ALBERT

Description: In this paper results from the application of an ocean data assimilation (ODA) system, combining a multivariate reduced-order optimal interpolator (OI) scheme with a global ocean general circulation model (OGCM), are described. The present ODA system, designed to assimilate in situ temperature and salinity observations, has been used to produce ocean reanalyses for the 1962–2001 period. The impact of assimilating observed hydrographic data on the ocean mean state and temporal variability is evaluated. A special focus of this work is on the ODA system skill in reproducing a realistic ocean salinity state. Results from a hierarchy of different salinity reanalyses, using varying combinations of assimilated data and background error covariance structures, are described. The impact of the space and time resolution of the background error covariance parameterization on salinity is addressed.

Description: This work has been funded by the ENACT Project (Contract EVK2-CT2001-00117) for A. Bellucci and P. Di Pietro, and partially by the ENSEMBLES Project (Contract GOCE-CT-2003-505539) for A. Bellucci.

Description: Published

Description: 3785-3807

Description: 3.7. Dinamica del clima e dell'oceano

Description: JCR Journal

Description: reserved

Description: Skill in ensemble-mean dynamical seasonal climate hindcasts with a coupled land-atmosphere model and specified observed sea surface temperature is compared to that for long multi-decade integrations of the same model where the initial conditions are far removed from the seasons of validation. The evaluations are performed for surface temperature and compared among all seasons. Skill is found to be higher in the seasonal simulations than the multi-decadal integrations except during boreal winter. The higher skill is prominent even beyond the first month when the direct influence of the atmospheric initial state elevates model skill. Skill is generally found to be lowest during the winter season for the dynamical seasonal forecasts, equal to that of the long integrations, which show some of the highest skill during winter. The reason for the differences in skill during the non-winter months is attributed to the severe climate drift in the long simulations, manifest through errors in downward fluxes of water and energy over land and evident in soil wetness. The drift presses the land surface to extreme dry or wet states over much of the globe, into a range where there is little sensitivity of evaporation to fluctuations in soil moisture. Thus, the land-atmosphere feedback is suppressed, which appears to lessen the model’s ability to respond correctly over land to remote ocean temperature anomalies.

Description: Center for Ocean-Land-Atmosphere Studies

Description: Published

Description: When the U. S. Bureau of Fisheries, in cooperation with the Museum of Comparative Zoology, commenced the oceanographic survey of the Gulf of Maine in the summer of 1912 (Bigelow, 1925-1927), it was in the hope that this might later be extended to the coastal waters thence southward; eventually even as far as the Gulf of Mexico. Cruises carried out in connection with investigations of the biology of the mackerel, by the Fisheries' steamer "Albatross II" from 1927 to 1932, supplemented by those of the research ship "Atlantis" of the Woods Hole Oceanographic Institution, have made it possible to extend the detailed examination of the physical oceanography of the continental shelf as far as the offing of Chesapeake Bay, and to the offing of Cape Hatteras for some of the months. The present account of the temperature of the region will, it is hoped, be followed shortly by corresponding accounts of salinity, of circulation and of the dominant planktonic communities.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September, 1975

Description: Erosion processes involving fine-grained marine sediments were studied by using an in situ flume to erode undisturbed bottom sediments on the sea floor in Buzzards Bay, a shallow marine embayment off the Massachusetts coast. Tte muddy sea floor in that area is characterized by a deposit-feeding infauna that reworks the sediments. Observations made with the in situ flume suggest that erosion resistance of compacted bottom sediments is up to twice as great as the erosion resistance of biogenically reworked sediments. Estimates of erosional bed shear stress from the in situ flume experiments are similar to estimates made during this study of bed shear stress developed in near-bottom tidal currents. It is inferred that erosion by the in situ flume produces reasonable estimates of bed shear stress necessary to erode undisturbed bottom sediments on the sea floor. Buzzards Bay muds were redeposited in a laboratory flume and eroded after various periods of reworking by the deposit-feeding organisms contained in them. Other Buzzards Bay mud samples were treated to remove organic matter, and the erosion resistance of flat beds of these sediments was also investigated in a laboratory flume. The surface of a biogenically reworked bed after two months was covered with mounds, burrows, trails, and aggregates composed of sediments and organic material. This bed was similar in appearance to many of the beds eroded by the in situ flume. The two month bed eroded at an erosional shear stress similar to the erosional shear stress necessary to erode the in situ Buzzards Bay muds (0.8 dynes/cm2 ) . Beds biogenically reworked for shorter periods had high values of erosional shear stress, up to twice that of the two month bed. The bed shear stress necessary to erode flat beds of Buzzards Bay sediments increased as the concentration of organic matter in the sediments increased. Deposit-feeders were absent in these beds, and the mode of deposition was kept uniform, so the increase of erosion resistance with increase in organic content is considered a reliable indication of sediment behavior, and not an artifact of experimental conditions. During the in situ experiments, lee drifts were created behind resistant roughness elements on the sea floor. A brief study of lee drift formation in the laboratory suggests that the formation of lee drifts from fine-grained sediments can be predicted to take place when the body Reynolds number of the resistant roughness elements is below a critical value.

