bluuebunny/crossref_metadata_2025 huggingface dataset:
Seasat A satellite scatterometer measurements of equatorial surface winds
Authors: Halpern David
Published: February 2008

Seasat A Satellite Scatterometer measurements of surface wind components were made under normal weather conditions with unsurpassed space and time resolutions during August and September 1978. Longitudinal distributions of the monthly mean zonal component were markedly different in each ocean: in the Pacific the zonal profile resembled a semicircle; a linear change occurred in the Atlantic, and quasi‐uniform values prevailed in the Indian Ocean. Only in the Atlantic and Pacific was the prevailing direction of the zonal component westward. In the Pacific the monthly mean standard deviations increased towards the west. This indicated that the larger day‐to‐day wind variability observed at the western islands compared to moored buoy measurements in the eastern region was a natural phenomenon and not caused by islands. The average monthly mean slope of the wave number spectra throughout the 550‐ to 2200‐km wavelength band was −1.7, which was approximately equal to the −5/3 power law associated with turbulent motions. That the spectra levels of the zonal wind, but not the meridional component, were substantially different in each equatorial ocean represents an enigma. Largest spectral values occurred in the Atlantic where variances were nearly 10 times greater than in the Pacific, which contained the smallest values.
Formulae for pressure gradients in one‐dimensional lake models
Authors: Svensson Urban , Sahlberg Jörgen
Published: February 2008

Formulae for barotropic and baroclinic pressure gradients, suitable for one‐dimensional lake models, are derived and explored. The derivation is based on the equation for free standing oscillations and a continuity relation. The formulae, which are easy to derive and use, give the correct external and internal seiche frequencies for nonrotating, rectangular one‐ or two‐layered basins of constant depth. When applied to situations with continuous stratification, further assumptions need to be introduced. These are discussed in the paper. Applications to a laboratory experiment, dealing with wind‐induced entrainment, show that pressure gradients for the entrainment process are significant and that the pressure formulae derived capture this effect. The formulae are also used in combination with a one‐dimensional lake model. Comparisons with field measurements of the seasonal stratification show that the inclusion of pressure gradients improves the predictive capabilities of the lake model.
The effect of wind on transport and circulation in Lake St. Clair
Authors: Schwab David J. , Clites Anne H. , Murthy C. R. , Sandall Joan E. , Meadows Lorelle A. , Meadows Guy A.
Published: February 2008

A numerical circulation and transport model is used to simulate currents and trajectories in Lake St. Clair. Results from the model are compared to three different types of measurements, namely, (1) mean currents from an array of fixed current meters, (2) currents measured from a ship during seven synoptic surveys of the lake, and (3) trajectories of satellite‐tracked drifting buoys during four different experiments. The model is then used to predict the effects of storms on the residence time of water entering the lake from eight tributaries and the probable horizontal distribution in the lake of water from a particular tributary. Results show that although the average residence time of the lake is about 9 days, actual residence times range from less than 2 days to over 30 days depending on wind conditions. The calculated distribution patterns of water from various tributaries coincide closely with observed distributions of some water quality parameters and biota.
Interpentadal variability of temperature and salinity at intermediate depths of the north Atlantic Ocean, 1970–1974 versus 1955–1959
Authors: Levitus Sydney
Published: February 2008

We have composited historical hydrographic observations for the world ocean for two pentads, 1955–1959 and 1970–1974, and have objectively analyzed these data for the purpose of studying the temporal variability of temperature and salinity in the North Atlantic Ocean. The subtropical gyre (500–1300 m) of the North Atlantic was colder and fresher during 1970–1974 compared to 1955–1959. The relatively large changes in temperature and salinity on constant‐depth surfaces in the subtropical gyre were due to upward displacements of isopycnal surfaces. At intermediate depths the eastern portion of the subarctic gyre was also colder and fresher during the 1970–1974 pentad. In contrast, the western portion of the subarctic gyre exhibited higher temperatures and salinities during 1970–1974 compared to 1955–1959. Characteristic differences in temperature and salinity for both gyres were 0.5°C and 0.025‰, respectively. Analysis of changes in potential vorticity indicates that in the subtropical gyre potential vorticity increased in all density layers above the 26.5 surface, suggesting that convection in this region may have been shallower in 1970–1974 compared to 1955–1959. Below the 26.5 surface, potential vorticity decreased through the 27.3 surface, indicating that deeper than normal convection may have formed the water masses in these layers. At 250‐m depth, relatively large changes in temperature and salinity occurred along the line of maximum climatological mixed layer depth that extends northeast to southwest across the North Atlantic Ocean.
Observations of near‐inertial waves in acoustic Doppler current profiler measurements made during the Mixed Layer Dynamics Experiment
Authors: Chereskin T. K. , Levine M. D. , Harding A. J. , Regier L. A.
Published: February 2008

