bluuebunny/crossref_metadata_2025 huggingface dataset:
Evidence for a geomagnetic excursion recorded in the sediments of Lac St. Front, France: A link with the Laschamp excursion?
Authors: Vlag P. , Thouveny N. , Williamson D. , Rochette P. , Ben‐Atig F.
Published: February 2004

Anomalous paleomagnetic directions, accompanied by low intensities of natural remanent magnetization are recorded at 17 m depth in the sediments cored from Lac St. Front (Massif Central, France). The rock magnetic properties and the parameters characterizing anisotropy of susceptibility of samples taken from the sediment horizons carrying these anomalous directions, spanning some 30–40 cm, are identical to those of samples taken from the other stratigraphic intervals, carrying a record of normal geomagnetic secular variation. Therefore the sequence of anomalous directions is interpreted as a geomagnetic excursion. Furthermore, it is concluded that this excursion occurred at a time when the paleointensity field was low. The excursion is dated at about 33 14C (∼37 ka B.P.) by correlating the susceptibility and paleomagnetic logs from Lac St. Front with the corresponding logs from the nearby Lac du Bouchet sedimentary sequence. The excursional directions in Lac St. Front can be averaged out with a triangular window of 20 cm. These smoothed excursional directions resemble the directional variations observed in the corresponding Lac du Bouchet interval. This finding indicates that the absence of this excursion in Lac du Bouchet may be due to a larger smoothing of the geomagnetic signal.
The free‐air gravity constraint on subcontinental mantle dynamics
Authors: Pari Giovanni , Peltier W. Richard
Published: February 2004

An outstanding geophysical issue concerns the nature, and dynamical role in the mantle general circulation, of the seismically fast body wave anomalies that have been tomographically imaged beneath coatinents. In this paper, we investigate the possibilities that these seismologically imaged “roots” represent either neutrally buoyant, chemically distinct material or cold, negatively buoyant, upper mantle and transition zone downwelling flow. In assessing these alternatives, we first construct disaggregated models of the seismic heterogeneity in which a component associated with subcontinental fast anomalies is isolated from the global tomographic models either by employing the “continent function” or a new “craton function”. We find that the use of the new craton function leads to geophysically more realistic chemical models of subcontinental heterogeneity. The thermal and chemical density fields reconstructed from the disaggregated tomographic models are employed to compute the long‐wavelength nonhydrostatic geoid, the free‐air gravity field and the upper mantle radial flow pattern within the framework of an anelastically compressible internal loading theory. We find that the radial component of flow velocity provides useful insight into the dynamical implications of the alternative density models. However, since this field is not directly observable, we consider the geoid and free‐air gravity anomaly as possible diagnostic discriminants and show that the free‐air gravity anomaly provides a sensitive discriminant of the gravitational differences that characterize the chemical and thermal models, whereas the geoid does not. By focusing on the free‐air gravity low over the Hudson Bay region of Canada, we are able to rule out the hypothesis that positively or neutrally buoyant subcontinental material that is chemically distinct from the surrounding mantle exists below the Laurentian craton. However, when the fast body wave anomaly is mapped into a high‐density downwelling flow beneath this region, we are able to fully explain the fraction of the anomalous free‐air gravity low which is inexplicable as a contribution associated with the existing degree of glacial isostatic disequilibrium due to the disintegration of the Laurentide ice sheet. This conclusion concerning the North American craton may be equally valid for other continental nuclei. We explore the general tectonophysical implications of this dynamical model.
Cosmogenic production of 7Be and 10Be in water targets
Authors: Nishiizumi K. , Finkel R. C. , Klein J. , Kohl C. P.
Published: February 2004

We have measured 10Be (t1/2 = 1.5 × 106 years) and 7Be (t1/2 = 53.28 days) concentrations in water targets exposed for 1 to 2 years at Echo Lake, Colorado (elevation = 3246 m) and at La Jolla, California (140 m). Neutron monitor data were used to normalize the measured concentrations in order to calculate production rates equivalent to the cosmic ray flux averaged over four solar cycles (43 years). The 7Be production rates thus obtained correspond to 6.03 ± 0.07 × 10−6 atom g−1·O s−1 at Echo Lake and 5.06 ± 0.20 × 10−6 atom g−1·O s−1 at La Jolla. The 10Be production rates correspond to 3.14 ± 0.18 × 10−6 atom g−1·O s−1 at Echo Lake and 2.68 ± 0.47 × 10−7 atom g−1·O s−1 at La Jolla. When compared with 10Be production rates determined in 10Be‐saturated rocks from the Antarctic and with theoretical calculations based on meteorite and lunar sample data, we find that the million‐year average production rate is about 14–17% greater than the present production rate averaged over the last four solar cycles. Comparison with production rates determined by measuring glacially polished rocks from the Sierra Nevada in California indicates that average production (based on a revised 13,000‐year deglaciation age and a geographic latitude correction) is about 11% greater than the average over the last four solar cycles. The measured 10Be/7Be production ratio in oxygen is 0.52 ± 0.03 at Echo Lake and 0.55 ± 0.07 at La Jolla.
Correlation between shear wave upper mantle structure and tectonic surface expressions: Application to central and southern Germany
Authors: Passier M. L. , Snieder R. K.
Published: February 2004