Description: The Office of Naval Research supported this research and provided salary support through grants to the Woods Hole Oceanographic Institution and the Massachusetts Institute of Technology.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2007

Description: This thesis presents three studies which apply geophysical tools to the task of better understanding mantle melting phenomena at the upper and lower boundaries of the mantle. The first study uses seafloor bathymetry and small variations in the gravitational acceleration over the Hawaii-Emperor seamount chain to constrain the changes in the igneous production of the hot spot melting in the mantle which has created these structures over the past 80 My. The second study uses multichannel seismic reflection data to constrain the location and depth of axial magma chambers at the Endeavour Segment of the Juan de Fuca spreading ridge, and then correlates these magma chamber locations with features of the hydrothermal heat extraction system in the upper crust such as microseismicity caused by thermal cracking and high temperature hydrothermal vent systems observed on the seafloor. The third study uses two-dimensional global pseudospectral seismic wave propagation modeling to characterize the sensitivity of the SPdKS seismic phase to two-dimensional, finite-width ultra-low velocity zones (ULVZs) at the core-mantle boundary. Together these three studies highlight the dynamic complexities of melting in the mantle while offering new tools to understand that complexity.

Description: This thesis was funded by a National Science Foundation Graduate Research Fellowship, NSF grant OCE-0002551 to theWoods Hole Oceanographic Institution (WHOI), the WHOI Academic Programs Office, the Earth, Atmospheric, and Planetary Science Department at MIT, and by the WHOI Deep Ocean Exploration Institute.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy and the Woods Hole Oceanographic Institution February 1982

