The Structure and Composition of the Annular Modes in an Aquaplanet General Circulation Model

The annular mode simulated by an atmospheric general circulation model with a zonally symmetric lower boundary is investigated. The annular mode, defined as the leading empirical orthogonal function (EOF) of the zonal-mean surface pressure, has a meridional structure consisting of a north–south dipole, similar to observations. The leading EOF of the zonally varying surface pressure has the same meridional structure and is also zonally symmetric. Because the lower boundary is zonally symmetric, composites of days with high projection onto the mode have, to within sampling error, no zonal structure. However, individual periods during which the zonalmean surface pressure projects strongly onto the annular mode are dominated by zonally localized structures. Thus, the model annular mode represents a zonally homogeneous distribution of zonally localized events with a similar meridional structure, rather than a zonally symmetric mode of variability per se. Individual annularmode events typically show a north–south teleconnection pattern whose meridional structure closely resembles the annular mode and whose zonal structure extends 608 to 908 in longitude, with a slight northwest–southeast offset between its centers of action. Similar structures are found for EOFs calculated over a subset of the domain corresponding to the width of the Atlantic basin. The spatial structure of both the teleconnection pattern and the regional EOFs resemble the observed North Atlantic Oscillation (NAO) pattern.