We have inverted fundamental mode Rayleigh waveforms in the frequency range from 20 to 100 s to obtain a three‐dimensional S velocity distribution beneath central and southern Germany. The data come from 27 teleseismic events recorded at 17 broadband stations in Germany and adjacent areas. This results in 56 paths between pairs of stations, each pair lying on the same great circle with an epicenter. The phase differences between the waveforms recorded at a pair of stations have been inverted directly for the path‐averaged S velocity perturbation between the two stations. All path models were then combined into a single three‐dimensional model. The most prominent features of the obtained model are (1) low velocities beneath the Eger Graben, which are an expression of the deep‐reaching rift structure, (2) high velocities between 40 and 120 km depth beneath northern Bavaria, caused by an apparent thickening of the lithosphere toward the south, (3) high velocities in the uppermost mantle beneath the southern Rhine Graben area (presently inactive with respect to rifting), suggesting the absence of present upwelling hot mantle material, and (4) low velocities beneath the presently experiencing rifting northern branch of the central European rift system, including the northern part of the Rhine Graben and the Rhenish Massif west of the river Rhine with the volcanic fields of the Eifel. These reduced velocities indicate a broad updoming of the lithosphere‐asthenosphere boundary. Our model suggests a strong correlation between upper mantle S wave structure at depths down to 200 km and tectonic surface expressions.
Observed discrepancy between Geodolite and GPS distance measurements
Authors: Savage J. C. , Lisowski M. , Prescott W. H.
Published: February 2004

Comparison of contemporaneous measurements of 84 distances in the range of 10 to 50 km by both Global Positioning System (GPS) and Geodolite (an electro‐optical distance‐measuring instrument) indicates that the Geodolite measurements are systematically longer by 0.283 ± 0.100 parts per million of the measured distance. Quoted uncertainty is 1 standard deviation. This amounts to 11.3 ± 4.0 mm at 40 km, which is near the maximum Geodolite range. The systematic difference is within the random uncertainty of an individual GPS‐Geodolite comparison and was detected only from an analysis of a large number (84) of such comparisons. The source of the systematic difference has not been identified.
A lower crustal extension to a midcrustal magma body in the Rio Grande Rift, New Mexico
Authors: Schlue John W. , Aster Richard C. , Meyer Robert P.
Published: February 2004

A prominent and spatially localized phase arriving approximately 4 s after the initial compressional arrival, repeatedly observed on seismograms recorded in the central Rio Grande Rift from intermediate‐ and deep‐focus earthquakes at distances near 90°, is identified as a probable compressional to shear wave conversion from a lower crustal root of the Socorro midcrustal magma body (SMB). This phase is inconsistent with previously determined crustal models containing a simple sill‐like midcrustal magma body with a negligible conduit system. Finite element synthetic seismogram modeling suggests that a partially molten root with a western boundary dipping steeply to the east extends several kilometers from the 19‐km‐deep upper surface of the magma body down into the lower crust of the southwestern portion of the Albuquerque‐Belen basin. The location and geometry of this feature suggest that the intracrustal intrusion of mantle magmas responsible for the ongoing inflation of the SMB and associated seismicity is occurring via an off‐axis conduit system located beneath the active margin of the Albuquerque‐Belen basin.
Postrifting anelastic deformation around the spreading plate boundary, north Iceland: 2. Implications of the model derived from the 1987–1992 deformation field
Authors: Hofton M. A. , Foulger G. R.
Published: February 2004

A decade‐long rifting episode that began in the Krafla volcanic system, north Iceland, in 1975 caused substantial, regional, postevent anelastic deformation. This was modeled as viscous relaxation in an elastic/viscoelastic structure by Hofton and Foulger [this issue]. The results from modeling the deformation detected in north Iceland have far‐reaching implications both for local and regional processes and for the fundamental behavior of deformation around spreading plate boundaries in general. Tilt in the vicinity of the Krafla volcano fits the model well after 1988/1989 which suggests that the volcano magma chamber stopped inflating/deflating in 1988. A viscosity of 0.8 × 1018 Pa s was required to match the local tilt data, less than that predicted for north Iceland as a whole. Vertical motion measured using the Global Positioning System (GPS) 1987–1992 around the ice cap Vatnajökull is inconsistent with isostatic uplift. Using the elastic‐viscoelastic model to predict motion in other regions of Iceland suggests that the deformation effects of the Krafla episode are significant in many parts of Iceland and should be taken into account when modeling deformation there. Though not a realistic plate boundary model, interesting complexities of the elastic‐viscoelastic model are highlighted by deformation modeling of an infinitely long dike. This predicts that the amount of horizontal displacement close to the dike may exceed the amount of initial dike opening early in the spreading cycle. A more realistic approximation to the plate boundary in north Iceland, involving five overlapping segments that experience dike emplacements at discrete intervals, suggests that the width of the zone within which transient, time‐dependent deformation occurs may be several hundred kilometers wide, considerably wider than the neovolcanic zone. A kinematic approach to describing plate motions is not appropriate close to spreading plate boundaries and elsewhere where the viscosity of the Earth is low.
Synthetic seismicity models for the Wellington Region, New Zealand: Implications for the temporal distribution of large events
Authors: Robinson Russell , Benites Raphael
Published: February 2004

