bluuebunny/crossref_metadata_2025 huggingface dataset:
Seismic folding in coastal south central Chile
Authors: Vita‐Finzi C. , Mann C. D.
Published: February 2004

The great (Mw = 9.3) Valdivia earthquake of May 21–22, 1960, was associated with ground deformation over a distance of over 800 km parallel to the Peru‐Chile trench. Downwarping by as much as 2.3 m affected a belt averaging a width of 100 km which was bordered by a major upwarped zone in the west, where uplift locally attained 5.7 m, and a minor upwarped zone in the east. The deformation has been explained by thin viscous sheet models or by dislocation models involving predominantly dip‐slip movement on thrust faults. We show that it can be modeled by elastic buckling of the overriding South American plate without the need to invoke specific fault geometries. The proposed buckling mechanism accounts for both zones of upwarping and for the exceptionally rapid uplift experienced by Mocha Island to the west. Moreover, elastic relaxation provides a simple explanation for the postseismic recovery reported by observers at several sites. It is also consistent with the results obtained by 14C dating of fossil shorelines within and outside the area affected by the 1960 events, which show that central and northern Chile has undergone little uplift during the last 30,000 years despite convergence between the South American and Nazca plates at over 90 mm/yr, although the presence of numerous older shorelines shows that coastal uplift was very active earlier in the Cenozoic.
Gradients in the composition of hydrothermal fluids from the Endeavour segment vent field: Phase separation and brine loss
Authors: Butterfield David A. , McDuff Russell E. , Mottl Michael J. , Lilley Marvin D. , Lupton John E. , Massoth Gary J.
Published: February 2004

Hydrothermal fluid samples collected in 1984, 1987, and 1988 from a large vent field near 47°57′N on the Endeavour segment of the Juan de Fuca Ridge (JFR) have been analyzed for major and minor elements and gases. There are of the order of 100 individual smoker vents on ∼10 large sulfide structures, which are localized along faults and fault intersections across the vent field. Each sulfide structure has a characteristic fluid composition, which varies very little from one vent orifice to the next, or from year to year, on a given structure. However, there are large gradients in fluid composition across the vent field, with endmember chlorinity increasing from ∼255 mmol/kg in the SW to 505 mmol/kg in the NE. End‐member concentrations of major elements are well correlated with chlorinity, and endmember volatile concentrations in the lowest chlorinity fluids are approximately twice as high as in the highest chlorinity fluids. The gradients in composition across the vent field and measured vent fluid temperatures >400°C are consistent with supercritical phase separation and loss of brine phase below the seafloor. The factor‐of‐2 variation in CO2 (and H2S) is larger than expected for loss of a very high‐chlorinity brine. Concentrations of iron and manganese are not positively correlated with chlorinity, suggesting that temperature and pH are more important in controlling metal solubility. Elevated ammonia and bromide/chloride ratios indicate that there has been subseafloor interaction between the hydrothermal fluids and organic matter, and high boron concentrations point to a sedimentary source.
Tidal variations in UT1 observed with very long baseline interferometry
Authors: Robertson Douglas S. , Ray Jim R. , Carter William E.
Published: February 2004

Nine years of very long baseline interferometry (VLBI) observations have been analyzed to determine the magnitudes of the tidal variations in UTl for periods between 5 and 35 days. Corrections for variations in atmospheric angular momentum (AAM) significantly reduce the scatter of the measured amplitudes across both time and frequency. The AAM corrections are found to reduce the scatter in the observed tidal amplitudes by as much as 60% for periods as short as 5.6 days; in contrast, earlier studies have shown a loss of coherence between AAM and length‐of‐day (LOD) for periods shorter than about 10 days. The tidal amplitude measurements place bounds on both the nonequilibrium ocean and mantle anelasticity effects. The in‐phase k/C determination is found to agree to better than 0.5% with the value of 0.944 from Yoder et al. (1981). The out‐of‐phase values are found to have a frequency dependence that can only be explained by nonequilibrium ocean effects. The observed slope is larger than the theoretical by about 3 times the expected error. This result may indicate that the ocean is farther out of equilibrium for the higher frequencies than present models permit. Significant improvements are needed in both ocean and atmosphere modeling to exploit the full capability of the VLBI observations.
Excess 222Rn above the Cleft segment of the Juan de Fuca Ridge
Authors: Gendron James F. , Todd James F. , Feely Richard A. , Baker Edward T. , Kadko David C.
Published: February 2004

