bluuebunny/crossref_metadata_2025 huggingface dataset:
A laboratory study of surface boundary currents: Application to the Algerian Current
Authors: D'Hieres G. Chabert , Didelle H. , Obaton D.
Published: February 2008

The problem of a gravity current along a coast is considered in the laboratory. The flow originates from a continuous pointsource and, when facing downstream, has the shore to its right. The shore is the vertical wall of a 13‐m‐diameter rotating basin. The density of the current, r1, is slightly less than that of the surrounding fluid. This buoyant flow is shown to depend upon three parameters; namely, the Burger and Ekman numbers and the aspect ratio of the current at its source. Disregarding friction and taking the flow as hydrostatic so that the current aspect ratio does not need to be considered, the stability characteristics of the current are shown to depend mostly upon the Burger number. To study its role in the flow, we performed several experiments by varying the Burger number Bu between 0.15 and 0.82. For large Burger numbers the current is stable, but as it decreases, increasing numbers of anticyclonic disturbances develop and grow along the axis of the current. Measurements of the normalized distance from the source, and times for these disturbances to develop, are given as functions of the Burger number. The dependence of the normalized separation distance of disturbances upon the Burger number, in cases of multiple instabilities, is also discussed. Finally, these laboratory results are compared with the Algerian Current, where both anticyclonic and cyclonic eddies have been observed. The experimental current simulates the Algerian Current when the Burger number is 0.15.
Dynamics of the Coastal Transition Zone through data assimilation studies
Authors: Walstad Leonard J. , Allen J. S. , Kosro P. Michael , Huyer Adriana
Published: February 2008

The dynamics of the coastal transition zone off Northern California during late May and early June 1987 are examined through assimilation modeling studies. A regional baroclinic quasi‐geostrophic model is driven by the data through initial and boundary conditions. These initial and boundary conditions are specified by objective analysis of hydrographic and acoustic Doppler current profiler data. The data assimilation is accomplished by varying the objective analysis parameters, numerical parameters, and subgrid‐scale parameters until the final solution of the model is in best agreement with the analysis of the data. The solution which best agrees with the data is regarded as a four dimensional field estimate of the coastal transition zone flow. An aspect of this study that is new to data assimilation modeling of mesoscale eddy fields is the use of acoustic Doppler current profiler data. These data prove to be very important to accurate description of the oceanic flow field as indicated by comparison with float trajectories. The complete data set provides an opportunity to study the ability of dynamical constraints to improve field estimates when acting upon a subset of the data (hydrography). Data assimilation modeling generates field estimates that are substantially better than those based upon the hydrographic data alone as indicated by comparison with the acoustic Doppler current profiler based analyses. The kinematics and energetics of this constrained (quasi‐geostrophic) field estimate are examined. Despite the relatively small region (108 by 324 km ) and short period of the field estimate (21 days), interesting processes are identified. A meandering barotropically unstable part of the jet is identified using the results of related idealized numerical studies and stability analyses. Similarly, this jet may be undergoing a simultaneous larger scale mixed instability. Another part of the jet interacts with an eddy and meanders in a much different manner. Characteristics of the energy balances are identified and compared with the results of linear analysis and of nonlinear studies utilizing idealized models of the observed jet as described in this issue by Pierce et al. and Allen et al. respectively.
Observations of amplified flows atop a large seamount
Authors: Eriksen Charles C.
Published: February 2008

Current meter records from a mooring atop a large seamount in the eastern North Pacific (Fieberling Guyot) are dominated by diurnal fluctuations at depths close to the seamount summit depth. These oscillations are driven by diurnal tides to have amplitudes of 0.2–0.4 m/s or more and exhibit characteristics of seamount trapped waves (i.e., coastal trapped waves for a submerged periodic coast). These waves rectify to produce anticyclonically directed low‐frequency currents in the vicinity of the seamount summit of magnitude comparable to the rms wave amplitude. Mean currents over several months are directed across isobaths at depths more than 200 m shallower than the seamount summit. Currents closer to the bottom have a component directed toward deeper water and are ageostrophic as a result of wave rectification. Internal wave band currents are more energetic with increasing proximity to the local bottom, presumably as a result of wave reflection and focussing.
Particulate organic matter δ13C variations across the Drake Passage
Authors: Rau G. H. , Takahashi T. , Des Marais D. J. , Sullivan C. W.
Published: February 2008

A 7 ‰ gradient in the δ13C of suspended particulate organic matter (POM) was observed in samples taken during two transects across the Drake Passage during March 1986. This POM δ13C transition from −23.2 ‰ at 53.3°S to values as low as −30.3 ‰ at >62°S does not track previously reported abrupt changes in water chemistry and plankton species composition associated with the Polar Front Zone that resides at approximately 58°S in this region. Also, the north‐south isotopic trend is not accompanied by significant changes in POM carbon or nitrogen concentrations, or in POM C/N. Differences in plankton standing crop or biochemistry (e.g., lipid content) therefore do not appear responsible for the isotopic trends observed. The latitudinal change in POM δ13C is, however, highly correlated with water temperature and with the calculated concentration of CO2 (aq) at equilibrium with atmospheric CO2. These observations are consistent with the hypothesis that [CO2 (aq)] significantly influences POM δ13C in ocean surface waters.
Tidal and buoyancy‐driven currents in Chignecto Bay, Bay of Fundy
Authors: Tee Kim T. , Amos Carl L.
Published: February 2008

