bluuebunny/crossref_metadata_2025 huggingface dataset:
Coupling between the California Current System and a coastal plain estuary in low riverflow conditions
Authors: Hickey B. M. , Zhang X. , Banas N.
Published: November 2002

Willapa Bay, a partially mixed coastal plain estuary, is located on the shoreward side of a narrow, deep continental shelf whose water properties fluctuate on several day scales in response to alternating periods of upwelling and downwelling. Hydrographic surveys as well as water property and velocity time series at a number of sites both within the estuary and on the adjacent coast are used to examine water property and circulation patterns in the estuary during a low runoff period. The data demonstrate that variability is significant (up to 3 psu, ∼2°C and 10 cm s−1 at 5 km from the estuary mouth) and that this variability is determined primarily by the variability in the coastal ocean rather than by estuarine processes such as changes in riverflow or neap‐spring variation in mixing. Density changes near the mouth of the estuary that result from upwelling or downwelling of coastal water are consistent with transmission to the estuary primarily through a gravity current mechanism, which modifies the along‐estuary density gradient and hence the gravitational circulation within the estuary. Tidal stirring is likely also important to the modification of estuary water properties. New water moves up the estuary at a rate on the order of 10 cm s−1. Associated Eulerian residual velocity fluctuations propagate up estuary about 50% faster than water properties, indicating that up‐estuary transmission of the ocean water perturbation may also have internal wave‐like characteristics. The modulations in estuarine circulation and water properties lag local wind stress fluctuations (hence upwelling or downwelling) by more than a day near the estuary mouth and several days farther up the estuary.
Oceanic nutrient and oxygen transports and bounds on export production during the World Ocean Circulation Experiment
Authors: Ganachaud Alexandre , Wunsch Carl
Published: November 2002

Large‐scale oceanic transports of nutrient and oxygen are estimated from selected hydrographic sections from the World Ocean Circulation Experiment spanning the world ocean. A so‐called geostrophic inverse box model is used to calculate the velocity field across the transoceanic sections. The circulation is required, a priori, to conserve mass, salt, top‐to‐bottom silicate, and subsurface heat and PO( = 170P + O2). The resulting estimate of the time mean circulation is used to compute horizontal and vertical nutrient transports and their residual sources and sinks associated with biogeochemical processes. Locally, the remineralization rate is, in general, below our uncertainty level, with magnitudes consistent with in situ measurements (0 ± 0.1mol N m−2 yr−1 to 0.7 ± 0.25 mol N m−2 yr−1 below about 100 m). Because of correlations between errors, the export production becomes significant when integrated globally, with 390 ± 240 kmol Si s−1 and 57 ± 40 kmol N s−1(420 ± 290 kmol C s−1) between 47°N and 30°S and at 2000 m. Examination of N* provides estimates for nitrate fixation and denitrification, either consistent (North Atlantic) or contradictory (Indian) with previous findings. Similarily, oxygen utilization rates and net air‐sea exchanges are provided. A net uptake of oxygen at high latitudes and an outgassing at low latitudes are found. Examination of the pseudotracer O* suggests an important role of dissolved organic matter in the oxygen budget. Oceanic oxygen and O* are found in global balance within uncertainties.
Experimental modeling of viscous inclusions in a circular high‐strain shear rig: Implications for the interpretation of shape fabrics and deformed enclaves
Authors: Piazolo Sandra , Passchier Cees W.
Published: November 2002

Deformation experiments with initially spherical and prolate viscous inclusions suspended in a viscous Newtonian matrix in a circular high strain annular shear rig provide insights on the shape development of inclusions in high strain shear zones during progressive deformation. Inclusions with a specific viscosity ratio with respect to the matrix material show distinct types of three‐dimensional shape development. For instance, at a high viscosity ratio between matrix and inclusion a pulsating ellipsoid develops, which both continuously rotates and changes its shape from a sphere to an ellipsoid and back to a sphere. The experiments show that the shape of an inclusion that has a viscosity different to the matrix material cannot be taken as a reliable indicator for the magnitude or type of finite strain. In naturally formed shear zones, in which strain rates are heterogeneous from boundary to center, viscosity contrast and power law flow will result in a large number of different shape developments trends for inclusions such as mafic and microgranitic enclaves, pebbles or mineral aggregates.
Three‐dimensional Kirchhoff migration: Imaging of the Jemez volcanic field using teleseismic data
Authors: Aprea Claudia María , Hildebrand Steve , Fehler Michael , Steck Lee , Baldridge W. Scott , Roberts Peter , Thurber Clifford H. , Lutter William J.
Published: November 2002

We computed a reflectivity image for the Earth beneath the Jemez volcanic field, New Mexico, using a novel adaptation of petroleum exploration seismic imaging. This image was obtained by applying the Kirchhoff wave field imaging method to digitally recorded teleseismic data. The volume imaged has a lateral extent of 30 km, extends to 45 km depth, and lies beneath the Valles caldera, to the west of Los Alamos, New Mexico. The derived picture of the Earth is a three‐dimensional (3‐D) map of the locations of impedance changes within the crust and upper mantle below the Jemez volcanic field, with a spatial resolution on the order of a kilometer. Significant features seen in the image include the base of the caldera fill; several reflectors in the crust we interpret to be associated with intrusions coming from the mantle and/or other crystallized chambers such as a low‐velocity zone seen in the tomographic image; two strong reflectors coincident with the crust‐mantle interface, and a zone of layered reflections at the base of the crust which may be basaltic underplating. This method proved to be a powerful tool for imaging the Earth's crust and mantle when combined with other methods, such as tomography.
The nature of orogenic crust in the central Andes
Authors: Beck Susan L. , Zandt George
Published: November 2002

