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Paul A. Hwang

Abstract

Many wind wave spectrum models provide excellent wave height prediction given the input of wind speed and wave age. Their quantification of the surface roughness, on the other hand, varies considerably. The ocean surface roughness is generally represented by the mean square slope, and its direct measurement in open ocean remains a challenging task. Microwave remote sensing from space delivers ocean surface roughness information. Satellite platforms offer global coverage in a broad range of environmental conditions. This paper presents low-pass mean square slope (LPMSS) data obtained by spaceborne microwave altimeters and reflectometers operating at L, Ku, and Ka bands (about 1.6, 14, and 36 GHz). The LPMSS data represent the spectrally integrated ocean surface roughness with 11, 95, and 250 rad m−1 upper cutoff wavenumbers, and the maximum wind speeds are 80, 29, and 25 m s−1, respectively. A better understanding of the ocean surface roughness is important to the goal of improving wind wave spectrum modeling. The analysis presented in this paper shows that over two orders of magnitude of the wavenumber range (0.3–30 rad m−1), the spectral components follow a power function relating the dimensionless spectrum and the ratio between wind friction velocity and wave phase speed. The power function exponent is about 0.38, which is considerably smaller than unity as expected from the classical equilibrium spectrum function. It may suggest that wave breaking is not only an energy sink but also a source of roughness generation covering a wideband of wavelengths about 20 m and shorter.

Significance Statement

This paper presents low-pass mean square slope (LPMSS) data obtained by spaceborne microwave altimeters and reflectometers operating at L, Ku, and Ka bands (about 1.6, 14, and 36 GHz). The LPMSS data represent the spectrally integrated ocean surface roughness with 11, 95, and 250 rad m−1 upper cutoff wavenumbers, and the maximum wind speeds are 80, 29, and 25 m s−1, respectively. A better understanding of the ocean surface roughness is important to the goal of improving wind wave spectrum modeling that is critical to the investigation of air–sea interaction and ocean remote sensing. The analysis presented in this paper suggests that wave breaking is not only an energy sink but also a generation source of surface roughness covering a wide band of wavelengths about 20 m and shorter.

Restricted access
Daniel B. Whitt

Abstract

The impacts of rainy days (>24 mm) on the physics of the surface atmosphere and upper ocean are characterized in the central Pacific Ocean (140°–170°W) on the equator, where deep-cycle turbulence substantially influences the sea surface temperature and air–sea heat flux on diurnal and longer time scales. Here, rainfall is relatively weak on average (1–3 mm day−1), and enough rain to substantially alter the diurnal cycle of upper-ocean buoyancy only occurs on the order of once in 100 days, albeit more frequently to the west and during El Niño and boreal winter. Rainy days are associated with multiple systematic changes in the surface atmosphere, but the freshwater and the reduction in daily downwelling shortwave radiation (by ∼50 W m−2) are codominant and drive systematic changes in the ocean during and the day after the rainy day. These two drivers explain ensemble average reductions in the upper-ocean salinity (−0.12 psu at 1 m) and temperature (−0.16°C at 1 m) and increases in buoyancy (+0.0005 m s−2 at 1 m), which are typically confined to a shallow fresh/warm mixing layer on the order of 10 m thick in the daytime. At deeper depths, the intrinsic ocean temperature, salinity, and velocity variability make it challenging to extract an ensemble average response to rainy days in observations, but some examples from observations and large-eddy simulations suggest that rainfall can significantly reduce the vertical extent and heat flux in the deep-cycle turbulence, although the bulk energetics and buoyancy flux of the turbulence are not necessarily modified by rain.

Significance Statement

Rain significantly impacts social and ecological systems in many ways that are readily apparent in populated areas, but the impacts of rain over the ocean are not as well known. In this paper, sustained in situ observations over decades and highly resolved numerical simulations of ocean turbulence during a few rain events are used to characterize the impacts of rainy days on the surface–atmosphere and upper-ocean physics in the center of action of El Niño in the central equatorial Pacific. These results contribute to broader efforts to observe, understand, and accurately model the surface atmosphere, the upper ocean, and air–sea interaction in the central Pacific and thereby improve long-range weather and climate observations and predictions.

