bluuebunny/crossref_metadata_2025 huggingface dataset:
Fast photometry of flickering in discrete auroral arcs
Authors: McHarg Matthew G. , Hampton Don L. , Stenbaek‐Nielsen Hans C.
Published: February 2004

We present high speed photometric measurements of the auroral intensity of flickering aurora. These measurements reveal the existence of intensity modulations in the discrete aurora above the Nyquist frequency of standard 30 frame per second TV cameras. The intensity fluctuations observed are primarily below 80 Hz, although frequencies up to 180 Hz have been detected. Changes in the spectral characteristics from essentially band‐limited to broadband intensity fluctuations and then to narrowband fluctuations are seen in individual discrete arcs within a few minutes. The center frequency of the observed fluctuations also change during this period. Data obtained from a standard narrow field TV camera observing the same arc show that only the low ( ≈ 10 Hertz) narrowband modulations would be considered standard flickering aurora.
The 1996 Peru tsunamigenic earthquake: Broadband source process
Authors: Ihmlé Pierre F. , Gomez Juan‐Martin , Heinrich Philippe , Guibourg Sandrine
Published: February 2004

We investigate the spatio‐temporal complexity of moment release of the February 21, 1996 Peru earthquake (Mw 7.5). We use a non‐linear source tomographic technique, based on simulated annealing, to invert surface wave source spectra for the slip distribution on a gently dipping fault plane. The spectra (5–65 mHz) are obtained using an empirical Green's function (EGF) method applied to first and second orbit fundamental mode Rayleigh waves. Spectra are well fit by a 110 km bilateral rupture, subparallel to the trench, updip of the hypocenter, with a total moment of 2.0×1020 Nm. The non‐linear inversion reveals a 30×30 km² major slip patch south the onset, where rupture velocities attain 1.5–2.0 km/s. Inversions of teleseismic broadband P and SH waves (10–500 mHz) indicate little or no directivity, consistent with the surface wave data. The average source time functions for both the surface and body wave data are similar in shape and in duration (∼50 s). Part of the slip during the 1996 Peru event occurred in a region of reduced background seismicity, as was the case for the 1992 Nicaragua slow event, suggesting that the seismogenic potential of low seismicity regions near the trench should be globally reassessed.
Reappraisal of Antarctic seasonal sea‐ice at the Last Glacial Maximum
Authors: Crosta Xavier , Pichon Jean‐Jacques , Burckle Lloyd H.
Published: February 2004

We used modern analog technique applied to Antarctic diatoms to quantitatively reconstruct seasonal sea‐ice extent at the Last Glacial Maximum. Winter maximum sea‐ice limit occurred around 48°S in the Atlantic and western Indian sectors, around 55°S in the eastern Indian and western Pacific sectors, and around 58–60°S in the eastern Pacific sector of the Southern Ocean. Summer maximum sea‐ice extents during the last ice age and today are similar, which contradicts CLIMAP's findings. This implies a reduced summer albedo feedback of the Southern Hemisphere and a greater transfer of heat and moisture from the ocean to the atmosphere than shown by previous qualitative studies.
Possible evidences of earthquake lightning accompanying the 1995 Kobe Earthquake inferred from the Nojima Fault Gouge
Authors: Enomoto Yuji , Zheng Zhong
Published: February 2004

The Kobe earthquake of January 17, 1995, which had a magnitude of 7.2, was accompanied by earthquake lightning (EQL). The fault gouge near ground level at the Nojima fault, near where the EQL was witnessed, was highly lithified and anomalously mag‐netized. The characterization of the fault gouge and the mudstone near the fault suggests that the anomalies might have been induced by an intense EQL current which passed through the fault plane.
S‐wave attenuation in the Marmara Region, northwestern Turkey
Authors: Gündüz Horasan , Ayşe Kaşlilar‐Özcan , Aysun Boztepe‐Güney , Niyazi Türkelli
Published: February 2004

The frequency‐dependent attenuation of shear waves in the crust is estimated by using two different methods based on the rate of time decay of coda‐wave amplitude, Qc−1, and the rate of decay of the S‐wave to coda wave amplitude ratio with distance, Qs−1. The epicentral distances of selected earthquakes range between 20 and 110 km and magnitudes are around 3. The quality factors, Qc, are calculated at frequencies of 1.5, 3., 6., 12., and 24. Hz and lapse times of 40, 50, 60, and 70 s. Estimated Qc values show a dependence on frequency and lapse time. We observed the similarity in the frequency dependency of Qc=(41±1.07)f1.08±0.03 and Qs=(50±1.7)f1.09±0.05 for a lapse time of 50 s indicating that the attenuation mechanism for coda waves is similar to that of the direct S waves.
Deformation following the 1994 Northridge Earthquake (M=6.7), Southern California
Authors: Savage J. C. , Svarc J. L. , Prescott W. H. , Hudnut K. W.
Published: February 2004