Circulations in the Chignecto Bay system (upper part of the Bay of Fundy) were studied using current meter moorings, tidal and diagnostic models, and salinity and suspended sediment concentration measurements. The current meter observations revealed that (1) the tides behaved basically as standing waves (tidal current leads tidal elevation by about 90°); (2) tidal currents were typically of the order of 1 m s−1 and were almost rectilinear; (3) residual currents were up to 0.17 m s−1 and the circulation pattern was quite complex; and (4) a two‐layer residual circulation with inflow in the upper layer and outflow in the lower layer existed at the entrance to the bay. Most of the residual currents in the upper part of the bay were driven by tidal rectification. The two‐layer residual circulation at the bay entrance was examined using a diagnostic model. It was found that the circulation was probably induced by horizontal density variations. Near the entrance to Cumberland Basin, Shepody Bay, and Chignecto Bay, the cross‐channel variation of surface suspended sediment concentration was consistent with the observed residual currents from current meter moorings and the estimated currents from the diagnostic model. The cross‐channel variation of salinity near the entrance to Chignecto Bay was also consistent with the observed and computed residual currents.
Long waves and eddies in the tropical Atlantic Ocean: 1984–1990
Authors: Steger John M. , Carton James A.
Published: February 2008

Weekly images of sea surface temperature (SST) have been constructed for the tropical Atlantic Ocean for May‐September, 1984–1990, from National Oceanic and Atmospheric Administration National Environmental Satellite, Data, and Information Service satellite‐derived sea surface temperature data. These images have been analyzed to determine weekly to seasonal changes in the position of the fronts between the cold equatorial water and the warmer waters to the north and south. We have found that long waves along the northern front (previously reported by Legeckis and Reverdin (1987)) are present each year, although their number, strength, and duration vary. The year 1987 was the most atypical, with only a few short‐lived waves seen. The long waves typically appear in SST in mid‐June and last into September. They form along the equatorial thermal front between 5°W and 30°W and propagate westward, sometimes reaching 45°W. Wave crests travel at speeds varying between 10 and 40 cm/s, but variations in amplitude reveal that the group velocity of the waves is eastward at 30 cm/s. Long‐wave crests are observed to be associated with mesoscale anticyclonic warm‐core eddies, which apparently play a significant role in cross‐equatorial heat transport. Instability features are also observed along the southern front of the equatorial cold tongue, although because of weaker thermal gradients they are less distinct. Southern waves propagate westward along the front at speeds and wavelengths similar to the long waves in the north. There is no simple phase relationship between long waves and the southern waves, which suggests that the southern waves are not the direct result of the northern long waves.
On the numerical dispersion relation of equatorial waves
Authors: Proehl Jeffrey A.
Published: February 2008

Numerical dispersion relations for equatorial wave modes are computed two ways: from equations for the pressure, p, and for the meridional velocity, v. These are compared with both the continuous and the analytic finite difference dispersion relations for the u‐v‐p system of equations on an Arakawa C‐grid derived by D. W. Moore (personal communication, 1990). For a particular meridional wave structure and grid resolution, the dispersion relation is approximated more accurately via the finite‐difference form of the v‐equation than by either the p‐equation or the u‐v‐p system. However, the finite difference v‐equation possesses an undesirable wave solution which has unbounded u and p, and it also lacks the equatorial Kelvin wave. On the other hand, the p‐equation, is found to possess additional numerical solutions due to the conversion of the apparent singularities of the continuous equation at the inertial latitudes (yil = ±σ/β) into regular singularities of the discrete equation due to numerical truncation. These additional solutions are well separated in dispersion space from the equatorial ones except at selected frequencies. The frequencies at which the wavelengths of these additional solutions to the p‐equation approach the continuous dispersion relations are found to be sensitive to the distance from the inertial latitude to the nearest gridpoint.
The development of a coupled ice‐ocean model for forecasting ice conditions in the Arctic
Authors: Riedlinger Shelley H. , Preller Ruth H.
Published: February 2008