Open access
Thomas Wilder
,
Xiaoming Zhai
,
David Munday
, and
Manoj Joshi

Abstract

Including the ocean surface current in the calculation of wind stress is known to damp mesoscale eddies through a negative wind power input and have potential ramifications for eddy longevity. Here, we study the spindown of a baroclinic anticyclonic eddy subject to absolute (no ocean surface current) and relative (including ocean surface current) wind stress forcing by employing an idealized high-resolution numerical model. Results from this study demonstrate that relative wind stress dissipates surface mean kinetic energy (MKE) and also generates additional vertical motions throughout the whole water column via Ekman pumping. Wind stress curl–induced Ekman pumping generates additional baroclinic conversion (mean potential to mean kinetic energy) that is found to offset the damping of surface MKE by increasing deep MKE. A scaling analysis of relative wind stress–induced baroclinic conversion and relative wind stress damping confirms these numerical findings, showing that additional energy conversion counteracts relative wind stress damping. What is more, wind stress curl–induced Ekman pumping is found to modify surface potential vorticity gradients that lead to an earlier destabilization of the eddy. Therefore, the onset of eddy instabilities and eventual eddy decay takes place on a shorter time scale in the simulation with relative wind stress.

Open access
Xiangyu Li
,
Marvin Lorenz
,
Knut Klingbeil
,
Evridiki Chrysagi
,
Ulf Gräwe
,
Jiaxue Wu
, and
Hans Burchard

Abstract

The relationship between the salinity mixing, the diffusive salt transport, and the diahaline exchange flow is examined using salinity coordinates. The diahaline inflow and outflow volume transports are defined in this study as the integral of positive and negative values of the diahaline velocity. A numerical model of the Pearl River Estuary (PRE) shows that this diahaline exchange flow is analogous to the classical concept of estuarine exchange flow with inflow in the bottom layers and outflow at the surface. The inflow and outflow magnitudes increase with salinity, while the net transport equals the freshwater discharge Qr after sufficiently long temporal averaging. In summer, intensified salinity mixing mainly occurs in the surface layers and around the islands. The patchy distribution of intensified diahaline velocity suggests that the water exchange through an isohaline surface can be highly variable in space. In winter, the zones of intensification of salinity mixing occur mainly in deep channels. Apart from the impact of freshwater transport from rivers, the transient mixing is also controlled by an unsteadiness term due to estuarine storage of salt and water volume. In the PRE, the salinity mixing and exchange flow show substantial spring–neap variation, while the universal law of estuarine mixing m = 2SQr (with m being the sum of physical and numerical mixing per salinity class S) holds over longer averaging period (spring–neap cycle). The correlation between the patterns of surface mixing, the vorticity, and the salinity gradients indicates a substantial influence of islands on estuarine mixing in the PRE.

Open access
Albion Lawrence
and
Jörn Callies

Abstract

Along-track wavenumber spectral densities of sea surface height (SSH) are estimated from Jason-2 altimetry data as a function of spatial location and calendar month to understand the seasonality of meso- and submesoscale balanced dynamics across the global ocean. Regions with significant mode-1 and mode-2 baroclinic tides are rejected, restricting the analysis to the extratropics. Where balanced motion dominates, the SSH spectral density is averaged over all pass segments in a region for each calendar month and is fit to a four-parameter model consisting of a flat plateau at low wavenumbers, a transition at wavenumber k 0 to a red power law spectrum k s , and a white spectrum at high wavenumbers that models the altimeter noise. The monthly time series of the model parameters are compared to the evolution of the mixed layer. The annual mode of the spectral slope s reaches a minimum after the mixed layer deepens, and the annual mode of the bandpassed kinetic energy in the ranges [2k 0, 4k 0] and [k 0, 2k 0] peak ∼2 and ∼4 months, respectively, after the maximum of the annual mode of the mixed layer depth. This analysis is consistent with an energization of the submesoscale by a winter mixed layer instability followed by an inverse cascade of kinetic energy to the mesoscale, in agreement with prior modeling studies and in situ measurements. These results are compared to prior modeling, in situ, and satellite investigations of specific regions and are broadly consistent with them within measurement uncertainties.

Significance Statement

This paper uses satellite observations to understand the source of ocean dynamics at the 1–100-km scales at which vertical motion becomes important and which are thus relevant for biology and for the exchange of heat and carbon with the atmosphere. The observations are consistent with a seasonal variation of dynamics at these scales, predicted by a specific theory of upper-ocean turbulence and confirmed by modeling studies and regional observations. We update prior satellite-based studies by excluding regions with competing effects, by our treatment of the noise, and by our characterization of the seasonality. This work provides a template for analyzing data from the upcoming Surface Water and Ocean Topography (SWOT) satellite.