The distributions of excess 222Rn above the North Cleft segment of the Juan de Fuca Ridge were determined from 1988 to 1991 in order to obtain a better understanding of the temporal variability of heat flux from this ridge segment. Based on the inventories of excess 222Rn, the hydrothermal heat flux was estimated to be 2.7±1.3 GW in 1990. The large uncertainty results from errors in calculating the standing crop of 222Rn, the difficulty in determining the 222Rn to heat ratio, and the problem of entrainment of low‐temperature waters into the rising plume. The high‐resolution sampling program carried out in 1990 permitted the mapping of apparent plume ages for different sections of the neutrally buoyant plume. About 3 km to the east of the ridge axis there is a sharp gradient in the apparent age of the plume, while to the west the plume appears to be younger. This plume age distribution could indicate a current flow to the northwest during the time this transect was sampled. The mean 222Rn/heat ratios for the high‐ and low‐temperature vent fluids were 0.0017 disintegrations per minute (dpm) g−1 °C−1 and 0.0044 dpm g−1 °C−1, respectively. The extrapolated end‐member concentrations of excess 222Rn in low‐temperature vent fluids are a factor of 6 greater than the high‐temperature fluids, based on the regressions of excess 222Rn activity and Mg concentration in North Cleft segment vent fluid samples.
The northern Mid‐Atlantic and Reykjanes Ridges: Spreading center morphology between 55°50′N and 63°00′N
Authors: Appelgate Bruce , Shor Alexander N.
Published: February 2004

SeaMARC II and Hydrosweep acoustic imagery and bathymetry reveal that the northern Mid‐Atlantic Ridge (MAR) and Reykjanes Ridge between 55°50′N and 63°00′N are segmented seafloor spreading centers that exhibit structural morphologies that vary with distance from Iceland. The northern 100 km of the MAR (55°50′N‐56°47′N) is oriented 005° (perpendicular to the spreading azimuth) and contains four third‐order spreading segments separated along strike by right‐ and left‐stepping axial discontinuities. The boundary between MAR and Reykjanes Ridge occurs at 56°47′N, where the Bight transform fault offsets the neo volcanic axis 15 km right laterally across a single linear fault oriented 092°. This is the only transform fault between 55°50′N and 63°00′N and is inferred to have generated the Bight Fracture Zone. The Reykjanes Ridge lies between the Bight transform and Iceland's Reykjanes Peninsula and is oriented ∼30° oblique to the spreading‐orthogonal direction. Axial faults and volcanic systems on the Reykjanes Ridge are arranged in a right‐stepping en echelon pattern and display strikes intermediate between the spreading‐orthogonal direction and the strike of the ridge, which is compatible with experimental models of oblique extension. Spreading segments on the northern MAR and Reykjanes Ridge are defined structurally and bathymetrically. At the smallest scale, the volcanic axis is segmented into individual volcanic systems 4–45 km long, which are separated along strike by intervolcano gaps or offsets. These features are analogous to fourth‐order spreading segments documented elsewhere on the MAR. Axial volcanic systems are superimposed on intermediate‐wavelength (13–65 km) axial topographic highs that constitute third‐ or second‐order spreading segments. Segment boundaries occur at axial depth maxima, which correspond to offsets in the volcanic axis. Four primary characteristics of the spreading axis (axial valley depth, axial boundary fault throw, relief along the neovolcanic axis, and degree of inter‐segment structural discontinuity) decrease toward the north. This variation is consistent with higher crustal temperatures near Iceland, due to either thicker crust or elevated mantle temperatures.
Interannual scales of variation of pigment concentrations from coastal zone color scanner data in the Benguela Upwelling system and the Subtropical Convergence zone south of Africa
Authors: Weeks Scarla J. , Shillington Frank A.
Published: February 2004

South African oceanographers were engaged in collecting hydrographic and biological sea truth data in order to calibrate the coastal zone color scanner (CZCS) measurements from the Nimbus 7 satellite over the Benguela Upwelling region near Cape Town, South Africa, during the period 1978 to 1981 (Shannon, 1985). We give a brief overview of this research and then concentrate on a more recent analysis of level III CZCS data obtained from NASA for the region 10–60°S, 10–100°E. This area includes the Benguela Upwelling system on the continental shelf and the southern ocean with the Subtropical Convergence zone south of Africa. High annual concentrations of chlorophyll (5 mg m−3) typical of other upwelling systems in the world ocean occurred in the Benguela shelf region, and the data show a strong interannual signal in the 7 years of composited data from 1978 to 1985. Two distinct regimes were found in the Benguela Upwelling system, the seasonal variations of pigment concentration in the northern and southern Benguete regions being out of phase. In the southern ocean, levels of chlorophyll were generally low (0.15 mg m−3), with the strongest signal (1.5 mg m−3) found at the southern border of the Agulhas retroflection region and its frontal boundary with the colder Subantarctic water to the south. The high levels of chlorophyll found in this region are 10 times those of the typical open southern ocean. There is a very clear interannual signal in the CZCS data for this Subtropical Convergence region, which has a low value in 1979, rises to a maximum in 1981, and then decreases to another low value in 1985. There appears to be no clear seasonal variation in the Subtropical Convergence data. Reasons for the strong signal in the surface chlorophyll concentrations at the front between the Agulhas Return Current and the southern ocean are discussed, and it is shown that the Agulhas Plateau sets up a topographic Rossby wave in the Agulhas Return Current, which can be clearly identified in the CZCS signal.
Comparison of the seasonal and interannual variability of phytoplankton pigment concentrations in the Peru and California Current systems
Authors: Thomas A. C. , Huang F. , Strub P. T. , James C.
Published: February 2004