Measurements of upper ocean shear made during the Mixed Layer Dynamics Experiment (MILDEX) provide evidence of large horizontal scale motion at near‐inertial frequency. The measurements consist of shipboard acoustic Doppler current profiles. Four large‐scale spatial surveys of 2–4 days duration were made by the R/V Wecoma as a set of boxes approximately 60 km per side around a drifting current meter buoy. Velocity time series from the drifting buoy and from sonar measurements made from FLIP also indicated the presence of motions at near‐inertial frequency. Horizontal length and time scales of the motion are estimated from the phase of the shear vector measured during the spatial surveys. Estimates of the length scale of the waves range from 500 to 1000 km, and the frequency is approximately 1.1f. The behavior of the phase is found to be consistent with a model of narrow‐band inertial waves with vertical structure such that there is a zero crossing in velocity at the base of the mixed layer (40–60 m).
Evolution of the climatological near‐surface thermal structure of the tropical Indian Ocean: 1. Description of mean monthly mixed layer depth, and sea surface temperature, surface current, and surface meteorological fields
Authors: Rao Rokkam R. , Molinari Robert L. , Festa John F.
Published: February 2008

Mean monthly mixed layer depth (MLD), sea surface temperature (SST), and surface current climatologies are generated for the tropical Indian Ocean. In addition, surface meteorological climatologies are produced for those variables which could influence the evolution of the MLD and SST fields. Only the MLD climatology is described in detail, as climatologies for the other variables have appeared previously in the literature. The sum of the annual and semiannual harmonics account for greater than 75% of the energy in the MLD time series over most of the basin. The amplitude of the annual signal is greater than 20 m between 10°S and 25°S, with deepest MLDs observed during the southern hemisphere winter. The south central Arabian Sea, between the equator and 10°N, and the northern Arabian Sea are also regions of larger annual harmonic amplitude (>15 m). The amplitude of the semiannual harmonic is largest in the central Arabian Sea (>25 m). Deepest MLDs are observed there during the height of the two monsoon seasons. Correlation coefficients are computed between MLD and SST and several other oceanographic and meteorological variables to explore possible causal relationships. Net energy flux through the sea surface can account for 75% of the variance in the SST and MLD time series over most of the region south of the equator. Large coefficients are also observed in the northwestern Arabian Sea. Correlations between SST and MLD and surface currents are in general small throughout the region, with maxima observed in the central Arabian Sea, in the vicinity of the South Equatorial Current and in the extreme eastern equatorial Indian Ocean. These correlations will be examined in more detail in part 2 of this study in which simple models of mixed layer dynamics are employed.
Passive microwave in situ observations of winter Weddell Sea ice
Authors: Comiso J. C. , Grenfell T. C. , Bell D. L. , Lange M. A. , Ackley S. F.
Published: February 2008

The microwave radiative characteristics of Antarctic sea ice during a winter period were investigated continuously from R/V Polarstern during the 1986 Winter Weddell Sea Project while the ship went through about 3000 km of ice from the marginal ice zone to the coastal region and back. Radiometer measurements at 6, 10, 18, 37, and 90 GHz in vertical and horizontal polarizations were complemented by visual and video observations and measurements at 60 stations of ice thickness, salinity, temperature, snow cover, density, and other physical characteristics. Two distinct types of ice cover were observed in the marginal ice zone: small pancakes evenly distributed during the southbound leg, and ice bands with wet pancakes during the northbound leg. Other ice types observed were first‐year ice covered by varying thicknesses and states of snow cover, and new and young ice found mainly in leads and polynyas. Analysis of the data shows a large variability in the multispectral microwave emissivities of these ice types, especially at 90 GHz. Over newly refrozen lead or polynya regions, several forms of new ice appear radiometrically distinct, while over thick consolidated ice with snow cover, the brightness temperatures observed at 90 GHz varied by as much as 100 K. Overall, however, at 18 GHz and lower frequencies, the emissivities of thick and cold first‐year ice are relatively stable with standard deviations of about ±0.02. At the marginal ice zone, the emissivity of the ice cover is a lot less predictable and could cause large uncertainties in ice concentration estimates. The use of the 90‐GHz channel in combination with a lower‐frequency channel shows strong potential for more detailed characterization of the ice cover including the identification of various forms of new ice and the quantification of varying snow cover and roughness.
An ice‐ocean coupled model
Authors: Mellor George L. , Kantha Lakshmi
Published: February 2008

