bluuebunny/crossref_metadata_2025 huggingface dataset:
The monsoonal current regimes of the tropical Indian Ocean: Observed surface flow fields and their geostrophic and wind‐driven components
Authors: Hastenrath Stefan , Greischar Lawrence
Published: February 2008

The annual cycle of current regimes in the tropical Indian Ocean (to 30°S and 120°E) is studied on the basis of long‐term observations of the surface wind field, ship drift measurements of surface currents, and subsurface temperature and salinity casts to 400 dbar depth. For the major current domains, the Ekman surface current is computed from wind observations under assumption of a drag coefficient and of an eddy viscosity coefficient; the geostrophic surface flow is calculated from the field of the geopotential anomaly of the ocean surface relative to 400 dbar, constructed from subsurface temperature and salinity casts; the observed current is compiled from ship drift measurements; the Ekman volume transport is estimated using the assumed drag coefficient; and the geostrophic volume transport is obtained from the field of mass transport function between zero and 400 m relative to 400 dbar, constructed from subsurface temperature and salinity casts. The monsoonal reversals of wind stress forcing, which are most dramatic in the northern Indian Ocean and the equatorial zone and more moderate in the southern part of the basin, have diverse consequences for the various current systems. Under the steady southeast trades the westward directed South Equatorial Current (SEC) is basically geostrophic, with an additional strong Ekman contribution in boreal summer. The eastward directed South Equatorial Countercurrent (SCC) is mainly geostrophic, but in boreal summer it is virtually eliminated by the then westward directed Ekman component. The westward/eastward flowing Northeast Monsoon Current/Southwest Monsoon Current (NEM and SWM) of boreal winter/summer are Ekman flows. The Eastward Equatorial Jets (EEJ) during the monsoon transitions have larger geostrophic than Ekman components, while in the northward East African Coastal Current (EAC) and the northeastward Somali Current (SCN) of boreal summer the Ekman prevails over the geostrophic flow. In the latter three current systems the Ekman flow creates mass distributions that in turn entail sympathetic geostrophic currents. The prevailing geostrophic annual mean transports are of order 5 sverdrups (Sv) for the SCC, NEM, and SWM and about 13 Sv for the SEC. A transport performance comparable to that of the perennial SEC is attained by the EEJ during the monsoon transitions.
The mean sea surface height and geoid along the Geosat subtrack from Bermuda to Cape Cod
Authors: Kelly Kathryn A. , Joyce Terrence M. , Schubert David M. , Caruso Michael J.
Published: February 2008

Measurements of near‐surface velocity and concurrent sea level along an ascending Geosat subtrack were used to estimate the mean sea surface height and the Earth's gravitational geoid. Velocity measurements were made on three traverses of a Geosat subtrack within 10 days, using an acoustic Doppler current profiler (ADCP). A small bias in the ADCP velocity was removed by considering a mass balance for two pairs of triangles for which expendable bathythermograph measurements were also made. Because of the large curvature of the Gulf Stream, the gradient wind balance was used to estimate the cross‐track component of geostrophic velocity from the ADCP vectors; this component was then integrated to obtain the sea surface height profile. The mean sea surface height was estimated as the difference between the instantaneous sea surface height from ADCP and the Geosat residual sea level, with mesoscale errors reduced by low‐pass filtering. The error estimates were divided into a bias, tilt, and mesoscale residual; the bias was ignored because profiles were only determined within a constant of integration. The calculated mean sea surface height estimate agreed with an independent estimate of the mean sea surface height from Geosat, obtained by modeling the Gulf Stream as a Gaussian jet, within the expected errors in the estimates: the tilt error was 0.10 m, and the mesoscale error was 0.044 m. To minimize mesoscale errors in the estimate, the alongtrack geoid estimate was computed as the difference between the mean sea level from the Geosat Exact Repeat Mission and an estimate of the mean sea surface height, rather than as the difference between instantaneous profiles of sea level and sea surface height. In the critical region near the Gulf Stream the estimated error reduction using this method was about 0.07 m. Differences between the geoid estimate and a gravimetric geoid were not within the expected errors: the rms mesoscale difference was 0.24 m rms.
Surface heating and patchiness in the coastal ocean off central California during a wind relaxation event
Authors: Ramp Steven R. , Garwood Roland W. , Davis Curtiss O. , Snow Richard L.
Published: February 2008

The difference between the temperature of the ocean at 4‐cm and 2‐m depth was continuously monitored during a cruise to the coastal transition zone off Point Arena, California (38°58′N, 123°45′W), during June 1987. The two temperatures were coincident most of the time but diverged during one nearshore leg of the cruise where large temperature differences (ΔT) of up to 4.7°C were observed between the 4‐cm and 2‐m sensors, in areas which were separated by regions where the two temperatures were coincident as usual. The spatial scale of this “patchy” thermal structure was about 5–10 km. The Naval Postgraduate School mixed layer model (Garwood, 1977) was used to simulate the near surface stratification when forced by the observed wind stress, surface heating, and optical clarity of the water. The model produced a thin strongly stratified surface layer at stations where exceptionally high turbidity was observed but did not produce such features otherwise. This simple model could not explain the horizontal patchiness in the thermal structure, which was likely due to patchiness in the near‐surface chlorophyll distributions or to submesoscale variability of the surface wind stress.
Drifting buoy in the field of flow of two eddies on East Thulean Rise (northeast Atlantic)
Authors: Pingree R. D. , Le Cann B.
Published: February 2008

