Journal of Physical Oceanography

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ISSN / EISSN : 0022-3670 / 1520-0485
Published by: American Meteorological Society (10.1175)
Total articles ≅ 8,423
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Lauren Hoffman, Matthew R. Mazloff, Sarah T. Gille, Donata Giglio, Aniruddh Varadarajan
Journal of Physical Oceanography, Volume -1; https://doi.org/10.1175/jpo-d-21-0272.1

Abstract:
Atmospheric rivers (ARs) result in precipitation over land and ocean. Rainfall on the ocean can generate a buoyant layer of fresh water that impacts exchanges between the surface and the mixed layer. These “fresh lenses” are important for weather and climate because they may impact the ocean stratification at all timescales. Here we use in situ ocean data, co-located with AR events, and a one-dimensional configuration of a general circulation model, to investigate the impact of AR precipitation on surface ocean salinity in the California Current System (CCS) on seasonal and event-based time scales. We find that at coastal and onshore locations the CCS freshens through the rainy season due to AR events, and years with higher AR activity are associated with a stronger freshening signal. On shorter time scales, model simulations suggest that events characteristic of CCS ARs can produce salinity changes that are detectable by ocean instruments (≥ 0.01 psu). Here, the surface salinity change depends linearly on rain rate and inversely on wind speed. Higher wind speeds (U > 8 m s−1) induce mixing, distributing freshwater inputs to depths greater than 20 m. Lower wind speeds (U ≤ 8 m s−1) allow freshwater lenses to remain at the surface. Results suggest that local precipitation is important in setting the freshwater seasonal cycle of the CCS and that the formation of freshwater lenses should be considered for identifying impacts of atmospheric variability on the upper ocean in the CCS on weather event time scales.
Journal of Physical Oceanography, Volume -1; https://doi.org/10.1175/jpo-d-22-0017.1

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 Z0o, distinct from the atmospheric bulk-flux roughness length Z0a, and 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 U10N ≤ 20 m s−1, the empirical Z0o ranges from 10−4 m 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 U10N ≤ 10 m s−1 and 20°-35° from downwind for 10 ≤ U10N ≤ 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.
Julie Jakoboski, Robert E. Todd, W. Brechner Owens, Kristopher B. Karnauskas, Daniel L. Rudnick
Journal of Physical Oceanography, Volume -1; https://doi.org/10.1175/jpo-d-21-0124.1

Abstract:
The Galápagos Archipelago lies on the equator in the path of the eastward flowing Pacific Equatorial Undercurrent (EUC). When the EUC reaches the archipelago, it upwells and bifurcates into a north and south branch around the archipelago at a latitude determined by topography. Since the Coriolis parameter (ƒ) equals zero at the equator, strong velocity gradients associated with the EUC can result in Ertel potential vorticity (Q) having sign opposite that of planetary vorticity near the equator. Observations collected by underwater gliders deployed just west of the Galápagos Archipelago during 2013 – 2016 are used to estimate Q and to diagnose associated instabilities that may impact the Galápagos Cold Pool. Estimates of Q are qualitatively conserved along streamlines, consistent with the 2.5-layer, inertial model of the EUC in Pedlosky (1987a). Q with sign opposite of ƒ is advected south of the Galápagos Archipelago when the EUC core is located south of the bifurcation latitude. The horizontal gradient of Q suggests that the region between 2°S and 2°N above 100 m is barotropically unstable, while limited regions are baroclinically unstable. Conditions conducive to symmetric instability are observed between the EUC core and the equator and within the southern branch of the undercurrent. Using two-month and three-year averages, e-folding timescales are 2 – 11 days, suggesting that symmetric instability can persist on those timescales.
, Sarah T. Gille, Teresa K. Chereskin, Eric Firing, Jules Hummon, Cesar B. Rocha
Journal of Physical Oceanography, Volume -1; https://doi.org/10.1175/jpo-d-21-0139.1

