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Kim, SY, Cornuelle BD, Terrill EJ, Jones B, Washburn L, Moline MA, Paduan JD, Garfield N, Largier JL, Crawford G, Kosro PM.  2013.  Poleward propagating subinertial alongshore surface currents off the US West Coast. Journal of Geophysical Research-Oceans. 118:6791-6806.   10.1002/jgrc.20400   AbstractWebsite

The network comprising 61 high-frequency radar systems along the U.S. West Coast (USWC) provides a unique, high resolution, and broad scale view of ocean surface circulation. Subinertial alongshore surface currents show poleward propagating signals with phase speeds of O(10) and O(100-300) kmd-1 that are consistent with historical in situ observations off the USWC and that can be possibly interpreted as coastally trapped waves (CTWs). The propagating signals in the slow mode are partly observed in southern California, which may result from scattering and reflection of higher-mode CTWs due to curvature of shoreline and bathymetry near Point Conception, California. On the other hand, considering the order of the phase speed in the slow mode, the poleward propagating signals may be attributed to alongshore advection or pressure-driven flows. A statistical regression of coastal winds at National Data Buoy Center buoys on the observed surface currents partitions locally and remotely wind-forced components, isolates footprints of the equatorward propagating storm events in winter off the USWC, and shows the poleward propagating signals year round.

Skarsoulis, EK, Cornuelle BD, Dzieciuch MA.  2011.  Second-Order Sensitivity of Acoustic Travel Times to Sound Speed Perturbations. Acta Acustica United with Acustica. 97:533-543.   10.3813/aaa.918434   AbstractWebsite

The second-order sensitivity of finite-frequency acoustic travel times to sound speed perturbations in range-independent environments is studied. Using the notion of peak arrivals and the normal-mode representation of the Green's function first- and second-order perturbation expressions are derived for the travel times in terms of the underlying perturbations in the Green's function and finally in the sound speed profile. The resulting theoretical expressions are numerically validated. Assuming small and local perturbations the non-linear effects appear to be strongest for sound speed perturbations taking place close to the lower turning depths of the corresponding eigenrays. At the upper turning depths - in the case of temperate propagation conditions - the effects are much weaker due to the larger sound speed gradients. The magnitude of the second-order sensitivity of travel times relative to the first-order sensitivity can be used to obtain an estimate for the limits of linearity.

Andrew, RK, Howe BM, Mercer JA, Group NPAL, Cornuelle B, Colosi J.  2005.  Transverse horizontal spatial coherence of deep arrivals at megameter ranges. Journal of the Acoustical Society of America. 117:1511-1526.   10.1121/1.1854851   AbstractWebsite

Predictions of transverse horizontal spatial coherence from path integral theory are compared with measurements for two ranges between 2000 and 3000 km. The measurements derive from a low-frequency (75 Hz) bottom-mounted source at depth 810 m near Kauai that transmitted m-sequence signals over several years to two bottom-mounted horizontal line arrays in the North Pacific. In this paper we consider the early arriving portion of the deep acoustic field at these arrays. Horizontal coherence length estimates, on the order of 400 m, show good agreement with lengths calculated from theory. These lengths correspond to about 1 degrees in horizontal arrival angle variability using a simple, extended, spatially incoherent source model, Estimates of scintillation index, log-amplitude variance, and decibel intensity variance indicate that the fields were partially saturated. There was no significant seasonal variability in these measures. The scintillation index predictions agree quite well with the dataset estimates; nevertheless, the scattering regime predictions (fully saturated) vary from the regime classification (partially saturated) inferred from observation. This contradictory result suggests that a fuller characterization of scattering regime metrics may be required. (c) 2005 Acoustical Society of America.