Description: In this thesis, seismic waves generated by sources ranging from 2.7 kg shots of TNT to magnitude 5 earthquakes are studied in order to determine the seismic activity and crustal structure of the Orozco transform fault. Most of the data were collected by a network of 29 ocean bottom seismometers (OBS) and hydrophones (OBH) which were deployed as part of project ROSE (Rivera Ocean Seismic Experiment). Additional information is provided by magnetic anomaly and bathymetric data collected during and prior to ROSE and by teleseismic earthquakes recorded by the WWSSN (Worldwide Seismic Station Network). In Chapter II, the tectonic setting, bathymetry and teleseismic history of the Orozco Fracture Zone are summarized. Covering an area of 90 x 90 km which includes ridges and troughs trending both parallel and perpendicular to the present spreading direction (approximately east-west), the bathymetry of the transform portion of the fracture zone does not resemble that of other transform faults which have been studied in detail. A detailed study of one of the largest teleseismic earthquakes (mb=5.1) indicates right lateral strike-slip faulting with a strike parallel to the present spreading direction and a focal depth of less than 5 km. The moment sum from teleseismic earthquakes suggests an average fault width of at most a few kilometers. Because the teleseismic earthquake locations are too imprecise to define the present plate boundary and the magnetic anomaly data are too sparse to resolve the recent tectonic history, more questions are raised than are answered by the results in this chapter. These questions provide the focus for the study of the ROSE data. Chapter III contains an examination of the transfer function between seafloor motion and data recorded by the MIT OBS. The response of the recording system is determined and the coupling of the OBS to the seafloor during tests at two nearshore sites is analysed. Applying these results to the ROSE data, we conclude that the ground motion in the absence of the instrument can be adequately determined for at least one of the MIT OBS deployed during ROSE. Hypocentral parameters for 70 earthquakes, calculated for an assumed laterally homogeneous velocity structure which was adapted from the results of several refraction surveys in the area, are presented in Chapter IV. Because of the large number of stations in the ROSE network, the epicentral locations, focal depths and source mechanisms are determined with a precision unprecedented in marine microseismic work. Relative to the assumed model, most horizontal errors are less than ±1 km; vertical errors are somewhat larger. All epicenters are within the transform region of the Orozco Fracture Zone. About half of the epicenters define a narrow line of activity parallel to the spreading direction and situated along a deep topographic trough which forms the northern boundary of the transform zone (region 1). Most well determined depths are very shallow (〈4km) and no shallowing of activity is observed as the rise-transform intersection is approached. In fact, the deepest depths (4-10km) are for earthquakes within 10 km of the intersection; these apparent depth differences are supported by the waveforms recorded a t the MIT OBS. First motion polarities for all but two of the earthquakes in region 1 are compatible with right lateral strike-slip faulting along a nearly vertical plane striking parallel to the spreading direct ion. Another zone of activity is observed in the central part of the transform (region 2). The apparent horizontal and vertical distribution of activity is more scattered than for the first group and the first motion radiation patterns of these events do not appear to be compatible with any known fault mechanism. No difference can be resolved between the stress drops or b values in the two regions. In Chapter V, lateral variations in the crustal structure within the transform region are determined and the effect of these structures on the results of the previous chapter is evaluated. Several data sources provide information on different aspects of the crustal structure. Incident angles and azimuths of body waves from shots and earthquakes measured at one of the MIT OSS show systematic deflections from the angles expected for a laterally homogeneous structure. The effect of various factors on the observed angles and azimuths is discussed and it is concluded that at least some of the deflection reflects regional lateral velocity heterogeneity. Structures which can explain the observations are found by tracing rays through three dimensional velocity grids. High velocities are inferred at upper mantle depths beneath a shallow, north-south trending ridge to the west of the OBS, suggesting that the crust under the ridge is no thicker, and perhaps thinner, than the surrounding crust. Observations from sources in region 2 suggest the presence of a low velocity zone in the central transform between the sources and the receiver. That the presence of such a body provides answers to several of the questions raised in Chapter IV about the hypocenters and mechanisms of earthquakes in region 2 is circumstantial evidence supporting this model. These proposed structures do not significantly affect the hypocenters and fault plane solutions for sources in region 1. The crustal velocity structure beneath the north-south trending ridges in the central transform and outside of the transform zone is determined by travel time and amplitude modeling of the data from several lines of small shots recorded at WHOI OBH. Outside of the transform zone, a velocity-depth structure typical of oceanic crust throughout the world oceans is found from three unreversed profiles: a 1 to 2 km thick layer in which the velocity increases from about 3 to 6.7 km/sec overlies a 4 to 4.5 km thick layer with a nearly constant velocity of 6.8 km/sec. A reversed profile over one of the north-south trending ridges, on the other hand, indicates an anomalous velocity structure with a gradient of 0.5 sec-1 throughout most of the crust ( from 5.25 km/sec to 7.15 km/sec over 3.5 km). A decrease in the gradient at the base of the crust to about 0.1 sec-1 and a thin, higher gradient layer in the upper few hundred meters are also required to fit the travel time and amplitude data. A total crustal thickness of about 5.4 km is obtained. An upper mantle velocity of 8.0 to 8.13 km/sec throughout much of the transform zone is determined from travel times of large shots of TNT recorded at MIT and WHOI instruments. "Relocations" of the large shots relative to the velocity model assumed in Chapter IV support the conclusion from the ray tracing that results from region 2 may be systematically biased because of lateral velocity heterogeneity whereas results from region 1 are not affected. In the last chapter, the results on crustal structure and seismicity are combined in order to define the present plate boundary and to speculate on the history of the present configuration.

Description: This research was supported by the Office of Naval Research, under contracts N00014-75-C-0291 and N00014-80-C-0273

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution October, 1975

Description: A geomagnetic variation study on mature oceanic lithosphere in the North Atlantic just south of Bermuda has revealed the presence of at least one layer of low electrical resistivity. The low resistivity layer of approximately 10 ohm-m has been found at three widely spaced seafloor sites with crustal ages of 85, 110 and 150 million years. There is also evidence that the resistivity increases to greater than 20 ohm-m below about 100 km. Apparent resistivity and phase versus period are calculated using the vertical gradient of the horizontal magnetic field variations to estimate the seafloor electric field. The vertical gradient method assumes that the seasurface magnetic variations can be estimated from a nearby land station and that no local magnetic induction occurs at either reference or seafloor site. Both assumptions are critically evaluated during the analysis. Seafloor observations are modeled using the Monte Carlo technique. Estimates of the smoothed resistivity structure as well as the resolution and precision of the estimates are made using the Backus-Gilbert method. Models are shown to be severely data limited. Resolution is found to be poor in the upper 30-40 km of the lithosphere due to the lack of reliable data at periods shorter than 30 minutes. The uncertainty involved in estimating the magnetic field at the seasurface and the large error estimates combine to give low overall precision. The diurnal results do not agree with the continuum results if the continuum is corrected for latitudinal variations of the source field between the reference station and seafloor sites. Data at periods as short as 10 minutes are required to resolve structures in the upper 30 km of the mantle. Artificial source fields may be necessary to obtain periods short enough to resolve crustal features. Periods longer than diurnal will be required to study sub-lithospheric resistivity variations.

