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Lianxin Zhang, Xuefeng Zhang, William Perrie, Changlong Guan, Bo Dan, Chunjian Sun, Xinrong Wu, Kexiu Liu, and Dong Li

Abstract

A coupled ocean–wave–sea spray model system is used to investigate the impacts of sea spray and sea surface roughness on the response of the upper ocean to the passage of the Super Typhoon Haitang. Sea spray–mediated heat and momentum fluxes are derived from an improved version of Fairall’s heat fluxes formulation and Andreas’s sea spray–mediated momentum flux models. For winds ranging from low to extremely high speeds, a new parameterization scheme for the sea surface roughness is developed, in which the effects of wave state and sea spray are introduced. In this formulation, the drag coefficient has minimal values over the right quadrant of the typhoon track, along which the typhoon-generated waves are longer, smoother, and older, compared to other quadrants. Using traditional interfacial air–sea turbulent (sensible, latent, and momentum) fluxes, the sea surface cooling response to Typhoon Haitang is overestimated by 1°C, which can be compensated by the effects of sea spray and ocean waves on the right side of the storm. Inclusion of sea spray–mediated turbulent fluxes and sea surface roughness, modulated by ocean waves, gives enhanced cooling along the left edges of the cooling area by 0.2°C, consistent with the upper ocean temperature observations.

Open access
Edward J. Walsh, C. W. Fairall, and Ivan PopStefanija

Abstract

The airborne NOAA Wide Swath Radar Altimeter (WSRA) is a 16-GHz digital beamforming radar altimeter that produces a topographic map of the waves as the aircraft advances. The wave topography is transformed by a two-dimensional FFT into directional wave spectra. The WSRA operates unattended on the aircraft and provides continuous real-time reporting of several data products: 1) significant wave height; 2) directional ocean wave spectra; 3) the wave height, wavelength, and direction of propagation of the primary and secondary wave fields; 4) rainfall rate; and 5) sea surface mean square slope (mss). During hurricane flights the data products are transmitted in real-time from the NOAA WP-3D aircraft through a satellite data link to a ground station and on to the National Hurricane Center (NHC) for use by the forecasters for intensity projections and incorporation in hurricane wave models. The WSRA is the only instrument that can quickly provide high-density measurements of the complex wave topography over a large area surrounding the eye of the storm.

Open access
Hossein A. Kafiabad, Jacques Vanneste, and William R. Young

Abstract

Anticyclonic vortices focus and trap near-inertial waves so that near-inertial energy levels are elevated within the vortex core. Some aspects of this process, including the nonlinear modification of the vortex by the wave, are explained by the existence of trapped near-inertial eigenmodes. These vortex eigenmodes are easily excited by an initial wave with horizontal scale much larger than that of the vortex radius. We study this process using a wave-averaged model of near-inertial dynamics and compare its theoretical predictions with numerical solutions of the three-dimensional Boussinesq equations. In the linear approximation, the model predicts the eigenmode frequencies and spatial structures, and a near-inertial wave energy signature that is characterized by an approximately time-periodic, azimuthally invariant pattern. The wave-averaged model represents the nonlinear feedback of the waves on the vortex via a wave-induced contribution to the potential vorticity that is proportional to the Laplacian of the kinetic energy density of the waves. When this is taken into account, the modal frequency is predicted to increase linearly with the energy of the initial excitation. Both linear and nonlinear predictions agree convincingly with the Boussinesq results.

Open access
J. Thomas Farrar, Theodore Durland, Steven R. Jayne, and James F. Price

Abstract

Measurements from satellite altimetry are used to show that sea surface height (SSH) variability throughout much of the North Pacific Ocean is coherent with the SSH signal of the tropical instability waves (TIWs) that result from instabilities of the equatorial currents. This variability has regular phase patterns consistent with freely propagating barotropic Rossby waves radiating energy away from the unstable equatorial currents, and the waves clearly propagate from the equatorial region to at least 30°N. The pattern of SSH variance at TIW frequencies exhibits remarkable patchiness on scales of hundreds of kilometers, which we interpret as being due to the combined effects of wave reflection, refraction, and interference. North of 40°N, more than 6000 km from the unstable equatorial currents, the SSH field remains coherent with the near-equatorial SSH variability, but it is not as clear whether the variability at the higher latitudes is a simple result of barotropic wave radiation from the tropical instability waves. Even more distant regions, as far north as the Aleutian Islands off of Alaska and the Kamchatka Peninsula of eastern Russia, have SSH variability that is significantly coherent with the near-equatorial instabilities. The variability is not well represented in the widely used gridded SSH data product commonly referred to as the AVISO or DUACS product, and this appears to be a result of spatial variations in the filtering properties of the objective mapping scheme.