The track of an Argos buoy, drogued at a depth of 70 m, over a year's deployment (from April 18, 1989 to April 18, 1990) in the NE Atlantic is described. The mean speed of the buoy over a year's period was 1.4 cm s−1 to the NE. For 320 days there was little net movement, and during this time the buoy spent 66 days in a cyclonic eddy and 130 days in an anticyclonic eddy. The anticyclonic eddy diameter was about 200 km, and the buoy completed 21 complete revolutions around the eddy center. At a radius of 55 km, the rotation rate was 10 days. In the eye of the anticyclonic eddy the rotation rate was 36 hours. The inertial period for the latitude (∼51.5°N) was 15.3 hours, and inertial motions were occasionally marked in the anticyclonic eddy; the observed inertial period was about half an hour longer than the inertial period corresponding to the latitude. Both eddies appeared to be partially trapped to East Thulean Rise (water depth, 3000 m) and moved at a mean speed of about 1 cm s−1 to the NW along the rise, following water depth contours, though the anticyclonic eddy was stationary for about 50 days over the more central region of the rise. The track of an imperfectly drogued buoy in a circular eddy field is then examined analytically. A simple balance of stress forces on the drogued assembly is assumed, and commonly adopted assumptions for the stress forces are made. With assumptions, the idealized study shows that under conditions of a steady surface stress, a surface wind or current for example, a buoy can remain trapped within an eddy field and show no tendency for steady translation in the direction of the surface stress. For a clockwise eddy, the displacement of the buoy's track is to the right of the surface stress direction. For larger surface stresses or time varying winds the buoy may be “blown out” of an eddy of limited extent.
Velocities and shear stresses in wave‐current flows
Authors: Sleath John F. A.
Published: February 2008

A model is presented for the velocity distributions in combined wave‐current flows over rough beds. The model is based on a new analysis of turbulence measurements in oscillatory flow. The resulting formulas for the velocity distribution are remarkably simple and show good agreement with experiment. The formula derived for friction factor also shows good agreement with experiment.
Measurements of nitrogen productivity in the equatorial Pacific
Authors: Wilkerson Frances P. , Dugdale Richard C.
Published: February 2008

During the R/V Wecoma WEC88 cruise that sampled a meridional transect along 150°W from 15°N to 15°S, uptake of nitrate and ammonium by phytoplankton was measured using the stable isotope 15N with simulated in situ bottle incubations and shipboard mass spectrometry. A set of 25 daily productivity stations showed the influence of equatorial upwelling on nitrate distribution and 15N uptake in a band from 6°N to 7.5°S compared with the oligotrophic waters to the north and south, with the highest values of nitrate uptake occurring at the equator (0.14 mg‐at. m−2 h−1). During a 5‐day time series at the equator, there was an increase in nitrate accompanied by increased nitrate uptake. Interestingly, nitrate uptake rates (equivalent to new production rates) at the equator were lower than those predicted by previous investigators. Holdover experiments and uptake versus irradiance curves showed that the phytoplankton there were in an early stage of metabolic adaptation and that may have been a contributing factor.
The phytoplankton bloom in the northwestern Arabian Sea during the southwest monsoon of 1979
Authors: Brock John C. , McClain Charles R. , Luther Mark E. , Hay William W.
Published: February 2008

The biological variability of the northwestern Arabian Sea during the 1979 southwest monsoon has been investigated by the synthesis of satellite ocean color remote sensing with analysis of in situ hydrographic and meteorological data sets and the results of wind‐driven modeling of upper ocean circulation. The phytoplankton bloom in the northwestern Arabian Sea peaked during August‐September, extended from the Oman coast to about 65°E, and lagged the development of open‐sea upwelling by at least 1 month. In total, the pigment distributions, hydrographic data, and model results all suggest that the bloom was driven by spatially distinct upward nutrient fluxes to the euphotic zone forced by the physical processes of coastal upwelling and offshore Ekman pumping. Coastal upwelling was evident from May through September, yielded the most extreme concentrations of phytoplankton biomass, and along the Arabian coast was limited to the continental shelf in the promotion of high concentrations of phytoplankton. Upward Ekman pumping to the northwest of the Somali Jet axis stimulated the development of a broad open‐sea phytoplankton bloom oceanward of the Oman shelf. Vertical mixing during the 1979 southwest monsoon was apparently not a primary cause of the regional‐scale phytoplankton bloom.
Linear systems analysis of momentum on the continental shelf and slope of the central Great Barrier Reef
Authors: Burrage Derek M. , Church John A. , Steinberg Craig R.
Published: February 2008

Current meter records from a mooring transect deployed across the continental shelf and slope of the central Great Barrier Reef, Australia during 1985 have been analyzed in a study of the subtidal momentum balance. In the 3–20 day wave band, a single‐input linear systems model of the subtidal along‐shelf flow, driven by across‐shelf pressure gradient (i.e., assuming semi‐geostrophic balance), explained over 70% of the variance on the shelf, but only 30% at the shelf break and upper slope. A two‐input model driven by along‐shelf horizontal pressure gradient and wind stress, and incorporating along‐shelf acceleration and bottom stress, explained approximately 60% of the variance on both the shelf and upper slope. The model responses evidently combine local wind‐driven circulation and freely‐propagating continental shelf waves. Linear resistance coefficients estimated from the two‐input model averaged 0.07 cm s−1, but were higher (0.09) within the reef matrix and lower (0.06) near the coast.
Structure and transport of the East African Coastal Current
Authors: Swallow John C. , Schott Friedrich , Fieux Michèle
Published: February 2008