An ice model, an ocean model, and a method of coupling the models are described. The ice model is a synthesis, with variations and extensions, of previous modeling ideas. Ice thickness, concentration, velocity, and internal energy are prognostic variables. The ice thermodynamics are represented by temperatures at the snow surface, ice surface, the interior, and the bottom surface. Melting and freezing rates are calculated at the ice‐atmosphere, ice‐ocean, and atmosphere‐ocean interfaces. A prescribed portion of summer meltwater can be stored on the surface and refrozen in the fall. The ocean model includes a second moment, turbulence closure submodel and enables one to solve for oceanic heat flux, the interfacial stress, and subsurface properties. In this paper the model is applied to one‐dimensional simulations, but the equations are cited in a form for implementation by two‐ and three‐dimensional models. In a companion paper (Kantha and Mellor, this issue) the model is used for two‐dimensional (vertical plane) simulations in the Bering Sea. Several one‐dimensional sensitivity studies are performed in the case where the ice model is decoupled from the ocean; here the oceanic heat flux and sea surface temperature are prescribed constants. The studies reveal the role and sensitivity of surface trapped meltwater, ice concentration, and ice divergence. With the coupled ice‐ocean model, the seasonally varying oceanic heat flux and mixed layer properties are determined by the model. Some comparisons with observations in the central Arctic ocean are possible. The role of the molecular sublayer immediately adjacent to the ice is examined; frazil ice production is related to the large disparity in the molecular diffusivities for temperature and salinity. The mixed layer model contains empirical constants which are known from turbulence data. The molecular sublayer parameterization requires one empirical parameter b, which is uncertain but, from this study, is assuredly greater than zero, the value implicit in previous models. The ice model requires the empirical parameters ΦF and ΦM to quantitatively account for freezing or melting processes in open leads; their values are also uncertain, but we present reasoning and sensitivity studies to suggest specific values. Finally, an empirical parameter G is introduced; it is the ratio of the value of the ice thickness used to represent average ice volume in the dynamic and thermodynamic equations to the value of the thickness needed in the heat conduction equation. Estimates of G are made from observed thickness distribution functions; sensitivity studies show it to be an important parameter.
On using significant wave height and radar cross section to improve radar altimeter measurements of wind speed
Authors: Monaldo Frank , Dobson Ella
Published: February 2008

Wind blowing across the ocean surface generates small, centimeter scale roughness. The radar cross section (RCS) measured by spaceborne, nadir‐looking altimeters operating at about 13.5 GHz is responsive to this roughness. Present altimeter algorithms use RCS to infer wind speed. We compare Geosat altimeter estimates of wind speed and nearly coincident estimates from NOAA data buoys to determine whether altimeter algorithms can be improved by using more of the information available from altimetry. We find that a marginal improvement in wind speed retrievals can be obtained by including additional RCS information and significant wave height in the retrieval algorithm. Perhaps most important, results also suggest that wind‐wave growth is suppressed in the presence of preexisting ocean swell.
Ambient and transient bubble spectral densities in quiescent seas and under spilling breakers
Authors: Medwin Herman , Breitz Nigel D.
Published: February 2008

Recently published observations, using laser holography near the ocean surface, have shown that the densities of 10 to 15 micron radius bubbles can be as high as 106 (per cubic meter per micron radius increment) within 3 m of the surface of quiescent seas, thereby supporting the acoustically derived values of two decades ago. The accumulated evidence suggests that these quiescent microbubble densities off the coast of California are proportional to a−4 for radius range 10 < a < 60 microns and a−2.5 for larger bubbles. A calibrated, floating, multi‐frequency, acoustical resonator has now been used to obtain the bubble spectrum for 9 radii from 30 to 240 microns, 25 cm under breaking waves in the open sea. At this depth, bubbles larger than 60 microns have a radius dependence approximately a−2.5 and smaller bubble densities vary approximately as a a−4. Our densities are in good agreement with recently published laboratory studies of bubbles, using laser scattering under wind‐blown surfaces with about the same surface frictional velocity U*. Older, photographically derived densities at ocean depth 75 cm are close to the new data only over the limited range 50 to 100 microns. Past inconsistencies between acoustical results and optical results were apparently due to an inability of some optical techniques to identify small bubbles in the difficult ocean environment.
Evolution of isotherm depth anomalies in the eastern tropical Pacific Ocean during the El Niño event of 1982–1983
Authors: Hansen Donald V. , Herman Alan
Published: February 2008

Subsurface temperature data collected from research vessels and aircraft and vessels of opportunity are used to describe the evolution of the major El Niño event in the tropical Pacific from September 1982 through September 1983. Optimum interpolation is used to create six analysis maps of the anomalous depths of the 20°C, 15°C, and 10°C isothermal surfaces. The region analyzed is 20°N to 20°S and 140°W to the coast of the Americas. Each analysis is based on data accumulated over about a month, and the analysis periods are separated by about a month. The depths of the 20°C isotherm are used to compute zonal thermoclinic transports across 100°W of the North and South Equatorial currents and the North Equatorial Countercurrent.
Quantitative properties of shelfbreak eddies
Authors: Garvine R. W. , Wong K.‐C. , Gawarkiewicz G. G.
Published: February 2008

