Anticyclonic Mesoscale Research Articles

Spontaneous near-inertial wave generation from mesoscale eddy: Energy transformation

The energy transformation between inertial oscillations (IOs), near-inertial waves (NIWs), and mesoscale eddies during spontaneous NIW generation is analyzed by the kinetic energy equations of the multiple temporal–spatial scale interaction between slow and fast motions and then quantitatively estimated by numerical simulations. The evolution of perturbed quasi-geostrophic mesoscale eddies is accompanied by IOs. The nonlinear interaction of IOs and mesoscale eddies results in spontaneous energy transfer from mesoscale eddies to NIWs. However, IOs act as catalysts in the spontaneous NIW energy transformation process, because there is no energy transfer between NIWs and IOs for longer than one inertial period and NIW energy is entirely transferred from mesoscale eddies. The energy conversion rate (ECR) from a propagating eddy to NIWs is significantly enhanced by the resonance of NIWs and the nonlinear coupling of IOs and mesoscale eddies during the spontaneous NIW generation. The time-averaged NIW ECR from a propagating an anti-cyclonic mesoscale eddy (AE) is approximately 5.725 × 10−6 Wm−2 and nearly 16 times higher than those from the standing AE. The magnitude of global near-inertial kinetic energy generated spontaneously from mesoscale eddies is approximately on the order of 2 GW. Therefore, the spontaneous adjustment of mesoscale eddies is a non-negligible NIW generation mechanism and plays a crucial role in the process of supplying energy from mesoscale eddies to the diapycnal mixing in the ocean interior.

How Atmospheric Forcing Frequency, Horizontal and Vertical Grid Resolutions Impact Mesoscale Eddy Evolution in a Numerical Model

AbstractSeasonal evolution of both surface signature and subsurface structure of a Mediterranean mesoscale anticyclones is assessed using the Coastal and Regional Ocean Community high‐resolution numerical model with realistic background stratification and fluxes. In good agreement with remote‐sensing and in‐situ observations, our numerical simulations capture the seasonal cycle of the anomalies induced by the anticyclone, both in the sea surface temperature (SST) and in the mixed layer depth (MLD). The eddy signature on the SST shifts from warm‐core in winter to cold‐core in summer, while the MLD deepens significantly in the core of the anticyclone in late winter. Our sensitivity analysis shows that the eddy SST anomaly can be accurately reproduced only if the vertical resolution is high enough (∼4 m in near surface) and if the atmospheric forcing contains high‐frequency. In summer with this configuration, the vertical mixing parameterized by the k − ϵ closure scheme is three times higher inside the eddy than outside the eddy, and leads to an anticyclonic cold core SST anomaly. This differential mixing is explained by near‐inertial waves, triggered by the high‐frequency atmospheric forcing. Near‐inertial waves propagate more energy inside the eddy because of the lower effective Coriolis parameter in the anticyclone core. On the other hand, eddy MLD anomaly appears more sensitive to horizontal resolution, and requires SST retroaction on air‐sea fluxes. These results detail the need of high frequency forcing, high vertical and horizontal resolutions to accurately reproduce the evolution of a mesoscale eddy.

Dynamics of diabatically forced anticyclonic plumes in the stratosphere

AbstractA new class of vortices has been observed in the stratosphere following extreme wildfires (Canada 2017, Australia 2020) and volcanic eruptions (Raikoke 2019). These vortices are long‐lived mesoscale anticyclones (hundreds to 1,000 km in diameter) trapping plumes of aerosols and combustion/volcanic compounds. Owing to their unusual composition, these anticyclonically trapped plumes (ATPs) are associated with a significant radiative heating, which fuels their ascent through the stratosphere. This article investigates the dynamics of ATPs using two complementary approaches: analytically, in a potential vorticity (PV) perspective, and through idealised numerical simulations with the Weather Research and Forecasting (WRF) model. In both cases, we consider the vortical flow forced by a heating Lagrangian tracer. By reformulating the problem in the potential radius–potential temperature coordinate system introduced for tropical cyclones, we first clarify that ATP formation is concomitant with the injection of air into the stratosphere at extratropical latitudes. Then, we derive a set of simplified one‐dimensional equations describing the subsequent evolution of the flow after the injection. The equation obtained for the tracer is a variant of the classical Burgers' equation. In qualitative agreement with the three‐dimensional WRF simulations, this theoretical model predicts that ATPs develop an upper tracer front associated with sustained near‐zero anticyclonic PV, followed by a smooth tracer tail of cyclonic PV. Radiative relaxation of the temperature perturbations induced by the anticyclone and the presence of an initial PV anomaly tend to stabilise ATPs during their ascent. Finally, we note that the theory predicts a similar relationship between the plume and anticyclonic PV for cooled ATPs, which is supported by three‐dimensional simulations and may apply to the 2022 Hunga volcanic plume.