Our previous synthetic seismicity model for multiple interacting faults in a three‐dimensional half‐space has been extended and applied to the Wellington region, New Zealand, generating long catalogs of earthquakes for studying the effects of elastic interactions on the temporal distribution of large events. The region is one of oblique plate convergence with the interface between the subducting and overlying plates at an average depth of 22 km. Faults included, besides the subduction thrust, are segments of the four major arc‐parallel, strike‐slip faults overlying the plate interface. We have used 40 different models, with geometric and mechanical parameters chosen at random within reasonable ranges, to generate catalogs of 200,000 years duration each. For comparison, each model was rerun with the elastic interactions suppressed. Considering events of magnitude 7.2 or more (“characteristic” events in the sense that they rupture most of a fault plane), the number of short (<10 years) interevent times is higher than for the corresponding case with no interactions, for all models but one: the ratio ranges from 0.94 to 37.21 (9.46 average). For longer interevent times (10 to 250 years) the relative numbers are in the opposite sense. For still longer interevent times (>250 years), the relative numbers are again mostly higher. Experiments with simple models indicate that this pattern requires both interfault enhancement and inhibition of large events. Mutual enhancement occurs most often between the subduction thrust and the overlying strike‐slip faults and between the two segments of the Wellington fault that almost join end to end. Mutual inhibition mostly occurs between the subparallel strike‐slip faults.
Continental collisions in wide aspect ratio and high Rayleigh number two‐dimensional mantle convection models
Authors: Lowman Julian P. , Jarvis Gary T.
Published: February 2004

Distinct rigidly moving oceanic and continental plates of finite thickness are incorporated into a two‐dimensional numerical model of mantle convection. We investigate upper mantle convection in models having aspect ratios as great as 24 and compare our findings with the results of earlier studies which were limited to aspect ratio 4 models. In addition, we implement models of whole mantle flow by specifying high Rayleigh number convection and thinner nondimensional plates. We are thus able to compare the results of continental collision models which include similarly sized continents in the cases of upper and whole mantle convection. For each case considered we model a pair of identical continents being carried toward a site of plate convergence by underlying counterrotating mantle convection cells. Upon collision, the continents form a motionless, rigid, model supercontinent, while oceanic plate material continues to recycle through the mantle. Following the continental collision, our models of upper mantle convection exhibit a reorganization of the convective planform below the model supercontinent into a smaller wavelength mode which is unable to generate the net stress needed to break apart the continent; alternating compressive and tensile deviatoric stress associated with the small scale flow results in a low integrated stress. In contrast the large scale of whole mantle convection enables flow reversals to produce shear stresses acting in a common direction over extensive areas of the base of a continent, the integrated effect of which is capable of causing continental rifting. The conventional view of the role of thermal blanketing in continental rifting does not apply in the whole mantle convection scenario.
Growth model of the inner core coupled with the outer core dynamics and the resulting elastic anisotropy
Authors: Yoshida Shigeo , Sumita Ikuro , Kumazawa Mineo
Published: February 2004

We present a growth tectonic model of Earth's inner core and the resulting model of the seismic anisotropy. The inner core grows anisotropically if the convection in the outer core is of Taylor column type. The anisotropic growth produces a flow field of the poloidal zonal order 2 type as a result of the isostatic adjustment of the viscous inner core. Crystals in the inner core align themselves under the stress field produced by the flow. We model the anisotropic structure of the inner core, using the theory of Kamb [1959] and elastic constants of Stixrude and Cohen [1995b]. We consider models for both hcp iron and fcc iron, which are the probable crystal structures for the inner core iron according to Stixrude and Cohen [1995a]. We have found that the c axis for hcp iron and [111] direction for fcc iron align in the polar direction. The alignment is consistent with seismic observations, which have revealed that the P wave velocity is faster in the polar direction. Our model predicts that the degree of the alignment decreases near the inner core boundary in accord with recent body wave observations. The radial dependence of the alignment would result from the following three effects: (1) crystals near the surface have not undergone stressed state long enough to acquire anisotropy after precipitation, (2) stress near the surface is different from that in the interior of the inner core due to shear stress free boundary condition, and (3) partially molten structure results in transversely isotropic stress condition near the inner core surface due to compaction. Thus the application of Kamb's theory successfully explains the seismic anisotropy in the inner core provided that the crystals have been subjected under the same stress condition for the timescale of the order of 109 years.
Scale‐invariant topography and porosity variations in fluvial sedimentary basins
Authors: Pelletier J. D. , Turcotte D. L.
Published: February 2004