The East African Coastal Current (EACC) runs northward throughout the year between latitudes 11°S and 3°S, with surface speeds exceeding 1 m s−1 in northern summer. Mean transport from five sections near 4°–5°S is 19.9 Sv (1 Sv ≡ 106 m3 s−1) northward in the upper 500 dbar, out to 120 km offshore. Below that, between 500 and 1000 dbar, there appears to be a weak variable transport of the order of 1 Sv. Comparing transports in the EACC with those in the boundary current north of Madagascar, it seems that most of the water in the upper 300 dbar of the northern branch of the South Equatorial Current goes into the EACC. Below 300 dbar there is an excess westward transport north of Madagascar which probably goes into the Mozambique Channel. At its northern end, in northern winter the EACC converges with the south‐going Somali Current to form the Equatorial Countercurrent, with an eastward transport of about 22 Sv (0–300 dbar). In northern summer, the EACC merges into the north‐going Somali Current.
Synthetic aperture radar observations of currents colocated with slicks
Authors: Ochadlick Andrew R. , Cho Peter , Evans‐Morgis Jeannette
Published: February 2008

During low wind conditions, our synthetic aperture radar (SAR) imagery of the ocean contains examples of strong perturbations in the geometry of persistent turbulent ship wakes at the intersection of wakes with dark slicklike features. The perturbations in the wakes are presumably caused by currents and are invariably colocated with slicklike submesoscale features. The current dynamics, influencing the geometry of turbulent wakes, are hypothesized to contribute to the efficient accretion of a biotic and abiotic microlayer in a localized surface zone, which is observed as a dark slick in our SAR imagery. Time displaced SAR imagery allows us to track the time evolution of the turbulent wake perturbations to determine the differential surface current responsible for the perturbations and to estimate the associated shear. In addition, the time displaced imagery also allows us to observe the temporal stability of the slicklike features.
The Peru upwelling and the ventilation of the south Pacific thermocline
Authors: Toggweiler J. R. , Dixon K. , Broecker W. S.
Published: February 2008

A reconstruction of the prebomb Δ14C distribution in the tropical Pacific using data from old coral heads shows that surface waters with the lowest Δ14C content are found distinctly south of the equator. Prebomb, low‐Δ14C surface water appears to owe its origin to the upwelling of ∼15°C water off the coast of Peru. The low‐Δ14C water upwelling off Peru is shown to be derived from the “13° Water” thermostad (11°–14°C) of the Equatorial Undercurrent. Untritiated water in the lower part of the undercurrent had nearly the same Δ14C content during the Geochemical Ocean Sections Study (GEOSECS) as the prebomb growth bands in Druffel's (1981) Galapagos coral. Similar Δ14C levels were observed in 9°–10°C water in the southwest Pacific thermocline in the late 1950s. We suggest that the low‐Δ14C water upwelling off Peru and the thermostad water in the undercurrent both originate as ∼8°C water in the subantarctic region of the southwest Pacific. This prescription points to the “lighter variety” of Subantarctic Mode Water (7°–10°C) as a possible source. Because prebomb Δ14C is so weakly forced by exchange of carbon isotopes with the atmosphere, thermocline levels Of Δ14C should be particularly unaffected by diapycnal mixing with warmer overlying water types. We argue that successively less dense features of the South Pacific thermocline, like the Subantarctic Mode Water, the equatorial 13°C Water, and the Peru upwelling, may be part of a single process of thermocline ventilation. Each evolves from the other by diapycnal alteration, while prebomb Δ14C is nearly conserved. Detailed comparisons are made between the coral Δ14C distribution and a model simulation of radiocarbon in Toggweiler et al. (1989). While the Δ14C data suggest a southern hemisphere thermocline origin for the equatorial Δ14C minimum, the model produces its Δ14C minimum by upwelling abyssal water to the surface via the equatorial divergence. In an appendix to the paper we present a new set of coral Δ14C measurements produced over the last 10 years at Lamont‐Doherty Geological Observatory and compile a post‐1950 set of published coral Δ14C measurements for use in model validation studies.
Turbulence, mixing, and heat flux in the ocean main thermocline
Authors: Gibson Carl H.
Published: February 2008