Enhanced Mixing Induced by Near-Inertial Waves Inferred by Glider Observation in the Northern South China Sea

Enhanced turbulence triggered by near-inertial wave (NIW) trapping by a mesoscale anticyclone and typhoon “Kompasu” was observed in the northern South China Sea. The observations provide evidence for the trapping of NIW packets of amplitude ~0.2 m/s near the base of an anticyclonic eddy, and of ~0.3 m/s after the passage of typhoon “Kompasu”. The wave energy was amplified in a layer located near the base of the anticyclonic eddy, between 150 and 300 m, while stronger NIWs triggered by the typhoon extended to depths > 500 m. Diffusivity was calculated by a fine-scale parameterization. A diffusivity elevated by one order of magnitude, the occurrence of high near-inertial velocity shears, and the low (≤1) Richardson numbers were consistent with turbulence production and mixing from the base of the anticyclonic eddy and following the passage of the typhoon, and were associated with the trapped NIWs. This study showed that, by serving as a bridge between mesoscale eddies and small-scale motion, NIWs are an important pathway for ocean energy transmission. Mesoscale-NIW interactions represent a significant source of NIWs as well as a sink of mesoscale energy.

Mesoscale Anticyclonic Eddies in the Primorye Current System of the Japan/East Sea in the Summer

This study identified a process of water supply due to discrete transport by anticyclonic eddies towards the formation region of the intermediate water in the northwestern Japan Sea in the summer season. The structures and dynamics of three anticyclonic mesoscale eddies were studied by means of the Aqualog moored profiler observational survey at the continental slope in the Primorsky Current region east of Peter the Great Bay in June–July 2015. It was shown that the eddy cores were of ellipsoidal shape with an elongated lower part. The anticyclones as isolated dynamic structures carried water enriched with dissolved oxygen in the west-southwest direction. In terms of the thermohaline characteristics the water transported was similar to the Japan Sea intermediate water at its formation region south of Peter the Great Bay.

Asymmetric Drifter Trajectories in an Anticyclonic Mesoscale Eddy

The influences of sea surface wind on the oceanic mesoscale eddy are complex. By integrating our self-developed surface drifters with satellite observations, we examined the influence of sea surface wind on the distribution of water masses and biomass within the interior of an anticyclonic eddy. Ten drifters were deployed in the northern South China Sea in the spring of 2021. Eventually, six were trapped in an anticyclonic mesoscale eddy for an extended period. Interestingly, the drifters’ trajectories were not symmetric around the eddy center, displaying a significant offset of the distance from the wind turns to the southerly wind. Particle tracking experiments demonstrated that this departure could mainly be attributed to wind-driven ageostrophic currents. This is due to the strength of wind-driven ageostrophic currents being more comparable to geostrophic currents when accompanied by a deflection between the directions of the wind-driven current and the eddy’s translation. The drifters’ derived data indicated that sub-mesoscale ageostrophic currents within the eddy contributed to this asymmetric trajectory, with Ekman and non-Ekman components playing a role. Furthermore, the evolution of ocean color data provided corroborating evidence of these dynamic processes, highlighting the importance of ageostrophic processes within mesoscale eddies.

