361 research outputs found
Wintertime convection and frontal interleaving in the Southern Ocean
Submitted in partial fulfillment of the requirements for the
degree of Doctor of Science at the Massachusetts Institute of Technology
and the Woods Hole Oceanographic Institution March, 1980The Southern Ocean as defined here is the body of water between the
Antarctic Continent and the Antarctic Polar Front, (APF). This ocean is
considered important in the global thermodynamic balance of the
ocean-atmosphere system because large planetary heat losses are believed
to occur at high latitudes. The ocean and atmosphere must transport
heat poleward to balance these losses. In the Southern Hemisphere, the
oceanic contribution to this flux involves a southward transport of heat
across the APF into the Southern Ocean where it is given up to the
atmosphere through air-sea interactions.
In Part I, the air-sea interactions and structure of the near
surface waters of the Southern Ocean are investigated with a three
dimensional time dependent numerical model. The surface waters in this
region in summer are characterized by a relatively warm surface mixed
layer with low salinity. Below this layer, a cold temperature extremum
is usually observed in vertical profiles which is believed to be the
remnant of a deep surface mixed layer produced in winter. The
characteristics of this layer, the surface mixed layer and the observed
distribution of wintertime sea ice are reproduced well by this model.
Unlike some other sea-ice models the air-sea heat exchange is a free
variable. Model estimates of the annual heat loss by the Southern Ocean
exhibit the observed meridional variation of heat gained by the ocean
along the APF with heat lost further south. The model's area average
heat loss is much smaller than that estimated with direct observations.
While several model parameterizations were made which could be in error,
the model results suggest that the Southern Ocean does give up vast
amounts of heat to the atmosphere away from the continental margins.
The model results and direct calculations of air-sea exchanges
suggest a southward heat flux must occur across the APF. The lateral
water mass transition across the front is not discontinuous but occurs
over a finite sized zone of fluid which is dominated by intrusive
finestructure. The characteristics and dynamics of these features are
investigated in Part II to try and assess their importance in the
meridional heat budget.
Observations made on two cruises to the APF are presented and the
space-time scales of the features and thermohaline characteristics are
discussed. It is suggested that double diffusive processes dominated by
salt fingering are active within the intrusions. An extension of
Stern's (1967) model of the stability of a thermohaline front to
intrusive finestructure driven by saltfingering where small scale
viscous processes are included, is presented to explain why intrusions
are observed in frontal zones. The model successfully predicts vertical
scales of intrusions observed in the ocean and the observed dependence
of the intrusions' slopes across density surfaces on the vertical
scale. Since the fastest growing intrusion is not strongly determined
by the model, though, it is likely that finite amplitude effects
determine the dominant scale of interleaving in the ocean.
The analysis predicts that intrusions transport heat, salt and
density down the mean gradients of the front. For the APF, this heat
flux is poleward which is the direction required by the global heat
budget. This model does not describe intrusions at finite amplitude or
in steady state and so cannot be used to estimate the magnitude of the
poleward heat flux due to intrusions in the APF.The research reported on here, and my support as a graduate student was provided by the National Science Foundation through grants OCE 75 14056. OCE 76 82036 and OCE 77 28355
The Antarctic Circumpolar Current and the oceanic heat and freshwater budgets
Hydrographic sections that span the Antarctic Circumpolar Current are used to estimate the zonal heat and freshwater transports south of Africa, New Zealand and America. These in tum are used to calculate the exchanges of heat and freshwater between the three major oceans...
Fine- and microstructure observations at Fieberling Guyot : R/V New Horizon cruise report
This report describes fine- and microstructure profile data taken on a cruise to Fieberling Guyot, a seamount in the northeast
subtropical Pacific Ocean. The work performed at sea, instruments used, data return and processing procedures will be summarized
here. This cruise took place between March 4 and March 28, 1991 on the R/V New Horizon. and was part of the interdisciplinary
Accelerated Research Initiative (ARI) for Abrupt Topography sponsored by the Office of Naval Research. An overall goal of the
ARI was to understand the physical, biological, and geological processes occurring near a seamount.
