Publication Date:
2022-05-25
Description:
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.
Description:
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.
Keywords:
Environmental acoustics
;
Internal waves
;
Kuroshio Current
;
Shelf circulation
Repository Name:
Woods Hole Open Access Server
Type:
Article
Format:
4467768 bytes
Format:
application/pdf
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