Following the 1994 Mw=6.7 Northridge earthquake, a 65‐km‐long, north‐south array of 11 geodetic monuments was established across the rupture. The array was surveyed with GPS ten times in the 4.25 yr after the earthquake. Although there is evidence for modest nonlinear postseismic relaxation in the first few weeks after the Northridge earthquake, the deformation in the subsequent four years can be adequately described by constant station velocities. The observed S70°E velocity components are consistent with the deformation expected from steady strain accumulation on the San Andreas fault. The N20°E velocity components indicate that the southern Northridge fault block is moving almost as a unit N20°E with repect to the northern fault block, the motion being accommodated by a zone of convergence (width 20 km) at the north end of the Northridge rupture.
Comparison of dark pixels observed by VIS and UVI in dayglow images
Authors: Parks G. , Brittnacher M. , Elsen R. , McCarthy M. , O'Meara J. M. , Germany G. , Spann J.
Published: February 2004

Frank and Sigwarth [1997a] claim that the dark pixels observed in dayglow images obtained by the Earth sensor of the Visible Imaging System (VIS) are due to bombardment of Earth by 20 to 40 ton cosmic snowballs. We have independently studied the VIS data and compared the dark pixels from the VIS images to those obtained from the overlapping images from the Ultraviolet Imager (UVI). We find the occurrence distributions of the dark pixels, single and multiple, from VIS and UVI are nearly identical. The distributions also do not show any altitude dependence. A search for evidence of spacecraft “wobble” motion, whose presence would indicate that the source is external to the camera, has found that pairs of dark pixel clusters are uniformly distributed in orientation and no preference is observed in the wobble direction. Instrument artifacts as the source of the dark pixels is the most likely explanation for these results. The conclusion of this study is that neither VIS nor UVI provide any scientific evidence that the dark pixels are geophysical.
Bipolar electrostatic structures in the shock transition region: Evidence of electron phase space holes
Authors: Bale S. D. , Kellogg P. J. , Larsen D. E. , Lin R. P. , Goetz K. , Lepping R. P.
Published: February 2004

We present observations of intense, bipolar, electrostatic structures in the transition region of the terrestrial bow shock from the Wind spacecraft. The electric field signatures are on the order of a tenth of a millisecond in duration and greater than 100 mV/m in amplitude. The measured electric field is generally larger on the smaller dipole antenna, indicating a small spatial size. We compare the potential on the two dipole antennas with a model of antenna response to a Gaussian potential profile. This result agrees with a spatial scale determined by convection and gives a characteristic scale size of 2–7 λd. We interpret the observations as small scale convecting unipolar potential structures, consistent with simulations of electron phase space holes and discuss the results in the context of electron thermalization at strong collisionless shocks.
Olivine crystallographic control and anisotropic melt distribution in ultramafic partial melts
Authors: Jung Haemyeong , Waff Harve S.
Published: February 2004

In the upper mantle, physical properties (permeability, electrical conductivity, and seismic velocity) are highly dependent on the melt geometry. Both flat and curved melt interfaces exist in olivine‐basalt partial melts. To further understand anisotropic wetting behavior of partial melting within the mantle, crystal orientation studies have been conducted to determine the Miller indices of flat interfaces, wetted by the melt in olivine‐basalt partial melt samples. The samples were equilibrated at pressures of 1‐2 GPa, and temperatures of 1350–1400°C for 142–549 hours. Crystal orientation studies show that lower Miller indices of crystals are associated with wetted‐flat melt interfaces and melt films. Specifically, (010), (110), and (021) olivine faces in these studies were dominantly wetted, indicating that they may be potentially large contributors to permeability and seismic anisotropy in the upper mantle where partial melts occur with high degrees of preferred crystallographic alignment.
The solar wind alpha‐particle content as a clue for the origin of slow flows
Authors: Bavassano B. , Bruno R. , Pietropaolo E.
Published: February 2004

Some aspects of the variation of the alpha‐particle abundance in the solar wind are studied using Ulysses data. We focus on the first mid‐latitude phase of the mission, when the spacecraft intermittently enters the fast high‐latitude wind, and on the region around the first perihelion passage, when low‐latitude wind in conditions of low solar activity is observed. In particular, we look for the existence of a relationship between the alpha‐particle abundance and the solar wind velocity. No evidence of correlation is found for the midlatitude wind, while at low latitudes a positive correlation emerges. This difference may be indicative of a different relevance, in the two regions, of the processes that can be at the origin of low‐speed solar wind. At mid latitudes the low velocity of the wind would be mainly due to the topology of the solar magnetic field, with a large expansion factor. Near the equator this effect would become weaker and a dominant role would be played by source region features related to the alpha‐particle content.
Pre‐ and syn‐eruptive geochemistry of volcanic gases from Soufriere Hills of Montserrat, West Indies
Authors: Hammouya G. , Allard P. , Jean‐Baptiste P. , Parello F. , Semet M. P. , Young S. R.
Published: February 2004

Soufriere Hills fumaroles contained magma‐derived volatiles before and during the eruption initiated in 1995 but also preserved a typical and quite steady hydrothermal composition. Chemical changes due to increased boiling and a greater input of oxidizing magmatic gas occurred only at Galway's Soufriere, the most active fumarolic field. Hydrothermal buffering of the fumaroles has been favoured by their remote location (1–2 km) from the eruptive vents and by a preferential degassing of the uprising magma through intrusive conduits under the crater. High temperature (720°C) gas collected from the extruding lava dome in Feb. 1996 was chemically and isotopically representative of the magmatic gas stream. Its composition allows assessment of average eruptive fluxes of H2O, CO2 and HCl which require the degassing of only 2.5–3 times more magma than erupted.
Observations of hybrid seismic events at Soufriere Hills Volcano, Montserrat: July 1995 to September 1996
Authors: White Randall A. , Miller Angus D. , Lynch Lloyd , Power John
Published: February 2004

