11 research outputs found
Investigation of regional variation in core flow models using spherical Slepian functions
Abstract By assuming that changes in the magnetic field in the Earth’s outer core are advection-dominated on short timescales, models of the core surface flow can be deduced from secular variation. Such models are known to be under-determined and thus require other assumptions to produce feasible flows. There are regions where poor knowledge of the core flow dynamics gives rise to further uncertainty, such as within the tangent cylinder, and assumptions about the nature of the flow may lead to ambiguous patches, such as if it is assumed to be strongly tangentially geostrophic. We use spherical Slepian functions to spatially and spectrally separate core flow models, confining the flow to either inside or outside these regions of interest. In each region we examine the properties of the flow and analyze its contribution to the overall model. We use three forms of flow model: (a) synthetic models from randomly generated coefficients with blue, red and white energy spectra, (b) a snapshot of a numerical geodynamo simulation and (c) a model inverted from satellite magnetic field measurements. We find that the Slepian decomposition generates unwanted spatial leakage which partially obscures flow in the region of interest, particularly along the boundaries. Possible reasons for this include the use of spherical Slepian functions to decompose a scalar quantity that is then differentiated to give the vector function of interest, and the spectral frequency content of the models. These results will guide subsequent investigation of flow within localized regions, including applying vector Slepian decomposition methods
On the annual and semi-annual components of variations in extent of Arctic and Antarctic sea-ice
The time series of northern hemisphere (NHSI) and southern hemisphere (SHSI)
sea-ice extent are submitted to singular spectral analysis (SSA). The
components are analyzed with Laplace's formulation of the Liouville-Euler
system. As already shown in a previous work, the trends observed in the time
series are quasi linear, decreasing for NHSI and increasing for SHSI. The
amplitude of annual variations in the Antarctic is double that in the Arctic,
they are in phase opposition, modulated. The semi-annual components are in
quadrature. The first 3 components of both NHSI and SHSI at 1, 1/2 and 1/3 yr
account for more than 95% of the signal variance. We complement previous
analyses of variations in pole position (PM = m1, m2) and length of day (lod).
Whereas SSA of lod is dominated by the same first 3 components as sea-ice, SSA
of PM contains only the 1 yr and the Chandler components. The 1 yr component of
NHSI is in phase with that of lod and in phase opposition with m1. The reverse
holds for the 1 yr component of SHSI. We note that the semi-annual component
appears in lod not in m1. The annual and semi-annual components of NHSI and
SHSI are much larger than the trends observed since 1978, that leads us to test
whether a first order geophysical or astronomical forcing should not be
preferred to the mechanisms generally suggested as a forcing factor of the
trends. The lack of modulation of the largest forced component suggests an
alternate mechanism. In Laplace's paradigm, the torques exerted by the Moon,
Sun and planets play the leading role as the source of forcing of many
geophysical phenomena. These forces lead to changes in the inclination of the
Earth's rotation axis, setting Earth masses in motion and resulting in thermal
dissipation. It is variations in inclination of the rotation axis that lead to
the large annual components of melting and re-freezing of sea-ice
Small-scale structure of the geodynamo inferred from Ørsted and Magsat satellite data
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Solar Influence on Global and Regional Climates
The literature relevant to how solar variability influences climate is vast—but
much has been based on inadequate statistics and non-robust procedures. The common
pitfalls are outlined in this review. The best estimates of the solar influence on the global
mean air surface temperature show relatively small effects, compared with the response to
anthropogenic changes (and broadly in line with their respective radiative forcings).
However, the situation is more interesting when one looks at regional and season variations
around the global means. In particular, recent research indicates that winters in Eurasia
may have some dependence on the Sun, with more cold winters occurring when the solar
activity is low. Advances in modelling ‘‘top-down’’ mechanisms, whereby stratospheric
changes influence the underlying troposphere, offer promising explanations of the observed
phenomena. In contrast, the suggested modulation of low-altitude clouds by galactic
cosmic rays provides an increasingly inadequate explanation of observations