A coupled ice‐ocean model has been developed to investigate how a better simulation of ice‐ocean interaction can improve sea ice forecasting capabilities. The coupling of the ice and ocean results in improved temporal variability of ocean circulation and heat and salt exchange between ice and ocean. The U.S. Navy's Polar Ice Prediction System is coupled to a diagnostic version of the Bryan‐Cox three‐dimensional ocean circulation model. A horizontal grid spacing of 127 km was used in the coupled model with 17 vertical levels from the surface to the ocean bottom. Atmospheric data from the Naval Operational Global Atmospheric Prediction System (NOGAPS) for 1986 were used to force the model. The ice‐ocean model simulation yielded realistic ice thickness distributions, ice drifts, and ocean currents. The model predicted accurate seasonal trends in ice growth and decay. Excess ice is often grown in the Greenland and Barents seas in fall and winter. This is due, in part, to the model's grid resolution which does not accurately resolve narrow currents, such as the West Spitsbergen Current. A sensitivity study of the heat transfer coefficients used in the ice model showed that the ice edge could be improved by using different coefficient values for thick ice, thin ice, and open water. Other sensitivity studies examined the effect of removing the “distorted” physics frequently used in the Bryan‐Cox ocean circulation model and the effect of the vertical eddy momentum coefficient on the surface ocean circulation. An additional simulation was made using 1989 NOGAPS forcing to examine what type of variability could occur when using different years of NOGAPS forcing in the diagnostic ocean model. Significant differences occurred between the 1989 and 1986 ice thickness distributions as well as the oceanic heat fluxes. These differences show that the forecast system, which presently uses an ocean “climatology,” can benefit from the variability allowed by the diagnostic ocean model.
Antarctic sea ice variations and seasonal air temperature relationships
Authors: Weatherly John W. , Walsh John E. , Zwally H. Jay
Published: February 2008

Monthly Antarctic station temperatures are used in conjunction with grids of sea ice coverage in order to evaluate the temporal trends and the strength of associations between the two variables at lags of up to several seasons. Over the 30‐year period 1958–1987 the trends of temperature are positive in all seasons. However, for the 15 years (1973–1987) for which ice data are available, the trends of temperature are predominantly positive only in winter and summer. The trends are most strongly positive over the Antarctic Peninsula. The spatially aggregated trend of temperature for this latter period is small but positive, while the corresponding trend of ice coverage is small but negative. Regional trends of both variables are larger. Cross correlations between concurrent anomalies of the two variables are negative over most of the continent and are strongest over the Antarctic Peninsula, especially in winter. Lag correlations between seasonal anomalies of the two variables are generally stronger with ice lagging the summer temperatures and with ice leading the winter temperatures. The implication is that summer temperatures predispose the near‐surface waters to above‐ or below‐normal ice coverage in the following autumn and winter. The conclusions show little dependence on the choice of the sea ice data source or of the measure of sea ice (extent or areal coverage), but they do depend considerably on the method by which the data are geographically aggregated.
The relationship of sea surface temperature to thermocline depth at annual and interannual time scales in the tropical Atlantic Ocean
Authors: Houghton Robert W.
Published: February 2008

Expendable bathythermograph data derived from SEQUAL/FOCAL and TOGA sponsored volunteer observing ship programs in the tropical Atlantic Ocean are used to study the relationship between the sea surface temperature (SST) and the underlying thermocline. Time series up to 8 years long allow investigation of both the annual and interannual variability between 2N and 2S along selected shipping lanes. For the annual harmonic a warm SST is correlated with a shallow thermocline everywhere except in the eastern equatorial region. Near the intertropical convergence zone (ITCZ) the 180° phase shift in the annual harmonic of both the SST and the thermocline depth occurs at slightly different latitudes. Over most of the tropical Atlantic the annual cycle of SST is linked to the phase of the net surface heat flux and only in the eastern equatorial and coastal upwelling regions does heat flux across the thermocline become a significant factor for the annual cycle of the SST. Canonical correlation analysis of the interannual variability reveal two distinct modes with structures similar to the annual cycle. The first, in which a warm SST is correlated with a shallow thermocline, is a dipole with centers of maximum amplitude off the equator at approximately 8S at 12N and a node at the ITCZ. The second mode, for which a warm SST is correlated with a deep thermocline, peaks at the equator. The predominance of the first mode suggests that for most of the tropical Atlantic Ocean the dynamical state beneath the surface mixed layer has only a limited influence on SST fluctuations.
Mesoscale air‐sea interactions related to tropical and extratropical storms in the Gulf of Mexico
Authors: Lewis James K. , Hsu S. A.
Published: February 2008

Data indicate that the climatological vorticity of the lower atmosphere of the northwestern Gulf of Mexico is relatively strong, with monthly means exceeding a Rossby number of 3 from October through March, with a maximum of 4.6 in December. It has been hypothesized that wintertime cyclogenesis over the northwestern gulf is enhanced by this lower level atmospheric vorticity field when it occurs simultaneously with appropriate upper level venting. Observations from November 1982 to mid‐February 1983 were studied in which seven significant cyclones were generated in the northwestern gulf. It was found that all seven storms occurred when the vorticity correlate of horizontal air temperature difference was ∼3–5°C above the climatological mean difference. Although these results corroborate the postulate that such lower level vorticity is related to cyclogenesis, the data from mid‐February through March 1983 were less supportive. During those months, only two of the six significant storms developed when the horizontal air temperature difference was above the climatological mean. Surprisingly, the location of the most unstable air within the atmospheric boundary layer is not found in the northwestern Gulf of Mexico but instead along the shelf break. Relatively strong free convection in the atmospheric boundary layer is found over the Loop Current and the deep western gulf as expected. The unstable regions of the deep gulf can enhance the potential for frontogenesis and cyclogenesis, but it is likely that the linear character of the strong shelf break free convection does not enhance cyclogenesis. It is shown that a maximum in the frequency of tropical storms within the Gulf of Mexico exists some 275 km south of the Mississippi delta at 27°N, 90°W. This maximum is a result of only those storms which originate within the gulf. Two plausible effects of the Loop Current and its rings on tropical storms are discussed. One is that these ocean features are large and consolidated heat and moisture sources from which a nearby, slowly moving atmospheric disturbance can extract energy. The second is that of the cyclonic vorticity that can be generated in the lower atmosphere by such oceanographic features.
A radiative transfer model for sea ice with vertical structure variations
Authors: Grenfell Thomas C.
Published: February 2008

