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Ryan D. Patmore
,
Paul R. Holland
,
Catherine A. Vreugdenhil
,
Adrian Jenkins
, and
John R. Taylor

Abstract

The ice shelf–ocean boundary current has an important control on heat delivery to the base of an ice shelf. Climate and regional models that include a representation of ice shelf cavities often use a coarse grid, and results have a strong dependence on resolution near the ice shelf–ocean interface. This study models the ice shelf–ocean boundary current with a nonhydrostatic z-level configuration at turbulence-permitting resolution (1 m). The z-level model performs well when compared against state-of-the-art large-eddy simulations, showing its capability in representing the correct physics. We show that theoretical results from a one-dimensional model with parameterized turbulence reproduce the z-level model results to a good degree, indicating possible utility as a turbulence closure. The one-dimensional model evolves to a state of marginal instability, and we use the z-level model to demonstrate how this is represented in three dimensions. Instabilities emerge that regulate the strength of the pycnocline and coexist with persistent Ekman rolls, which are identified prior to the flow becoming intermittently unstable. When resolution of the z-level model is degraded to understand the gridscale dependencies, the degradation is dominated by the established problem of excessive numerical diffusion. We show that at intermediate resolutions (2–4 m), the boundary layer structure can be partially recovered by tuning diffusivities. Last, we compare replacing prescribed melting with interactive melting that is dependent on the local ocean conditions. Interactive melting results in a feedback such that the system evolves more slowly, which is exaggerated at lower resolution.

Open access
Junde Li
and
Moninya Roughan

Abstract

Examining eddy–mean flow interactions in western boundary currents is crucial for understanding the mechanisms of mesoscale eddy generation and the role of eddies in the large-scale circulation. However, this analysis is lacking in the East Australian Current (EAC) system. Here we show the detailed three-dimensional structure of the eddy–mean flow interactions and energy budget in the EAC system. The energy reservoirs and conversions are greatest in the upper 500 m, with complex vertical structures. Strong mean kinetic energy is confined within a narrow band (24.5°–32.5°S) in the EAC jet. Most energy is contained in the eddy fields instead of the mean flow in the EAC typical separation and extension regions (south of 32.5°S). Strong barotropic instability is the primary source of eddy kinetic energy north of 36°S, while baroclinic instability dominates the eddy kinetic energy production in the EAC southern extension, which peaks in the subsurface. The mean flow transfers 5.22 GW of kinetic energy and 3.33 GW of available potential energy to the eddy field in the EAC typical separation region. The largest conversion term is from available potential energy conversion from the mean flow to the eddy field through baroclinic instability, dominating between 29° and 35.5°S. Nonlocal eddy–mean flow interactions also play a role in the energy exchange between the mean flow and the eddy fields. This study provides the mean state of the eddy–mean flow interactions in the EAC system, paving the way for further studies exploring seasonal and interannual variability and provides a baseline for assessing the impact of environmental change.

Open access
Xiao Ma
,
Hailong Liu
, and
Xidong Wang

Abstract

This study reveals the role of the tropical Atlantic variability in modulating barrier layer thickness (BLT) in peak seasons. Based on reanalysis data during 1980–2016, statistical and dynamical analyses are performed to investigate the mechanism of BLT variability associated with the tropical Atlantic modes. The regions with significant correlation between BLT and tropical Atlantic modes are located on the northwest and southeast coasts of the tropical Atlantic, which are consistent with BLT maximum variability regions. In boreal spring, BLT decreases in the northwest because less latent heat release affected by weak trade wind related to the Atlantic meridional mode (AMM) shoals the isothermal layer depth (ITLD). In the south equatorial Atlantic, deepened mixed layer depth (MLD) is controlled by the decreasing freshwater input brought by a northward shift of the intertropical convergence zone (ITCZ) and further leads to a thinner barrier layer (BL). However, a shoaling MLD appears in the north equatorial Atlantic, which results from excessive freshwater input, causing a thick BL there. In boreal summer, positive runoff anomaly caused by the Atlantic equatorial mode (AEM) leads to upper warming of the tropical northwest Atlantic and a shallowing ITLD, favoring a thinner BL there. However, a southward shift of ITCZ brings more freshwater into the south equatorial Atlantic, inducing a shallowing MLD as well as a thicker BL. AEM-driven horizontal heat advection of the south equatorial current contributes to a thick ITLD in the central southern tropical Atlantic and thus increases BLT.

