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Daniel P. Dauhajre, M. Jeroen Molemaker, James C. McWilliams, and Delphine Hypolite

Abstract

Idealized simulations of a shoaling internal tide on a gently sloping, linear shelf provide a tool to investigate systematically the effects of stratification strength, vertical structure, and internal wave amplitude on internal tidal bores. Simulations that prescribe a range of uniform or variable stratifications and wave amplitudes demonstrate a variety of internal tidal bores characterized by shoreward-propagating horizontal density fronts with associated overturning circulations. Qualitatively, we observe three classes of solution: 1) bores, 2) bores with trailing wave trains, and 3) no bores. Very strong stratification (small wave) or very weak stratification (large wave) inhibits bore formation. Bores exist in an intermediate zone of stratification strength and wave amplitude. Within this intermediate zone, wave trains can trail bores if the stratification is relatively weak or wave amplitude large. We observe three types of bore that arise dependent on the vertical structure of stratification and wave amplitude: 1) a “backward” downwelling front (near uniform stratification, small to intermediate waves), 2) a “forward” upwelling front (strong pycnocline, small to large waves), and 3) a “double” bore with leading up and trailing downwelling front (intermediate pycnocline, intermediate to large waves). Visualization of local flow structures explores the evolution of each of these bore types. A frontogenetic diagnostic framework elucidates the previously undiscussed yet universal role of vertical straining of a stratified fluid that initiates formation of bores. Bores with wave trains exhibit strong nonhydrostatic dynamics. The results of this study suggest that mid-to-outer shelf measurements of stratification and cross-shore flow can serve as proxies to indicate the class of bore farther inshore.

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Scott D. Bachman

Abstract

The identification of vortices in a fluid flow is a dynamically interesting problem that has practical applications in oceanography due to the outsized role eddies play in water mass, heat, and tracer transport. Here a new Eulerian scheme is developed to detect both vortices and strongly strained fronts, which are both ubiquitous in the World Ocean. The new scheme is conceptually linked to the well-known Okubo–Weiss parameter, but is extended to quasigeostrophic flows by recognizing the strong role played by vertical shear in ocean dynamics. Adapted from the λ 2 criterion for vortex identification, the scheme considers the curvature of the pressure field as the differentiator between vortical and strained flow structures, and it is shown that its underlying geometry also exhibits characteristics of quasigeostrophic flow. The uses and skill of the scheme are demonstrated using a high-resolution regional ocean simulation, and prospects for its use with observational products are discussed.

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Alexis K. Kaminski, Eric A. D’Asaro, Andrey Y. Shcherbina, and Ramsey R. Harcourt

Abstract

A crucial region of the ocean surface boundary layer (OSBL) is the strongly sheared and strongly stratified transition layer (TL) separating the mixed layer from the upper pycnocline, where a diverse range of waves and instabilities are possible. Previous work suggests that these different waves and instabilities will lead to different OSBL behaviors. Therefore, understanding which physical processes occur is key for modeling the TL. Here we present observations of the TL from a Lagrangian float deployed for 73 days near Ocean Weather Station Papa (50°N, 145°W) during fall 2018. The float followed the vertical motion of the TL, continuously measuring profiles across it using an ADCP, temperature chain, and salinity sensors. The temperature chain made depth–time images of TL structures with a resolution of 6 cm and 3 s. These showed the frequent occurrence of very sharp interfaces, dominated by temperature jumps of O(1)°C over 6 cm or less. Temperature inversions were typically small (10 cm), frequent, and strongly stratified; very few large overturns were observed. The corresponding velocity profiles varied over larger length scales than the temperature profiles. These structures are consistent with scouring behavior rather than Kelvin–Helmholtz–type overturning. Their net effect, estimated via a Thorpe-scale analysis, suggests that these frequent small temperature inversions can account for the observed mixed layer deepening and entrainment flux. Corresponding estimates of dissipation, diffusivity, and heat fluxes also agree with previous TL studies, suggesting that the TL dynamics is dominated by these nearly continuous 10-cm-scale mixing structures, rather than by less frequent larger overturns.

