21 research outputs found
Shelf-edge frontal structure in the central East China Sea and its impact on low-frequency acoustic propagation
Author Posting. © IEEE, 2004. This article is posted here by permission of IEEE for personal use, not for redistribution. The definitive version was published in IEEE Journal of Oceanic Engineering 29 (2004): 1011-1031, doi:10.1109/JOE.2004.840842.Two field programs, both parts of the Asian Seas
International Acoustics Experiment (ASIAEX), were carried out
in the central East China Sea (28 to 30 N, 126 30 to 128 E)
during April 2000 and June 2001. The goal of these programs was
to study the interactions between the shelf edge environment and
acoustic propagation at a wide range of frequencies and spatial
scales. The low-frequency across-slope propagation was studied
using a synthesis of data collected during both years including conductivity-
temperature-depth (CTD) and mooring data from 2000,
and XBT, thermistor chain, and wide-band source data from 2001.
The water column variability during both years was dominated
by the Kuroshio Current flowing from southwest to northeast
over the continental slope. The barotropic tide was a mixed
diurnal/semidiurnal tide with moderate amplitude compared to
other parts of the Yellow and East China Sea. A large amplitude
semidiurnal internal tide was also a prominent feature of the data
during both years. Bursts of high-frequency internal waves were
often observed, but these took the form of internal solitons only
once, when a rapid off-shelf excursion of the Kuroshio coincided
with the ebbing tide. Two case studies in the acoustic transmission
loss (TL) over the continental shelf and slope were performed.
First, anchor station data obtained during 2000 were used to study
how a Kuroshio warm filament on the shelf induced variance in
the transmission loss (TL) along the seafloor in the NW quadrant
of the study region. The corresponding modeled single-frequency
TL structure explained the significant fine-scale variability in time
primarily by the changes in the multipath/multimode interference
pattern. The interference was quite sensitive to small changes in
the phase differences between individual paths/modes induced by
the evolution of the warm filament. Second, the across-slope sound
speed sections from 2001 were used to explain the observed phenomenon
of abrupt signal attenuation as the transmission range
lengthened seaward across the continental shelf and slope. This
abrupt signal degradation was caused by the Kuroshio frontal
gradients that produced an increasingly downward-refracting
sound-speed field seaward from the shelf break. This abrupt
signal dropout was explained using normal mode theory and was predictable and source depth dependent. For a source located
above the turning depth of the highest-order shelf-trapped mode,
none of the propagating modes on the shelf were excited, causing
total signal extinction on the shelf
Internal solitons in the northeastern south China Sea. Part I: sources and deep water propagation
Author Posting. © IEEE, 2004. This article is posted here by permission of IEEE for personal use, not for redistribution. The definitive version was published in IEEE Journal of Oceanic Engineering 29 (2004): 1157-1181, doi:10.1109/JOE.2004.840839.A moored array of current, temperature, conductivity,
and pressure sensors was deployed across the Chinese
continental shelf and slope in support of the Asian Seas International
Acoustics Experiment. The goal of the observations was to
quantify the water column variability in order to understand the
along- and across-shore low-frequency acoustic propagation in
shallow water. The moorings were deployed from April 21âMay
19, 2001 and sampled at 1â5 min intervals to capture the full range
of temporal variability without aliasing the internal wave field.
