Western Boundary Research Articles

Paleoenvironmental characterisation of the Campos Basin slope (SE Brazil) in front of Cape São Tomé based on benthic foraminiferal biofacies succession since MIS 5

The distribution of benthic foraminifera and the assemblage dynamics were analysed on piston core GL-54 to understand the changes in paleoecology since Marine Isotopic Stage 5 (MIS 5). The core was drilled on the upper slope of the Campos Basin on the Rio de Janeiro (Brazil) coast. Benthic foraminifera biofacies, ecological indices, lithological facies, biostratigraphy, and isotopic analyses were undertaken to characterise the paleoenvironment in front of Cape of São Tomé. Two distinct biofacies were identified in the GL-54 sedimentary record since the Last Interglacial Period (MIS 5). These two biofacies point to seasonal organic matter and a well-oxygenated environment, however, Biofacies 2 can be differentiated from Biofacies 1 by the presence of significant bottom water currents. Biofacies 2 has the same species as Biofacies 1, except for Miliolinella subrotunda. Benthic foraminiferal species and biofacies from GL-54 were compared to the ones present in the upper portion of the GL-39 piston core, which was drilled from the middle slope of the same basin. Species such as Globocassidulina crassa, Islandiella norcrossi, Alabaminella weddellensis, Bolivina paula, M. subrotunda, Bulimina aculeata, Globocassidulina subglobosa, Rotorbinella lepida, and Uvigerina peregrina were found in both piston cores. The biofacies of both piston cores were correlated, pointing to similarities between the upper slope and the middle slope in the Campos Basin since MIS 5. In general, the upper and middle slopes of the Campos Basin are characterised as a well-oxygenated environment with variation in the deposition of organic matter. In addition to these environmental factors, there is evidence for a strong bottom current in MIS 2 at both sites, probably due to the displacement of Brazil Current and Intermediate Western Boundary Current.

Dwindling Effective Radiative Forcing of Large Volcanic Eruption: The Compensation Role of Ocean Latent Heat Flux

AbstractClimatic impacts of historical volcanism are principally tied to the eruption size, while observation versus model discrepancies have been commonly attributed to the uncertainties in paleo‐reconstruction or malpresentation of volcanic aerosols in models. Here we present convergent evidence for significant compensation effect of ocean latent heat (LH) in balancing the tropical volcanic‐induced heat loss, by introducing an effective perturbation ratio which is found to decrease with increasing eruption magnitude. Four LH compensation hot spots overlapping with the trade wind regions are identified, together with three western boundary currents regions with intensified LH loss. Comparison between the 1258 Samalas and 1452 Unidentified eruptions suggests considerable modulation of the concurring El Nino‐Southern Oscillation on LH anomaly, which is further verified by CESM large ensemble sensitivity experiments. This study depicts how the interplay between the ocean and the atmosphere could contribute to the overall resilience of the climate system in the face of volcanic disturbances.

Offshore Plankton Blooms via Mesoscale and Sub‐Mesoscale Interactions With a Western Boundary Current

AbstractWe investigate mesoscale circulations in an oligotrophic western boundary current, the East Australian Current, during a sustained offshore plankton bloom. Using a series of high resolution hydrographic sections taken a few days apart, supplemented with in situ samples of nutrients, satellite and long‐term mooring measurements, we describe a dynamic situation by which the East Australian Current's velocity core and associated front, interacts with a mesoscale eddy, migrating zonally by approximately 100 km over the course of 10 days. This interaction between the boundary current and mesoscale eddy field occurred concurrently with a sustained offshore plankton bloom. Sub‐mesoscale upwelling motions on the inshore flank of the boundary current core coincides with increased nutrient and plankton concentrations in the near surface. Calculations based on the quasi‐geostrophic omega equation and finite size lyapunov exponents suggest that these vertical motion arise from the interaction of the mesoscale with the East Australian Current. Frontolysis (the destruction of horizontal buoyancy gradients) leads to a thermally indirect ageostrophic secondary circulation that has the potential to supply nutrients to the near surface ocean. Using satellite data, we investigate the mesoscale conditions associated with all offshore phytoplankton blooms identified by an automated method, finding similar mesoscale patterns to those observed during the field campaign. We conclude that the interaction between the East Australian Current and mesoscale eddies is a recurrent catalyst for the complex sub‐mesoscale dynamics we observed, and is likely a fundamental processes in driving offshore biological productivity in the region.