We consider a model of a floodplain evolving with channel avulsion, deposition, and erosion. Avulsion is modeled as a random process in space and time. Sediment transport is modeled by the diffusion equation (Culling's model). The power spectrum of variations in the topographic profiles predicted by the model S (k) is proportional to k−2 (where k is the wavenumber). This is the Brown noise often observed in topography. The power spectrum of variations in the local elevation in time is proportional to f−3/2 (where f is the frequency). The model prediction of Brown noise floodplain topography is roughly consistent with spectral analyses of microtopography measured with laser altimetry. We inferred Brown noise paleotopography by comparing the pair correlation function of showing wells in the Denver and Powder River basins with a synthetic oil field based on a caprock with Brown noise topography. Topographic control of variations in the grain size of deposited sediment suggests that porosity variations may exhibit the scale invariance predicted for the topographic profile. To illustrate vertical scale invariance in porosity variations, we computed the power spectrum of vertical porosity well logs in 15 offshore wells in the Gulf of Mexico. At spatial scales above 3 m we find an average power spectral exponent of −1.4, close to our model prediction of −1.5.
Low‐frequency fluctuations in the Indonesian throughflow through Lombok Strait
Authors: Arief Dharma , Murray Stephen P.
Published: February 2004

A significant component of the Indonesian throughflow, apparently about 25%, passes through the Lombok Strait. Direct observations in 1985 reported a ∼2 Sv annual average, with an annual cycle of amplitude ∼2 Sv. There are also significant fluctuations in this transport in the 0.01–0.1 cpd frequency band. Shallow pressure gauge data (sea level) inside the strait during the current meter observations were of limited use in explaining the large fluctuations in currents. Sea level data at Cilacap, 720 km west (upcoast), however, overlap the current observations for 5 months and correlate exceedingly well (r = 0.87) with the observations inside the strait at these frequencies. These sea level oscillations in the Indian Ocean force fluctuations in the Lombok throughflow that reach 50–70 cm/s, equivalent to 2–3 Sv. Lagged regression analysis indicates Cilacap sea level leads Lombok currents by 1–2 days, suggesting a low‐frequency, progressive wave. Simultaneous data in 1989 from four stations extending from the near‐equatorial station at Padang at 1°S to Benoa in the Lombok Strait (2000 km downcoast) clearly show the persistent propagation of low‐frequency waves of 20‐ to 40‐cm range along this coast. Lagged correlation on station pairs indicates a phase speed consistent with coastally trapped internal Kelvin waves. We speculate that further eastward progression of these waves to the Timor Passage of Ombai Strait will further modulate the throughflow. The forcing of these waves is not yet identified, but it appears likely that intraseasonal oscillations in the equatorial Indian Ocean winds, as demonstrated by Enfield [1987] for the Pacific, are a probable mechanism. Improved wind data quality in 1991 due to the assimilation of satellite data (special sensor microwave/imager) will allow investigation of remote forcing on more recent data sets.
Large‐scale sea level, thermocline, and wind variations in the Indonesian throughflow region
Authors: Bray Nancy A. , Hautala Susan , Chong Jackson , Pariwono John
Published: February 2004

The Indonesian throughflow is presumed to be driven by a sea level gradient from the Pacific to the Indian Ocean. Deep throughflow transport may also be driven by a steric gradient between the two basins. The sea level gradient, in turn, is thought to be maintained by the differing wind patterns in the two basins: monsoonal in the Indian Ocean and trades in the western equatorial Pacific. In the interaction between sea level, wind stress, and thermocline depth as identified from historical measurements, we find (1) over the Indian, Indonesian, and equatorial Pacific basins and specifically within the throughflow region, sea level, and thermocline seasonal variations are negatively correlated (sea level rise corresponding to thermocline deepening) and sea level and meridional wind stress are also correlated; (2) the expected strong seasonal gradients in sea level through the eastern throughflow region (near the island of Timor) are found, though without an accompanying thermocline depth gradient; (3) seasonal convergence in baroclinic, upper ocean throughflow transport previously identified [Meyers et al., 1995] in the Timor Sea is associated with changes in sea level as well as upper ocean dynamic height at annual period but not at semiannual; (4) interannual variability explains more of the sea level variance in the eastern throughflow region than is explained by seasonal harmonics; however, there does not appear to be a strong interannual signal in the sea level gradient to drive fluctuations in the upper ocean throughflow. We hypothesize that seasonal variability in the upper layer throughflow and interannual variability in the deep throughflow are the predominant results of the complex interaction of forcing mechanisms.
Stability at M2 critical latitude in the Barents Sea
Authors: Furevik T. , Foldvik A.
Published: February 2004

The Barents Sea is the site for one of the largest biomass productions in the world oceans. This is mainly due to the combined effect of winter thermohaline convection and a strong seasonal pycnocline during spring and summer. Secondary bloomings are due to wind‐induced entrainment of nutrients to the upper layer, which may be especially effective near 74.5°N, the critical latitude for the M2 constituent. At this latitude, the inertial frequency equals the M2 frequency, resulting in a strongly depth‐dependent tidal current and a thick benthic boundary layer. Our investigations of current meter measurements in the Barents Sea confirm these theoretical predictions. Statistical treatment of about 800 conductivity‐temperature‐depth stations between 70°N and 79°N reveals a stability minimum in the central part of the Barents Sea. This may be due to enhanced tidal mixing near the critical latitude. Observations of nutrients partly support this conclusion.
Observations of the vertical structure of the oceanic boundary layer
Authors: Weller Robert A. , Plueddemann Albert J.
Published: February 2004

