Much of the energy that feeds the extra-tropical atmospheric systems originates in the excess of energy in the tropics,
which is meridionally transported to the extra-tropics by the Hadley circulation. Superimposed on the Hadley
Circulation, one observes throughout the equatorial region, a series of zonal direct thermal cells named the Walker
Circulation, its ascending branches corresponding to regions of strong superficial convergence and intense precipitation
located over Indonesia and west Pacific, Amazonia and Southern Africa.
Perturbations of these important general circulation mechanisms can have consequences on the climate and its
variability (on an inter-annual and in longer temporal scales) in some regions of the planet. In particular, it is known
the influence of the El Niño-Southern Oscillation phenomenon (ENSO) in the structure of the Walker Circulation. Less
addressed has been the impact of these changes on the Hadley Circulation, particularly in the Euro-Atlantic region.
Much of the research carried out so far on the influence of ENSO on European climate links the propagation of the
ENSO signal through of a wave train which propagates across the Pacific, North America and North Atlantic. Recent
concerns on the desertification of the Mediterranean and southern Europe regions, which may or may no be related to
ENSO, resulted in a number of local climate studies of these regions seem to confirm this tendency. However, the
physical mechanisms behind these changes remain obscure. Thus, it seems a reasonable scientific hypothesis the
existence of physical mechanisms related to the variability of the meridional circulation, which force extra-tropical low-
frequency variability.
In winter, when the westerly winds predominate in the extra-tropical stratosphere, the troposphere and stratosphere
are dynamically coupled. Thus, the effect of the tropical/extra-tropical connection could be different, depending on the
characteristics of the coupling between the two atmospheric layers.
In this project, one intends to perform a series of 20 simulations with the Melbourne University General Circulation
Model, to study the atmospheric inter-annual climate variability of the Euro-Atlantic tropics/extratropics interface
region. In these simulations the atmosphere is forced with lower boundary conditions (sea surface temperature and
sea-ice) observed during the 1979-96 period. Each of these simulations differs in the initial conditions. With this
approach one can separate the total variability into forced (due to the temporal evolution of the boundary conditions),
and internal variability. This allows also the separate study of the anomalous energy excess (El Niño) and deficit (La
Niña) in the tropics to which the extra-tropical circulation may not respond linearly.
The effect of the dynamical coupling between the troposphere and stratosphere will be studied. The analysis will be
performed also on observed data and, if necessary, on simulations available from other models.