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Airflow and Water-Drop Trajectories at Instrument Sampling Points around the Beechcraft King Air and Lockheed Electra
No full textDue to distortion of airflow streamlines, flow velocities and droplet size distributions measured around a moving aircraft can differ from freestream conditions. This can complicate measurements made from aircraft platforms. Potential flow calculations were used to predict airflow characteristics and the spatial distribution of different-sized droplets around the Lockheed Electra L-188 and Beechcraft King Air-200 aircraft at a variety of instrument mounting locations. Large deviations from freestream conditions were found to occur at certain locations on both aircraft near the fuselage and in regions of strong curvature. The number concentration of droplets 100–200 µm in diameter is most seriously affected by flow distortion effects. Calculation results were in reasonable agreement with measurements at a forward mounting location on the King Air
The Dependence of Boundary-Layer Shear on Diurnal Variation of Stability
No full textBoundary-layer data from several different geographical locations are analyzed to document the behavior of boundary-layer shear above the surface. The influence of diurnal variation of stability is emphasized. The applicability of the power law for use in shear estimates is examined
A summary of ODP leg 141 hydrogeologic, geochemical and thermal results
Get PDFThe subduction of the oceanic spreading center at the Chile Triple Junction is marked by a substantial thermal perturbation
and marked changes in the hydrogeologic and aqueous geochemical regimes in the overthrust plate. Ridge subduction substantially
changes the fluid chemistry in the wedge through variably hydrating the oceanic basement, accretionary wedge, and continental
backstop. This generates positive anomalies in salinity and chloride values with respect to sea water. The wedge immediately above
the subducted ridge also experiences greatly enhance diagenesis and cementation together with the influx of primordial mantle
derived ⁴He.
Linear temperature and pore fluid chemistry profiles suggest a predominantly diffusive/conductive regime predominates in
the interior eastern portion of the wedge and continental backstop region. In contrast, a vigorous and transient hydrogeolgic system
within 5 km of the toe of the wedge at both Sites 859 and 863 generates spatially narrow, large, and complex anomalies in
temperature and fluid chemistry. At the toe the vigorous hydrogeologic system may be variably influenced by the episodic
expulsion of fluid from both the deeper parts of the wedge and oceanic basement driven convection systems. Structural and
diagenetic observations are also consistent with a hydrogeologic regime that both evolves with time and that is dominated by
episodic processes. In particular, studies of cements, mineralized veins, deformation bands, and Fe sulfide distribution suggest
that above the subducting ridge (i.e., Site 863) the lithification in the wedge is greatly enhanced and that and periods of enhanced
fluid expulsion are associated with local hydrofracture and dilation episodes
North-south variability in the history of deformation and fluid venting across Hydrate Ridge, Cascadia margin
Get PDFHydrate Ridge is an accretionary thrust ridge located on the lower slope of the central Cascadia convergent margin. Structural mapping based on two-dimensional and three-dimensional multichannel seismic reflection profiles and gridded bathymetry coupled with deep-towed sidescan sonar data and Ocean Drilling Program (ODP) biostratigraphy suggests that seafloor fluid venting patterns are likely controlled by the seaward-vergent (SV) structural style at northern Hydrate Ridge (NHR) and by the dominantly landward-vergent (LV) structural style at southern Hydrate Ridge (SHR). North-south structural variability across Hydrate Ridge is coincident with the seafloor authigenic carbonate distribution, which varies from aerially extensive authigenic carbonate crusts at NHR to a minor focused occurrence of authigenic carbonate at SHR. The older stratigraphy exposed at the seafloor at NHR (>1.6–1.7 Ma) has likely been subjected to a longer history of sediment compaction, dewatering, and deformation than the younger slope basin strata preserved at SHR (1.7 Ma to recent), suggesting the extent of carbonates at NHR may result from a longer history of fluid flow and/or more intense venting through a more uplifted, lithified, and fractured NHR sequence. Furthermore, recent work at SHR shows that the major seafloor fluid venting site there is fed by fluid flow through a volcanic ash–bearing turbidite sequence, suggesting stratigraphic conduits for fluid flow may be important in less uplifted, LV-dominated portions of Hydrate Ridge. In addition, the variability in structural style observed at Hydrate Ridge may have implications for the distributions and concentrations of fluids and gas hydrates in other accretionary settings and play a role in the susceptibility of accretionary ridges to slope failure
