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Extracting the internal tide from data: Methods and observations from the Mixed Layer Dynamics Experiment
Get PDFSeveral methods are developed for analyzing data containing a highly variable internal tide. In particular, the methods are aimed at the analysis of moored observations with relatively few measurements in the vertical. The analysis depends upon an "elliptical decomposition" that is a generalization of the familiar "rotary decomposition." The technique is applied to velocity and temperature observations in the upper ocean made during the Mixed Layer Dynamics Experiment (MILDEX) in the northeast Pacific Ocean, about 700 km west of Santa Barbara, California, during October-November 1983. The observed propagation direction and amplitude of the internal tide was highly variable in time. It was anticipated that the wave could be propagating from the continental shelf where it is presumed to be generated. However, most of the time the internal tide appears to be propagating parallel to the coast. This result suggests the importance of density and velocity structure at mesoscale and frontal scale in affecting the propagation of the internal tide
Spatial anatomy of species driven survival: Effects of predation and climate-driven environmental variability
No full textThe majority of survival analyses focus on temporal scales. Consequently, there is a limited understanding of how species survival varies over space and, ultimately, how spatial variability in the environment affects the temporal dynamics of species abundance. Using data from the Barents Sea, we study the spatiotemporal variability of the juvenile Atlantic cod (Gadus morhua) survival. We develop an index of spatial survival based on changes of juvenile cod distribution through their first winter of life (from age-0 to age-1) and study its variability in relation to biotic and abiotic factors. Over the 25 years analyzed (1980– 2004), we found that, once the effect of passive drift due to dominant currents is accounted for, the area where age-0 cod survival was lowest coincided with the area of highest abundance of older cod. Within this critical region, the survival of age-0 cod was negatively affected by its own abundance, by that of older cod, and by bottom depth. Furthermore, during cold years, age-0 cod survival increased in the eastern and coldest portion of the examined area, which was typically avoided by older conspecifics. Based on these results we propose that within the examined area top-down mechanisms and predation-driven density dependence can strongly affect the spatial pattern of age-0 cod survival. Climate-related variables can also influence the spatial survival of age-0 cod by affecting their distribution and that of their predators. Results from these and similar studies, focusing on the spatial variability of survival rates, can be used to characterize species habitat quality of marine renewable resources
Rupture process of the MacQuarie Ridge earthquake of May 23, 1989
Get PDFBroadband body waves recorded at 15 digital seismic stations worldwide are used to study the rupture process of the May 23, 1989 Macquarie Ridge earthquake. The centroidal solution (strike 211°, dip 86°, rake 180°, and depth of 10 km below the seafloor) indicates shallow rupture with pure right-lateral strike-slip motion along the Pacific-Australia plate boundary, in agreement with motion predicted by plate tectonic models. The total seismic moment is 13.4x10²⁰ Nm, 80% of which was released in the first 24 s of the rupture process. Modeling favors a bilaterally propagating rupture with slightly different dip and rake for the northward and southward fault segments and similar moment release along both directions. The estimated fault length is quite short, about 90 km, and the derived stress drop of 180 bar and average displacement of 17 m are unusually high. The bathymetry in the epicentral region shows topographic segmentation of the ridge, possibly indicating fault segmentation which confines ruptures to short segments
Estimation of surface winds from upward looking acoustic doppler current profilers
Get PDFThree upward looking acoustic Doppler current profilers (ADCP) were deployed beneath meteorological buoys in the Gulf of Tehuantepec, Mexico, during winter 1988-1989. Hourly averaged wind speed data from the buoys and from ship when in the vicinity were compared with surface acoustic backscatter intensity recorded at the ADCPs. The backscatter was found to be a significant predictor of wind speeds from both buoy and ship, the latter when within 50 km of the mooring site. There was no apparent saturation of the backscatter signal at the maximum wind speeds (< 15 m/s). The results cast doubt on the ability of near-surface Doppler directional information to provide reliable estimates of wind direction
Mid-Pliocene shifts in ocean overturning circulation and the onset of Quaternary-style climates
