and challenges early-warning systems because events are short-lived and geographically confined. Here we assess the predictability of the Chile Niño Index (CNI) using a hierarchy of data-driven inverse models, ranging from a baseline Linear Inverse Model to Seasonally Varying and nonlinear extensions, and a lightweight hybrid scheme in which a Long Short-Term Memory network is trained to correct systematic forecast residuals. We also evaluate Gross Coastal Warming (GCW), a complementary metric of total coastal SST anomalies over the Chile Niño region, to distinguish the predictability of total coastal warming from that of the residual coastal mode represented by the CNI. We first show that the Linear Inverse-Model framework reproduces key characteristics of coastal sea-surface temperature variability associated with Chile Niño, supporting its suitability for predictability assessment. Deterministic and probabilistic verification identifies a clear window of forecast skill for austral-autumn initializations (April–May) at short lead times (1–3 months), together with a secondary but shorter-lived enhancement for early-winter initializations (June–July) that is largely confined to 1–2-month leads. Within these windows, the Extended CS-LIM yields the strongest correlations, the lowest normalized errors, and the most reliable probabilities of warm and cold coastal conditions. The hybrid correction does not substantially increase the short-lead forecast skill maximum, but it provides consistent relative improvements at intermediate lead times (4–8 months), particularly when warm eastern Pacific events precede the target period. The GCW experiment shows higher forecast skill than the CNI and remains skillful through lead 4, indicating that part of coastal-warming predictability is associated with the basin-scale component retained in total coastal SST anomalies. A case study of the 2017 event, one of the strongest on record, illustrates the added value of the hybrid correction: the inverse model captures the onset of coastal warming but underestimates its magnitude and delays the phase transition, whereas the hybrid scheme better maintains warm persistence and improves the timing of the reversal. Overall, our results indicate that a seasonally explicit stochastic Inverse-Model framework, augmented with a parsimonious data-driven correction, provides a physically interpretable and computationally efficient basis for diagnosing and forecasting coastal warming in the southeast Pacific.

Oceanic coastal trapped waves transmit energy over long distances and influence coastal variability, affecting sea level, currents, temperature, and nutrient fluxes, with consequences for marine ecosystems. Previous studies in the tropical Atlantic have demonstrated the conversion of equatorial Kelvin waves into coastal trapped waves at the eastern boundary. However, the connection between the western tropical Atlantic and the equatorial band has received less attention. Here, we identify an intercontinental oceanic teleconnection linking South America and Africa through the equatorial and coastal waveguides in the Atlantic using a global ocean reanalysis. Intraseasonal (40-130 days) coastal trapped waves propagate northward along the South American coast and occasionally reach the equator, where they convert into equatorial Kelvin waves. These signals cross the basin and subsequently generate coastal trapped waves along the African coast, forming a continuous trapped-wave corridor connecting the continental margins. Episodically, the connection extends further, with waves traced back to the equatorial Pacific. Coastal trapped wave activity exhibits strong seasonal and interannual modulation, controlled by background circulation and large-scale atmospheric forcing associated with the Madden-Julian Oscillation, ENSO, and the Antarctic Oscillation. Our results reveal an unreported pathway for oceanic connectivity across the tropical Atlantic, with potential implications for Atlantic Niño preconditioning. A continuous pathway for oceanic trapped waves bridges the Atlantic basin, linking the South American and African coast, according to analyses of sea surface height data.

Extreme coastal flooding often arises when large-scale climate patterns and local ocean–atmosphere variability combine to magnify water levels beyond what communities can withstand. Understanding and anticipating these interactions is essential for protecting vulnerable coastlines. Here we aim to determine how two major modes of climate variability—the El Niño/Southern Oscillation and the North Atlantic Oscillation—individually and jointly influence extreme coastal water levels worldwide. Using global observational and reanalysis datasets spanning 1958–2023, we analyse their separate effects and diagnose potential nonlinear interactions through statistical and process-based methods. We show that specific, seasonally aligned phases of these two climate modes interact nonlinearly, producing coastal water levels far higher than expected from either mode alone. These combinations enhance storm activity and wave conditions from the eastern seaboard of North America to western Europe and the Mediterranean. We further show that incorporating these nonlinear interactions into a conceptual climate model enables skilful seasonal predictions of coastal flooding hazards several months in advance, demonstrating the feasibility of reliable early-warning systems for coastal risk reduction. Co-occurring phases of the El Niño/Southern Oscillation and North Atlantic Oscillation can interact nonlinearly, amplify coastal water levels and increase flood risk along the United States East Coast and in Europe, according to climate data over 1958–2023.

