Journal of Physical Oceanography

Wang, Yang; Legg, Sonya

doi: 10.1175/jpo-d-23-0045.1pmid: N/A

AbstractThe dissipation of low-mode internal tides as they propagate through mesoscale baroclinic eddies is examined using a series of numerical simulations, complemented by three-dimensional ray tracing calculations. The incident mode-1 internal tide is refracted into convergent energy beams, resulting in a zone of reduced energy flux in the lee of the eddy. The dissipation of internal tides is significantly enhanced in the upper water column within strongly baroclinic (anticyclonic) eddies, exhibiting a spatially asymmetric pattern, due to trapped high-mode internal tides. Where the eddy velocity opposes the internal tide propagation velocity, high-mode waves can be trapped within the eddy, whereas high modes can freely propagate away from regions where eddy and internal wave velocities are in the same direction. The trapped high modes with large vertical shear are then dissipated, with the asymmetric distribution of trapping leading to the asymmetric distribution of dissipation. Three-dimensional ray tracing solutions further illustrate the importance of the baroclinic current for wave trapping. Similar enhancement of dissipation is also found for a baroclinic cyclonic eddy. However, a barotropic eddy is incapable of facilitating robust high modes and thus cannot generate significant dissipation of internal tides, despite its strong velocities. Both energy transfer from low to high modes in the baroclinic eddy structure and trapping of those high modes by the eddy velocity field are therefore necessary to produce internal wave dissipation, a conclusion confirmed by examining the sensitivity of the internal tide dissipation to eddy radius, vorticity, and vertical scale.Significance StatementThe oceanic tides drive underwater waves at the tidal frequency known as internal tides. When these waves break, or dissipate, they can lead to mixing of oceanic heat and salt which impacts the ocean circulation and climate. Accurate climate predictions require computer models that correctly represent the distribution of this mixing. Here we explore how an oceanic eddy, a swirling vortex of order 100–400 km across, can locally enhance the dissipation of oceanic internal tides. We find that strong ocean eddies can be hotspots for internal tide dissipation, for both clockwise and anticlockwise rotating vortices, and surface-enhanced eddies are most effective at internal tide dissipation. These results can improve climate model representations of tidally driven mixing, leading to more credible future predictions.

Choi, Jang-Geun; Pringle, James; Lippmann, Thomas

doi: 10.1175/jpo-d-22-0191.1pmid: N/A

AbstractA perturbative solution of simplified primitive equations for nonlinear weakly stratified upwelling over a frictional slope is found that resolves the vertical structure of velocity fields and can satisfy Ertel’s potential vorticity conservation in the stratified inviscid interior. The solution uses assumptions consistent with the model proposed by Lentz and Chapman, including a steady-state, constant cross-shore density gradient, no alongshore gradients, laterally inviscid, and consideration of cross-shore advection of alongshore momentum. The solution resolves the vertical structure of velocity fields (including subsurface maxima of compensational flow, not resolved by Lentz and Chapman) and can satisfy Ertel’s potential vorticity conservation in the stratified inviscid interior. The dynamics are similar to Lentz and Chapman; bottom stress balances alongshore wind stress in a homogeneous density ocean and is replaced by nonlinear cross-shore transport of alongshore momentum as the Burger number (S = αN/f, where α, N, and f are the bottom slope, buoyancy frequency, Coriolis frequency, respectively) increases. When the solution uses the empirical relation between cross-shore and vertical density gradients proposed by Lentz and Chapman, vorticity conservation is not satisfied and the nonlinear momentum transport estimated by the solution linearly increases with S, asymptotically matching Lentz and Chapman for S < 1. When the solution conserves interior potential vorticity, the momentum transport is proportional to S2 for S < 1 and is in better agreement with numerical simulations.