Independent temperature microstructure samples of Gregg (1977) in the main thermocline of the mid‐Pacific in different years and seasons are used to infer space‐time average vertical heat fluxes and space‐time average turbulence and temperature dissipation rates. Lognormality plots of these data suggest that mixing rates in thermocline layers are approximately lognormal and very intermittent, with intermittency factors σ2lnC̄150m that increase with depth from 3 to 6 as Kz increases from ≈ 0.2 to 1.6 cm2 s−11, and ϵ ≈ (0.6–1.6)×10−4 cm2 s−3. Decorrelation length scale ranges for turbulence with such large intermittency factors should be large by the Kolmogorov intermittency (third universal similarity) hypothesis. Available microstructure data suggest that mixing concentrates near fronts and is dominated by powerful bursts of turbulence that fossilize and radiate breaking internal waves. Equations for the estimation of vertical heat flux from temperature dissipation rate measurements are rederived by a control volume rather than Reynolds averaging method that emphasizes the necessity for large averaging length and time scales and does not exclude frontal mixing and nonturbulent mixing processes that are neglected by most applications of the Osborn and Cox (1972) model. The indicated heat fluxes qz are about 6 and 3 W m−2 at 0.1 and 1 km depths, consistent with deeper bulk flow estimates of qz ≈ 2 W m−2 from Munk (1966) but much larger than those inferred from the Gregg (1989) thermocline ε correlation that decrease from qz ≈ 1 to 0.1 W m−2 in this depth range. It is suggested that the Gregg (1989) values of qz, ̄2lnϵ ≈ 0.04–0.9, Kz ≈ 0.06 cm2s−1, and ϵ ≈ 7×10−6 cm2 s−3 for the ocean main thermocline are underestimates due to a combination of undersampling errors.
Wind ambiguity removal by the use of neural network techniques
Authors: Badran F. , Thiria S. , Crepon M.
Published: February 2008

This paper deals with the removal of ambiguities existing on the direction of the wind measured by satellite scatterometers. It is shown that neural networks are well adapted to solving this problem. Results using ERS. 1 simulated scatterrometric wind fields are presented. It is found that 99% of the ambiguities can be removed by imposing a rate of 25% ambiguity at 180° and 90°.
Analysis of collinear passes of satellite altimeter data
Authors: van Gysen Herman , Coleman Richard , Morrow Rosemary , Hirsch Bernd , Rizos Chris
Published: February 2008

The analysis of altimetry measurements along repeated satellite ground tracks, as in the Geosat Exact Repeat Mission, provides an excellent way of determining temporal variability in the sea surface height. This collinear analysis is usually carried out in two steps: first, by determining a mean sea level profile and then by removing a radial orbit error. We propose a new unified analysis method for the simultaneous estimation of orbit error, mean sea surface height, and residual sea surface heights (variability) with respect to each pass. All passes are treated in exactly the same way; the consequent rank or datum defect in the model is overcome by appending a minimal set of conditions, chosen either to give no overall distortion of the orbits or otherwise a best fit to a reference profile, such as the geoid. Gaps in the altimeter data are easily accommodated. Numerical results of the method are shown using data from the first year of the Geosat Exact Repeat Mission. Some consideration is given to the choice of radial orbit error model. The results indicate that for orbit error removal, a sinusoid model is the most appropriate over a one‐revolution cycle. For passes less than one revolution in length, a component of the longer wavelength ocean signal is lost if using the sinusoid model. If passes longer than about two revolutions (some 200 mins) are analyzed, additional terms are needed to eliminate nongravitational perturbations.
Surface‐based passive microwave studies of multiyear sea ice
Authors: Grenfell T. C.
Published: February 2008

Multifrequency passive microwave observations are reported from surface based field experiments carried out over multiyear sea ice in the eastern Arctic Basin, the Beaufort Sea, the Canadian archipelago and the northern Greenland Sea. Dual polarized brightness temperatures at 6.7, 10, 18.7, 37, and 90 GHz were obtained at selected multiyear ice sites covering a wide range of surface properties. Considerable variation was found in the observed spectra over short distances on individual floes under winter conditions where values of [ev(37 GHz) −ev(18 GHz)]/[ev(37 GHz) +ev(18 GHz)] ranged from nearly zero to approximately −0.08. Concurrent surface characterization observations consisting of temperature, density, salinity, and grain size profiles in the snow and the upper meter of the ice were carried out frequently in conjunction with Cold Regions Research and Engineering Laboratory personnel. The present analysis shows that the magnitude of the spectral gradient of emissivity is directly related to the existence and thickness of a decomposed surface ice layer with very high porosity. Spectra for melt ponds with a frozen surface layer closely resembled those of lake ice and show a positive spectral gradient. As soon as the ponds were completely frozen, their spectra became nearly the same as those for first‐year ice. Fine‐grained winter snow produced a mild decrease in the emissivities at 90 GHz with increasing snow thickness from 0 to 0.2 m at which point the snow appears to be optically thick. During the melt season the ice emissivitieseνdepend most strongly on whether the surface layers are experiencing temporary freezing or not. Under melting conditions,eνdepends on the liquid water content only over a very small range (e.g., 0–5% by mass at 90 GHz). If the liquid water in the surface layers is freezing, which can occur at intervals throughout the melt season, the spectral gradient is negative with a slope which depends on the thickness of the refrozen layer. The variance among emissivity spectra for multiyear ice appears to be influenced primarily by the distribution of melt ponds and by the presence of significant amounts of scattering inhomogeneities greater than 1 mm in diameter in the snow and in the upper 20–30 cm of the ice. We expect the signatures of multiyear ice to be regionally dependent, perhaps yielding environmental information about the Arctic ice pack.
Hysteresis of turbulent kinetic energy in nonrotational tidal flows: A model study
Authors: Baumert Helmut , Radach Günther
Published: February 2008

