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Mika P. Malila
,
Francesco Barbariol
,
Alvise Benetazzo
,
Øyvind Breivik
,
Anne Karin Magnusson
,
Jim Thomson
, and
Brian Ward

Abstract

Wave crests of unexpected height and steepness pose a danger to activities at sea, and long-term field measurements provide important clues for understanding the environmental conditions that are conducive to their generation and behavior. We present a novel dataset of high-frequency laser altimeter measurements of the sea surface elevation gathered over a period of 18 years from 2003 to 2020 on an offshore platform in the central North Sea. Our analysis of crest height distributions in the dataset shows that mature, high sea states with high spectral steepness and narrow directional spreading exhibit crest height statistics that significantly deviate from standard second-order models. Conversely, crest heights in developing sea states with similarly high steepness but wide directional spread correspond well to second-order theory adjusted for broad frequency bandwidth. The long-term point time series measurements are complemented with space–time stereo video observations from the same location, collected during five separate storm events during the 2019/20 winter season. An examination of the crest dynamics of the space–time extreme wave crests in the stereo video dataset reveals that the crest speeds exhibit a slowdown localized around the moment of maximum crest elevation, in line with prevailing theory on nonlinear wave group dynamics. Extending on previously published observations focused on breaking crests, our results are consistent for both breaking and nonbreaking extreme crests. We show that wave crest steepness estimated from time series using the linear dispersion relation may overestimate the geometrically measured crest steepness by up to 25% if the crest speed slowdown is not taken into account.

Significance Statement

Better understanding of the statistics and dynamical behavior of extreme ocean surface wave crests is crucial for improving the safety of various operations at sea. Our study provides new, long-term field evidence of the combined effects of wave field steepness and directionality on the statistical distributions of crest heights in storm conditions. Moreover, we show that the dynamical characteristics of extreme wave crests are well described by recently identified nonlinear wave group dynamics. This finding has implications, for example, for wave force calculations and the treatment of wave breaking in numerical wave models.

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Mengmeng Li
,
Chongguang Pang
,
Xiaomei Yan
,
Linlin Zhang
, and
Zhiliang Liu

Abstract

Using the recently developed multiscale window transform and multiscale energy and vorticity analysis methods, this study diagnoses the climatological characteristics of the nonlinear mutual interactions among mesoscale eddies, low-frequency (seasonal to interannual) fluctuations, and the decadally modulating mean flow in the Agulhas Retroflection Current System (ARCS). It is found that mesoscale eddies are generated primarily in the retroflection region by mixed barotropic and baroclinic instabilities. The barotropic instability dominates the generation of eddy kinetic energy (EKE) here, contributing power roughly 10 times larger than the baroclinic one. These locally generated eddies are transported away. In the rings drift and meanders regions, the nonlocal transport serves as an important energy source for the eddy field, making a contribution comparable to that of the baroclinic instability for the EKE production. Contrarily, in the stable region, the EKE is generated mainly due to the baroclinic instability. In most of the ARCS area, the kinetic energy (KE) is further transferred inversely from mesoscale eddies to other lower-frequency motions. In particular, in the retroflection, rings drift, and stable regions, the inverse KE cascade plays a leading role in generating seasonal–interannual fluctuations, providing roughly 3–5 times as much power as the forward KE cascade from the mean flow and the advection effect. In the meanders region, however, the forward cascade contributes 4 times more KE to the low-frequency variabilities than the inverse one. All the results provide a model-based benchmark for future studies on physical processes and dynamics at different scales in the ARCS.

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Han Zhang

Abstract

The ocean temperature response to tropical cyclones (TCs) is important for TC development, local air–sea interactions, and the global air–sea heat budget and transport. The modulation of the upper ocean vertical temperature structure after a fast-moving TC was studied at the observation stations in the northern South China Sea, including TCs Kalmaegi (2014), Rammasun (2014), Sarika (2016), and Haima (2016). The upper ocean temperature and heat response to the TCs mainly depended on the combined effect of mixing and vertical advection. Mixing cooled the sea surface and warmed the subsurface, while upwelling (downwelling) reduced (increased) the subsurface warm anomaly and cooled (warmed) the deeper ocean. An ideal parameterization that depends on only the nondimensional mixing depth (HE ), nondimensional transition layer thickness (HT ), and nondimensional upwelling depth (HU ) was able to roughly reproduce sea surface temperature (SST) and upper ocean heat change. After TCs, the subsurface heat anomalies moved into the deeper ocean. The air–sea surface heat flux contributed little to the upper ocean temperature anomaly during the TC forcing stage and did not recover the surface ocean back to pre-TC conditions more than one and a half months after the TC. This work shows how upper ocean temperature and heat content varies by a TC, indicating that TC-induced mixing modulates the warm surface water into the subsurface, and TC-induced advection further modulates the warm water into the deeper ocean and influences the ocean heat budget.