Spontaneous near-inertial wave generation from mesoscale eddy: Nonlinear forcing mechanism

The spontaneous generation of near-inertial waves (NIWs) from quasi-geostrophic mesoscale eddies in a stratified fluid is investigated and the process and mechanism clarified by numerical modeling and a theoretical analysis. Mesoscale eddies, as slow balance flows, are inevitably accompanied by unbalanced near-inertial oscillations (NIOs) in their long period evolution. A mesoscale eddy supplies kinetic energy to NIOs through its interaction with perturbations, which exist universally in the world oceans. Afterward, NIWs are generated spontaneously via the nonlinear coupling of the eddy and NIOs. The baroclinicity of mesoscale eddies is an essential condition of this spontaneous NIW generation mechanism. The resonance of the NIWs and NIOs enables the internal wave continuum to be dominated by NIWs, which share the same horizontal wavenumbers with the eddy. After generation, the NIWs radiate energy toward the eddy center and remain stationary as a whole, relative to the propagating eddy. Generally, an anticyclonic mesoscale eddy can emit much stronger NIWs than does a cyclonic eddy. The NIW intensity strengthens exponentially with the Rossby number. The essence of this spontaneous NIW generation mechanism is the nonlinear interaction between slow balance flows and the accompanying fast motions. It represents an effective pathway for mesoscale eddy energy sinks and makes a non-negligible contribution to the global NIW energy.

Divergence Observation in a Mesoscale Eddy during Chla Bloom Revealed in Submesoscale Satellite Currents

Oceanic mesoscale eddies continuously regulate the horizontal and vertical transport of mass, heat, salt, carbon, and nutrients throughout the ocean system owing to their ubiquity, three-dimensionality, and long-term persistence. Although satellites have been the main platforms used to observe mesoscale eddies and chlorophyll-a (Chla) distributions, they cannot support submesoscale physical–biological interactions. Contemporary satellite observations of Eulerian velocity fields are unable to resolve submesoscale processes that govern vertical migration and mixing, which are crucial for controlling the nutrients and light for phytoplankton in the surface layer. We explored the physical–biological interaction between the anticyclonic mesoscale eddy and the Chla secondary bloom that occurred after the spring bloom in the East/Japan Sea using the Geostationary Ocean Color Imager (GOCI). The GOCI currents were generated using GOCI Chla data and were used to map streamlines, vorticity, and divergence to characterize the surface current near the eddy. In the early spring bloom period, the eddy interior showed Chla depletion as the eddy was trapped externally. We found that the second bloom period coincided with a higher divergence or upwelling period in the eddy core, and a sharp Chla peak was observed when wind-induced Ekman suction was pronounced. This study describes the first satellite observation of surface layer divergence inside an anticyclonic mesoscale eddy with internal Chla blooms, utilizing a submesoscale-permitting GOCI-based surface current.

Cross-Slope Transport by a Mesoscale Anticyclone in the Northern South China Sea

Cross-slope eddies play an important role in the exchange of water, salt, heat, nutrients, chlorophyll, phytoplankton and other biogeochemical elements between basin and shelf in the South China Sea. The cross-slope transport process by a mesoscale anticyclonic eddy is studied by ROMS model and satellite data. The 1000 m isobath was considered as a proxy for the slope. The anticyclone shows different features at different places on the slope: (a) the volume transport at the northeast of the slope was off-slope, while at the southwest was on-slope; (b) both on and off-slope transports were greatly enhanced during the cross-slope process, and gradually weakened after crossing the slope. The total cross-slope water transport was 5.97 Sv, which was higher than the along-slope component with −0.58 Sv. The Eulerian results also showed that enhanced cross-slope transport was related to the distance between the eddy and slope, the eddy radius, and the eccentricity of the eddy. The offline passive tracer experiment showed that particles were floating up during and after the crossing process, mainly due to the strong Ekman pumping.

Floating plastic accumulation and distribution around Kuroshio Current, western North Pacific

The distribution of floating plastic debris around the Kuroshio Current which transports plastics from the coastal waters of Asian countries to North Pacific subtropical gyre, was investigated in 2014. The mean abundance and weight of plastic debris on the sea surface were 100,376 counts/km2 and 446.16 g/km2, respectively. Intensive plastic accumulation was observed in the frontal area between the northern edge of the Kuroshio and coastal waters off Shikoku, while a relatively higher abundance in the south of Kuroshio was generally associated with anticyclonic mesoscale eddies. Such an accumulation resulted from the eddy-Kuroshio interactions which are specifically associated with the offshore non-large meandering Kuroshio path. Overall, white, fragmented, small-sized (≤1 mm) particles with polyethylene and polypropylene polymers were dominant. In the southern area of Kuroshio, the contribution of polystyrene and larger-sized plastic was higher, suggesting a rapid influx of fresh particles from western Japan to offshore by the northwest monsoon.