For horizontally homogeneous barotropic conditions, nonrotational tidal (M2) cycles of hydrodynamic fields are computed by the standardk— ε turbulence model for large Reynolds numbers. The characteristic tidal time lag between current and turbulent kinetic energy at the flow reversal turned out to be close to measurements in shallow tidal rivers reported in the literature. Using a scale transformation of the model equations, the phase shift between the surface and the bottom flow is shown to be related to the dimensionless Strouhal number. In shallow waters this number is large and the phase shift is negligible. In deeper water the Strouhal number is much smaller, the shift becomes significant, and the lag between current and turbulence quantities is even one order of magnitude larger than the shift. The results indicate that thek— ε turbulence model can reproduce the essential physical details of tidal flows.
The sizes, frequencies, and freeboards of East Greenland icebergs observed using ship radar and sextant
Authors: Dowdeswell Julian A. , Whittington Robert J. , Hodgkins Richard
Published: February 2008

The Scoresby Sund fjord system, East Greenland, contains the most productive fast‐flowing outlet glaciers draining east from the Greenland Ice Sheet, calving 18 km3 a−1 of icebergs. The sizes, frequencies, and freeboards of 1900 icebergs were measured from F.S. Polarstern, using ship X‐band radar and sextant. Radar beam spreading exaggerates iceberg width by 60 m per nautical mile of range beyond the first mile. Data sets on iceberg size (e.g., that collated for Antarctic icebergs) collected using ship radars which do not take this effect into account will overestimate iceberg dimensions significantly. The location and concentration of icebergs within the fjord complex can be explained by (1) the locations of the principal source glaciers and (2) fjord topography and bathymetry. Iceberg concentration (maximum 0.6 icebergs km−2) declines with distance from the major iceberg sources. We found that 69% of icebergs within the fjord system are <200 m in width. Only five are >1 km in length. The largest is 2.7 km long. Icebergs become spread more evenly over the range of size classes in the outer fjord and shelf. Modal iceberg keel depth, calculated from freeboard measurements, is 4–500 m in the inner fjords, shifting to lower values in the outer fjords, reflecting shallower bathymetry. Radar measurements of iceberg width cannot be used to infer keel depths accurately, because width and keel depth are only weakly correlated. Comparison between freeboards and keel depths for icebergs from East Greenland and the Barents Sea indicates that the iceberg source (i.e., floating or grounded) exerts a fundamental control on iceberg dimensions. The drift pattern of icebergs is from the head to the mouth of the fjord system, although fjord bifurcations, bathymetry, and currents provide additional complications. The trends in observed iceberg size and frequency, and in inferred keel depth, in the Scoresby Sund region are likely to be applicable to other fjords and shelves around Greenland.
On the calculation of suspended load for noncohesive sediments
Authors: McLean S. R.
Published: February 2008

Calculation of suspended sediment transport rates requires integration of the product of sediment concentration times velocity over the depth of the flow; hence accurate profiles of these quantities are required especially near the sediment bed. Although this would appear to be a straightforward task, many effects can cause significant deviation of these quantities from those values expected from simple known relationships. For example, density stratification by suspended particulates can dampen turbulence fluctuations and significantly reduce the effective viscosity and diffusivity of the flow. The momentum and suspended sediment equations are solved herein, including the effects of stratification by suspended sediment and the effects due to size distribution and bedforms. The results show that stratification and bedforms generally reduce the transport capacity of the flow, whereas treating the sediment as an assemblage of different sizes rather than a single size (D50) increases the expected transport.
Isopycnal and diapycnal circulation of the upper equatorial Atlantic Ocean in 1983–1984
Authors: Gouriou Yves , Reverdin Gilles
Published: February 2008

Isopycnal circulation and diapycnal processes in the equatorial Atlantic Ocean are investigated using eight cruises from November 1982 to July 1984. An eastward decrease of the transport of the equatorial undercurrent is observed averaging 18.2 106 m3 s−1 at 35°W to 10.2 106 m3 s−1 at 4°W. There is also a meridional convergence at the undercurrent level and a larger meridional divergence in the surface layer of 15 106 m3 s−1 between 35°W and 4°W. In the eastern equatorial Atlantic, the undercurrent core density as well as the salinity above the undercurrent experience a large seasonal cycle. During the season where the eastern Atlantic thermocline is closest to the surface and surface waters are coldest, the current peaks in denser waters and subsurface salinity maximum are weaker. An analysis of the salinity on isopycnal surfaces indicates that there is significant diapycnal mixing in the upper thermocline. In the eastern Atlantic, the upper thermocline vertical heat diffusivity coefficient scaled over 1° of latitude varies between 3 cm2 s−1 during the upwelling season and nearly 0 early in the year. This seasonal mixing is an important element of undercurrent dynamics. The turbulent heat flux at the base of the surface layer averages 50 W m−2 between 1.5°N and 1.5°S. The surface layer heat budget implies an average oceanic gain of 60 W m−2 between 1.5°N and 1.5°S which could result from exchanges with the atmosphere. The heat flux and fresh water seasonal cycles needed to close the budgets are not realistic, however. The eastern equatorial Atlantic thermocline seasonal upwelling is associated with a large inflow into the surface layer. Over the two years, this flow averaging 11–12 × 106 m3 s−1 originates from the upper thermocline, and diapycnal transports near the core of the undercurrent or below are found to be small. Below the core of the undercurrent, we also find that mixing is not intense: for instance, at σθ = 26.5, in the upper part of the thermostad, vertical heat diffusivity is only 0.6 cm2 s−1 (assuming that mixing takes place over 1° of latitude). The uncertainties on these budgets are however large, and assumptions on the dynamics that were used could not be checked.
A model for the generation of two‐dimensional surf beat
Authors: List Jeffrey H.
Published: February 2008

