South Atlantic explosive cyclones in 2014-2015: study employing NCEP2 and MERRA-2 reanalyses

Abstract

An analysis of explosive cyclone cases was produced by comparing the reanalysis of MERRA-2 (high spatial resolution) and NCEP2 (low spatial resolution) to South Atlantic in the 2014-2015 period. A total of 51 cases were found, of which 49 were detected by the first reanalysis and 33 by the second (2 cases identified by NCEP2 were not identified by MERRA-2). Spring was the dominant season in the formation of the cases in both reanalyses. It was observed that most systems are formed preferentially eastward of a preexisting trough at higher levels, while others are formed under an almost zonal upper airstream. This difference is more evident in the NCEP2. It was also diagnosed that the MERRA-2 shows more clearly the diffluence in the 250 hPa flow. The analysis of the composite fields revealed a negative horizontal tilt of the trough in 500 hPa, influenced by intense convection as the system develops. Besides, it pointed to a more pronounced jet stream in intense explosive cyclones and more prominent diffluence in non-intense cases. Since the NCEP2 reanalysis detected fewer cases (and only 2 intense) than MERRA-2, it was considered that the former is less suited to the analysis of this type of event.

Key words
Bombogenesis; composite fields; diffluent flow; extratropical cyclone