The scientific objective of the Seamount Mixing Cruise was to collect data describing the oceanic fine-scale velocity and
density fields, as well as the related turbulence and mixing in the vicinity of the seamount. The High Resolution Profiler (HRP)
was deployed 95 times above and around the seamount. As well, two test dives were conducted on the way to the site, and eight
deployments completed in deep basdins off the southern California coast before returning to port. The near-synoptic surveys of
the seamount were completed with the deployment of 128 Expendable Current Profilers (XCP's). The temperature field of the
upper 760 meters of water within a 50 kilometer radius of the seamount was mapped using 144 Expendable Bathythermographs
(XBT's).Funding was provided by the Office of Naval Research through
Grant No. NOOOI4-89-J-1073
Interannual sea level variability in the western North Atlantic : regional forcing and remote response
© The Author(s), 2013. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Geophysical Research Letters 40 (2013): 5915–5919, doi:10.1002/2013GL058013.Annually averaged sea level (1970–2012) measured by tide gauges along the North American east coast is remarkably coherent over a 1700 km swath from Nova Scotia to North Carolina. Satellite altimetry (1993–2011) shows that this coherent interannual variability extends over the Middle Atlantic Bight, Gulf of Maine, and Scotian Shelf to the shelf break where there is a local minimum in sea level variance. Comparison with National Center for Environmental Prediction reanalysis winds suggests that a significant fraction of the detrended sea level variance is forced by the region's along-shelf wind stress. While interannual changes in sea level appear to be forced locally, altimetry suggests that the changes observed along the coast and over the shelf may influence the Gulf Stream path downstream of Cape Hatteras.M. Andres
gratefully acknowledges support from the Woods Hole Oceanographic
Institution’s Coastal Ocean Institute. G. Gawarkiewicz acknowledges the
support of NSF grant OCE-1129125
A New Discrete Analytic Signal for Reducing Aliasing in the Discrete Wigner-Ville Distribution
It is not possible to generate an alias-free discrete Wigner--Ville distribution (DWVD) from a discrete analytic signal. This is because the discrete analytic signal must satisfy two mutually exclusive constraints. We present, in this article, a new discrete analytic signal that improves on the commonly used discrete analytic signal's approximation of these two constraints. Our analysis shows that---relative to the commonly used signal---the proposed signal reduces aliasing in the DWVD by approximately 50%. Furthermore, the proposed signal has a simple implementation and satisfies two important properties, namely, that its real component is equal to the original real signal and that its real and imaginary components are orthogonal
Accurate and efficient implementation of the time-frequency matched filter
The discrete time--frequency matched filter should replicate the continuous time--frequency matched filter. But the methods differ. To avoid aliasing the discrete method transforms the real-valued signal to the complex-valued analytic signal. The theory for the time--frequency matched filter does not consider the discrete case using the analytic signal. We find that the performance of the matched filter degrades when using the analytic, rather than real-valued, signal. This performance degradation is dependent on the signal to noise ratio and the signal type. In addition, we present a simple algorithm to efficiently compute the time--frequency matched filter. The algorithm with the real-valued signal, comparative to using the analytic signal, requires one-quarter of the computational load. Hence the real-valued signal---and not the analytic signal---enables an accurate and efficient implementation of the time--frequency matched filter
Near-inertial internal wave field in the Canada Basin from ice-tethered profilers
Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 44 (2014): 413–426, doi:10.1175/JPO-D-13-0117.1.Salinity and temperature profiles from drifting ice-tethered profilers in the Beaufort gyre region of the Canada Basin are used to characterize and quantify the regional near-inertial internal wave field over one year. Vertical displacements of potential density surfaces from the surface to 750-m depth are tracked from fall 2006 to fall 2007. Because of the time resolution and irregular sampling of the ice-tethered profilers, near-inertial frequency signals are marginally resolved. Complex demodulation is used to determine variations with a time scale of several days in the amplitude and phase of waves at a specified near-inertial frequency. Characteristics and variability of the wave field over the course of the year are investigated quantitatively and related to changes in surface wind forcing and sea ice cover.The ITP program and J. Toole’s contributions were supported
by the National Science Foundation Office of Polar
Programs Arctic Observing Network. We acknowledge
the support of the Office of Naval Research (Grant
N00014-11-1-0454) for this study. Support for H. Dosser was also provided by the Natural Sciences and Engineering
Research Council of Canada.2014-08-0
Proofs for Discrete Time-Frequency Distribution Properties
This technical report contains proofs for a set of mathematical properties of a recently proposed discrete time-frequency distribution class
A computationally efficient implementation of quadratic time-frequency distributions
Time-frequency distributions (TFDs) are computationally intensive methods. A very common class of TFDs, namely quadratic TFDs, is obtained by time-frequency (TF) smoothing the Wigner Ville distribution (WVD). In this paper a computationally efficient implementation of this class of TFDs is presented. In order to avoid artifacts caused by circular convolution, linear convolution is applied in both the time and frequency directions. Four different kernel types are identified and separate optimised implementations are presented for each kernel type. The computational complexity is presented for the different kernel types
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