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Paul A. Sanders
,
Martijn D. Dorrestijn
, and
Theo Gerkema

Abstract

The along-slope propagation of subinertial trapped internal tides is studied for the configuration of a simple step. It is revealed that they form a beam structure in the along-slope direction that is evanescent above the top of the step; these beams lack strict periodicity in the along-slope direction. As in classical internal Kelvin waves, they become less sharp away from the step, as higher modes decay more rapidly in the cross-slope direction. We discuss implications for abyssal mixing and outline the necessary ingredients for their generation.

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Carlos Conejero
,
Lionel Renault
,
Fabien Desbiolles
,
J. C. McWilliams
, and
Hervé Giordani

Abstract

Current feedback (CFB) and thermal feedback (TFB) have been shown to strongly influence both atmospheric and oceanic dynamics at the oceanic mesoscale (10–250 km). At smaller scales, oceanic submesoscale currents (SMCs; 0.1–10 km) have a major influence on the ocean’s energy budget, variability, and ecosystems. However, submesoscale air–sea interactions are not well understood because of observational and modeling limitations related to their scales. Here, we use a realistic submesoscale-permitting coupled oceanic and atmospheric model to quantify the spatiotemporal variability of TFB and CFB coupling in the northwest tropical Atlantic Ocean. While CFB still acts as a submesoscale eddy killer by inducing an energy sink from the SMCs to the atmosphere, it appears to be more efficient at the submesoscale by approximately 30% than at the mesoscale. Submesoscale CFB affects the surface stress, however, the finite time scale of SMCs for adjusting the atmospheric boundary layer results in a diminished low-level wind response, weakening partial ocean reenergization by about 70%. Unlike at the mesoscale, submesoscale CFB induces stress/wind convergence/divergence, influencing the atmospheric boundary layer through vertical motions. The linear relationship between the surface stress derivative or wind derivative fields and sea surface temperature gradients, widespread at the mesoscale, decreases by approximately 35% ± 7% or 77% ± 10%, respectively, at the submesoscale. In addition, submesoscale TFB induces turbulent heat fluxes comparable to those at the mesoscale. Seasonal variability in meso- and submesoscale CFB and TFB coupling is mostly related to background wind speed. Also, disentangling submesoscale CFB and TFB is challenging because they can reinforce or counteract each other.

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Yifan Wang
,
Shoude Guan
,
Zhiwei Zhang
,
Chun Zhou
,
Xin Xu
,
Chuncheng Guo
,
Wei Zhao
, and
Jiwei Tian

Abstract

Based on yearlong observations from three moorings at 12°, 14°, and 16°N in the northwest Pacific, this study presents observational evidence for the occurrence and behavior of parametric subharmonic instability (PSI) of diurnal internal tides (ITs) both in the upper and abyssal ocean around the critical latitudes (O1 IT: 13.44°N; K1 IT: 14.52°N), which is relatively less explored in comparison with PSI of M2 ITs. At 14°N, near-inertial waves (NIWs) feature a “checkerboard” pattern with comparable upward- and downward-propagating components, while the diurnal ITs mainly feature a low-mode structure. The near-inertial kinetic energy at 14°N, correlated fairly well with the diurnal KE, is the largest among three moorings. The bicoherence analysis, and a causality analysis method newly introduced here, both show statistically significant phase locking between PSI triads at 14°N, while no significant signals emerge at 12° and 16°N. The estimated PSI energy transfer rate shows a net energy transfer from diurnal ITs to NIWs with an annual-mean value of 1.5 × 10−10 W kg−1. The highly sheared NIWs generated by PSI result in a 2–6 times larger probability of shear instability events at 14°N than 12° and 16°N. Through swinging the local effective inertial frequency close to either O1 or K1 subharmonic frequencies, the passages of anticyclonic and cyclonic eddies both result in elevated NIWs and shear instability events by enhancing PSI efficiency. Particularly, different from the general understanding that cyclonic eddies usually expel NIWs, enhanced NIWs and instability are observed within cyclonic eddies whose relative vorticity can modify PSI efficiency.