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Sultan Hameed, Christopher L. P. Wolfe, and Lequan Chi

Abstract

Previous work by Meinen and coworkers to find an association between variations of annually averaged Florida Current transport (FCT) and the North Atlantic Oscillation (NAO) has yielded negative results. Here we show that the Florida Current in winter is impacted by displacements in the positions of the Azores high and the Icelandic low, the constituent pressure centers of the NAO. As a one-dimensional representation of North Atlantic atmospheric circulation, the NAO index does not distinguish displacements of the pressure centers from fluctuations in their intensity. FCT is significantly correlated with Icelandic low longitude with a lag of less than one season. We carried out perturbation experiments in the ECCOv4 model to investigate these correlations. These experiments reveal that east–west shifts of the Icelandic low perturb the wind stress in midlatitudes adjacent to the American coast, driving downwelling (through longshore winds) and offshore sea level anomalies (through wind stress curl) that travel to the Straits of Florida within the same season. FCT is also correlated with the latitude variations of both the Icelandic low and the Azores high with a lag of 4 years. Regression analysis shows that latitude variations of the Icelandic low and the Azores high are associated with positive wind stress curl anomalies over extended regions in the ocean east of Florida. Rossby wave propagation from this region to the Straits of Florida has been suggested as a mechanism for perturbing FCT in several previous studies by various researchers, as detailed in sections 4b and 5.

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Shuo Li, Alexander V. Babanin, Fangli Qiao, Dejun Dai, Shumin Jiang, and Changlong Guan

Abstract

The CO2 gas transfer velocity (KCO2) at air–sea interface is usually parameterized with the wind speed, but to a great extent it is defined by waves and wave breaking. To investigate the direct relationship between KCO2 and waves, laboratory experiments are conducted in a wind-wave flume. Three types of waves are forced in the flume: modulational wave trains generated by a wave maker, wind waves with 10-m wind speed ranging from 4.5 to 15.5 m s−1, and (mechanically generated) modulational wave trains coupled with superimposed wind force. The wave height and wave orbital velocity are found to be well correlated with KCO2 whereas wind speed alone cannot adequately describe KCO2. To reconcile the measurements, nondimensional equations are established in which gas transfer velocity is expressed as a main function of wave parameters and an additional secondary factor to account for influence of the wind.

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Sijia Zou, Amy S. Bower, Heather Furey, Robert S. Pickart, Loïc Houpert, and N. Penny Holliday

Abstract

Recent mooring measurements from the Overturning in the Subpolar North Atlantic Program have revealed abundant cyclonic eddies at both sides of Cape Farewell, the southern tip of Greenland. In this study, we present further observational evidence, from both Eulerian and Lagrangian perspectives, of deep cyclonic eddies with intense rotation (ζ/f > 1) around southern Greenland and into the Labrador Sea. Most of the observed cyclones exhibit strongest rotation below the surface at 700–1000 dbar, where maximum azimuthal velocities are ~30 cm s−1 at radii of ~10 km, with rotational periods of 2–3 days. The cyclonic rotation can extend to the deep overflow water layer (below 1800 dbar), albeit with weaker azimuthal velocities (~10 cm s−1) and longer rotational periods of about one week. Within the middepth rotation cores, the cyclones are in near solid-body rotation and have the potential to trap and transport water. The first high-resolution hydrographic transect across such a cyclone indicates that it is characterized by a local (both vertically and horizontally) potential vorticity maximum in its middepth core and cold, fresh anomalies in the deep overflow water layer, suggesting its source as the Denmark Strait outflow. Additionally, the propagation and evolution of the cyclonic eddies are illustrated with deep Lagrangian floats, including their detachments from the boundary currents to the basin interior. Taken together, the combined Eulerian and Lagrangian observations have provided new insights on the boundary current variability and boundary–interior exchange over a geographically large scale near southern Greenland, calling for further investigations on the (sub)mesoscale dynamics in the region.

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Mohammad Hadi Bordbar, Volker Mohrholz, and Martin Schmidt

Abstract

Spatial and temporal variations of nutrient-rich upwelled water across the major eastern boundary upwelling systems are primarily controlled by the surface wind with different, and sometimes contrasting, impacts on coastal upwelling systems driven by alongshore wind and offshore upwelling systems driven by the local wind stress curl. Here, concurrently measured wind fields and satellite-derived chlorophyll-a concentration, along with a state-of-the-art ocean model simulation, spanning 2008–18 are used to investigate the connection between coastal and offshore physical drivers of the Benguela upwelling system (BUS). Our results indicate that the spatial structure of long-term mean upwelling derived from Ekman theory and the numerical model is fairly consistent across the entire BUS and is closely followed by the chlorophyll-a pattern. The variability of the upwelling from the Ekman theory is proportionally diminished with offshore distance, whereas different and sometimes opposite structures are revealed in the model-derived upwelling. Our result suggests the presence of submesoscale activity (i.e., filaments and eddies) across the entire BUS with a large modulating effect on the wind-stress-curl-driven upwelling off Lüderitz and Walvis Bay. In Kunene and Cape Frio upwelling cells, located in the northern sector of the BUS, the coastal upwelling and open-ocean upwelling frequently alternate each other, whereas they are modulated by the annual cycle and are mostly in phase off Walvis Bay. Such a phase relationship appears to be strongly seasonally dependent off Lüderitz and across the southern BUS. Thus, our findings suggest that this relationship is far more complex than is currently thought and seems to be sensitive to climate changes, with short- and far-reaching consequences for this vulnerable marine ecosystem.

