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Yunwei Yan, Lei Zhang, Xiangzhou Song, Guihua Wang, and Changlin Chen

Abstract

Diurnal variation in surface latent heat flux (LHF) and the effects of diurnal variations in LHF-related variables on the climatological LHF are examined using observations from the Global Tropical Moored Buoy Array. The estimated amplitude of the climatological diurnal LHF over the Indo-Pacific warm pool and the equatorial Pacific and Atlantic cold tongues is remarkable, with maximum values exceeding 20.0 W m−2. Diurnal variability of sea surface skin temperature (SSTskin) is the primary contributor to the diurnal LHF amplitude. Because the diurnal SSTskin amplitude has an inverse relationship with surface wind speed over the tropical oceans, an inverse spatial pattern between the diurnal LHF amplitude and surface wind speed results. Resolving diurnal variations in the SSTskin and wind improves the estimate of the climatological LHF by properly capturing the daytime SSTskin and daily mean wind speed, respectively. The diurnal SSTskin-associated contribution is large over the warm pool and equatorial cold tongues where low wind speeds tend to cause strong diurnal SSTskin warming, while the magnitude associated with the diurnal winds is large over the highly dynamic environment of the Inter-Tropical Convergence Zone. The total diurnal contribution is about 9.0 W m−2 on average over the buoy sites. There appears to be a power function (linear) relationship between the diurnal SSTskin-associated (wind-associated) contribution and surface mean wind speed (wind speed enhancement from diurnal variability). The total contribution from diurnal variability can be estimated accurately from high-frequency surface wind measurements using these relationships.

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Zhumin Lu, Guihua Wang, and Xiaodong Shang

Abstract

As a key to modulate the negative feedback to tropical cyclone (TC) intensity, the TC-induced inner-core sea surface cooling (SSCIC) is poorly understood. Using a linear two-layer theory and OGCM experiments, this study illustrates that the pattern of the inner-core mixing can be well interpreted by the wind-driven currents in the mixed layer (ML). This interpretation is based on: 1) the mixing is triggered by the ML bulk shear instability; 2) the lag of upwelling makes the inner-core bulk shear equivalent to the inner-core wind-driven currents. Overall, the patterns of the inner-core bulk shear and mixing resemble the crescent body of a sickle. As an accumulative result of mixing, the SSCIC is clearly weaker than the maximum cold wake because of the weaker mixing ahead of the inner core and nearly zero mixing in a part of the inner core. The SSCIC induced by a rectilinear-track TC is mainly dominated by the inner-core mixing. Only for a slow-moving case, upwelling and horizontal advection can make minor contributions to the SSCIC by incorporating them with mixing. The SSCIC strength is inversely proportional to the moving speed, suggesting the mixing time rather than the mixing strength dominates the SSCIC. Despite inability in treating the mixing strength, this study elucidates the fundamental dynamical mechanisms of SSCIC, especially emphasizes the different roles of mixing, upwelling and horizontal advection for fast- and slow-moving TCs, and thus provides a good start point to understand SSCIC.

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Delphine Hypolite, Leonel Romero, James C. McWilliams, and Daniel P. Dauhajre

Abstract

A set of realistic coastal simulations in California allows for the exploration of surface gravity wave effects on currents (WEC) in an active submesoscale current regime. We use a new method that takes into account the full surface gravity wave spectrum and produces larger Stokes drift than the monochromatic peak-wave approximation. We investigate two high wave events lasting several days — one from a remotely generated swell and another associated with local wind-generated waves — and perform a systematic comparison between solutions with and without WEC at two submesoscale-resolving horizontal grid resolutions (dx = 270 m and 100 m). WEC results in the enhancement of open-ocean surface density and velocity gradients when the averaged significant wave height H S is relatively large (> 4.2m). For smaller waves, WEC is a minor effect overall. For the remote swell (strong waves and weak winds), WEC maintains submesoscale structures and accentuates the cyclonic vorticity and horizontal convergence skewness of submesoscale fronts and filaments. The vertical enstrophy ζ 2 budget in cyclonic regions (ζ/f > 2) reveals enhanced vertical shear and enstrophy production via vortex tilting and stretching. Wind-forced waves also enhance surface gradients, up to the point where they generate a small-submesoscale roll-cell pattern with high vorticity and divergence that extends vertically through the entire mixed layer. The emergence of these roll-cells results in a buoyancy gradient sink near the surface that causes a modest reduction in the typically large submesoscale density gradients.