Swarms of small repetitive events with similar waveforms and magnitudes are often observed during the emplacement of lava domes. Over 300,000 such events were recorded in association with the emplacement of the lava dome at Soufriere Hills Volcano, Montserrat, from August 1995 through August 1996. These events originated < 2–3 km deep. They exhibited energy ranging over ˜ 1.5–4.5 Hz and were broader band than typical long‐period events. We term the events “hybrid” between long‐period and volcano‐tectonic. The events were more impulsive and broader band prior to, compared with during and after, periods of inferred increased magma flux rate. Individual swarms contained up to 10,000 events often exhibiting very similar magnitudes and waveforms throughout the swarm. Swarms lasted hours to weeks, during which inter‐event intervals generally increased, then decreased, often several times. Long‐duration swarms began about every two months starting in late September 1995. We speculate that the events were produced as the magma column degassed into adjacent cracks.
Seasonality of low‐frequency variability in early‐instrumental European temperatures
Authors: Shabalova Marina V. , Weber Susanne L.
Published: February 2004

The seasonality of low‐frequency temperature variability is studied by application of multichannel singular spectrum analysis to 7 long early‐instrumental European temperature records. Focus is on timescales longer than 50 years. We find that the temporal pattern of low‐frequency variability is clearly season‐dependent. Opposing winter/summer tendencies result in a weak variability on timescales longer than 50 yr in the annual‐mean data. Summer temperature variations seem to exhibit a preferential timescale in the range 60–80 years. An oscillation with this timescale is found to be significant against the red noise surrogates when analysing 4 long European paleo proxy records for summer temperature. The present results stress the necessity of using seasonally homogeneous datasets of paleo proxies for reconstructing low‐frequency variability patterns.
Simultaneous, dual‐point, in situ measurements of ionospheric structures using space tethers: TSS‐1R observations
Authors: Indiresan R. S. , Gilchrist B. E. , Basu S. , Lebreton J.‐P. , Szuszczewicz E. P.
Published: February 2004

First ever simultaneous, dual‐point, in situ measurements of natural ionospheric structures using widely spaced tethered sensors, flying in formation, were made during the reflight of the Tethered Satellite System (TSS‐1R) mission. A “target‐of‐opportunity” observation provided a direct comparison of structured ionospheric irregularity features at two altitudes near the South American geomagnetic equator at approximately 2000 hours local time and at an altitude of ≈300 km. With the TSS‐1R satellite and space shuttle separated by a vertical distance of 10 km, correlated plasma signatures detected by plasma instruments at each end indicated a strong eastward displacement in the irregularity features and possible growth of steepened features at the higher altitude. Observations made by a SUNDIAL ground station ionosonde located north of the flight path also indicated considerable spread‐F activity at the time. A SUNDIAL corrected ionospheric model indicated that the shuttle was flying near the F‐peak. Therefore, the strongly correlated in situ observations were most likely associated with irregularities in their early development or modulations near the F‐peak due to equatorial spread‐F (ESF). While the TSS‐1R system was not optimized for dual‐point in situ ionospheric measurements, and the tether break eliminated additional equatorial zone observations planned for later in the mission, the results reported here nevertheless indicate that vertically correlated plasma features can exist and can also have strong structural variations as a function of altitude. Such variations need closer examination in order to understand their effects on radiowave scintillation. These observations also demonstrated the feasibility of tethered sensors in the study of ionospheric irregularities using controlled vertical sampling.
Decadal climate oscillations in the Arctic: A new feedback loop for atmosphere‐ice‐ocean interactions
Authors: Mysak Lawrence A. , Venegas Silvia A.
Published: February 2004

A combined complex empirical orthogonal function analysis of 40 years of annual sea ice concentration (SIC) and winter sea level pressure (SLP) data reveals the existence of an approximately 10‐year climate cycle in the Arctic and subarctic. The cycle is characterized by a clockwise propagating signal in the SIC anomalies and a standing oscillation in the SLP anomalies, the latter being linked to a fluctuation between the two phases of the North Atlantic Oscillation. To describe the formation and evolution of the SIC and SLP anomalies associated with the cycle, a simple feedback loop is proposed.
Solar irradiance since 1874 revisited
Authors: Solanki S. K. , Fligge M.
Published: February 2004

We reconstruct the solar irradiance since 1874 employing an evolved version of previously published models and improved sunspot and facular data. A good correlation between reconstructed irradiance and Earth's global air temperature on time scales longer than the solar cycle is obtained and, in contrast to many earlier models, solar irradiance does not on average lag behind global temperature prior to 1975, although the exact time lag between the two quantities depends somewhat on details of the reconstruction. Since that epoch, however, air temperatures have increased by 0.2 K, whereas solar irradiance has risen a disproportionately smaller amount. Therefore, unless the influence of solar variability on Earth is very strongly non‐linear, at least this most recent temperature increase reflects the influence of man‐made greenhouse gases or non‐solar sources of natural variability.
On the role of an anisotropic gravity wave spectrum in maintaining the circulation of the middle atmosphere
Authors: Medvedev A. S. , Klaassen G. P. , Beagley S. R.
Published: February 2004