Mesoscale oceanic eddies ((100)km) play a key role in regulating climate by affecting the transport of heat and water mass properties. However, most climate models struggle to simulate these eddies due to coarse horizontal resolution. Using a 120-year ultra-high-resolution simulation with 1/10 horizontal ocean resolution, conducted with the Community Earth System Model, this study examines how eddy activity off Peru changes during Eastern Pacific (EP) El Ni & ntilde;o events. The model shows a meridional dipole in eddy activity during EP El Ni & ntilde;o, with increased energy north of similar to 14S and decreased energy to the south, matching satellite-data except for the extreme 1997/1998 El Ni & ntilde;o. This pattern is caused by baroclinic instabilities linked to El Ni & ntilde;o-driven changes in the equatorial undercurrent. The study also finds a nonlinear relationship between the strength of the dipole pattern and El Ni & ntilde;o-related equatorial influences, providing new insights into the equatorial oceanic teleconnection along the coast of Peru.

A large proportion of intertidal benthic marine species have a planktonic larval stage that enables connectivity among populations, with the duration of the planktonic larval period (PLD) closely reflecting population genetic structure of these species along the Southeast Pacific coast. Species with direct or short planktonic development exhibit genetic structure coinciding with major biogeographic breaks, whereas species with long planktonic development show no differentiation. Species with intermediate development have been less studied. In this study, we examined the population genetic structure of Petrolisthes violaceus along the Chilean coast, which exhibits intermediate PLD. We analyzed samples collected from eight sites between 2023 and 2024, spanning from 20 degrees S to 42 degrees S (approximately 2500 km). A total of 5443 Single Nucleotide Polymorphisms (SNPs) from 165 individuals were analyzed. Our results suggest the presence of two genetically differentiated populations: a northern population extending from Iquique (20 degrees S) to Vi & ntilde;a del Mar (33 degrees S), and a southern population ranging from Tom & eacute; (36.6 degrees S) to Ancud (41.8 degrees S). Connectivity analyses revealed high gene flow among sites within each population, and biophysical model experiments revealed different dispersal patterns in the two populations. Along the Chilean coast, species with larval development lasting > 6 weeks typically exhibit no spatial population genetic structure, whereas species with abbreviated development (< 2 weeks) tend to display several genetic populations along the coast. Consistent with this pattern, P. violaceus, with an intermediate PLD of approximately 25 days, exhibits genetic differentiation in its population, with a northern and southern population spanning approximately 1400 km and 590 km, respectively.

Coastal ecosystems face multiple stressors, and their responses are magnified by global stressors associated with climate change, such as warming and ocean acidification (OA). Oyster farming are vulnerable to the effects of these stressors. Environmental monitoring technologies have been proposed as an adaptive strategy to OA. This study examined the perceptions of the oyster farmers in Bahia San Quintin, Mexico, toward this strategy. Through surveys and workshops, we identified the main challenges oyster farmers face in their industry, their level of awareness about OA, and their openness to adopting new technologies. Most respondents (66 %) did not recognize OA, which suggests that they had a low perception of its risks and its potential consequences for their activities. The most frequent problems were environmental issues (48 %), such as extreme temperature events, biofouling, and predation, followed by limited technical and financial resources (34 %). Recognizing the negative effect that high temperatures have on their activity, especially during El Nino Southern Oscillation (ENSO) events, is a positive outcome, as it allows them to adopt strategies to cope with OA. The main barriers to adopt new technologies were related to management issues (56 %), including a lack of economic resources. We recommend that interactions between oyster producers, academia, and governmental actors must be strengthened to promote environmental monitoring, thus improving their adaptive capacity and reducing potential impacts of stressors on their industry, such as climate change and OA. This study case is a valuable reference for other oyster farming communities in similar environmental and socio-economic contexts.