The vertical structure of the surface layer of the ocean is examined at inertial and lower frequencies using observations made from the research platform FLIP in the late winter of 1990. The analysis isolates the shear supported by density gradients at the base of the mixed layer from that found within the layer. Near‐inertial frequency shear variability is most energetic at the base of the shallow, well‐mixed isothermal surface layer. At lower frequencies, significant shear is found within the surface layer. The wind‐driven transport is in good agreement with the Ekman transport and is divided almost equally between the surface layer and a less well mixed layer just below. The vertical structure of the wind‐driven flow within the surface mixed layer varies with frequency; it is slab‐like near the inertial frequency but strongly sheared at the lowest frequencies.
Development of seasonal pack ice in the Beaufort Sea during the winter of 1991–1992: A view from below
Authors: Melling Humfrey , Riedel David A.
Published: February 2004

Subsea sonars moored in the Beaufort Sea acquired a spatial section of draft across 941 km of sea ice during the winter of 1991–1992. These observations document the development of seasonal sea ice from an open sea surface in October to an average accumulation exceeding 2.5 m draft by early April. Initially, level ice occupied 85% of the profile, but continued ridging reduced this fraction to 50% in late winter. Three modes of level ice were tracked: that of seasonal ice whose growth was initiated at freeze‐up, and two others created during openings of the ice field in January and in March. The growth of these modes can be closely matched by calculations based on a slab model of ice formation forced using surface meteorological data. Initially, ridge keels were small and widely spaced. By midwinter, an exponential dependence of keel frequency on draft was obvious (e‐folding scale, 2.16 m), and the frequency and mean draft of keels had stabilized. The maximum keel draft was a modest 17.4 m. The low incidence of deep keels at all times is a statistically significant departure from the exponential dependence valid at lesser drafts. The truncation point of the exponential relation is related to the draft of the thinnest level ice present. In late winter, the formation of deeper keels from thick first‐year ice was apparently precluded by the presence of younger, thinner ice, which limited the force available for ridge building. Through calculation of the seaward transport of ice over the sonars, the total production of ice in the coastal flaw lead during 1991–1992 was determined to be about 60% of earlier indirect estimates. In general, the observations revealed winter ice conditions significantly less severe than those found on the periphery of the polar pack only a few tens of kilometers to the north, but more so than conditions in a marginal first‐year ice zone at the same latitude in Davis Strait.
A Lagrangian approach to the seasonal variation of salinity in the mixed layer of the Indonesian Seas
Authors: Miyama Toru , Awaji Toshiyuki , Akitomo Kazunori , Imasato Norihisa
Published: February 2004

We have investigated the seasonal variation of the salinity in the mixed layer of the Indonesian Seas from a Lagrangian viewpoint using the “particle‐tracking method.” Numerous labeled particles are tracked in a velocity field obtained using the robust diagnostic general circulation model with seasonal forcing. Salinities tagged to these particles are subsequently changed by freshwater flux through sea surface during their transit. Finally, the salinity in the “mixed layer” (upper 50 m) of each basin comprising the Indonesian Seas is calculated by monthly averaging the salinities of all particles located in the mixed layer of each basin. The seasonal salinity variation in the mixed layer obtained with this method shows similar features to that observed. Since this method has the advantage of clarifying the origins and histories of waters affecting the salinity variation in the mixed layer, a close examination of the result by the particle‐tracking method revealed the major factors causing seasonal salinity variation in each basin of the Indonesian Seas. Seasonal variation of the salinity in the mixed layer of the Indonesian Seas is determined by the seasonal variations of net air‐sea freshwater flux, three‐dimensional velocity field, and their combined effect.
Spectral reflectance of sea foam in the visible and near‐infrared: In situ measurements and remote sensing implications
Authors: Frouin Robert , Schwindling Myriam , Deschamps Pierre‐Yves
Published: February 2004

The spectral reflectance of sea foam was measured at the Scripps Institution of Oceanography Pier, La Jolla, California, by viewing the sea surface radiometrically in a region of breaking waves. Foam reflectance was found to decrease substantially with wavelength in the near‐infrared, contrary to the findings of previous studies, theoretical as well as experimental. Values in the visible (0.44 μm) were reduced by typically 40% at 0.85 μm, 50% at 1.02 μm, and 85% at 1.65 μm. The spectral effect was explained by the nature of the foam, which is composed of large bubbles of air separated by a thin layer of water (foam stricto sensu) and of bubbles of air injected in the underlayer. The presence of bubbles in the underlayer enhances water absorption and thus reduces reflectance in the near‐infrared. For ocean color remote sensing, affected by the presence of foam and aerosols, the consequences of neglecting the spectral dependence of foam are dramatic. With only a small amount of foam, in the presence of aerosols or not and thus irrespective of aerosol type, the errors in the retrieved water reflectance at 0.44 μm are above 0.01, which does not meet the accuracy goal of 0.001 for biological applications. Since under normal conditions the effect of foam may have the same magnitude as the effect of aerosols, atmospheric corrections will be inaccurate (and useless) in many cases, even taking into account the spectral dependence of the foam reflectance. Space observations potentially contaminated by an effective foam reflectance (product of reflectance and fractional coverage) above 0.001, i.e., corresponding to wind speeds above 8 m s−1, should be eliminated systematically. Utilization of near‐infrared wavelengths above 0.9 μm for atmospheric corrections of ocean color, possible with the moderate‐resolution imaging spectrometer (MODIS), would aggravate the problem. The measurements also indicated that foam significantly affects the retrieval of aerosol turbidity at 0.85 and 1.02 μm for wind speeds above 10 m s−1 but impacts minimally turbidity estimates at 1.65 μm. Over the oceans the spectral range above 1 μm is definitely recommended for remote sensing of tropospheric aerosol load and type from space.
Monsoon/El Niño‐Southern Oscillation relationships in a simple coupled ocean‐atmosphere model
Authors: Wainer Ilana , Webster Peter J.
Published: February 2004

