Connecting Regional SSTA With Brazil/Argentina and Australia 15-Day Rainfall Outlooks

Fig. 1: The GFS ENS 15-day upper air pattern forecast identifies a prevailing upper trough off the Southeast Brazil coast.

South America Outlook: The GFS ENS indicates a semi-permanent upper trough just-off the East Coast of Brazil in the 15-day outlook (Fig. 1). The upper trough (combined with a semi-permanent weaker upper ridge) combines to produce dramatically different rainfall patterns in Brazil/Argentina the next 2 weeks. The regional SSTA patterns mirrors this pattern as cooler waters prevail east of Southern Brazil where the upper trough produces more water-cooling cloudiness (Fig. 2). The 15-day outlook is characteristic of the projected summer pattern which finds difficulty in eliminating the upper trough pattern.

The upper trough certainly increases wet weather risk over the northern half and eastern Brazil. The upper trough engages super-wet trade winds made-so by the continued record warm waters of the South Atlantic tropics. However, compensating dryness is not far away and settles on far Southeast Brazil to Northern Argentina. This small region is susceptible both short-term and long-term drought heading into December (and beyond).

Fig. 2: South Atlantic SSTA pattern identifying emerging cool waters east of Southeast Brazil.

Australia Outlook: The ECM ENS maintains a soaking wet forecast across East Australia in the 15-day outlook (Fig. 3). Additionally, wet weather may become important over Southwest Australia. The SSTA pattern in the South Pacific identifies the wet weather-driver. The semi-permanent “warm blob” of SSTA near and east of New Zealand (Fig. 4) is reflected in the upper air pattern by a semi-permanent amplified high-pressure ridge. Upstream the upper ridge is compensated for by a semi-permanent wet low-pressure trough with anchor dropped on East Australia. The pattern is locked-in for now with any significant change slow to occur.  

Fig. 3: The ECM ENS 15-day percent of normal rainfall forecast for Australia.

Fig. 4: The South Pacific SSTA analysis identifies the semi-permanent New Zealand “warm blob”.