Temporal changes of the Oyashio water distribution east of Japan under the changing climate: development of an objective evaluation method and its application

AbstractThe Oyashio, a southern part of the western boundary current in the North Pacific subarctic gyre, carries cold and fresh seawater with abundant nutrients southward from the high-latitude, influencing regional climate in the East Asia and marine environment in the western mid-latitude North Pacific. Previously, a distribution of the Oyashio water has been evaluated by empirical temperature thresholds; for example, in spring (March–May) when the Oyashio intrudes southward into the east of Japan, the Oyashio water is defined at 100-m depth as ≤ 5 °C. However, this method is not necessarily adequate under the changing climate because upper ocean temperature may change over time due to some causes unrelated to cold water transport by the Oyashio (e.g., surface heat fluxes). In this study, we developed an objective method to evaluate the Oyashio water distribution applicable under the changing climate with a focus on a thermohaline front located at the warm- and salty-side boundary of the Oyashio water. We identified isohalines at 100-m depth best corresponding to the thermohaline front in each month and used them as the Oyashio water threshold. Using the developed method, we further investigated the springtime Oyashio water distribution east of Japan (in the North Pacific south of 43°N, 141–148°E). The area of the Oyashio water shows inter-annual variation and significant long-term decrease. It was suggested that these temporal variation and change reflect changes in a distribution of anti-cyclonic meso-scale eddies off Hokkaido, which block the southward Oyashio intrusion into the east of Japan.

Probabilistic reconstruction of sea-level changes and their causes since 1900

Abstract. Coastal communities around the world are increasingly exposed to extreme events that have been exacerbated by rising sea levels. Sustainable adaptation strategies to cope with the associated threats require a comprehensive understanding of past and possible future changes. Yet, many coastlines lack accurate long-term sea-level observations. Here, we introduce a novel probabilistic near-global reconstruction of relative sea-level changes and their causes over the period from 1900 to 2021. The reconstruction is based on tide gauge records and incorporates prior knowledge about physical processes from ancillary observations and geophysical model outputs, allowing us, for the first time, to resolve individual processes and their uncertainties. We demonstrate good agreement between the reconstruction and satellite altimetry and tide gauges (if local vertical land motion is considered). Validation against steric height estimates based on independent temperature and salinity observations over their overlapping periods shows moderate to good agreement in terms of variability, though with larger reconstructed trends in three out of six regions. The linear long-term trend in the resulting global-mean sea-level (GMSL) record is 1.5 ± 0.19 mm yr−1 since 1900, a value consistent with central estimates from the 6th Assessment Report of the Intergovernmental Panel on Climate Change. Multidecadal trends in GMSL have varied; for instance, there were enhanced rates in the 1930s and near-zero rates in the 1960s, although a persistent acceleration (0.08 ± 0.04 mm yr−2) has occurred since then. As a result, most recent rates have exceeded 4 mm yr−1 since 2019. The largest regional rates (>10 mm yr−1) over the same period have been detected in coastal areas near western boundary currents and the larger tropical Indo-Pacific region. Barystatic mass changes due to ice-melt and terrestrial-water-storage variations have dominated the sea-level acceleration at global scales, but sterodynamic processes are the most crucial factor locally, particularly at low latitudes and away from major melt sources. These results demonstrate that the new reconstruction provides valuable insights into historical sea-level change and its contributing causes, complementing observational records in areas where they are sparse or absent. The Kalman smoother sea-level reconstruction dataset can be accessed at https://doi.org/10.5281/zenodo.10621070 (Dangendorf, 2024).