The central Andes (16°–22°S) are part of an active continental margin mountain belt and the result of shortening of the weak western edge of South America between the strong lithospheres of the subducting Nazca plate and the underthrusting Brazilian shield. We have combined receiver function and surface wave dispersion results from the BANJO‐SEDA project with other geophysical studies to characterize the nature of the continental crust and mantle lithospheric structure. The major results are as follows: (1) The crust supporting the high elevations is thick and has a felsic to intermediate bulk composition. (2) The relatively strong Brazilian lithosphere is underthrusting as far west (65.5°W) as the high elevations of the western part of the Eastern Cordillera (EC) but does not underthrust the entire Altiplano. (3) The subcrustal lithosphere is delaminating piecemeal under the Altiplano‐EC boundary but is not completely removed beneath the central Altiplano. The Altiplano crust is characterized by a brittle upper crust decoupled from a very weak lower crust that is dominated by ductile deformation, leading to lower crustal flow and flat topography. In contrast, in the high‐relief, inland‐sloping regions of the EC and sub‐Andean zone, the upper crust is still strongly coupled across the basal thrust of the fold‐thrust belt to the underthrusting Brazilian Shield lithosphere. Subcrustal shortening between the Altiplano and Brazilian lithosphere appears to be accommodated by delamination near the Altiplano‐EC boundary. Our study suggests that orogenic reworking may be an important part of the “felsification” of continental crust.
Lithospheric structure and its relationship to seismic and volcanic activity in southwest China
Authors: Huang Jinli , Zhao Dapeng , Zheng Sihua
Published: November 2002

The Sichuan‐Yunnan region in southwest China is located in the boundary area between the active Tibetan Plateau to the west and the stable South China platform to the east. This region is characterized by complex Cenozoic structures and active seismotectonics. In this study, we have used over 30,000 arrival times from 1315 local earthquakes recorded by 172 seismic stations to determine a detailed three‐dimensional (3‐D) P wave velocity structure of the lithosphere down to 85 km depth in this region. We have taken into account the complex morphology of the Moho discontinuity to conduct the tomographic inversions, which leads to a better result than that with a flat Moho as in the previous studies. Our results show that large velocity variations of up to 7% exist in the crust and upper mantle in the Sichuan‐Yunnan region. The velocity image of the upper crust correlates with the surface geological features. The Sichuan basin is imaged as a prominent low‐velocity zone, while the Panzhihua mining district is imaged as a high‐velocity feature. Velocity changes are visible across some of the large fault zones, and the faults and some large crustal earthquakes seem to occur at the boundary areas between slow and fast velocity anomalies. Some of the faults, such as the Red River fault, may have cut through the crust and reached up to the upper mantle. Under the Tengchong volcanic area, strong low‐velocity zones are visible down to 85 km depth, with a lateral extent of about 100 km, suggesting the existence of magma chambers under the volcano. It is unclear how the Tengchong intraplate volcanism was generated. It may be related to the collision processes between the Indian plate, Burma microplate and the Eurasian plate, and the possible subduction of the Burma microplate under the Eurasian plate. Another possibility is that it was caused by the extensional fractures of the lithosphere and the upward intrusion of the hot asthenospheric materials. It is also possible that the Tengchong volcanism represents a hot spot with a lower mantle origin.
Observed variability of water properties and transports on the World Ocean Circulation Experiment SR1b section across the Antarctic Circumpolar Current
Authors: García Marc A. , Bladé Ileana , Cruzado Antonio , Velásquez Zoila , García Hernán , Puigdefàbregas Joan , Sospedra Joaquim
Published: November 2002

R/V Hespérides occupied the World Ocean Circulation Experiment SR1b repeat section across the Scotia Sea in February 1995, February 1996, and February 1998. On each cruise the same set of 21 hydrographic stations with characteristic spacing <20 nautical miles was visited. The results of the three surveys display a characteristic zonation of water masses in the region, which is defined by four hydrographic fronts. The net geostrophic transport of the Antarctic Circumpolar Current (ACC) across SR1b, computed with reference to the deepest common depth of each pair of adjacent stations, is similar in all three cruises, 144 Sv in February 1995, 131 Sv in February 1996, and 134 Sv in February 1998, and compares well with earlier computations of the ACC transport across Drake Passage. A close comparison of the vertical distributions of water properties on SR1b reveals changes in the structure of the ACC that are linked to undersampled current mesoscale activity, as suggested by infrared satellite images contemporary to the cruises. The most remarkable features are several “hydrographic discontinuities” observed in the Antarctic Zone south of the Southern ACC Front (SACCF), which are believed to be signatures of eddies produced east of the Shackleton Fracture Zone. The paper further addresses the different contribution of each ACC zonal band to the net geostrophic transport across SR1b on each Hespérides occupation.
Winter snow cover on the sea ice of the Arctic Ocean at the Surface Heat Budget of the Arctic Ocean (SHEBA): Temporal evolution and spatial variability
Authors: Sturm Matthew , Holmgren Jon , Perovich Don K.
Published: November 2002