The spectral parameterization of gravity wave momentum deposition proposed by Medvedev and Klaassen has been incorporated into the Canadian Middle Atmosphere Model. Numerical experiments show that an anisotropic gravity wave source significantly improves the middle atmosphere circulation compared to an isotropic source. These results suggest that, at least in midlatitudes, the gravity waves primarily responsible for the momentum deposition at mesospheric heights are launched with observed phase velocities which are primarily in the same direction as the mean wind at tropospheric (source) heights.
A shock associated (SA) radio event and related phenomena observed from the base of the solar corona to 1 AU
Authors: Bougeret J.‐L. , Zarka P. , Caroubalos C. , Karlický M. , Leblanc Y. , Maroulis D. , Hillaris A. , Moussas X. , Alissandrakis C. E. , Dumas G. , Perche C.
Published: February 2004

We present for the first time an almost complete frequency coverage of a Shock Associated (SA) radio event and related phenomena observed on May 6, 1996 at 9:27 UT. It is observed from the base of the solar corona up to almost 1 Astronomical Unit (AU) from the Sun by the following radio astronomical instruments: the Ondřejov spectrometer operating between 4.5 GHz and 1 GHz (radiation produced near the chromosphere); the Thermopyles Artemis‐IV spectrograph operating between 600 MHz and 110 MHz (distance range about 1.1–1.4 R⊙ from sun center); the Nançay Decameter Array operating between 75 and 25 MHz (distance range about 1.4–2 R⊙); and the RAD2 and RAD1 radio receivers on the WIND spacecraft covering the range from 14 MHz to about 20 kHz (distance range between 3 R⊙ and about 1 AU). Observations at the Nançay Decameter Array clearly show that the SA event starts from a coronal type II radio burst which traces the progression of a shock wave through the corona above 1.8 R⊙–2 R⊙ from the sun center. This SA event has no associated radio emission in the decimetric‐metric range, thus there is no evidence for electron injection in the low/middle corona.
Effects of subglacial geothermal activity observed by satellite radar interferometry
Authors: Jónsson Sigurjón , Adam Nico , Björnsson Helgi
Published: February 2004

We use one day Synthetic Aperture Radar (SAR) interferograms from data of the Earth Remote Sensing Satellites ERS‐1 and ERS‐2 to study ice flow and uplift of two surface depressions within the Vatnajökull ice cap, Iceland. The ice cauldrons are created by melting at sub‐glacial geothermal areas. Meltwater accumulates in a reservoir under the cauldrons over 2 to 3 years until it drains in a jökulhlaup under the ice dam surrounding the reservoir. The ice surface in the depressions drops down by several tens of meters during these draining events but rises again, as ice flows into the depressions, until a jökulhlaup occurs again. Using SAR interferograms we quantify an uplift rate of about 2 to 18 cm/day within the jökulhlaup cycle varying with the surface slope of the depressions. The uplift rate is high during the first months after a jökulhlaup when the cauldron is relatively deep with steep slopes, but the uplift rate decreases as the cauldron is gradually filled. A simple axisymmetric model simulating the ice‐flow into one of the depressions describes quantitatively the filling rate of the cauldron and qualitatively the shape of the ice flow field. The best‐fit model has an ice flow law parameter A0 that is about one order of magnitude lower than typically estimated for temperate glaciers.
Improved optical properties of the Martian atmospheric dust for radiative transfer calculations in the infrared
Authors: Forget François
Published: February 2004

A simple synthetic model of the single scattering properties of the atmospheric Martian dust suitable for radiative transfer calculations in the infrared is presented. The model is built by modifying the long‐standing Toon et al. [1977] model in order to reproduce accurately the Mariner 9 observations of the 1971 dust storm, without concern to the actual dust composition. Compared to previous Martian dust models which were based on known Earth‐analog material, this model should allow more accurate radiative transfer calculations, until new data from the upcoming spacecraft missions becomes available. Integrated values suitable for broad‐band climate models are also provided.
CME geometry in relation to cosmic ray anisotropy
Authors: Bieber John W. , Evenson Paul
Published: February 2004

Strong enhancements of the cosmic ray anisotropy were observed before and during the January 1997 CME/magnetic cloud. From a multi‐station analysis of neutron monitor data, we conclude that “B × ∇n” drift is a primary source of CME‐related anisotropies for 5 GeV cosmic rays. Evolution of the cosmic ray density and density gradients is closely linked to magnetic properties of the ejecta, and provides information on the magnetic cloud and related features as they approach and pass Earth. Strong enhancement of the field‐aligned anisotropy was observed primarily during the 9 hours prior to shock arrival. If typical, this phenomenon should prove useful for space weather forecasting.
Bay of Bengal currents during the Northeast Monsoon
Authors: Hacker Peter , Firing Eric , Hummon Julia , Gordon Arnold L. , Kindle J. C.
Published: February 2004

