Search Results
Abstract
Measurements of vertical shear and strain were acquired from the research platform FLIP during the PATCHEX experiment in October, 1986 (34°N, 127°W). Vertical sheer was shear from two separate Doppler sonar systems. A long-range sonar, with independent estimates every 18 m, sampled from 150–1200 m in depth. A short-range sonar measured fine-scale shear over 150–180 m depth, with 1.5 m vertical resolution. Vertical strain, ∂η/∂z, was estimated from two repeatedly profiling CTDs. These sampled to 560 m once every three minutes. The time variation of the strain field is monitored in both Eulerian (fixed-depth) and semi-Lagrangian (isopycnal-following) reference frames, from 150–406 m depth.
Eulerian vertical wavenumber-frequency (m, ω) spectra of vertical shear and strain exhibit a frequency dependency which is a strong function of wavenumber (ω−2–ω0 for m = 0.01–0.3 cpm). In contrast the semi-Lagrangian strain spectrum is more nearly separable in frequency and wavenumber, in closer agreement with the Garrett–Munk (GM) internal wave spectral model.
When a simulated GM shear field is vertically advected by a GM isopycnal displacement field, the resultant Eulerian vertical wavenumber–frequency spectrum exhibits the same qualitative, nonseparable, form as the PATCHEX shear spectrum: The dominant near-inertial waves are Doppler-shifted across all frequency bands, resulting in a “while” frequency spectrum at high wavenumbers. Measured ratios of Eulerian shear/strain variance support this interpretation. Higher shear-low strain variances (characteristic of near-inertial waves) are seen at high wavenumber, high encounter frequencies. The conclusion is that internal wave vertical self-advection strongly alters the observed frequency at high vertical wavenumbers in an Eulerian reference frame.
Abstract
Measurements of vertical shear and strain were acquired from the research platform FLIP during the PATCHEX experiment in October, 1986 (34°N, 127°W). Vertical sheer was shear from two separate Doppler sonar systems. A long-range sonar, with independent estimates every 18 m, sampled from 150–1200 m in depth. A short-range sonar measured fine-scale shear over 150–180 m depth, with 1.5 m vertical resolution. Vertical strain, ∂η/∂z, was estimated from two repeatedly profiling CTDs. These sampled to 560 m once every three minutes. The time variation of the strain field is monitored in both Eulerian (fixed-depth) and semi-Lagrangian (isopycnal-following) reference frames, from 150–406 m depth.
Eulerian vertical wavenumber-frequency (m, ω) spectra of vertical shear and strain exhibit a frequency dependency which is a strong function of wavenumber (ω−2–ω0 for m = 0.01–0.3 cpm). In contrast the semi-Lagrangian strain spectrum is more nearly separable in frequency and wavenumber, in closer agreement with the Garrett–Munk (GM) internal wave spectral model.
When a simulated GM shear field is vertically advected by a GM isopycnal displacement field, the resultant Eulerian vertical wavenumber–frequency spectrum exhibits the same qualitative, nonseparable, form as the PATCHEX shear spectrum: The dominant near-inertial waves are Doppler-shifted across all frequency bands, resulting in a “while” frequency spectrum at high wavenumbers. Measured ratios of Eulerian shear/strain variance support this interpretation. Higher shear-low strain variances (characteristic of near-inertial waves) are seen at high wavenumber, high encounter frequencies. The conclusion is that internal wave vertical self-advection strongly alters the observed frequency at high vertical wavenumbers in an Eulerian reference frame.
Abstract
A new autonomous instrument collected 76 profiles of temperature microstructure over a ten-day period in the eastern subtropical North Atlantic as part of the North Atlantic Tracer Release Experiment. The data between 200-m and 350-m depth was used to determine the mean rate of temperature variance dissipation 〈χ〉. The estimated diapycnal diffusivity is Ky = 1.4×10−5 m2 s−1. The distribution of χ is approximately lognormal, suggesting that the 95% confidence limits on 〈χ〉 are ±4%. This uncertainty is less than that caused by the imperfectly known probe response, possible noise spikes on the probes, and variability in the degree of microstructure anisotropy; the latter two effects were estimated from a pair of closely spaced probes. Each of these uncertainties is about ±15%. Statistically significant low-frequency variability of χ is observed with 〈χ〉 decreasing by a factor of 2 between the first and second half of the observation. This low-frequency variability is likely the largest cause of error in estimating a seasonally averaged diapycnal diffusivity.
Abstract
A new autonomous instrument collected 76 profiles of temperature microstructure over a ten-day period in the eastern subtropical North Atlantic as part of the North Atlantic Tracer Release Experiment. The data between 200-m and 350-m depth was used to determine the mean rate of temperature variance dissipation 〈χ〉. The estimated diapycnal diffusivity is Ky = 1.4×10−5 m2 s−1. The distribution of χ is approximately lognormal, suggesting that the 95% confidence limits on 〈χ〉 are ±4%. This uncertainty is less than that caused by the imperfectly known probe response, possible noise spikes on the probes, and variability in the degree of microstructure anisotropy; the latter two effects were estimated from a pair of closely spaced probes. Each of these uncertainties is about ±15%. Statistically significant low-frequency variability of χ is observed with 〈χ〉 decreasing by a factor of 2 between the first and second half of the observation. This low-frequency variability is likely the largest cause of error in estimating a seasonally averaged diapycnal diffusivity.
Abstract
“Spray” gliders, most launched from small boats near shore, have established a sustainable time series of equatorward transport through the Solomon Sea. The first 3.5 years (mid-2007 through 2010) are analyzed. Coast-to-coast equatorward transport through the Solomon Sea fluctuates around a value of 15 Sv (1 Sv ≡ 106 m3 s−1) with variations approaching ±15 Sv. Transport variability is well correlated with El Niño indices like Niño-3.4, with strong equatorward flow during one El Niño and weak flow during two La Niñas. Mean transport is centered in an undercurrent focused in the western boundary current; variability has a two-layer structure with layers separated near 250 m (near the core of the undercurrent) that fluctuate independently. The largest variations are in midbasin, confined to the upper layer, and are well correlated with ENSO. Analysis of velocity and salinity on isopycnals shows that the western boundary current within the Solomon Sea consists of a deep core coming from the Coral Sea and a shallow core that enters the Solomon Sea in mid basin. Analysis of the structure of transport and its fluctuations is presented.
Abstract
“Spray” gliders, most launched from small boats near shore, have established a sustainable time series of equatorward transport through the Solomon Sea. The first 3.5 years (mid-2007 through 2010) are analyzed. Coast-to-coast equatorward transport through the Solomon Sea fluctuates around a value of 15 Sv (1 Sv ≡ 106 m3 s−1) with variations approaching ±15 Sv. Transport variability is well correlated with El Niño indices like Niño-3.4, with strong equatorward flow during one El Niño and weak flow during two La Niñas. Mean transport is centered in an undercurrent focused in the western boundary current; variability has a two-layer structure with layers separated near 250 m (near the core of the undercurrent) that fluctuate independently. The largest variations are in midbasin, confined to the upper layer, and are well correlated with ENSO. Analysis of velocity and salinity on isopycnals shows that the western boundary current within the Solomon Sea consists of a deep core coming from the Coral Sea and a shallow core that enters the Solomon Sea in mid basin. Analysis of the structure of transport and its fluctuations is presented.
