On the relationship between SAM and frost days as represented by an ensemble of IPCC AR4 models

MENÉNDEZ C. G. AND A. F. CARRIL

CLARIS Newsletter

CLARIS

Lugar: Paris, Francia; Año: 2006 vol. 5 p. 21 - 25

1669-3264

The Southern Annular Mode (SAM) is the dominant pattern of climate variability in the southern extra-tropics. It is characterized by a meridional seesaw in atmospheric mass between Antarctica and the mid-latitudes, accompanied by an out-of phase relation in the strength of the zonal flow along 60°S and 40°S. The SAM exerts a strong influence on the extratropical climate, not only over the Southern Ocean and Antarctica (e.g. Carril et al. 2005), but over the southern continental regions as well (e.g. Silvestri and Vera, 2003).   Frich et al. (2002) defined ten indices related to extreme weather in temperature and precipitation. These "extreme indices" (annual indicators derived from daily temperature and precipitation time series) were computed by some modeling centers for climate change simulations in the framework of the IPCC Fourth Assessment Report (AR4). Meehl et al. (2005) and Tebaldi et al. (2006) provide a first overview of projected changes in climate extremes from the IPCC AR4 model ensemble. This recent literature analyses the trends in globally averaged values of extreme indices and the global spatial patterns of changes under different emissions rates.  In these studies, the indices have been regarded only over continental areas.   The extent to which the SAM is linked to the extreme events is not yet clear. The SAM could shape not only the mean conditions, but also the daily variability. Consequently extreme events could be sensitive to SAM-related changes in the characteristics of extratropical storms, blocking highs and cold air outbreaks throughout the southern mid- to high latitudes. Our aim is to assess if the climate extremes provided by the AR4 coupled models are sensitive to the SAM and to examine the projections of these extremes for the end of the 21st century for the SRES A1B scenario over the Southern Ocean and embedded land areas.   For the sake of brevity, in this letter we only present results related with a particular index: the ‘‘total number of frost days’’ (“Fd” as named by Frich et al., 2002). Fd is defined as the total number of days per year with absolute minimum temperature below 0˚C. This index is particularly pertinent to the extratropical regions and it is associated with anomalies in the length of spring and fall seasons. We consider that selecting Fd is helpful as a basis to examine processes related with generating the geographic patterns of changes in temperature extremes. Other indices (including precipitation indices) will be discussed elsewhere.