71 research outputs found
Lateral entrainment in baroclinic currents II
The strong mesoscale velocity fluctuations (“eddies”) observed in the deep Gulf Stream region may be dynamically necessary to incorporate the Recirculation Gyres which account for the downstream increase in transport. The eddy-current interaction leading to entrainment is studied in a two layer quasi-geostrophic model with piecewise uniform potential vorticity. It is shown that the volume entrained by a bottom eddy of strength Γ*2 initially located near the edge of a bottom current depends mainly on the lateral shear s2 of this current, rather than on the much stronger shear of the upper layer. It is suggested that the “mean entrainment velocity” into an isopycnal layer of nearly uniform vertical thickness is proportional to (Γ*2s2)1/2, where Γ*2 (cm2/sec) is proportional to the integrated potential vorticity anomaly of the eddy
Lateral entrainment in baroclinic currents
A two layer shear flow model with piecewise uniform potential vorticity is used to show that circulation or free exchange of parcels in an isopycnal layer of uniform potential vorticity can be greatly inhibited by a strong potential vorticity front in another isopycnal layer. On the other hand, a net mass transfer across the edge of a shear flow can be produced by the initial presence of a strong mesoscale eddy. The entrainment resulting from this eddy-shear flow interaction is defined and quantified for an ensemble of initial realizations. It is suggested that dynamically similar entrainment processes, occurring in more realistic potential vorticity distributions, are important in coupling the recirculation gyres to the Gulf Stream, thereby providing the observed downstream increase in transport
Wave Extremes in the North East Atlantic from Ensemble Forecasts
A method for estimating return values from ensembles of forecasts at advanced
lead times is presented. Return values of significant wave height in the
North-East Atlantic, the Norwegian Sea and the North Sea are computed from
archived +240-h forecasts of the ECMWF ensemble prediction system (EPS) from
1999 to 2009.
We make three assumptions: First, each forecast is representative of a
six-hour interval and collectively the data set is then comparable to a time
period of 226 years. Second, the model climate matches the observed
distribution, which we confirm by comparing with buoy data. Third, the ensemble
members are sufficiently uncorrelated to be considered independent realizations
of the model climate. We find anomaly correlations of 0.20, but peak events
(>P97) are entirely uncorrelated. By comparing return values from individual
members with return values of subsamples of the data set we also find that the
estimates follow the same distribution and appear unaffected by correlations in
the ensemble. The annual mean and variance over the 11-year archived period
exhibit no significant departures from stationarity compared with a recent
reforecast, i.e., there is no spurious trend due to model upgrades.
EPS yields significantly higher return values than ERA-40 and ERA-Interim and
is in good agreement with the high-resolution hindcast NORA10, except in the
lee of unresolved islands where EPS overestimates and in enclosed seas where it
is biased low. Confidence intervals are half the width of those found for
ERA-Interim due to the magnitude of the data set.Comment: 27 pp, 10 figures, J Climate (in press
Sea-state contributions to sea-level variability in the European Seas
The contribution of sea-state-induced processes to sea-level variability is investigated through ocean-wave coupled simulations. These experiments are performed with a high-resolution configuration of the Geestacht COAstal model SysTem (GCOAST), implemented in the Northeast Atlantic, the North Sea and the Baltic Sea which are considered as connected basins. The GCOAST system accounts for wave-ocean interactions and the ocean circulation relies on the NEMO (Nucleus for European Modelling of the Ocean) ocean model, while ocean-wave simulations are performed using the spectral wave model WAM. The objective is to demonstrate the contribution of wave-induced processes to sea level at different temporal and spatial scales of variability. When comparing the ocean-wave coupled experiment with in situ data, a significant reduction of the errors (up to 40% in the North Sea) is observed, compared with the reference. Spectral analysis shows that the reduction of the errors is mainly due to an improved representation of sea-level variability at temporal scales up to 12 h. Investigating the representation of sea-level extremes in the experiments, significant contributions (> 20%) due to wave-induced processes are observed both over continental shelf areas and in the Atlantic, associated with different patterns of variability. Sensitivity experiments to the impact of the different wave-induced processes show a major impact of wave-modified surface stress over the shelf areas in the North Sea and in the Baltic Sea. In the Atlantic, the signature of wave-induced processes is driven by the interaction of wave-modified momentum flux and turbulent mixing, and it shows its impact to the occurrence of mesoscale features of the ocean circulation. Wave-induced energy fluxes also have a role (10%) in the modulation of surge at the shelf break.publishedVersio