Velocity and property observations were made during February and March 1995 as part of the World Ocean Circulation Experiment (WOCE) Hydrographic Program (WHP) expedition in the Indian Ocean. The observed circulation in the upper 300 m of the ocean during the northeast monsoon is compared to the output of a high‐resolution, 3‐layer, nonlinear model forced by European Center for Medium Range Weather Forecasting (ECMWF) winds. The data identify several new features in the Bay of Bengal: a shelf‐break coastal current in the northeast corner, an eddy and subsurface jet near the South Preparis Channel, and an eastward countercurrent extending from 80°E to the eastern boundary. The countercurrent transports high‐salinity surface water from the northwest Indian Ocean into the bay, and separates the historically observed North Equatorial Current into two currents with separate sources and water properties. The data also detail the spatial structure of the South Equatorial Current and Countercurrent, and show the Equatorial Undercurrent in the eastern half of the Indian Ocean. The model compares well enough with the data to suggest that such realistic models may provide a useful temporal context for the WHP snapshot.
Present‐day crustal motion in the Solomon Islands from GPS observations
Authors: Tregoning Paul , Tan Francis , Gilliland John , McQueen Herbert , Lambeck Kurt
Published: February 2004

Site velocities in the Solomon Islands from Global Positioning System measurements spanning two years provide direct evidence of active deformation between the Pacific Plate and the Solomon Arc block. Convergence is occurring at the San Cristobal Trench at a rate of ∼52±4 mm/yr, with no apparent local deformation occurring in the Australian Plate at a distance of ∼100 km from the trench. The islands of Guadalcanal and Makira are in a first approximation moving with the Pacific Plate although there is evidence of small but significant decoupling from the Pacific Plate of 14–23 mm/yr in a direction of 75–85°.
Models overestimate diffuse clear‐sky surface irradiance: A case for excess atmospheric absorption
Authors: Halthore Rangasayi N. , Nemesure Seth , Schwartz Stephen E. , Imre Dan G. , Berk Alexander , Dutton Ellsworth G. , Bergin Michael H.
Published: February 2004

Radiative transfer models consistently overestimate surface diffuse downward irradiance in cloud‐free atmospheres by 9 to 40% at two low altitude sites while correctly calculating direct‐normal Solar irradiance. For known systematic and random measurement errors and for realistic aerosol optical properties, the discrepancy can be resolved by a reduction in the vertical aerosol optical thickness (AOT) inferred from sunphotometric measurements by an average 0.02 ± 0.01 for 32 cases examined, together with a compensating increase in a continuum‐like atmospheric absorptance over the solar spectrum of ∼5.0% ± 3.0%. This phenomenon is absent at two high altitude sites, where models and measurements agree to within their mutual uncertainties. Examination of apparent AOT at several locations around the globe also indicates presence of such excess atmospheric absorption. The proposed absorption and corresponding reduction in AOT would have important consequences for climate prediction and remote sensing.
Abyssal circulation in the Argentine Basin
Authors: Smythe‐Wright Denise , Boswell Stephen
Published: February 2004

The abyssal water mass in the Argentine Basin, generically called Antarctic Bottom Water, is of southern origin. Its source is primarily Weddell Sea Deep Water (with characteristics of −0.7° to 0.2°C and salinity 34.65–34.68), which flows out of the Weddell gyre into the South Sandwich trench and continues north beyond the Falklands Escarpment into the Argentine Basin. The abyssal water cannot have originated in the Antarctic Circumpolar Current (ACC) since waters colder than 0.2°C are not associated with the ACC in the Drake Passage. Once the abyssal water has entered the Argentine Basin through gaps in the Falklands Escarpment or around the Islas Orcadas Rise it is generally assumed that it turns sharply to the west, in accordance with the Stommel and Arons [1960] theory of ocean circulation, and flows along the base of the Falklands Escarpment before flowing north along the continental slope as a deep western boundary current off South America. Results from the World Ocean Circulation Experiment (WOCE) All section, however, suggest that the newest most CFC‐rich water occurs on the eastern side of the Argentine Basin rather than the west. We therefore conclude that in addition to the western boundary current there is a more dominant anticlockwise circulation around the Zapiola Rise, which is likely related to the migratory pattern of the ACC.
Selective absorption of solar radiation and upper ocean temperature in the equatorial western Pacific
Authors: Sui C.‐H. , Li X. , Lau K.‐M.
Published: February 2004

The solar radiation absorbed by seawater in the upper 0.45 m within the Tropical Ocean‐Global Atmosphere (TOGA) Coupled Ocean‐Atmosphere Response Experiment (COARE) domain is estimated from the observed surface heat fluxes and subsurface temperature during the TOGA COARE Intensive Observation Period. On the average, more than 39% of the net surface solar irradiance is estimated to be absorbed within the first 0.45 m. This value is higher than previous estimates. The observed absorption is applied to modify the parameterized transmittance profile representing the COARE domain, which is bordered by the optical water types IA, IB, and II [Jerlov, 1968]. The original and modified profiles of solar irradiance are prescribed in a one‐dimensional ocean model to test their effects on the model responses to the observed surface fluxes. The modified profiles for the three types of water all produce better simulation results than the corresponding original profiles in terms of the diurnal and intraseasonal sea surface temperature amplitudes. The model results support our estimate that the near‐surface absorption of solar radiation in the COARE region is stronger than that suggested by the water types IA, IB, and II.
Separating baroclinic flow from tidally induced flow in estuaries
Authors: Li Chunyan , Valle‐Levinson Arnoldo , Wong Kuo Chuin , Lwiza Kamazima M. M.
Published: February 2004

