174 research outputs found
FOWD: A Free Ocean Wave Dataset for Data Mining and Machine Learning
The occurrence of extreme (rogue) waves in the ocean is for the most part
still shrouded in mystery, as the rare nature of these events makes them
difficult to analyze with traditional methods. Modern data mining and machine
learning methods provide a promising way out, but they typically rely on the
availability of massive amounts of well-cleaned data.
To facilitate the application of such data-hungry methods to surface ocean
waves, we developed FOWD, a freely available wave dataset and processing
framework. FOWD describes the conversion of raw observations into a catalogue
that maps characteristic sea state parameters to observed wave quantities.
Specifically, we employ a running window approach that respects the
non-stationary nature of the oceans, and extensive quality control to reduce
bias in the resulting dataset.
We also supply a reference Python implementation of the FOWD processing
toolkit, which we use to process the entire CDIP buoy data catalogue containing
over 4 billion waves. In a first experiment, we find that, when the full
elevation time series is available, surface elevation kurtosis and maximum wave
height are the strongest univariate predictors for rogue wave activity. When
just a spectrum is given, crest-trough correlation, spectral bandwidth, and
mean period fill this role
On the pathways of the return flow of the meridional overturning circulation in the tropical Atlantic
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2002A numerical model of the tropical Atlantic ocean is used to investigate the upper
layer pathways of the Meridional Overturning Circulation (MOC) in the tropical
Atlantic. The main focus of this thesis is on those parts of the tropical circulation
that are thought to be important for the MOC return flow, but whose dynamics
have not been understood yet.
It is shown how the particular structure of the tropical gyre and the MOC act
to inhibit the flow of North Atlantic water into the equatorial thermocline. As a
result, the upper layers of the tropical Atlantic are mainly fed by water from the
South Atlantic. The processes that carry the South Atlantic water across the tropical
Atlantic into the North Atlantic as part of the MOC are described here, and
three processes that were hitherto not understood are explained as follows:
The North Brazil Current rings are created as the result of the reflection of
Rossby waves at the South American coast. These Rossby waves are generated by
the barotropically unstable North Equatorial Countercurrent. The deep structure of
the rings can be explained by merger of the wave's anticyclones with the deeper intermediate
eddies that are generated as the intermediate western boundary current
crosses the equator.
The bands of strong zonal velocity in intermediate depths along the equator have
hitherto been explained as intermediate currents. Here, an alternative interpretation
of the observations is offered: The Eulerian mean flow along the equator is negligible
and the observations are the signature of strong seasonal Rossby waves. The
previous interpretation of the observations can then be explained as aliasing of the
tropical wave field.
The Tsuchyia Jets are driven by the Eliassen-Palm flux of the tropical instability
waves. The equatorial current system with its strong shears is unstable and generates
tropical instability waves. These waves cause a poleward temperature flux
which steepens the isotherms which in turn generates are geostrophically balanced
zonal flow. In the eastern part of the basin this zonal flow feeds the southeastward
flow of the equatorial gyre.NASA and ONR ~ho generously funded me with their respective grants NAG5-
7194 and N00014-98-10881
Multistability and Intermediate Tipping of the Atlantic Ocean Circulation
Tipping points (TP) in climate sub-systems are usually thought to occur at a
well-defined, critical forcing parameter threshold, via destabilization of the
system state by a single, dominant positive feedback. However, coupling to
other sub-systems, additional feedbacks, and spatial heterogeneity may promote
further small-amplitude, abrupt reorganizations of geophysical flows at forcing
levels lower than the critical threshold. Using a primitive-equation ocean
model we simulate a collapse of the Atlantic Meridional Overturning Circulation
(AMOC) due to increasing glacial melt. Considerably prior to the collapse,
various abrupt, qualitative changes in AMOC variability occur. These
intermediate tipping points (ITP) are transitions between multiple stable
circulation states. Using 2.75 million years of model simulations, we uncover a
very rugged stability landscape featuring parameter regions of up to nine
