Introduction
In “The Structure and Composition of the Annular Modes in an Aquaplanet General Circulation Model” (Cash, Kushner, and Vallis 2002) and “Zonal Asymmetries, Teleconnections, and Annular Patterns in a GCM” (Cash, Kushner, and Vallis 2005) (CKV from now on), the authors analyse simulations on an aquaplanet model (in the former) and with some zonal asymmetries (in the latter). In particular, they study the configuration of their “annular modes” or, rather, their leading EOF. Their conclusion is that annular modes as seen from the leading EOF are a statistical artifact and that, in fact, they actually describe localised events.
If this is correct and is also valid for the real atmosphere (as opposed to their simple models), then it throws a monkey wrench to any attempts to understand the Souther Annular Mode –it doesn’t even exist as a physically meaningful entity.
Here, I use reanalysis data to show that, while CKV raises important points, the concept of the global SAM is safe.
CKV analysis
CKV ran an aquaplanet model and they point out that the first EOF of sea level pressure describes an almost perfect annular mode.
Figure 1: Figure 4 from Cash, Kushner, and Vallis (2002): Leading EOF of winter season surface pressure. (a) Normalized, nondimensional EOF of the zonal-mean surface pressure. (b) Regression map of zonal-mean surface pressure against the principal component of the leading EOF. Amplitude is the response to 1 std dev in the principal component. (Units: mb.) (c) As in (b), except for the zonally varying pressure. Solid lines are positive, dashed lines are negative, and a heavy contour denotes the zero line. Contour interval is 2 mb
The problem is that when the look at particular events of high / low values of EOF1, they are nowhere near “annular.”