Monthly composite images from the global coastal zone color scanner (CZCS) data set are used to provide an initial illustration and comparison of seasonal and interannual variability of phytoplankton pigment concentration along the western coasts of South and North America in the Peru Current system (PCS) and California Current system (CCS). The analysis utilizes the entire time series of available data (November 1978 to June 1986) to form a mean annual cycle and an index of interannual variability for a series of both latitudinal and cross‐shelf regions within each current system. Within 100 km of the coast, the strongest seasonal cycles in the CCS are in two regions, one between 34° and 45°N and the second between 24° and 29°N, each with maximum concentrations (>3.0 mg m−3) in May–June. Weaker seasonal variability is present north of 45°N and in the Southern California Bight region (32°N). Within the PCS, in the same 100‐km‐wide coastal region, highest (>45°S) and lowest (<20°S) latitude regions have a similar seasonal cycle with maximum concentrations (>1.5 mg m−3) during the austral spring, summer, and fall, matching that evident throughout the CCS. Between these regions, off northern and central Chile, the seasonal maximum occurs during July–August (austral winter), contrary to the influence of upwelling favorable winds. Within the CCS, the dominant feature of interannual variability in the 8‐year time series is a strong negative concentration anomaly in 1983, an El Niño year. The relative value of this negative anomaly is strongest off central California and is followed by an even stronger negative anomaly in 1984 off Baja California. In the PCS, strong negative anomalies during the 1982–1983 El Niño period are evident only off the Peruvian coast and are evident there only in the regions 100 km or more from the coast. Although negative anomalies associated with the El Niño were not present at higher latitudes (more than approximately 20°S) in the PCS, the extremely sparse sampling weakens our confidence in the results of the interannual analysis in this region. An upper estimate of the systematic winter bias remaining in the global CZCS data after reprocessing with the multiple scattering algorithm is given in the appendix.
Surface layer variations observed in multiyear time series measurements from the western equatorial Pacific
Authors: Sprintall Janet , McPhaden Michael J.
Published: February 2004

Mooring measurements at 0°, 165°E, for the period November 1988 to August 1991 indicate that surface layer structure was characterized by two distinct climatic regimes associated with dramatic differences in large‐scale atmospheric and oceanic conditions. La Niña conditions existed from November 1988 to November 1989, during which time the easterly trades were strong, Ekman divergence and upwelling were pronounced, surface velocity was strongly westward, and rainfall was low. The surface layer was cold, salty and well mixed down to 100‐m depth, with density variations controlled primarily by temperature. In contrast, from November 1989 to August 1991, the zonal winds were on average westerly and punctuated by frequent westerly wind bursts, the surface currents reversed and flowed eastward in the upper 50 m, and rainfall was high. Compared to the La Niña period, the surface layer was warmer and fresher, and the density mixed layer was shallower than the isothermal layer owing to the presence of a 30‐m‐thick mean halocline (or barrier layer) between 55‐ and 85‐m depth. Moreover, density variations in the mixed layer were determined primarily by salinity. During the November 1988 to November 1989 La Niña period, variability in sea surface temperature was influenced by local upwelling and zonal advection. However, during November 1989 to August 1991, the presence of the barrier layer effectively prevented the entrainment of cooler, saltier water from the thermocline into the surface layer. Local air‐sea heat fluxes were therefore more likely to be prominent in the surface layer temperature balance. The barrier layer thickness, which varied with a dominant time scale of 12–25 days, appears to have been affected by variations in zonal advection of low‐salinity water past the mooring.
The effect of short‐scale wind variations on shelf currents
Authors: Brink K. H. , LaCasce J. H. , Irish J. D.
Published: February 2004

Most existing models of shelf currents (such as coastal‐trapped wave models) usually disagree with observations, in that the cross‐shelf currents are weaker than observed, and that the predicted coherence length scales of cross‐shelf currents are much longer. We seek to test the hypothesis that these inconsistencies can be resolved by including, in the forcing, more realistic wind variations with alongshore length scales down to about 10 km. We use the Coastal Ocean Dynamics Experiment (CODE) Northern California, 1982, wind observations to drive a linear, stratified stochastic ocean model which includes bottom friction. The model results are expressed as statistics of the flow field which can be checked against observations. CODE aircraft winds from 1982 low‐level alongshore flight tracks were converted to stresses and then subjected to wave number spectral analysis. Results show about 1 order of magnitude more energy at wavelengths shorter than 50–100 km than would be expected from the extrapolation of larger‐scale spectral estimates based on buoy wind time series. Thus, for the CODE region, the forcing is energetic at relatively short length scales. The model results for cross‐shelf velocity support our hypothesis, in that the modeled alongshore coherence length scales are indeed much shorter than those from a large‐scale‐only model. For example, at a 7‐day period, the present calculations predict a coherence squared of 0.3 at a 35‐km separation, while the large‐scale‐only model predicts 0.3 at 250 km. The observations show a 35–50 km scale for 0.3 coherence squared. However, while cross‐shelf current variances are increased, they are still about a factor of 5 or more smaller than the CODE shelf observations over the shelf.
Evidence of surface wave enhancement in the southwest Indian Ocean from satellite altimetry
Authors: Gründlingh Marten L.
Published: February 2004