Anticyclonic mesoscale eddy induced mesopelagic biomass hotspot in the oligotrophic ocean

Increasing evidence suggests that mesoscale eddies have a strong influence on the distribution and behavior of zooplankton and micronekton. However, the mechanisms driving their responses to mesoscale eddies are not yet fully understood, largely due to the low resolution of previous observations. Underwater gliders equipped with acoustic equipment and environmental sensors provide an ideal means to obtain observations with high temporal and spatial resolutions. In this study, two underwater gliders were used to record environmental parameters and acoustic intensities across an anticyclonic eddy in the oligotrophic western North Pacific subtropical gyre in September 2019. Based on the variations in the sea surface current and sea surface height anomaly (SSHA) along the underwater glider tracks, the study area was divided into three subregions: eddy core (SSHA >27 cm), eddy periphery (27 cm > SSHA >20 cm), and ambient water (SSHA <20 cm). More intense backscattering occurred in the upper 800 m of the eddy core with mean area backscattering coefficient values that were 35.8% higher than those of the eddy periphery and 19.8% higher than those of the ambient water. The convergence associated with the eddy could explain that the largest animal congregations occurred in the upper 100 m of the eddy core at night. Downwelling of warmer and oxygen-rich water in the eddy core induced animals to accumulate toward the deep of the eddy core, while migrating downwards during the day. Both these results and the lack of spatial heterogeneity of chlorophyll -a in and around the anticyclonic eddy indicated that horizontal and vertical distributions of zooplankton and micronekton in the anticyclonic eddy in the oligotrophic ocean were less related to spatial variations in the phytoplankton, but might be associated with the combined effect of the physical process in the anticyclonic eddy and the diel vertical migration behavior of animals. Our results suggest that anticyclonic eddies can dynamically modulate the spatial distribution of biomass, leading to the formation of mesopelagic biomass hotspots in the oligotrophic oceans and strongly affecting the ecology and behaviors of top predators that prey on small animals. • Spatial heterogeneity in biomass distribution was observed in an anticyclonic eddy. • Redistribution of biomass occurs within anticyclonic eddies. • Anticyclonic eddies in oligotrophic oceans support mesopelagic biomass hotspots.

USV-Observed Turbulent Heat Flux Induced by Late Spring Cold Dry Air Incursion over Sub-Mesoscale Warm Regions off Sanriku, Japan.

We performed oceanic and atmospheric observations in the region off the Sanriku coast, Japan, from May 11 to 5 July 2022, using a wave-propelled unmanned surface vehicle, a Wave Glider (WG). Despite the severe weather conditions of atmospheric low-pressure system crossings, we successfully measured wind, air temperature, humidity, and sea surface temperature over the course of 55 days to calculate the turbulent heat flux. The WG observed that the atmosphere became more humid due to the southerly wind along the northwestern rim of the North Pacific subtropical high. The warm Kuroshio water expanded to the southeast of Hokkaido as a result of the northward shedding of an anticyclonic mesoscale (~100 km) eddy, called a warm-core ring, from the Kuroshio Extension. The WG traversed smaller (sub-mesoscale) water regions that were warmer and saltier than the surrounding Kuroshio water. The observations indicate that cold, dry air masses advected by northerly winds following the passage of atmospheric low-pressure systems generate a substantial upward turbulent heat flux over sub-mesoscale warm water regions, contrasting to no heat flux in the surrounding Kuroshio water region.

Temperature Variability and Eddy‐Flow Interaction in the South of Oyashio Extension

AbstractBased on observations and eddy‐resolving model products, this study investigates the temperature variability south of the Oyashio Extension (OE). During 2016–2021, six dramatic cold events, which significantly impact the local ecosystems, are captured by in situ and satellite observations. It reveals that interaction between the OE front (OEF) and anticyclonic mesoscale eddies originating from the Kuroshio Extension is responsible for these cold events. Accompanied by intense eddy‐flow baroclinic energy exchange, the OEF meanders to the south and favors a southward cold‐water intrusion. Further analysis illustrates that the cold events tend to occur during spring and autumn, which is associated with the seasonal evolutions of OEF and anticyclonic eddies.