Get PDFA major tipping point of Earth's history occurred during the mid-Pliocene: the onset of major Northern-Hemisphere Glaciation (NHG) and of pronounced, Quaternary-style cycles of glacial-to-interglacial climates, that contrast with more uniform climates over most of the preceding Cenozoic and continue until today (Zachos et al., 2001). The severe deterioration of climate occurred in three steps between 3.2 Ma (warm MIS K3) and 2.7 Ma (glacial MIS G6/4) (Lisiecki and Raymo, 2005). Various models (sensu Driscoll and Haug, 1998) and paleoceanographic records (intercalibrated using orbital age control) suggest clear linkages between the onset of NHG and the three steps in the final closure of the Central American Seaways (CAS), deduced from rising salinity differences between Caribbean and the East Pacific. Each closing event led to an enhanced North Atlantic meridional overturning circulation and this strengthened the poleward transport of salt and heat (warmings of +2–3°C) (Bartoli et al., 2005). Also, the closing resulted in a slight rise in the poleward atmospheric moisture transport to northwestern Eurasia (Lunt et al., 2007), which probably led to an enhanced precipitation and fluvial run-off, lower sea surface salinity (SSS), and an increased sea-ice cover in the Arctic Ocean, hence promoting albedo and the build-up of continental ice sheets. Most important, new evidence shows that the closing of the CAS led to greater steric height of the North Pacific and thus doubled the low-saline Arctic Throughflow from the Bering Strait to the East Greenland Current (EGC). Accordingly, Labrador Sea IODP Site 1307 displays an abrupt but irreversible EGC cooling of 6°C and freshening by ~2 psu from 3.25/3.16–3.00 Ma, right after the first but still reversible attempt of closing the CAS
Mode 2 waves on the continental shelf : ephemeral components of the nonlinear internal wavefield
Get PDFShoreward propagating, mode 2 nonlinear waves appear sporadically in mooring records obtained off the coast of New Jersey in the summer of 2006. Individual mode 2 packets were tracked between two moorings separated by 1 km; however, packets could not be tracked between moorings separated by greater distances from one another (∼10 km). The inability to track individual packets large distances through the mooring array combined with detailed observations from a ship suggest that these waves are short lived. The evolution of the ship‐tracked wave group was recorded using acoustic backscatter, acoustic Doppler current profilers, and turbulence profiling. The leading mode 2 wave quickly changed form and developed a tail of short, small‐amplitude mode 1 waves. The wavelength of the mode 1 oscillations agreed with that expected for a copropagating tail on the basis of linear theory. Turbulent dissipation in the mixed layer and radiation of the short mode 1 waves contributed to rapid energy loss in the leading mode 2 wave, consistent with the observed decay rate and short life span of only a few hours. The energy in the leading mode 2 wave was 10–100 times smaller than the energy of mode 1 nonlinear internal waves observed during the experiment; however, the magnitudes of wave‐localized turbulent dissipation were similar
The Relative Importance of Wind Strength and Along-Shelf Bathymetric Variations on the Separation of a Coastal Upwelling Jet
Get PDFA high-resolution numerical model with idealized topography is used to investigate the degree to which a coastal upwelling jet separates from the shelf as it flows around a submarine bank depending on the wind strength and the horizontal scale of the bank. Experiments were run using several wind forcing magnitudes and submarine banks with different geometries, so as to explore a wide range of the flow strength as measured by the Rossby number (Ro) and the ratio of the squares of the internal Rossby radius of deformation and curvature of the topography as denoted by the Burger number (Bu). The intensity of the jet separation is strongly dependent on both parameters, with maximum separation with increasing Ro and Bu close to 1, when large amounts of upwelled water are exported toward deeper waters. For small Bu, separation is minimal and independent of Ro. For high Ro, the degree of separation decreases at large Bu since the bank acts only as a small perturbation to the flow. Term balances in the along-shelf momentum equation reveal that the primary balance over the bank is between the nonlinear and the ageostrophic terms. In an asymmetric bank, the radius of curvature in the upstream half of the bank dominates in terms of determining the offshore deflection of a water particle at the surface. The asymmetry increases the cross-isobath transport but not the offshore deflection of the jet
Turbulence in a Sheared, Salt-Fingering-Favorable Environment: Anisotropy and Effective Diffusivities