The evolution and spatial distribution of the snow cover on the sea ice of the Arctic Ocean was observed during the Surface Heat Budget of the Arctic Ocean (SHEBA) project. The snow cover built up in October and November, reached near maximum depth by mid‐December, then remained relatively unchanged until snowmelt. Ten layers were deposited, the result of a similar number of weather events. Two basic types of snow were present: depth hoar and wind slab. The depth hoar, 37% of the pack, was produced by the extreme temperature gradients imposed on the snow. The wind slabs, 42% of the snowpack, were the result of two storms in which there was simultaneous snow and high winds (>10 m s−1). The slabs impacted virtually all bulk snow properties emphasizing the importance of episodic events in snowpack development. The mean snow depth (n = 21,169) was 33.7 cm with a bulk density of 0.34 g cm−3 (n = 357, r2 of 0.987), giving an average snow water equivalent of 11.6 cm, 25% higher than the amount record by precipitation gauge. Both depth and stratigraphy varied significantly with ice type, the greatest depth, and the greatest variability in depth occurring on deformed ice (ridges and rubble fields). Across all ice types a persistent structural length in depth variations of ∼20 m was found. This appears to be the result of drift features at the snow surface interacting with small‐scale ice surface structures. A number of simple ways of representing the complex temporal and spatial variations of the snow cover in ice‐ocean‐atmosphere models are suggested.
A physical model for the narrowing of the directional sea wave spectra in the short gravity to gravity‐capillary range
Authors: Caudal Gérard
Published: November 2002

This paper gives the description of a theoretical model of the sea wave spectrum for short gravity to gravity‐capillary waves. It accounts for wind forcing, viscous damping, dissipation due to white capping, and nonlinear resonant three‐wave interactions, which are expected to be dominating over the four‐wave interactions near the gravity‐capillary transition. It is based upon the approach used by Janssen et al. [1998] for their physical omnidirectional spectral model of the sea surface. It is shown that by adding the momentum balance equation to the energy balance equation, the approach used by Janssen et al. to produce their omnidirectional sea wave spectral model may be generalized to obtain a two‐dimensional sea wave spectrum. The azimuthal behavior of the inferred sea wave spectrum is examined and shown to be consistent with spectral anisotropy of the sea wave spectrum observed at decimeter to centimeter wavelengths.
Aerial observations of the evolution of ice surface conditions during summer
Authors: Perovich D. K. , Tucker W. B. , Ligett K. A.
Published: November 2002

During spring and summer, the Arctic pack ice cover undergoes a dramatic change in surface conditions, evolving from a uniform, reflective surface to a heterogeneous mixture of bare ice, melt ponds, and leads. This transformation is accompanied by a significant decrease in areally averaged, integrated albedo. The key factors contributing to this reduction in albedo are the melting of the snow cover, the formation and growth of the melt ponds, and the increase in the open water fraction. To document these changes and enable quantification of the evolution of the ponds throughout the melt season, a program of aerial photography was carried out at the main site of the Surface Heat Budget of the Arctic Ocean (SHEBA) program. A modified square pattern, 50 km on a side, surrounding the SHEBA site was flown at altitudes ranging from 1220 to 1830 m. Twelve of these aerial survey photography flights were completed between 20 May and 4 October 1998. The flights took place at approximately weekly intervals at the height of the melt season, with occasional gaps as long as 3 weeks during August and September due to persistent low clouds and fog. In addition, flights on 17 May and 25 July were flown in a closely spaced pattern designed to provide complete photo coverage of a 10‐km square centered on the SHEBA main site. Images from all flights were scanned at high resolution and archived on CD‐ROMs. Using personal computer image processing software, we have measured ice concentration, melt pond coverage, statistics on size and shape of melt ponds, lead fraction, and lead perimeter for the summer melt season. The ponds began forming in early June, and by the height of the melt season in early August the pond fraction exceeded 0.20. The temporal evolution of pond fraction displayed a rapid increase in mid‐June, followed by a sharp decline 1 week later. After the decline, the pond fraction gradually increased until mid‐August when the ponds began to freeze. By mid‐September the surface of virtually all of the ponds had frozen. The open water fraction varied between 0.02 and 0.05 from May through the end of July. In early August the open water fraction jumped to 0.20 in just a few days owing to ice divergence. Melt ponds were ubiquitous during summer, with number densities increasing from 1000 to 5000 ponds per square kilometer between June and August.
Mesoscale patterns in the Cape São Vicente (Iberian Peninsula) upwelling region
Authors: Relvas Paulo , Barton Eric D.
Published: November 2002