A radiative transfer model designed for application to snow‐covered sea ice with vertical layering is presented which removes several of the computational problems associated with media whose optical properties are strong functions of wavelength and depth. With this model it is possible to treat the effects of biological activity in the ice as well as vertical variations in brine volume and bubble density. Transmission spectra can also be calculated as a function of depth in the water column beneath the ice. Accurate results are achieved for both direct and diffuse incident irradiance for single scattering albedos ranging from 0.0 to 0.99999999. Calculated albedos and extinction coefficients for snow and sea ice are consistent with both observations and previous models, and the model results resolve apparent inconsistencies among certain observational data sets. Refraction is included for all layer interfaces and is found to reduce the albedo, while the transmissivity responds to changes in the optical depth in addition to redirection of the radiation. For bare sea ice, ignoring refraction can introduce albedo errors as great as 24% and transmissivity errors of more than 70%. Absorption by algae in the ice is shown to reduce the transmitted radiation by more than a factor of 10 at certain visible wavelengths. The predicted azimuthal dependence of the emergent radiances is very weak even for large solar incidence angles.
Surface films and wind wave growth
Authors: Creamer Dennis B. , Wright Jon A.
Published: February 2008

The generation and growth of surface water waves by the wind and including modified boundary conditions appropriate for the presence of surfactants at the surface are reconsidered using linear instability theory to describe the process. The new features of our work are the inclusion of both surfactants and a wind drift layer. We allow a coupled mean flow in both the air and water; the depth‐dependent mean flow is considered to be wind‐induced. The complex wave frequencies are determined numerically. Uncontrolled numerical errors associated with a direct numerical solution of the Orr‐Sommerfeld equation for the coupled mean flow are avoided by solving an associated equation obtained by a Riccati transformation of the original equation, allowing a straightforward and efficient determination of the eigenvalues of the linear stability problem. A different aspect of our analysis is the change of surface boundary conditions due to contamination of the surface, notably, the change of tangential stress at the surface due to surface tension gradients induced by the film (known as the Marangoni effect). Comparisons are made with previous work on both film‐free and contaminated surfaces. The dominant effect of introducing film boundary conditions is a large change in the wave number of the most unstable wave with an attendant reduction in the growth rate. Implications for wind wave growth on the ocean are discussed.
Seasonal variations in the West Spitsbergen Current estimated from bottom pressure measurements
Authors: Morison James
Published: February 2008

From 1982 to 1985 two pressure gauges were deployed across the continental shelf northwest of Spitsbergen to determine the seasonal variations in the barotropic component of the West Spitsbergen Current. The variation in current velocity was calculated after analysis and removal of the tidal signals and correction of the pressure difference across the 47‐km section for bottom buoyancy effects. The pressure‐derived velocity fluctuates about 10 cm s−1 peak to peak with the maximum northward flow occurring in the winter. The maximum appears to the correlated with the seasonal cycle of wind stress curl in the northern Greenland Sea. In fact, the magnitude of the velocity variation corresponds to the Sverdrup transport due to the fluctuation in wind stress curl at 79°N, 7.5°E. The idea that the observed variation might also be due to the spin‐up of the basin by wind stress curl is explored with two simple analytical models. Basin spin‐up does appear to be a possible cause. However, East Greenland Current velocity data from Muench et al. (1986) indicate that the fluctuation about the mean in the deep currents on the western side of the basin is northward in winter, while basin spin‐up would produce a southward fluctuation. This supports the idea that Sverdrup transport dominates basin spin‐up, at least at the margins of the basin. The fluctuations in transport and heat flux though the pressure gauge section are estimated to be 2 Sv and 30 to 60 TW peak to peak. These values are major fractions of historical estimates of the mean transport and heat flux values in the West Spitsbergen Current, and they are significant fractions of the total oceanic fluxes into the Arctic Ocean. This suggests that long time series are required to make meaningful estimates of the heat budget of the Arctic.
The dependence of wind stress on wave height and wind speed
Authors: Blake Reginald A.
Published: February 2008

Three near‐neutral boundary layer data sets were investigated with the aim of finding a dependence of wind stress on both wind speed and significant wave height. The data set most representative of open ocean wave height, wind speed, and momentum flux conditions, was selected and analyzed by means of the least squares method to produce a new parameterization for the wind stress as a function of both wind speed and significant wave height. This study shows that the wind stress, and consequently the drag coefficient, decreases with increasing wave height for a fixed wind speed. The study also shows that the curvature of the wind profile decreases with increasing wave height and that the CDN = A + BŪ(10) form for the drag coefficient parameterization is inadequate. A drag coefficient that applies to both smooth and rough flows is proposed. These results are more applicable for open ocean, deep water conditions and less applicable for sheltered, closed, shallow water sites.
NASA sea ice validation program for the Defense Meteorological Satellite Program special sensor microwave imager
Authors: Cavalieri Donald J.
Published: February 2008