Wave height data from Geosat over the period November 1986 to February 1989 were used to survey the occurrence of wave amplification/attenuation in the southwest Indian Ocean. More than half of the wave profiles that were inspected revealed some enhancement in the Agulhas Current, with 17% of the profiles showing fractional amplification of more than 40% (relative to the background wave height). Other aspects that were considered included attenuation of wave height in the shelf region and the characterization of the Agulhas Return Current by wave height variations.
Tidal and subtidal variability in the sloping benthic boundary layer
Authors: White M.
Published: February 2004

Observations are presented of the benthic boundary layer (BBL) structure for two sites on the continental slope, west of the British Isles. Variability at the tidal (M2) and subtidal frequencies is discussed. A mean poleward, along‐isobath current is present at both sites, with periodic (5–8 days) reversals observed in the flow. These reversals cause a change in the water column stratification close (<100 m) to the seabed. The relationship between the direction of the along‐ and cross‐slope flow, stratification, and the change of temperature in the frictional layer, relative to that above it, is consistent with Ekman veering induced up/downwelling close to the slope. The BBL temperature (density) structure is highly variable at the tidal frequency. There is a periodic variability in the current shear and associated Richardson number (Ri), particularly for the region where the bottom slope is close to critical for the M2 internal tide. Mixed or gravitationally unstable density layers are generated about one buoyancy period after the minimum Ri are observed. The mixed layers are transient, however, and do not persist throughout the tidal cycle. Indirect estimates of the vertical turbulent eddy diffusivity (Kυ) were made from conductivity‐temperature‐depth (CTD) yoyo casts, and its variability through a tidal cycle measured. A background level of 10−4 m2 s−1 is estimated with high values O(10−2 m2 s−1) measured for the times associated with the generation of mixed or inverted density layers. A mean value ofKυ = 15–50×10−4 m2 s−1 is found. The observations are compared to recent models of the sloping BBL and laboratory experiments, with particular emphasis on the transient nature of the BBL and the influence of internal wave reflection from the slope.
Does the Earth's rotation influence the location of the shelf sea fronts?
Authors: Simpson J. H. , Sharples J.
Published: February 2004

Two alternative hypotheses which attempt to explain the position of the shelf sea fronts are (1) control by the energetics of tidal and wind mixing and (2) limitation of the penetration of tidal stresses by the effects of rotation. A turbulence‐closure (T/C) point model has been used to investigate the interaction of these two constraints and predict the seasonal cycle of stratification and frontal position for varying current polarization and water depths of h = 70 m and h = 100 m. For clockwise‐rotating tidal currents the frontal positions are in close accord with the previous prescriptive model, which is based only on the energy constraint, and with the observed positions of fronts at critical values of χ = log (h/〈u3〉) = 2.7 ± 0.4 (where h = depth and u = depth mean tidal current amplitude for M2, and angle brackets denote a time mean). For rectilinear tidal currents (equal clockwise and anticlockwise components), there is a small displacement (∼0.25) toward lower values of χ. When the currents are predominantly anticlockwise, the frontal positions predicted by the T/C model deviate significantly from the constant χ condition with a maximum discrepancy, when polarization P = +1, of Δχ = 0.86 and 1.56 at h = 70 and 100 m, respectively. This displacement is in the same sense as, but considerably less than, that predicted by the combined boundary layer theory. It may be detectable in the few areas of the shelf where P may reach values of up to 0.7 in depths of ∼100 m. For most regions of the European shelf, where −0.3 < P < 0.3, the model suggests that the influence of rotation will be small and control of frontal boundaries will rest primarily with the energy constraint.
Surface gravity waves at equilibrium with a steady wind
Authors: Glazman Roman E.
Published: February 2004

Observations of wave fields' spatial evolution and of gravity wave spectra S(ω) are analyzed on the basis of the data reported by several research groups as well as on a 2‐year data set of wind and wave measurements by stationary National Oceanic and Atmospheric Administration buoys near the Hawaiian Islands. We seek to clarify the role of the wave energy advection (with the wave group velocity) in the overall energy balance. This advective transfer appears to be no less important than the local (breaking wave induced) dissipation as a factor of wind‐wave equilibrium. The advection is found to manifest itself in the shape of wave spectra by reducing the rate at which the spectral density of the wave energy, S(ω) ∼ ω−p, falls off as the frequency increases away from the spectral peak. This and other conclusions are derived by comparing the field observations with theoretical predictions of the weak turbulence theory for a spatially inhomogeneous, statistically stationary, wave field. The observations also indicate that the typical wave age ξ = C0/U in the open ocean is much greater than the limiting value 1.2 attributed to the “fully developed sea.” Although the observed spectra can be approximated by a power law with a single “effective” exponent, this apparent exponent, p, is found to depend on the wave age. At high ξ and at frequencies below the generation range, −p tends to −3 rather than the value of −11/3 predicted by the Zakharov‐Zaslavskii theory. This deviation is interpreted as pointing to a nonconservative nature of the inverse cascade, the latter including a leakage of energy to low‐wavenumber modes. Dependence of the overall effective exponent on ξ is shown to be responsible for variation in the coefficients b, B, c, C appearing in empirical fetch laws, such as
and
, where
and e are the dimensionless fetch and wave energy, respectively.
Surface wave period modulations in near surface ambient sound
Authors: Zedel Len , Farmer David
Published: February 2004