The coastal upwelling region near Cape São Vicente, the southwestern tip of the Iberian Peninsula where the southern zonal coast meets the meridional western coast, was studied using over 1200 advanced very high resolution radiometer (AVHRR) satellite images of sea surface temperature and time series of sea level height, wind velocities, and nearshore sea surface temperature recorded at coastal sites within 200 km of the cape. Summer upwelling is more intense and persistent off the western coast than off the southern coast, where a recurrent warm coastal countercurrent flows westward, and at times turns northward along the western coast after reaching the cape. In this region the equatorward current jet of cold water upwelled off the western coast is no longer bounded by a coast. Three preferred directions for the spreading of this water are identified. The most persistent is eastward along the southern shelf break and slope, possibly merging with waters previously upwelled locally, which becomes separated from shore by the coastal progression of the warmer counterflow. The second preferred direction results in the southward development of a cold filament feature fed by cold waters upwelled farther north and represents the southernmost extent of the intense coastal upwelling jet, which overshoots the cape. The least frequent feature to develop is a cold filament that grows westward at the latitude of the cape, appearing to result from the meandering of the equatorward jet. The coastal countercurrent is seen to interact with the equatorward jet at times of relaxation, not only by separating the cold upwelled water from the coast but, when it is energetic enough, breaking westward offshore through the equatorward cold flow and separating the eastward and southward cold features from the upcoast cold waters. Empirical evidence shows the presence of an alongshore pressure gradient, stronger in summer, driving the coastal progression of the warm counterflow. Wind forcing plays an important role in the circulation by augmenting or diminishing the effect of the preexisting alongshore pressure gradients. The extent of the progression of the warm coastal countercurrent along the southern and western coast is dictated by the the strength of the upwelling favorable wind stress, which is able to balance and reverse the alongshore flow, at least in the upper layers.
Tropical monsoons around Africa: Stability of El Niño–Southern Oscillation associations and links with continental climate
Authors: Jury Mark R. , Enfield David B. , Mélice Jean‐Luc
Published: November 2002

Interannual fluctuations of monsoons around Africa and the stability of associations with the El Niño–Southern Oscillation (ENSO) and African rainfall are studied. The statistical analysis employs sea surface temperature (SST), surface and upper winds, and surface pressure averaged over key monsoon areas of the tropical Atlantic and Indian Oceans. The time series span the period 1958–1998, and wavelet analysis is applied to localize relationships in time as well as in frequency and enable us to examine how the amplitude and time delay at interannual scales varies through the record. Comparisons are made with Niño3 SST and other known ENSO signals in the African hemisphere. It is found that upper zonal winds over the tropical Atlantic are an integral part of the global ENSO. Zonal winds are associated with SST changes in the equatorial east Atlantic, which are antiphase to those in the west‐central Indian Ocean. A composite analysis of warm and cool events in the Indian Ocean reveals that evaporation, radiative fluxes, and wind curl interact constructively. Anticyclonic curl (depression of isotherms) leads warm events, while cool events may initiate from oceanic advection and are sustained by evaporative fluxes. Rainfall fluctuations across Africa are analyzed, and three coherent areas are identified: West (Sahel‐Guinea), Southern (Kalahari‐Zambezi), and East (Kenya‐Tanzania). Multivariate regression algorithms are fitted to the continuous filtered rainfall series over the period 1958–1988. Using three monsoon indices in a multivariate model, about 40% of the variance is explained at zero lag. An influential variable for most African rainfall areas is the zonal wind over the tropical Atlantic. The north‐south SST gradient in the tropical Atlantic modulates rainfall in West Africa as expected. At 6 month lead, surface pressure in the north Indian Ocean is a key determinant for West African climate. For southern African rainfall, SST in the southwest Indian Ocean and monsoon indices in the west‐central Indian Ocean play significant roles. East African rainfall fluctuations are linked with zonal winds in the east Indian Ocean. The findings address current Climate Variability and Predictability program (CLIVAR) priorities for understanding how continental climate interacts with ENSO and other regional modes of variability.
Principal component analysis of Arctic solar irradiance spectra
Authors: Rabbette Maura , Pilewskie Peter
Published: November 2002

During the First International Satellite Cloud Climatology Regional Experiment Phase III (FIRE III) Arctic Cloud Experiment (ACE) and coincident Surface Heat Budget of the Arctic Ocean (SHEBA) campaign, detailed moderate‐resolution solar spectral measurements were made to study the radiative energy budget of the coupled Arctic Ocean–atmosphere system. The National Aeronautics and Space Administration (NASA) Ames Solar Spectral Flux Radiometers (SSFR) were used to measure moderate‐resolution solar spectral irradiance in the spectral range 380–2200 nm on board the NASA ER‐2 (flying at 21 km) and on the surface at the SHEBA ice camp. Principal Component Analysis (PCA) of SSFR spectra was used to quantify the variability in the data, to determine its causes, to assess our ability to separate cloud and surface effects, and to compare PCA in the Arctic with similar analysis of spectra obtained at a midlatitude continental site. For both the upwelling and downwelling spectra, nearly all of the variance was contained in the first six principal components (PC). The variability in the Arctic downwelling solar irradiance spectra was explained by a few fundamental components including cloud absorption, surface reflectance, molecular scattering, water vapor, and ozone. PCA of the SSFR upwelling spectra separated the influence of the ice surface from that of clouds.
Positive feedbacks between the Antarctic Circumpolar Wave and the global El Niño‐Southern Oscillation Wave
Authors: White Warren B. , Chen Shyh‐Chin , Allan Rob J. , Stone Roger C.
Published: November 2002