Open access
Jonathan A. Baker, Andrew J. Watson, and Geoffrey K. Vallis

Abstract

The response of the meridional overturning circulation (MOC) to changes in Southern Ocean (SO) zonal wind forcing and Pacific Ocean basin vertical diffusivity is investigated under varying buoyancy forcings, corresponding to “warm,” “present day,” and “cold” states, in a two-basin general circulation model connected by a southern circumpolar channel. We find that the Atlantic MOC (AMOC) strengthens with increased SO wind stress or diffusivity in the model Pacific, under all buoyancy forcings. The sensitivity of the AMOC to wind stress increases as the buoyancy forcing is varied from a warm to a present-day or cold state, whereas it is most sensitive to the Pacific diffusivity in a present-day or warm state. Similarly, the AMOC is more sensitive to buoyancy forcing over the Southern Ocean under reduced wind stress or enhanced Pacific diffusivity. These results arise because of the increased importance of the Pacific pathway in the warmer climates, giving an increased linkage between the basins and so the opportunity for the diffusivity in the Pacific to affect the overturning in the Atlantic. In cooler states, such as in glacial climates, the two basins are largely decoupled and the wind strength over the SO is the primary determinant of the AMOC strength. Both wind- and diffusively driven upwelling sustain the AMOC in the warmer (present day) state. Changes in SO wind stress alone do not shoal the AMOC to resemble that observed at the last glacial maximum; changes in the buoyancy forcing are also needed to decouple the two basins.

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Lixin Qu, Leif N. Thomas, and Robert D. Hetland

Abstract

This study describes a specific type of critical layer for near-inertial waves (NIWs) that forms when isopycnals run parallel to sloping bathymetry. Upon entering this slantwise critical layer, the group velocity of the waves decreases to zero and the NIWs become trapped and amplified, which can enhance mixing. A realistic simulation of anticyclonic eddies on the Texas–Louisiana shelf reveals that such critical layers can form where the eddies impinge onto the sloping bottom. Velocity shear bands in the simulation indicate that wind-forced NIWs are radiated downward from the surface in the eddies, bend upward near the bottom, and enter critical layers over the continental shelf, resulting in inertially modulated enhanced mixing. Idealized simulations designed to capture this flow reproduce the wave propagation and enhanced mixing. The link between the enhanced mixing and wave trapping in the slantwise critical layer is made using ray tracing and an analysis of the waves’ energetics in the idealized simulations. An ensemble of simulations is performed spanning the relevant parameter space that demonstrates that the strength of the mixing is correlated with the degree to which NIWs are trapped in the critical layers. While the application here is for a shallow coastal setting, the mechanisms could be active in the open ocean as well where isopycnals align with bathymetry.

Open access
Peng Wang, James C. McWilliams, and Yusuke Uchiyama

Abstract

Coastal fronts impact cross-shelf exchange of materials, such as plankton and nutrients, that are important to the ecosystems in continental shelves. Here, using numerical simulation we demonstrate a nearshore front induced by wave streaming. Wave streaming is a bottom Eulerian current along the surface wave direction, and it is caused by the wave bottom dissipation. Wave streaming drives a Lagrangian overturning circulation in the inner shelf and pumps up deep and cold water into the overturning circulation. The water inside the overturning circulation is quickly mixed and cooled because of the wave-streaming-enhanced viscosity. However, the offshore water outside the overturning circulation remains stratified and warmer. Hence, a front develops between the water inside and outside the overturning circulation. The front is unstable and generates submesoscale shelf eddies, which cause the offshore transport across the front. This study presents a new mechanism for coastal frontogenesis.