We present a quantitative analysis of properties of the eddy field associated with the shelfbreak front south of New England off the U.S. east coast. We employ records from current meters, drifting buoys, and hydrography obtained during June 1984. Mean currents during the study period showed the across‐shelf cyclonic shear in the alongshelf velocity to be similar to that found in earlier studies when eddies were probably absent most of the time. Our mean across‐shelf currents, however, differed from earlier findings and showed a marked divergence of the across‐shelf flow for the upper 65 m of water that implied high rates of upwelling from beneath. This across‐shelf divergence was also consistent with observed spreading of surface frontal isotherms in the study area. Records from both current meters and drifting buoys enabled us to compute both the lagged Eulerian and Lagrangian velocity autocorrelations. The former showed a decorrelation time of about 4 days and lacked any negative lobe, but the latter showed a strong negative lobe. Using a simple model for a band‐limited process in frequency, we inferred a rotational period for fluid particles in trajectories within eddies of 4 days and a Lagrangian decorrelation time of 2.6 days, comparable to the Eulerian time. The corresponding estimate of horizontal diffusivity was zero. These results were consistent with the generation close upstream of discrete, coherent vortices with dominantly deterministic character. We could find no evidence that across‐shelf exchange of heat or salt by the eddies was enhanced above estimates of climatic mean exchange, despite having a detailed alongshelf hydrographic transect across the strongest eddy features that we found.
Interpentadal variability of temperature and salinity in the deep North Atlantic, 1970–1974 versus 1955–1959
Authors: Levitus Sydney
Published: February 2008

Historical hydrographic observations have been composited and objectively analyzed for the North Atlantic Ocean for two pentads, 1955–59 and 1970–74, for the purpose of studying the temporal variability of the thermohaline structure of the deep North Atlantic Ocean. Statistically significant differences between the two pentads are found in the deep ocean. At 1750 m depth most of the North Atlantic increased in temperature (approximately 0.1°C) and salinity (0.025‰) from the earlier to the later period. An exception to this was a region in the eastern Atlantic between 33°N and 50°N where a cooling (0.1°C) and freshening (0.025‰) occurred. Changes at other depths as evidenced by difference fields of temperature along 24.5°N and 36.5°N are in agreement with differences found by Roemmich and Wunsch (1984) who described changes at these latitudes between sections taken in the late 1950's and sections taken in 1981.
The near‐surface flow in the subtropical gyre south of the Azores
Authors: Maillard Catherine , Käse Rolf
Published: February 2008

This study presents evidence that the Azores Current (AZC) is a permanent feature of the North Atlantic subtropical gyre and that it feeds the North Equatorial Current (NEC). The AZC path is located south of the Azores; its velocity is mostly geostrophic, and the energy level in the mesoscale range is higher than in the NEC. The NEC velocity appears to be mostly ageostrophic, and the wind driven surface velocity is at about 15° to the right of the mean wind field.
The wind‐driven seasonal circulation in the southern tropical Indian Ocean
Authors: Woodberry Karen E. , Luther Mark E. , O'Brien James J.
Published: February 2008

A numerical model of the Indian Ocean, driven by climatological monthly mean winds, realistically simulates the major features of the large scale upper ocean circulation observed in the southern hemisphere and equatorial regions. The principal feature in the tropical Indian Ocean is a basin‐wide clockwise southern hemisphere (cyclonic) gyre comprised of the South Equatorial Current to the south, the South Equatorial Countercurrent to the north, and the East African Coastal Current in the west. Rossby waves propagate westward in the shear zone between the South Equatorial Current and the South Equatorial Countercurrent, and are obstructed and partially reflected by the banks along the Seychelles‐Mauritius Ridge (60°E). A region of high eddy activity northwest of Madagascar is an extension of the tropical gyre and is a tropical analog to the Gulf Stream recirculation region. Oscillations in meridional transport at the equator have westward phase speed and eastward group velocity and are the result of mixed Rossby‐gravity (Yanai) waves forced by oscillations in the highly nonlinear western boundary current region. Oscillations with 40‐ to 50‐day periods are seen in most currents. These oscillations cannot be atmospherically forced, as the shortest period in the mean monthly wind forcing is 60 days. Mean transports in the western basin agree with observations. Small (2 Sv) mean throughflow from the Pacific to the Indian Ocean at the eastern open boundary is due to wind‐forced Indian Ocean dynamics alone and is within the range of observations of throughflow from the Pacific.
Residual circulation in shallow estuaries: 2. Weakly stratified and partially mixed, narrow estuaries
Authors: Jay David A. , Smith J. Dungan
Published: February 2008

Long‐wave theory is used herein to analyze circulation in weakly stratified and partially mixed estuaries. Unlike the highly stratified systems considered in part 1, the flows considered here have only a minimal tidal‐frequency internal wave component. These estuaries may therefore be modeled as the sum of weakly interacting barotropic and baroclinic modes. The dominant factors driving the residual flow are finite amplitude barotropic effects in weakly stratified estuaries and a combination of barotropic effects and steady horizontal density gradient forcing in partially mixed estuaries. The dominant vertical exchange mechanism in the weakly stratified case is bottom boundary‐induced turbulence, and that in partially mixed estuaries is believed to be random internal wave interactions. A model of the Columbia River Estuary under weakly stratified conditions accurately predicts the observed residual velocity and salinity fields, and the tidal amplitude at which a transition to a highly stratified state takes place. The partially mixed state is unstable in the Columbia River Estuary because the tides and buoyancy input are both too strong.
Wind specification based on Seasat A satellite scatterometer and conventional winds for driving an ocean wave model
Authors: Lalbeharry R. , Khandekar M. L. , Peteherych S.
Published: February 2008