The Humboldt eastern boundary upwelling system, due to its proximity to the equator is highly sensitive to equatorial Pacific disturbances, particularly those associated with the El Niño Southern Oscillation (ENSO). This has consequences for the so-called oxygen minimum zone, an extensive area of low oxygen waters at intermediate depths that impacts marine biota. During warm El Nino events, the OMZ tends to shrink in its upper margin along both Peru and central Chile but the magnitude of the change is sensitive to the characteristics of El Niño events. Here, we focus on the subtropical OMZ off Chile (18°-38°S) and document OMZ volume and pattern changes during extreme El Niño events (1982/83 and 1997/1998) based on a regional coupled model simulation. This is contrasted to changes that occurred in 1972/73 that correspond to the occurrence of a moderate El Niño event in the tropical Pacific. The results indicated that the volume of the subtropical OMZ off Chile decreased on average by 27-48% during these El Niño events, which was associated with a coastal oxycline deepening that peaked during the development phase of the events. However, we find that the magnitude of the change varies a lot between events with in particular the 1972/73 El Niño exhibiting the largest changes in volume. The model analyses reveal that the OMZ volume reduction resulted from a combined effect of changes in the poleward transport oxygen-poor waters by the Peru-Chile undercurrent and the contribution of Ekman pumping (negative wind stress curl) and mesoscale eddy fluxes.

Coastal El Niño events in the Eastern Boundary Upwelling system off Peru have garnered significant attention due to their substantial societal impacts. The recent events, 2017 and 2023, rank amongst the strongest on record, raising concerns about their future behavior. This study relies on the CESM2 Large Ensemble (CESM2-LE) to explore how the frequency, intensity and spatial patterns of coastal events may evolve throughout the 21st century. Initially, an evaluation of the model revealed a pattern bias associated with a too energetic South Pacific Meridional Mode (SPMM) and a weaker North Pacific Meridional Mode (NPMM), both patterns known to affect coastal warming. Nevertheless, the model realistically simulates precipitation during coastal events in both their cold and warm phases and captures a strong link to Pacific Meridional Modes (PMMs). At the end of the 21st century, warm coastal events are expected to become 40% less frequent but are associated with a precipitation increase of approximately 2 mm/day due to increased sea surface temperatures in the mean state. Future climatological precipitation levels during February-March-April (FMA) from the third decade of the 21st century onward are projected to match those currently seen during extreme events, such as the 2017 Coastal El Niño episode. Coastal La Niña, conversely, exhibits no meaningful change in frequency or intensity, but may serve as intervals of moderate rather than extreme precipitation in the future.

Extreme weather and climate events result from complex interactions between physical processes at different scales. The convergence of multiple factors, including large-scale environmental conditions and local climate variability, can amplify the effects, resulting in significant societal impacts. Coastal regions are particularly vulnerable to sea level rise and changes in coastal water levels (CWL) due to climate variability, ocean circulation, and atmospheric conditions. The El Niño/Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) are key drivers of interannual CWL variability in the Northern hemisphere, influencing storm activity, flooding, and erosion, with ENSO affecting the Pacific and NAO the Atlantic. While studies have extensively analyzed their independent effects, their combined influence on coastal hazards remain underexplored. This study uses diverse observational datasets to assess the modulation of extreme CWL and associated hazards by different phases of ENSO and NAO. We show that the frequent occurrence of La Niña conditions, although relatively weak in terms of severity, and the comparatively rare but exceptionally strong extreme El Niño events make the world's coastlines more vulnerable to flooding overall. However, the picture is different regionally, especially in the Euro-Atlantic sector, where the co-occurrence of El Niño events and different phases of the NAO tends to exacerbate extreme CWL compared to the local NAO variability alone due to the strengthening of the Pacific-Atlantic jet stream teleconnections either in the high or mid latitudes, depending on the ENSO type and the NAO phase. These results highlight the climate modes’ compounded risks to coastal populations that allows us to produce skillful seasonal forecasting of coastal hazards using the newly developed XRO model.

The El Niño-Southern Oscillation (ENSO) is characterized by a zonal (longitudinal) displacement of the background Walker circulation across the equatorial Pacific, or spatial shifting. The warm pool edge position (WPEP) commonly measures the intensity of this shifting. Hereby organizing sea surface temperature (SST) maps according to WPEP quantiles, we construct a synthetic, continuous sequence of patterns—termed the pattern continuum—that captures the SST evolution along the movement, namely the gradual transition from broad Central Pacific La Niña cooling to Central then Eastern Pacific El Niño warming. This pattern continuum can also be approximated by a “shifted-mean” framework, wherein a fixed spatial structure shifts zonally. The present diagnosis provides insight into key dynamical features of the ENSO, such as its spatial diversity, asymmetry, and nonlinearity, including the quadratic relationship between SST principal components. It may also be useful for practical model evaluation and intercomparison.