Significance Statement

Tropical cyclones can cause a strong ocean response that modulates the upper ocean temperature structure and contributes to the local heat budget and transport. This manuscript shows how mixing and vertical advection modulate upper ocean temperature after four fast-moving tropical cyclones, and then gives a parameterization of how sea surface temperature and upper ocean heat change depend on the two mechanisms. The temperature anomalies can propagate into deeper ocean after the tropical cyclones, and sea surface heat flux is not important for upper ocean temperature response during a tropical cyclone. These results show how the upper ocean temperature responses to a tropical cyclone, and influences the local heat budget.

Open access
Mika N. Siegelman
,
Eric Firing
,
Mark A. Merrifield
,
Janet M. Becker
, and
Ruth C. Musgrave

Abstract

Motivated by observations of enhanced near-inertial currents at the island chain of Palau, the modification of wind-generated near-inertial oscillations (NIOs) by the presence of an island is examined using the analytic solutions of Longuet-Higgins and a linear, inviscid, 1.5-layer reduced-gravity model. The analytic solution for oscillations at the inertial frequency f provides insights into flow adjustment near the island but excludes wave dynamics. To account for wave motion, the numerical model initially is forced by a large-scale wind field rotating at f, where the forcing is increased then decreased to zero. Numerical simulations are carried out over a range of island radii and the ocean response detailed. Near the island, wind energy in the frequency band near f can excite subinertial island-trapped waves and superinertial Poincaré waves. In the small-island limit, both the Poincaré waves and the island-trapped waves are very near f, and their sum resembles the Longuet-Higgins analytic solution but with increased amplitude near the island. The flow field can be viewed as primarily a far-field NIO locally deflected by the island plus an island-trapped contribution, leading to enhanced near-inertial currents near the island, on the scale of the island radius. As the island size is increased, the island-trapped wave frequency deviates further from f and its amplitude depends strongly on the frequency bandwidth and wavenumber structure of the wind forcing. In the large-island limit, the island-trapped wave resembles a Kelvin wave, and the sum of incident and reflected Poincaré waves suppresses the near-inertial current amplitude near the island.

Significance Statement

Strong, impulsive winds over the ocean excite currents that rotate in the opposite direction to Earth’s rotation. This work examines how these wind-generated currents, known as near-inertial oscillations (NIOs), are modified by the presence of an island. Around small islands, the primary response is locally enhanced near-inertial currents. Alternatively, around large islands, near-inertial currents are weaker. Understanding how these currents behave should provide insight into the physical processes that drive current variability near islands and spur local mixing.

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Yue Wu
,
Eric Kunze
,
Amit Tandon
, and
Amala Mahadevan

Abstract

While lee-wave generation has been argued to be a major sink for the 1-TW wind work on the ocean’s circulation, microstructure measurements in the Antarctic Circumpolar Currents find dissipation rates as much as an order of magnitude weaker than linear lee-wave generation predictions in bottom-intensified currents. Wave action conservation suggests that a substantial fraction of lee-wave radiation can be reabsorbed into bottom-intensified flows. Numerical simulations are conducted here to investigate generation, reabsorption, and dissipation of internal lee waves in a bottom-intensified, laterally confined jet that resembles a localized abyssal current over bottom topography. For the case of monochromatic topography with |kU 0| ≈ 0.9N, where k is the along-stream topographic wavenumber, |U 0| is the near-bottom flow speed, and N is the buoyancy frequency; Reynolds-decomposed energy conservation is consistent with linear wave action conservation predictions that only 14% of lee-wave generation is dissipated, with the bulk of lee-wave energy flux reabsorbed by the bottom-intensified flow. Thus, water column reabsorption needs to be taken into account as a possible mechanism for reducing the lee-wave dissipative sink for balanced circulation.