Restricted access
Anda Vladoiu
,
Ren-Chieh Lien
, and
Eric Kunze

Abstract

Horizontal and vertical wavenumbers (kx , kz ) immediately below the Ozmidov wavenumber (N 3/ε)1/2 are spectrally distinct from both isotropic turbulence (kx , kz > 1 cpm) and internal waves as described by the Garrett–Munk (GM) model spectrum (kz < 0.1 cpm). A towed CTD chain, augmented with concurrent Electromagnetic Autonomous Profiling Explorer (EM-APEX) profiling float microstructure measurements and shipboard ADCP surveys, are used to characterize 2D wavenumber (kx , kz ) spectra of isopycnal slope, vertical strain, and isopycnal salinity gradient on horizontal wavelengths from 50 m to 250 km and vertical wavelengths of 2–48 m. For kz < 0.1 cpm, 2D spectra of isopycnal slope and vertical strain resemble GM. Integrated over the other wavenumber, the isopycnal slope 1D kx spectrum exhibits a roughly +1/3 slope for kx > 3 × 10−3 cpm, and the vertical strain 1D kz spectrum a −1 slope for kz > 0.1 cpm, consistent with previous 1D measurements, numerical simulations, and anisotropic stratified turbulence theory. Isopycnal salinity gradient 1D kx spectra have a +1 slope for kx > 2 × 10−3 cpm, consistent with nonlocal stirring. Turbulent diapycnal diffusivities inferred in the (i) internal wave subrange using a vertical strain-based finescale parameterization are consistent with those inferred from finescale horizonal wavenumber spectra of (ii) isopycnal slope and (iii) isopycnal salinity gradients using Batchelor model spectra. This suggests that horizontal submesoscale and vertical finescale subranges participate in bridging the forward cascade between weakly nonlinear internal waves and isotropic turbulence, as hypothesized by anisotropic turbulence theory.

Open access
R. M. Samelson

Abstract

A simple dynamical model is proposed for the near-surface drift current in a homogeneous, equilibrium sea. The momentum balance is formulated for a mass-weighted mean in curvilinear surface-conforming coordinates. Stokes drifts computed analytically for small wave slopes by this approach for inviscid linear sinusoidal and Pollard–Gerstner waves agree with the corresponding Lagrangian means, consistent with a mean momentum balance that determines mean parcel motion. A wave-modified mixing length model is proposed, with a depth-dependent eddy viscosity that depends on an effective ocean surface roughness length z 0 o , distinct from the atmospheric bulk-flux roughness length z 0 a , and an additional wave-correction factor ϕw . Kinematic Stokes drift profiles are computed for two sets of quasi-equilibrium sea states and are interpreted as mean wind drift profiles to provide calibration references for the model. A third calibration reference, for surface drift only, is provided by the traditional 3%-of-wind rule. For 10-m neutral wind U 10 N ≤ 20 m s−1, the empirical z 0 o ranges from 10−4 to 10 m, while ϕw ranges from 1.0 to 0.1. The model profiles show a shallow log-layer structure at the smaller wind speeds and a nearly uniform near-surface shear at the larger wind speeds. Surface velocities are oriented 10°–20° from downwind for U 10 N ≤ 10 m s−1 and 20°–35° from downwind for 10 ≤ U 10 N ≤ 20 m s−1. A small correction to the drag coefficient is implied. The model predictions show a basic consistency with several sets of previously published near-surface current measurements, but the comparison is not definitive.