Abstract:
Kinetic energy associated with inertia-gravity waves (IGWs) and other ageostrophic phenomena often overwhelms kinetic energy due to geostrophic motions for wavelengths on the order of tens of kilometers. Understanding the dependencies of the wavelength at which balanced (geostrophic) variability ceases to be larger than unbalanced variability is important for interpreting high-resolution altimetric data. This wavelength has been termed the transition scale. This study uses Acoustic Doppler Current Profiler (ADCP) data along with auxiliary observations and a numerical model to investigate the transition scale in the eastern tropical Pacific and the mechanisms responsible for its regional and seasonal variations. One-dimensional kinetic energy wavenumber spectra are separated into rotational and divergent components, and subsequently into vortex and wave components. The divergent motions, most-likely predominantly IGWs, account for most of the energy at wave-lengths less than 100 km. The observed regional and seasonal patterns in the transition scale are consistent with those from a high-resolution global simulation. Observations, however, show weaker seasonality, with only modest wintertime increases in vortex energy. The ADCP-inferred IGW wavenumber spectra suggest that waves with near-inertial frequency dominate the unbalanced variability, while in model output, internal tides strongly influence the wavenumber spectrum. The ADCP-derived transition scales from the eastern tropical Pacific are typically in the 100–200 km range.
Zheen Zhang, , Xueen Chen
Journal of Physical Oceanography, Volume -1; https://doi.org/10.1175/jpo-d-21-0247.1

Abstract:
The characteristics and variability of intraseasonal internal coastal Kelvin waves (CKWs) along the Bay of Bengal (BoB) waveguide are investigated in the context of global warming by employing a regional ocean model. The analyzed period covers 120 years from 1980 to 2099, which includes the historical scenario and the RCP8.5 scenario. CKW information is successfully extracted from the temperature anomalies along the pycnocline by applying a newly developed methodology. The analysis reveals that intraseasonal CKWs in the BoB are highly in accordance with the intraseasonal zonal wind stress in the western equatorial Indian Ocean; the downwelling CKW lags the equatorial intraseasonal westerly winds, and the upwelling CKW lags the equatorial intraseasonal easterly winds. The CKWs significantly affect subsurface characteristics at the eastern BoB boundary; and the weakening of CKWs near the Irrawaddy Delta tip is a general feature occurring in the subsurface. With respect to the long-term scale, the occurrence of significant CKWs is predicted to be more frequent in the future under the high emissions pathway. Remarkably, the monthly climatology of CKWs varies over time; unlike the first two 30-year analyzed periods, significant CKWs are predicted to mainly occur around August during the last two 30-year periods due to the corresponding variabilities in the equatorial wind field, suggesting that the BoB characteristics may greatly deviate from the current climatological state.
, E. Mauri, R. Gerin, R. Martellucci, P. Zuppelli, P. M. Poulain
Journal of Physical Oceanography, Volume -1; https://doi.org/10.1175/jpo-d-21-0108.1

Abstract:
The deepwater formation in the northern part of the South Adriatic Pit (Mediterranean Sea) is investigated using a unique oceanographic data set. In situ data collected by a glider along the Bari-Dubrovnik transect captured the mixing and the spreading/re-stratification phase of the water column in winter 2018. After a period of about two weeks from the beginning of the mixing phase, a homogeneous convective area of ∼ 300 m depth breaks up due to the baroclinic instability process in cyclonic cones made of geostrophically adjusted fluid. The base of these cones is located at the bottom of the mixed layer and they extend up to the theoretical critical depth Zc. These cones, with a diameter of the order of internal Rossby radius of deformation (∼ 6 km) populate the ∼ 110-km wide convective site, develop beneath it and have a short life time of weeks. Later on, the cones extend deeper and intrusion from deep layers makes their inner core denser and colder. These observed features differ from the long-lived cyclonic eddies sampled in other ocean sites and formed at the periphery of the convective area in a post-convection period. So far, to the best of our knowledge, only theoretical studies, laboratory experiments and model simulations have been able to predict and describe our observations and no other in-situ information has yet been provided.
, John R. Taylor, Peter E. D. Davis, Keith W. Nicholls, Paul R. Holland, Adrian Jenkins
Journal of Physical Oceanography, Volume -1; https://doi.org/10.1175/jpo-d-21-0166.1