A simple method is used to separate the tidally induced and density‐driven subtidal flows in a coastal plain estuary. This method is applicable to weak wind conditions and to systems with appreciable fortnightly variation of tidal amplitude. The baroclinic density‐driven motion is assumed to depend on the river discharge, which generates a horizontal density gradient, and is weakened by vertical mixing, which in turn depends on tidal forcing. The barotropic tidally induced motion is assumed to be a function of the tidal amplitude. By Taylor series expansions, two equations are obtained. These equations show the dependence of the tidally induced flow component on the tidal amplitude and the dependence of the density‐driven flow component on the ratio between river discharge and tidal amplitude, respectively. The method is applied to water velocity data obtained in the James River, Virginia, in October‐November 1996. The data cover two spring tidal cycles and two neap tidal cycles. The vertical structures, as well as the depth mean, of both tidally induced and density‐driven components of the subtidal flow are obtained. Results show that the tidally induced component has a predominant seaward flow in the channel and a landward flow over the shoals. The density‐driven exchange flow is seaward over the shoals and landward in the channel. These results are consistent with theoretical model results which show that the tidally induced component and density‐driven component compete against each other. The increased tidal mixing and tidally induced exchange flow during spring tides reduce density‐driven motion, which results in a weak net subtidal flow. In contrast, during neap tides, both the tidally induced flow component of the subtidal flow and tidal mixing are weak, and the tidally induced flow is overwhelmed by the density‐driven flow component, which results in a stronger subtidal flow. By extending the proposed method, we suggest that future studies use a least squares fitting technique to obtain an optimal estimate for the tidally induced and density‐driven subtidal flow components.
Long‐period lunar fortnightly and monthly ocean tides
Authors: Kantha Lakshmi H. , Stewart J. Scott , Desai Shailen D.
Published: February 2004

Long‐period lunar fortnightly (Mf) and monthly (Mm) tides in the global oceans are of particular interest to geophysicists because of their impact on the Earth's rotation, and they are of interest to oceanographers from the point of view of the response characteristics of the global oceans to low‐frequency forcing. Long‐period tides have long been quite controversial, but precision altimetry is providing an accurate means of measuring and modeling them in the global oceans. Here we describe Mf and Mm tides in the global oceans extracted from a 1° barotropic global hydrodynamic tidal model, which assimilates ocean tides estimated from cycles 10–130 of TOPEX altimetric data by an empirical tide model. The model results enable a better understanding of the energetics of these tides. Compared to the short‐period ocean tides, which dissipate a total of roughly 3490 GW of lunisolar gravitational energy, the dissipation rates for Mf and Mm, 0.369 and 0.023 GW, respectively, are quite insignificant from the point of view of the dissipation of tidal energy in the global oceans. Their energy content is also low, 0.381 and 0.049 PJ, respectively (compared to 580 PJ in short‐period tides). Also compared to the high quality factor Q values for short‐period tides (∼23 for semidiurnal and ∼13 for diurnal), the Q values for these tides are 5.9 and 6.2 for Mf and Mm, respectively. The corresponding unassimilated model Q value is 5.8 for both. These Q values correspond to a decay timescale of the order of a period and hence indicate that unlike its response to short‐period tidal forcing, the oceanic response to barotropic forcing at these low frequencies is like a heavily damped system. They are also consistent with the low values of Q (∼5.5) observed in the global oceans on timescales of several days. The relative potential energy to kinetic energy ratios for Mf and Mm tides (0.59 and 2.91 for assimilated and 0.83 and 3.20 for unassimilated, for Mf and Mm, respectively) are consistent with the dynamics of long‐wavelength Rossby waves. At amplitudes of ∼0.115 ms in universal time, UT1, for both Mf and Mm, the impact of these long‐period tides on the length of day fluctuations of the Earth is considerable.
Atmospheric boundary layer response to sea surface temperatures during the SEMAPHORE experiment
Authors: Giordani Hervé , Planton Serge , Benech Bruno , Kwon Byung‐Hyuk
Published: February 2004

The sensitivity of the marine atmospheric boundary layer (MABL) subjected to sea surface temperatures (SST) during the Structure des Echanges Mer‐Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE) experiment in 1993 has been studied. Atmospheric analyses produced by the Action de Recherche, Petite Echelle, Grande Echelle (ARPEGE) operational model at the French meteorological weather service assimilated data sets collected between October 7 and November 17, 1993, merged with the Global Telecommunication System (GTS) data. Analyses were validated against independent data from aircraft instruments collected along a section crossing the Azores oceanic front, not assimilated into the model. The responses of the mean MABL in the aircraft cross section to changes in SST gradients of about 1°C/100 km were the presence of an atmospheric front with horizontal gradients of 1°C/100 km and an increase of the wind intensity from the cold to the warm side during an anticyclonic synoptic situation. The study of the spatiotemporal characteristics of the MABL shows that during 3 days of an anticyclonic synoptic situation the SST is remarkably stationary because it is principally controlled by the Azores ocean current, which has a timescale of about 10 days. However, the temperature and the wind in the MABL are influenced by the prevailing atmospheric conditions. The ocean does not appear to react to the surface atmospheric forcing on the timescale of 3 days, whereas the atmospheric structures are modified by local and synoptic‐scale advection. The MABL response appears to be much quicker than that of the SSTs. The correlation between the wind and the thermal structure in the MABL is dominated by the ageostrophic and not by the geostrophic component. In particular, the enhancement of the wind on either side of the SST front is mainly due to the ageostrophic component. Although the surface heat fluxes are not the only cause of ageostrophy, the surface buoyancy flux Qb appears to be an important local source.
Global upper ocean heat storage response to radiative forcing from changing solar irradiance and increasing greenhouse gas/aerosol concentrations
Authors: White Warren B. , Cayan Daniel R. , Lean Judith
Published: February 2004