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Soeren Thomsen, Xavier Capet, and Vincent Echevin

Abstract

Coastal upwelling rates are classically determined by the intensity of the upper-ocean offshore Ekman transport. But (sub-)mesoscale turbulence modulates offshore transport, hence the net upwelling rate. Eddy effects generally oppose the Ekman circulation, resulting in so-called “eddy cancellation”, a process well studied in the Southern Ocean. Here we investigate how air-sea heat/buoyancy fluxes modulate eddy cancellation in an idealized upwelling model. We run CROCO simulations with constant winds but varying heat fluxes with and without submesoscale-rich turbulence. Eddy cancellation is consistently evaluated with three different methods that all account for the quasi-isopycnal nature of ocean circulation away from the surface. For zero heat fluxes the release of available potential energy by baroclinic instabilities is strongest and leads, near the coast, to nearly full cancellation of the Ekman cross-shore circulation by eddy effects, i.e., zero net mean upwelling flow. With increasing heat fluxes eddy cancellation is reduced and the transverse flow progressively approaches the classical Ekman circulation. Sensitivity of the eddy circulation to synoptic changes in air-sea heat fluxes is felt down to 125 m depth despite short experiments of tens of days. Mesoscale dynamics dominate the cancellation effect in our simulations which might also hold for the real ocean as the relevant processes act below the surface boundary layer. Although the idealized setting overemphasis the role of eddies and thus studies with more realistic settings should follow, our findings have important implications for the overall understanding of upwelling system dynamics.

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Antoine Villefer, Michel Benoit, Damien Violeau, Christopher Luneau, and Hubert Branger

Abstract

A series of experiments were conducted in a wind-wave tank facility in Marseilles (France) to study the effects of preexisting swell conditions (represented by long mechanically-generated waves) on wind-wave growth with fetch. Both monochromatic and irregular (JONSWAP-type) long wave conditions with different values of wave steepness have been generated in the presence of a constant wind forcing, for several wind velocities. A spectral analysis of temporal wave signals combined with airflow measurements allowed to study the evolution of both wave systems with the aim of identifying the interaction mechanisms transportable to prototype scale. In particular, a specific method is used to separate the two wave systems in the measured bimodal spectra. In fetch-limited conditions, pure wind-wave growth is in accordance with anterior experiments, but differs from the prototype scale in terms of energy and frequency variations with fetch. Monochromatic long waves are shown to reduce the energy of the wind-waves significantly, as it was observed in anterior laboratory experiments. The addition of JONSWAP-type long waves instead results in a downshift of the wind-wave peak frequency but no significant energy reduction. Overall, it is observed that the presence of long waves affects the wind-wave energy and frequency variations with fetch. Finally, in the presence of JONSWAP-type long waves, variations of wind-wave energy and peak frequency with fetch appear in close agreement with the wind-wave growth observed at prototype scale both in terms of variations and nondimensional magnitude.

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Qinbiao Ni, Xiaoming Zhai, Xuemin Jiang, and Dake Chen

Abstract

Mesoscale eddies are ubiquitous features of the global ocean circulation and play a key role in transporting ocean properties and modulating air–sea exchanges. Anticyclonic and cyclonic eddies are traditionally thought to be associated with anomalous warm and cold surface waters, respectively. Using satellite altimeter and microwave data, here we show that surface cold-core anticyclonic eddies (CAEs) and warm-core cyclonic eddies (WCEs) are surprisingly abundant in the global ocean—about 20% of the eddies inferred from altimeter data are CAEs and WCEs. Composite analysis using Argo float profiles reveals that the cold cores of CAEs and warm cores of WCEs are generally confined in the upper 50 m. Interestingly, CAEs and WCEs alter air–sea momentum and heat fluxes and modulate mixed layer depth and surface chlorophyll concentration in a way markedly different from the traditional warm-core anticyclonic and cold-core cyclonic eddies. Given their abundance, CAEs and WCEs need to be properly accounted for when assessing and parameterizing the role of ocean eddies in Earth’s climate system.

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Yoeri M. Dijkstra and Henk M. Schuttelaars

Abstract

The classification diagram developed by Hansen and Rattray is one of the classic papers on classification of estuarine salinity dynamics. However, we found several inconsistencies in both their stratification–circulation and estuarine classification diagrams. These findings considerably change the interpretation of their work. Furthermore, while their classification includes salt wedge estuaries, the model used to derive this is only applicable to well-mixed and partially mixed estuaries. Here, we identify and solve these inconsistencies, and we propose new adjusted and extended stratification–circulation and classification diagrams. To this end, we summarize the model of Hansen and Rattray and extend their work to find analytical model solutions and an adjusted stratification–circulation diagram. Using this new diagram, it is shown that Hansen and Rattray incorrectly discussed the behavior of dispersion-dominated estuaries and that several parts of the diagram correspond to physically unrealistic model solutions. This is then used to demonstrate that several estuarine classes identified by Hansen and Rattray correspond to physically unrealistic model solutions and can therefore not be interpreted. A new and extended classification is proposed by using a recently developed model that extends the work of Hansen and Rattray to salt wedge estuaries. This results in an extended estuarine classification including examples of the location of 12 estuaries in this new diagram.

