Vertical Heat Transport Research Articles

Seasonality of Submesoscale Vertical Heat Transport Modulated by Oceanic Mesoscale Eddies in the Kuroshio Extension

AbstractEnergetic mesoscale eddies are often accompanied by strong submesoscale variability, which plays a significant role in connecting mesoscale and turbulent motions in the ocean and leads to strong vertical motions. The product of a high‐resolution (1/48°) oceanic numerical model, the LLC4320, is employed to investigate the seasonal variations of vertical heat transport induced by submesoscale processes within multiple mesoscale eddies in the Kuroshio Extension (KE) region. In different seasons, the submesoscale vertical heat transport exhibits a consistent upward pattern, with notably higher magnitudes observed during winter. In winter, the maxima value of submesoscale vertical heat flux (SVHF) can account for approximately 60% of the total vertical heat flux (VHF). This is equivalent to the average net sea surface heat flux in a single eddy region. In summer and autumn, the maxima absolute value of submesoscale vertical heat flux can account for approximately 30% of the total VHF. Energy analysis reveals that baroclinic instability associated with vertical buoyancy flux has a crucial effect on generating submesoscale processes within the eddy region. The submesoscale motions are influenced by the mixed layer instability, strain‐induced frontogenesis, turbulent thermal wind and turbulent thermal wind‐induced frontogenesis within the upper mixed layer, while they are largely associated with the strain‐induced frontogenesis in the ocean interior. Furthermore, the upward low‐frequency submesoscale vertical heat transport is generated by submesoscale secondary circulation at eddy peripheries.

The impact of turbulent transport efficiency on surface vertical heat fluxes in the Arctic stable boundary layer predicted from similarity theory and machine-learning

Abstract We analyzed 14 days of observations from sonic anemometry and high-resolution fiber optic distributed sensing collected in the stable polar boundary layer (SBL). The study sought to evaluate if and under which conditions the sensible heat flux is related to the temperature gradient. Machine learning methods were employed to identify drivers of and model heat fluxes. We found the recently proposed coupling metric Ω defined as the ratio of the buoyancy length scale and measurement height to delineate physically meaningful transport regimes. The regime transition marks the point where static stability in addition to the vertical turbulence strength control the heat transport, which is rather gradual than abrupt. The maximum downward heat flux is reached when one third of turbulent eddies exceed the opposing buoyancy forces in the SBL. We found evidence that even for large Ω a substantial fraction of the turbulent transport is non-equilibrium. The non-dimensional temperature gradient is better explained by variations in Ω than ζ = zL−1 from Monin-Obukhov Similarity theory. Its continuous organization with Ω across stabilities suggest that the vertical heat transport always remains coupled to the surface, but its efficiency and the resulting flux vary. 43% of the total enthalpy is exchanged during conditions of limited transport efficiency in the very SBL despite the small flux magnitude of ≤ 7 W m−2, which underlines the importance of quantifying the weak surface exchange for polar regions. When predicting sensible heat fluxes using mean quantities from weather stations, the net longwave radiative forcing and the horizontal wind speed are the most important predictors representing stratification and bulk shear.

Surface Quasi Geostrophic Reconstruction of Vertical Velocities and Vertical Heat Fluxes in the Southern Ocean: Perspectives for SWOT

AbstractMesoscale currents account for 80% of the ocean's kinetic energy, whereas submesoscale currents capture 50% of the vertical velocity variance. SWOT's first sea surface height (SSH) observations have a spatial resolution an order of magnitude greater than traditional nadir‐looking altimeters and capture mesoscale and submesoscale features. This enables the derivation of submesoscale vertical velocities, crucial for the vertical transport of heat, carbon and nutrients between the ocean interior and the surface. This work focuses on a mesoscale energetic region south of Tasmania using a coupled ocean‐atmosphere simulation at km‐scale resolution and preliminary SWOT SSH observations. Vertical velocities (w), temperature anomalies and vertical heat fluxes (VHF) from the surface down to 1,000 m are reconstructed using effective surface Quasi‐Geostrophic (sQG) theory. An independent method for reconstructing temperature anomalies, mimicking an operational gridded product, is also developed. Results show that sQG reconstructs 90% of the modeled w and VHF rms at scales down to 30 km just below the mixed layer and 50%–70% of the rms for scales larger than 70 km at greater depth, with a spatial correlation of 0.6. The reconstruction is spectrally coherent for scales larger than 30–40 km at the surface, slightly degrading ( 0.55) at depth. Two temperature anomaly data sets yield similar results, indicating the dominance of w on VHF. The RMS of sQG and VHF derived from SWOT are twice as large as those derived from conventional altimetry, highlighting the potential of SWOT for reconstructing energetic meso and submesoscale dynamics in the ocean interior.