Troitskaya, Yuliya; Kandaurov, Alexander; Zotova, Anna; Korsukova, Evgenia V.; Sergeev, Daniil

doi: 10.1175/jpo-d-23-0037.1pmid: N/A

AbstractRecent studies indicate that the dominant mechanism for generating sprays in hurricane winds is a “bag breakup” fragmentation. This fragmentation process is typically characterized by inflation and consequent bursting of short-lived objects, referred to as “bags” (sail-like pieces of water film surrounded by a rim). Both the number of spray droplets and their size distribution substantially affect the air–sea heat and momentum exchange. Due to a lack of experimental data, the early spray generation function (SGF) for the bag breakup mechanism was based on the assumed similarity with resembling processes. Here we present experimental results for the case with a single isolated bag breakup fragmentation event. These experiments revealed several differences from similar fragmentation events that control the droplet sizes, such as secondary disintegration of droplets in gaseous flows and bursting of bubbles. In contrast to the bubble bursting, the film thickness of the bag canopy is not constant but is random with lognormal distribution. Additionally, its average value does not depend on the canopy radius but is determined by the wind speed. The lognormal size distribution of the canopy droplets is observed in conjunction with the established mechanism of liquid film fragmentation. The rim fragmentation results in two types of droplets, and their size distribution has been found to be lognormal distribution. The constructed SGF is verified by comparing it with experimental data from the literature. The perspectives of transferring the results from laboratory to field environment have also been discussed.Significance StatementThe “bag breakup” fragmentation is the dominant mechanism for generating spray in hurricane winds. The number and the sizes of the spray droplets substantially affect the heat transport from the ocean to the atmosphere and, thereby, the development of hurricanes. This paper presents experimental data and analysis that demonstrate how droplet formation occurs during bag breakup fragmentation. It also shows analysis of the quantity and size of droplets formed during a single fragmentation event. This work demonstrates how obtained experimental results can be applied to real field conditions in the context of hurricane prediction models.

Yang, Xiaoting; Cessi, Paola

doi: 10.1175/jpo-d-23-0010.1pmid: N/A

AbstractMultidecadal variability on time scales between 20 and 70 years have been observed in the time series of North Atlantic SST. Many mechanisms have been proposed to explain multidecadal variabilities in the Atlantic. Generally, it is the interaction between the meridional overturning circulation (MOC) and North Atlantic surface buoyancy distribution that sustains this variability, with buoyancy anomalies either due to ocean-only processes or to air–sea interactions. In this context, the role of the Arctic Ocean, especially its freshwater flux into the North Atlantic, has been underappreciated. Bering Strait, the only oceanic pathway between the Pacific Ocean and the Arctic Ocean, has been found important in Arctic Ocean freshwater budget and in modulating the time-averaged state and long-term response of the MOC to high-latitude buoyancy forcing anomalies, via freshwater transport between the Pacific and Atlantic Oceans. In this paper, we use idealized configurations that include a Pacific-like wide basin and an Atlantic-like narrow basin. The two basins are connected both in the south and north to longitudinally periodic channels, representing the Southern Ocean and the Arctic Ocean, respectively. The Pacific-like basin is opened to the north only through a shallow and narrow strait, while the Atlantic-like basin is fully open to the north. With the goal of studying the role of Bering Strait in the multidecadal variability, we find that the freshwater transport from the Bering Strait forms a tongue structure along the western boundary of the narrow basin, which enhances the local horizontal density gradient. The western boundary region becomes unstable to large-scale baroclinic anomalies, giving rise to multidecadal variability.

