The Amundsen Sea Semi-Permanent Low-Pressure Trough

Amundsen Sea low-pressure trough discussion: Beginning in 2020, a large area of low-pressure formed in the Amundsen Sea off the Antarctic Coast well southwest of South America. The upper trough has intensified and persisted through 2021 (Fig. 1). Compensating for the deep upper-level trough is an equally intense upper-level high-pressure ridge just east of New Zealand. The semi-permanent upper-level trough/ridge pattern(s) are well-correlated to the cold SSTA regime north of Antarctica and southwest of South America and warm SSTA near and east of New Zealand (sometimes referred to as the South Pacific “warm blob”).

Last winter, cold periods featuring coffee crop freeze events were observed in Brazil (Fig. 2). Likely is the generation of these cold air masses caused by shortwave energy emitted northeastward from the semi-permanent Amundsen Sea upper-level low-pressure trough (Fig. 3). In May of 2022, the Amundsen Sea trough not only remains in-place (Fig. 4).

The latest NCEP CFS V2 upper air forecast for JUN/JUL/AUG 2022 maintains this feature and the upper ridge near and east of New Zealand (Fig. 5). The high-pressure ridge stretching across Argentina by the NCEP CFS V2 (model) implies a mild winter for Argentina. However, occasional cold interruptions that can surge northward and reach Brazil, similar to last winter are anticipated. A highly changeable (warm and cold) winter climate pattern for Argentina to Brazil! However, cold extremes including freeze events for Brazil’s coffee-growing area is likely.

Fig. 1: Emergence of the 2020-21 Amundsen Sea semi-permanent upper-level low-pressure trough.

Fig. 2-3: Last winter featured cold episodes reaching the coffee growing areas of Brazil. The chilly air masses were associated with upper-level shortwave energy emitted northeastward from the Amundsen Sea upper-level low-pressure trough.

Fig. 4-5: The Amundsen Sea upper trough persists in May 2022 and is forecast to remain in-place by NCEP CFS V2 for JUN/JUL/AUG 2022.

ENSO and other regional SSTA: La Nina persists as mid-2022 approaches. Waters off the West Coast of South America are also prohibitively cold (Fig. 6). The tendency for cool waters off the West Coast and La Nina climate entering its 3^rd year leaves much of South America with dry soils or drought conditions (Fig. 7). La Nina may weaken during southern hemisphere winter, but La Nina climate is expected to continue with at least moderate intensity. A significant contribution to sustaining the La Nina climate is the expected evolution of a negative Indian Ocean Dipole (Fig. 8) which increases the convection patterns in the West Pacific/East Indian Ocean tropics that encourages trade winds in the central equatorial Pacific. The presence of this regime should maintain the La Nina climate despite some marginal warming of the far eastern equatorial Pacific Ocean.

Fig. 6-7: Current persistence of cool SSTA west and northwest of South America is a major contributor to large areas of soil moisture deficit across South America.

The tropical South Atlantic index has warmed to weak warm phase in recent weeks. Forecast models are indicating the +TSA regime will intensify during the next few months. The +TSA regime should increase wet risk across northern South America.

Fig. 8: The ECMWF global SSTA forecast for July 2022.

Forecast methodology: The Climate Impact Company month 1-3 ahead climate forecast for South America is based on a constructed analog which involves presence of a La Nina climate (connected to a developing -IOD pattern), developing +TSA regime and presence of the Amundsen Sea semi-permanent low-pressure area.

June 2022: Meteorological winter begins warmer than normal across Brazil. The pattern across Brazil is most likely drier than normal. A wet zone associated with persistence of stalled frontal systems stretches across Uruguay and vicinity. Argentina is mostly dry and temperate although persistent chilly temperatures emerge across Peru into Chile and far western Argentina.

Fig. 9-10: The Climate Impact Company constructed analog forecast of temperature and precipitation anomalies for South America valid for June 2022.

July 2022: Mid-winter is cold, possibly record cold. The outlook is reliant on a general cooler than normal climate forecast for mid-winter associated with a La Nina climate enhanced by the expected emittance of shortwave energy northeastward from the titanic Amundsen Sea low-pressure system. Cold weather causing a freeze in coffee-growing areas of Brazil is certainly at above normal risk. Northern South America trends wetter otherwise seasonable dryness is expected.

Fig. 11-12: The Climate Impact Company constructed analog forecast of temperature and precipitation anomalies for South America valid for July 2022.

August 2022: The mid-winter chill eases in Brazil but may remain quite intense over Argentina plus Peru into Chile. The cold has a choppy character interrupted by warm periods. Dryness is prominent in Southeast Brazil to Northeast Argentina. Drought patterns in Argentina and Brazil certainly continue into next spring.

Fig. 13-14: The Climate Impact Company constructed analog forecast of temperature and precipitation anomalies for South America valid for August 2022.