bluuebunny/crossref_metadata_2025 huggingface dataset:
Microearthquake Characteristics of a Mid‐Ocean Ridge along‐axis high
Authors: Kong Laura S. L. , Solomon Sean C. , Purdy G. M.
Published: February 2008

We report results from a 3‐week microearthquake survey of the segment of the Mid‐Atlantic Ridge axis near 26°N. The segment is centered on an along‐axis median valley bathymetric high that includes the site of the TAG hydrothermal field. The seismic network, consisting of seven ocean bottom hydrophones and two ocean bottom seismometers, spanned the median valley inner floor and eastern valley wall. Hypocenters were determined for 189 earthquakes, with good resolution of focal depth obtained for 105 events. Almost all events occurred at depths between 3 and 7 km beneath the seafloor, with earthquakes occurring at shallower depths (less than 4 km) beneath the along‐axis high. No events were detected in the immediate vicinity of the hydrothermal field. The along‐axis high is the site of a midcrustal low‐velocity zone, significant attenuation of P wave energy, and high b values; the low‐velocity volume extends about 10 km south of the high to the vicinity of volcano within the axial neovolcanic zone. Fault plane solutions indicate high‐angle (or very low angle) normal faulting beneath the along‐axis high and the base of the adjacent western wall, reverse faulting beneath the axial volcano, and a more conventional normal‐faulting geometry for earthquakes beneath the eastern wall. The distribution of seismicity and the diversity of faulting styles suggest a spatially variable tectonic state for the ridge segment at 26°N. These variations are likely a signature of along‐axis differences in thermal structure and state of stress. We suggest that the low‐velocity volume beneath the along‐axis high is the site of a relatively recent crustal injection of magma. Continued cooling of the now largely solid but still hot intrusion, and associated thermal stress and fracturing in the immediately surrounding crust, can account generally for the distribution of areas of most intense earthquake activity, the diversity of observed faulting mechanisms, and the presence of the high‐temperature vent field. These results are supportive of the spreading cell model for segmentation of magmatism and thermal structure along a slowly spreading ridge.
Geometry and relative motion of the Philippine Sea Plate and Pacific Plate beneath the Kanto‐Tokai District, Japan
Authors: Ishida Mizuho
Published: February 2008

The recent acquisition of high‐density and improved seismic data by the Kanto‐Tokai (K‐T) Observational Network of the National Research Institute of Earth Science and Disaster Prevention requires the revision of conventional plate configuration models for the K‐T district. We propose a new interpretation of the configuration and relative motion of the Philippine Sea (PHS) plate, Pacific (PAC) plate, and Eurasian (EUR) plate on the basis of the distribution of hypocenters, velocity structure, and focal mechanisms in the K‐T district. The new model for the configuration of the PHS and PAC slabs clearly delineates the PHS slab subducting northwestward beneath the K‐T district from the Sagami and Suruga troughs and the PAC slab subducting westward beneath the PHS slab from the Japan trench. The thickness of the descending PHS slab is estimated to be 30 ± 5 km. The proposed configuration of the PHS slab is significantly different from those obtained by other researchers, including our own previous study. The new model is fully consistent with global plate motion data. The new model leads to an interpretation of the tectonic processes taking place beneath the K‐T district. For the Sagami trough wing of the PHS slab, the northeastern portion of the slab is dipping northwestward in direct contact with the upper surface of the PAC slab; the southwestern portion carrying the Izu Peninsula has been bent upward, because of its buoyancy, producing the observed contorted shape. For the Suruga trough wing, the dip direction of the PHS slab gradually changes from northwestward at the eastern portion to northward at the western portion. The new model also indicates the presence of offset within the PHS slab between the Sagami and Suruga trough wings. This area is characterized by a low level of seismic activity, the absence of a high‐velocity structure, and the presence of Quaternary volcanoes. It seems reasonable to speculate that the distribution of Quaternary volcanic edifice construction is controlled by the location of the offset within the PHS slab. In view of this new model, the tectonic processes of large earthquakes which occurred recently in the studied area are also reexamined.
Estimation of ground surface temperatures from borehole temperature data
Authors: Wang Kelin
Published: February 2008

To infer past climatic changes from temperatures measured in boreholes, one must obtain reliable estimates of ground surface temperature (GST) histories from these data. This paper presents a method that uses a Bayesian inverse technique to estimate the GST in the Fourier frequency domain. By assuming the a priori GST to be stationary, with a prescribed standard deviation and a cutoff period, the time series is constrained to be bounded and smooth. Because accounting for conductivity variations with depth is crucial to the estimation of GST, a layered Earth medium has been used, for which the forward analytical solution of one‐dimensional heat conduction is available. Borehole data from Flin Flon and Lac Dufault in Canada are inverted with this method. The Flin Flon hole, logged to a depth of 2900 m, provides an opportunity to study long‐term GST variations, and the estimated GST from this hole shows some effects of deglaciation at the end of Pleistocene. At Lac Dufault, similar GSTs were obtained independently from three holes ranging from 550 to 900 m, and hence the results are considered more reliable. In the Lac Dufault GSTs, there is a warm period centered about 1000 years BP and a cold period about 400 years B.P., confirming the presence of the Little Climatic Optimum and the Little Ice Age, respectively. The Flin Flon result does not show the Little Climatic Optimum, and the Little Ice Age occurs about 100–200 years earlier. However, in both locations, the GSTs show another brief cold period around the turn of the century followed by rapid warming until 1940–1950, in good agreement with the trend of northern hemisphere surface air temperatures.
Seismicity and geometry of a 110‐km‐long blind thrust fault 2. Synthesis of the 1982–1985 California Earthquake Sequence
Authors: Stein Ross S. , Ekström Göran
Published: February 2008