Open access
Sydney Sroka and Kerry Emanuel

Abstract

The intensity of tropical cyclones is sensitive to the air–sea fluxes of enthalpy and momentum. Sea spray plays a critical role in mediating enthalpy and momentum fluxes over the ocean’s surface at high wind speeds, and parameterizing the influence of sea spray is a crucial component of any air–sea interaction scheme used for the high wind regime where sea spray is ubiquitous. Many studies have proposed parameterizations of air–sea flux that incorporate the microphysics of sea spray evaporation and the mechanics of sea spray stress. Unfortunately, there is not yet a consensus on which parameterization best represents air–sea exchange in tropical cyclones, and the different proposed parameterizations can yield substantially different tropical cyclone intensities. This paper seeks to review the developments in parameterizations of the sea spray–mediated enthalpy and momentum fluxes for the high wind speed regime and to synthesize key findings that are common across many investigations.

Open access
B. Praveen Kumar, Eric D’Asaro, N. Sureshkumar, E. Pattabhi Rama Rao, and M. Ravichandran

Abstract

We use profiles from a Lagrangian float in the north Indian Ocean to explore the usefulness of Thorpe analysis methods to measure vertical scales and dissipation rates in the ocean surface boundary layer. An rms Thorpe length scale L T and an energy dissipation rate ε T were computed by resorting the measured density profiles. These are compared to the mixed layer depth (MLD) computed with different density thresholds, the Monin–Obukhov (MO) length L MO computed from the ERA5 reanalysis values of wind stress, and buoyancy flux B 0 and dissipation rates ε from historical microstructure data. The Thorpe length scale L T is found to accurately match MLD for small (<0.005 kg m−3) density thresholds, but not for larger thresholds, because these do not detect the warm diurnal layers. We use ξ = L T/|L MO| to classify the boundary layer turbulence during nighttime convection. In our data, 90% of points from the Bay of Bengal (Arabian Sea) satisfy ξ < 1 (1 < ξ <10), indicating that wind forcing is (both wind forcing and convection are) driving the turbulence. Over the measured range of ξ, ε T decreases with decreasing ξ, i.e., more wind forcing, while ε increases, clearly showing that ε/ε T decreases with increasing ξ. This is explained by a new scaling for ξ ≪ 1, ε T = 1.15B 0 ξ 0.5 compared to the historical scaling ε = 0.64B 0 + 1.76ξ −1. For ξ ≪ 1 we expect ε = ε T. Similar calculations may be possible using routine Argo float and ship data, allowing more detailed global measurements of ε T, thereby providing large-scale tests of turbulence scaling in boundary layers.

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Henry Potter and Johna E. Rudzin

Abstract

Strong winds in tropical cyclones (TCs) mix the ocean, causing cooler water from below the thermocline to be drawn upward, reducing sea surface temperature (SST). This decreases the air–sea temperature difference, limits available heat energy, and impacts TC intensity. Part of TC forecast accuracy therefore depends upon the ability to predict sea surface cooling; however, it is not well understood how underlying ocean conditions contribute to this cooling. Here, ~4400 Argo profiles in the Gulf of Mexico were used in a principal component analysis to identify the modes of variability in upper-ocean temperature, and a 1D mixed layer model was used to determine how the modes respond to surface forcing. It was found that the first two modes explain 75% of the variance in the data, with high mode-1 scores being broadly characterized as having warm SST and deep mixed layer and mode-2 scores being characterized as having high SST and a shallow mixed layer. Both modes have distinct seasonal and spatial variability. When subjected to the same model forcing, mode-1- and mode-2-characteristic waters with equal tropical cyclone heat potential (TCHP) respond very differently. Mode-2 SST cools faster than mode 1, with the difference being most pronounced at lower wind speeds and when comparing early-season storms with late-season storms. The results show that using TCHP as a marker for SST response during TC forcing is insufficient because it does not fully capture subsurface ocean thermal structure. This result underscores the need for continual subsurface monitoring so as to accurately initialize the upper ocean in coupled TC models.

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