Dynamics of submesoscale processes and their influence on vertical heat transport in the southeastern tropical Indian Ocean

Dynamics of submesoscale processes and their influence on vertical heat transport in the southeastern tropical Indian Ocean

Turbulence, Richardson number (Ri) distributions, and parametric instabilities in the turbopause region (96–105 km) from Na LIDAR measurements at the Andes Lidar Observatory (ALO)

Turbulence in the Mesosphere and Lower Thermosphere (MLT) region is responsible for vertical mixing and transport of constituents and heat and the formation of the turbo-pause. A study of turbulence at the Andes Lidar Observatory (ALO) by Philbrick et al. (2021) found, for 25 nights of lidar observations, the probability of Ri < 1/4 decreased with altitude above 100 km, whereas the power in turbulence increased. The objective of this study is to understand the atmospheric process responsible for the observed increase in turbulence power with altitude. Conventionally turbulence is caused by instabilities due to convection (Ri < 0), and Kelvin-Helmholtz Instabilities (KHI), 0 < Ri < 1/4. These criteria are based on laminar flow, a waveless basic state. However, wave-modulated states requiring Floquet theory may dominate the MLT region and can generate instabilities and turbulence under more stable conditions (Ri > 1/4, Klostermeyer, 1990). It was determined in this study the probability of Ri > 1/4 to be >70% at 105 km, consistent with parametric instability (PI) where large tidal induced wind shears and gravity wave presence are contributing factors.

Mesoscale Ocean–Atmosphere Coupling Effects on the North Pacific Subtropical Mode Water

Abstract Subtropical mode water (STMW) is a thick layer of water mass characterized by homogeneous properties within the main pycnocline, important for oceanic oxygen utilization, carbon sequestration, and climate regulation. North Pacific STMW is formed in the Kuroshio Extension region, where vigorous mesoscale eddies strongly interact with the atmosphere. However, it remains unknown how such mesoscale ocean–atmosphere (MOA) coupling affects the STMW formation. By conducting twin simulations with an eddy-resolving global climate model, we find that approximately 25% more STMW is formed with the MOA coupling than without it. This is attributable to a significant increase in ocean latent heat release primarily driven by higher wind speed over the STMW formation region, which is associated with the southward deflection of storm tracks in response to oceanic mesoscale imprints. Such enhanced surface latent heat loss overwhelms the stronger upper-ocean restratification induced by vertical eddy and turbulent heat transport, leading to the formation of colder and denser STMW in the presence of MOA coupling. Further investigation of a multimodel and multiresolution ensemble of global coupled models reveals that the agreement between the STMW simulation in eddy-present/rich coupled models and observations is superior to that of eddy-free ones, likely due to more realistic representation of MOA coupling. However, the ocean-alone model simulations show significant limitations in improving STMW production, even with refined model resolution. This indicates the importance of incorporating the MOA coupling into Earth system models to alleviate biases in STMW and associated climatic and biogeochemical impacts. Significance Statement North Pacific subtropical mode water (STMW) is a distinct pycnostad within the main thermocline located south of the Kuroshio Extension. As short-term heat and carbon silos, STMW is traditionally thought to be driven by the basin-scale atmospheric forcing. The role of air–sea interactions at mesoscales residing in the Kuroshio Extension region has been overlooked. Here, we demonstrate that the strong thermal feedback of mesoscale sea surface temperature anomalies is not negligible for the STMW formation. This is achieved by accelerating wind and consequently promoting ocean latent heat release. Our results pinpoint the significance of accounting for the role of oceanic mesoscale feedback in improving the simulation of STMW as well as its climatic and biogeochemical impacts in Earth system models.