A coupled ocean‐atmosphere model is used to investigate the equatorial Indian and Pacific Oceans' response to the seasonally varying monsoon winds and the relationship between monsoon variations and the El Niño‐Southern Oscillation phenomenon (ENSO). The atmosphere is a simple linear shallow water system driven by a mass source/sink term that is proportional to the sea surface temperature (SST) over the oceans and the heat balance over land. The ocean is modeled using the Anderson and McCreary [1985] reduced‐gravity transport model driven by atmospheric wind stress forcing and a parameterized heat flux. The model domain includes both the Indian and Pacific Oceans and land masses to represent Asia and Africa. Results show that variations in the model's monsoon circulation (evolving somehow from other influences) induce changes in the large‐scale circulation associated to ENSO. In this way the year‐to‐year differences in the monsoon impact on the longer‐period, coupled ocean‐atmosphere dynamics of the near‐equatorial Pacific basin. By changing the amplitude of the monsoon forcing the interval between ENSO‐like warm events varies, while the variability in the annual cycle over the Indian Ocean is hardly affected.
Seasonal evolution of sea ice cover and shelf water off Labrador simulated in a coupled ice‐ocean model
Authors: Ikeda M. , Yao T. , Yao Q.
Published: February 2004

The sea ice model developed by Hibler [1979] is coupled with the ocean general circulation model developed by Bryan [1969] and Cox [1984] to examine the seasonal ice cover over the Labrador Shelf as well as dynamic and thermodynamic roles of the shelf/slope water. The model covers the area between 40°N and 65°N, extends nearly to the European coast, and has realistic bottom topography. The model is initialized with the temperature and salinity compiled by Levitus [1982], with fresh and cold shelf water imbedded, and then forced by observed atmospheric data. Simulation is first carried out for individual ice seasons, i.e., 1970/1971 season for 6 months from December 1, 1970. Several case runs are carried out with uncertain parameters varied within their possible ranges. Interannual variabilities are examined in additional simulations for the 1968/1969 and 1971/1972 seasons. General characteristics of the seasonal ice growth and decay are duplicated in the model. However, some discrepancies are observed in the 1970/1971 simulation, including the maximum ice cover about 20 days earlier in the model than in the observation. The earlier start of ice decay is related to a rapid ice melting in March, which could be reduced by increasing water albedo. This modification is justified, because thin ice is forced to merge immediately with thick ice in the present two‐category ice model. Heat flux to the shelf area due to advection of the warm offshore water induces long‐term warming effects and is also responsible for the excess of ice decay in spring. The interannual variability among the light, medium and heavy ice seasons is well reproduced in the three runs for the 1968/1969, 1970/1971, and 1971/1972 seasons, respectively. The ice‐cover variability is related to that in the air‐sea heat flux. However, a major part of the heat flux variability is retained in the water, while the ice cover absorbs a minor part. The timing variability among the three seasons is duplicated in the simulations as well.
An improved algorithm for the retrieval of ocean wave spectra from synthetic aperture radar image spectra
Authors: Hasselmann S. , Brüning C. , Hasselmann K. , Heimbach P.
Published: February 2004

An earlier algorithm for retrieving two‐dimensional wave spectra from synthetic aperture radar (SAR) image spectra is improved by using a modified cost function and introducing an additional iteration loop in which the first‐guess input spectrum is systematically updated. For this purpose a spectral partitioning scheme is applied in which the spectrum is decomposed into a finite number of distinct wave systems. At each iteration step, the individual wave systems of the partitioned nth‐guess wave spectrum are adjusted to agree in mean energy, frequency, and direction with the corresponding mean values of the associated wave systems of the SAR‐inverted wave spectrum. The algorithm retrieves smooth wave spectra, avoiding the discontinuities which tended to arise in the previous algorithm in the transition region near the azimuthal wavenumber cutoff of the SAR image spectrum. The azimuthal cutoff of the SAR spectrum is also reproduced more accurately. The greatest improvement of the new retrieval algorithm is obtained when the discrepancies between the initial first‐guess wave spectrum and the observed SAR spectrum are large. In this case the additional updating loop for the input spectrum enables the retrieved spectrum to adjust such that the simulated SAR spectrum matches more closely the observed SAR spectrum. The overall correlation of a large set of simulated SAR spectra with the measured SAR spectra is found to be significantly higher than with the previous algorithm, indicating that the algorithm not only overcomes isolated shortcomings of the earlier algorithm but also yields retrieved wave spectra which are generally more consistent with the input SAR data. An additional practical advantage of the new algorithm is that it returns spectral partioning parameters which can be used in SAR wave data assimilation schemes.
Intensive surveys of the Azores Front: 2. Inferring the geostrophic and vertical velocity fields
Authors: Rudnick Daniel L.
Published: February 2004