A finite difference model predicting group‐forced long waves in the nearshore is constructed with two interacting parts: an incident wave model providing time‐varying radiation stress gradients across the nearshore, and a long‐wave model which solves the equations of motion for the forcing imposed by the incident waves. Both shallow water group‐bound long waves and long waves generated by a time‐varying breakpoint are simulated. Model‐generated time series are used to calculate the cross correlation between wave groups and long waves through the surf zone. The cross‐correlation signal first observed by Tucker [1950] is well predicted. For the first time, this signal is decomposed into the contributions from the two mechanisms of leaky mode forcing. Results show that the cross‐correlation signal can be explained by bound long waves which are amplified, though strongly modified, through the surf zone before reflection from the shoreline. The breakpoint‐forced long waves are added to the bound long waves at a phase of π/2 and are a secondary contribution owing to their relatively small size.
Polycyclic aromatic and organochlorine compounds in the atmosphere of northern Ellesmere Island, Canada
Authors: Patton G. W. , Walla M. D. , Bidleman T. F. , Barrie L. A.
Published: February 2008

In February–April 1988 we collected air samples at Alert in the Canadian Arctic (82.5°N, 62.3°W) to determine the types, concentrations, and vapor‐particle relationships for polycyclic aromatic hydrocarbons (PAH) and oxygenated compounds, Organochlorine (OC) pesticides, and polychlorinated biphenyls (PCB). Samples were taken using a glass fiber filter‐polyurethane foam train and were analyzed by capillary gas chromatography using mass selective and electron capture detection. PAH and oxygenated compounds included dibenzofuran, biphenyl, fluorene, phenanthrene, 9‐fluorenone, fluoranthene, benzofluoranthenes, pyrene, chrysene, benzopyrenes, indeno[cd]pyrene, benzo[ghi]perylene, 2‐methyl phenanthrene, benz[a]anthracene, and anthracene (given in order of relative abundance, highest to lowest). OC compounds included hexachlorocyclohexanes (HCH), hexachlorobenzene, pentachlorobenzene, PCB, polychlorocamphenes, chlordanes, and the dichlorodiphenyl‐trichloroethane (DDT) group (given as above). The concentration ratios of α‐HCH/γ‐HCH (5.2–9.8) andtrans‐ tocis‐chlordane (0.78–1.29) are reported. Compounds having estimated liquid‐phase saturation vapor pressure (pL0) ≥ 10‐3Pa at the average sampling temperature (245 K) were almost entirely gaseous. Those from 10‐6≤pL0≤ 10‐3Pa were distributed between the particle and gas phases, whereas little or no gaseous component was evident for compounds havingpL0≤ 10‐6Pa. The particle‐vapor distribution of PAH and OC compared favorably to the Junge‐Pankow model.
Atmospheric water balance in the Amazon basin: An isotopic evapotranspiration model
Authors: Gat J. R. , Matsui E.
Published: February 2008

An increase of 3‰ in the “d” value of the isotopic composition of precipitation over the Amazon Basin suggests that an isotopically fractionated evapotranspiration flux contributes to the atmospheric water balance over the region. A steady state evapotranspiration model for the Amazon Basin was developed, and it is shown that the observed increase in the “d” value corresponds to a situation where 20–40% of the total evapotranspiration flux is accompanied by an isotopic fractionation, such as by evaporation from an open water surface. The site of this fractionation, whether by evaporation from the flood plains, canopy interception or lakes, cannot be resolved with the presently available data set.
Ion‐assisted tropospheric OH measurements
Authors: Eisele F. L. , Tanner D. J.
Published: February 2008

A highly sensitive real‐time ion‐assisted OH measurement technique has recently been developed and tested. It offers an OH detection sensitivity of about 1×105 molecules/cm3 with a 300‐s integration time and provides useful OH concentration measurements on a 10‐s time scale. Measurements of OH concentration are seen to track solar flux during both short and long periods of cloudiness. Good temporal correspondence between changes in OH and O3 concentration as differing air masses pass the measurement site are also observed and discussed. The present technique provides a new opportunity for fast sensitive OH measurement using a totally new nonoptical measurement scheme.
Upper limit for sea ice albedo feedback contribution to global warming
Authors: Covey Curt , Taylor Karl E. , Dickinson Robert E.
Published: February 2008