Observations of near‐surface, high‐frequency ambient sound in the deep ocean demonstrate modulations at surface wave frequencies. Ambient sound was recorded at frequencies of 2 to 20 kHz at 30 m depth for a total of 65 hours through a variety of sea state and wind conditions. Throughout these observations, modulations in ambient sound level occur at surface wave frequencies and are in phase with the surface displacements directly above the acoustic observations. The modulations are present at all frequencies within the recorded bandwidth but are most obvious at higher frequencies; at 8 kHz, fluctuations in signal level of about 5 dB occur. A model which assumes continuous sound generation along the ocean surface with source levels adjusted in proportion to wave parameters is developed. Results from this model suggest that sound sources must be closely spaced compared to the dominant surface wave lengths to reproduce the observed fluctuations. For the data analyzed, sound sources must be continuous on a scale of 10 m or less to account for the observations. A possible cause for sound generation consistent with these observations is small wave breaking events triggered either by long wave/short wave interactions, or by variations in wind stress over the long waves. Such sound sources should be distinguished from discrete sources that have been associated with larger breaking waves at lower acoustic frequencies.
Surface temperatures in the polar regions from Nimbus 7 temperature humidity infrared radiometer
Authors: Comiso Josefino C.
Published: February 2004

Monthly surface temperatures in the Arctic and Antarctic regions have been derived from the 11.5‐μM thermal infrared channel of the Nimbus 7 temperature humidity infrared radiometer (THIR) for a whole year in 1979 and for a winter and a summer month from 1980 through 1985. The data set shows interannual variability and provides spatial details that allow identification of temperature patterns over sea ice and ice sheet surfaces. For example, the coldest spot in the southern hemisphere is observed to be consistently in the Antarctic plateau in the southern hemisphere, while that in the northern hemisphere is usually located in Greenland, or one of three other general areas: Siberia, the central Arctic, or the Canadian Archipelago. Also, in the southern hemisphere, the amplitude of the seasonal fluctuation of ice sheet temperatures is about 3 times that of sea ice, while in the northern hemisphere, the corresponding fluctuations for the two surfaces are about the same. The main sources of error in the retrieval are cloud and other atmospheric effects. These were minimized by first choosing the highest radiance value from the set of measurements during the day taken within a 30 km by 30 km grid of each daily map. Then the difference of daily maps was taken, and where the difference is greater than a certain threshold (which in this case is 12°C), the data element is deleted. Overall, the monthly maps derived from the resulting daily maps are spatially and temporally consistent, are coherent with the topography of the Antarctic continent and the location of the sea ice edge, and are in qualitative agreement with climatological data. Quantitatively, THIR data are in good agreement with Antarctic ice sheet surface air temperature station data with a correlation coefficient of 0.997 and a standard deviation of 2.0°C. The absolute values are not as good over the sea ice edges, but a comparison with Russian 2‐m drift station temperatures shows very high correlation (with correlation coefficient at 0.998) and a standard deviation of 1.1°C. Overall, the rms error is estimated to be from 1° to 2°C, depending on the surface, while the average bias when compared with in situ data is less than 2°C.
The eddy field associated with the Azores Front east of the Mid‐Atlantic Ridge as observed by the Geosat altimeter
Authors: Le Traon P.‐Y. , De Mey P.
Published: February 2004

Two years of Geosat data (November 1986 to December 1988) for the Azores‐Madeira region of the North Atlantic are analyzed. The objective is to study the eddy field associated with the terminal retroflection of the Azores current. The rms variability of sea level anomaly is 4 to 8 cm, with maximum values between 33° and 35°N being associated with the Azores current and located on its northern flank. Higher energy is observed during winter and during the year November 1987 to November 1988. Analysis of velocities at crossovers reveals anisotropy with larger meridional velocity variances. This is the signature of meanders and/or eddies elongated in the north‐south direction. The eddy‐mean flow interaction is studied through the analysis of the Reynolds tensor. Meridional and zonal accelerations of the mean by the eddy field are consistent with retroflection of the Azores Current to the south and to the west. Spectral analyses show that the dominant signals have periods and wavelengths larger than 100 days and 300 km, respectively. Propagation velocities are westward but slow (1.5 km/d). Synoptic mapping of the eddy field shows the wavelike structure of the variability on the northern flank of the current. It is likely that the wave generation is not directly due to an instability of the Azores current but is attributable to a cause more to the east. Once formed, the wave interacts with the current in winter and forms meanders. Seasonal variations of the surface circulation are finally analyzed from the 2‐year‐long time series of synoptic maps. There is a clear seasonally in the surface circulation which generally appears to be consistent with historical in situ measurements. In winter there is a strong meandering and a southward branching, while in summer the mean front is narrower and penetrates further to the east.
A study of the seasonal cycle of sea surface temperature in the tropical Pacific Ocean using reduced gravity models
Authors: Chang Ping
Published: February 2004