Restricted access
Hui Zhou, Hengchang Liu, Shuwen Tan, Wenlong Yang, Yao Li, Xueqi Liu, Qiang Ren, and William K. Dewar

Abstract

The structure and variations of the North Equatorial Countercurrent (NECC) in the far western Pacific Ocean during 2014–16 are investigated using repeated in situ hydrographic data, altimeter data, Argo data, and reanalysis data. The NECC shifted ~1° southward and intensified significantly with its transport exceeding 40 Sv (1 Sv ≡ 106 m3 s−1), nearly double its climatology value, during the developing phase of the 2015/16 El Niño event. Observations show that the 2015/16 El Niño exerted a comparable impact on the NECC with that of the extreme 1997/98 El Niño in the far western Pacific Ocean. Baroclinic instability provided the primary energy source for the eddy kinetic energy (EKE) in the 2015/16 El Niño, which differs from the traditional understanding of the energy source of EKE as barotropic instability in low-latitude ocean. The enhanced vertical shear and the reduced density jump between the NECC layer and the North Equatorial Subsurface Current (NESC) layer renders the NECC–NESC system baroclinically unstable in the western Pacific Ocean during El Niño developing phase. The eddy–mean flow interactions here are diverse associated with various states of El Niño–Southern Oscillation (ENSO).

Open access
Xingchi Wang and Tobias Kukulka

Abstract

Turbulence driven by wind and waves controls the transport of heat, momentum, and matter in the ocean surface boundary layer (OSBL). For realistic ocean conditions, winds and waves are often neither aligned nor constant, for example, when winds turn rapidly. Using a large-eddy simulation (LES) method, which captures shear-driven turbulence (ST) and Langmuir turbulence (LT) driven by the Craik–Leibovich vortex force, we investigate the OSBL response to abruptly turning winds. We design idealized LES experiments in which winds are initially constant to equilibrate OSBL turbulence before abruptly turning 90° either cyclonically or anticyclonically. The transient Stokes drift for LT is estimated from a spectral wave model. The OSBL response includes three successive stages that follow the change in direction. During stage 1, turbulent kinetic energy (TKE) decreases as a result of reduced TKE production. Stage 2 is characterized by TKE increasing, with TKE shear production recovering and exceeding TKE dissipation. Transient TKE levels may exceed their stationary values because of inertial resonance and nonequilibrium turbulence. Turbulence relaxes to its equilibrium state at stage 3, but LT still adjusts as a result of slowly developing waves. During stages 1 and 2, greatly misaligned wind and waves lead to Eulerian shear TKE production exceeding Stokes drift shear TKE production. A Reynolds stress budget analysis and Reynolds-averaged Navier–Stokes equation models indicate that Stokes drift shear production furthermore drives the OSBL response. The Coriolis effects result in asymmetrical OSBL responses to wind turning directions. Our results suggest that transient wind conditions play a key role in understanding realistic OSBL dynamics.

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Olivier Marchal and Ning Zhao

Abstract

Radiocarbon dates of fossil carbonates sampled from sediment cores and the seafloor have been used to infer that deep ocean ventilation during the last ice age was different from today. In this first of two companion papers, the time-averaged abyssal circulation in the modern Atlantic is estimated by combining a hydrographic climatology, observational estimates of volume transports, Argo float velocities at 1000 m, radiocarbon data, and geostrophic dynamics. Different estimates of modern circulation, obtained from different prior assumptions about the abyssal flow and different errors in the geostrophic balance, are produced for use in a robust interpretation of fossil records in terms of deviations from the present-day flow, which is undertaken in Part II. We find that, for all estimates, the meridional transport integrated zonally and averaged over a hemisphere, ⟨V k⟩, is southward between 1000 and 4000 m in both hemispheres, northward between 4000 and 5000 m in the South Atlantic, and insignificant between 4000 and 5000 m in the North Atlantic. Estimates of ⟨V k⟩ obtained from two distinct prior circulations—one based on a level of no motion at 4000 m and one based on Argo float velocities at 1000 m—become statistically indistinguishable when Δ14C data are considered. The transport time scale, defined as τk=Vk/Vk, where Vk is the volume of the kth layer, is estimated to about a century between 1000 and 3000 m in both the South and North Atlantic, 124 ± 9 yr (203 ± 23 yr) between 3000 and 4000 m in the South (North) Atlantic, and 269 ± 115 yr between 4000 and 5000 m in the South Atlantic.

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