Negative Atlantic Meridional Mode Increases Risk of a Wet Spring in Brazil

Highlight: Negative Atlantic meridional mode increases risk of a wet spring in Brazil.

Fig. 1: The Atlantic meridional mode has shifted into a strong negative phase which should benefit Brazil for increased risk of a wet spring ahead.

Discussion: The Atlantic meridional mode (AMM) has shifted into a strong negative phase during mid-year 2021 (Fig. 1). -AMM is present when the water surface is warmer than normal south of the equator while waters to the north of the equator are much cooler, near normal or cooler than normal (Fig. 2). Surface wind direction is tilted toward the warmer waters where surface pressure is lower. The cross-equatorial wind from the north to south hemisphere increases the effectiveness of moisture increasing rainfall risk into northern Brazil. Coupled with an evolving La Nina, climate forecasts are favoring a wet spring for much of Brazil (Fig. 3).

Fig. 2: Along and south of the Atlantic equator the SSTA pattern is very warm.

Fig. 3: A wet spring climate forecast for the northern half of South America by the NCEP CFS V2 model.