Significance Statement

This research aims to reveal how the tropical Atlantic meridional and equatorial interannual climatic modes affect barrier layer thickness (BLT). These two climate modes can affect the wind field, ocean current, and precipitation through air–sea interaction processes, and further affect mixing, heat–salt transport, and stratification in the upper ocean and thus BLT. This finding is important because the barrier layer restricts the exchange of heat, momentum, mass, and nutrients between the mixed layer and the thermocline, thereby impacting local and remote weather events, the ecological environment, and the climate. Our results provide guidance for interpreting the interannual variability of BLT in the tropical Atlantic.

Restricted access
Jianing Li
,
Qingxuan Yang
,
Hui Sun
,
Shuwen Zhang
,
Lingling Xie
,
Qingye Wang
,
Wei Zhao
, and
Jiwei Tian

Abstract

This study focuses on the statistical features of dissipation flux coefficient Γ in the upper South China Sea (SCS). Based on the microscale measurements collected at 158 stations in the upper SCS and derived dissipation rates of turbulent kinetic energy and temperature variance ε and χT , via a modified method, we estimate Γ and analyze its spatiotemporal variation in an energetic and a quiescent region. We show that Γ is highly variable, which scatters over three orders of magnitude from 10−2 to 101 in both regions. Ιn the energetic region, Γ is slightly greater than in the quiescent region; their median values are 0.23 and 0.17, respectively. Vertically, Γ presents a clear increasing tendency with depth in both regions, though the increasing rate is greater in the energetic region than in the quiescent region. In the upper SCS, Γ positively depends on the buoyancy Reynolds number Re b and negatively depends on the ratio of the Ozmidov scale to the Thorpe scale R OT and is scaled as Γ Re b 1 / 2 R OT 4 / 3 , which holds for both regions. The vertical decreasing of R OT is observed, which yields parameterization of R OT = 10−0.002 z ; this parameterization improves the performance of the Thorpe-scale method by reducing at least 50% of the bias between the observed and parameterized ε. These results shed new light on the spatiotemporal variability and modulating mechanism of Γ in the upper ocean.

Significance Statement

The great global ocean conveyor is maintained by vertical mixing. Turbulent kinetic energy released by local internal wave breaking goes into two parts: one part is used to furnish this vertical mixing, and the rest is dissipated into irreversible heat. The ratio of these two parts is termed as the dissipation flux coefficient and is usually treated as a constant. Our measurements suggest that this coefficient is highly spatiotemporally variable. Specific relationships are obtained when scaling this coefficient with other parameters, and mechanisms modulating this coefficient are also explored. This study sheds light on how much turbulent kinetic energy contributes to elevating the potential energy and its associated influences not only in marginal seas but also in open oceans.

Restricted access
Erick R. Olvera-Prado
,
Efraín Moreles
,
Jorge Zavala-Hidalgo
, and
Rosario Romero-Centeno

Abstract

The study of the relationship between the upper and lower layers in the Gulf of Mexico (GoM) has experienced a lot of progress in recent years. Nevertheless, an examination of their coupling for the entire GoM in a statistically consistent manner is still needed. Layer thickness data from a GoM 21-yr free-running simulation are used to examine the coupling between the upper (<250 m) and lower (>1000 m) layers, focusing on the dominant modes of variability through a Hilbert empirical orthogonal function (HEOF) analysis. The results show that the three leading modes are associated with the coupling between both layers during the life cycle of the Loop Current (LC) and the LC eddy (LCE) separation process, consistent with previous observational studies. These modes are cyclical, with periodicities in agreement with the mean LCE separation period, indicating recurrence of the circulation patterns. The fourth mode of the upper layer is associated with the translation of LCEs and their dissipation in the northwestern GoM, while in the lower layer it captures variability related to the strengthening of the circulation along the Sigsbee Gyre western branch. This mode does not show cyclicity, suggesting persistence of the associated circulation patterns with a dominant time scale of 14 months. Evidence and corroboration of recently observed lower-layer circulation features are provided. The application of the HEOF technique used here can complement the three-dimensional oceanic assimilation methods by projecting surface information to depth in a statistically consistent manner.

Significance Statement

The purpose of this study is to better understand the relationship between the upper and lower ocean layers in the Gulf of Mexico. While much is known about their variability separately, an examination of the coupling through the whole region and in a long time period is still needed. We use a free numerical simulation of the circulation in the Gulf to accomplish this goal. The dominant circulation patterns in the lower layer are tied to the upper ones and are governed by the same temporal scales. Our findings point to a way to better understand the response of deep circulation to upper circulation, and may contribute to a better prediction of ocean dynamics.

Restricted access
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.

Restricted access
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.

Restricted access
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.

Restricted access
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