Heat transport into the interior ocean induced by water-mass subduction

The subduction of oceanic water masses provides a crucial pathway for anthropogenic heat to enter the subsurface ocean, thereby shaping deep-reaching warming signatures. Analyzing data from eight ocean and atmosphere reanalysis datasets, we show that the average annual subduction rate of the global ocean (excluding 10° S–10° N) is 312.4 ± 27.9 Sv, resulting in a mean heat transport of 20.2 ± 2.1 PW towards the subsurface ocean. This subduction-driven heat transport has exhibited an increase of 0.09 ± 0.08 PW/decade since 1970. The increase predominantly stems from the overall enhancement of subduction within the latitudes of 30° S–50° S, dictated by intensified westerly winds that lead to the deepening of the local mixed layer depth. Our findings underscore the essence of wind-driven changes in the Southern Ocean subduction, which wield considerable influence over the global climate by regulating the vertical transport of heat and carbon from the sea surface to the deep waters.

Submesoscale Vertical Heat Transport at the Kuroshio Extension: Characteristics and Mechanisms

AbstractSubmesoscale processes significantly influence the air‐sea interaction, heat budget and the distribution of physical and biogeochemical tracers in the upper‐ocean. We study the spatiotemporal characteristics and generation mechanisms of submesoscale vertical heat transport (SVHT) at the Kuroshio Extension using a submesoscale‐permitting simulation. Compared with the commonly used Boxcar and Hanning filters, the clean‐cut feature of the Butterworth filter in the wavenumber domain makes it a proper filter to diagnose SVHT. SVHT is systematically upward, peaking in winter. Through causality analysis, we find that, among the basic factors (mixed layer depth, strain rate, surface wind stress, and horizontal buoyancy gradient) from the conventional submesoscale generation mechanism scalings, mixed layer depth plays the leading role in modulating the intraseasonal variation of SVHT. Using the budget of submesoscale temperature variance and causality analysis, we find that advection also plays an important role in regulating SVHT. This work suggests that the choice of appropriate spatial filter is important for SVHT diagnosis and including advection may help improve submesoscale parameterization schemes.

Tethered balloon measurements reveal enhanced aerosol occurrence aloft interacting with Arctic low-level clouds

Low-level clouds in the Arctic affect the surface energy budget and vertical transport of heat and moisture. The limited availability of cloud-droplet-forming aerosol particles strongly impacts cloud properties and lifetime. Vertical particle distributions are required to study aerosol–cloud interaction over sea ice comprehensively. This article presents vertically resolved measurements of aerosol particle number concentrations and sizes using tethered balloons. The data were collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition in the summer of 2020. Thirty-four profiles of aerosol particle number concentration were observed in 2 particle size ranges: 12–150 nm (N12−150) and above 150 nm (N&amp;gt;150). Concurrent balloon-borne meteorological measurements provided context for the continuous profiles through the cloudy atmospheric boundary layer. Radiosoundings, cloud remote sensing data, and 5-day back trajectories supplemented the analysis. The majority of aerosol profiles showed more particles above the lowest temperature inversion, on average, double the number concentration compared to below. Increased N12−150 up to 3,000 cm−3 were observed in the free troposphere above low-level clouds related to secondary particle formation. Long-range transport of pollution increased N&amp;gt;150 to 310 cm−3 in a warm, moist air mass. Droplet activation inside clouds caused reductions of N&amp;gt;150 by up to 100%, while the decrease in N12−150 was less than 50%. When low-level clouds were thermodynamically coupled with the surface, profiles showed 5 times higher values of N12−150 in the free troposphere than below the cloud-capping temperature inversion. Enhanced N12−150 and N&amp;gt;150 interacting with clouds were advected above the lowest inversion from beyond the sea ice edge when clouds were decoupled from the surface. Vertically discontinuous aerosol profiles below decoupled clouds suggest that particles emitted at the surface are not transported to clouds in these conditions. It is concluded that the cloud-surface coupling state and free tropospheric particle abundance are crucial when assessing the aerosol budget for Arctic low-level clouds over sea ice.

