Sources and impacts of the zonally asymmetric component of the Southern Annular Mode

SAM ENSO general circulation

Explanation of our recent paper on asymmetric and symmetric parts of the Southern Annular Mode.

Elio Campitelli (Centro de Investigaciones del Mar y la Atmósfera)

Today I got my first paper published! 🎉

Along with my PhD advisors, Leandro Díaz and Carolina Vera, we wrote “Assessment of zonally symmetric and asymmetric components of the Southern Annular Mode using a novel approach” (because it wouldn’t be an academic publication if the title was shorter), and this post is a general explanation of the main methods and takeaways of the paper.

What’s the deal with the SAM

Climatologists love to think of the climate as a series of different modes. That is, take all the incomprehensible complexity of the evolving climate and distil it to a handful of phenomena that we can understand. For example, instead of having to think about all the variability of the Equatorial Pacific Ocean, which is a 3D field with multiple variables, you can think about the El Niño–Southern Oscillation (ENSO) phenomenon, which can be characterised by a single time series. Other members of the “Climate Oscillations Hall of Fame,” and their respective acronyms, are the Indian Ocean Dipole (IOD), the Northern Annular Mode (NAM), the North Atlantic Oscillation (NAO) and, important for this paper, the Southern Annular Mode (SAM).

The SAM describes an oscillating pattern of alternating low and high pressure anomalies over Antarctica and in the middle latitudes. To have a clear picture, the typical field of pressure anomalies when the SAM is on it’s positive phase looks like Figure 1.

Typical pressure anomalies of the positive phase of the SAM. Lower pressure than usual over the Antarctic and higher pressure than usual in the mid-latitudes.

Figure 1: Typical pressure anomalies of the positive phase of the SAM. Lower pressure than usual over the Antarctic and higher pressure than usual in the mid-latitudes.

As you can see, this pattern lives up to its name in that the positive pressure anomalies form a ring (or, for the hoity-toity scientists, an annulus) around the negative pressure anomalies. In fact, almost every paper on the topic starts the introduction with a sentence along the lines of “The SAM is approximately zonally symmetric…” (Fogt and Marshall 2020) –where “zonally symmetric” means that it doesn’t depend on longitude. The word “approximately” is doing a lot of heavy lifting: the zonally symmetric ring is clearly deformed by zonally asymmetric anomalies.

Most papers basically ignore these deviations from zonal symmetry and think of the SAM as zonally symmetric. This is fine as a first order approximation, but many aspects of the SAM are actually tied to its asymmetric nature. For example, the SAM is associated with anomalies in meridional (north–south) wind, which is not possible for a zonally symmetric pattern. And these meridional wind anomalies clearly are related to impacts in precipitation in over South America (Silvestri and Vera 2009) and temperature over the Antarctic Peninsula (Fogt, Jones, and Renwick 2012). These anomalies are also conspicuously similar to the effect of ENSO on these higher latitudes (e.g. Clem and Fogt 2013).

The understanding of the SAM as zonally symmetric also influences the theories behind the positive trend of the SAM index seen during the austral summer. Simulations show that increased concentrations of greenhouse gases and changes in stratospheric ozone combine to produce a zonally symmetric change of lower pressures over the poles and higher pressures at lower latitudes (Figure 2 from Arblaster and Meehl (2006)). Since these changes are similar to the positive phase of the SAM, then it’s only logical to identify this simulated changes with the observed SAM trend. But, of course, this only works for a zonally symmetric SAM.