Significance Statement

Parametric subharmonic instability (PSI) effectively transfers energy from low-mode internal tides (ITs) to high-mode near-inertial waves (NIWs), triggering elevated mixing around critical latitudes. This study provides observational evidence for the occurrence of PSI of diurnal ITs in the northwest Pacific and its role in enhancing shear instability. Generally, anticyclonic eddies act to trap NIWs while cyclonic eddies tend to expel NIWs. Here we document elevated NIWs and shear instability within both anticyclonic and cyclonic eddies, which shift the local effective inertial frequency close to either O1 or K1 subharmonic frequencies, thereby enhancing PSI efficiency. Processes associated with PSI and the modulation of PSI efficiency by mesoscale eddies have significant implications for improving mixing parameterizations in ocean circulation and climate models.

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Sheng Chen
,
Wen Zheng Jiang
,
Yuhuan Xue
,
Hongyu Ma
,
Yong Qing Yu
,
Zhanli Wang
, and
Fangli Qiao

Abstract

The large scatter of the drag coefficient CD at a given wind speed and its discrepancy in coastal regions and open oceans have received increasing attention. However, the parameterization of CD is still an open question, especially in coastal regions. Therefore, this study systematically investigated the influence of surface waves on wind stress based on in situ observations of surface waves and air–sea fluxes on three coastal tower-based platforms in different regions. A formulation that is a function of only wind speed was established in the wind speed range of 1–20 m s−1, and when extended to 30 m s−1, it could predict the saturation of coastal CD at a 20 m s−1 wind speed and then the attenuation. However, this wind-based formulation does not simulate the scatter of CD in practice. By further analyzing the effect of wave states on wind stress, the parameters of wave age and directionality of wind and waves were incorporated into the wind-based formulation, and a new wave-state-based parameterization on CD was proposed, which can estimate the widely spread CD values to a large extent and the saturation of CD . The RMSE between this new parameterization and observations reduce approximately 20% and 9% relative to the COARE and wind-based formula. The applicability of this new parameterization was further demonstrated through a comparison between the newly parameterized CD and observed asymmetric CD in different quadrants of a tropical cyclone. The wave-state-based parameterization scheme requires three parameters, wind speed U 10, wave age β, and wave off-wind angle θ, and it is expected to be applied to coastal regions.

Significance Statement

Wind stress over the ocean plays an important role in numerical simulations for both the atmosphere and ocean, which requires accurate parameterization. However, parameterization of wind stress or drag coefficient CD is still an open question due to the complexity of the potential factors behind wind stress, especially for coastal regions. This manuscript provided a new wave-state-based parameterization scheme at low to high wind speeds for coastal regions, based on field observations on three coastal towers. This new parameterization can predict the saturation of CD at a wind speed of 20 m s−1 and then the attenuation, agreeing well with the previous coastal observations, and simulate the large scatter of CD to a large extent. Furthermore, it can predict the asymmetric CD in different quadrants of a tropical cyclone, consistent with the observations. This parameterization scheme requires only three parameters, wind speed, wave age, and misalignment angle between wind and wave, which can be conveniently applied to the numerical models.

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Jie Peng
,
Miaohua Mao
, and
Meng Xia

Abstract

The dynamics of typhoon-induced waves in semienclosed seas become an interesting topic with the increase of typhoon intensity. Based on the calibrated Simulating Waves Nearshore (SWAN) model, wave dynamics were investigated under distinct typhoon tracks [e.g., Matmo (2014), Rumbia (2018), and Lekima (2019)] in the Bohai Sea. Distributions of significant wave heights (SWHs) are affected by the typhoon wind fields and are directly related to the typhoon tracks. The classical JONSWAP wave spectra were adopted for the analysis of sea states (e.g., wind seas or swells) to further explain variations in wave heights. Results indicate that the dominant sea state with higher energy experiences significant spatiotemporal variability under distinct tracks. For typhoons passing through the central part of the Bohai Sea (e.g., Rumbia), high-energy waves are observed under swell-dominated and mixed sea states, which are subjected to the fetch limitation in the semienclosed sea and rapid changes in typhoon winds. The high energy waves induced by other typhoons passing along the edges of the Bohai Sea correspond to the wind-sea-dominated sea state. Spatiotemporal variability of the sea state exhibits a high correlation with its position relative to the typhoon center. Therefore, a reference frame based on the radius of the maximum wind speed was established to discuss the sea states in this semienclosed sea. Further investigations reveal that swells (wind seas) dominate the regions within the radius of the maximum wind speed (elsewhere), and the double-peaked wave spectra tend to appear in the left quadrants.