The geostrophic and vertical velocity fields are inferred using density and horizontal velocity data from three SeaSoar/acoustic Doppler current profiler surveys of the Azores Front. The analysis is a two‐step procedure consisting of (1) objective analyses to reflect the observed length scales and (2) dynamical adjustments so that the density field is statically stable and the velocity field is in geostrophic balance. The vertical velocity is inferred using a version of the quasi‐geostrophic omega equation in which the stratification is allowed to vary in the horizontal. The resulting vertical velocity peaks at 2 × 10−4 m s−1 at 200–300 m. There is a tendency for the denser water to the north of the front to be downwelled, while the warmer water to the south is upwelled. The implied heat flux exceeds 10 W m−2 near 100‐m depth, so the upper 100 m is warmed while the lower water column is cooled, tending to stratify the upper ocean. Ageostrophic horizontal flow is toward the dense side of the front at the surface. The dominant terms in the density balance are time rate of change and horizontal advection. The inferred circulation cells may be an important mechanism of subduction in the upper ocean.
Flow around a coastal bend: A model of the Santa Barbara Channel eddy
Authors: Oey Lie‐Yauw
Published: February 2004

A steady, equatorward wind stress is applied over a two‐layer ocean (infinitely deep lower layer) west of an otherwise straight meridional coast with a right‐angle bend. Initial (t ≈ 10 days) response consists of an equatorward current (Kelvin wave) that triggers a cyclone around the bend through viscous production and advection of vorticity, a process akin to eddy shedding in flows without rotation. The response at large times is governed by a Kelvin wave forced by the equatorward weakening of the (assumed positive) wind stress curl, which produces a poleward current near the coast. Application to the Santa Barbara Channel cyclone is discussed, and the cyclone‐formation process is further demonstrated with a three‐dimensional model with topography and stratification.
Temporal and spatial patterns in the Ross Sea: Phytoplankton biomass, elemental composition, productivity and growth rates
Authors: Smith Walker O. , Nelson David M. , DiTullio Giacomo R. , Leventer Amy R.
Published: February 2004

The temporal and spatial patterns of phytoplankton biomass, productivity, and particulate matter composition in the Ross Sea were assessed during cruises in January 1990 and February 1992. Biomass and primary productivity in the southern Ross Sea were greatest during mid‐January, with surface chlorophyll concentrations, particulate organic carbon levels, and integrated primary productivity averaging 4.9 μg L−1, 0.54 mg L−1 and 2.63 g C m−2 d−1, respectively. Comparable mean concentrations and rates for February were 1.1 μg L−1, 0.29 mg L−1, and 0.78 g C m−2 d−1 (decreases of 76, 46, and 70%, respectively), indicative of the scale of temporal changes. A distinct south‐north transition also was observed both in productivity and phytoplankton biomass, with the lowest values occurring in the northern Ross Sea. East‐west gradients in phytoplankton biomass and composition occurred within the southern Ross Sea. The areal productivity of the Ross Sea ranged from 0.15 to 2.85 g C m−2 d−1 and is among the highest found in the entire Antarctic. Carbon:chlorophyll ratios were uniformly high but were highest (150) in 1990 in the diatom‐dominated western Ross Sea. Surface growth rates were modest, averaging less than 0.2 day−1 during both seasons. We hypothesize that the marked seasonality in the region provides an environment in which net growth rates, although slow, are maximized through low loss rates and which allows biomass to accumulate in the surface layer. Furthermore, the temporal variations are quantitatively similar to the observed spatial variations. Therefore the dominant determinant of phytoplankton biomass and productivity at any one point on the Ross Sea continental shelf is the stage of the seasonal growth cycle.
Responses of the tropical Pacific to wind forcing as observed by spaceborne sensors and simulated by an ocean general circulation model
Authors: Liu W. Timothy , Tang Wenqing , Atlas Robert
Published: February 2004

In this study, satellite observations, in situ measurements, and model simulations are combined to assess the oceanic response to surface wind forcing in the equatorial Pacific. The surface wind fields derived from observations by the spaceborne special sensor microwave imager (SSM/I) and from the operational products of the European Centre for Medium‐Range Weather Forecasts (ECMWF) are compared. When SSM/I winds are used to force a primitive‐equation ocean general circulation model (OGCM), they produce 3°C more surface cooling than ECMWF winds for the eastern equatorial Pacific during the cool phase of an El Niño‐Southern Oscillation event. The stronger cooling by SSM/I winds is in good agreement with measurements at the moored buoys and observations by the advanced very high resolution radiometer, indicating that SSM/I winds are superior to ECMWF winds in forcing the tropical ocean. In comparison with measurements from buoys, tide gauges, and the Geosat altimeter, the OGCM simulates the temporal variations of temperature, steric, and sea level changes with reasonable realism when forced with the satellite winds. There are discrepancies between model simulations and observations that are common to both wind forcing fields, one of which is the simulation of zonal currents; they could be attributed to model deficiencies. By examining model simulations under two winds, vertical heat advection and uplifting of the thermocline are found to be the dominant factors in the anomalous cooling of the ocean mixed layer.
Surface water fCO2 in the eastern equatorial Pacific during the 1992–1993 El Niño
Authors: Wanninkhof Rik , Feely Richard A. , Chen Hua , Cosca Cathy , Murphy Paulette P.
Published: February 2004