Two versions of the National Center for Atmospheric Research Community Climate Model (CCM) are used to calculate the increase in solar energy absorbed by the Earth‐atmosphere system if all sea ice on the planet were to melt. The increase in solar energy is determined at several time points in the seasonal cycle by brief integrations of the models with the surface albedo of sea ice changed to that of open ocean; temperature, cloudiness, and other climate parameters are unchanged during the short integrations, so that our results isolate the climatic effect of sea ice albedo changes in the absence of other processes and feedbacks. (In particular, we do not include the effects of removing the insulation between ocean underneath sea ice and the atmosphere above it.) We find that the globally and annually averaged enhancement of absorbed solar flux due to removal of sea ice is 2–3 W m−2; a simple calculation indicates that most of the difference between model versions is due to differences in the surface albedo of sea ice. About half the albedo reduction at the surface is masked at the top of the atmosphere by clouds, even though the CCM versions we use tend to underestimate cloudiness. Our upper limit is significant compared to the direct radiative forcing of a doubling of atmospheric carbon dioxide, but it suggests that for greenhouse gas warming equivalent to doubling of CO2 or greater, the sea ice albedo feedback is likely to be smaller than that from water vapor and potentially that from clouds.
The effect of liquid water on thunderstorm charging
Authors: Saunders C. P. R. , Keith W. D. , Mitzeva R. P.
Published: February 2008

Laboratory studies have shown that thunderstorm charging caused by the interactions of ice crystals and graupel pellets is affected in sign and magnitude by temperature and cloud liquid water content; the presence of water droplets is a requirement for substantial charge transfer. Relationships showing the dependence of charge transfer on ice crystal size and velocity have previously been reported and now, in a continuation of the laboratory studies, the effect of liquid water content on the charge transfer has been investigated. The experiments show that positive graupel charging occurs at temperatures above a “charge sign reversal temperature” with negative charging at lower temperatures. The reversal temperature moves to lower temperatures when the liquid water content is increased. However, at low values of liquid water content, the sign of the graupel charging is inverted being positive at low temperatures and negative at higher temperatures. A discussion is presented of the various charge transfer theories. The results are consistent with the idea of two competing mechanisms whose relative success depends on the temperature and liquid water content. Positive graupel charging occurs when the graupel surface grows from the vapor and the crystals interact with a negative surface charge caused by a temperature gradient across the rime ice surface layer. Negative graupel charging occurs when the surface growth effect is swamped by freezing droplets which create either a pseudo contact potential with which the crystals interact, or a positive surface charge, due to dislocations in the rime ice, which is removed during glancing crystal interactions. Relationships between charge transfer, liquid water content, temperature, ice crystal size, and velocity have been determined and the equations may be used in numerical models of the development of thunderstorm electric fields. A one‐dimensional model indicates that the charge separation rates noted here are adequate to account for thunderstorm electrification. Use of the equations with cloud parameter values obtained in a thunderstorm research flight, leads to a predicted charge reversal level around −13°C, which is in agreement with the analysis of the electric field in the thunderstorm studied.
Validation of SAGE II NO2 measurements
Authors: Cunnold D. M. , Zawodny J. M. , Chu W. P. , Pommereau J. P. , Goutail F. , Lenoble J. , McCormick M. P. , Veiga R. E. , Murcray D. , Iwagami N. , Shibasaki K. , Simon P. C. , Peetermans W.
Published: February 2008

Stratospheric aerosol and gas experiment (SAGE) II satellite‐borne measurements of the stratospheric profiles of NO2 at sunset have been made since October 1984. The measurements are made by solar occultation and are derived from the difference between the absorptions in narrow bandwidth channels centered at 0.448 and 0.453 μm. The precision of the profiles is approximately 5% between an upper altitude of 36 km and a latitude‐dependent lower altitude at which the mixing ratio is 4 ppbv (for example, approximately 25 km at mid‐latitudes and 29 km in the tropics). At lower altitudes the precision is approximately 0.2 ppbv. The profiles are nominally smoothed over 1 km except at altitudes where the extinction is less than 2×10−5/km. (approximately 38 km altitude), where 5 km smoothing is employed. The profile measurement noise has an autocorrelation distance of 3–5 km for 1 km smoothing and more than 10 km for 5 km smoothing. The absolute accuracy of the measurements is estimated to be 15% based on uncertainties in the absorption cross‐sections and their temperature dependence. Comparisons against two sets of balloon profiles and atmospheric trace molecules spectroscopy experiment (ATMOS) measurements show agreement within approximately 10% over the altitude range of 23 to 37 km at mid‐latitudes. SAGE II NO2 measurements are calculated to be approximately 20% smaller at the mixing ratio peak than average limb infrared monitor of the stratosphere (LIMS) measurements in the tropics in 1979. They show acceptable agreement with SAGE I sunset NO2 measurements in the tropics in 1979–1981 when the limited resolution and precision of the SAGE I measurements and the differences between the two measurement techniques are considered.
A study of the diabiatic heating associated with the Madden‐Julian oscillation
Authors: Chen Tsing‐Chang , Yen Ming‐Cheng
Published: February 2008