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Allan J. Clarke
and
Sean Buchanan

Abstract

Past work has shown that interannual California coastal sea level variability is mostly of equatorial origin, and decades of satellite sea surface height (SSH) and in situ dynamic height observations indicate that this interannual signal propagates westward from the California coast as nondispersive Rossby waves (RWs). These observations agree with standard linear vertical mode theory except that even when mean flow and bottom topography are considered, the fastest baroclinic vertical mode RW in each case is always much slower (1.6–2.3 cm s−1) than the observed 4.2 cm s−1. This order-1 disagreement is only resolved if the standard bottom boundary condition that the vertical velocity w′ = 0 is replaced by perturbation pressure p′ = 0. Zero p′ is an appropriate bottom boundary condition because south of San Francisco the northeastern Pacific Ocean boundary acts approximately like an impermeable vertical wall to the interannual equatorial wave signal, and therefore equatorial quasigeostrophic p′ is horizontally constant along the boundary. Thus, if equatorial p′ = 0 at the bottom, then this condition also applies off California. The large-scale equatorial ocean boundary signal is due to wind-forced eastward group velocity equatorial Kelvin waves, which at interannual and lower frequencies propagate at such a shallow angle to the horizontal that none of the baroclinic equatorial Kelvin wave signal reaches the ocean floor before striking the eastern Pacific boundary. Off California this signal can thus be approximated by a first baroclinic mode with p′ = 0 at the bottom, and hence the long RW speed there agrees with that observed (both approximately 4.2 cm s−1).

Significance Statement

The California Current System is one of the most biologically rich and best-documented coastal regions in the world. In this region coastal sea level propagates westward from the coast at about 110 km month−1, slow enough to enable us to make large-scale ocean climate forecasts of the California Current ecosystem using coastal sea level. Although the westward speed seems slow, theoretically it is about double what we would expect. Offered here is an explanation of why this speed is “too fast” by linking the California wave signal to the equator, El Niño, and the shallow equatorial ocean response to the wind.

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Luigi Cavaleri
,
Sabique Langodan
,
Paolo Pezzutto
, and
Alvise Benetazzo

Abstract

We have explored the earliest stages of wind wave generation in the open sea, from the first initial wavelets appearing on an otherwise flat surface or low, smooth undulations until the practically fully developed conditions for the very low range of wind speeds we have considered. We suggest the minimal wind speed for the appearance of the first wavelets to be close to 1.8 m s−1. The peculiar conditions associated with the development of coastal sea breezes allow us to consider the local waves as generated under time-limited conditions. The 2D spectra measured during these very early stages provide the first evidence of an active Phillips process generation in the field. After appearing in these very early stages, wavelets quickly disappear as soon as the developing wind waves take a leading role. We suggest that this process is due to the strong spatial gradients in the surface orbital velocity, which impedes the instability mechanism at the base of their formation, while at a later stage of development, these gradients decrease and wavelets reappear. On a decadal perspective, the progressive decrease of the intensity of the sea breezes in the northern Adriatic Sea, where we have carried out our measurements, is associated with the steadily milder winters, and therefore not sufficiently cold local sea temperatures in early summer.