The launch of the first Defense Meteorological Satellite Program (DMSP) special sensor microwave imager (SSM/I) on June 19, 1987, was of particular significance to the polar research community in that it extended the series of polar‐orbiting passive microwave imager s which began with the Nimbus 5 electrically scanning microwave radiometer (ESMR) in 1972 and continued until August 1987 with the Nimbus 7 scanning multichannel microwave radiometer (SMMR). Six additional SSM/I instruments will be launched over the next 2 decades as part of the DMSP program.

In 1982, in recognition of the importance of the SSM/I to the polar research community, NASA established a program to process, archive, and distribute gridded SSM/I microwave radiances and derived sea ice concentrations for both polar regions. A key element of this effort was the initiation of a sea ice validation program for the purpose of providing the user community a measure of the precision and accuracy of the derived sea ice products. Sea ice remote sensing specialists were invited to serve on the NASA sea ice validation team to define and implement the program [Cavalieri and Swift, 1987]. A parallel program which included other geophysical parameters was established by the Department of Defense under the leadership of J. P. Hollinger at the Naval Research Laboratory [Hollinger, 1989].
Ocean surface topography measured by the Geosat radar altimeter during the Frontal Air‐Sea Interaction Experiment
Authors: Bisagni James J.
Published: February 2008

An objective analysis (OA) technique was used to analyze Geosat Geodetic Mission residual sea height anomalies (RSHA) for the Frontal Air‐Sea Interaction Experiment (FASINEX), conducted in the western North Atlantic. Preprocessing of the Geosat data included removal of a mean sea surface and orbit error and corrections for tides, wave height, inverse barometer, and ionospheric and dry tropospheric effects. Geosat data were not corrected for water vapor. Central to the analysis was the computation of a spatially anisotropic, time‐dependent autocorrelation function directly from the Geosat data. Results show a single autocorrelation function peak, having a diameter of ∼270 km at 0‐day time lag. A 15‐month time series of RSHA topography was constructed at 10‐day intervals, forming a set of 47 OA maps. These maps show a field of topographic features propagating to the west at ∼4 km d−1. The features exhibited spatial and temporal scales of ∼200 km and ∼50 days, respectively. The root‐mean‐square (rms) of the OA‐derived feature amplitudes fluctuated slowly between 0.05 and 0.09 m with a period of ∼200 days over the 15‐month interval. These fluctuations appeared to be related to the passage of large‐scale, northeast‐southwest trending sea surface temperature (SST) anomaly features reported for FASINEX, which were surface manifestations of large‐scale, first‐mode baroclinic Rossby waves. The rms amplitude of the OA‐derived features was maximal (minimal) during periods of negative (positive) SST anomaly, when the depths of the mixed layer and main thermocline in the upper ocean shoaled (deepened). The fluctuations also appeared to be correlated with the structure of the field. Periods of decreased feature amplitudes corresponded to a “closely packed” structure, while during times of increased amplitudes, the field structure became “zonal” with regions of alternating sign. During FASINEX (January–June 1986) the field was characterized by the closely packed structure and decreasing feature amplitudes. This corresponded to the period of positive SST anomaly and high upper ocean heat content reported for FASINEX.
Analysis of the Niagara River plume dynamics
Authors: Masse Ann K. , Murthy C. R.
Published: February 2008

The Niagara River discharges into Lake Ontario at a typical rate of 6500 m3/s, and is the lake's principal source of suspended and dissolved materials. The discharge plume is frequently warmer and thus more buoyant than the surrounding lake surface water. Using thermal satellite imagery collected over a 4‐year period, it was found that the plume executes an anticyclonic turn after exiting the river mouth in 53% of the images in which the plume is visible, while in 26% of the images the plume moves offshore. The cyclonic plume was the least common trajectory occurring in only 9% of the images. Temperature and velocity data are used to examine the buoyant anticyclonic plume. Estimates of the bulk shear Froude number suggest that the plume is unstable at the offshore edges of the turning region and stable in the interior. The stability of the interior is enhanced by the presence of a strong seasonal thermocline directly beneath the plume. The momentum balance within the plume is examined using a local coordinate system. Near the river mouth the plume is vertically mixed to the bottom and the large initial flow deceleration is balanced by bottom stress. Away from the river mouth the principal balance is between Coriolis acceleration, relative acceleration (inertial turning), and the cross‐stream baroclinic pressure gradient.
Geosat altimeter observations of long waves in the equatorial Atlantic
Authors: Musman Steven
Published: February 2008