Tidal and atmospheric forcing of the upper ocean in the Gulf of California, Part 2: Surface heat flux
No full textSatellite infrared imagery and coastal meteorological data for March 1984 through February 1985 are used to estimate the net annual surface heat flux for the northern Gulf of California. The average annual surface heat flux for the area north of Guaymas and Santa Rosalia is estimated to be +74 W m-2 for the 1984–1985 time period. This is comparable to the +20–50 W m-2 previously obtained from heat and freshwater transport estimates made with hydrographic surveys from different years and months. The spatial distribution of the net surface heat flux shows a net gain of heat over the whole northern gulf. Except for a local maximum near San Esteban Island, the largest heat gain (+110–120 W m-2) occurs in the Ballenas and Salsipuedes channels, where strong tidal mixing produces anomalously cold sea surface temperatures (SSTs) over much of the year. The lowest heat gain occurs in the Guaymas Basin (+40–50 W m-2), where SSTs are consistently warmer. In the relatively shallow northern basin the net surface heat flux is fairly uniform, with a net annual gain of approximately +70 W m-2. A local minimum in heat gain (approximately +60 W m-2) is observed over the shelf in the northwest, where spring and summer surface temperatures are particularly high. A similar minimum in heat gain over the shelf was observed in a separate study in which historical SSTs and 7 years (1979–1986) of meteorological data from Puerto Penasco were used to estimate the net surface heat flux for the northern basin. In that study, however, the heat fluxes were higher, with a gain of +100 W m-2 over the shelf and +114 W m-2 in the northern basin. These larger values are directly attributable to the higher humidities in the 1979–1986 study compared to the 1984-1985 satellite study. Significant interannual variations in humidity appear to occur in the northern gulf, with relatively high humidities during El Niño years and low humidities during anti-El Niño years. High humidities reduce evaporation and the associated latent heat loss, promoting a net annual heat gain. In the northern Gulf of California, however, tidal mixing appears to play a key role in the observed gain of heat. Deep mixing in the island region produces a persistent pool of cold water which is mixed horizontally by the large-scale circulation, lowering surface temperatures over most of the northern gulf. These cold SSTs decrease evaporation by reducing the saturation vapor pressure of the overlying air. As a result, heat loss is substantially reduced, even when humidities are low. By removing heat from the surface, tidal mixing alters the time scale of air-sea interaction and reduces or possibly even inhibits the formation of deep water masses via convection. Over climatological timescales, it may be tidal mixing that ultimately maintains the estuarinelike circulation in the northern Gulf of California, differentiating it from the Mediterranean and Red seas, which lose heat to the atmosphere
Chlorophyll enchancement and mixing associated with meanders of the shelf break front in the Mid-Atlantic Bight
Get PDFMeanders of the shelf break front in the Mid-Atlantic Bight (MAB) during April and May of 1997 were associated with chlorophyll enhancement along a hydrographic and a topographic feature. The hydrographic feature was the surface outcrop of the front, which ranged from ~10 to >100 km seaward of the shelf break owing to the meanders. The topographic feature was the shelf break (100-m isobath). Chlorophyll enhancement was observed by a satellite instrument, the ocean color and temperature sensor, and by a fluorometer in situ. It developed in near-surface waters typically nutrient depleted during late spring, thus local nutrient enrichment of near-surface waters was probable. Observations of sufficient resolution to define processes were available only for the region of shelf break chlorophyll enhancement. Along two meander troughs( shoreward extremities near the shelf break), we observed shoaling of cold shelf water. Shelf water shoaled >20 m along frontal-isopycnals, and phytoplankton absorption maxima coincided directly with the shoaled water. Thus local nutrient enrichment by along-isopycnal upwelling was the supported mechanism of chlorophyll enhancement at the shelf break. The basis for along-isopycnal upwelling was seaward flow of shelf water forced by meander circulation near the shelf break. Strong cross-isobath flow and mixing developed as these meanders propagated along the shelf break front of the MAB at a relatively constant rate of ~9 km day¯¹