Atmospheric and oceanic teleconnections link the Antarctic Circumpolar Wave (ACW) in the Southern Ocean [White and Peterson, 1996] and the global El Niño‐Southern Oscillation (ENSO) wave (GEW) in the tropical Indo‐Pacific Ocean [White and Cayan, 2000], both signals characterized by eastward phase propagation and 3‐ to 5‐year‐ period variability. We extend the tropical standing mode of ENSO into the extratropics by regressing the Niño‐3 sea surface temperature (SST) index against sea level pressure (SLP) anomalies over the globe, finding the Pacific‐South America (PSA) pattern in SLP anomaly [Cai and Baines, 2001] straddling Drake Passage in the Southern Ocean. The amplitude of this PSA pattern is ∼1/3 that of the ACW in this domain and thus cannot be considered its principal driver. On the other hand, suppressing the tropical standing mode of ENSO in interannual ST (surface temperature) and SLP anomalies over the globe allows the GEW to be observed much more readily, whereupon its eastward phase propagation across the Warm Pool is found to remotely force the ACW in the eastern Pacific and western Atlantic sectors of the Southern Ocean through atmospheric teleconnections [Sardeshmukh and Hoskins, 1988] which propagate along with it. Subsequently, the ACW propagates this imposed GEW signal throughout the remainder of the Southern Ocean as a coupled wave in covarying ST and SLP anomalies, whereupon entering the Indian sector 1.5 to 2.5 years later it spawns a northern branch which takes another 1.5 to 2.5 years to propagate the ACW signal equatorward into the Warm Pool south of Indonesia. There it interferes constructively with the GEW. Thus the two forms of teleconnection, one fast and directed from the tropics to the high southern latitudes via the atmosphere and the other slow and directed from the high southern latitudes to the tropics via the ocean, complete a global circuit of 3‐ to 5‐year duration that reinforces both the ACW and GEW and influences the tropical standing mode of ENSO.
Tracer studies of pathways and rates of meltwater transport through Arctic summer sea ice
Authors: Eicken H. , Krouse H. R. , Kadko D. , Perovich D. K.
Published: November 2002

At the Surface Heat Budget of the Arctic Ocean (SHEBA) program's field site in the northern Chukchi Sea, snow and ice meltwater flow was found to have a strong impact on the heat and mass balance of sea ice during the summer of 1998. Pathways and rates of meltwater transport were derived from tracer studies (H218O, 7Be, and release of fluorescent dyes), complemented by in situ sea‐ice permeability measurements. It was shown that the balance between meltwater supply at the surface (averaging between 3.5 and 10.5 mm d−1) and ice permeability (between <10−11 and >10−9 m2) determines the retention and pooling of meltwater, which in turn controls ice albedo. We found that the seasonal evolution of first‐year and multiyear ice permeability and surface morphology determine four distinct stages of melt. At the start of the ablation season (stage 1), ponding is widespread and lateral melt flow dominates. Several tens of cubic meters of meltwater per day were found to drain hundreds to thousands of square meters of ice through flaws and permeable zones. Significant formation of underwater ice, composed between <30 and >50% of meteoric water, formed at these drainage sites. Complete removal of snow cover, increase in ice permeability, and reductions in hydraulic gradients driving fluid flow mark stage 2, concurrent with a reduction in pond coverage and albedo. During stage 3, maximum permeabilities were measured, with surface meltwater penetrating to 1 m depth in the ice and convective overturning and desalination found to dominate the lower layers of first‐year and thin multiyear ice. Enhanced fluid flow into flaws and permeable zones was observed to promote ice floe breakup and disintegration, concurrent with increases in pond salinities and 7Be. Advective heat flows of several tens of watts per square meter were derived, promoting widening of ponds and increases in pond coverage. Stage 4 corresponds to freeze‐up. Roughly 40% of the total surface melt was retained by the ice cover within the ice matrix as well as in surface and under‐ice ponds (with a total net retention of 15%). Based on this work, areas of improvement for fully prognostic simulations of ice albedo are identified, calling for parameterizations of sea‐ice permeability and the integration of ice topography and refined ablation schemes into atmosphere‐ice‐ocean models.
Eddy mean flow decomposition and eddy diffusivity estimates in the tropical Pacific Ocean: 2. Results
Authors: Bauer S. , Swenson M. S. , Griffa A.
Published: November 2002