Open access
Shota Katsura
,
Janet Sprintall
,
J. Thomas Farrar
,
Dongxiao Zhang
, and
Meghan F. Cronin

Abstract

Formation and evolution of barrier layers (BLs) and associated temperature inversions (TIs) were investigated using a 1-yr time series of oceanic and air–sea surface observations from three moorings deployed in the eastern Pacific fresh pool. BL thickness and TI amplitude showed a seasonality with maxima in boreal summer and autumn when BLs were persistently present. Mixed layer salinity (MLS) and mixed layer temperature (MLT) budgets were constructed to investigate the formation mechanism of BLs and TIs. The MLS budget showed that BLs were initially formed in response to horizontal advection of freshwater in boreal summer and then primarily maintained by precipitation. The MLT budget revealed that penetration of shortwave radiation through the mixed layer base is the dominant contributor to TI formation through subsurface warming. Geostrophic advection is a secondary contributor to TI formation through surface cooling. When the BL exists, the cooling effect from entrainment and the warming effect from detrainment are both significantly reduced. In addition, when the BL is associated with the presence of a TI, entrainment works to warm the mixed layer. The presence of BLs makes the shallower mixed layer more sensitive to surface heat and freshwater fluxes, acting to enhance the formation of TIs that increase the subsurface warming via shortwave penetration.

Restricted access
Samuel M. Kelly
and
Sebastine Ogbuka

Abstract

Coastal trapped waves (CTWs) transport energy along coastlines and drive coastal currents and upwelling. CTW modes are nonorthogonal when frequency is treated as the eigenvalue, preventing the separation of modal energy fluxes and quantification of longshore topographic scattering. Here, CTW modes are shown to be orthogonal with respect to energy flux (but not energy) when the longshore wavenumber is the eigenvalue. The modal evolution equation is a simple harmonic oscillator forced by longshore bathymetric variability, where downstream distance is treated like time. The energy equation includes an expression for modal topographic scattering. The eigenvalue problem is carefully discretized to produce numerically orthogonal modes, allowing CTW amplitudes, energy fluxes, and generation to be precisely quantified in numerical simulations. First, a spatially uniform K1 longshore velocity is applied to a continental slope with a Gaussian bump in the coastline. Mode-1 CTW generation increases quadratically with the amplitude of the bump and is maximum when the bump’s length of coastline matches the natural wavelength of the CTW mode, as predicted by theory. Next, a realistic K1 barotropic tide is applied to the Oregon coast. The forcing generates mode-1 and mode-2 CTWs with energy fluxes of 6 and 2 MW, respectively, which are much smaller than the 80 MW of M2 internal-tide generation in this region. CTWs also produce 1-cm sea surface displacements along the coast, potentially complicating the interpretation of future satellite altimetry. Prospects and challenges for quantifying the global geography of CTWs are discussed.

Restricted access
Lisa Maillard
,
Julien Boucharel
, and
Lionel Renault

Abstract

Tropical instability waves (TIWs) are oceanic features propagating westward along the northern front of the Pacific cold tongue. Observational and modeling studies suggest that TIWs may have a large impact on the eastern tropical Pacific background state from seasonal to interannual time scales through heat advection and mixing. However, observations are coarse or limited to surface data, and modeling studies are often based on the comparison of low- versus high-resolution simulations. In this study, we perform a set of regional high-resolution ocean simulations (CROCO 1/12°) in which we strongly damp (NOTIWs-RUN) or not (TIWs-RUN) TIW propagation, by nudging meridional current velocities in the TIW region toward their monthly climatological values. This approach, while effectively removing TIW mesoscale activity, does not alter the model internal physics in particular related to the equatorial Kelvin wave dynamics. The impact of TIWs on the oceanic mean state is then assessed by comparing the two simulations. While the well-known direct effect of TIW heat advection is to weaken the meridional temperature gradient by warming up the cold tongue (0.34°C month−1), the rectified effect of TIWs onto the mean state attenuates this direct effect by cooling down the cold tongue (−0.10°C month−1). This rectified effect occurs through the TIW-induced deepening and weakening of the Equatorial Undercurrent, which subsequently modulates the mean zonal advection and counterbalances the TIWs’ direct effect. This approach allows quantifying the rectified effect of TIWs without degrading the model horizontal resolution and may lead to a better characterization of the eastern tropical Pacific mean state and to the development of TIW parameterizations in Earth system models.

Significance Statement

Tropical instability waves (TIWs), meandering features at the surface of the equatorial Pacific Ocean, have long been recognized as a key component of the climate system that can even impact marine ecosystems. Yet, they are still hardly simulated in coupled global climate models. Here, we introduce a new framework to isolate and quantify their complex influence on the tropical Pacific background climate. This approach allows revealing a so far overlooked effect of TIWs on the mean circulation and heat transport in this region that should be accounted for in the next generation of global coupled climate models through parameterization or increased resolution.

Open access