Open access
Tobias Kukulka
and
Robert J. Chant

Abstract

Buoyant material, such as floating debris, marine organisms, and spilled oil, is aggregated and trapped within estuaries. Traditionally, the aggregation of buoyant material is assumed to be a consequence of converging Eulerian surface currents, often associated with lateral (cross-estuary) density gradients that drive baroclinic lateral circulations. This study explores an alternative aggregation mechanism due to tidally driven Lagrangian residual circulations without Eulerian convergence zones and without lateral density variation. In a tidally driven estuary, the depth-dependent tidal phase of the lateral velocity varies across the estuary. This study demonstrates that the lateral movement of surface trapped material follows the tidal phase, resulting in a lateral Lagrangian residual circulation known as Stokes drift for small-amplitude motions. For steeper bathymetry, the lateral change in tidal phase is greater and the corresponding lateral Lagrangian residual flow faster. At local depth extrema, e.g., in the thalweg, depth does not vary laterally, so that the associated tidal phase is laterally constant. Therefore, the Stokes drift is weak near depth extrema resulting in Lagrangian convergence zones where buoyant material concentrates. These ideas are evaluated employing an idealized analytic model in which the along-estuary tidal flow is driven by an imposed barotropic pressure gradient, whereas cross-estuary flow is induced by the Coriolis force. Model results highlight that convergence zones due to Lagrangian residual velocities are efficient in forming persistent aggregation regions of buoyant material along the estuary.

Significance Statement

Our study focuses on the aggregation of buoyant material (e.g., debris, oil, organisms) in estuaries. Traditionally, the aggregation of buoyant material is assumed to be a consequence of converging Eulerian surface currents, often associated with lateral (cross-estuary) density gradients that drive baroclinic lateral circulations. Our study explores an alternative aggregation mechanism due to tidally driven Lagrangian residual circulations without Eulerian convergence zones and without lateral density variation. Our results highlight that convergence zones due to Lagrangian residual velocities are efficient in forming persistent aggregation regions of buoyant material along the estuary.

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Nicola Jane Brown
,
Cecilie Mauritzen
,
Camille Li
,
Erica Madonna
,
Pål Erik Isachsen
, and
J. H. LaCasce

Abstract

We explore drivers of variability in the Norwegian Atlantic Slope Current, which carries relatively warm Atlantic Water toward the Barents Sea and Arctic Ocean, using Copernicus Marine Environment Monitoring Service (CMEMS) satellite altimetry data and TOPAZ4 ocean reanalysis data. Previous studies have pointed to a variety of causes, on a variety of time scales. We use data with daily resolution to investigate day-to-day changes in ocean transport across three sections crossing the shelf-slope of Norway (Svinøy, Gimsøy, and the Barents Sea Opening). The highest (lowest) extremes in transport at all sections develop over two days as a cyclonic (anticyclonic) atmospheric pressure system approaches from the southwest, piling up (extracting) water at the coast of Norway. The actual peak is reached when the pressure system passes the site of measurement, and the transport then relaxes for the next two days as the system continues northward along the coast. Other sources of short-term variability, such as propagating continental shelf waves and baroclinic instability, are unlikely to yield covariability over large separations. Monthly variability in the current can also be explained by passing weather systems since their numbers and intensity vary greatly from month to month. Many studies of longer-term variability, especially in the Barents Sea Opening, have pointed to the North Atlantic Oscillation (NAO) as the main cause of variability. Our results show that passing weather systems offer a better explanation of month-to-month variability.