Eddy diffusivity of the surface velocity field in the tropical Pacific Ocean was estimated using satellite‐tracked drifting buoys (1979 through mid‐1996). The tropical Pacific surface current system is characterized by nonstationarity, strong meridional shear, and an energetic mesoscale velocity field. Eddy diffusivity may be defined as the integral of the autocovariance of Lagrangian eddy velocities, requiring both stationary and homogeneous statistics of the eddy field. Eddy velocities were obtained by removing a splined mean field to eliminate mean shear from observations binned (1) spatially to group data that have similar dispersion characteristics and (2) temporally to create stationary eddy statistics. Zonal diffusivity estimates are up to ≈7 times larger than meridional diffusivity estimates in the high eddy energy regions. This anisotropy is associated with the meridional mesoscale wave motion (i.e., by equatorial and tropical instability waves) that increases eddy variance but does not lead to a proportional increase in water parcel diffusion because of the coherent character of the trajectory motion, at least for initial time lags. Simple autoregressive models of first and second order are used to describe and classify the resulting eddy statistics. An independent confirmation of the diffusivity estimate in the central/eastern Pacific was obtained by comparing tracer flux divergence computed from a parameterization using diffusivity estimates of our analysis with that from direct eddy Reynolds stress flux divergence. Our results show that diffusivity can be estimated for regions not considered previously either because of sparse data or the complexities of the velocity field.
Observed reflectivity of the western boundary of the equatorial Pacific Ocean
Authors: Zang Xiaoyun , Fu Lee‐Lueng , Wunsch Carl
Published: November 2002

Published claims that the reflectivity of the western boundary of the equatorial Pacific is near 100% are examined by using TOPEX/Poseidon data from October 1992 to February 2000. We perform a Fourier analysis of sea surface height measurements and define the variability consistent with the Rossby wave and Kelvin wave dispersion relationship as the energy of Rossby wave and Kelvin wave, respectively. Basin‐scale Rossby waves dominate in the period ranges of 180–250 and 270–450 days. Their largest amplitudes are at 4° and 6°N, respectively. Kelvin waves dominate in distinguishable period ranges of 67–76 and 106–170 days. Because the wavelengths of the Kelvin waves generated from the reflection of low‐frequency Rossby waves are much longer than the width of the basin, they cannot be readily differentiated from large‐scale forced motions. An upper bound on the reflection coefficient of the Rossby wave is thus estimated as 31% for Rossby waves with periods of 180–250 days. The duration of TOPEX/Poseidon data is too short for studying the reflectivity of the lower‐frequency Rossby waves.
Dependence of the Arctic Ocean ice thickness distribution on the poleward energy flux in the atmosphere
Authors: Björk Göran , Söderkvist Johan
Published: November 2002

The sensitivity of the Arctic Ocean ice cover on the atmospheric poleward energy flux, D, is studied using a coupled column model of the ocean, ice, and atmosphere. In the model, the ice cover is described by a thickness distribution and the atmosphere is a simple two‐stream gray body in radiative equilibrium. It is shown that the thickness distribution, in combination with the albedo function, gives a strong nonlinear response to positive perturbations of D. The response on D is sensitive to the albedo parameterization and the shape of the thickness distribution, controlled by ridging and divergence. An increase of about 9 W m−2 from a standard value of D = 103 W m−2 has a dramatic effect, reducing the ice thickness by more than 2 m, and generates a large open‐water fraction during summer. The reduction of ice thickness is characterized by a clear transition between two regimes, going from a regime where first‐year ice survives the next summer melt period to a seasonal ice regime where the first‐year ice melts completely. It is shown that the existence of seasonal ice regime is dependent on the surface mixed layer thickness. The model enters a completely ice‐free state if the thickness of the mixed layer is increased above a threshold value. The adjustment timescale for the ice cover is 6 years for small positive and negative perturbations in D. For larger positive perturbations of about 10 W m−2, the adjustment timescale is up to 20 years.
A case study of sea surface temperature–cooling dynamics near the Indian tip during May 1997
Authors: Luis Alvarinho J. , Kawamura Hiroshi
Published: November 2002

The sea surface temperature (SST) off the southwest coast of India is important to the Indian summer monsoon onset, which generally occurs in late May off this coast. It is known that both locally and remotely forced coastal processes contribute to the SST cooling. In this paper we focus on the latter by studying the SST‐cooling dynamics near the Indian tip (IT) during 1–15 May, using advanced very high resolution radiometer (AVHRR) SST, NASA scatterometer (NSCAT) winds, National Centers for Environmental Prediction/National Center for Atmospheric Research surface meteorological variables, and the Princeton Ocean Model. High spatial resolution (9 km) daily mean AVHRR SST maps indicate that the SST cooling near the IT is a recurrent phenomenon, wherein the cold SST water occupies the southwest India shelf and extends southward of the IT. During the 15 day period the NSCAT‐observed winds increased from near zero, the net surface heat flux into the ocean changed by ∼62 W/m2, and the SST decreased by 1.8°C near the IT. Under these conditions the model integration is carried out as an initial value problem with realistic bottom topography, climatological density stratification, and observed NSCAT winds. The results show that the Ekman dynamics along the west India shelf promote upwelling and initiate SST cooling. Shallow and spatially variable shelf topography near the IT enhance wind mixing, while sharp gradients in the isobaths at the shelf break and stratification enhance upwelling/downwelling intensity through cross‐isobath flow. It is inferred that the SST cooling near the IT is a locally forced process that is dictated by shelf topography.
Radiative and microphysical interactions between marine stratocumulus clouds and Saharan dust 1. Remote sensing observations
Authors: Pradelle F. , Cautenet G. , Jankowiak I.
Published: November 2002