This study examines the utility of remotely sensed Seasat A satellite scatterometer (SASS) winds for driving a spectral ocean wave model. The SASS winds are integrated with the conventional ship winds using an objective analysis scheme which employs the method of successive correction, modified to account for the variable data density, data quality, and the application of an elliptical weighting function. The analyzed winds are produced initially on a Cartesian grid over the North Atlantic and are then transformed onto a spherical orthogonal grid for input to a spectral ocean wave model. The model products are examined and validated against the observed ship and remotely sensed wave data to assess the impact of SASS winds on ocean wave analysis and modeling. Our study demonstrates that the integration of SASS winds into the wind analyses can provide improved wind inputs for validating wave models in a hindcast mode. The model wave heights when validated against the comparison data show a reduction in the root mean square error of up to 25% with the assimilation of SASS winds in the wind analyses. The ultimate goal of this study is to develop suitable algorithms for incorporating satellite‐sensed wind and wave data into operational ocean wave models.
A comparison of seasonal winds with currents on the continental slope of the Catalan Sea (northwestern Mediterranean)
Authors: Font J.
Published: February 2008

The waters of the northwestern region of the Mediterranean Sea follow the continental shelf break in a flow that appears to be linked to a permanent shelf/slope density front. Several studies are underway to evaluate the role of the possible forces that drive this circulation. The continental margin located off the Ebro River is an area of persistent and high‐intensity winds. Long‐term representative seasonal wind and current meter data are presented here in a comparative analysis of the temporal variability of both fields. The data seem to indicate that the local wind field is not responsible for the characteristics of the marine circulation. The wind is only able to develop significant inertial oscillations in spring and may contribute to the intensification of the currents only during the cold season. In addition, the variability of the current field shows no relationship to wind variability. Important mesoscale activity is probably associated with variations of the density structure and not with the wind. In essence, the wind provides no appreciable direct contribution to modifying the general circulation.
Atlantic water in the strait of Sicily
Authors: Manzella G. M. R. , Hopkins T. S. , Minnett P. J. , Nacini E.
Published: February 2008

Historical and recently acquired hydrographic and current meter data are used to study the behavior and the annual cycle of the surface layer of Atlantic Water (AW) within the Sicilian Channel section of the Strait of Sicily. From January to March the surface layer isotherms and isohalines are much more steeply sloped toward the African coast. The transition from winter to summer stratification occurs during the April to May period, and by July both temperature and salinity isolines are quite horizontal. The seasonal variations in temperature and salinity can be characterized by corresponding values of mean velocity, mean kinetic energy, and eddy kinetic energy. During winter these values increased 2–3 times their summer values. Hydrographic and satellite data show that the stream of warmer and less saline AW enters the Strait of Sicily close to the Tunisian coast. South of Cap Bon, the AW appears to decelerate, forming meanders and detached parcels or small rings. These structures are seen in satellite images more often during winter than summer, when the AW current in the Strait of Sicily is more energetic.
An assessment of the transport of atmospheric CO2 into the Arctic Ocean
Authors: Anderson Leif G. , Dyrssen David , Jones E. Peter
Published: February 2008

Data on concentration of total carbonate together with calcium, total alkalinity, salinity, and temperature from the Canadian Expedition to Study the Alpha Ridge (CESAR) Ice Camp, the Ymer 80 expedition and the 1984 F. S. Polarstern Marginal Ice Zone Experiment (MIZEX 84) are used to assess the transport of CO2 into the different water masses of the Arctic Ocean. Most of this CO2 goes into the surface mixed layer and halocline waters. A small flux to the Atlantic layer is discernible. No flux to the deep water could be observed. It is possible to separate the carbon input to the Arctic Ocean into three components: carbon from the atmosphere fixed in river drainage basins (40±20×1012 g C a−1), carbon from dissolved mineral calcium carbonate in river drainage basins (40±20×1012 g C a−1), and carbon from the atmosphere fixed over the continental shelves (129±65×1012 g C a−1). From these we estimate the transport of decayed organic carbon from the tundra to the Arctic Ocean and the “new production” of 45±20 g C m−2 a−1 for biogenic carbon over the continental shelves of the Arctic Ocean.
Effects of height acceleration on Geosat heights
Authors: Hancock David W. , Brooks Ronald L. , Lockwood Dennis W.
Published: February 2008