Significance Statement

We have explored for the first time the earliest stages of wind wave generation (millimeter scale) in the open sea. This was possible with the combination of the daily sea breeze development and the availability of an oceanographic tower 15 km offshore. The minimum wind speed for wave generation was 1.8 m s−1, lower than previously assumed. The data provide strong indications on the different stages of the generation process, offering measured and visual evidence, under these very light wind conditions, of the Phillips one. The presence of wind-related ripples, essential for remote sensing measurements, turns out to be dependent on the stage of generation.

Open access
Audrey Delpech
,
Roy Barkan
,
Kaushik Srinivasan
,
James C. McWilliams
,
Brian K. Arbic
,
Oladeji Q. Siyanbola
, and
Maarten C. Buijsman

Abstract

Oceanic mixing, mostly driven by the breaking of internal waves at small scales in the ocean interior, is of major importance for ocean circulation and the ocean response to future climate scenarios. Understanding how internal waves transfer their energy to smaller scales from their generation to their dissipation is therefore an important step for improving the representation of ocean mixing in climate models. In this study, the processes leading to cross-scale energy fluxes in the internal wave field are quantified using an original decomposition approach in a realistic numerical simulation of the California Current. We quantify the relative contribution of eddy–internal wave interactions and wave–wave interactions to these fluxes and show that eddy–internal wave interactions are more efficient than wave–wave interactions in the formation of the internal wave continuum spectrum. Carrying out twin numerical simulations, where we successively activate or deactivate one of the main internal wave forcing, we also show that eddy–near-inertial internal wave interactions are more efficient in the cross-scale energy transfer than eddy–tidal internal wave interactions. This results in the dissipation being dominated by the near-inertial internal waves over tidal internal waves. A companion study focuses on the role of stimulated cascade on the energy and enstrophy fluxes.

Open access
Kunxiang Wang
,
Dongliang Yuan
, and
Kaixin Ren

Abstract

The seasonal and interannual variations of the Mindanao Current retroflection are studied using surface geostrophic currents of satellite altimeters covering January 1993 through December 2019. The results show that the Mindanao Current mainstream retroflects back to the Pacific Ocean north of the Talaud Island in boreal summer, and intrudes into the northern Maluku Sea in boreal winter. The variation of the Mindanao Current retroflection has resulted in the seasonal movement of the sea surface color fronts at the entrance of the Indonesian seas, both of which are highly correlated to the seasonal transport variations of the North Equatorial Countercurrent, lagging the latter due to the westward propagation of the seasonal Rossby waves. The MC retroflection and sea surface color fronts are found to move synchronously on interannual time scales at the Pacific entrance of the Indonesian seas, with the Niño 3.4 index lagging by about 2 months. The MC retroflection intrudes anomalously deeper than the seasonal cycle into the northern Maluku Sea in El Niño winters, while tends to take a leaping path in La Niña winters. During El Niño summers, the leaping path of the MC is changed into a penetrating path sometimes.

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Anirban Guha
and
Akanksha Gupta

Abstract

By providing mathematical estimates, this paper answers a fundamental question – “what leads to Stokes drift”? Although overwhelmingly understood for water waves, Stokes drift is a generic mechanism that stems from kinematics and occurs in any non-transverse wave in fluids. To showcase its generality, we undertake a comparative study of the pathline equation of sound (1D) and intermediate-depth water (2D) waves. Although we obtain a closed-form solution x(t) for the specific case of linear sound waves, a more generic and meaningful approach involves the application of asymptotic methods and expressing variables in terms of the Lagrangian phase θ. We show that the latter reduces the 2D pathline equation of water waves to 1D. Using asymptotic methods, we solve the respective pathline equation for sound and water waves, and for each case, we obtain a parametric representation of particle position x(θ) and elapsed time t(θ). Such a parametric description has allowed us to obtain second-order-accurate expressions for the time duration, horizontal displacement, and average horizontal velocity of a particle in the crest and trough phases. All these quantities are of higher magnitude in the crest phase in comparison to the trough, leading to a forward drift, i.e. Stokes drift. We also explore particle trajectory due to second-order Stokes waves and compare it with linear waves. While finite amplitude waves modify the estimates obtained from linear waves, the understanding acquired from linear waves is generally found to be valid.

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