Abstract
Microstructure measurements in Drake Passage and on the flanks of Kerguelen Plateau find turbulent dissipation rates
ε
on average factors of 2–3 smaller than linear lee-wave generation ...predictions, as well as a factor of 3 smaller than the predictions of a well-established parameterization based on finescale shear and strain. Here, the possibility that these discrepancies are a result of conservation of wave action
E
/
ω
L
=
E
/|
kU
| is explored. Conservation of wave action will transfer a fraction of the lee-wave radiation back to the mean flow if the waves encounter weakening currents
U
, where the intrinsic or Lagrangian frequency
ω
L
= |
kU
| ↓ |
f
| and
k
the along-stream horizontal wavenumber, where
kU
≡
k
⋅
V
. The dissipative fraction of power that is lost to turbulence depends on the Doppler shift of the intrinsic frequency between generation and breaking, hence on the topographic height spectrum and bandwidth
N
/
f
. The partition between dissipation and loss to the mean flow is quantified for typical topographic height spectral shapes and
N
/
f
ratios found in the abyssal ocean under the assumption that blocking is local in wavenumber. Although some fraction of lee-wave generation is always dissipated in a rotating fluid, lee waves are not as large a sink for balanced energy or as large a source for turbulence as previously suggested. The dissipative fraction is 0.44–0.56 for topographic spectral slopes and buoyancy frequencies typical of the deep Southern Ocean, insensitive to flow speed
U
and topographic splitting. Lee waves are also an important mechanism for redistributing balanced energy within their generating bottom current.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Deep cycle turbulence (DCT) is a diurnally oscillating turbulence that penetrates into a stratified shear layer below the surface mixed layer, which is often observed in the eastern Pacific and ...Atlantic above the Equatorial Undercurrent (EUC). Here we present the simulation of DCT by a global ocean general circulation model (OGCM) for the first time. As the k‐ε vertical mixing scheme is used in the OGCM, the simulation of observed DCT structure based on in situ microstructure measurements can be explicitly demonstrated. The simulated DCT is found in all equatorial ocean basins, and its characteristics agree very well with observations. Zonal and meridional variations of DCT in the entire equatorial Pacific and Atlantic are described through constructing the composite diurnal cycle. In the central Pacific where the maximum shear associated with EUC is deep, the separation of DCT from the surface mixed layer is much more prominent than other areas.
Plain Language Summary
Deep cycle turbulence (DCT) is a nighttime intensified turbulence that develops in the stratified layer below the base of the surface mixed layer. It is often observed below the equatorial Pacific and Atlantic cold tongue regions above the Equatorial Undercurrent (EUC). Mixing caused by DCT is essential in modulating sea surface temperature (SST), which could have a large impact on air‐sea interaction and thus global climate variability. However, simulations of DCT in global ocean models have not been demonstrated so far, and the spatial variation of DCT characteristics in the entire equatorial oceans is not well known. This study presents the first global ocean general circulation model simulation of DCT, demonstrated by the comparison of simulated turbulence with that derived from in situ observations. The simulated DCT is found in all equatorial ocean basins, and its characteristics agree very well with observations. Large‐scale spatial variability of DCT in the equatorial Pacific and Atlantic is described through the analysis of model output. The DCT layer completely separated from surface mixed layer is found at locations where the EUC is deep, such as the central equatorial Pacific near the dateline.