Mesoscale eddies in the Gulf of Mexico: A three-dimensional characterization based on global HYCOM

The Gulf of Mexico (GoM) is characterized by strong mesoscale eddy activities that have been studied extensively, yet the comprehensive three-dimensional (3-D) kinematic properties of GoM eddies are still not well documented. In this study, the 3-D mesoscale eddy activities in the upper layer (0–800 m) of the GoM are characterized using 14-year (1997–2010) global Hybrid Coordinate Ocean Model (HYCOM) outputs. Most eddies in the upper layer (both cyclonic and anticyclonic) have radii of ∼30–60 km and lifespans shorter than 30 days. The spatial distributions of GoM eddies do not vary much with depth, while their intensity decreases with depth. The size of cyclonic eddies does not vary much with depth, while the size of anticyclonic eddies decreases slightly with depth. Cyclonic eddies are often found to be generated in the eastern GoM (especially in the Loop Current region), the Bay of Campeche, and on the continental slope of the Campeche Bank, while anticyclonic eddies are often generated on the northeastern and northwestern GoM continental slopes, and in the central GoM (near 24°N) and the Bay of Campeche (92–94°W). In addition, long-lived GoM eddies (e.g., lifespan >150 days) tend to have intermediate eddy intensity (e.g., 0.13–0.32 for cyclonic eddies at the 10 m level). Both cyclonic and anticyclonic eddies are found to play an important role in the horizontal and vertical transport of heat and salt, and eddy-induced anomalies of water temperature and salinity at both surface and subsurface are generally more pronounced in the eastern GoM than in the western GoM.

Impact of Atmospheric Compressibility and Stokes Drift on the Vertical Transport of Heat and Constituents by Gravity Waves

AbstractVertical transport of heat and atmospheric constituents by gravity waves plays a crucial role in shaping the thermal and constituent structure of the middle atmosphere. We show that atmospheric mixing by non‐breaking waves can be described as a diffusion process where the potential temperature (KH) and constituent (KWave) diffusivities depend on the compressibility of the wave fluctuations and the vertical Stokes drift imparted to the atmosphere by the wave spectrum. KH and KWave are typically much larger than the eddy diffusivity (Kzz), arising from the turbulence generated by breaking waves, and can exceed several hundred m2s−1 in regions of strong wave dissipation. We also show that the total diffusion of heat and constituents caused by waves, turbulence, and the thermal motion of molecules, is enhanced in the presence of non‐breaking waves by a factor that is proportional to the variance of the wave‐driven lapse rate fluctuations. Diffusion enhancements of both heat and constituents of 50% or more can be experienced in regions of low atmospheric stability, where the lapse rate fluctuations are large. These important transport effects are not currently included in most global chemistry‐climate models, which typically only consider the eddy diffusion that is induced when the unresolved, but parameterized waves, experience dissipation. We show that the theoretical results compare favorably with observations of the mesopause region at midlatitudes and describe how the theory may be used to more fully account for the unresolved wave transport in global models.

Observations reveal vertical transport induced by submesoscale front

Submesoscale fronts, with horizontal scale of 0.1–10 km, are key components of climate system by driving intense vertical transports of heat, salt and nutrients in the ocean. However, our knowledge on how large the vertical transport driven by one single submesoscale front can reach remains limited due to the lack of comprehensive field observations. Here, based on high-resolution in situ observations in the Kuroshio-Oyashio Extension region, we detect an exceptionally sharp submesoscale front. The oceanic temperature (salinity) changes sharply from 14 °C (34.55 psu) to 2 °C (32.7 psu) within 2 km across the front from south to north. Analysis reveals intense vertical velocities near the front reaching 170 m day−1, along with upward heat transport up to 1.4 × 10−2 °C m s−1 and salinity transport reaching 4 × 10−4 psu m s−1. The observed heat transport is much larger than the values reported in previous observations and is three times as that derived from current eddy-rich climate models, whereas the salinity transport enhances the nutrients concentration with prominent implications for marine ecosystem and fishery production. These observations highlight the vertical transport of submesoscale fronts and call for a proper representation of submesoscale processes in the next generation of climate models.