Description: Most of this work was supported through the National Science Foundation Grants GA 42651 and DES74-l2730, Office of Naval Research Contracts N00014-66-C-0241; NR 083-004 and N00014-77-C-0262; NR 083-004, and the Education Office of the Woods Hole Oceanographic Institution.

Description: Submitted in partial fulfillment of the requirements for Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2005

Description: A moored profiler record from the western tropical North Atlantic provides the first continuous time series of temperature, salinity and velocity profiles in a thermohaline staircase. Variations in the intensity of layering and the evolution of layer properties are well documented during the 4.3 month record. Such staircases are the result of strong salt fingering at the interfaces between the mixed layers, and these data provide unique insights into the dynamics of salt fingers. In particular, a striking linear correlation between the temperature and salinity of the layers may be interpreted as resulting from vertical salt finger flux divergences. Data from this record allow new interpretations of previous work on this topic by McDougall (1991).

Description: This research was supported by the National Science Foundation under grants OCE-0081502 and OCE-0350743

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February, 1983

Description: Altimetric, gravimetric and oceanographic data over the North Atlantic are combined -using techniques of optimum estimation- to infer the surface expression of the time averaged circulation (ζ) and to estimate the marine geoid (γ), both in the wavelength band 100 km-2000 km. Optimum inverse methods in geophysics are reviewed. They are then used to analyze the estimation of the geoid from gravity data, emphasizing the wavenumber spectrum of resolution functions. It is found that accurate bandpassed versions of the geoid can be recovered from restricted data sets. The accuracy and distribution of publicly available gravity data are shown to define an estimate γ whose expected errors, σγ, range between 30 and 260 cm, assuming the Wagner and Colombo (1978) spectrum describes the average geoid behaviour. The σγ underestimate the actual differences between 'y and an altimetric surface (s) derived from Seasat, but the spatial variation of σγ follows closely the differences s-γ. The discrepancy is attributable to a partial failure of the spectral model at short wavelengths. The differences s-γ are dominated by geoid error that masks much of the signal ζ. The main North Atlantic gyre emerges clearly only after the σγ and the simplest model for ζ -as a spatially uncorrelated process with (30 cm)2 variance- are taken into account. To obtain a corrected geoid, a hydrographic estimate of ζ is combined with sand γ, and their expected errors.

Description: NASA's research Grant NAG6-9 funded this work

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1978

Description: The design and operation of a unique flow measuring instrument for bottom boundary layer studies in the marine environment is documented. The effectiveness of the instrument in acquiring data with which models of near bottom flows in the ocean can be tested is demonstrated by the results of a field experiment in Vineyard Sound. The instrument uses four sensors which measure the mean and fluctuating parts of the three components of the velocity vector at four heights above the sea bed. The sensors employ the acoustic travel time difference technique, and are designed to minimize sensor-induced flow disturbances. BASS, an acronym for Benthic Acoustic Stress Sensor, has a resolution of .033 cm/sec per least bit, a range of ±62 cm/sec, noise of .07 cm/sec in 10 sec, and an estimated accuracy of ±.5 cm/sec, referred to an in situ zero point. A complete set of velocity measurements is made every .750 seconds, each measurement being the vector component averaged over 15 cm. The data is internally recorded on digital cassette tape. Eight hours of continuous data can be recorded. BASS was deployed in a tidal flow in Vineyard Sound at a depth of 10 m where a time series of u, v, and w velocities at 26 cm, 46 cm, 96 cm, and 210 cm above the bottom was recorded. The mean velocity was determined by fitting each 6 hour series with a sixth order polynomial and the deviations from the polynomial, the fluctuating velocity components, were correlated to produce Reynolds stress profiles. The stress series shows very few negative stress events while the dominant positive events have an average duration of 5 seconds and exceed 30 dynes/cm2. Zero offset was removed from the mean by assuming a log profile at maximum ebb. Deviations from a log profile developed when the current dropped below 40% of maximum, i.e., when the flow could no longer be considered steady. A break in the Reynolds stress profile at 1 m suggested a larger length scale than the 1 cm bottom roughness was present in the flow. A value of u* was determined by using the quadratic drag law (u* = 1.56 cm/sec), the log profile method (u* = 1.60 cm/sec), and the eddy correlation method (u* = 1.91 cm/sec). Integral length scales of 5 m cross-stream, and 2.5 m vertically were identified by correlation calculations. Two length scales were present in the downstream direction, 5 m within 1 meter of the wall and 8 m further from the wall.

Description: Support from the National Science Foundation is acknowledged.