A simple model of the upper ocean is used to simulate the seasonal cycle of sea surface temperature (SST) in the tropical Pacific Ocean. The model is an extension of the conventional 1½‐layer reduced gravity system that includes the physics of the surface mixed layer and allows prediction of the sea surface temperature. Two types of mixed layer models, a constant depth mixed layer model of Cane and Zebiak and a variable depth mixed layer model of Kraus and Turner, are examined. The numerical simulations indicate that the simple models are capable of capturing the essential seasonal SST variability in the tropical Pacific when forced with climatological surface forcing. The sensitivity study of model SST response to changes in the surface forcing demonstrates that although the response to surface heat flux forcing dominates the seasonal cycle of SST in most areas of the tropical Pacific, the dynamical response of the ocean to changes in the wind also makes a significant contribution to the seasonal variation of SST in the eastern equatorial Pacific. The seasonal variation of the winds is particularly important in maintaining the phase structure of the annual SST signal along the equator. Moreover, the sensitivity of the SST response depends on the form of vertical entrainment parameterization in the model. It is shown that the Cane‐Zebiak type of model has a bias toward the dynamical response of the ocean to the winds, whereas the Kraus‐Turner type of model has bias toward the surface heat flux variations. The simple model results are also compared with those from an ocean general circulation model.
Effects of viscosity and bottom friction on recirculating flows
Authors: Denniss Tom , Middleton Jason H.
Published: February 2004

A recent set of coastal experiments (Pattiaratchi et al., 1987), found that the horizontal eddy Reynolds number did not adequately describe the wakes found in the lee of coastal islands. Other flow parameters formulated on bottom friction were found to be much better at describing these wakes. In this paper it is suggested that these findings are caused by the extraction of energy from the flow due to bottom friction, which is the net result of the vertical viscous transfer of momentum through the water column to the seabed. This is in contrast to the horizontal viscous transfer of momentum which results in negligible energy loss to the coast, because of the far larger horizontal scales. It is hypothesized that this difference results in fundamentally different flow regimes and this hypothesis is investigated both analytically and numerically. Conditions for the existence of steady zones of barotropic recirculation under the influence of both horizontal viscosity and bottom friction in the wake behind a bluff body are examined analytically. It is found that such steady recirculation requires the presence of horizontal viscosity and is hindered by the loss of energy due to bottom friction. A numerical model is used to verify and quantify this analytical result for the case of two‐dimensional flow around a circular cylinder, and it is concluded that an appropriate bottom friction number, based on the ratio of scales of the nonlinear and bottom friction terms, may often govern the nature of the recirculation in the lee of coastal islands, instead of the eddy Reynolds number based on horizontal eddy viscosity, as is sometimes assumed.
Carbon trace gas fluxes along a successional gradient in the Hudson Bay lowland
Authors: Klinger Lee F. , Zimmerman Patrick R. , Greenberg James P. , Heidt Leroy E. , Guenther Alex B.
Published: February 2004

Patterns and controls of carbon trace gas emissions from wetlands may vary depending upon the spatial and temporal scale being examined. The factors affecting these emissions are thought to be hierarchically related according to their respective scales of importance. A hierarchical model of processes controlling methane emissions from wetlands is presented and examined here. During the 1990 Northern Wetlands Study (NOWES) methane (CH4), carbon dioxide (CO2), and non‐methane hydrocarbon (NMHC) fluxes were measured in static chambers along a 100 km transect in the Hudson Bay lowland (HBL). Environmental variables, vegetation abundance, and ecosystem age and structure were also quantified at each sampling site. The findings indicate that CH4 emissions from peatlands (e.g., bogs and fens) and other wetlands (e.g., salt marshes) in the region were low, and were nil or negative (i.e., CH4 uptake) in forests and bog forests dominated by aspen and black spruce. Site to site variations in mean CH4 flux appeared to be most closely related to mean water table and sedge productivity, both of which are intercorrelated. Seasonal changes in CH4 flux tend to follow soil temperature fluctuations. Instantaneous CO2 and CH4 daytime fluxes exhibit a negative correlation, suggesting that photosynthetic assimilation of carbon may be related to CH4 emissions, although the processes of CO2 and CH4 production are occurring at somewhat different temporal scales. No diurnal variations in CH4 flux could be detected. While soil water pH trends are not fully explored, there is some indication that high CH4 fluxes are concentrated around pH 4 and pH 7. Soil temperature closely follows the seasonal progression of CH4 flux. Estimated CH4 seasonal flux (1.5–3.9 g CH4 m−2 season−1) and estimated aboveground net primary productivity (NPP) (90–400 g dry weight m−2 season−1) show systematic changes along a successional sequence which are consistent with patterns predicted from successional theory. Estimated seasonal NMHC emissions (0.5–1.4 g C m−2 season−1) exhibit an increase along the succession from salt marsh to Sphagnum bog communities. Data from several studies were combined to estimate seasonal CO2 flux from three sites. The estimated fluxes range from a net uptake of 23 g CO2 m−2 season−1 to a net loss of 77 g CO2 m−2 season−1, although there are large uncertainties in these estimates. It is inferred from the assessment of ecosystem age and structure that disturbance effects and successional changes occurring over hundreds to thousands of years in the HBL strongly control regional CH4, CO2 and NMHC emissions by influencing NPP, species composition, community structure, soil (peat) development, and landscape hydrology. Given this, it is likely that models of carbon trace gas flux based on succession models may be useful in predicting climate change‐landscape change feedbacks.
Ecosystem and physiological controls over methane production in northern wetlands
Authors: Valentine David W. , Holland Elisabeth A. , Schimel David S.
Published: February 2004