Anthropogenic climate change (CC) has triggered a cascade of impacts on marine ecosystems, often referred to as the ‘deadly trio’: warming, acidification, and deoxygenation. While these stressors will globally lead to the compression of marine habitats, their regional effects vary significantly and remain understudied. This is particularly true for the southeastern Pacific (SEP), which supports rich pelagic and benthic ecosystems closely linked to a complex seafloor featuring archipelagos and extensive seamount chains. Using model simulations from Phase 6 of the Coupled Model Intercomparison Project, this study examines future regional-scale environmental changes in the SEP. Our analysis builds on the observation that the South Pacific Ocean Gyre is among the regions experiencing the least warming globally and that the epipelagic zone within the oxygen minimum zone (OMZ) may oxygenate in the future. These conditions may promote habitat expansion, which we assess using the climate velocities for temperature, oxygen, and pH. Estimates of climate velocities from the ensemble model mean under a pessimistic near future (2015-2050) yield values ranging from –730 to 449 km/year, exhibiting greater absolute climate velocities for oxygen than pH. Over the longer-term horizon (2015–2100), the area of zones where absolute climate velocity exceeded the 75th percentile increased by 65%, 72%, and 215% for temperature, oxygen, and pH, respectively. The strongest velocities (absolute value) occur in the equatorial sector and in the Humboldt system. While all regions mostly show a climate-driven habitat loss due to surface-to-200 m pH decline, two broad areas benefit from conservation below the surface: a region in the tropics extending from 10°S–100°W to the east of Rapa Nui and the coastal region of Peru and Chile, extending up to the Desventuradas and Juan Fernández archipelagos. While the former is due to the slow warming rates (<2.9 km yr ^−1 ), the latter results from both slow deoxygenation and oxygenation climate velocities (between −2.9 and 2.9 km yr ^−1 ) along the coast of those countries, a zone that overlaps with the lowest changes in pH in the SEP, giving them a unique conservation value. We demonstrate that epipelagic ecosystems within the OMZ may be less impacted by CC than those outside of it. These findings highlight key areas for conservation under future ocean warming, deoxygenation and pH changes.

The El Ni & ntilde;o (EN) event of 2023 exhibited a unique evolution, starting with an extremely warm coastal episode off Peru followed by a moderate basin-scale event. We addressed the associated oceanographic and biological conditions (chlorophyll and zooplankton biomass) in the Humboldt Archipelago. It is part of the Coquimbo upwelling system (29 degrees-30 degrees S), within the Humboldt Current System. Eight (8) campaigns between November 2022 and December 2023, over a deep canyon surrounding the archipelago, provided hydrographic profiles and samples for determinations of chlorophyll (Chl) and particulate organic carbon (POC) concentrations and large- and small-sized mesozooplankton biomass. Oceanographic variability over the period was analyzed through reanalysis products and satellite observations, including data of sea level, surface wind, sea surface temperature (SST) and sea surface chlorophyll, geostrophic currents, and mixed layer depth (MLD). Upwelling was promoted by high-frequency variability in winds and deeper MLD associated with the basin-scale EN. The EN also fosters the arrival of a Kelvin wave in June and July, leading to positive anomalies in SST and sea level, elevated oxygen levels, and increased pH in the upper 100 m. The lowest Chl concentration was recorded after the warming event, while POC concentration and mesozooplankton biomass exhibited temporal and vertical stability. However, a significant surface increase for both size fractions was observed during the spring. Zooplankton biomass was correlated to food resources and transport, suggesting stronger regulation by local drivers. Current findings are discussed in the context of recent studies that have documented the local circulation patterns in this region.