During 1982–1985, three 5.4 ≤ Mw ≤ 6.5 earthquakes migrated 65 km along the north half of a string of Quaternary folds at the east front of the California Coast Ranges. There is no surface fault associated with these earthquakes, but the fold chain, oriented parallel to the San Andreas fault 30 km to the west, takes up contraction normal to the San Andreas fault. Identification of active blind thrust faults is hindered by the absence of a fault trace but may be revealed by the presence of growing folds at the surface and by earthquakes at depth. Because of excellent seismic, geodetic, and geologic data, this earthquake sequence illuminates the geometry of the blind faults and enables us to probe their mechanics. We relocate seismicity and compute focal mechanisms in a three‐dimensional velocity space and model the vertical deformation associated with the 1983 M = 6.5 Coalinga earthquake. The aftershock zones abut at echelon offsets in the fold axes, and the mainshocks display reverse slip perpendicular to the axes, suggesting that the folds conceal a contiguous, segmented thrust fault. Background seismicity concentrates at bends and breaks in the fold chain, sites that may correspond to tears and ramps in the thrust fault at depth. Seismic reflection profiles reveal thrust and reverse faults dipping toward the San Andreas fault at depths of 5–10 km with several kilometers of cumulative slip and high‐angle reverse faults in the anticlines with several hundred meters cumulative slip. Coseismic fold uplift accompanied the Coalinga and Kettleman Hills North Dome earthquakes, suggesting that fold growth is episodic and coupled closely to repeated earthquakes on the underlying thrusts. The north half of the fold chain has been the site of several 6 ≤ M ≤ 6.5 earthquakes since 1885. Thus Kettleman Hills Middle Dome, the next fold segment to the south, has an elevated seismic potential. The aftershock zones of the three main shocks are diffuse and occupy a region much larger than the site of seismic slip. Aftershocks occur 5–7 km from the fault, regions where, on the basis of a boundary element model, the shear strain increase caused by the fault slip exceeds ∼20 ppm (equivalent to about 0.7 MPa). We argue that the broad aftershock zone is a product of high, sustained off‐fault stress caused by repeated displacement on faults that do not cut the Earth's surface. The fault tip stresses lead to the formation of secondary faults, which can become sites of aftershocks and postseismic creep.
Rapid resolution of crustal motion at short ranges with the global positioning system
Authors: Genrich Joachim F. , Bock Yehuda
Published: February 2008

On the basis of the analysis of Global Positioning System (GPS) data collected at the U.S. Geological Survey Kennedy Ranch House alinement array near Parkfield, California, in November 1990 and February 1991, we assess the temporal resolution of crustal deformation for GPS receiver systems operating as strain meters and provide guidelines for achieving millimeter‐level precision with short‐range, kinematic‐type GPS measurements. Site occupation of about one‐half hour is sufficient for day‐to‐day repeatability of submillimeter precision in relative position during good satellite geometry. Signal multipath effects which dominate dispersive baseline errors are essentially averaged out and need not be modeled. However, as occupation times decrease, baseline solutions are increasingly sensitive to multipath effects which together with nondispersive receiver measurement noise, are of millimeter to centimeter magnitude in epoch‐by‐epoch solutions. On repeatedly monitored baselines we can account for both of these noise sources separately. The mostly random receiver noise is effectively suppressed by low‐pass filters with corner frequencies of about 20 mHz. The multipath noise is filtered using the fact that it essentially repeats from day to day. Residual errors in filtered 1‐s epoch‐by‐epoch solutions are below 2 mm during periods of good satellite geometry. Hence for a continuously monitored baseline, actual deformation in excess of this threshold can be detected with a time lag comparable to the group delay of the low‐pass filter, i.e., typically within a minute. In terms of strain along a typical short baseline of 1 km sampled once per second we can detect 1–2 μs in l min with multipath modeling. Without multipath modeling, it takes 10 min to resolve the same magnitude of deformation. In 30 min we can detect 0.1 μs independent of multipath modeling. These results have important implications for kinematic‐type GPS surveys. For millimeter‐level precision without impractical multipath modeling it is necessary to occupy a site during good satellite geometry for a period longer than that of the multipath signature. Our experience in both benign and difficult multipath environments indicates that approximately 10 min are sufficient. The measurements at the Kennedy Ranch House alinement array suggest that this segment of the San Andreas fault appears to be virtually locked at the surface during the 3‐month period between the two surveys.
A global mean sea surface based upon GEOS 3 and Seasat altimeter data
Authors: Marsh J. G. , Koblinsky C. J. , Zwally H. J. , Brenner A. C. , Beckley B. D.
Published: February 2008