Peat chemistry appears to exert primary control over methane production rates in the Canadian Northern Wetlands Study (NOWES) area. We determined laboratory methane production rate potentials in anaerobic slurries of samples collected from a transect of sites through the NOWES study area. We related methane production rates to indicators of resistance to microbial decay (peat C:N and lignin:N ratios) and experimentally manipulated substrate availability for methanogenesis using ethanol (EtOH) and plant litter. We also determined responses of methane production to pH and temperature. Methane production potentials declined along the gradient of sites from high rates in the coastal fens to low rates in the interior bogs and were generally highest in surface layers. Strong relationships between CH4 production potentials and peat chemistry suggested that methanogenesis was limited by fermentation rates. Methane production at ambient pH responded strongly to substrate additions in the circumneutral fens with narrow lignin:N and C:N ratios (∂CH4/∂EtOH = 0.9–2.3mg g−1) and weakly in the acidic bogs with wide C:N and lignin:N ratios (∂CH4/∂EtOH = −0.04–0.02 mg g−1). Observed Q10 values ranged from 1.7 to 4.7 and generally increased with increasing substrate availability, suggesting that fermentation rates were limiting. Titration experiments generally demonstrated inhibition of methanogenesis by low pH. Our results suggest that the low rates of methane emission observed in interior bogs during NOWES likely resulted from pH and substrate quality limitation of the fermentation step in methane production and thus reflect intrinsically low methane production potentials. Low methane emission rates observed during NOWES will likely be observed in other northern wetland regions with similar vegetation chemistry.
Summertime distribution of PAN and other reactive nitrogen species in the northern high‐latitude atmosphere of eastern Canada
Authors: Singh H. B. , Herlth D. , O'Hara D. , Zahnle K. , Bradshaw J. D. , Sandholm S. T. , Talbot R. , Gregory G. L. , Sachse G. W. , Blake D. R. , Wofsy S. C.
Published: February 2004

Aircraft measurements of key reactive nitrogen species (NO, NO2, HNO3, PAN, PPN, NO3‐, NOy), C1 to C6 hydrocarbons, acetone, O3, chemical tracers (C2Cl4 CO), and important meteorological parameters were performed over eastern Canada during July to August 1990 at altitudes between 0 and 6 km as part of an Arctic Boundary Layer Expedition (ABLE3B). In the free troposphere, PAN was found to be the single most abundant reactive nitrogen species constituting a major fraction of NOy and was significantly more abundant than NOx and HNO3. PAN and O3 were well correlated both in their fine and gross structures. Compared to data previously collected in the Arctic/subarctic atmosphere over Alaska (ABLE3A), the lower troposphere (0–4 km) over eastern Canada was found to contain larger reactive nitrogen and anthropogenic tracer concentrations. At higher altitudes (4–6 km) the atmospheric composition was in many ways similar to what was seen over Alaska and supports the view that a large‐scale reservoir of PAN (and NOy) is present in the upper troposphere over the entire Arctic/subarctic region. The reactive nitrogen budget based on missions conducted from the North Bay site (missions 2–10) showed a small shortfall, whereas the budget for data collected from the Goose Bay operation (missions 11–19) showed essential balance. It is calculated that 15–20 ppt of the observed NOx may find its source from the available PAN reservoir. Meteorological considerations as well as relationships between reactive nitrogen and tracer species suggest that the atmosphere over eastern Canada during summer is greatly influenced by forest fires and transported industrial pollution.
An algorithm for retrieving atmospheric motion from satellite measurements of tracer behavior
Authors: Salby Murry L. , Juckes Martin N.
Published: February 2004

