Abstract
An atmospheric general circulation model is used to investigate the influence of the North Atlantic Ocean on atmospheric variability. The study covers the period from 1950 to 1994. The observed sea surface temperature and sea ice extension are used to force the atmospheric model. Several configurations of the oceanic boundary conditions were made to isolate the role of the North Atlantic and to study its non-linear interaction with forcings from other oceanic basins. The multi-realization character of the experiments distinguishes between the internal random part and the external forced part of the total variability. The potential predictability can thus be evaluated. The response of the atmosphere is also studied with a modal approach in terms of hemispheric teleconnection patterns. The North Atlantic Ocean has a direct influence on both the Northern Hemisphere annular mode and the Pacific-North-America pattern, leading to a weak predictability. However the direct response is largely modulated by forcings from other oceanic basins. The non-linearity of the system compensates the predictable component of the annular mode induced by the North Atlantic forcing. Furthermore it reduces the forced component of the Pacific-North-America pattern, increasing its chaoticity.
Highlights
Atmospheric circulation of the Northern Hemisphere exhibits variability in a wide range of timescales, from days to decades. This variability is dominated by a small number of large-scale coherent teleconnection patterns (Wallace and Gutzler, 1981), such as the PNA (Pacific-North-America pattern) or the NAO (North Atlantic Oscillation)
When an atmospheric general circulation model is used to investigate the second kind of predictability, the boundary conditions are the SST and sea-ice variation
In this work we address the problem of the influence of the oceanic boundary conditions on atmospheric variability in the Northern Hemisphere
Summary
Predictability of the climate at seasonal to decadal timescales arises from two distinct sources as described by Lorenz (1975). Rodwell et al (1999) reported a high hindcast skill of the NAO, originated from the North Atlantic decadal- scale variation by using an atmospheric general circulation model forced by the observed SST for the second half of the 20th century. Mid-latitude atmospheric circulation is characterized by a strong transient circulation with large-scale wave-like structures Response of such a regime of atmospheric circulation to the boundary conditions is much more complex. The North Atlantic ocean can have a direct impact on the atmospheric variability, but its influence can be non linear and the direct impact can be modulated by other oceanic forcing. This non-linearity can act on the internal variability.
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