The fugacity of CO2 in surface water (fCO2w) was measured in the eastern equatorial Pacific (EEP) during the boreal spring and fall of 1992 and in the spring of 1993. A prolonged El Niño occurred during this period with anomalously warm sea surface temperatures (SST) during the spring of 1992 and 1993. Correspondingly, the fCO2w values were lower than historical non‐El Niño values at the equator. However, the fCO2w in the spring of 1993 was up to 50 µatm higher than in the spring of 1992, despite similar SSTs. The trend is attributed to the slower response times of factors causing fCO2w decrease compared to rapid increase of fCO2w by upwelling of cold water with high carbon content and subsequent heating. During the fall of 1992, SSTs south of the equator were 5°C cooler than in the spring, which is indicative of vigorous upwelling of water with high CO2 content from below the thermocline. Decreases in fCO2w due to net biological productivity and gas exchange take of the order of months, causing the fCO2w levels during the spring of 1993 to be elevated compared to the spring of the previous year. Our data and data obtained in 1986 and 1989 along 110°W suggest that fCO2 maxima in the equatorial Pacific can be either associated with temperature minima or temperature maxima. Despite the multitude of factors which influence fCO2w, most of the variance can be accounted for with changes in nitrate and SST. A multilinear regression of fCO2w with SST and nitrate for the 1992 data has a standard error in predicted fCO2w of 10 µatm. Air‐sea fluxes of CO2 in the EEP were estimated to be 30% higher in the spring of 1993 and 10% higher in the fall of 1992 than in the spring of 1992.
Fractal dimension as an indicator for turbulent mixing in the thermocline
Authors: Wijesekera Hemantha W.
Published: February 2004

The complexity of small‐scale density fluctuations is investigated experimentally, with special emphasis on their scale‐similar (fractal) facets. Using empirical observations, it is shown that the fractal dimension of density at scales between 0.1 and 3 m is closely related to the turbulent kinetic dissipation rate ε, the local buoyancy frequency N, and the buoyancy Reynolds number ε/νN2 in the upper equatorial Pacific thermocline.
Frontogenesis with ageostrophic vertical shears and horizontal density gradients: Gulf Stream meanders onto the continental shelf
Authors: Mied Richard P. , Shen Colin Y. , Evans Thomas E. , Lindemann Gloria J.
Published: February 2004

This paper deals with frontogenesis in the presence of ageostrophic vertical current shears and horizontal density gradients. The problem has broad application to the situation encountered in tidal fronts and current system meanders, but specific focus here is on Gulf Stream meander crests and filaments that advance onto the continental shelf just north of Cape Hatteras. These occur typically every few days as Gulf Stream meanders progress northeastward through the South Atlantic Bight and past Cape Hatteras. We model the submesoscale evolution of the interface between the continental shelf water and these Gulf Stream features while they are on the continental shelf. We assume the region to be characterized by an initial condition consisting of a horizontal density transition region and an ageostrophic, surface‐intensified horizontal flow. The ensuing frontogensis process is modeled numerically with an f plane calculation employing the full nonlinear equations in the depth/cross‐front plane; flow is assumed out of this plane (along the front), but no variation of the flow in this direction is allowed. A pseudospectral model is employed using trigonometric functions in the horizontal and Chebyshev polynomials in the vertical. Many different scenarios are investigated by changing the width, shape, and relative positions of the density transition and velocity jet. In the majority of cases a propagating hydraulic jump is formed. Simultaneously, the initial surface jet evolves to a subsurface‐intensified jet while it weakens and ultimately changes directions. The presence of this strong velocity jet can substantially enhance the rate of jump formation or completely inhibit frontogenesis. Supporting analytical calculations are used to show that the presence of vertical ageostrophic shear can augment or oppose the usual frontogenesis mechanism present when the collapsing horizontal density gradient is acted on by the resulting convergent surface current. The outcome of the shear/density gradient interaction depends upon the position of each field with respect to the other. In the vicinity of the nose of the hydraulic jump for the cases investigated, the density is seen to have a qualitatively similar dependence upon the stream function in the translating frame, irrespective of the initial condition from which it evolved.
Properties of nonuniform grids used in ocean general circulation models
Authors: Treguier A. M. , Dukowicz J. K. , Bryan K.
Published: February 2004

Ocean general circulation models frequently use nonuniform grids, especially in the vertical direction. This paper clarifies the implications of using such grids on the consistency and accuracy of numerical schemes. It is emphasized that numerical schemes maintain their order of accuracy on a nonuniform grid provided the grid can be related to a smooth mapping. Additional metric terms appear in the truncation error, which should not be interpreted simply as a numerical diffusion.