Rare subtropical storm to the southeast of Brazil
April 21 2021:
A rare subtropical storm named Subtropical Storm Potira formed to the southeast of Brazil.
Such systems are given names from the list of tropical cyclone names in Atlantic and Eastern Pacific basins but not in the Australian Region.
Such subtropical cyclones with similar anatomy, appearance and mechanism of formation form around Australia but the BOM doesn't name them and call them "subtropical cyclones", instead the BOM refers to such systems which form off the east coast as "East coast lows" or those that form elsewhere as "cut off lows".
Such subtropical cyclones differ from both tropical cyclones and extratropical cyclones. They often form equatorward of the subtropical high pressure ridge in a region of upper level divergence ahead of an upper level trough and form in sea surface temperatures (SSTs) slightly lower than that associated with tropical cyclones. These non-frontal lows have their strongest winds and heaviest rainfall polewards of the centre, by contrast tropical cyclones have relatively symmetric wind field near the centre and extratropical cyclones typically have their strongest winds equatorward of the centre.
Tropical cyclones derive their energy primarily from the latent heat release whereas extratropical cyclones derive their energy primarily from baroclinic energy and subtropical cyclones are reliant on both latent heat energy and baroclinic energy.
Satellite image:
A rare subtropical storm named Subtropical Storm Potira formed to the southeast of Brazil.
Such systems are given names from the list of tropical cyclone names in Atlantic and Eastern Pacific basins but not in the Australian Region.
Such subtropical cyclones with similar anatomy, appearance and mechanism of formation form around Australia but the BOM doesn't name them and call them "subtropical cyclones", instead the BOM refers to such systems which form off the east coast as "East coast lows" or those that form elsewhere as "cut off lows".
Such subtropical cyclones differ from both tropical cyclones and extratropical cyclones. They often form equatorward of the subtropical high pressure ridge in a region of upper level divergence ahead of an upper level trough and form in sea surface temperatures (SSTs) slightly lower than that associated with tropical cyclones. These non-frontal lows have their strongest winds and heaviest rainfall polewards of the centre, by contrast tropical cyclones have relatively symmetric wind field near the centre and extratropical cyclones typically have their strongest winds equatorward of the centre.
Tropical cyclones derive their energy primarily from the latent heat release whereas extratropical cyclones derive their energy primarily from baroclinic energy and subtropical cyclones are reliant on both latent heat energy and baroclinic energy.
Satellite image:
Image courtesy of NASA.
MSLP chart:
MSLP chart:
Image courtesy of Meteologix.
Wind chart:
Wind chart:
Image courtesy of Meteologix.