Geosat altimeter observations covering most of the equatorial Atlantic between March and November 1987 were examined. Six ground tracks were selected with equator crossings between 15°W and 23°W longitude. At these locations there is evidence of westward propagating long waves during four repeat passes between July and October. Best fit parameters are phase velocity of 45 ± 6 cm s−1 and period of 25 ± 2 days. Waves are likely present over a wider range in longitude, but the observations are restricted by scan lines intersecting Africa to the east and noise associated with geoid undulations near the mid‐Atlantic ridge to the west. A composite mean wave pattern was constructed between 8°N and 8°S latitude. The pattern resembles a somewhat distorted free mode. There are two off‐equatorial maxima of 3.3 ± 1.0 cm near 4°N and 2.6 ± 0.7 cm near 4°S latitude. The northern maxima has a distinct NW‐SE tilt of about 30°. This is direct evidence of the transport of energy from the mean zonal flow into the wave. The kinetic energy replacement time is about 17 days. For comparison, an example of a shear instability wave in a simple rotating system is shown with the same sort of tilt as well as a free mode without a tilt.
Encounter of a deep western boundary current with the equator: A numerical spin‐up experiment
Authors: Kawase Mitsuhiro , Rothstein Lewis M. , Spriinger Scott R.
Published: February 2008

The encounter of a deep western boundary current with the equator is studied using an ultra‐high resolution numerical model of the equatorial ocean in a study of the origin of the deep equatorial current system. The integration was carried out from a state of rest to 3500 days. The spin‐up experiment shows that the deep western boundary current, upon encountering the equator, first turns along the equator. The time scale of spin‐up is fast along the equator and the boundaries (about 250 days) but is considerably slower off the equator. After this initial response, the western boundary current slowly begins to penetrate into the opposite hemisphere, as expected from the linear theory. The eastward equatorial current gradually slows down: the total water mass transport in the equatorial branch decreases steadily until by day 3000 it has dwindled to zero and all the mass transport in the western boundary current crosses into the other hemisphere. However, a vestige of the equatorial branch remains until the end of the integration with the speed of less than 3 cm/s and with intermittent meandering. An analysis of the momentum balance indicates that this meander may be a free equatorial inertial oscillation. We postulate that a permanent, steady bifurcation of the mid‐depth western boundary current that carries substantial mass does not occur and eventually all of the mass transport will continue into the opposite hemisphere. The asymptotic flow field seems incompatible with the tracer field observed in the Atlantic, which clearly shows a splitting at the equator of the tracer signal associated with the Upper North Atlantic Deep Water. We speculate that an inherently time‐dependent explanation of the tracer tongue is called for.
Ozone trend in the northern hemisphere: A numerical study
Authors: Pitari Giovanni , Visconti Guido
Published: February 2008

A two‐dimensional model has been used to study the possible effects of heterogeneous chemistry on the ozone concentration in the northern high latitudes. The model uses a diabatic circulation which is consistent with the calculated ozone. The temperature field is prescribed in such a way to realistically reproduce the thermal conditions in the polar lower stratosphere during the winter season. A complete chemical code is included that produces a realistic nitric acid concentration and then the formation of polar stratospheric clouds of nitric acid tri‐hydrate particles. NOx and ClOx families are explicitly predicted as a function of time‐dependent N2O, CH4 and CFC mixing ratios from the simulated year 1960 until 2000. The increase of the total chlorine amount in the stratosphere is shown to be a possible explanation for the observed trend of ozone depletion in the high‐latitude northern region. The presence of polar stratospheric clouds is found to be important for the seasonal behavior of the ozone secular trend.
Homogeneity of spatial correlation statistics of tropical oceanic rainfall
Authors: Shin Kyung‐Sup , North Gerald R.
Published: February 2008

The possibility of uniform horizontal correlation scales for tropical oceanic rainfall has been examined by a study of satellite‐observed microwave data as a proxy measure of rain rates. From the brightness temperatures from the electrically scanning microwave radiometer on Nimbus 5 near nadir during the year of 1974, the mean spatial autocorrelation function as a function of simultaneous pixel separation (25 km pixel contiguous center‐to‐center spacing) was calculated in each 5° by 5° grid box over the tropical Pacific and Atlantic for each season. The equal‐time spatially lagged correlations were compared for geographical dependence to investigate the hypothesis of homogeneity. A simple model of the spatial statistics of the microwave brightness temperatures was used, consisting of a mixture of uncorrelated spatial white noise incoherently superimposed on a spatially correlated field (spatial red noise). The red noise signals are presumed to be generated by convective activity in the tropical atmosphere. The parameters of the red noise are consistent with this scheme over the tropical oceans, yielding a uniform spatial scale of about 50 km throughout. We believe the red noise field in the microwave data is due mainly to rainfall, but some contributions from cloud liquid water and water vapor amount in the atmosphere are also possible. Due to the inherent limitation of our approach, it is impossible to test the homogeneity hypothesis rigorously. However, there are no reasons to reject it, and the likelihood of a uniform spatial scale of tropical rainfall over oceans is enhanced through this study.
Scene identification and its effect on cloud radiative forcing in the Arctic
Authors: Li Zhanqing , Leighton H. G.
Published: February 2008