Get PDFDirect numerical simulations (DNS) of a shear layer with salt-fingering-favorable stratification have been performed for different Richardson numbers Ri and density ratios R[subscript]p. In the absence of shear (Ri = oo), the primary instability is square planform salt fingering, alternating cells of rising and sinking fluid. In the presence of shear, salt fingering takes the form of salt sheets, planar regions of rising and sinking fluid, aligned parallel to the sheared flow. After the onset of secondary instability, the flow becomes turbulent. The continued influence of the primary instability distorts the late-stage structure and hence biases isotropic estimates of the turbulent kinetic energy dissipation rate . In contrast, thermal and saline gradients evolve to become more isotropic than velocity gradients at their dissipation scales. Thus, the standard observational methodology of estimating the turbulent kinetic energy dissipation rate from vertical profiles of microscale gradients and assuming isotropy can underestimate its true value by a factor of 2–3, whereas estimates of thermal and saline dissipation rates using this approach are relatively accurate. Likewise, estimates of G from vertical profiles overestimate the true G by roughly a factor of 2. Salt sheets are ineffective at transporting momentum. Thermal and saline effective diffusivities decrease with decreasing Ri, despite the added energy source provided by background shear. After the transition to turbulence, the thermal to saline flux ratio and the effective Schmidt number remain close to the values predicted by linear theory
Small-Scale Sea Surface Temperature Structure
No full textObservations of sea surface temperature and wave height were made from a large, manned spar buoy (R/P FLIP) ~100 km off the coast of Baja California. Surface temperature was measured with a radiation thermometer which viewed a disc on the surface 12 cm in diameter. The instrument responded to frequencies up to 3 Hz. Wave height was measured with a resistance gage located close to the field of view of the radiometer.
Log-log plots of spectra of sea surface temperature exhibit a plateau between 0.05 and 0.5 Hz, followed by a rapid decrease in energy at frequencies >1 Hz. A coherence of 0.5 between waves and surface temperature occurs at the same frequency as the peak in the wave spectrum. Phase spectra show that warm temperatures associated with the thinning of the surface viscous layer occur systematically upwind of the crests of the dominant gravity waves and downwind of the crests of steeply sloping, shorter period gravity waves. The warm temperatures are hypothesized to be caused by enhanced wind stress upwind from the crests and by surface instability and surface convergence downwind from the crests.
The magnitude of the mean temperature difference between the surface and the warmer, well-mixed water below is estimated from the surface temperature record. It is assumed that the warmest surface temperatures observed are associated with thinning of the viscous layer and are representative of the well-mixed water below. The dimensionless constant in a formula due to Saunders (1967), which relates the temperature difference to wind stress and heat flux, is found to be seven
Response Characteristics of the VACM Compass and Vane Follower
No full textSeveral simple laboratory experiments have been conducted to study the dynamic behavior of the vector-averaging current meter (VACM) compass and vane follower. They demonstrate that the behavior of the compass and vane follower can be modeled as a damped linear harmonic oscillator for small-amplitude forcing. The combined eddy-current and bearing-friction torque nearly critically damps the free oscillation of the compass and vane follower. This frictional torque is proportional to the angular-velocity difference between the instrument magnet assembly and housing. Dynamic experiments on five compasses indicate a mean (undamped) resonant period of 3–5 s at 41°N. Similar experiments on two vane followers indicate a resonant period of 2–3 s.
For the VACM dynamic compass experiments, frictional torque allowed an angular oscillation of the compass housing to drive an oscillation of the compass magnet, and at resonant forcing, the compass magnet oscillates exactly in phase with its housing. For the VACM vane-follower experiments, angular motion of the vane magnet directly drove the vane follower. For resonant forcing of the vane magnet, the vane-follower oscillation overshoots the forcing slightly and is 90° out of phase with the forcing. The damped linear harmonic oscillator model suggests that a small-amplitude angular forcing of the compass or vane-follower housing (which may occur in the field due to mooring motion) should not cause any error in the vector-averaged headings. However, periodic angular oscillations near the resonant frequencies of the compass or vane follower could cause an error in the magnitude of the vector-averaged velocity. Forcing at frequencies lower than the well-defined resonant frequencies of the instruments should have little effect since directional errors do not exceed the angular resolution of the instruments at periods of ≥10 s