Key Points
Deep cycle turbulence is well simulated for the first time by a global ocean general circulation model in all equatorial ocean basins
Large‐scale zonal and meridional variations of the deep cycle turbulence in equatorial Pacific and Atlantic are fully described
Separation of deep cycle turbulence from the surface mixed layer is most prominent in the central Pacific near the dateline
Atmospheric cold pools are frequently observed during the Madden‐Julian Oscillation events and play an important role in the development and organization of large‐scale convection. They are generally ...associated with heavy precipitation and strong winds, inducing large air‐sea fluxes and significant sea surface temperature (SST) fluctuations. This study provides a first detailed investigation of the upper ocean response to the strong cold pools associated with the Madden‐Julian Oscillation, based on the analysis of in situ data collected during the Dynamics of the Madden‐Julian Oscillation (DYNAMO) field campaign and one‐dimensional ocean model simulations validated by the data. During strong cold pools, SST drops rapidly due to the atmospheric cooling in a shoaled mixed layer caused by the enhanced near‐surface salinity stratification generated by heavy precipitation. Significant contribution also comes from the component of surface heat flux produced by the cold rain temperature. After the period of heavy rain, while net surface cooling remains, SST gradually recovers due to the enhanced entrainment of warmer waters below the mixed layer.
Plain Language Summary
The Madden‐Julian Oscillation (MJO) is an eastward traveling intraseasonal (30–60 days) disturbance of clouds, rainfall, and winds in the tropical atmosphere, which has a strong impact on the tropical and extratropical climate. Atmospheric cold pools are pools of air cooled by rain evaporation, moving downward and spreading out as gust front upon reaching the surface. During MJO events, cold pools are often observed and accompanied with heavy rain and strong winds, changing sea surface temperature (SST) substantially. As such changes could have large impacts on air‐sea interaction, understanding oceanic processes that influence SST may lead to improving MJO prediction. This study provides a first detailed investigation of upper ocean response to strong atmospheric cold pools associated with the MJO. The analysis of the data collected during the international field campaign indicates a rapid SST drop and subsequent gradual recovery (warming) associated with strong cold pool events. We demonstrated key oceanic processes controlling SST fluctuations observed during the field campaign based on a series of numerical ocean model simulations. The important oceanic processes include strong salinity stratification created by heavy rain, the mixing of near‐surface seawater with colder rainwater, and the mixing of cold surface water with warmer waters below.
Key Points
A first detailed investigation of upper ocean response to strong atmospheric cold pool events associated with the MJO is presented
A rapid cooling of SST is produced by the intensified salinity stratification generated by heavy rain and by the enhanced sensible heat flux due to cold rain temperature
A subsequent gradual SST recovery (warming) occurs due to the mixing of warmer waters below the mixed layer produced by strong winds
Abstract
Five large-amplitude internal solitary waves (ISWs) propagating westward on the upper continental slope in the northern South China Sea were observed in May–June 2011 with nearly full-depth ...measurements of velocity, temperature, salinity, and density. As they shoaled, at least three waves reached the convective breaking limit: along-wave current velocity exceeded the wave propagation speed C. Vertical overturns of ~100 m were observed within the wave cores; estimated turbulent kinetic energy was up to 1.5 × 10−4 W kg−1. In the cores and at the pycnocline, the gradient Richardson number was mostly <0.25. The maximum ISW vertical displacement was 173 m, 38% of the water depth. The normalized maximum vertical displacement was ~0.4 for three convective breaking ISWs, in agreement with laboratory results for shoaling ISWs. Observed ISWs had greater available potential energy (APE) than kinetic energy (KE). For one of the largest observed ISWs, the total wave energy per unit meter along the wave crest E was 553 MJ m−1, more than three orders of magnitude greater than that observed on the Oregon Shelf. Pressure work contributed 77% and advection contributed 23% of the energy flux. The energy flux nearly equaled CE. The Dubriel–Jacotin–Long model with and without a background shear predicts neither the observed APE > KE nor the subsurface maximum of the along-wave velocity for shoaling ISWs, but does simulate the total energy and the wave shape. Including the background shear in the model results in the formation of a surface trapped core.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The drag coefficient, often used to parameterize the surface wind stress τ, beneath tropical cyclones (TCs) is a critical but poorly known factor controlling TC intensity. Here, τ is estimated using ...current measurements taken by 12 Electromagnetic Autonomous Profiling Explorer floats beneath the forward half of five TCs. Combining estimates of τ and aircraft measurements of winds U10, the downwind drag coefficient
C∥~ and the angle ϕ clockwise orientation from U10 to τ are computed. At |U10| = 25–40 m/s,
C∥~ and ϕ vary over (0.8–3.1) × 10−3 and −15–40°, respectively. A new nondimensional parameter “effective wind duration,” a function of |U10|, storm translation speed, and positions in TCs, predicts
C∥~ to within 25%. The largest
C∥~ and smallest ϕ occur at high winds, in the forward right quadrant of fast‐moving storms. These dependences are explained by variations in surface wave age and breaking under different wave forcing regimes.