A radar altimeter tracking loop, such as that utilized by Geosat, produces height errors in the presence of persistent height acceleration ha. The correction factor for the height error is a function of both the loop feedback parameters and the height acceleration. The correction, in meters, to the sea surface height derived from Geosat is −0.16ha, where ha is in meters per second per second. The errors induced by accelerations are produced primarily by changes in along‐track geoid slopes. The nearly circular Geosat orbit and dynamic ocean topography produce small height acceleration values. Methods of using sea surface tracking data from Geosat and altimeter tracker simulation to calculate ha have been evaluated over various time intervals and topographically interesting geoid areas. One of the areas studied in some detail encompasses the Marianas Trench and the Challenger Deep, in the west central Pacific Ocean. Histograms of sea surface height corrections due to range accelerations have also been determined from 24‐hour segments of Geosat global data. The findings are that 20% of the Geosat measurements have acceleration‐induced errors of ≥2 cm, while 8% have errors of ≥3 cm.
Path length variations caused by atmospheric water vapor and their effects on the measurement of mesoscale ocean circulation features by a radar altimeter
Authors: Monaldo Frank
Published: February 2008

Spaceborne radar altimeters make high‐precision measurements of the range between the satellite and the ocean surface. Atmospheric water vapor will lengthen the range as measured by an altimeter. It is theoretically possible for horizontal changes in atmospheric water vapor to be misinterpreted as changes in sea surface height (SSH) or to mask SSH changes that are present. Scanning multichannel microwave radiometer (SMMR) data from the entire 3‐month lifetime of the Seasat satellite are used to estimate water vapor path length corrections and to assess the probability that water vapor will significantly degrade the interpretation of Geosat altimeter data. It is shown that if one considers Geosat‐observed mesoscale circulation features of 10 cm or greater, particularly in the context of a regional circulation model, water vapor is not generally a problem. For mesoscale circulation features 5 cm or smaller, potential contamination by water vapor gradients becomes significant. Cumulative distributions describing water vapor statistics for a variety of spatial scales and in specific geographical regions, as well as the entire globe, are presented. These distributions are compared with ones derived using Fleet Numerical Oceanography Center (FNOC) predictions. It is demonstrated that FNOC‐derived water vapor path lengths, although providing a good mean correction, will not serve to compensate for small horizontal scale changes in water vapor.
Gravity field of the southeast central Pacific from Geosat Exact Repeat Mission Data
Authors: McAdoo David C.
Published: February 2008

Efficient techniques for mapping the marine gravity field with Geosat Exact Repeat Mission (ERM) data are demonstrated. Geosat has completed more than fifty‐two 17‐day, global ERM repeat cycles and has collected roughly 50 million observations of geoid undulations or, more precisely, sea surface topography. Because the global ground track repeats every 17 days, the ERM observations are highly redundant. By averaging the repeat cycles, very precise profiles of alongtrack geoid undulations are obtained for a region of the southeast central Pacific. Without any crossing‐arc, orbit adjustments, these profiles are shown to be repeatable to within 10 cm over their entire 3000km length (i.e., to three parts in 108). This southeast central Pacific region contains a portion of the East Pacific Rise as well as some cross‐grain gravity field lineations and is therefore of considerable geophysical interest. In addition to profiles of alongtrack deflections of the vertical, regularly gridded gravity anomalies are derived for this region and portrayed in color. The procedure used for recovering gravity has two stages: in stage 1, alongtrack deflections of vertical are derived by fitting cubic splines to alongtrack heights, computing alongtrack slopes at standardized latitudes, and then averaging slopes for coincident passes; in stage 2, gridded, band‐limited gravity anomalies are recovered from deflections by using fast Fourier transform techniques to accomplish the necessary inverse Vening Meinesz transformation. High‐precision GEM‐T1 orbits are used and crossover adjustments are not performed. The operational Navy Astronautics Group orbits are shown to have systematic errors which may be as large as 10 m.
Reflection of annual Rossby waves at the maritime western boundary of the tropical Pacific
Authors: White Warren B. , Graham Nicholas , Tai C.‐K.
Published: February 2008

In this study the examination of altimetric sea level time series from the first 17 months of the Geosat Geodetic Mission provides the first evidence of the Rossby wave reflection process operating at the maritime western boundary of the tropical Pacific in both the northern and southern hemispheres. In an earlier study, Tai et al. (1989) analyzed altimetric sea level time series derived from crossover differences during the first 17 months of the Geosat Geodetic Mission from April 1985 to September 1986, finding statistically significant agreement with in situ measurements of sea level differences (i.e., island sea level and relative dynamic height), In the present study, two‐dimensional spectral analysis verified the existence of both off‐equatorial baroclinic Rossby wave activity and equatorial baroclinic Kelvin wave activity in the altimetric sea level data set, establishing also that the annual cycle dominated the variability over this 17‐month period. Next, extended empirical orthogonal function analysis (Graham et al., 1987) determined the evolutionary behavior of the readjustment process that occurred in the sea surface topography on the annual cycle over this 17‐month period. Both the Philippines archipelago in the northern hemisphere and the Solomon archipelago‐Bismarck archipelago‐New Guinea complex in the southern hemisphere were found to reflect incident baroclinic Rossby wave activity of 1‐year period, particularly manifested in the October–April transition, generating equatorial Kelvin wave activity that conducted the annual signal along the equatorial waveguide into the eastern equatorial Pacific. At the eastern boundary of South America, reflection of the incident equatorial Kelvin wave activity was associated with coastal Kelvin wave activity and equatorial‐trapped Rossby wave activity in the equatorial waveguide, the latter propagating back to the west. However, the equatorial‐trapped Rossby wave activity thus generated was not observed to propagate westward past the Galapagos archipelago, a result determined theoretically by Yoon (1981).
Continuous assimilation of Geosat altimetric sea level observations into a numerical synoptic ocean model of the California Current
Authors: White Warren B. , Tai Chang‐Kou , Holland William R.
Published: February 2008