Linear Inverse Models (LIMs) are widely used data-driven tools for studying El Ni & ntilde;o Southern Oscillation (ENSO). However, standard LIMs struggle to simulate the observed asymmetry and diversity of ENSO events. Observations reveal that strong Central Pacific (CP) La Ni & ntilde;as and extreme Eastern Pacific (EP) El Ni & ntilde;os occur more frequently than their counterparts, a feature standard LIMs fail to capture. We introduce a modified model, the Non-Gaussian LIM (NG-LIM), which transforms the LIM variables to better simulate ENSO asymmetry and diversity. Specifically, the NG-LIM reproduces the spatial pattern of sea surface temperature (SST) skewness and the inverted U-shaped relationship between the first two principal components of Tropical Pacific SST anomalies, reflecting more frequent strong CP La Ni & ntilde;as and extreme EP El Ni & ntilde;os. NG-LIM simulations also show El Ni & ntilde;os that are stronger and evolve more rapidly than La Ni & ntilde;as. This improved inverse model generates synthetic events to supplement the limited observational record.

Abstract. Predicting the ocean state in support of human activities, environmental monitoring, and policymaking across different regions worldwide is fundamental. To properly address physical, dynamical, ice, and biogeochemical processes, numerical strategies must be employed. The authors provide an outlook on the status of operational ocean forecasting systems in eight key regions including the global ocean: the West Pacific and Marginal Seas of South and East Asia, the Indian Seas, the African Seas, the Mediterranean and Black Sea, the North East Atlantic, South and Central America, North America (including the Canadian coastal region, the United States, and Mexico), and the Arctic. The authors initiate their discussion by addressing the specific regional challenges that must be addressed and proceed to discuss the numerical strategy and the available operational systems, ranging from regional to coastal scales. This compendium serves as a foundational reference for understanding the global offering, demonstrating how the diverse physical environment – ranging from waves to ice – and the biogeochemical features besides ocean dynamics can be systematically addressed through regular, coordinated prediction efforts.

Fronts are ephemeral structures in the ocean that mark the boundaries between water masses of different properties, attracting a wide range of marine organisms, from plankton to whales. Despite their fundamental role in marine ecosystem functioning, the association with biodiversity has mainly focused on single species in regions with high data availability. Here, using multidecadal datasets on dynamical and thermal fronts, satellite tracking, and aerial observations, we assess marine megafauna associations with ocean fronts in the ecologically rich yet highly turbulent Mozambique Channel. We find that a diverse array of species associate with various ocean fronts, although the strength and type of affinity vary across taxa. Downscaled climate change simulations predict significant spatial shifts in front-rich areas by the end of the century. As climate change reshapes ocean front dynamics, adaptive management strategies will be essential to balance conservation and resource use in these critical ecosystems. Teaser Ocean fronts attract marine megafauna, but climate change might alter these habitats, requiring adaptive conservation strategies. ### Competing Interest Statement The authors have declared no competing interest. National Science Foundation French National Research Agency (ANR), ANR-20-BFOC-0006-04 European FEDER Fund, 1166-39417 Excellence Initiative of Aix-Marseille University (AMU) - AMIDEX, “Investissements d’Avenir”, N/A European Space Agency, 4000141547/23/IDT OMNCG Federation of the University of Reunion (SPY Program), N/A Waterlust, Aqua-Firma, and the Shark Foundation, N/A Australian Research Council (ARC), DE210100367 Agencia Nacional de Investigación y Desarrollo, R20F0008-CEAZA, FB210021 (COPAS COASTAL) Australian Research Council (ARC), DP210103091