A deep ocean acoustic noise floor, 1–800 Hz
Author Posting. © Acoustical Society of America, 2018. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 143 (2018): 1223, doi:10.1121/1.5025042.The ocean acoustic noise floor (observed when the overhead wind is low, ships are distant, and marine life silent) has been measured on an array extending up 987 m from 5048 m depth in the eastern North Pacific, in what is one of only a few recent measurements of the vertical noise distribution near the seafloor in the deep ocean. The floor is roughly independent of depth for 1–6 Hz, and the slope (∼ f−7) is consistent with Longuet-Higgins radiation from oppositely-directed surface waves. Above 6 Hz, the acoustic floor increases with frequency due to distant shipping before falling as ∼ f−2 from 40 to 800 Hz. The noise floor just above the seafloor is only about 5 dB greater than during the 1975 CHURCH OPAL experiment (50–200 Hz), even though these measurements are not subject to the same bathymetric blockage. The floor increases up the array by roughly 15 dB for 40–500 Hz. Immediately above the seafloor, the acoustic energy is concentrated in a narrow, horizontal beam that narrows as f−1 and has a beam width at 75 Hz that is less than the array resolution. The power in the beam falls more steeply with frequency than the omnidirectional spectrum.The
OBSANP cruise was funded by the Office of Naval
Research under Grant Nos. N00014-10-1-0987, N00014-14-
1-0324, N00014-10-1-0510, and N00014-10-1-0990
Wind sea behind a cold front and deep ocean acoustics
Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 46 (2016): 1705-1716, doi:10.1175/JPO-D-15-0221.1.A rapid and broadband (1 h, 1 < f < 400 Hz) increase in pressure and vertical velocity on the deep ocean floor was observed on seven instruments comprising a 20-km array in the northeastern subtropical Pacific. The authors associate the jump with the passage of a cold front and focus on the 4- and 400-Hz spectra. At every station, the time of the jump is consistent with the front coming from the northwest. The apparent rate of progress, 10–20 km h−1 (2.8–5.6 m s−1), agrees with meteorological observations. The acoustic radiation below the front is modeled as arising from a moving half-plane of uncorrelated acoustic dipoles. The half-plane is preceded by a 10-km transition zone, over which the radiator strength increases linearly from zero. With this model, the time derivative of the jump at a station yields a second and independent estimate of the front’s speed, 8.5 km h−1 (2.4 m s−1). For the 4-Hz spectra, the source physics is taken to be Longuet-Higgins radiation. Its strength depends on the quantity , where Fζ is the wave amplitude power spectrum and I the overlap integral. Thus, the 1-h time constant observed in the bottom data implies a similar time constant for the growth of the wave field quantity behind the front. The spectra at 400 Hz have a similar time constant, but the jump occurs 25 min later. The implications of this difference for the source physics are uncertain.The OBSANP cruise was funded by the Office of Naval Research under Grants N00014-10-1-0987, N00014-14-1-0324, N00014-10-1-0510, and N00014-10-1-0990
Wind and Wave Extremes over the World Oceans from Very Large Ensembles
Global return values of marine wind speed and significant wave height are
estimated from very large aggregates of archived ensemble forecasts at +240-h
lead time. Long lead time ensures that the forecasts represent independent
draws from the model climate. Compared with ERA-Interim, a reanalysis, the
ensemble yields higher return estimates for both wind speed and significant
wave height. Confidence intervals are much tighter due to the large size of the
dataset. The period (9 yrs) is short enough to be considered stationary even
with climate change. Furthermore, the ensemble is large enough for
non-parametric 100-yr return estimates to be made from order statistics. These
direct return estimates compare well with extreme value estimates outside areas
with tropical cyclones. Like any method employing modeled fields, it is
sensitive to tail biases in the numerical model, but we find that the biases
are moderate outside areas with tropical cyclones.Comment: 28 pages, 16 figure
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