coexisting stable states. The path to an AMOC collapse via a sequence of ITPs
depends on the rate of change of the meltwater input. This challenges our
ability to predict and define safe limits for TPs
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Artifacts from manganese reduction in rock samples prepared by focused ion beam (FIB) slicing for X-ray microspectroscopic analysis
Abstract. Manganese (Mn)-rich natural rock coatings, so-called rock varnishes, are discussed controversially regarding their genesis. Biogenic and abiogenic mechanisms, as well as a combination of both, have been proposed to be responsible for the Mn oxidation and deposition process. We conducted scanning transmission X-ray microscopy - near edge X-ray absorption fine structure spectroscopy (STXM-NEXAFS) measurements to examine the abundance and spatial distribution of the different oxidation states of Mn within these nano- to micrometer thick crusts. Such microanalytical measurements of thin and hard rock crusts require sample preparation with minimal contamination risk. Focused ion beam (FIB) slicing, a well-established technique in geosciences, was used in this study to obtain 100–200 nm thin slices of the samples for X-ray transmission spectroscopy. However, even though this preparation is suitable to investigate element distributions and structures in rock samples, we observed that, using standard parameters, modifications of the Mn oxidation states occur in the surfaces of the FIB slices. Based on our results, the preparation technique likely causes the reduction of Mn4+ to Mn2+/3+. We draw attention to this issue, since FIB slicing, SEM imaging, and other preparation and visualization techniques operating in the keV range are well-established in geosciences, but researchers are often unaware of the potential for reduction of Mn and possibly other elements in the samples’ surface layers
Beyond the bipolar seesaw: toward a process understanding of interhemispheric coupling
The thermal bipolar ocean seesaw hypothesis was advanced by Stocker and Johnsen (2003) as the ‘simplest possible thermodynamic model’ to explain the time relationship between Dansgaard–Oeschger (DO) and Antarctic Isotope Maxima (AIM) events. In this review we combine palaeoclimate observations, theory and general circulation model experiments to advance from the conceptual model toward a process understanding of interhemispheric coupling and the forcing of AIM events. We present four main results: (1) Changes in Atlantic heat transport invoked by the thermal seesaw are partially compensated by opposing changes in heat transport by the global atmosphere and Pacific Ocean. This compensation is an integral part of interhemispheric coupling, with a major influence on the global pattern of climate anomalies. (2) We support the role of a heat reservoir in interhemispheric coupling but argue that its location is the global interior ocean to the north of the Antarctic Circumpolar Current (ACC), not the commonly assumed Southern Ocean. (3) Energy budget analysis indicates that the process driving Antarctic warming during AIM events is an increase in poleward atmospheric heat and moisture transport following sea ice retreat and surface warming over the Southern Ocean. (4) The Antarctic sea ice retreat is itself driven by eddy-heat fluxes across the ACC, amplified by sea-ice–albedo feedbacks. The lag of Antarctic warming after AMOC collapse reflects the time required for heat to accumulate in the ocean interior north of the ACC (predominantly the upper 1500 m), before it can be mixed across this dynamic barrier by eddies
Seasonal influence of Indonesian Throughflow in the southwestern Indian Ocean
Abstract The influence of the Indonesian Throughflow (ITF) on the dynamics and the thermodynamics in the southwestern Indian Ocean (SWIO) is studied by analyzing a forced ocean model simulation for the Indo-Pacific region. The warm ITF waters reach the subsurface SWIO from August to early December, with a detectable influence on weakening the vertical stratification and reducing the stability of the water column. As a dynamical consequence, baroclinic instabilities and oceanic intraseasonal variabilities (OISVs) are enhanced. The temporal and spatial scales of the OISVs are determined by the ITF-modified stratification. Thermodynamically, the ITF waters influence the subtle balance between the stratification and mixing in the SWIO. As a result, from October to early December, an unusual warm entrainment occurs and the SSTs warm faster than just net surface heat flux driven warming. In late December and January, signature of the ITF is seen as a relatively slower warming of SSTs. A conceptual model for the processes by which the ITF impacts the SWIO is proposed.
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