Impacts of strong positive Indian Ocean Dipole on the generation of the Great Whirl

The Great Whirl (GW) is an anticyclonic mesoscale eddy, forming periodically in the tropical western Indian Ocean. The lifespan of the GW exhibits a marked interannual variability, which has a late generation time and short lifetime due to the abnormal local wind field in a strong positive IOD year. The anomalous northeasterly wind inputs positive wind stress curl into the ocean, which is not conducive to establishing a stable anticyclonic circulation at the early stage of GW formation. In addition, oceanic Rossby waves exert impacts on the GW generation. The abnormal Rossby waves account for the earlier generation time of the GW in 2019 despite the unfavorable local wind field. This study identifies the atypical lifespan of the GW associated with a positive IOD event and reveals the contribution of local Ekman pumping and Rossby wave propagation.

Turbulent Dissipation in the Surface Mixed Layer of an Anticyclonic Mesoscale Eddy in the South China Sea

AbstractClarifying contributions to the surface mixed layer (SML) dissipation from dynamic processes including winds, waves, buoyancy forcing and submesoscales is of significance for quantifying exchanges between the atmosphere and the ocean. Based on two observation sections across an anticyclonic eddy in the South China Sea, the contributions from different dynamic processes to the SML dissipation rate of turbulence are quantified. The potential vorticity indicates instability events including symmetric instability (SI), gravitational instability and centrifugal instability at the eddy. Despite of a dominant role of wind‐ and wave‐induced dissipation rates, SI is highlighted by a mean estimated depth‐integrated dissipation rate of 4.3 × 10−6 W m kg−1 with a maximum up to 3.2 × 10−5 W m kg−1. The SI dissipation is believed to play a role in the eddy kinetic energy budget by extracting energy from the vertical geostrophic shear at the eddy.

Impact of a long-lived anticyclonic mesoscale eddy on seawater anomalies in the northeastern tropical Pacific Ocean: a composite analysis from hydrographic measurements, sea level altimetry data, and reanalysis model products

Abstract. Using observational data, satellite altimeters, and reanalysis model products, we have investigated eddy-induced seawater anomalies and heat and salt transport in the northeastern tropical Pacific Ocean. An eddy detection algorithm (EDA) was used to identify eddy formation at the Mexican Tehuantepec Gulf (TT) in July 2018 during an unusually strong summer wind event. The eddy separated from the coast with a mean translation velocity of 11 cm s−1 and a mean radius of 115 km and traveled 2050–2400 km westwards off the Central American coast, where it was followed at approx 114∘ W and 11∘ N for oceanographic observation between April and May 2019. The in situ observations show that the major eddy impacts are restricted to the upper 300 m of the water column and are traceable down to 1500 m water depth. In the eddy core at 92 m water depth an extreme positive temperature anomaly of 8.2 ∘C, a negative salinity anomaly of −0.78 psu, a positive fluorescence anomaly of +0.8 mg m−3, and a positive dissolved oxygen concentration anomaly of 137 µmol kg−1 are observed. Compared with annual climatological averages in 2018, the water trapped within the eddy is estimated to transport an average positive westward zonal heat anomaly of 85×1012 W and an average westward negative salt anomaly of -2.1×106 kg s−1. The heat transport is the equivalent of 1 % of the total annual zonal eddy-induced heat transport at this latitude in the Pacific Ocean. Understanding the dynamics of long-lived mesoscale eddies that may reach the seafloor in this region of the Pacific Ocean is especially important in light of potential deep-sea mining activities that are being targeted on this area.

Observed Equatorward Propagation and Chimney Effect of Near‐Inertial Waves in the Midlatitude Ocean

AbstractThe propagation characteristics of near‐inertial waves (NIWs) and how mesoscale and submesoscale processes affect the waves' vertical penetration are investigated using observations from a mooring array located in the northeast Atlantic. The year‐long observations show that near‐inertial motions are mainly generated by local wind forcing, and that they radiate equatorward and downward following several strong wind events (wind stress ≳0.5 N m−2). Observational estimates of horizontal group speed typically exceed those of vertical group speed by two orders of magnitude, consistent with predictions from the dispersion relation. Enhanced near‐inertial kinetic energy and vertical shear are found only in mesoscale anticyclones with Rossby number of O(0.1). By contrast, submesoscale motions with order one Rossby number have little effect on the trapping and vertical penetration of NIWs, due to their smaller horizontal scales, shorter time scales, and confined vertical extent compared to mesoscale eddies.