This work emphasizes the radiative impact of Saharan dust particles in the presence of clouds, over the tropical North Atlantic Ocean. A first analysis based upon recent European Centre for Medium‐Range Weather Forecasts data confirms the classical observations; that is, in summer, the dust plume lies over the trade wind inversion, that is to say, over the main cloud layer. In winter, however, dust particles generally are transported in the trade wind layer, namely, where the major part of clouds is developing. In a second part, we present a statistical analysis based upon a 6‐year series of daily VIS and IR Meteosat data. Our study reveals, at any season, a decrease in the apparent cloud reflectivity derived from satellite measurements. Minimum values of this albedo are found in geographical areas where dust particles are most often observed and where they have their highest values in optical thickness. This albedo decrease can exceed 20% during spring months, when the dust outbreaks are the most frequent. A final discussion puts forward some physical processes that may explain the observations. In summer, dust presence may result in a change of the reflectance of the system, due to an increase of the absorption of the surface‐cloud‐dust plume system. On the other hand, during the winter season, the cloud albedo decrease may be in part explained by the drying of the atmosphere due to the dry air masses associated with the dust outbreaks coming from the Saharan desert. However, an interesting and complementary explanation is put forward for the winter case: Because of its lower altitude, the dust plume is likely to mix with cloud water. Consequently, in‐cloud processes may lead to a change in droplet spectrum size, which in turn, could affect the cloud reflectivity and therefore the reflectance measured at the top of the atmosphere. Therefore, whatever the physical processes involved, the dust outbreaks seem always to lead to a decrease in the cloud cover albedo over the tropical North Atlantic Ocean. In part 2 of this study, the observations will be completed by numerical modeling dealing with radiation, microphysics, and mesoscale transport, in order to better explain and understand this radiative effect.
Radiative and microphysical interactions between marine stratocumulus clouds and Saharan dust 2. Modeling
Authors: Pradelle F. , Cautenet G.
Published: November 2002

In the first part of this study, we attempted to highlight some radiative effects of dust particles in the presence of clouds using satellite observations. A statistical study over 6 years of daily Meteosat images revealed a minimum in the apparent cloud albedo (ocean‐cloud‐atmosphere system) ranging between 10% and 20%, near the West African coast. These decreases were observed at any season, and over the areas where the dust outbreaks are the most frequent. In order to explain these observations, we begin this second part by radiative simulations using the ECMWF radiative code. It appears that the presence of dust over the cloud cover (i.e., summer configuration) leads to a decrease in albedo of the ocean‐cloud‐atmosphere system reaching about 15% on average, i.e., a value comparable to the one observed by satellite. In winter, however, dust is found in the trade wind layer, at the same level as the cloud cover, but a simple direct effect (superposition effect) cannot explain the observed albedo decrease. According to earlier experimental studies, dust aging is frequently observed in the atmosphere. Complex interactions involving other particles and chemical species enhance the soluble fraction of dust: in‐cloud processing, chemical reactions on dust surface, transfer of small sulfate particles toward the large dust particles. Microphysical simulations using an air parcel model reveal that dust presence in a stratiform cloud field may lead to a decrease in the initial cloud condensation nucleus number, an increase of the effective droplet radius, and eventually, a reduction of the cloud albedo. This albedo decrease may reach more than 10% under certain conditions. On average, the albedo of the system is found to be reduced by a value ranging between 3% and 10%. Simulations performed with a mesoscale model reveal the presence at altitude of carbonaceous particles emitted by the African savanna burning in winter. Dust microphysical effects associated with the presence of absorbent carbonaceous particles at altitude would decrease the system albedo by more than 10% and so would explain the satellite observations during the winter season too in the southern part of the area under study. All these computations are in rather good agreement with the satellite observations and confirm the assumption that dust particles play an important climatic role to the west of the African coast. According to our estimates, the radiative forcing associated with the dust in the presence of clouds is (1) globally always positive along the column and (2) located either near the surface (winter case) or at altitude (summer case) according to the season. In the last case (summer), the radiative forcing remains negative near the surface but will be overcompensated by positive values at altitude (dust level).
Optical closure for an aerosol column: Method, accuracy, and inferable properties applied to a biomass‐burning aerosol and its radiative forcing
Authors: Fiebig Markus , Petzold Andreas , Wandinger Ulla , Wendisch Manfred , Kiemle Christoph , Stifter Armin , Ebert Martin , Rother Tom , Leiterer Ulrich
Published: November 2002

