24 research outputs found
Relating Agulhas leakage to the Agulhas Current retroflection location
The relation between the Agulhas Current
retroflection location and the magnitude of Agulhas leakage,
the transport of water from the Indian to the Atlantic Ocean,
is investigated in a high-resolution numerical ocean model.
Sudden eastward retreats of the Agulhas Current retroflection
loop are linearly related to the shedding of Agulhas rings,
where larger retreats generate larger rings. Using numerical
Lagrangian floats a 37 year time series of the magnitude of
Agulhas leakage in the model is constructed. The time series
exhibits large amounts of variability, both on weekly and annual
time scales. A linear relation is found between the magnitude
of Agulhas leakage and the location of the Agulhas
Current retroflection, both binned to three month averages.
In the relation, a more westward location of the Agulhas Current
retroflection corresponds to an increased transport from
the Indian Ocean to the Atlantic Ocean. When this relation
is used in a linear regression and applied to almost 20 years
of altimetry data, it yields a best estimate of the mean magnitude
of Agulhas leakage of 13.2 Sv. The early retroflection
of 2000, when Agulhas leakage was probably halved, can be
identified using the regression
A geological fence diagram for England and Wales
This report contains a brief description of the data and methodology used to
compile the National Geological Fence Diagram (NGFD). The NGFD comprises
a network of intersecting geological cross-sections covering England and
Wales, to a minimum depth of 1 km and a total section length of approximately
5,500 km. It was compiled by the British Geological Survey (BGS) on behalf of
the Environment Agency (EA) and is based upon lines of section agreed
between both parties at the inception of the project. The model includes
generalised bedrock strata based principally on the BGS 1:625 000 scale digital
geological data and superficial deposits greater than 10m in thickness.
Additional sources of model data were also considered, largely taken from
published BGS data holdings. In total 41 cross-sections were constructed and
geologically correlated within the GSI3D software
Uncertainty in mapped geological boundaries held by a national geological survey: eliciting the geologists' tacit error model
It is generally accepted that geological line work, such as mapped boundaries, are uncertain for various reasons. It is difficult to quantify this uncertainty directly, because the investigation of error in a boundary at a single location may be costly and time consuming, and many such observations are needed to estimate an uncertainty model with confidence. However, it is recognized across many disciplines that experts generally have a tacit model of the uncertainty of information that they produce (interpretations, diagnoses, etc.) and formal methods exist to extract this model in usable form by elicitation. In this paper we report a trial in which uncertainty models for geological boundaries mapped by geologists of the British Geological Survey (BGS) in six geological scenarios were elicited from a group of five experienced BGS geologists. In five cases a consensus distribution was obtained, which reflected both the initial individually elicited distribution and a structured process of group discussion in which individuals revised their opinions. In a sixth case a consensus was not reached. This concerned a boundary between superficial deposits where the geometry of the contact is hard to visualize. The trial showed that the geologists' tacit model of uncertainty in mapped boundaries reflects factors in addition to the cartographic error usually treated by buffering line work or in written guidance on its application. It suggests that further application of elicitation, to scenarios at an appropriate level of generalization, could be useful to provide working error models for the application and interpretation of line work
K'Mudku A-type magmatism in the southernmost Guyana Shield, central-north Amazon Craton (Brazil): the case of Pedra do Gavião syenogranite
Lagrangian validation of numerical drifter trajectories using drifting buoys: Application to the Agulhas system
The skill of numerical Lagrangian drifter trajectories in three numerical models is assessed by comparing these numerically obtained paths to the trajectories of drifting buoys in the real ocean. The skill assessment is performed using the two-sample Kolmogorov–Smirnov statistical test. To demonstrate the assessment procedure, it is applied to three different models of the Agulhas region. The test can either be performed using crossing positions of one-dimensional sections in order to test model performance in specific locations, or using the total two-dimensional data set of trajectories. The test yields four quantities: a binary decision of model skill, a confidence level which can be used as a measure of goodness-of-fit of the model, a test statistic which can be used to determine the sensitivity of the confidence level, and cumulative distribution functions that aid in the qualitative analysis. The ordering of models by their confidence levels is the same as the ordering based on the qualitative analysis, which suggests that the method is suited for model validation. Only one of the three models, a 1/10° two-way nested regional ocean model, might have skill in the Agulhas region. The other two models, a 1/2° global model and a 1/8° assimilative model, might have skill only on some sections in the regio