Plain Language Summary
The forecast of tropical cyclone intensification is critical to the protection of coastlines, involving the complicated tropical cyclone‐ocean interaction. The wind of storms can force strong near‐inertial current via surface wind stress (often parameterized by a drag coefficient Cd), and then induce the upper ocean cooling due to the shear instability. The transferred momentum and reduced heat supply can both restrict tropical cyclones' development. In other words, the Cd can affect the prediction of momentum and thermal response under storms, and thereby the forecast on storm intensity. This study investigates the spatial variability of downwind drag coefficient Cd under five different tropical cyclones, by integrating the storm‐induced ocean momentum because previous results of Cd as a function of wind speed |U10| are scattered significantly at |U10|=25‐40 m/s. Here, larger Cd in the front‐right sector of faster storms than that of slower stoms is found, presumably due to the surface wave effect. A new parameterization of Cd using the surface wave properties under tropical cyclones is proposed, which largely improves the conventional parameterization of Cd(|U10|). Future studies on the tropical cyclone‐wave‐ocean interaction and storm intensification forecast will be benefited from this new parameterization.
Key Points
Drag coefficients under five different tropical cyclones
New data‐based parameterization of drag coefficients using surface wave effects
Abstract
Twenty Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats in the upper-ocean thermocline of the summer Sargasso Sea observed the temporal and vertical variations of Ertel ...potential vorticity (PV) at 7–70-m vertical scale, averaged over
O
(4–8)-km horizontal scale. PV is dominated by its linear components—vertical vorticity and vortex stretching, each with an rms value of ~0.15
f
. In the internal wave frequency band, they are coherent and in phase, as expected for linear internal waves. Packets of strong, >0.2
f
, vertical vorticity and vortex stretching balance closely with a small net rms PV. The PV spectrum peaks at the highest resolvable vertical wavenumber, ~0.1 cpm. The PV frequency spectrum has a red spectral shape, a −1 spectral slope in the internal wave frequency band, and a small peak at the inertial frequency. PV measured at near-inertial frequencies is partially attributed to the non-Lagrangian nature of float measurements. Measurement errors and the vortical mode also contribute to PV in the internal wave frequency band. The vortical mode Burger number, computed using time rates of change of vertical vorticity and vortex stretching, is 0.2–0.4, implying a horizontal kinetic energy to available potential energy ratio of ~0.1. The vortical mode energy frequency spectrum is 1–2 decades less than the observed energy spectrum. Vortical mode energy is likely underestimated because its energy at vertical scales > 70 m was not measured. The vortical mode to total energy ratio increases with vertical wavenumber, implying its importance at small vertical scales.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Trains of large Kelvin‐Helmholtz (KH) billows within the Kuroshio current at ~230 m depth off southeastern Taiwan and above a seamount were observed by shipboard instruments. The trains of large KH ...billows were present in a strong shear band along the 0.55 m s−1 isotach within the Kuroshio core; they are presumably produced by flow interactions with the rapidly changing topography. Each individual billow, resembling a cat's eye, had a horizontal length scale of 200 m, a vertical scale of 100 m, and a timescale of 7 min, near the local buoyancy frequency. Overturns were observed frequently in the billow cores and the upper eyelids. The turbulent kinetic energy dissipation rates estimated using the Thorpe scale had an average value of O(10−4) W kg−1 and a maximum value of O(10−3) W kg−1. The turbulence mixing induced by the KH billows may exchange Kuroshio water with the surrounding water masses.