A Comparison of Diagnostics for AMOC Heat Transport Applied to the CESM Large Ensemble

AbstractAtlantic time‐mean heat transport is northward at all latitudes and exhibits strong multidecadal variability between about 30°N and 55°N. Atlantic heat transport variability influences many aspects of the climate system, including regional surface temperatures, subpolar heat content, Arctic sea‐ice concentration and tropical precipitation patterns. Atlantic heat transport and heat transport variability are commonly partitioned into two components: the heat transport by the Atlantic Meridional Overturning Circulation (AMOC) and the heat transport by the gyres. In this paper we compare four different methods for performing this partition, and we apply these methods to the Community Earth System Model Large Ensemble at 34°N, 26°N and 5°S. We discuss the strengths and weaknesses of each method. The four methods all give significantly different estimates for the proportion of the time‐mean heat transport performed by AMOC. One of these methods is a new physically‐motivated method based on the pathway of the northward‐flowing part of AMOC. This paper presents a preliminary version of our method that works only when the AMOC follows the western boundary of the basin. All the methods agree that at 26°N, 80%–100% of heat transport variability at 2–10 years timescales is performed by AMOC, but there is more disagreement between methods in attributing multidecadal variability, with some methods showing a compensation between the AMOC and gyre heat transport variability.

Eastward extension jet driven by vorticity anomaly at the western boundary: characteristics and the effects of southward background flow

Eastward extension jet driven by vorticity anomaly at the western boundary: characteristics and the effects of southward background flow

Satellite land surface temperature for mapping the structurally controlled geothermal anomalies in the Cretaceous rift, South Egypt

ABSTRACT Exploration of new geothermal systems often requires regional-scale investigation. Satellite Land Surface Temperature (LST) products are essential in geothermal anomaly identification and mapping. In this study, seven thermal anomalies covering Egypt were observed using the Sentinel-3 LST images acquired in 2019 and confirmed using the 2020 LST images. In addition, the LST analysis reveals three new anomalies covering the southern part of Egypt (Kalabsha, Natash, and Wadi Allaqi), which are structurally controlled. Nuqra-Kharit-Natash is a good site for geothermal energy owing to the following reasons: 1) the high geothermal gradient beneath the Wadi Natash region (heat flow ~100 mW/m2); 2) the presence of many geothermal anomalies retrieved from the analyses of the Landsat-8 acquired in August 2018; 3) existence of surface geothermal manifestation at the western boundary of the basin includes warm springs and the tufa deposits. Intricate structural patterns, including NW-oriented normal faults, ENE-to-WNW-oriented right-lateral faults, and NNE-oriented sinistral strike-slip faults, characterise Wadi Natash. Wadi Natash LST anomaly shows a solid relation to fault trends. High values of LST are arranged along NW and NNE faults and recorded the peak at the intersection of two fault trends. Recent seismic activity along the NNE fault system enhances the permeability along this trend and its intersection with the NW-oriented fault.

Southwest Pacific Ocean Warming Driven by Circulation Changes

AbstractAn area of ocean centered on 179°E, 46°S has warmed to full depth since 2006, with surface warming around 5 times the global rate. This Subtropical Front area is associated with a confluence of warm, salty, subtropical water from the north carried in a western boundary current and cold, fresh, subantarctic water from the south carried in the northernmost branch of the Antarctic Circumpolar Current. Temperature and salinity changes observed from Argo floats indicate that the Subtropical Frontal Zone has moved west ∼120 km, creating this area of strong warming analogous to changes in extension regions of other western boundary currents. The warming is a result of changes in the local flows of subantarctic water, evident in satellite altimeter data and 1,000 m Argo trajectories, which in turn likely result from changes in meridional ocean heat content and winds. The warming has placed this biologically‐significant region in almost perpetual marine heatwave conditions.