The dominant oceanographic signal by far was in fact the highly
nonlinear internal waves (or solitons) which were generated near
the Batan Islands in the Luzon Strait and propagated 485 km
across deep water to the observation region. Dubbed trans-basin
waves, to distinguish them from other, smaller nonlinear waves
generated locally near the shelf break, these waves had amplitudes
ranging from 29 to greater than 140 m and were among the largest
such waves ever observed in the worldâs oceans. The waves arrived
at the most offshore mooring in two clusters lasting 7â8 days each
separated by five days when no waves were observed.Within each
cluster, two types of waves arrived which have been named type-a
and type-b. The type-a waves had greater amplitude than the
type-b waves and arrived with remarkable regularity at the same
time each day, 24 h apart. The type-b waves were weaker than
the type-a waves, arrived an hour later each day, and generally
consisted of a single soliton growing out of the center of the
wave packet. Comparison with modeled barotropic tides from
the generation region revealed that: 1) The two clusters were
generated around the time of the spring tides in the Luzon strait;
and 2) The type-a waves were generated on the strong side of the
diurnal inequality while the type-b waves were generated on the
weaker beat. The position of the Kuroshio intrusion into the Luzon
Strait may modulate the strength of the waves being produced. As
the waves shoaled, the huge lead solitons first split into two solitons
then merged together into a broad region of thermocline depression
at depths less than 120 m. Elevation waves sprang up behind
them as they continued to propagate onshore. The elevation waves
also grew out of regions where the locally-generated internal tide
forced the main thermocline down near the bottom. The âcritical pointâ where the upper and lower layers were equal was a good
indicator of when the depression or elevation waves would form,
however this was not a static point, but rather varied in both space
and time according to the presence or absence of the internal tides
and the incoming trans-basin waves themselves.The planning,
execution, and analysis of this work was supported by the U.S. Office of Naval
Research Ocean Acoustics and Physical Oceanography Programs. Significant
funding contributions were also made by the National Science Council of
Taiwan
Ice loss from the East Antarctic Ice Sheet during late Pleistocene interglacials
Understanding ice sheet behaviour in the geological past is essential for evaluating the role of the cryosphere in the climate system and for projecting rates and magnitudes of sea level rise in future warming scenarios1,2,3,4. Although both geological data5,6,7 and ice sheet models3,8 indicate that marine-based sectors of the East Antarctic Ice Sheet were unstable during Pliocene warm intervals, the ice sheet dynamics during late Pleistocene interglacial intervals are highly uncertain3,9,10. Here we provide evidence from marine sedimentological and geochemical records for ice margin retreat or thinning in the vicinity of the Wilkes Subglacial Basin of East Antarctica during warm late Pleistocene interglacial intervals. The most extreme changes in sediment provenance, recording changes in the locus of glacial erosion, occurred during marine isotope stages 5, 9, and 11, when Antarctic air temperatures11 were at least two degrees Celsius warmer than pre-industrial temperatures for 2,500 years or more. Hence, our study indicates a close link between extended Antarctic warmth and ice loss from the Wilkes Subglacial Basin, providing ice-proximal data to support a contribution to sea level from a reduced East Antarctic Ice Sheet during warm interglacial intervals. While the behaviour of other regions of the East Antarctic Ice Sheet remains to be assessed, it appears that modest future warming may be sufficient to cause ice loss from the Wilkes Subglacial Basin
Systematic exploration of essential yeast gene function with temperature-sensitive mutants
Conditional temperature-sensitive (ts) mutations are valuable reagents for studying essential genes in the yeast Saccharomyces cerevisiae. We constructed 787 ts strains, covering 497 (~45%) of the 1,101 essential yeast genes, with ~30% of the genes represented by multiple alleles. All of the alleles are integrated into their native genomic locus in the S288C common reference strain and are linked to a kanMX selectable marker, allowing further genetic manipulation by synthetic genetic array (SGA)âbased, high-throughput methods. We show two such manipulations: barcoding of 440 strains, which enables chemical-genetic suppression analysis, and the construction of arrays of strains carrying different fluorescent markers of subcellular structure, which enables quantitative analysis of phenotypes using high-content screening. Quantitative analysis of a GFP-tubulin marker identified roles for cohesin and condensin genes in spindle disassembly. This mutant collection should facilitate a wide range of systematic studies aimed at understanding the functions of essential genes
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The effects of rainfall on temperature and salinity in the surface layer of the equatorial Pacific
Measurements of temperature and salinity in the upper 5 m of the ocean
along the equator showed cool fresh anomalies due to rain showers. The
measurements were made between 140 W and 110 W during April 1987, an
El Nino year. The eastern equatorial Pacific was characterized by weak winds
(3 m/s average), high rainfall (1.6 cm/day), and warm surface temperatures
(28.4 C). Measurements of temperature were made from a catamaran float
at 0.5 and 1 m depth and at 5 m depth from the ship. Salinity was measured
at a depth of 1 m from the float and 5 m from the ship. The float was towed
off of the port side of the ship outside of the bow wake. Near-surface low
temperature and low salinity anomalies due to cool rainfall were encountered.