In this study, real Geosat altimetric sea level observations for the 1‐year period extending from January to December 1987 were assimilated into a realistic wind‐driven numerical synoptic ocean model of the California Current. The objective was to evaluate the effectiveness of using a realistic synoptic ocean model to interpolate (dynamically) real altimetric sea level observations from the Geosat ERM onto a regular grid. First, the model mean sea level was shown to simulate qualitatively the observed mean sea level found in in situ CaICOFI hydrographic data. Second, the model was shown to simulate approximately the space‐time statistics of mesoscale eddy variability contained within the altimetric sea level observations. Third, prior to assimilation the altimetric sea level observations were referenced using this model mean. Then the referenced altimetric sea level observations were assimilated into the model, with the resulting sea level residuals compared with estimates from in situ (expendable bathythermograph) observations. This comparison yielded nearly exact agreement at low frequency (i.e., the semiannual cycle), but less agreement on month‐to‐month time scales of variability (yet still significant), probably owing to the unaltered nature of the in situ estimates. Subsequent comparisons of the dynamically interpolated altimetric sea level residuals with those obtained from the statistical interpolation yielded marked improvement in the latter over the former; i.e., dynamical interpolation allowed observed mesoscale variability to be conducted from the Geosat ERM repeat tracks into the regions between tracks with little change in magnitude and structure, not possible with the statistical interpolation. The latter tended to produce extrema on grid points that coincided with the track lines. As such, dynamical interpolation demonstrated marked improvement in resolution, time‐space continuity, intensity, and gradient structure of mesoscale eddy activity over that possible with statistical interpolation. Model‐data assimilation also allowed for the determination of the vertical structure of the mesoscale eddy activity. Moreover, it yielded a forecasting capability that had a statistically significant improvement over persistence in specifying mesoscale eddy activity 2–3 weeks into the future.
The role of secondary circulation in boundary mixing
Authors: Garrett Chris
Published: February 2008

Enhanced mixing near a sloping boundary reduces the stratification and hence the vertical diffusive buoyancy transport. An additional advective buoyancy flux, however, is associated with the bidirectional secondary flow driven by buoyancy forces near the boundary. Some integral relations and simple physical arguments show that, while advection increases the downslope buoyancy transport, it decreases the vertical transport; the overall effectiveness of boundary mixing involves the square of the reduction factor that would occur for the diffusive flux alone in a region of reduced stratification. The secondary circulation may increase the effectiveness of boundary mixing, indirectly, however, by restoring the stratification, and hence the diffusive fluxes, unless this restratification also suppresses the mixing. Integral relations are also derived for the along‐slope flow which is part of the solution rather than arbitrary, and the relationship of the theory to Ekman layer dynamics is discussed.
Continuous assimilation of simulated Geosat altimetric sea level into an eddy‐resolving numerical ocean model: 1. Sea level differences
Authors: White Warren B. , Tai Chang‐Kou , Holland William R.
Published: February 2008

Continuous assimilation of altimetric sea level differences from the simulated Geosat exact repeat mission (ERM) into a three‐layer, quasi‐geostrophic, eddy‐resolving numerical ocean box model at mid‐latitude was conducted using the method of optimal interpolation (Lorenc, 1981). In the linear portion of the model domain, the assimilation of simulated Geosat altimetric sea level differences constrained the detectable periods/wavelengths of mesoscale eddy activity, within the limits of accuracy of the initialization, nearly indefinitely. However, in the nonlinear portion of the model domain, the constraint of these otherwise detectable periods/wavelengths degraded with time from the point of initialization, due to the influence of nonlinear processes and aliasing upon the assimilation process. Hence reinitialization at period intervals was indicated. The Nyquist wavelength of mesoscale eddy activity that could be constrained by the simulated Geosat ERM (i.e., 140‐km track separation at mid‐latitudes, 45‐km sampling interval along track) in the nonlinear portion of the model domain was determined to be the track separation, and not the sampling interval along track; hence in both the linear and nonlinear portions of the model domain, wavelengths greater than 280 km were constrained, but wavelengths shorter than 280 km were not. Therefore decreasing the track separation in future satellite altimeter strategies will be the only way that smaller wavelength scales of mesoscale eddy activity can be detected. This should also increase the capability of the model to constrain the nowcast of the mesoscale eddy activity in the nonlinear portions of the model domain beyond initialization. It was established that the constraint of mesoscale eddy activity in the middle and lower layer stream function fields of the model was similar to that in the upper layer stream function field, responding to the same Nyquist criteria.
Dimethyl sulfide in the Baltic Sea: Annual variability in relation to biological activity
Authors: Leck Caroline , Larsson Ulf , Bågander Lars Erik , Johansson Sif , Hajdu Susanna
Published: February 2008