Variability and Dynamics of the Kuroshio and Mindanao Current during the 2010/11 La Niña and in Late 2012

Abstract Variability of two Pacific western boundary currents (WBCs)—the Kuroshio and the Mindanao Current—during the strong 2010/11 La Niña event is investigated using ship-based hydrographic observations and moored current-meter data collected off the east coasts of the Philippines. The geostrophic currents calculated using the hydrographic data show that, during the 2010/11 La Niña winter, the Kuroshio decreased by ∼10 Sv (1 Sv ≡ 106 m3 s−1), whereas the Mindanao Current increased by ∼5–10 Sv, relative to the normal winter in late 2012. The interannual variability based on the hydrographic data is confirmed by moored current-meter measurements and satellite altimeter geostrophic currents. A coastally trapped Kelvin wave model is used to explain the interannual variability of the two WBCs during the different ENSO phases. The good comparison of the simulated sea level anomalies around the Philippines with the altimeter data suggests that the interannual variability of the WBCs is associated with Kelvin wave propagation from the Sulawesi–Sulu Seas clockwise around the Philippine Archipelago. We identified that the Kelvin waves are excited by downwelling equatorial Rossby waves propagating into the Indonesian Seas during the La Niña. The transport anomalies of the WBCs are comparable to the total meridional transport anomalies integrated across the interior North Pacific Ocean, suggesting the importance of the WBCs in the heat charge–discharge processes of the western Pacific warm pool during ENSO events. Significance Statement The two western boundary currents (WBCs)—the Kuroshio and the Mindanao Current—play the role of closing the subtropical and tropical gyre circulation of the Pacific Ocean. Their variability during ENSO is unknown. Existing studies based on numerical modeling suggest that their variability is highly correlated with ENSO, with the Kuroshio stronger and Mindanao Current weaker during La Niña and vice versa during El Niño. Here, we use in situ hydrographic observations combined with mooring and satellite altimeter data to show that the Kuroshio transport decreases and the Mindanao Current transport increases during the 2010/11 La Niña, the dynamics of which are controlled by the Kelvin wave propagation from the Sulawesi–Sulu Seas clockwise around the Philippine Archipelago. The result is important for the warm pool dynamics during ENSO.

Meteorology Modulates the Impact of GCM Horizontal Resolution on Underestimation of Midlatitude Ocean Wind Speeds

AbstractWe utilize ocean 10‐m wind speed (U10m) from the microwave Multi‐sensor Advanced Climatology data set to examine the coupling between convective cloud and precipitation processes, synoptic state, and U10m and to evaluate the representation of U10m in global climate models (GCMs). We find that midlatitude U10m is underestimated by GCMs relative to observations. We examine two potential mechanisms to explain this model behavior: cold pool formation in cold air outbreaks (CAOs) associated with downdrafts that enhance U10m and sea surface temperature (SST) gradients affecting U10m through thermally forced surface winds at regional scales. When the effects of the CAO index (M) and SST gradients on U10m are accounted for, a relationship between GCM horizontal resolution and U10m appears. The strongest correlation between resolution and U10m is over the western boundary currents characterized by frequent CAOs atop strong SST gradients which drives the strongest surface fluxes on Earth.