These anomalies were on average cool and fresh by 0.02 C and 0.2 PSTJ
with maximum values of 0.5 C and 1.6 PSU. The horizontal extent of the
anomalies ranged from less than 10 to more than 100 km. Rainfall depths
estimated from salt conservation agreed roughly with shipboard rain-gauge
measurements. The characteristic lifetime of the anomalies, estimated from
the ratio of the average rain depth to average rain rate, was about 10 hrs.
Rainfall temperatures were computed from the T-S mixing curves for three
large, newly-formed anomalies. The average rainfall temperature was 21 C.
Ocean buoyancy fluxes estimated for intense rain showers were an order of
magnitude larger than the fluxes in the absence of rain
Oceanographic and atmospheric conditions on the continental shelf north of the Monterey Bay during August 2006
A comprehensive data set from the ocean and atmosphere was obtained just north of the Monterey Bay as part of the Monterey Bay 2006 (MB06) field experiment. The wind stress, heat fluxes, and sea surface temperature were sampled by the Naval Postgraduate Schoolâs TWIN OTTER research aircraft. In situ data were collected using ships, moorings, gliders and AUVs. Four data-assimilating numerical models were additionally run, including the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPSÂź) model for the atmosphere and the Harvard Ocean Prediction System (HOPS), the Regional Ocean Modeling System (ROMS), and the Navy Coastal Ocean Model (NCOM) for the ocean.
The scientific focus of the Adaptive Sampling and Prediction Experiment (ASAP) was on the upwelling/relaxation cycle and the resulting three-dimensional coastal circulation near a coastal promontory, in this case Point Año Nuevo, CA. The emphasis of this study is on the circulation over the continental shelf as estimated from the wind forcing, two ADCP moorings, and model outputs. The wind stress during August 2006 consisted of 3â10 day upwelling favorable events separated by brief 1â3 day relaxations. During the first two weeks there was some correlation between local winds and currents and the three modelsâ capability to reproduce the events. During the last two weeks, largely equator-ward surface wind stress forced the sea surface and barotropic poleward flow occurred over the shelf, reducing model skill at predicting the circulation. The poleward flow was apparently remotely forced by mesoscale eddies and alongshore pressure gradients, which were not well simulated by the models. The small, high-resolution model domains were highly reliant on correct open boundary conditions to drive these larger-scale poleward flows. Multiply-nested models were no more effective than well-initialized local models in this respect
Understanding the remote influences of ocean weather on the episodic pulses of particulate organic carbon flux
The biological carbon pump has been estimated to export ~5â15 Gt C yrâ1 into the deep ocean, and forms the principal deep-sea food resource. Irregular, intense pulses of particulate organic carbon (POC) have been found to make up about one-third of the overall POC fluxes at a long-term deep-sea research station influenced by coastal upwelling of the California Current, Station M (34°50âČN, 123° W, 4000 m depth). However, the drivers of these pulses have been challenging to quantify. It has long been recognized that ocean currents can result in particles being advected while sinking to the point of collection by a sediment trap. Thus, a sediment trap time series can incorporate material from a dynamic catchment area, a concept sometimes referred to as a statistical funnel. This concept raises many questions including: what are the day-to-day conditions at the source locations of the sinking POC? And, how might such âocean weatherâ and related ecosystem factors influence the intense variation seen at the seafloor? Here we analyzed three-dimensional ocean currents from a Regional Ocean Modeling System (ROMS) model from 2011 to 2017 to trace the potential source locations of particles sinking at 1000, 100, and 50 m dâ1 from an export depth of 100 m. We then used regionally tailored satellite data products to estimate export flux of POC from these locations. For the 100 m dâ1 speed, the particles had origins of up to ~300 km horizontal distance from the sediment trap location, moored at Station M at 3400 m depth., and nearly 1000 km for the 50 m dâ1 speed. Particle tracking indicated that, there was considerable inter-annual variation in source locations. Particle source locations tended to originate from the east in the summer months, with higher export and POC fluxes. Occasionally these locations were in the vicinity of highly productive ocean features nearer to the coast. We found significant correlations between export flux of organic carbon from the estimated source locations at 100 m depth to trap-estimated POC fluxes at 3400 m depth. These results set the stage for further investigation into sinking speed distributions, conditions at the source locations, and comparisons with mechanistic biogeochemical models and between particle tracking model frameworks