A mean sea surface relative to the International Union of Geodesy 1980 Geodetic Reference System reference ellipsoid has been derived from Seasat and GEOS 3 altimeter measurements. This surface, called MSS‐9012, has been computed on a grid of 1/8° resolution. Each elevation value was calculated by fitting all data within 111 km to a local biquadratic surface using Bayesian least squares. Individual data points were weighted inversely to the square of the distance to the grid location in the gridding process. The surface covers the global ocean between 70°N and 72°S. In the vicinity of sea ice the altimeter heights have been corrected for the on‐board tracker error that occurs over non‐Gaussian surfaces. Comparisons are made between MSS‐9012 and ocean bathymetry. The eastern extent of the Chain Fracture Zone in the Gulf of Guinea is more apparent in the altimetry than in the bathymetry data, as are many other features. The combination of data from the two satellites has successfully retrieved more information about the sea surface than was previously possible using only Seasat data.
Observations of the Mindanao Current during the western equatorial Pacific Ocean circulation study
Authors: Lukas Roger , Firing Eric , Hacker Peter , Richardson Philip L. , Collins Curtis A. , Fine Rana , Gammon Richard
Published: February 2008

The Western Equatorial Pacific Ocean Circulation Study (WEPOCS) III expedition was conducted from June 18 through July 31, 1988, in the far western equatorial Pacific Ocean to observe the low‐latitude western boundary circulation there, with emphasis on the Mindanao Current. This survey provides the first quasi‐synoptic set of current measurements which resolve all of the important upper‐ocean currents in the western tropical Pacific. Observations were made of the temperature, salinity, dissolved oxygen, and current profiles with depth; of water mass properties including transient tracers; and of evolving surface flows with a dense array of Lagrangian drifters. This paper provides a summary of the measurements and a preliminary description of the results. The Mindanao Current was found to be a narrow, southward‐flowing current along the eastward side of the southern Philippine Islands, extending from 14°N to the south end of Mindanao near 6°N, where it then separates from the coast and penetrates into the Celebes Sea. The current strengthens to the south and is narrowest at 10°N. Direct current measurements reveal transports in the upper 300 m increasing from 13 Sv to 33 Sv (1 Sverdrup = 1 × 106 m3 s−1) between 10°N and 5.5°N. A portion of the Mindanao Current appears to recurve cyclonically in the Celebes Sea to feed the North Equatorial Countercurrent, merging with waters from the South Equatorial Current and the New Guinea Coastal Undercurrent. Another portion of the Mindanao Current appears to flow directly into the NECC without entering the Celebes Sea. The turning of the currents into the NECC is associated with the Mindanao and Halmahera eddies.
Synthetic aperture radar imaging of upper ocean circulation features and wind fronts
Authors: Johannessen Johnny A. , Shuchman Robert A. , Johannessen Ola M. , Davidson Kenneth L. , Lyzenga David R.
Published: February 2008

Airborne C band synthetic aperture radar (SAR) data were collected during the Norwegian Continental Shelf Experiment (NORCSEX) carried out in March 1988. Unique signature variations in the SAR backscatter due to upper ocean circulation features and wind fronts were found. It is documented that a current jet of 0.3 m s−1 with a shear of about 0.3 × 10−3 s−1 is correlated with a narrow, bright curved line in the backscatter. In comparison a rapid wind shift of 9 m s−1 resulted in a sharp transition from dark to brighter backscatter regions. The corresponding profiles of backscatter modulation across these features are expressed by a peak of about 2 dB in contrast to a steplike increase of 5–8 dB. This suggests that SAR image expressions of upper ocean circulation features and wind fronts can be distinguished and classified. On the basis of this classification, we attempt to quantify the dominating marine geophysical variables. This method for systematic interpretation of SAR images should be further validated with the use of airborne or satellite data such as from the first European Space Agency remote sensing satellite, ERS 1.
The Frontal Air‐Sea Interaction Experiment in perspective
Authors: Charnock H. , Businger J. A.
Published: February 2008

The Frontal Air‐Sea Interaction Experiment addressed an important aspect of the coupling between the boundary layers of the ocean and the atmosphere, concentrating as far as possible on the effect of oceanic fronts. An excellent data set has been obtained, which has led to a collected series of papers. The results presented in these papers are placed in the general perspective of our understanding of air‐sea interaction and of the larger‐scale processes in the ocean and atmosphere. Suggestions are made for further analysis of the FASINEX data set and for future research in this area.
Biological and geochemical signatures associated with the water circulation through the Strait of Gibraltar and in the western Alboran Sea
Authors: Minas Hans Joachim , Coste Bernard , Le Corre Pierre , Minas Monique , Raimbault Patrick
Published: February 2008