Bhagtani, Dhruv; Hogg, Andrew McC.; Holmes, Ryan M.; Constantinou, Navid C.

doi: 10.1175/jpo-d-23-0016.1pmid: N/A

AbstractGyres are central features of large-scale ocean circulation and are involved in transporting tracers such as heat, nutrients, and carbon dioxide within and across ocean basins. Traditionally, the gyre circulation is thought to be driven by surface winds and quantified via Sverdrup balance, but it has been proposed that surface buoyancy fluxes may also contribute to gyre forcing. Through a series of eddy-permitting global ocean model simulations with perturbed surface forcing, the relative contribution of wind stress and surface heat flux forcing to the large-scale ocean circulation is investigated, focusing on the subtropical gyres. In addition to gyre strength being linearly proportional to wind stress, it is shown that the gyre circulation is strongly impacted by variations in the surface heat flux (specifically, its meridional gradient) through a rearrangement of the ocean’s buoyancy structure. On shorter time scales (∼10 years), the gyre circulation anomalies are proportional to the magnitude of the surface heat flux gradient perturbation, with up to ∼0.15 Sv (1 Sv ≡ 106 m3 s−1) anomaly induced per watt per square meter change in the surface heat flux. On time scales longer than a decade, the gyre response to surface buoyancy flux gradient perturbations becomes nonlinear as ocean circulation anomalies feed back onto the buoyancy structure induced by the surface buoyancy fluxes. These interactions complicate the development of a buoyancy-driven theory for the gyres to complement the Sverdrup relation. The flux-forced simulations underscore the importance of surface buoyancy forcing in steering the large-scale ocean circulation.Significance StatementOcean gyres are large swirling circulation features that redistribute heat across ocean basins. It is commonly believed that surface winds are the sole driver of ocean gyres, but recent literature suggests that other mechanisms could also be influential. We perform a series of numerical simulations in which we artificially change either the winds or the heating at the ocean’s surface and investigate how each factor independently affects the ocean gyres. We find that gyres are steered by both winds and surface heating, and that the ocean circulation responds differently to heating on short and long time scales. In addition, the circulation depends on where the heating is applied at the ocean’s surface. Through these simulations, we argue that a complete theory about ocean gyres must consider heating at the ocean’s surface as a possible driver, in addition to the winds.

Senjyu, Tomoharu

doi: 10.1175/jpo-d-22-0209.1pmid: N/A

AbstractThe short-period current fluctuations (topographic wave fluctuations, TWFs) on the southern rim slope of the abyssal Sea of Japan were investigated using current meter datasets from closely spaced mooring arrays. The TWFs occurred almost continuously throughout the year with short periods in a narrow band (1.5–5 days), showing a seasonal modulation in their amplitude. The TWFs were attributable to alternate passage of cyclonic and anticyclonic eddies on the rim slope, which propagated eastward at a speed of 0.15–0.23 m s−1. In addition, the TWFs showed a bottom-intensified characteristic, along with the two-layer structure consisting of an almost barotropic lower layer and a marginally baroclinic upper layer. The lowest topographic Rossby mode, which is a normal mode of the topographic Rossby waves prescribed by the two ridges on the rim slope, was considered as a cause of the TWFs because of its eastward-propagating eddy train structure along the rim slope and the eigenperiod (3–5 days) near the TWF band. In addition, the local time-dependent Sverdrup balance was considered as a mechanism of the TWF generation, since the TWFs significantly correlated with the wind stress curl variations over the observation area with time lags. That is, the current fluctuations near the eigenperiod were selectively amplified via the resonance between the lowest topographic Rossby mode and the Ekman pumping variations induced by the TWF-band wind stress curl. We concluded that the observed TWFs were a manifestation of the wind-induced lowest topographic Rossby mode prescribed by the bottom topography.Significance StatementThe dispersion relation teaches us that short-period (<10 days) Rossby waves have a very long wavelength (>103 km). However, as atmospheric forcing with both such period and wavelength is absent, the short-period Rossby waves excited by a local forcing generally dissipate quickly in a limited area. Nevertheless, we observed short-period (1.5–5 days) current fluctuations occurring continuously throughout the year in the abyssal (>1000 m) Sea of Japan. The deep current fluctuations were attributable to the propagation of cyclonic and anticyclonic eddy trains on the zonally extended slope. This is the wind-induced lowest topographic Rossby normal mode prescribed by the bottom topography. This study suggests that short-period current fluctuations can occur everywhere if appropriate topographic and atmospheric conditions were established.