Key Points
Large KH billows induced by flow‐seamount interactions were observed within the Kuroshio core
The KH billows produce energetic and persistent turbulent mixing in the Kuroshio stream
Properly parameterizing small‐scale processes in simulations of the Kuroshio is needed
Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and ...the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis, sediment and pollutant transport and acoustic transmission; they also pose hazards for man-made structures in the ocean. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects. For over a decade, studies have targeted the South China Sea, where the oceans' most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SBMB, SIK, UILJ, UKNU, UL, UM, UPUK
Large internal solitary waves with subsurface cores have recently been observed in the South China Sea. Here fully nonlinear solutions of the Dubreil–Jacotin–Long equation are used to study the ...conditions under which such cores exist. We find that the location of the cores, either at the surface or below the surface, is largely determined by the sign of the vorticity of the near-surface background current. The results of a numerical simulation of a two-dimensional shoaling internal solitary wave are presented which illustrate the formation of a subsurface core.
Given the increasing attention in forecasting weather and climate on the subseasonal time scale in recent years, National Oceanic and Atmospheric Administration (NOAA) announced to support Climate ...Process Teams (CPTs) which aim to improve the Madden‐Julian Oscillation (MJO) prediction by NOAA’s global forecasting models. Our team supported by this CPT program focuses primarily on the improvement of upper ocean mixing parameterization and air‐sea fluxes in the NOAA Climate Forecast System (CFS). Major improvement includes the increase of the vertical resolution in the upper ocean and the implementation of General Ocean Turbulence Model (GOTM) in CFS. In addition to existing mixing schemes in GOTM, a newly developed scheme based on observations in the tropical ocean, with further modifications, has been included. A better performance of ocean component is demonstrated through one‐dimensional ocean model and ocean general circulation model simulations validated by the comparison with in‐situ observations. These include a large sea surface temperature (SST) diurnal cycle during the MJO suppressed phase, intraseasonal SST variations associated with the MJO, ocean response to atmospheric cold pools, and deep cycle turbulence. Impact of the high‐vertical resolution of ocean component on CFS simulation of MJO‐associated ocean temperature variations is evident. Also, the magnitude of SST changes caused by high‐resolution ocean component is sufficient to influence the skill of MJO prediction by CFS.
Plain Language Summary
The idea of Climate Process Teams (CPTs) has been suggested in early 2000 to accelerate the development of numerical models for prediction of weather and climate. Members of CPTs consist of observationalists, theoreticians, process‐oriented modelers, and scientists at modeling centers, and thus knowledge obtained from observational and process‐oriented researches can be transferred to the improvement of physical process representations in global climate models. The CPT program initiated by NOAA in 2015 specifically aims to improve prediction of Madden‐Julian Oscillation (MJO) which is the major intraseasonal (30–90 days) fluctuation in the tropical atmosphere. Our CPT primarily focuses on improving the representation of upper ocean processes relevant to the MJO in NOAA's operational climate prediction system: Climate Forecast System (CFS). Performance of the improved ocean component of CFS is evaluated through a comparison of model simulations with high‐quality in‐situ data collected during the recent field campaign which was designed to monitor ocean and atmospheric variability associated with the MJO. The improvement includes the realistic model simulation of large upper ocean warming during daytime through implementing high vertical resolution mixing schemes near the surface. The results demonstrate a significant impact of the high‐vertical resolution ocean component in CFS on the MJO prediction skill.
Key Points
Upper ocean processes relevant to MJO simulations in the Climate Forecast System are improved through our Climate Process Team project
Realistic model simulations of diurnal warming and its dependence on mixing schemes are demonstrated by the comparison with in situ data
A series of CFS simulations indicate a positive impact of high vertical resolution near the ocean surface on MJO prediction skills
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