We constructed gridded fields of diabatic heat storage changes in the upper ocean from 20°S to 60°N from historical temperature profiles collected from 1955 to 1996. We filtered these 42 year records for periods of 8 to 15 years and 15 to 30 years, producing depth‐weighted vertical average temperature (DVT) changes from the sea surface to the top of the main pycnocline. Basin and global averages of these DVT changes reveal decadal and interdecadal variability in phase across the Indian, Pacific, Atlantic, and Global Oceans, each significantly correlated with changing surface solar radiative forcing at a lag of 0±2 years. Decadal and interdecadal changes in global average DVT are 0.06°±0.01°K and 0.04°K±0.01°K, respectively, the same as those expected from consideration of the Stefan‐Boltzmann radiation balance (i.e., 0.3°K per W m−2) in response to 0.1% changes in surface solar radiative forcing of 0.2 W m−2 and 0.15 W m−2, respectively. Global spatial patterns of DVT changes are similar to temperature changes simulated in coupled ocean‐atmosphere models, suggesting that natural modes of Earth's variability are phase‐locked to the solar irradiance cycle. A trend in global average DVT of 0.15°K over this 42 year record cannot be explained by changing surface solar radiative forcing. But when we consider the 0.5 W m−2 increase in surface radiative forcing estimated from the increase in atmospheric greenhouse gas and aerosol (GGA) concentrations over this period [Intergovernmental Panel on Climate Change, 1995], the Stefan‐Boltzmann radiation balance yields this observed change. Moreover, the sum of solar and GGA surface radiative forcing can explain the relatively sharp increase in global and basin average DVT in the late 1970's.
Observations of the Eastern Maine Coastal Current and its offshore extensions in 1994
Authors: Pettigrew Neal R. , Townsend David W. , Xue Huijie , Wallinga John P. , Brickley Peter J. , Hetland Robert D.
Published: February 2004

Cold surface temperatures, reflecting Scotian Shelf origins and local tidal mixing, serve as a tracer of the Eastern Maine Coastal Current and its offshore extensions, which appear episodically as cold plumes erupting from the eastern Maine shelf. A cold water plume emanating from the Eastern Maine Coastal Current in May 1994 was investigated using advanced very high resolution radiometer (AVHRR) imagery, shipboard surveys of physical and biochemical properties, and satellite‐tracked drifters. Evidence is presented that suggests that some of the plume waters were entrained within the cyclonic circulation over Jordan Basin, while the major portion participated in an anticyclonic eddy at the distal end of the plume. Calculations of the nitrate transported offshore by the plume show that this feature can episodically export significant quantities of nutrients from the Eastern Maine Coastal Current to offshore regions that are generally nutrient depleted during spring‐summer. A series of AVHRR images is used to document the seasonal along‐shelf progression of the coastal plume separation point. We speculate on potential causes and consequences of plume separation from the coastal current and suggest that this feature may be an important factor influencing the patterns and overall biological productivity of the eastern Gulf of Maine.
On the variability of Baltic Sea deepwater mixing
Authors: Axell Lars B.
Published: February 2004

Historical oceanographic data from the period 1964–1997 from two deep subbasins (the Gotland Deep and the Landsort Deep) in the Baltic Sea have been analyzed, by using a budget method on stagnant periods, with respect to vertical diffusion and vertical energy flux density in the deep water. It was found that the rate of deepwater mixing varied with the seasons, with higher rates in fall and winter compared to spring and summer. Further, according to the analyzed data, the downward flux density of energy available for vertical diffusion decreased with increasing depth in the Gotland Deep. In the Landsort Deep, however, the flux density increased somewhat, probably because of topographic concentration of the energy, before decreasing toward the bottom. Moreover, the vertical energy flux densities were compared with the expected flux density from the local wind. It is proposed that in the Gotland Deep, which is outside the coastal boundary layer, the observed deepwater mixing is dominated by the energy input from the wind via inertial currents and internal waves. In the Landsort Deep, however, which is within the coastal boundary layer, the expected flux density of energy from the local wind cannot explain the observed rate of work against the buoyancy forces. It is proposed that the active coastal boundary layer plays a central role in the transfer of energy to mixing processes in the deep water.
A model of the circulation on the outer Scotian Shelf with open boundary conditions inferred by data assimilation
Authors: Thompson Keith R. , Griffin David A.
Published: February 2004

