Dramatic change in Atlantic Meridional Mode coupled with weak warm ENSO could trigger a drier climate into mid-summer for Brazil crop areas
Discussion: An interesting climate situation has just-emerged in the tropical Atlantic Ocean also affecting SSTA off the East Coast of Brazil. During the past month the wind component of the Atlantic meridional mode (AMM) has flipped from the positive phase to strong negative phase. The change is dramatic and similarly observed at this time of the year in 2018, 2015, 2014 and 2002 (Fig. 1) taken from the 20-year climatology.
Fig. 1: One of the strongest flips on record for OCT to NOV of the wind component of Atlantic meridional mode has just occurred. Identified is a change in the wind flow patterns across the Atlantic equatorial zone from toward the No. Atlantic Ocean to into the So. Atlantic/toward So. America.
Most apparent, given the AMM/wind index shift is the dramatic change in regional SSTA in the South Atlantic basin. Off the East Coast of Brazil, waters are much cooler than normal (Fig. 2) and all of that cool appearance has generated in the past 30 days (Fig. 3). In the south equatorial Atlantic, surface water has warmed generously the past month as surface wind is now blowing across the equator into the southern hemisphere. Waters off the West Coast of Africa are also much warmer.
Fig. 2-3: Due to the change in low-level wind flow over the past month a dramatic change in the SSTA pattern has taken place off the coast of Brazil (much cooler), West Africa (much warmer) and the south equatorial region (much warmer).
Fig. 4: Since 2000 a dramatic change of the AMM to strong negative phase in November has occurred on 5 occasions each when El Nino was present, developing or trying to develop.
Interestingly, each dramatic +AMM to –AMM change occurring during the last 20 years in the months of OCT to NOV took place during El Nino or when El Nino was developing. In 2019 a weak El Nino is trying to redevelop in the eastern equatorial Pacific. During the 4 analog years when varying intensity of warm ENSO was present coupled with the dramatic –AMM/wind flip the following months of DEC/JAN were quite dry across Brazil while a very wet zone was observed over Uruguay to Northeast Argentina (Fig. 5).
Using 2018-19 and 2014-15 as analogs when warm ENSO was weak (as it is in 2019) the precipitation result is a little different although similar in character (Fig. 6). The Brazil dryness shifts farther east and is stronger while the Uruguay/Northeast Argentina wet anomaly is stronger. Additionally there is a wet anomaly across Interior Northwest South America.
Fig. 5-6: Since 2000 a strong flip from +AMM to an intense –AMM (wind index) occurring OCT to NOV occurred in 2018, 2015, 2014 and 2002. Each year ENSO was approaching El Nino or in a strong El Nino. The South America rainfall anomalies for following DEC/JAN are indicated (left). When ENSO was neutral or weak El Nino (like 2019) the South America rainfall pattern for DEC/JAN is indicated (right).
Conclusion: The Atlantic meridional mode is a measure of the interaction between the northern and southern hemisphere low atmospheric climate pattern across the equatorial region defined by sea surface temperature anomalies and cross-equatorial wind index. The cross-equatorial wind index flipped from +4.99 in October to -9.08 in November representing the largest OCT-to-NOV phase change on record. As a result the low level wind flow pattern change has caused dramatic warming of the ocean surface in the tropics just south of the equator while waters off Brazil have also cooled in stunning character. Waters off the West Coast of Africa have warmed. Normally, a –AMM regime cause anomalous wet climate into north/northeast Brazil. However, analogs indicate presence of strong –AMM when a version of El Nino is present produces a dry climate across Brazil and limited (although potent) area of wet weather in Northeast Argentina to Uruguay for the combination of December and January.
Result: Analysts should be on the look-out for a drier pattern across Brazil crop areas over the next 6-8 weeks.