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Zhiling Liao, Shaowu Li, Ye Liu, and Qingping Zou

Abstract

The theoretical model for group-forced infragravity (IG) waves in shallow water is not well established for nonbreaking conditions. In the present study, analytical solutions of the group-forced IG waves at O(β 1) (β 1 = h x/(Δkh), h x = bottom slope, Δk = group wavenumber, h = depth) in intermediate water and at O(β11) in shallow water are derived separately. In case of off-resonance [β 1 μ −1 = O(β 1), where μ=1cg2/(gh) is the resonant departure parameter, c g = group speed] in intermediate water, additional IG waves in quadrature with the wave group forcing (hereinafter, the nonequilibrium response or component) are induced at O(β 1) relative to the equilibrium bound IG wave solution of in phase with the wave group. The present theory indicates that the nonequilibrium response is mainly attributed to the spatial variation of the equilibrium bound IG wave amplitude instead of group-forcing. In case of near-resonance [β 1 μ −1 = O(1)] in shallow water; however, both the equilibrium and nonequilibrium components are ~O(β11) at the leading order. Based on the nearly-resonant solution, the shallow water limit of the local shoaling rate of bound IG waves over a plane sloping beach is derived to be ~h −1 for the first time. The theoretical predictions compare favorably with the laboratory experiment by and the present numerical model results generated using SWASH. Based on the proposed solution, the group-forced IG waves over a symmetric shoal are investigated. In case of off-resonance, the solution predicts a roughly symmetric reversible spatial evolution of the IG wave amplitude, while in cases of near to full resonance the IG wave is significantly amplified over the shoal with asymmetric irreversible spatial evolution.

Open access
P. Vélez-Belchí, V. Caínzos, E. Romero, M. Casanova-Masjoan, C. Arumí-Planas, D. Santana-Toscano, A. González-Santana, M. D. Pérez-Hernández, and A. Hernández-Guerra

Abstract

Poleward undercurrents are well-known features in eastern boundary upwelling systems. In the California Current upwelling system, the California poleward undercurrent has been widely studied, and it has been demonstrated that it transports nutrients from the equatorial waters to the northern limit of the subtropical gyre. However, in the Canary Current upwelling system, the Canary intermediate poleward undercurrent (CiPU) has not been properly characterized, despite recent studies arguing that the dynamics of the eastern Atlantic Ocean play an important role in the Atlantic meridional overturning circulation, specifically on its seasonal cycle. Here, we use trajectories of Argo floats and model simulations to characterize the CiPU, including its seasonal variability and its driving mechanism. The Argo observations show that the CiPU flows from 26°N, near Cape Bojador, to approximately 45°N, near Cape Finisterre and flows deeper than any poleward undercurrent in other eastern boundaries, with a core at a mean depth of around 1000 dbar. Model simulations manifest that the CiPU is driven by the meridional alongshore pressure gradient due to general ocean circulation and, contrary to what is observed in the other eastern boundaries, is still present at 1000 dbar as a result of the pressure gradient between the Antarctic Intermediate Waters in the south and Mediterranean Outflow waters in the north. The high seasonal variability of the CiPU, with its maximum strength in autumn and minimum in spring, is due to the poleward extension of AAIW, forced by Ekman pumping in the tropics.

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Ewa Jarosz, Hemantha W. Wijesekera, and David W. Wang

Abstract

Velocity, hydrographic, and microstructure observations collected under moderate to high winds, large surface waves, and significant ocean-surface heat losses were utilized to examine coherent velocity structures (CVS) and turbulent kinetic energy (TKE) budget in the mixed layer on the outer shelf in the northern Gulf of Mexico in February 2017. The CVS exhibited larger downward velocities in downwelling regions and weaker upward velocities in broader upwelling regions, elevated vertical velocity variance, vertical velocity maxima in the upper part of the mixed layer, and phasing of crosswind velocities relative to vertical velocities near the base of the mixed layer. Temporal scales ranged from 10 to 40 min, and estimated lateral scales ranged from 90 to 430 m, which were 1.5–6 times as large as the mixed layer depth. Nondimensional parameters, Langmuir and Hoenikker numbers, indicated that plausible forcing mechanisms were surface-wave-driven Langmuir vortex and destabilizing surface buoyancy flux. The rate of change of TKE, shear production, Stokes production, buoyancy production, vertical transport of TKE, and dissipation in the TKE budget were evaluated. The shear and Stokes productions, dissipation, and vertical transport of TKE were the dominant terms. The buoyancy production term was important at the sea surface, but it decreased rapidly in the interior. A large imbalance term was found under the unstable, high-wind, and high–sea state conditions. The cause of this imbalance cannot be determined with certainty through analyses of the available observations; however, our speculation is that the pressure transport is significant under these conditions.

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