Abstract:
The melt rate of Antarctic ice shelves is of key importance for rising sea levels and future climate scenarios. Recent observations beneath Larsen C Ice Shelf revealed an ocean boundary layer that was highly turbulent and raised questions on the effect of these rich flow dynamics on the ocean heat transfer and the ice shelf melt rate (Davis and Nicholls 2019). Directly motivated by the field observations, we have conducted large-eddy simulations (LES) to further examine the ocean boundary layer beneath Larsen C Ice Shelf. The LES was initialised with uniform temperature and salinity (T/S) and included a realistic tidal cycle and a small basal slope. A new parameterization based on Vreugdenhil and Taylor (2019) was applied at the top boundary to model near-wall turbulence and basal melting. The resulting vertical T/S profiles, melt rate and friction velocity matched well with the Larsen C Ice Shelf observations. The instantaneous melt rate varied strongly with the tidal cycle, with faster flow increasing the turbulence and mixing of heat towards the ice base. An Ekman layer formed beneath the ice base and, due to the strong vertical shear of the current, Ekman rolls appeared in the mixed layer and stratified region (depth ≈ 20–60m). In an additional high-resolution simulation (conducted with a smaller domain) the Ekman rolls were associated with increased turbulent kinetic energy, but a relatively small vertical heat flux. Our results will help with interpreting field observations and parameterizing the ocean-driven basal melting of ice shelves.
Qi Li, , Shoude Guan, Haiyuan Yang, Zhao Jing, Yongzheng Liu, Bingrong Sun, Lixin Wu
Journal of Physical Oceanography, Volume -1; https://doi.org/10.1175/jpo-d-21-0160.1

Abstract:
Shipboard observations of upper-ocean current, temperature/salinity, and turbulent dissipation rate were used to study near-inertial waves (NIWs) and turbulent diapycnal mixing in cold-core eddy (CE) and warm-core eddy (WE) in the Kuroshio Extension (KE) region. The two eddies shed from the KE were energetic, with the maximum velocity exceeding 1 m s−1 and relative vorticity magnitude as high as 0.6 f. The Mode Regression Method was proposed to extract NIWs from the shipboard-ADCP velocities. The NIW amplitudes were 0.15 and 0.3 m s−1 in the CE and WE, respectively, and their constant phase lines were nearly slanted along the heaving isopycnals. In the WE, the NIWs were trapped in the negative vorticity core and amplified at the eddy base (at 350–650 m), which was consistent with the “inertial chimney” effect documented in existing literature. Outstanding NIWs in the background wavefield were also observed inside the positive vorticity core of the CE, despite their lower strength and shallower residence (above 350 m) compared to the counterparts in the WE. Particularly, the near-inertial kinetic energy efficiently propagated downward and amplified below the surface layer in both eddies, leading to an elevated turbulent dissipation rate of up to 10−7 W kg−1. In addition, bidirectional energy exchanges between the NIWs and mesoscale balanced flow occurred during NIWs’ downward propagation. The present study provides observational evidence for the enhanced downward NIW propagation by mesoscale eddies, which has significant implications for parameterizing the wind-driven diapycnal mixing in the eddying ocean.
, XiaoMing Zhai, David Munday, Manoj Joshi
Journal of Physical Oceanography, Volume -1; https://doi.org/10.1175/jpo-d-22-0044.1

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 spin-down of a baroclinic anticyclonic eddy subject to absolute (no ocean surface current) and relative (including ocean surface current) wind stress forcing by employing an idealised 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 destabilisation of the eddy. Therefore, the onset of eddy instabilities and eventual eddy decay takes place on a shorter timescale in the simulation with relative wind stress.
, Ren-Chieh Lien, Eric Kunze
Journal of Physical Oceanography, Volume -1; https://doi.org/10.1175/jpo-d-21-0111.1

Abstract:
Horizontal and vertical wavenumbers (kx, kz) immediately below the Ozmidov wavenumber (N3)1/2 are spectrally distinct from both isotropic turbulence (kx, kz > 1 cpm) and internal waves as described by the Garrett-and-Munk (GM) model spectrum (kz < 0.1 cpm). Towed CTD chain, augmented with concurrent 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 of 50 m - 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.
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