Title: Modeling and predicting the short term evolution of the Geomagnetic field,
Abstract: The coupled evolution of the magnetic field and the flow at the Earth's core
mantle boundary is modeled within the 1900.0-2014.0 time period. To constraint
the dynamical behavior of the system with a core field model deriving from
direct measurements of the Earth's magnetic field we used an Ensemble Kalman
filter algorithm. By simulating an ensemble of possible states, access to the
complete statistical properties of the considered fields is available.
Furthermore, the method enables to provide predictions and to assess their
reliability. In this study, we could highlight the cohabitation of two distinct
flow regimes. One associated with the large scale part of the eccentric gyre,
which evolves slowly in time and posses a very long memory of its past, and a
faster one associated with the small scale velocity field. We show that the
latter can exhibit rapid variations in localized areas. The combination of the
two regimes can predict quite well the decadal variations in length of day, but
it can also explain the discrepancies between the physically predicted and the
observed trend in these variations. Hindcast tests demonstrate that the model
is well balanced and that it can provide accurate short term predictions of a
mean state and its associated uncertainties. However, magnetic field
predictions are limited by the high randomization rate of the different
velocity field scales, and after approximately 2000 years of forecast, no
reliable information on the core field can be recovered.