Previous hydrological investigations of the Gibraltar Strait have left some questions unanswered as regards the composition of the Mediterranean outflow into the Atlantic Ocean. In the present study, nutrient, oxygen, and chlorophyll distributions are used to characterize the main water masses flowing through the Strait of Gibraltar and the Alboran Sea, with particular reference to Levantine Intermediate Water (LIW) and Western Mediterranean Deep Water (WMDW). High chlorophyll a concentrations, reaching 3 μg L−1 at the subsurface, are observed in the northern part of the anticyclonic Alboran Sea gyre. The salinity subsurface minimum (≃36), located temporarily in the strait, corresponds to high nutrient concentrations (7–8 μg at. L−1). Diagram analysis shows that in the upwelling area off the Spanish coast, nutrients are largely transported into the photic zone, and that photosynthetic nitrate consumption actually takes place. Consequently, the formation of the Alboran Sea O2 extraminimum can be related to the enhanced new production. Most of the lowest O2 values (<4.00 mL L−1) also correspond to a slight nutrient maximum (PO4 and NO3). Comparative diagram analysis of biologically affected parameters (oxygen and nutrients) versus salinity as a conservative property confirms and expands results from the classical θ‐S analysis. Oxygen and silicate exhibit the best tracer qualities for the circulation pattern, demonstrating in particular that during the period of observations, water rich in LIW feeds the Mediterranean outflow.
An apparent wave height dependence in the sea‐state bias in Geosat altimeter range measurements
Authors: Witter Donna L. , Chelton Dudley B.
Published: February 2008

The sea‐state bias in Geosat altimeter range measurements expressed as a percentage of significant wave height (SWH) is examined as a function of SWH. The bias is shown to be approximately a fixed −3.5% of SWH for SWH smaller than about 4 m. For larger SWH, the bias decreases in magnitude approximately linearly to a value of about −1.5% of SWH for 6 m SWH. Because of this “saturation effect” associated with large SWH, estimates of the Geosat sea‐state bias which have been based on the assumption of a fixed percentage of SWH are too small by up to 1.5% of SWH. The saturation effect is so large at high latitudes in the southern hemisphere (where about 1/4 of the SWH values exceed 4 m) that it apparently overwhelms any wind speed dependence in the sea‐state bias suggested by several recent studies. The apparent SWH dependence found in this study appears to be the result of attitude and sea‐state errors in the Geosat altimeter on‐board tracking algorithm.
Medium‐ to large‐scale atmospheric variability during the Frontal Air‐Sea Interaction Experiment
Authors: Davidson K. L. , Boyle P. J. , Gautier C. , Hanson H. P. , Khalsa S. J. S.
Published: February 2008

Shipboard, aircraft, and satellite atmospheric data are examined to determine the representativeness of the Frontal Air‐Sea Interaction Experiment (FASINEX) intensive measurement period, Phase II, in terms of climatology, atmospheric forcing, the general structure of the marine atmospheric boundary layer (MABL), and variability in boundary layer properties affecting air‐ocean dynamics and thermodynamics. With regard to climatology, conditions observed during the intensive period were typical in terms of air‐sea temperature differences, surface pressure patterns, cloud cover, and storm tracks. Storm system variabilities, such as the air temperature behind cold fronts, wind stress maxima occurring after frontal passage, the times for clockwise vector wind shifts associated with the frontal systems, and the systems' speeds, are estimated and emphasized in synoptic‐scale characterizations, since these can be related to observed ocean responses. The local ocean surface variability was observed to have an influence on regional boundary layer properties and on air‐sea interaction parameters even in the presence of the atmospheric storms.
A bio‐optical model of Antarctic sea ice
Authors: Arrigo Kevin R. , Sullivan Cornelius W. , Kremer James N.
Published: February 2008

Biogenic particulate material in sea ice can substantially influence the spectral irradiance within the ice sheet and underlying seawater. In order to simulate accurately seasonal changes in light conditions in situ, the biomass changes of the sea ice microbial community must be considered. Here we attempt to provide an improved description of the optical regime within sea ice by combining information provided by models of radiative transfer in sea ice and snow and models of solar spectral irradiance with formulations describing the attenuation of spectral irradiance by particulates observed in sea ice in McMurdo Sound, Antarctica. Emphasis has been placed on the role of biogenic particles in visible light attenuation with the intent of developing a bio‐optical model that more rigorously describes their influence on radiative transfer processes as they occur in nature. Model results simulating seasonal changes in both photosynthetically active radiation and its spectral distribution agree well with measured under‐ice spectral irradiance. Results reveal how changes in microalgal concentrations, as well as their photophysiological characteristics influence both the quantity and quality of downwelled light in sea ice and in the upper layers of the ice‐covered oceans.
Currents and water masses of the Coastal Transition Zone off northern California, June to August 1988
Authors: Huyer Adriana , Kosro P. Michael , Fleischbein Jane , Ramp Steven R. , Stanton Tim , Washburn Libe , Chavez Francisco P. , Cowles Timothy J. , Pierce Stephen D. , Smith Robert L.
Published: February 2008