Submesoscale processes-induced vertical heat transport modulated by oceanic mesoscale eddies

Oceanic submesoscale processes mainly arise from phenomena such as jets, density fronts, and mesoscale eddies. Previous studies suggest that the overall submesoscale processes transport heat vertically upgradient, i.e. from cold to warm. However, it is not clear whether the submesoscale processes-induced vertical heat transport (VHT) in mesoscale eddies remains upgradient. Therefore, the present study focuses on submesoscale-induced VHT modulated by mesoscale eddies. A high-resolution oceanic numerical model product is applied to examine the VHT induced by submesoscale processes associated with a pair of cyclonic and anticyclonic mesoscale eddies in the Kuroshio Extension (KE) region. Eddies at different time steps are collocated in terms of the eddy centers to reconstruct a time series of a new eddy dataset that presents eddy-modulated submesoscale processes. The frequency-wavenumber spectra, Rossby numbers, and strain rates in the pair of eddies reveal the existence of submesoscale motions surrounding mesoscale eddies. The variables are decomposed into monthly mean mesoscale and submesoscale components to calculate submesoscale VHT at low-frequency (LF) and high-frequency (HF). The results indicate that submesoscale VHT is modulated by mesoscale eddies mainly along the eddy boundaries. The LF submesoscale VHT (upward in both cyclonic and anticyclonic eddies) dominates the monthly-averaged submesoscale VHT. The magnitude of the hourly HF submesoscale VHT is larger than the LF component, while its monthly average becomes negligible because it is caused by periodical internal waves. The submesoscale VHT along the eddy edges is related to strong frontogenesis which develops the vertical flux.

Cross‐Slope Buoyancy Fluxes Cause Intense Asymmetric Generation of Submesoscale Eddies on the Periphery of the Black Sea Mesoscale Anticyclones

AbstractSubmesoscale dynamics significantly affect the turbulence regime and is characterized by pronounced time variability, the reasons of which are still poorly known. In our study, we use high‐resolution (1 km) numerical model NEMO and satellite data in the Black Sea to demonstrate that one of the main processes impacting on the intensity of submesoscale dynamics in the marginal seas is the cross‐slope advection by mesoscale anticyclones. Such large eddies entrain the brackish shelf waters into their orbital motion and cause the strong rise of vertical and horizontal gradients of buoyancy. The emergence of baroclinic instability causes the generation of periodic submesoscale structures, among which cyclonic eddies have the largest vorticity. Our analysis shows that the presented mechanism is one of the main reasons for the intraannual variability of submesoscale dynamics over the continental slope of the Black Sea. This process also causes a strong asymmetry in the vertical structure of the mesoscale anticyclones. The generation of the submesoscale eddies and related vertical and horizontal exchange are significantly more intense on that side of the anticyclone, where current is directed offshore. Intensification of submesoscale motions in the zone of the brackish water entrainment is one of the probable reasons for the rapid and effective mixing of the shelf and open sea waters, often observed in satellite data.

Strong Long‐Lived Anticyclonic Mesoscale Eddies in the Balearic Sea: Formation, Intensification, and Thermal Impact

AbstractAnticyclonic mesoscale eddies are often observed in the Balearic Sea (BS) toward the end of summer and autumn. In some years, these eddies become strong and persistent, modifying the local water mass properties. In this study, we analyze two of the most significant recent long‐lived anticyclonic eddies, occurring in 2010 and 2017, using data from a high‐resolution circulation model, altimetry and satellite‐borne sea surface temperature observations. These eddies lasted around 2 and 4 months, respectively, with a radius varying between 40 and 75 km. The generation and intensification mechanisms of these long‐lived anticyclonic eddies are studied by means of (a) energy conversion terms associated with eddy‐mean flow interaction and (b) model sensitivity tests. Results show that these eddies were formed and intensified through mixed barotropic and baroclinic instabilities. The former are produced under the action of intense northwesterly (NW) winds. The latter are related to the existence of an intense summer thermal front between the BS and the Gulf of Lion, and to northward inflows of relatively lower salinity waters. Both the wind events and the presence of the thermal front are necessary for the formation of the eddies. The intensification process varied between both events. While in 2010 it was driven by significant salinity gradients produced by northwards inflows, in 2017 it was produced by additional intense NW winds. Both long‐lived anticyclonic eddies created long‐lasting surface temperature anomalies up to 2.5°C, which have characteristics of local marine heatwaves.