The circulation on Western Bank is described using data collected in spring 1991 and 1992 as part of an interdisciplinary study of the early life history of Atlantic cod (Gadus morhua). The mean circulation over the crest of the bank is weak, of the order of a few centimeters per second, and is flanked to the south and west by an anticyclonic flow with a speed of about 10 cm s−1. A thermal wind calculation shows the mean circulation is due primarily to horizontal variations in the density field. The M2 tidal constituent is dominant and reaches speeds of about 20 cm s−1. The tidal residuals have a standard deviation of about 10 cm s−1 and characteristic time and length scales of several days and tens of kilometers, respectively. In order to help collect and interpret biological data from Western Bank, we developed a hydrodynamic model of the near‐surface flow that could be used operationally. The model is based on a decomposition of the flow into components driven by (1) local wind stress, (2) horizontal density gradients, and (3) flows through the open boundaries of the model. The wind‐driven component is calculated using a simple slab model driven by the observed wind. The other two components are estimated through the assimilation of observed bottom pressures, dynamic heights, and currents into the hydrodynamic model. The hindcast skill of the model is quantified by cross validation and shown to be higher than that of four simple, statistically based schemes.
The circulation of the subtropical South Pacific derived from hydrographic data
Authors: Tsimplis M. N. , Bacon S. , Bryden H. L.
Published: February 2004

The circulation and distribution of water masses of the subtropical South Pacific are described, based on salinity and temperature data from two trans‐Pacific zonal sections at 32°S and 17°S (World Ocean Circulation Experiment sections P6 and P21, respectively). A circulation scheme is found using an inverse model solved by singular value decomposition. Approximately 12 Sv (1 Sv = 106 m3 s−1) of Circumpolar Deep Water that is transported northward through the Southwest Pacific Basin, mainly in the deep western boundary current, is returned southward at intermediate depths. The zonally integrated flow near the surface is northward. The southward transport of the East Australian Current is estimated to be 26 Sv at 25°S and 43 Sv at 32°S. Assuming zero net mass transport, the derived southward heat fluxes are −0.17±0.25 PW and −0.04±0.25 PW at 17°S and 32°S, respectively. Thus there is an implied net loss of heat of 0.13 PW between the two sections, equivalent to an average heat loss to the atmosphere of 6.5±5.8 W m−2. When the net mass transport is assumed to be nonzero, in accordance with a westward Indonesian Throughflow from the Pacific to the Indian Ocean, the sign of the temperature transport changes but the meridional divergence remains essentially the same. The vertical distribution of the assumed net mass transport is crucial in determining the temperature flux. In particular, when the net mass transport is assumed to be 6 Sv above 250 dbar and 4 Sv between 250 and 1000 dbar, the temperature fluxes are directed northward with magnitudes of 0.33±0.25 PW and 0.44±0.25 PW for the 17°S and the 32°S section, respectively. The freshwater fluxes suggest that there is a net evaporation of 0.39 Sv over the region enclosed by the two sections, which is about twice as large as the climatological estimates.
Energy conversion in the Cape Verde Frontal Zone
Authors: Erasmi Wolfgang , Siedler Gerold , Onken Reiner
Published: February 2004

Mechanical energy terms are calculated from moored current meter data in the Cape Verde Frontal Zone (about 20°N, 25°W) and compared with those derived from a mesoscale model of this frontal region. The model is of the Bleck and Boudra [1981] type with isopycnal coordinates. An initially zonal jet, representing the Canary Current, is allowed to develop under the influence of baroclinic and barotropic instability processes. We find reasonable agreement in magnitudes, somewhat smaller in the model, and similar distributions in the vertical. This leads to the conclusion that the energy transfer terms from the model can be expected to be sufficiently close to reality. Determination of the transfer terms confirms that the energy transfer in the zone is dominated by baroclinic instability processes while barotropic instability is of minor importance. Average baroclinic instability energy transfer terms reach values of 2–3 μW m−3 in the pycnocline. Peak layer mean values are of the order 10 μW m−3. It is shown that the spatial distribution of active transfer regions is closely related to the structure of the transient eddy field in the frontal zone and that strong instability processes are restricted to the pycnocline.
An empirical algorithm for light absorption by ocean water based on color
Authors: Lee Z. P. , Carder K. L. , Steward R. G. , Peacock T. G. , Davis C. O. , Patch J. S.
Published: February 2004

Empirical algorithms for the total absorption coefficient and absorption coefficient by pigments for surface waters at 440 nm were developed by applying a quadratic formula that combines two spectral ratios of remote‐sensing reflectance. For total absorption coefficients ranging from 0.02 to 2.0 m−1, a goodness of fit was achieved between the measured and modeled data with a root‐mean‐square difference between the measured and modeled values for logl0 scale (RMSDlog10) of 0.062 (15.3% for linear scale, number of samples N = 63), while RMSDlog10 is 0.111 (29.1% for linear scale, N = 126) for pigment absorption (ranging from 0.01 to 1.0 m−1). As alternatives to pigment concentration algorithms, the absorption algorithms developed can be applied to the coastal zone color scanner and sea‐viewing wide‐field‐of‐view sensor data to derive inherent optical properties of the ocean. For the same data sets, we also directly related the chlorophyll a concentrations to the spectral ratios and obtained an RMSDlog10 value of 0.218 (65.2% for linear scale, N = 120) for concentrations ranging from 0.06 to 50.0 mg m−3. These results indicate that it is more accurate to estimate the absorption coefficients than the pigment concentrations from remotely sensed data. This is likely due to the fact that for the broad range of waters studied the pigment‐specific absorption coefficient at 440 nm ranged from 0.02 to 0.2 m2 (mg chl)−1. As an indirect test of the algorithms developed, the chlorophyll a concentration algorithm is applied to an independent global data set and an RMSDlog10 of 0.191 (55.2% for linear scale, N = 919) is obtained. There is no independent global absorption data set available as yet to test the absorption algorithms.