Identification and Climatology of Southern Hemisphere Mobile Fronts in a Modern Reanalysis

Abstract

Presented here is an objective approach to identify, characterize, and track Southern Hemisphere mobile fronts in hemispheric analyses of relatively modest resolution, such as reanalyses. Among the principles in its design were that it should be based on broadscale synoptic considerations and be as simple and easily understood as possible. The resulting Eulerian scheme has been applied to the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA)–Interim and a climatology of frontal characteristics, at both the 10-m and 850-hPa levels, derived for the period 1 January 1989–28 February 2009. The knowledge of the character of these features is central to understanding weather and climate over the hemisphere. In both summer and winter the latitude belt 40°–60°S hosts the highest frequency of frontal points, but there are significant zonal asymmetries within this band. The climatology reveals that the longest fronts are in the Indian Ocean where mean lengths exceed 2000 km. The mean frontal intensity over the hemisphere tends to be greater at 850 hPa than at 10 m, and greater in winter than in summer. The frontal intensity also shows its maximum in the Indian Ocean. In the mean, the meridional tilt of these fronts is northwest–southeast over much of the midlatitudes and subtropics, and increases with latitude toward the equator. The tilts are of overwhelmingly opposite sign in the coastal Antarctic and subantarctic regions. Broadly speaking, the number of fronts and their mean length and mean intensity exhibit maxima in winter in the midlatitudes (30°–50°S), but show a sizeable semiannual variation (maxima in fall and spring) during the year at higher latitudes.

Climate change science and Victoria

“…Fronts are associated with strong winds and increased fire danger (Mills 2005). Recent climatologies indicate the prevalence of fronts as rain bearing systems, particularly cold fronts, across southern Australia (Berry et al 2011, Simmonds et al 2012. Any trends in observed winds are difficult to establish due to sparse coverage of wind observations, difficulties with instruments and the changing circumstances of anemometer sites (Jakob 2010).…”

A Global Climatology of Baroclinically Influenced Tropical Cyclogenesis

“…4e,f and 4g,h), consistent with the need for the moderate background baroclinicity that serves to discriminate between these pathways. This baroclinicity may arise from one of several sources: subtropical atmospheric fronts (Todd and Washington 1999) such as those that are uniformly present in the Southern Hemisphere summer at 308S (Simmonds et al 2012), precursor baroclinic vortices (Davis and Bosart 2002), or meridional SST gradients that exist poleward of the equatorial SST maxima (Reynolds et al 2002).…”

Seasonal and regional signature of the projected southern Australian rainfall reduction

“…Frontal systems are very important for rainfall in southern Australia (Berry et al, 2011;Simmonds et al, 2012) particularly in the southeast in autumn (Catto et al, 2012) and the southwest in winter, where they have been found to be declining in number (Hope et al, 2014). Fronts in an example GCM, ACCESS1.3, tend to be well represented in their frequency and the proportion of precipitation associated with them (Catto et al, 2013), although the intensity of that precipitation is underestimated.…”