Tropical Pacific Quasi-Decadal Variability Suppressed by Submesoscale Eddies

Abstract Tropical Pacific quasi-decadal (TPQD) climate variability is characterized by quasi-decadal sea surface temperature (SST) variations in the central Pacific (CP). This low-frequency climate variability is suggested to influence extreme regional weather and substantially impact global climate patterns and associated socioeconomics through teleconnections. Previous studies mostly attributed the TPQD climate variability to basin-scale air–sea coupling processes. However, due to the coarse resolution of the majority of the observations and climate models, the role of subbasin-scale processes in modulating the TPQD climate variability is still unclear. Using a long-term high-resolution global climate model, we find that energetic small-scale motions with horizontal scales from tens to hundreds of kilometers (loosely referred to as equatorial submesoscale eddies) act as an important damping effect to retard the TPQD variability. During the positive TPQD events, compound increasing precipitation and warming SST in the equatorial Pacific intensifies the upper ocean stratification and weakens the temperature fronts along the Pacific cold tongue. This suppresses submesoscale eddy growth as well as their associated upward vertical heat transport by inhibiting baroclinic instability (BCI) and frontogenesis; conversely, during the negative TPQD events, the opposite is true. Using a series of coupled global climate models that participated in phase 6 of the Coupled Model Intercomparison Project with different oceanic resolutions, we show that the amplitude of the TPQD variability becomes smaller as the oceanic resolution becomes finer, providing evidence for the impacts of submesoscale eddies on damping the TPQD variability. Our study suggests that explicitly simulating equatorial submesoscale eddies is necessary for gaining a more robust understanding of low-frequency tropical climate variability. Significance Statement Submesoscale ocean eddies inhibit the development of quasi-decadal climate variability in the equatorial central Pacific, according to a high-resolution global climate simulation.

Enhanced Upper Ocean Warming Projected by the Eddy‐Resolving Community Earth System Model

AbstractOcean warming is a key factor impacting future changes in climate. Here we investigate vertical structure changes in globally averaged ocean heat content (OHC) in high‐ (HR) and low‐resolution (LR) future climate simulations with the Community Earth System Model (CESM). Compared with observation‐based estimates, the simulated OHC anomalies in the upper 700 and 2,000 m during 1960–2020 are more realistic in CESM‐HR than ‐LR. Under RCP8.5 scenario, the net surface heat into the ocean is very similar in CESM‐HR and ‐LR. However, CESM‐HR has a larger increase in OHC in the upper 250 m compared to CESM‐LR, but a smaller increase below 250 m. This difference can be traced to differences in eddy‐induced vertical heat transport between CESM‐HR and ‐LR in the historical period. Moreover, our results suggest that with the same heat input, upper‐ocean warming is likely to be underestimated by most non‐eddy‐resolving climate models.

Validating Finescale Parameterizations for the Eastern Arctic Ocean Internal Wave Field

