A 30-day oscillation over the North Atlantic

Abstract

A spectral peak at a period of 30 days found in a certain EOF series is studied. A westward-moving wave local to the North Atlantic has been found. The wave is often present and is rather intense. In half of the cases, it is associated with a blocking. The wave can then be described as a retrograde blocking in the Scandinavian-Atlantic sector. It is shown that the oscillation can be viewed as a mode of the non-linear barotropic atmosphere. Three fields are needed in order to simulate the oscillation, i.e., the mean field, an initial anomaly field and a forcing field. All three fields are determined with the aid of the adjoint method by requiring that the oscillation patterns simulated and observed must be as alike as possible. The three fields mentioned have been the unknowns of the problem and the oscillation observed has been the only known variable.