It has been inferred from numerous studies analyzing out going long wave radiation (OLR) data in the tropics that cumulus convection injects energy through released latent heat into the global‐scale, 30–60 day (Madden‐Julian, MJ) oscillation of divergent circulation, which is well portrayed by velocity potential (χ) anomalies. To substantiate this inference regarding the global scope of the diabatic hearing effect, we propose an χ‐maintenance equation χ = χQ ‐ χHA. The values χQ and χHA are the contributions of vertical differential diabatic and adiabatic heating to χ, respectively. The 4‐year (1979 and 1985–1987) upper‐air daily data generated by the Global Data Assimilation System of the National Meteoroligical Center were used to compute global χ, χQ, and χHA anomalies, and the global‐scale χ MJ oscillation is shown to be related to diabatic heating by HVD (= ∇2 χQ) anomalies. To understand the geographic relation between the differential diabatic heating effect and the global‐scale χ MJ oscillation, composite charts of χ and HVD anomalies during various stages of this oscillation were constructed. Centers of positive (negative) HVD anomalies coincided with divergent (convergent) centers of χ anomalies. It was also found that the geographic distributions of our HVD anomalies and the OLR anomalies from previous studies resembled each other. These findings support the inferences from previous studies that maintaining the global‐scale χ MJ oscillation is linked to diabatic heating through the latent heat released by cumulus convection.
Volcanic disturbances in the stratospheric aerosol layer over Tsukuba, Japan, observed by the National Institute for Environmental Studies lidar from 1982 through 1986
Authors: Hayashida‐Amano S. , Sasano Y. , Iikura Y.
Published: February 2008

The stratospheric aerosol layer has been monitored from 1982 through 1986 by a lidar in NIES, Tsukuba (36°N, 140°E). A strong enhancement in the signals from the stratospheric aerosol layer was observed in 1982 and 1983 as a result of the volcanic aerosols from the El Chichon (Mexico) eruption in March and April 1982. Multiplex thin aerosol layers with ∼1‐km thickness were observed in the autumn of 1982. Another enhancement in the amount of stratospheric aerosols was observed in the winter of 1985/1986. The origin of the new aerosol layer can be attributed to the eruptions of Nevado del Ruiz (4°53′N, 75°22′W) on November 13, 1985. The long‐term decay of the integrated backscattering coefficient (IBC) from 1982 through 1986 will also be discussed. The optical thickness converted from IBC observed at Tsukuba in late 1982 was up to about 0.2 which was larger than that observed over North America in the same period.
Using sparse raingages to test satellite‐based rainfall algorithms
Authors: Morrissey Mark L.
Published: February 2008

A statistical method is presented which uses sparsely distributed raingages to obtain calibrations for satellite‐based rainfall algorithms. The method can also be used for the development and verification of these algorithms. It is assumed that the distributions of areal rainfall, given a value of a satellite areal rainfall estimate, are homogeneous and stationary over a region encompassing a network of sparse raingages. The method may best be applied over open ocean regions to supplement the limited number of dense raingage/radar networks there. A description of the method is given, followed by an illustration of the method using simulated rain fields. Finally, the method is used to calibrate a simple satellite rainfall algorithm using Pacific atoll monthly raingage data.
A study of heterogeneous oxidation of SO2in summer clouds
Authors: Husain Liaquat , Dutkiewicz V. A. , Hussain M. M. , Khwaja H. A. , Burkhard E. G. , Mehmood G. , Parekh P. P. , Canelli E.
Published: February 2008

Experimental determination of in‐cloud oxidation of SO2has been lacking because of an inability to distinguish SO4in cloud water, derived from aerosol scavenging (i.e., produced from homogeneous gas phase oxidation), from that due to in‐cloud SO2oxidation, (SO4)in. A tracer technique has been developed in this laboratory that uses trace elements (M) to resolve the two SO4components. Extensive data are reported in this paper on the use of Se as a tracer. Preliminary results are also included to test As and Sb as potential tracers. The technique is based on simultaneous determination of SO4and M in cloud water (cw) and in out‐of‐cloud air (aa), and the knowledge of the relative scavenging efficiencies α and β of SO4and M‐bearing aerosols, respectively: (SO4)in= [(SO4/M)cw‐ α/β (SO4/M)aa] (M)cw. Data from two events are presented which yield the scavenging ratio of SO4/Se as 1.04±0.29 and 1.04±0.19. The technique has been used to quantitatively determine in situ SO2oxidation in six summer cloud systems during 1987–1989 at Whiteface Mountain, New York. Initially, cloud water samples were collected hourly, but in later sampling campaigns this interval was reduced to 15 min to better delineate the relationship between chemistry and meteorology in clouds. Aerosol collection was also reduced from 12 to 2 hours. Five cloud systems with pH ranging from 2.8 to 4.0 processed air which, based on our understanding of elemental signatures, had passed through the high‐pollution emitting areas of the midwest. In individual samples, (SO4)invaried from 0 to 39%. The amount of (SO4)indid not vary dramatically from one sample to the next. The data appeared to be consistent with a flow‐through reactor at the site through which updrafted air is passing. Where available, SO2data in clear air and within the cloud yielded a mass balance consistent with our experimentally determined in situ SO4formation. A sixth cloud event, a storm system which processed air largely from the north‐northwest (“clean air”), yielded pH of 4.3 to 5.0 in the cloud water. Our tracer techniques based on both Se and As revealed no in situ oxidation of SO2. This was confirmed by our observation that SO2concentration was low, and values in the cloud and in cloud‐free air were equal. When data from all six summer cloud systems were pooled together, (SO4)indecreased with increasing pH with negligible oxidation above pH 4. Also, (SO4)indecreased smoothly with increasing aerosol SO4/Se ratio. Coal combustion is the primary source of S and Se in rural atmosphere in the northeast. Relative proportions of SO2and SO4are dependent on atmospheric oxidation. Enhancement in oxidation yields higher SO4/Se ratio and at the same time leaves less SO2in the air for oxidation. A simple model is developed that enables an estimation of SO2oxidation in clouds.