During the Lindenberg Aerosol Characterization Experiment (LACE 98), airborne measurements of aerosol size distribution, fine‐particle concentration, particle absorption coefficient, backscatter coefficient, depolarization, and chemical composition as well as ground‐based measurements of spectral particle optical depth and of spectral backscatter and extinction coefficients were performed in the aerosol column above Lindenberg, Germany. We compare the measured optical parameters with calculations from the size distributions, which assume the aerosol to consist of sulfuric acid near the tropopause and mixtures of ammonium sulfate and soot in the remaining column. We obtain closure to within 25% for the optical depth of a column, which includes a biomass‐burning aerosol of North American origin, and infer a soot volume fraction of 35% for this aerosol. Assuming spheroidal particles of prolate shape and the average aspect ratio of the particles to be 1.3 in the biomass‐burning aerosol layer, the calculated depolarization agrees with the lidar measurement, whereas comparing the spectral backscatter coefficient shows the soot to be externally mixed with the nonabsorbing particles. With the two‐stream approximation, we estimate the local, instantaneous, cloud‐free radiative forcing of the biomass‐burning aerosol at the tropopause to −5.8 W/m2 with a corresponding optical depth of 0.09 at 710 nm wavelength and solar zenith angle of 56°. The radiative forcing for the biomass‐burning aerosol is as sensitive to a change in state of mixture, either external or internal, as to a change in surface albedo, ocean to coniferous forest.
Lindenberg Aerosol Characterization Experiment 1998 (LACE 98): Overview
Authors: Ansmann Albert , Wandinger Ulla , Wiedensohler Alfred , Leiterer Ulrich
Published: November 2002

Backscattering and absorption of solar radiation by aerosol particles are an important source of uncertainty in climate predictions. Integrated research on the radiative properties of aerosol may reduce this uncertainty. The Lindenberg Aerosol Characterization Experiment 1998 (LACE 98) contributes to this aim. LACE 98 took place between 13 July and 12 August 1998, near Berlin, Germany. The Lindenberg Meteorological Observatory (52.2°N, 14.1°E) was chosen as the central field site because of its long record with aerosol optical‐depth data. Measurements were performed from three aircraft, with one airborne and four ground‐based lidars, and at a ground station. The meteorological situations in which intensive observations were carried out included clean and polluted air masses as characterized by low and high aerosol optical depths. This introductory paper gives an overview of the LACE 98 goals, instrumentation, meteorological and aerosol properties, and reports on the key findings as a guide to the results presented in the more detailed papers that follow. A very remarkable finding should be mentioned beforehand because of its unique character: on 9–10 August 1998, a free‐tropospheric aerosol layer was observed that originated from forest fires in western Canada.
Validation of the coupled Eta/SSiB model over South America
Authors: Chou Sin Chan , Tanajura Clemente A. S. , Xue Yongkang , Nobre Carlos A.
Published: November 2002

Two 1‐month integrations were performed with the regional Eta model coupled with the Simplified Simple Biosphere model (SSiB) over South America. The goal of the present work is to validate the model and to investigate its biases and skill on the simulations of South American climate. This is an initial step on the use of this model for climate research. The Eta model was set up with 80‐km horizontal resolution and 38 vertical layers over the South American continent and part of the adjacent oceans. Analyses from the National Centers for Environmental Prediction (NCEP) were used as initial and lateral boundary conditions. The selected months were August and November 1997, which are in opposite phases of the precipitation annual cycle observed in the central part of South America. The model was integrated continuously for each 1‐month period. Monthly means and daily variations of simulated precipitation and surface temperature compare well with observations. The patterns of simulated outgoing longwave radiation are also similar to the observed ones. However, a positive bias is verified in the simulations. The model shows a positive bias in latent and sensible heat surface fluxes due to an excessive shortwave incoming radiation at the surface. Comparisons with a version of the Eta model coupled with the bucket model shows that the Eta/SSiB version improves the surface temperature and increases precipitation in the interior of the continent during wet months.
Multisensor analysis of a squall line in the Amazon Region
Authors: Pereira Filho Augusto J. , Silva Dias Maria A. F. , Albrecht Rachel I. , Pereira Luis G. P. , Gandu Adilson W. , Massambani Oswaldo , Tokay Ali , Rutledge Steve
Published: November 2002

Mesoscale features of the 26 January 1999 squall line are described with measurements made during the field experiment in Rondônia, Brazil, in the wet season (WET) of the Amazon region (AMC) as part of the Large‐Scale Biosphere‐Atmosphere Experiment in the Amazon (LBA) and the Tropical Rainfall Measurement Mission (TRMM); henceforth referred to as the WET AMC and TRMM‐LBA field experiment. The squall line moved through the experiment area from northeast to southwest with high rainfall rates in its leading edge and a poorly defined trailing stratiform precipitating area. Polarimetric and Doppler measurements from the Portable Polarimetric S‐band Radar (S‐POL) were analyzed in conjunction with surface and upper level data, satellite visible (VIS) and infrared (IR) measurements. These remote and local measurements of variables such as cloud spatial and temporal distribution, pressure, temperature, moisture, precipitation, and wind fields are consistent with each other as well as with similar mesoscale dynamics and thermodynamics features measured, analyzed, and modeled elsewhere [e.g., Rotunno et al., 1988; Garstang et al., 1994; Houze et al., 1990]. This tropical squall line is similar to its cousins in the midlatitudes with finer‐scale structural and dynamic features such as rotation and divergence. Results suggest that the cold pool has its origins in midlevels between 400 and 600 hPa from where evaporative cooling and drop‐dragging bring down colder air to the surface. The convective region is dominated by warm microphysics, while the stratiform region is dominated by cold microphysics. Moreover, both regions are characterized by monomodal drop spectra centered around 2.0 and 1.0 mm, respectively. Horizontal circulation associated with strong updrafts tends to increase cloud growth efficiency in the leading edge of the squall line.