Santos-Ferreira, A. M.; Silva, J. C. B. da; St-Denis, B.; Bourgault, D.; Maas, L. R. M.

doi: 10.1175/jpo-d-22-0165.1pmid: N/A

AbstractThe equatorial cold tongue in the Pacific Ocean has been intensely studied during the last decades as it plays an important role in air–sea interactions and climate issues. Recently, Warner et al. revealed gravity currents apparently originating in tropical instability waves. Both phenomena have strong dissipation rates and were considered to play a significant role in cascading energy from the mesoscale to smaller horizontal scales, as well as to vertical scales less than 1 m. Here, we present Sentinel-3 satellite observations of internal solitary waves (ISWs) in the Pacific cold tongue near the equator, in a zonal band stretching from 210° to 265°E, away from any steep bottom topography. Within this band these waves propagate in multiple directions. Some of the waves’ characteristics, such as the distance between wave crests, crest lengths, and time scales, are estimated from satellite observations. In total we identify 116 ISW trains during one full year (2020), with typical distances between crests of 1500 m and crest lengths of hundreds of kilometers. These ISW trains appear to be generated by buoyant gravity currents having sharp fronts detectable in thermal infrared satellite images. A 2D numerical model confirms that resonantly generated nonlinear internal waves with amplitudes of O(10) m may be continuously initiated at the fronts of advancing gravity currents.Significance StatementSatellite imagery reveals the repeated occurrence of internal solitary waves in the near-equatorial region of the east Pacific, despite the absence of topography. These waves appear to be resonantly generated over the sheared Equatorial Undercurrent by gravity currents that propagate as frontal zones of 1000-km scale tropical instability waves, providing a physical link with viscous mixing scales.

Zheng, Hua; Zhu, Xiao-Hua; Zhao, Ruixiang; Chen, Juntian; Wang, Min; Ren, Qiang; Liu, Yansong; Nan, Feng; Yu, Fei; Park, Jae-Hun

doi: 10.1175/jpo-d-22-0136.1pmid: N/A

AbstractTyphoon Mangkhut crossed the northeastern South China Sea (SCS) in September 2018 and induced energetic near-inertial waves (NIWs) that were captured by an array of 39 current- and pressure-recording inverted echo sounders and two tall moorings with acoustic Doppler current profilers and current meter sensors. The array extended from west of the Luzon Strait to the interior SCS, with the path of the typhoon cutting through the array. NIWs in the interior SCS had lower frequency than those near the Luzon Strait. After the typhoon crossed the SCS, Mangkhut-induced near-inertial currents in the upper ocean reached over 50 cm s−1. NIWs traveled southward for hundreds of kilometers, dominated by modes 2 and 3 in the upper and deep ocean. The horizontal phase speeds of mode 2 were ∼3.9 and ∼2.5 m s−1 north and south of the typhoon’s track, respectively, while those of mode 3 were ∼2.1 and ∼1.7 m s−1, respectively. Mode 5 was only identified in the north with a smaller phase speed. Owing to different vertical group velocities, the energy of mode-2 NIWs reached the deep ocean in 20 days, whereas the higher-mode NIWs required more time to transfer energy to the bottom. NIWs in the north were trapped and carried by a westward-propagating anticyclonic eddy, which enhanced the near-inertial kinetic energy at ∼300 m and lengthened the duration of energetic NIWs observed in the north.Significance StatementNear-inertial waves (NIWs), generally caused by wind (e.g., typhoons and monsoons) in the upper ocean, are one of the two types of energetic internal waves widely observed in the ocean. After their generation near the surface, energetic NIWs propagate downward and equatorward, thereby significantly contributing to turbulent mixing in the upper and deep ocean and acting as a mechanism of energy transfer from the surface to the deep ocean. The unprecedented NIW observations in the South China Sea describe the generation, propagation, and vertical normal modes of typhoon-induced NIWs in the upper and deep oceans, and contribute to knowledge regarding the dynamic responses of abyssal processes to typhoons.