A revolution in understanding SE Asia geodynamics since 20.5–18 Ma

We present an updated evaluation of SE Asian geodynamics that includes the interactions of the South China Sea (SCS) marginal basin with surrounding plates since the end of SCS spreading 20.5–18 Ma. Newly available Ar39/Ar40 ages of SCS oceanic crust drilled at IODP U1431 near the SCS East basin extinct spreading center are older than 18 Ma. Conversely, the oldest ages of the Luzon arc and forearc at Taiwan's Lanyu island, Coastal range and Lichi mélange are 17–18 Ma, suggesting that onset of the Manila subduction zone may have begun a few m.y. earlier. Before ∼20.5 Ma, the northern part of the Manila transcurrent fault (MTF), considered as the western boundary of the Ryukyu subduction zone, was a left-lateral lithospheric-scale shear zone. From ∼20.5–18 Ma to ∼7 Ma, this portion of MTF was connected to the Manila trench. Since ∼7 Ma, the MTF extended into the Taiwan Longitudinal valley and continued southwards to north Luzon island as near-vertical, left-lateral shear zone. Today, south of ∼24°N, the MTF protrudes down to 30 km depths and terminates above the deeper Manila slab. Since ∼7 Ma, the whole MTF shifted 400 km westward with respect to Eurasia and rotated ∼23° clockwise to become oriented ∼NS north of 16°N latitude. We identify a tear fault in the Eurasian (EU) plate north of the Ryukyu trench that is located south of the Myako and Yonaguni islands. Since ∼10 Ma, the tear continuously progressed westward within EU crust, with the Philippine Sea plate progressively subducting northwestward between the two lips of the tear fault. A RFF (ridge-fault-fault) triple junction was active in the EU crust before 20.5 Ma, from 10 to 7 Ma, and since 2 Ma. This triple junction was always located on the MTF with one branch of the MTF on each side of the triple junction, and the third branch being the spreading center.

Interannual variability of the East African Coastal Current associated with the El Niño-Southern Oscillation

Abstract The East African Coastal Current (EACC) is an important western boundary current of the tropical South Indian Ocean and plays an important role in the ocean circulation and biogeochemical cycles in the Indian Ocean. This study investigates the interannual variability of the EACC and its dynamical mechanisms. The result shows that the EACC has interannual variability associated with the El Niño-Southern Oscillation (ENSO) during 1993-2017. The EACC shows a significantly positive correlation with the Niño3.4 index with a correlation coefficient of 0.65, lagging the Niño3.4 index by 18 months. During the decaying phases of El Niño (La Niña) events, the negative (positive) sea level anomaly (SLA) propagates westward as upwelling (downwelling) Rossby waves from the southeast Indian Ocean to the southwest Indian Ocean, and then strengthens (weakens) the EACC due to zonal SLA gradient off the East African coast under geostrophic equilibrium. The SLA gradually weakens in the southeast Indian Ocean during its westward propagation but strengthens in the southwest Indian Ocean promoted by local wind stress curl anomaly. This study can improve our understanding of the relationship between the western boundary current of the tropical South Indian Ocean and large-scale ENSO air-sea processes, and is important for managing marine fisheries and ecosystems on the East African coast.

Seasonal and interannual variabilities of the thermal front east of Gulf of Thailand

Based on satellite-observed and reanalysis data, this study investigates a thermal front east of the Gulf of Thailand (TFEGT) during the winter from 1982 to 2021. TFEGT exhibits distinct seasonal and interannual variation, emerging in December, peaking in February, gradually diminishing in March, and completely dissipating in April. Notably, the occurrence probability, area, and intensity of the thermal front are significantly higher in January and February compared with December and March. Through the application of a mixed temperature equation, we identify that geostrophic advection, driven by wind-induced western boundary current in the South China Sea (SCS), plays a crucial role in the formation of the TFEGT. In winter, the prevailing northeast monsoon propels the western boundary current through wind stress curl, causing the southward transport of cold water from north to south. This cold water encountered warm water within the Gulf of Thailand (GoT), leading to the formation of TFEGT. Furthermore, the interannual variation of TFEGT is closely associated with the El Niño-Southern Oscillation (ENSO). In El Niño (La Niña) years, the northeast monsoon weakens (enhances), resulting in a weaker (stronger) western boundary current, ultimately influencing the weakening (enhancement) of TFEGT.