In summer 1988, we made repeated mesoscale surveys of a grid extending 200 km offshore between 37°N and 39°N in the coastal transition zone off northern California, obtaining continuous acoustic Doppler current profiler data and conductivity‐temperature‐depth data at standard stations 25 km apart on alongshore sections 40 km apart. All surveys showed a baroclinic equatorward jet, with core velocities of >50 cm s−1 at the surface decreasing to about 10 cm s−1 at 200 m, a width of 50–75 km, and a baroclinic transport of about 4 Sv. The core of the jet lay between the 8.6 and 9.4 m2 s−2 contours of geopotential anomaly (relative to 500 dbar). Three current meter moorings, deployed at 25‐km separation across the jet at the beginning of the survey sequence, provided time‐series of the velocity; throughout the 37‐day deployment, at least one mooring was within the core defined by the 8.6 and 9.4 m2 s−2 contours. The jet flowed southwestward across the grid from late June until mid‐July 1988, when the jet axis moved offshore in the north and onshore in the southern portion of the grid. Temperature‐salinity analysis shows that jet waters can be distinguished from both the freshly upwelled coastal waters and the offshore waters. Isopycnal maps indicate alongshore advection of relatively fresh, cool water from farther north, as well as small‐scale patchiness not resolved by our survey grid. The baroclinic jet observed here may be continuous with the core of the California Current off central California. The later surveys clearly showed a poleward‐flowing undercurrent adjacent to the continental slope, with core velocities up to 20 cm s−1 at depths of 150–250 m. Its baroclinic transport (relative to 500 dbar) increased from <0.5 Sv to >1.0 Sv between late June and early August 1988. Within the survey grid, there was a definite onshore gradient in the characteristics of North Pacific Intermediate Water. The subsurface waters adjacent to the continental margin were warmer and more saline than those offshore, indicating net northward advection by the California Undercurrent over the inshore 100 km and equatorward advection farther from shore.
The Norwegian Continental Shelf Experiment prelaunch ERS 1 investigation
Authors: Johannessen Johnny A.
Published: February 2008

The Norwegian Continental Shelf Experiment (NORCSEX '88), a pre‐launch ERS 1 field investigation, was carried out during a 25‐day period in March 1988 on the continental shelf off the coast of Norway centered at 64°N with participation from Canada, France, Norway, the United States, and the Federal Republic of Germany. The overall goal was aimed at investigation of the capability of the first European Space Agency (ESA) remote sensing satellite (ERS 1) type active microwave sensors to measure marine variables such as near‐surface wind, waves and ocean surface current and their interaction in weather conditions ranging from moderate to extreme. The capability of the ERS‐I‐type active microwave instruments to sense these variables was investigated by combined use of the Canadian CV‐580 C band synthetic aperture radar (SAR), the U.S. Geosat radar altimeter, and a shipmounted scatterometer. Complementary in situ measurements of characteristic quantities in the lower atmosphere and upper ocean were simultaneously collected from research vessels, moorings, and drifting buoys.
Variations of zonal currents in the central tropical Pacific during 1970 to 1987: Sea level and dynamic height measurements
Authors: Taft Bruce A. , Kessler William S.
Published: February 2008

The annual and interannual variations of the major zonal currents of the central tropical Pacific for the period 1970–1987 are described by analyzing two data sets: sea level data from a few selected island stations and ship‐of‐opportunity expendable bathythermograph (XBT) observations. The annual cycles of geostrophic surface current indices (dynamic height or sea level difference between the ridge and the trough defining the current) in the North Equatorial Current (NEC) and North Equatorial Countercurrent (NECC) are found to be in phase (maxima in November), to be strong, and to explain about 50% of the total variance. The annual cycle of volume transport relative to 400 dbar in the NECC is also strong, but NEC volume transport has a weak annual cycle. The discrepancy between the NEC surface current and the transport is due to the variation of the surface current being larger than that of the deep currents. The annual cycle of the transport of the South Equatorial Current (SEC) is 6 months out of phase with that of the NEC and NECC. Phases of the annual cycles of the sea level and dynamic height indices of flow agree well in general, but there are significant differences between the annual amplitudes of the two types of measurements. These differences arise from a variety of sources: dynamic height reference level assumptions, longitudinal spacing of sea level gauges, meridional resolution of sea level array, and different signal to noise ratios for the two sets of indices. The NECC shows marked flow maxima during the peaks of the strong Niños of 1972 and 1982, which were followed by near disappearance of the current the following year; these fluctuations are clearly present in both the surface current indices and the volume transport. In 1976 and 1986–1987 (moderate Niños) the same pattern is not marked. The El Niño signal in NEC transport is weak. There was a strong rise of equatorial sea level during the early phases of the 1972 and 1982 Niños followed by a fall the next year. This resulted in a decrease of the transport of the SEC in the first year followed by an increase to a maximum during the second year; the maximum coincides with a minimum in NECC transport.
On the seasonal phytoplankton concentration and sea surface temperature cycles of the Gulf of Mexico as determined by satellites
Authors: Müller‐Karger Frank E. , Walsh John J. , Evans Robert H. , Meyers Mark B.
Published: February 2008