Measurements of cloud radiative forcing in polar regions are less reliable than at lower latitudes because of the difficulty in distinguishing between clouds and ice‐ or snow‐covered surfaces. Scene identification can, however, be improved by using multispectral narrow‐band radiances. Comparisons were made between scenes identified by the Earth Radiation Budget Experiment (ERBE) algorithm and those deduced from Advanced Very High Resolution Radiometer (AVHRR) radiances in the Arctic for 4 days in July 1985. It was found that they differ significantly from each other both in geotype and in cloud cover. For instance, regions of clear fractional sea ice according to the AVHRR analysis are assigned as being cloudy over open ocean by the ERBE analysis owing to incorrect specification of the sea ice extent. Zonal averages over 2.5° wide bands, of fluxes over clear and cloudy regions and hence also cloud forcing, are determined over the Arctic using radiances measured by the AVHRR on NOAA 9 to identify the nature of the scene and the ERBE radiometer to provide broadband radiances. Results are compared with fluxes and cloud forcing determined solely from ERBE radiometer data. Over the southern portion of the domain, where fractional sea ice was prevalent, the ERBE‐based values of net cloud forcing were as much as 50 W m−2 smaller than values determined when the scene identification was based on AVHRR measurements. At higher latitudes the ERBE‐estimated cloud forcing was larger than that from the AVHRR, and the magnitude of the difference was smaller.
Ground‐based infrared measurements of HNO3 total column abundances: Long‐term trend and variability
Authors: Rinsland C. P. , Zander R. , Demoulin P.
Published: February 2008

The long‐term trend and variability of the total column amount of atmospheric nitric acid (HNO3) have been investigated based on time series of infrared solar absorption spectra recorded at two remote high‐altitude sites, the International Scientific Station of the Jungfraujoch (ISSJ) in the Swiss Alps (altitude 3.6 km, latitude 46.5°N, longitude 8.0°E) and the National Solar Observatory McMath solar telescope facility on Kitt Peak (altitude 2.1 km, latitude 31.9°N, longitude 111.6°W), southwest of Tucson, Arizona. The HNO3 ν5 band Q branch at 879.1 cm−1 and three P branch manifolds near 869 cm−1 were analyzed using a nonlinear least squares spectral fitting technique and a consistent set of spectroscopic line parameters. The ISSJ measurements evaluated in the present work consist of two solar spectra recorded with a grating spectrometer in June 1951 and a set of observations obtained with a high‐resolution Fourier transform spectrometer between June 1986 and June 1990. The modern ISSJ measurements show a ∼20% peak‐to‐peak amplitude seasonal cycle with a winter maximum superimposed on significant variability and a summer minimum; the June results from 1986 to 1990 are both higher and lower than the two retrieved June 1951 HNO3 total column amounts. The fitted trend, (−0.16±0.50)% yr−1, 2σ, indicates that there has been no detectable change in the HNO3 total column over the last 4 decades. The Kitt Peak measurements, recorded with a high‐resolution Fourier transform spectrometer between December 1980 and June 1990, also show marked variability in the HNO3 total column, but in contrast to the ISSJ measurements, no obvious seasonal cycle is observed. The deduced trend in the total column above Kitt Peak, (−0.8±1.6)% yr−1, 2σ, is consistent with the ISSJ time series of measurements, in that no significant HNO3 long‐term trend has been found. The sets of measurements from the two sites are compared with each other and with previously published results, with emphasis on the reported variability of HNO3 and the changes in the HNO3 total column with season and latitude.
Model simulation of chemical depletion of Arctic ozone during the winter of 1989
Authors: McConnell John C. , Evans Wayne F. J. , Templeton Edna M. J.
Published: February 2008

Ozone depletion in the Arctic vortex has become an important issue. As part of a Canadian research program designed to study the effect of polar stratospheric clouds on the chemistry of the Arctic ozone layer, project CANOZE (Canadian Arctic Northern OZone Experiment) 4, a series of ozonesondes were flown during January and February 1989 from Alert at 82.5°N. This time series of ozone profiles indicated a decrease of ozone during this period. We have applied a simple chemical model to estimate the possible ozone depletions that could have occurred during this period. The results suggest a chemical depletion of ∼ 4% per week is possible, in approximate agreement with the observed depletion rate at 20 km of 5% per week. The major features of the analysis confirm earlier suggestions that thermal dissociation of Cl2O2 could limit the amount of O3 destruction. For our simulation conditions of largely unperturbed O3 densities the ClO+O reaction is a major loss process for O3 above 19 km, while Cl2O2 photolysis is the major loss process below 19 km, with the BrO+ClO reaction contributing about 14% of the loss in both cases. The branch of the BrO+ClO reaction that results in the formation of BrCl also acts to limit the nighttime O3 destruction via BrO+ClO since BrO is rapidly converted to BrCl at twilight. The photolytic release of NO and NO2 from HNO3 at the latitudes encountered by the air parcels driving these late winter/early spring conditions is sufficiently slow that it has no major impact on O3 destruction by Clx and Brx on the time scale of a ∼ 1 month.
Stratospheric water vapor measurement in the tropical zone by means of a frost point hygrometer on board long‐duration balloons
Authors: Ovarlez Joëlle
Published: February 2008

