The Southern Annular Mode Seen through Weather Regimes

Abstract

This article investigates the prominent features of the Southern Hemisphere (south of 208S) atmospheric circulation when extracted using EOF analysis and a k-means clustering algorithm. The focus is on the southern annular mode (SAM), the nature of its recent trend, and the zonal symmetry of associated spatial patterns. The study uses the NCEP-Department of Energy Atmospheric Model Intercomparison Project II Reanalysis (NCEP-2) (period 1979-2009) to obtain robust patterns over the recent years and the Twentieth Century Reanalysis Project (period 1871-2008) to document decadal changes. Also presented is a comparison of these signals against a station-based reconstruction of the SAM index and a gridded interpolated dataset [Hadley Centre Sea Level Pressure dataset version 2 (HadSLP2)].Over their common period, both reanalyses are in fair agreement, both in terms of spatial patterns and temporal variability. In particular, both datasets show weather regimes that can be interpreted as the opposite phases of the SAM. At the decadal time scale, the study shows that the trend toward the positive SAM phase (as inferred from the usual EOF-based index) is related more to an increase in the frequency of clusters corresponding to the positive phase, with little changes in the frequency of the negative SAM events. Similarly, the long-term tropospheric warming trend already discussed in the literature is shown to be related more to a decrease in the number of abnormally cold days, with little changes in the number of abnormally warm days. The cluster analysis therefore allows for complement descriptions based on simple indexes or EOF decompositions, highlighting the nonlinear nature of the decadal changes in the Southern Hemisphere atmospheric circulation and temperature.

SST subseasonal variability in the central Benguela upwelling system as inferred from satellite observations (1999–2009)

“…[58] A striking feature of the intraseasonal composite is a pronounced negative surface pressure anomaly over Antarctica that appears approximately 10 days before the event and last during the two following weeks. A similar negative anomaly of geopotential height is found at 500 hPa (Figure 11a, contour) and 200 hPa (not shown), suggesting an impact of the intraseasonal component of the Antarctic oscillation (AAO) [Rogers and van Loon, 1982;Pohl and Fauchereau, 2012], also called Southern Annular Mode or High Latitude Mode. The evolution of the SLP and Z500 composites (not shown) can be interpreted as an interchange of air mass between middle and high latitudes, a well-known feature of the AAO.…”

An original way to evaluate daily rainfall variability simulated by a regional climate model: the case of South African austral summer rainfall

“…For evaluation purposes, we analysed the ratio of the total daily rainfall variance explained by the partitioning into recurrent daily rainfall patterns. Successfully used by Pohl and Fauchereau (2012), this ratio quantifies the value of the clustering approach in synthesizing daily rainfall variability. It is defined as follows, for each grid-point within tropical South Africa:…”

Simulating regime structures in weather and climate prediction models

“…On the other hand, the geometry of the climate attractor is not determined solely by its first two moments. In particular, considerable evidence has emerged over the last decade or more, for quasi‐persistent weather regimes over different regions of the world [ Straus et al , 2007; Straus , 2010; Woollings et al , 2010a; Pohl and Fauchereau , 2012]. The existence of such regimes is not only of interest in its own right: there is evidence that in a dynamical system with regime structure, the time‐mean response of the system to some imposed forcing, which here could be thought of as enhanced greenhouse gas concentration, is in part determined by the change in frequency of occurrence of the naturally occurring regimes [ Palmer , 1999, 1993; Corti et al , 1999].…”