Monthly climatologies of near‐surface phytoplankton pigment concentration and sea surface temperature (SST) were derived for the Gulf of Mexico from multiyear series of coastal zone color scanner (CZCS) (November 1978 to November 1985) and advanced very high resolution radiometer (AVHRR) (January 1983 to December 1987) images. We complement these series with SST from the comprehensive ocean‐atmosphere data set (1946–1987) and Climate Analysis Center (1982–1990), and hydrographic profile data from the NOAA National Oceanographic Data Center (1914–1985). The CZCS ocean color satellite data provide the first climatological time series of phytoplankton concentration for the region. The CZCS images show that seasonal variation in pigment concentration seaward of the shelf is synchronous throughout the gulf, with highest values (>0.18 mg m−3) in December to February and lowest values (∼0.06 mg m−3) in May to July. Variation in SST is also synchronous throughout the gulf, with maxima in July to September and minima in February to March, The amplitude of the SST variation in the western gulf is about twice that observed in the eastern gulf, and SST maxima and minima persist longer in the west. Larger amplitudes in SST variation are also observed toward the margins. While annual cycles of SST and pigment concentrations are out of phase relative to each other, the phases of mixed layer depth change and pigment concentration change are similar. Model simulations suggest that the single most important factor controlling the seasonal cycle in surface pigment concentration is the depth of the mixed layer. The combined use of ocean color and infrared images permits year‐round observation of spatial structure of the surface circulation in the gulf and the pattern of dispersal of the Mississippi River plume. Infrared images are most useful between November and mid‐May, when strong SST gradients occur. During this time, pigment concentrations are high and can be horizontally homogeneous. In contrast, between late May and October, SST fields are uniform, but the Loop Current and large anticyclonic eddies could be traced with the CZCS. Three anticyclonic eddies were observed in 1979, and at least two were observed in 1980. No eddies were observed during summers of subsequent years in the CZCS time series, but this may be a result of the dramatic decrease in the satellite sampling rate. The series of color images showed that small parcels of Mississippi River water were frequently (2–4 times a year) entrained in the cyclonic edge of the Loop Current, stretched along the Current, and carried to the southeast along the western Florida shelf. However, most of the Mississippi River water flowed to the west, following the Louisiana‐Texas coast as far south as the Mexico‐United States border. Here, a persistent cyclone may reside, exporting shelf constituents to deeper regions of the gulf.
Use of simulated drifter tracks to investigate general transport patterns and residence times in the Coastal Transition Zone
Authors: Hofmann Eileen E. , Hedström Katherine S. , Moisan John R. , Haidvogel Dale B. , Mackas David L.
Published: February 2008

General transport patterns and residence times in the upper 100 m of the coastal transition zone (CTZ) were studied using simulated Lagrangian drifter experiments. The circulation fields used for the drifter experiments were obtained from a regional primitive equation model that incorporates coastal geometry and bottom topography that is representative of the CTZ region. The simulated circulation fields show velocity patterns that are consistent with those associated with the filaments and jets observed in the CTZ region. Drifters were released at 880 points in the model domain in a pattern that bracketed, both horizontally and vertically, the region in which offshore‐flowing filaments were observed to form. At each location, drifters were released at depths of 30, 60, 90 m and tracked for 30 days. The depth of these drifters varied along their trajectories in response to the vertical velocities experienced by the drifters. An additional set of drifters was released at 30 m and was constrained to remain at this depth. The composite drifter trajectories show that transport patterns and residence times in the upper 100 m are determined by the proximity of the drifter release point to the offshore‐flowing filament. Drifters released inshore of the filament are transported southward with the mean flow of the model California Current. Drifters released near the region in which the filament originates are transported rapidly offshore. The point at which this difference in drifter fate occurs is approximately 100 km offshore. The time required for a drifter to be transported from the nearshore region to the outer boundary of the area covered in the CTZ model domain is approximately 20 days. Comparison of the free‐floating drifters with the 30‐m fixed‐depth drifters shows that the deeper drifters are displaced southward as they are transported offshore in the jet. The magnitude of the southward displacement for the 90‐m drifters is about 15 km, implying substantial cross‐jet exchange. Comparison of the simulated drifter trajectories with the trajectory followed by a drifter released during the 1988 CTZ field sampling program support the hypothesis that the deepening of the abundance maxima observed in the distributions of Dolioletta gegenbauri, juvenile forms of Euphausia pacifica and Eucalanus californicus resulted from these populations being downwelled as they were transported offshore by the flow associated with the filament observed in 1988.
Interaction of short surface and electromagnetic waves with ocean fronts
Authors: Lyzenga David R.
Published: February 2008

The interaction of short surface waves with shearing and converging currents is investigated by means of an approximate analytical solution of the wave action spectral transport equation. Spectral perturbations of less than 10% are predicted at centimeter wavelengths for moderately strong ocean fronts, although much larger perturbations are expected at wavelengths of the order of 1 m. These results, when combined with a simple Bragg electromagnetic scattering model, do not explain the large backscatter variations observed at X band and C band in the vicinity of ocean fronts. A two‐scale electromagnetic scattering model provides an effective coupling between the long‐wave spectral perturbations and the radar backscatter at small incidence angles, but this mechanism becomes less effective at intermediate incidence angles. Observation of large backscatter variations at these angles may thus be an indication of additional hydrodynamic and/or electromagnetic scattering mechanisms that are not yet accounted for.
The nature of the cold filaments in the California Current system
Authors: Strub P. Ted , Kosro P. Michael , Huyer Adriana
Published: February 2008