Wang, Zhuoqun; Liu, Yonggang; Yin, Xunqiang; Zhang, Ming; Zhang, Jian; Qiao, Fangli

doi: 10.1175/jpo-d-23-0026.1pmid: N/A

AbstractWe investigate the mechanisms with which the sea surface temperature (SST) in the tropical Pacific responds to the perturbation of an exponential form to the background vertical mixing of the upper ocean. For a surface value of 0.005 m2 s−1 and a scale depth of 10 m (as typically used in the so-called nonbreaking wave parameterization), it is found that only ocean temperature within the equatorial eastern Pacific (EEP) is directly impacted; surface cooling and thermocline warming anomalies are produced. These signals propagate poleward as coastal Kelvin waves and then westward as equatorial Rossby waves. The surface cooling is severely damped while the thermocline warming is able to reach the western coast. This warm anomaly is brought up to the surface by equatorial upwelling more strongly around 110°W than at other places. In the coupled model, such equatorial warming induces an El Niño–like large-scale warming through Bjerknes feedback. Increasing the surface value of vertical mixing by a factor of 10 does not increase the equatorial surface warming while increasing the scale depth to 20 m does. Increasing the scale depth generates thermocline warming also in the subtropical region, which then propagates to the equatorial thermocline and enhances the warming there. Moreover, the off-equatorial cooling is enhanced, which makes the final warming anomaly narrower meridionally compared to an El Niño pattern.

Gou, Ruijian; Li, Pusheng; Wiegand, Kevin N.; Pennelly, Clark; Kieke, Dagmar; Myers, Paul G.

doi: 10.1175/jpo-d-23-0004.1pmid: N/A

AbstractEddies generated off the west Greenland coast modulate the deep convection in the Labrador Sea, while there are still open questions related to their formation mechanisms. Using 11 years (2008–18) of output from a NEMO model configured with a 1/60° nest in the Labrador Sea, we present the patterns of baroclinic and barotropic instability off the west Greenland coast. We highlight the generation of Irminger Rings at Cape Desolation and boundary current eddies at the location of the Overturning in the Subpolar North Atlantic Program (OSNAP) West section. In between these formation sites, eddy energy attenuation occurs along the West Greenland Current (WGC). Overall, baroclinic instability dominates in the upper 1000 m and is twice as strong as the barotropic instability. Seasonally, the instabilities are generally twice as strong in winter compared to summer. Interannually from 2008 to 2018, the instabilities generally show a strengthening trend, with values in 2018 two to three times as strong as those in 2008. We found that on an interannual time scale, the strengthening of WGC and the steepening of its velocity contours enhance the barotropic instability, and the intrusion of the upper Irminger Sea Intermediate Water (uISIW) on the Irminger Water enhances the baroclinic instability by increasing the horizontal density gradient. On a seasonal time scale, variability of the eddy momentum and density fluxes modulate the barotropic and baroclinic instability, respectively. From observation-based datasets, we also found that the downstream eddy kinetic energy is highly correlated with the uISIW transports, suggesting that the amount of uISIW affects the eddy formation. Using a very high-resolution numerical model, our study provides new insight into the variability and mechanisms of eddy formation along the west Greenland coast.Significance StatementVigorous eddy activity exists off the west Greenland coast. The eddies flux buoyancy to the interior Labrador Sea and thus weaken the convection, which feeds the lower limb of the Atlantic meridional overturning circulation. Given uncertainties in the eddy formation mechanisms, by using an ocean model with very high resolution that resolves those eddies, we show the factors that control the production and variability of the eddy formation off the western coast of Greenland. The eddy formation generally strengthens over the years 2008–18, which is a result of the intrusion of intermediate water on the continental slope and a stronger boundary current. The eddy formation shows a seasonal cycle—it is generally the strongest in winter and weakest in summer, which is modulated by the seasonal variability of eddy momentum and density fluxes.