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Weicong Cheng
and
Jianping Gan

Abstract

We used a high-resolution cross-shelf two-dimensional numerical model to investigate the response of coastal wind-driven upwelling circulation to barotropic tidal forcing and lateral buoyant discharge over a broad continental shelf. We found that the tidally amplified asymmetric friction effect arising from the interaction between tidal and subtidal currents modulated the upwelling structure across the shelf. The interaction weakened the water outcropping (upwelling) in the inner shelf due to tidally amplified mixing, but enhanced cross-shore velocity offshore due to tidally induced asymmetric friction effect and nonlinear advection. The enhanced mixing changed the density in the bottom boundary layer and subsequently in the upwelling front, which eventually weakened the geostrophic alongshore flow. The mass and stratification inputs of the lateral buoyant discharge weakened or even reversed geostrophic dynamics for alongshore and upslope transports. The reversed cross-shore density and elevation gradient induced by the buoyant influx weakened the alongshore current and the associated bottom friction effect. The upslope cross-shore transport was reduced due to weakened alongshore flow and the associated bottom Ekman transport. The mass of buoyant influx compensated for the wind-driven offshore transport in the upper layer. The upwelling could be reversed to downwelling when the transport of lateral influx exceeded the wind-driven offshore transport. The responses of upwelling circulation to tidal and lateral buoyancy forcing highlighted in this process-oriented study are fundamental for interpreting more complex wind-driven shelf circulation.

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Heriberto J. Vazquez
,
Ganesh Gopalakrishnan
, and
Julio Sheinbaum

Abstract

The Gulf of Mexico (GoM) surface circulation variability is dominated by the Loop Current (LC) and the episodically released anticyclonic Loop Current eddies (LCEs). The Yucatan Current feeds the LC through the Yucatan Channel (YC), and its flow structure at the YC is hypothesized to affect the LC evolution critically. This study examines the impact of assimilating YC subsurface velocity observations from a tall mooring array across the YC on the GoM circulation. State estimates and forecasts of the LC circulation were produced using a regional implementation of the Massachusetts Institute of Technology general circulation model (MITgcm) and its adjoint-based four-dimensional variational (4DVAR) assimilation system. The estimates were constrained by combinations of the YC observations and satellite-derived sea surface height (SSH) and sea surface temperature (SST). The results show that assimilation of both moored and satellite data improves the model hindcasts and forecasts for all LC phases. Additionally, one realization of the state estimate that assimilates only moored data matches the LCE detachment timing with that of AVISO SSH. Observations from the moorings close to the Yucatan Peninsula significantly impact the LCE detachment. A finite-time Lyapunov exponent analysis reveals the differences among the assimilation experiments, such as eddylike structures intruding into the GoM through the YC, and its relation to the typical LC sudden growth. Finally, an adjoint sensitivity analysis is used to verify the dynamic link between the LC extension and the intrusion of eddylike structures into the GoM.

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Abigail S. Bodner
,
Baylor Fox-Kemper
,
Leah Johnson
,
Luke P. Van Roekel
,
James C. McWilliams
,
Peter P. Sullivan
,
Paul S. Hall
, and
Jihai Dong

Abstract

Current submesoscale restratification parameterizations, which help set mixed layer depth in global climate models, depend on a simplistic scaling of frontal width shown to be unreliable in several circumstances. Observations and theory indicate that frontogenesis is common, but stable frontal widths arise in the presence of turbulence and instabilities that participate in keeping fronts at the scale observed, the arrested scale. Here we propose a new scaling law for arrested frontal width as a function of turbulent fluxes via the turbulent thermal wind (TTW) balance. A variety of large-eddy simulations (LES) of strain-induced fronts and TTW-induced filaments are used to evaluate this scaling. Frontal width given by boundary layer parameters drawn from observations in the General Ocean Turbulence Model (GOTM) are found qualitatively consistent with the observed range in regions of active submesoscales. The new arrested front scaling is used to modify the mixed layer eddy restratification parameterization commonly used in coarse-resolution climate models. Results in CESM-POP2 reveal the climate model’s sensitivity to the parameterization update and changes in model biases. A comprehensive multimodel study is in planning for further testing.

Significance Statement

The ocean surface plays a major role in the climate system, primarily through exchange in properties, such as in heat and carbon, between the ocean and atmosphere. Accurate model representation of ocean surface processes is crucial for climate simulations, yet they tend to be too small, fast, or complex to be resolved. Significant efforts lie in approximating these small-scale processes using reduced expressions that are solved by the model. This study presents an improved representation of the ocean surface in climate models by capturing some of the synergy that has been missing between the processes that define it. Results encourage further testing across a wider range of models to comprehensively evaluate the effects of this adjustment in climate simulations.

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