Oceanic eddies are ubiquitous features of the circulation thought to be involved in transporting water mass properties over long distances from their source region. Among these is a particular type that has a core within the thermocline with little surface expression. Despite their significance, their role in ocean circulation remains largely undocumented by observations. This study characterizes the variations in internal biogeochemistry, disparities with external properties, and processes influencing the dissolved oxygen budget of poleward undercurrent eddies (PUDDIES) during their transit to oceanic waters. Employing a high-resolution coupled simulation of the Southeast Pacific, we document biogeochemical properties and processes associated with the nitrogen cycle inside PUDDIES and contrast them with those of the surrounding environment. Our findings reveal that PUDDIES capture a biogeochemical signal contingent upon their formation location along the coast, particularly associated with the core of the Peru-Chile Undercurrent at the core of the oxygen minimum zone (OMZ). While permeability at the periphery facilitates exchange with external waters, thereby modulating the original properties, the core signal retains negative oxygen (O-2) anomalies and positive anomalies of other biogeochemical tracers. These anomalous conditions result in tracer values exceeding the 90th percentile of their distribution in the open ocean, in contrast to the formation zones, where anomalies only surpass the 50th percentile. This indicates that PUDDIES may play a role in modulating the average properties of the open ocean. Suboxic (O-2<20 M) cores are prevalent near the coast but decrease in abundance with distance from shore, giving way to a predominance of hypoxic (20 mu M < O-2<45 M) cores (predominating at 60 % in the open sea), suggesting core ventilation during transit. The principal mechanism governing O-2 input to or output from the eddy core entails lateral and vertical advection, with vertical mixing supplying O-2 to a lesser extent. Biological activity consumes O-2 inside PUDDIES for around 6 to 12 months, especially intensely for the first 100 d, thus facilitating the persistence of low O-2 conditions and extending the lifetime of biogeochemical anomalies within the core (up to 800 km offshore). Ammonium and nitrite deplete earlier in the eddy core with a decay rate greater than that of nitrate and nitrous oxide, while these accumulate in the open sea (up to 16 % and 100 % higher than the mean state, respectively). Our results suggest that southern regions of the southeast Pacific OMZ undergo greater deoxygenation and nutrient enrichment due to PUDDIES compared to northern regions. However, the combination of various physical conditions can generate zones with more pronounced changes in the nitrite (subsurface water masses due to interactions with the PUDDIES, such as at 30 degrees S). The maximum contribution of NO takes place in particular along this latitude, with a 460 % increase compared to the mean state, near the coastal zone. In summary, PUDDIES formed along the Chilean coast capture distinct biogeochemical "signatures" depending on where they form. In the north, minimal ventilation fosters suboxic conditions and denitrification - leading to deficits of nitrate (NO) and nitrous oxide (N2O) but high NO and ammonium (NH) - whereas central and southern subregions show increased NO and higher N2O. Moreover, cross-shore exchange between Equatorial Subsurface Water and Subantarctic Water further amplifies this variability, giving rise to eddies with diverse nutrient and oxygen properties as they move offshore.

Knowledge about connectivity between populations is essential for the fisheries management of commercial species. The lobster Jasus frontalis inhabits two oceanic island groups, the Juan Fernández Archipelago and the Desventuradas Islands, separated by 800 km. Since this species is primarily exploited in the Juan Fernández Archipelago, knowledge of the connectivity patterns among islands is foundational for species management. Here, we used variability at single-nucleotide polymorphisms (SNPs) and individual-based modeling (IBM) to estimate the genetic structure and connectivity between J. frontalis populations in these island groups. The variability at 9090 SNPs suggests two genetic populations, one in the Juan Fernández Archipelago and one in the Desventuradas Islands. Furthermore, IBM suggests an asymmetric connectivity pattern, with particles moving from the Juan Fernández Archipelago to the Desventuradas Islands but not vice versa. Since the IBM analysis suggests asymmetric larval movement between the islands, and the genetic analysis indicates isolation between the Juan Fernández Archipelago and the Desventuradas Islands, larval retention mechanisms such as small-scale oceanographic processes or behavior could hinder larval movement between islands. This study highlights the importance of using more than one methodology to estimate population connectivity.

Abstract As a consequence of on-going global warming, the ocean is losing oxygen, which has implications not only in terms of marine resources management and food supply but also in terms of the potentially important feedback on the global carbon cycle and climate. Of particular scrutiny are the extended zones of already low levels of oxygen called the oxygen minimum zones (OMZs) embedded in the subsurface waters of the productive Eastern Boundary Upwelling Systems (EBUS). These OMZs are currently diversely simulated by state-of-the-art Earth System Models (ESM) hampering a reliable projection of ocean deoxygenation on marine ecosystem services in these regions. Here we focus on the most emblematic EBUS OMZs of the planet, that of the South Eastern Pacific (SEP), which is under the direct influence of the El Niño Southern Oscillation (ENSO), the main climate mode on interannual timescales at global scale. We show that, despite the low consensus among ESM long-term projections of oxygen levels, the sensitivity of the depth of the upper margin (oxycline) of the SEP OMZ to El Niño events in an ensemble of ESMs can be used as a predictor of its long-term trend, which establishes an emergent constraint for the SEP OMZ. Because the oxycline along the coast of Peru and Chile deepens during El Niño events, the upper bound of the SEP OMZ is thus likely to deepen in the future climate, therefore oxygenating the SEP OMZ. This has implications not only for understanding the nitrogen and carbon cycles at global scale but also for designing adaptation strategies for regional upper-ocean ecosystem services.