Data from the Coastal Transition Zone (CTZ) experiment are used to describe the velocity fields and water properties associated with cold filaments in the California Current. Combined with previous field surveys and satellite imagery, these show seasonal variability with maximum dynamic height ranges and velocities in summer and minimum values in late winter and early spring. North of Point Arena (between 39°N and 42°N) in spring‐summer the flow field on the outer edge of the cold water has the character of a meandering jet, carrying fresh, nutrient‐poor water from farther north on its offshore side and cold, salty, nutrient‐rich water on its inshore side. At Point Arena in midsummer, the jet often flows offshore and continues south without meandering back onshore as strongly as it does farther north. The flow field south of Point Arena in summer takes on more of the character of a field of mesoscale eddies, although the meandering jet from the north continues to be identifiable. The conceptual model for the May‐July period between 36°N and 42°N is thus of a surface jet that meanders through and interacts with a field of eddies; the eddies are more dominant south of 39°N, where the jet broadens and where multiple jets and filaments are often present. At the surface, the jet often separates biological communities and may appear as a barrier to cross‐jet transport, especially north of Point Arena early in the season (March‐May). However, phytoplankton pigment and nutrients are carried on the inshore flank of the jet, and pigment maxima are sometimes found in the core of the jet. The biological effect of the jet is to define a convoluted, 100 to 400‐km‐wide region next to the coast, within which much of the richer water is contained, and also to carry some of that richer water offshore in meanders along the outer edge of that region.
Large‐scale response of the ocean to atmospheric forcing at low frequencies
Authors: Wunsch Carl
Published: February 2008

Estimates of the time‐dependent sea surface topography on large space (500–2500 km) and time (30 days to 2 years) scales from altimetry and tide gauge data are compared with estimates of the surface meteorological fields (winds and pressure) on the same scales. The comparisons are made in the North Pacific and North Atlantic. Despite the obvious remaining high noise level in the sea surface topography, there is a significant correlation between the atmosphere and the time‐varying ocean circulation. These relations are quantified using simple multichannel regression models. With no time lags, typically 50% of the sea level variance can be described by the wind curl and divergence and the pressure field. Some areas produce classical static inverted barometer effects, but others suggest an amplified response. The motions are believed to characterize most of the water column.
Deglacial meltwater discharge, North Atlantic Deep Circulation, and abrupt climate change
Authors: Keigwin L. D. , Jones G. A. , Lehman S. J. , Boyle E. A.
Published: February 2008

High‐resolution paleogeochemical data from the North Atlantic Ocean indicate that in the interval 15,000 to 10,00014C years before present (B.P.) North Atlantic Deep Water (NADW) production was decreased or eliminated four times: at about 14,500 (and probably older), 13,500, 12,000 and 10,500 years B.P. Each of these changes occurred at the same time as abrupt events of meltwater discharge to the surface ocean (inferred from oxygen isotope studies of planktonic foraminifera and from glacial geological studies on land). In addition, each of these times may be associated with brief episodes of cooler climate in the North Atlantic region, the best example of which is the Younger Dryas cooling of 10,500 years ago. These results support models linking meltwater discharge, decreased NADW production, and decreased North Atlantic heat flux.
Green's law revisited: Tidal long‐wave propagation in channels with strong topography
Authors: Jay D. A.
Published: February 2008

Green's Law states that tidal long‐wave elevation ζ and tidal transport Q vary with width b and depth h according to ζ ≌ b−1/2h−1/4 and Q ≌ b+1/2h+/4. This solution is of limited utility because it is restricted to inviscid, infinitesimal waves in channels with no mean flow and weak topography (those with topographic scale L ≫ wavelength λ). An analytical perturbation model including finite‐amplitude effects, river flow, and tidal flats has been used to show that (1) wave behavior to lowest order is a function of only two nondimensional parameters representing, respectively, the strength of friction at the bed and the rate of topographic convergence/divergence; (2) two different wave equations with nearly constant coefficients can be derived that together cover most physically relevant values of these parameters, even very strong topography; (3) a single, incident wave in a strongly convergent or divergent geometry may mimic a standing wave by having a ≡ 90° phase difference between Q and ζ and a very large phase speed, without the presence of a reflected wave; (4) channels with strong friction and/or strong topography (L ≪ λ) show very large deviations from Green/s Law; and (5) these deviations arise because both frictional damping and the direct dependence of |Q| and |ζ| on topography (topographic funnelling) must be considered.
Spatial and temporal variability of global surface solar irradiance
Authors: Bishop James K. B. , Rossow William B.
Published: February 2008