AbstractIn the Arctic Ocean, vertical transport of heat by turbulent mixing is ultimately coupled to the sea‐ice cover, with immediate and far‐reaching impacts on the climate and ecosystem. Unfortunately, direct observations of mixing are difficult, expensive and sparse. Finescale Parameterization (FS) of turbulent energy dissipation rate (ɛ) allows for the quantification of turbulence from breaking internal waves using standard measurements, such as profiles of hydrography and velocity. While FS proved to be reliable in mid‐latitudes, the Arctic Ocean internal wave field is distinct in terms of composition and energy level, rendering the applicability of FS uncertain. To test FS in a wide range of eastern Arctic conditions, we compiled data from eight cruises. All profiles used to calculate FS were collocated with in‐situ measurements of ɛ obtained from microstructure profilers. FS was applied between 50 and 450 m below the surface. Results show a satisfactory performance of FS, with 84% of FS‐derived ɛ being within a factor of 5 to observations. This improved to 90% when using lower‐noise velocity profiles of lowered current meters instead of ship‐mounted current meters. In our data, FS performance is independent of the shear‐strain ratio (Rω) and internal wave field bandwidth (N/f), but there is evidence that highly stratified environments with large potential energy, low turbulence and substantially non‐white shear spectra are less suitable for FS. A widely used formulation of FS using only hydrography and a prescribed Rω = 7 results in 73% of FS estimates being within a factor of 5 to observations.

The Inhomogeneity Effect. III. Weather Impacts on the Heat Flow of Hot Jupiters

The interior flux of a giant planet impacts atmospheric motion, and the atmosphere dictates the interior’s cooling. Here we use a non-hydrostatic general circulation model (Simulating Non-hydrostatic Atmospheres on Planets) coupled with a multi-stream multi-scattering radiative module (High-performance Atmospheric Radiation Package) to simulate the weather impacts on the heat flow of hot Jupiters. We found that the vertical heat flux is primarily transported by convection in the lower atmosphere and regulated by dynamics and radiation in the overlying radiation-circulation zone. The temperature inversion occurs on the dayside and reduces the upward radiative flux. The atmospheric dynamics relay the vertical heat transport until the radiation becomes efficient in the upper atmosphere. The cooling flux increases with atmospheric drag due to increased day–night contrast and spatial inhomogeneity. The temperature dependence of the infrared opacity greatly amplifies the opacity inhomogeneity. Although atmospheric circulation could transport heat downward in a narrow region above the radiative-convective boundary, the opacity inhomogeneity effect overcomes the dynamical effect and leads to a larger overall interior cooling than the local simulations with the same interior entropy and stellar flux. The enhancement depends critically on the equilibrium temperature, drag, and atmospheric opacity. In a strong-drag atmosphere hotter than 1600 K, a significant inhomogeneity effect in three-dimensional (3D) models can boost interior cooling several-fold compared to the 1D radiative-convective equilibrium models. This study confirms the analytical argument of the inhomogeneity effect in the companion papers by Zhang. It highlights the importance of using 3D atmospheric models in understanding the inflation mechanisms of hot Jupiters and giant planet evolution in general.

Deepening of Southern Ocean Gateway Leads to Abrupt Onset of a Deep‐Reaching Meridional Overturning Circulation

AbstractDuring the Eocene and the Eocene‐Oligocene transition, the lower cell of the meridional overturning circulation (MOC), associated with bottom water formation, underwent changes associated with the geological evolution of Southern Ocean gateways. These are important for the Cenozoic climate transition from Greenhouse to Icehouse, but their dynamics still remain elusive. We demonstrate, using an idealized eddying ocean model, that the opening of a gateway leads to the abrupt onset of a vigorous, deep‐reaching, MOC. This MOC has a maximum transport for a shallow gateway, and decreases with further deepening of the gateway. This abrupt change in the MOC can be explained through the ability with which standing meanders—turbulent features located downstream of the gateway—can induce deep vertical heat transport at high latitudes where bottom waters are produced. Our results demonstrate the crucial role of turbulent processes in setting the strength of the global ocean's deep‐reaching MOC.