Aircraft measurements of high average charges on cloud drops in layer clouds
Get PDFThe first reliable aircraft measurements of characteristic cloud drop charges were obtained by utilizing a counterflow virtual impactor to substantially increase charge sensitivity and eliminate spurious contact charging that contaminated previous aircraft measurements. We find average drop charges more than an order of magnitude larger than expected from mountain surface measurements in similar clouds. Our evaluation of the data indicates that the high average charges on cloud drops originate in charge layers at the cloud boundaries and are carried into the cloud layer by vertical motions. These initial aircraft results demonstrate that cloud drop charges in layer clouds may be high enough to influence microphysical processes that promote precipitation
Nitrogenated organic aerosols as cloud condensation nuclei
Get PDFOne important role of anthropogenic aerosol particles is their influence on climate by acting as cloud condensation nuclei. However, these particles are diverse in composition and mixing state, and our knowledge of which particle types act as cloud condensation nuclei is incomplete. Here we present direct measurements of individual organic particles that nucleated cloud droplets in the atmosphere. These results indicate that nitrogenated organic aerosol particles can act as cloud condensation nuclei without being mixed with inorganic material, and thus influence climate through cloud formation
Defining the word "Seamount"
Get PDFReading through this issue of Oceanography, it will become
apparent that researchers in different disciplines see their
seamounts in quite different ways. The term seamount has been
defined many times (e.g., Menard, 1964; Wessel, 2001; Schmidt and
Schmincke, 2000; Pitcher et al., 2007; International Hydrographic
Organization, 2008; Wessel et al., 2010) but there is no “generally
accepted” definition. Instead, most definitions serve the particular
needs of a discipline or a specific paper. Inconsistencies are
common among different publications and, most notably, differ
from the recommendations of the International Hydrographic
Organization and International Oceanographic Commission
(International Hydrographic Organization, 2008). It is not the
goal of this note to arbitrate or remedy these inconsistencies.
However, as seamount researchers begins to coalesce into one
broad, multidisciplinary research community, it is important
to: (1) have a simple definition that explains which features are
included under the umbrella of seamount research and which are
not, providing an essential condition for defining the seamount
research community, and (2) respect and be aware of differences
among disciplinary definitions, as they may stand in the way of
consistently applying one disciplinary data set to another
Atomspheric driving forces for the Agulhas Current in the subtropics
No full textThe Agulhas Current is the western boundary current of the South Indian Ocean and is thought to play an important role in the global overturning circulation. In this study, we investigate the contribution from the wind stress field over each ocean basin of the southern hemisphere to the variability of Agulhas Current transport. We ran a series of experiments using the Modular Ocean Model 2. The model grid extends from 20°S to 70°S and has a horizontal resolution of equation image° with 25 levels in the vertical. The first experiment was forced with monthly means of the wind stress field from the project ERA 40 from ECMWF. In three other sensitivity experiments, the model was forced with the climatological mean over the whole domain plus the monthly wind stress anomalies (Jan/1979–Dec/2001) over one of the three ocean basins to whit: the South Atlantic, the South Indian and the South Pacific. The results show that inter‐annual variations in the Agulhas Current transport are due largely to the wind field over the South Indian Ocean, whereas annual variations are driven by the wind field over both the South Atlantic and South Indian oceans. The annual signal from the South Atlantic is shown to move equatorward along the southeastern coast of Africa through coastally trapped waves