The in situ variation of dimethyl sulfide (CH3SCH3, DMS) at a fixed station in a coastal area of the Baltic Sea has been studied for a period of time (January 1987 to June 1988) covering the annual biological cycle. DMS in the surface waters of the brackish Baltic Sea showed a clear seasonal variation, ranging from 2 to 200 ng S L−1. Lowest concentrations were in winter, peak values followed the spring bloom, and a pronounced maximum was found during the summer (July–August). Concentrations above low winter levels occurred only when the trophogenic layer was depleted of inorganic nitrogen. From our data it is clear that the seasonal variation in DMS concentration is related to biological activity. However, we did not find any correlations between DMS concentration and gross parameters such as chlorophyll a, total phytoplankton biomass, or primary production on an annual basis. Further, we were not able to relate high DMS concentrations to any particular phytoplankton species or species assemblages. It appears that DMS production is primarily associated with phytoplankton growth under nitrogen‐limited conditions and not with certain species. We found a significant correlation of ambient DMS concentration with copepod and total Zooplankton biomass, suggesting Zooplankton grazing pressure as the major factor responsible for the liberation of β‐dimethylsulfoniopropionate (DMSP) from phytoplankton cells and thus for the DMS production. The turnover time of DMS in the water column was calculated to be of the order of 2 days, and the most effective sink process seems to be of microbiological and/or chemical origin. Previous to this study, no in situ data set has been available to test the relative importance of the various factors responsible for the DMS production in seawaters. We have demonstrated that variations in DMS concentration must be looked upon as the result of complex physiological as well as ecological interactions.
Observations of long Rossby waves in the northern tropical Pacific
Authors: Kessler William S.
Published: February 2008

Long baroclinic Rossby waves are potentially important in the adjustment of the tropical Pacific pycnocline to both annual and interannual wind stress curl fluctuations. Evidence for such waves is found in variations of the depth of the 20°C isotherm in the northern tropical Pacific during 1970–1987. A total of 199,067 bathythermograph profiles have been compiled from the archives of several countries; the data coverage is dense enough that westward propagating events may be observed with a minimum of zonal interpolation. After extensive quality control, 20°C depths were gridded with a resolution of 2° latitude, 5° longitude, and bimonths; statistical parameters of the data were estimated. A simple model of low‐frequency quasi‐geostrophic pycnocline variability allows the physical processes of Ekman pumping, the radiation of long (nondispersive) Rossby waves due to such pumping in midbasin, and the radiation of long Rossby waves from the observed eastern boundary pycnocline depth fluctuations. Although the wind stress curl has very little zonal variability at the annual period in the northern tropical Pacific, an annual fluctuation of 20°C depth propagates westward as a long Rossby wave near 5°N and 14°–18°N in agreement with the model hindcast. Near the thermocline ridge at 10°N, however, the annual cycle across the basin is dominated by local Ekman pumping. The wave‐dominated variability at 5°N weakens the annual cycle of geostrophic transport of the North Equatorial Countercurrenl in the western Pacific. El Niño events are associated with westerly wind anomalies concentrated in the central equatorial Pacific; up welling wind stress curl is generated in the extraequatorial tropics by these westerlies. Long up welling Rossby waves forced by this curl pattern were observed to raise the western Pacific thermocline well outside the equatorial waveguide in the later stages of El Niños, consistent with the simple long‐wave model. The observations suggest that although simple reflection of the long Rossby waves from the western boundary is not the major process affecting subsequent development on the equator, it is likely that the extraequatorial waves play some role in setting the timescale of ENSO events.
Interpentadal variability of steric sea level and geopotential thickness of the north Atlantic Ocean, 1970–1974 versus 1955–1959
Authors: Levitus Sydney
Published: February 2008

We have composited and objectively analyzed historical hydrographic observations for the North Atlantic Ocean for two periods, 1955–1959 and 1970–1974. Difference fields of steric sea level and geopotential thickness for the North Atlantic Ocean in the 0‐ to 1500‐m depth range between the 1955–1959 and 1970–1974 pentads were computed. We find that in the central portion of the subtropical gyre, steric sea level decreased by up to 17.5 dyn cm, whereas in the western subarctic gyre (north of the Gulf Stream) steric sea level increased by an amount up to 7.5 dyn cm from the earlier to the later pentad. A decrease in steric sea level (∼5 dyn cm) occurred along the eastern boundary of the North Atlantic.