Koopman neural operator as a mesh-free solver of non-linear partial differential equations

The lacking of analytic solutions of diverse partial differential equations (PDEs) gives birth to a series of computational techniques for numerical solutions. Although numerous latest advances are accomplished in developing neural operators, a kind of neural-network-based PDE solver, these solvers become less accurate and explainable while learning long-term behaviors of non-linear PDE families. In this paper, we propose the Koopman neural operator (KNO), a new neural operator, to overcome these challenges. With the same objective of learning an infinite-dimensional mapping between Banach spaces that serves as the solution operator of the target PDE family, our approach differs from existing models by formulating a non-linear dynamic system of equation solution. By approximating the Koopman operator, an infinite-dimensional operator governing all possible observations of the dynamic system, to act on the flow mapping of the dynamic system, we can equivalently learn the solution of a non-linear PDE family by solving simple linear prediction problems. We validate the KNO in mesh-independent, long-term, and zero-shot predictions on five representative PDEs (e.g., the Navier-Stokes equation and the Rayleigh-Bénard convection) and three real dynamic systems (e.g., global water vapor patterns and western boundary currents). In these experiments, the KNO exhibits notable advantages compared with previous state-of-the-art models, suggesting the potential of the KNO in supporting diverse science and engineering applications (e.g., PDE solving, turbulence modeling, and precipitation forecasting).

Delineating petrological feature and geochemical characteristics of volcanic rocks from the Herson-Kilkis Area, Northern Greece

The geochemical data of this study confirm a previous suggestion that the Herson—Kilkis rhyolites are the crystallization products of a magma generated by the partial fusion of K-rich schists or para-gneisses and erupted along a dip-slip fault at the western boundary of the Serbo—Macedonian massif. High-K rhyolites occur in the area around Kilkis town along the western boundary of the Serbo-Macedonian massif. The chemistry of the rocks is characterized by high K2O contents (7–12 Wt %), high K2O/Na2O ratios (2–22), low CaO (0.04–0.16%) and Sr (0–83 mg Kg−1) contents and high Rb/Sr ratios (4–40). The trace elements (in mg Kg−1), Nb (Not Detected–41), Nd (5–87), Ce (16–119), La (9–67), Rb (Not Detected–358) and Y (10–87) present higher contents than those of rhyolites derived from a more basic magma by low-pressure fractional crystallization.

Inertial Effect and Its Dependency on the Topographic Geometries in Barotropic Channel Models under the Weakly Nonlinear Regime

Abstract Recent studies have shown that the sensitivity of the circumpolar transport of channels to the westerlies is controlled by wind-driven gyre circulations. Although the form stress associated with the gyres has been shown to be controlled by eddies, bottom friction, and topographic width, the role of inertial effects has not been fully understood. In this study, we conduct a series of sensitivity analyses using the barotropic model with and without the advection term (hereinafter, the model without the advection term is denoted as linear model). Experiments showed that the sensitivity of the circumpolar transport decreased under the westerly winds compared to the linear model, while it increased under the easterly winds. We show that the inertial effect of western boundary currents generates anomalous anticyclonic circulations over the topography, producing the westward topographic form stress anomalies regardless of the wind directions. In addition, we discuss the sensitivity of the inertial effect mechanism to topographic height, width, and geometries. The inertial effect mechanism is robust as long as the gyre circulations dominate while its relative importance changes. We also found that the dynamics of the barotropic channel strongly depend on the geometries of geostrophic contours f/h. Therefore, we conclude that the dynamics of barotropic channel models might be interpreted with caution to understand the dynamics of the Southern Ocean. Significance Statement Previous studies have studied baroclinic and barotropic channel models as a benchmark of the Southern Ocean, but the nonlinear dynamics of channels have not been fully understood. In this paper, we show that the inertial effect by the mean flow works to decrease the sensitivity to the westerly winds in the barotropic channel models using numerical experiments. We also show that the inertial effect is robust as long as gyre circulations exist, while its relative importance differs.