An algorithm is developed for determining three‐dimensional atmospheric motion from global measurements of tracer behavior. The latter constitute observations of the material field, which underlies essential dynamical budgets that govern the circulation. Because it treats the material behavior as an observable, the algorithm provides a more direct means of determining atmospheric motion from space than the traditional scheme, in which material behavior must be derived from remote measurements of temperature. Incorporating observations of an ensemble of tracers (e.g., from multiple instruments or multiple orbiting platforms) leads to a variational problem for the Lagrangian displacement field, the solution of which determines the global distribution of air motion. The algorithm's direct relationship to material behavior circumvents well‐known limitations of the traditional scheme for inferring atmospheric motion from space. Further, since it is based on kinematic constraints that follow directly from observables, the retrieved motion is not artificially biased towards approximate forms of the governing equations and uncertainties accompanying them, e.g., as are inherent in assimilations based on numerical models. For this reason, motion in the tropics is determined as reliably as elsewhere on the globe. Calculations under realistic conditions illustrate that the accuracy of the retrieved motion is limited chiefly by the number and quality of tracer observations. Those calculations also demonstrate that the accuracy can be improved dramatically by increasing the number of orbiting platforms from which tracer measurements are provided.
Time scales in energy balance climate models: 2. The intermediate time solutions
Authors: Morantine Michael C. , Watts Robert G.
Published: February 2004

We demonstrated in a companion paper (Watts et al., this issue) that the transient response of a globally averaged upwelling diffusion energy balance climate model depends upon three time scales: the mixed‐layer thermal response time scale, the thermocline thermal‐response time scale, and the upwelling time scale. These time scales are defined and interpreted physically in that paper. The asymptotic solution for the case of linear radiative forcing was shown to depend upon the sum of the thermocline and mixed‐layer time scales, while the solution for small values of time depended only upon the mixed‐layer time scale. When the land fraction is included in the model, two additional time scales emerge: the land and ocean redistribution time scales. It has previously been shown by Kim et al. (1992) in a two‐dimensional energy balance model that the thermal‐response time scales are functions of position. The short‐time solution (for the responses to both a sudden increase in radiative forcing and a linear increase in radiative forcing) is, however, accurate for only very short times (on the order of 1 year). Here we present closed‐form, exact, analytical solutions for the response to both a sudden and a linear increase in radiative forcing. In addition to the mixed‐layer and thermocline time scales, these depend upon the upwelling time scale. We show here that for constant mixed‐layer and thermocline time scales, the response of the system is faster in intermediate stages when the upwelling time scale is longer.
Natural atmospheric sources and sinks of nitrous oxide: 1. An evaluation based on 10 laboratory experiments
Authors: Prasad Sheo S.
Published: February 2004

This paper discusses potentially potent new atmospheric sources and sinks of nitrous oxide (N2O), which is an important species by virtue of its being the dominant source of odd nitrogen (NOx) in the stratosphere and also a greenhouse gas. It is suggested that some of the excited nascent ozone from O, O2 recombination may form N2O via its reaction with N2. It is also conceivable that other metastable states of O3 accessible by optical pumping from the ground state might also be a potential source of N2O in the natural atmosphere. Additional atmospheric sources of N2O include reactions of excited NO3(Ā2E'), NO2(Ā2B1, 2B2) and OH(A) with N2, and of N2(A3∑) with O2. Highly vibrationally excited O2†(ν > 12) could be an important new sink of N2O in the stratosphere. This new N2O sink might also be a new NOx source in the stratosphere, depending on what fraction of the reactions of O2†(high ν) with N2O produces NO and NO2. At the lower altitudes, where O2†(ν > 12) density would become too small to matter, reactions of NO2 with N2O might be a potential N2O sink. The new sources need not conflict with the observed vertical profile of N2O, because: (1) the new O2†(high ν) sink could ensure the observed decrease of N2O mixing ratios in the upper atmosphere, and (2) the sources in the troposphere would be indistinguishable from the surface sources due to strong mixing and N2O long photochemical lifetime in this region. While some of these new sources and sinks are based on direct experimental data, others are based on new theoretical analysis of relevant experiments. Even so it is encouraging that the new sources could help us in removing the source deficit in the current N2O budget. So long as a prospect exists that NOxmight be produced in reactions of O2†(high ν) with N2O, it is constructive to speculate that the “new” N2O chemistry might have an effects on our current perceptions of NOx catalyzed stratospheric ozone depletion. If an important new source of NOx has been missed then an equally potent sink of NOx has also been missed, and it is possible that the inclusion of the latter into the current assessment models might affect our current perception of the atmospheric environmental effects of many aerospace activities.
Identification of turbulence structures above a forest canopy using a wavelet transform
Authors: Turner B. J. , Leclerc M. Y. , Gauthier M. , Moore K. E. , Fitzjarrald D. R.
Published: February 2004

The wavelet transform is used to identify scales of large coherent structures present in atmospheric turbulence above the subarctic forest at Schefferville. Individual coherent structures contributing to much of the exchange between the forest and the atmosphere are depicted in terms of both scale and location using contour diagrams of wavelet transform coefficients. Three typical case studies of turbulence and flux observations were selected to examine the physical characteristics of these flux‐filled events and their evolution With distance away from the forest canopy. A wavelet transform spectral technique is applied to vertical velocity, temperature, and turbulent heat flux data observed over the sparse coniferous forest to extract the relative importance of each scale present in those data series. The scale of turbulence structures in relation with their characteristic spacing is discussed.