Numerical simulation of upper ocean responses to Typhoon Maria (2018) in the Northwest Pacific: Behavior of sub-mesoscale processes

A deeper understanding of the upper ocean response to typhoons is beneficial to the further improvement of typhoon forecasting. However, because of model horizontal resolution limitations, we still do not understand how sub-mesoscale processes in the ocean may modulate the response of the upper ocean to typhoons. In this study, we set up one-way online nested parent (mesoscale resolving: Exp_9 km) and child (sub-mesoscale permitting: Exp_3 km) models to investigate how the 15–45-km sub-mesoscale processes in the ocean affected the thermal and dynamical response processes of the upper ocean to Super Typhoon Maria (2018). It was found that the sub-mesoscale processes and their induced vertical heat transport (VHT) were mainly concentrated at the eddy, and most of the sub-mesoscale processes and their accompanying VHT disappeared after the typhoon, indicating that typhoons may be destroyers of sub-mesoscale processes. The differences in the sea temperature response between Exp_3 km and Exp_9 km can reached 1.5 °C in the eddy-rich sea surface region and 3 °C at the thermocline. This thermal response difference could be attributed to the difference in vertical diffusion and the difference in total advection processes simulated by the two models. Furthermore, the sub-mesoscale VHT played an important role in causing smaller sea surface cooling (SSC) and larger subsurface warming to the right side of the typhoon, and considering more sub-mesoscale processes improved the simulation accuracy of the SSC on the right side of the typhoon. The difference in near-inertial kinetic energy (NIKE) and the difference in fD1 (a peak frequency approximately equal to the sum of f and D1) kinetic energy between the two experiments were mainly found in the eddy-rich region to the right side of the typhoon, accounting for, at most, 15% of Exp_3 km's NIKE and 50% of Exp_3 km's fD1 kinetic energy, respectively, and resolving more sub-mesoscale processes could limit the lateral propagation of NIKE, especially in warm eddy. The kinetic energy and vertical shear of the near-inertial currents (NICs) and fD1 currents of Exp_3 km were larger than those of Exp_9 km, suggesting that sub-mesoscale processes may enhance ocean vertical mixing. Moreover, sub-mesoscale processes promoted the downward propagation of NIKE, which may be of great significance for promoting NICs-induced ocean mixing into the deep ocean and accelerating the decay of NICs. This work is of great importance for improving the understanding of typhoon-induced thermal and dynamical responses, and thus for improving the typhoon prediction.

Multi‐Approach Analysis of Baroclinic Internal Tide Perturbation in the Ionian Sea Abyssal Layer (Mediterranean Sea)

AbstractDespite being widely recognized, the importance of deep layers thermohaline and mixing processes in the ocean circulation and variability is still poorly investigated, especially in the Mediterranean Sea. This limits understanding and parametrizing deep dynamics, which result in evident biases in the global circulation representation by observations and numerical ocean simulations. Having access to hydrological datasets, collected on a whole water column, we investigated the abyssal stratification and its variability of the Ionian Sea (Central Mediterranean). Applying multiple analyses, we found a tidal‐period oscillation and the resulting activation of mixing, pointing out that the combined effect of stratification, morphology, and tides has a key role in enhancing local diapycnal diffusivity in the deepest layers, being a mechanism that connects the whole water column with a compelling impact on the vertical transport of heat and tracers.

Lagrangian studies of coherent sets and heat transport in constant heat flux-driven turbulent Rayleigh–Bénard convection

We explore the mechanisms of heat transfer in a turbulent constant heat flux-driven Rayleigh–Bénard convection flow, which exhibits a hierarchy of flow structures from granules to supergranules. Our computational framework makes use of time-dependent flow networks. These are based on trajectories of Lagrangian tracer particles that are advected in the flow. We identify coherent sets in the Lagrangian frame of reference as those sets of trajectories that stay closely together for an extended time span under the action of the turbulent flow. Depending on the choice of the measure of coherence, sets with different characteristics are detected. First, the application of a recently proposed evolutionary spectral clustering scheme allows us to extract granular coherent features that are shown to contribute significantly less to the global heat transfer than their spatial complements. Moreover, splits and mergers of these (leaking) coherent sets leave spectral footprints. Second, trajectories which exhibit a small node degree in the corresponding network represent objectively highly coherent flow structures and can be related to supergranules as the other stage of the present flow hierarchy. We demonstrate that the supergranular flow structures play a key role in the vertical heat transport and that they exhibit a greater spatial extension than the granular structures obtained from spectral clustering.