Sohn, RA, Webb SC, Hildebrand JA, Cornuelle BD.  1997.  Three-dimensional tomographic velocity structure of upper crust, CoAxial segment, Juan de Fuca ridge: Implications for on-axis evolution and hydrothermal circulation. Journal of Geophysical Research-Solid Earth. 102:17679-17695.   10.1029/97jb00592   AbstractWebsite

Three-dimensional models of compressional velocity and azimuthal anisotropy from tomographic inversions using 23,564 ocean bottom seismometer P wave arrivals define systematic lateral variations in seismic structure of the CoAxial segment of the Juan de Fuca Ridge (JdFR). Over much of the segment the across-axis structure is roughly axisymmetric, characterized by a progressive increase in dike velocities moving away from the ridge axis. This trend is most apparent in the basal dikes, where on-axis velocities are about 800 m/s slower than those measured elsewhere within the rift valley. The on-axis sheeted dikes also exhibit ridge-oriented azimuthal anisotropy, with a peak-to-peak amplitude of about 600 m/s. Outboard of the rift valley, beneath ridge flanks with fault scarps, velocities in the upper 1500 m of crust are reduced. The maximum amplitude of this anomaly is about 700 m/s, located near the top of the sheeted dikes. Variations in the three-dimensional velocity model are believed to reflect changes in crustal porosity, from which we infer an axisymmetric porosity model for seismic layer 2 of the CoAxial segment. As the crust ages, the evolution of layer 2 porosity could occur in the following way: (1) the porosity of zero-age, on-axis dikes is set at formation by the contraction of molten material, (2) hydrothermal alteration fills pore spaces as the dikes move away from the center of the axial valley, and (3) normal faulting on the ridge flank scarps opens fractures and increases porosity of the upper dikes as they move off-axis. At the north end of the segment, dike velocities are several hundred meters per second slower, on average, and the across-axis structure is lost. The transition from a coherent, aligned seismic structure to a less distinct pattern with reduced velocities may represent a shift from magmatic to amagmatic extension moving away from the Cobb hotspot on the ridge axis. The porosity structure we have derived for the CoAxial segment suggests an alternative to the usual hydrothermal circulation model of cross-axis convection cells. A circulation model with along-axis convection cells located entirely within the axial valley appears to be more compatible with our data.

Morris, M, Roemmich D, Cornuelle B.  1996.  Observations of variability in the South Pacific subtropical gyre. Journal of Physical Oceanography. 26:2359-2380.   10.1175/1520-0485(1996)026<2359:oovits>;2   AbstractWebsite

Variability of the subtropical gyre in the South Pacific Ocean was investigated using high-resolution expendable bathythermograph sections along a repeated track between New Zealand and Hawaii. The southern part of the section sampled most of the zonal flow in the subtropical gyre with the eastward flowing branch between New Zealand and Fiji and the westward branch extending north of Fiji to approximately 10 degrees S. The time series began in September 1987 and extended through 1994, averaging four cruises every year. The geostrophic shear field was calculated, relative to 800 m, with the aid of a mean T-S relationship. Variability was present at a broad range of spatial and temporal scales but annual fluctuations were particularly prominent. The authors conclude that 30 snapshots of temperature, measured over a period of seven years, are sufficient to resolve the annual cycle of the gyre scale circulation along the transect. The shape and intensity of the gyre varied seasonally throughout the water column (0-800 m). Geostrophic transport was most intense (15 Sv, where Sv=10(6)m(3)s(-1)) in November. At this time, the northern edges of eastward dow at the surface and in the thermocline were closest together and the ratio of thermocline to surface transport was highest. Most intense flow occurred approximately two to three months after the basinwide seasonal peak in Ekman pumping. Transport was weakest(ll Sv) in May and was associated with an increase in the poleward slant of the gyre center with depth and a decrease in the ratio of thermocline to surface transport. Seasonal wind forcing was considered as a possible mechanism for the observed annual intensification of the gyre-scale circulation. A simple linear model of thermocline response to local changes in wind stress curl explained a significant fraction of the observed annual variability. Conservation of potential vorticity q yielded an estimate for the absolute mean how (-1 cm s(-1) at 800 m), consistent with direct measurements in the region. Interannual variability, possibly related to the El Nino-Southern Oscillation cycle, was observed. The cold event of 1988/89 appeared to be associated with relatively weak gyre-scale transport. After 1991, gyre-scale transport was more intense and a prominent change in the small-scale circulation occurred, with a shift in the alongtrack wavenumber spectral energy to higher wavenumbers.