Two long‐duration balloon flights were performed during austral summer in the southern hemisphere tropical zone, between 23°S and 10°S, for stratospheric water vapor measurement by means of a frost point hygrometer. The observations show a mixing ratio minimum of about 3.5 ppmv near 50 hPa that corroborates the existence of the hygropause in low‐latitude areas. Also observed was the approach of saturation conditions near a particularly cold tropopause in the highly convective area around New Hebrides, where cloud top temperatures as low as −84°C were seen with GMS satellite data.
A two‐dimensional study of ethane and propane oxidation in the troposphere
Authors: Kanakidou M. , Singh H. B. , Valentin K. M. , Crutzen P. J.
Published: February 2008

The oxidation chemistry of ethane (C2H6) and propane (C3H8) in the troposphere was studied using a global two‐dimensional model, adopting observed surface volume mixing ratios of C2H6 and C3H8 as a function of latitude and season. From the calculated distribution of OH, C2H6, and C3H8 the source strengths, which compensate the chemical loss of these hydrocarbons in the atmosphere, are estimated at 16 Tg C2H6/a and 23 Tg C3 H8/a. Uncertainties involved in the calculations are discussed. The resulting seasonal and latitudinal distribution of various organic compounds, such as acetaldehyde, acetone, peroxyacetyl nitrate (PAN), peroxypropyl nitrate (PPN), and alkyl nitrates were derived. The contribution of various nitrogen species to the unidentified NOy observed during measurement campaigns was examined. C2−C3 alkyl nitrates and HNO4 formed at mixing ratios of a few tens of pptv could account only for some of the unidentified NOy. PAN is calculated to be the most abundant organic nitrate, with mixing ratios exceeding 100 pptv at mid‐latitudes to high latitudes in spring in the northern hemisphere. These values are low compared to observations, however. Regionally, up to 10 times more odd nitrogen may be transported in the form of PAN than NOx. The influence of C2H6 and C3H8 chemistry on calculated mean tropospheric NOx mixing ratios and, subsequently, on O3 and OH concentrations is limited. Therefore major effects on global O3 and OH concentrations must be due to PAN formation in the low troposphere from NOx and reactive hydrocarbons other than C2H6 and C3H8. Such hydrocarbons are required to explain the observed high PAN mixing ratios.
The attenuation of concentration fluctuations in turbulent flow through a tube
Authors: Massman William J.
Published: February 2008

A transfer function describing the attenuation of concentration fluctuations caused by turbulent flow through a tube is developed. The mathematical formulation of the transfer function is posed as an eigenvalue problem with a turbulent diffusivity and a mean velocity profile appropriate to turbulent flow. The resulting differential equation is solved numerically for the lowest eigenvalue, which in turn is used to derive the transfer function. The attenuation is shown to be strongly dependent upon Reynolds number and possible solute interaction at the tube wall. A single, rather simple expression for the transfer function which includes the effects of solute removal at the wall is presented. However, when there are no interactions between the solute and the tube wall, in general, turbulent tube flow (for any Reynolds number greater than about 6000) will attenuate fluctuations less than laminar tube flow (for Reynolds numbers greater than 300). Comparisons between different models' predictions for the attenuation of fluctuations and the observed attenuation show that the model of Lee and Gill (1977) produce the best fit to the tube attenuation data of Lenschow and Raupach (this issue). Also discussed is the minimum tube aspect ratio (ratio of tube length to tube radius) necessary to insure that the transfer function is accurate.
The reaction probabilities of ClONO2 and N2O5 on 40 to 75% sulfuric acid solutions
Authors: Hanson David R. , Ravishankara A. R.
Published: February 2008

A wetted‐wall flow tube was used to measure the reaction probabilites, γ, of ClONO2 and N2O5 on sulfuric acid surfaces. The reaction probabilities were determined from first‐order wall loss rate coefficients measured over sulfuric acid slowly flowing down the inside wall of the flow tube. Sulfuric acid content was varied from 40 to 75% by weight, and measurements were conducted at temperatures characteristic of the stratosphere. We determined the reaction probabilities of N2O5 to be 0.12±0.03 over 40% acid at 215 K, 0.14±0.03 over 60% acid at 215 K and 0.10±0.02 over 70 and 75% acid at 220 and 230 K, respectively. The γ for ClONO2 increased from 2×10−4 over 75% acid at 230 K to 6.4×10−2 over 40% acid at 215 K; these are a factor of 2 lower than a previous determination performed under the same conditions. When we introduced HCl into the flow tube, we detected negligible uptake over the 65 and 70% acid solutions, γ < 3 × 10−5, and a small uptake, γ = 4 × 10−4, for HCl onto 60% acid at 215 K. We did not detect a significant enhancement in the first‐order loss rate coefficients for ClONO2 over these solutions in the presence of HCl. Thus the reaction ClONO2 + HCl occurring on 40–75 wt. % sulfuric acid solutions will not be a significant source of activated chlorine in the stratosphere; however, loss of N2O5 and ClONO2 could affect the NOx and ClOx concentrations.