A fast scheme for computing surface solar irradiance using data from the International Satellite Cloud Climatology Project (ISCCP) is described. Daily mean solar irradiances from the fast scheme reproduce the detailed global results from full radiative transfer model calculations to within 6 and 10 W m−2 over the ocean and land, respectively. In particular, the fast scheme reproduces the same dependence of surface irradiance on solar zenith angle which is critical for proper calculation of daily, seasonal, and latitudinal variability. Validation of both model results is limited because globally distributed data sets of high quality are lacking, particularly over the oceans. However, comparison of calculated monthly mean results using 5 months of ISCCP data (July 1983 to July 1984) with climatology from the 1970s at six temperate latitude ocean weather stations shows agreement within published estimates of interannual variability of monthly means at the individual stations. A further test against a 17‐day time series at a continental site (43°N, 90°W, October–November 1986; 13–170 W m−2 range of irradiance), where ground and satellite data were spatially and temporally coincident, showed an accuracy of better than 9 W m−2 on a daily basis and less than 4% bias in the 17‐day mean. Frequently used bulk formulae for solar irradiance were also evaluated in each of these tests. All suffered in comparison because they did not include a parameterization of the effects of the global variability in mean cloud optical thickness. Data from July (1983 and 1984) and January (1984 and 1985) were used to examine the spatial and temporal variability of surface irradiance and its potential impact on biospheric processes. Results show that the oceans and land experience fundamentally different light regimes, with continents receiving significantly greater irradiance. In summer, major interocean differences in zonally averaged irradiance are found in the northern hemisphere with the Atlantic greater than Pacific by up to 80 W m−2; in the southern hemisphere, interocean differences are small. Regional interannual variability (July 1983 versus 1984) ranged between +100 and −100 W m−2. The variability, perhaps due to the 1982–1983 El Niño event, occurred mostly in the Pacific but extended beyond the tropics over the entire north Pacific basin. The nutrient‐rich northern and southern ocean waters are almost perpetually cloud covered; however, there is a correspondence between higher than average surface irradiance and productivity in nutrient‐rich areas of the southwest Atlantic and Weddell Sea sector of the circumpolar current. This suggests that solar irradiance must be considered as an important factor governing the productivity of these waters.
A one‐dimensional model of ice shelf‐ocean interaction
Authors: Jenkins Adrian
Published: February 2008

Large‐scale oceanic circulation beneath Antarctic ice shelves is driven by the thenmohaline differences which result from mass and energy exchange at the ice‐ocean interface. Dense, saline waters are drawn underneath the ice shelves and emerge, cooled and diluted, as plumes of Ice Shelf Water. A simple, one‐dimensional model of this process has been developed, in which the Ice Shelf Water plume is treated as a turbulent gravity current, initiated at the inland margin by a flow of fresh meltwater emerging from beneath the grounded ice. Subsequent evolution of the plume, as it ascends along an ice shelf base of specified geometry, can be simulated. The model has been applied to a flow line on Ronne Ice Shelf, Antarctica, to explain the observed distribution and rate of basal melting and freezing. Calculations indicate that the present mean melt rate of 0.6 m yr−1 would increase to 2.6 m yr−1 if the underlying water were to warm by 0.6°C. This would not only lead to significant thinning of the ice shelf but could also cause a profound change in ocean circulation on the open continental shelf.
Tracing lower North Atlantic deep water across the equator
Authors: Speer Kevin G. , McCartney Michael S.
Published: February 2008

Water whose ultimate source is the overflow at the Denmark Strait can be traced using maps of salinity, dissolved oxygen, and silica concentration on deep density surfaces south along the American continent to at least 5°S. Several property extrema whose resolution is at the limit of accuracy of the measurements occur on these density surfaces, which nevertheless may indicate areas of stronger mixing along the crest of the Mid‐Atlantic Ridge. Transport estimates for that part of the southward flow within 600 km of the western boundary near 10°N suggest that 10±3×106 m3 s−1 of Lower North Atlantic Deep Water moves south there. The total southward deep water transport near 10°N is estimated to be closed to 25×106 m3 s−1.
On the turbulent fluxes of buoyancy, heat and moisture at the air‐sea interface at low wind speeds
Authors: Godfrey J. S. , Beljaars A. C. M.
Published: February 2008

Under low‐wind, unstable conditions over water, the Liu, Katsaros and Businger (LKB) estimate of the buoyancy flux into the atmosphere has the form ν1/3 ΔB4/3 g(ξ), where 22.7ξ is the ratio of the Monin‐Obukhov length to the calm‐water roughness length; ΔB is the buoyancy difference between air just above the ocean surface and the ambient air above, and ν is viscosity. An analytic expression for g(ξ) is obtained, for low winds; g(ξ) has a minimum at about ξ=40 (corresponding over tropical oceans to wind speeds of about 0.3–0.6 m s−1). For lower values of x the assumptions underlying the LKB algorithm are no longer valid. For a reasonable choice of dimensionless constants, g(ξ) varies only by 8% over 40<ξ<200, being just above the minimum. Thus the LKB flux estimates are nearly independent of wind speed at their lower limit of applicability; it seems reasonable to identify this LKB minimum flux with laboratory observations of fluxes at zero wind. These show that the buoyancy flux is given by C* Pr−⅔ ν1/3 ΔB4/3, where measured values of C* are about 0.14±0.01 (Golitsin and Grachev, 1986). With Pr=0.71 for air, this implies that g(0) = 0.19 — well within the allowable range of values for the minimum of g(ξ). However, the minimum in g(ξ) varies as (γAT)1/3, where γ is the dimensionless constant in the Dyer‐Hicks‐Businger stability profile and AT the constant in the scalar roughness length for low winds; this can be used to fix AT. In field applications, buoyant motions in the planetary boundary layer ensure that r.m.s. wind speeds wG just above the surface are generally greater than the wind speed at the minimum of g(ξ), even for dry convection — though this would not be true if the planetary boundary layer were typically 100 m thick, instead of 1000 m. This “gustiness” is allowed for by replacing mean wind speed U by (U2+wG2)½ in the bulk formulae.