The Importance of Contemporaneous Wind and pCO2 Measurements for Regional Air‐Sea CO2 Flux Estimates

AbstractFew observational platforms are able to sustain direct measurements of all the key variables needed in the bulk calculation of air‐sea carbon dioxide (CO2) exchange, a capability newly established for some Uncrewed Surface Vehicles (USVs). Western boundary currents are particularly challenging observational regions due to strong variability and dangerous sea states but are also known hot spots for CO2 uptake, making air‐sea exchange quantification in this region both difficult and important. Here, we present new observations collected by Saildrone USVs in the Gulf Stream during the winters of 2019 and 2022. We compared Saildrone data across co‐located vehicles and against the Pioneer Array moorings to validate the data quality. We explored how CO2 flux estimates differ when all variables needed to calculate fluxes from the bulk formulas are simultaneously measured on the same platform, relative to the situation where in situ observations must be combined with publicly‐available data products. We systematically replaced variables in the bulk formula with those often used for local and regional flux estimates. The analysis revealed that when using the ERA‐5 reanalysis wind speed in place of in situ observations, the ocean uptake of CO2 is underestimated by 8%; this underestimate grows to 9% if the NOAA Marine Boundary Layer atmospheric CO2 product and ERA‐5 significant wave height are also used in place of in situ observations. Overall our findings point to the importance of collecting contemporaneous observations of wind speed and ocean pCO2 to reduce biases in estimates of regional CO2 flux, especially during high wind events.

Significant Reduction of Potential Exposure to Extreme Marine Heatwaves by Achieving Carbon Neutrality

AbstractMarine heatwave (MHW), a prolonged period of anomalously warm seawater, has a catastrophic repercussion on marine ecosystems. With global warming, MHWs have become increasingly frequent, intense, and prolonged. To avoid irreversible damages from such extreme events, net‐zero carbon emissions by the 2050s, called carbon neutrality, were proposed. Here, we evaluate the impact of carbon neutrality on MHWs in the late 21st century using multi‐model projections from the Coupled Model Intercomparison Project Phase 6 (CMIP6) Shared Socioeconomic Pathway (SSP) 1–1.9 and SSP3‐7.0 scenarios. It is found that if the current regional rivalry over carbon emissions continues (i.e., SSP3‐7.0), the MHWs in the late 21st century will become stronger and longer than historical ones, especially in the western boundary current and equatorial current regions. Approximately 68% of the global ocean will be exposed to permanent MHWs, regionally 93% in the Indian Ocean, 76% in the Pacific Ocean, 68% in the Atlantic Ocean, 65% in the Coastal Ocean, and 48% in the Southern Ocean. Such MHWs can be significantly reduced by achieving carbon neutrality (i.e., SSP1‐1.9). In particular, the spatial proportion of the ocean exposed to permanent MHWs can be reduced to as low as 0.02%–0.07%, depending on the regions. This result underscores the critical importance of ongoing efforts to achieve net‐zero carbon emissions to reduce the potential ecological risks induced by extreme MHWs.

Role of Japan Sea Throughflow in the spatial variability of the long-term sea surface temperature trend

The Japan Sea shows a much stronger warming of long-term sea surface temperature (SST) than surrounding oceans. The warming trend possesses a meridionally alternating zonal band pattern, with weak trends along the paths of the Japan Sea Throughflow and strong trends in the remaining interior region. Using idealized models of the Japan Sea Throughflow and atmospheric heating, this study examines the process behind the formation of such spatial patterns in the SST trend. We find that zonal band structures form in a flat rectangular coastline model, and heat budget analysis shows that horizontal heat transport, due to throughflow, reduces the warming effect created by the surface heat flux. A weak SST trend appears around the jet, while a strong SST trend appears elsewhere. Bathymetric effects are also examined using a model with realistic coastline settings. The location of the western boundary current stabilizes, and the coastal branch begins to disconnect from the Japanese coastline toward the north, allowing a more stable SST warming region to form in the southern interior region. Lagrangian particle tracking experiments confirm that a weak (strong) SST trend corresponds to a short (long) residence time, and eddies in the Japan Sea prolong the residence time in interior regions. The model results suggest that the accumulation time of surface heating is essential to the spatial distribution of the long-term SST warming trend.