Highlight: Changing Atlantic Meridional Mode to Govern Summer South America Rainfall Pattern Discussion: Cross-equatorial flow in the Atlantic basin is likely to shift to northern-to-southern hemisphere (negative Atlantic meridional mode) for meteorological summer influencing South America rainfall patterns. The 30-day SSTA trend is much cooler in the North Atlantic tropics while warming sharply in the southern hemisphere tropics (Fig. 1-2). Wind direction tends to flow from cooler to warmer waters. The latest NMME global SSTA forecast confirms a shift toward near normal waters north of the equator while warmer waters are to the south and representative of –AMM (Fig. 3). Analog years from the past 20-year climatology representing a shift of the AMM to the negative phase during southern hemisphere summer are 2016-17, 2014-15, 2011-12, 2007-08, 2006-07 and 2002-03. The consensus of the analog years favors anomalous heat for Eastern Brazil and beneficial rains Mato Grosso to northeast Argentina during the summer season (Fig. 4-5). Fig. 1-2: 30-day SSTA change in the tropical northern and southern Atlantic Ocean. Fig. 3: Global SSTA forecast for DEC/JAN/FEB 2019-20 by the NMME model. Fig. 4-5: CIC constructed analog climate forecasts for DEC/JAN/FEB 2019-20 across South America based on AMM analog years.

The Positive Atlantic Meridional Mode & South America Rainfall Fig. 1: Positive phase of the Atlantic Meridional Mode (+AMM) is present when surface wind extends from the southern hemisphere across the equator into the northern hemisphere causing piling warm water in the North Atlantic tropics and affecting northern South America rainfall patterns. Fig. 2: The surface wind component of AMM is well-established in the positive phase. Recently, the SSTA component has shifted into the positive phase. The positive phase is likely to continue in late 2019. Fig. 3-4: Nov. 1, 2019 South America soil moisture has been enhanced drier due to the +AMM regime of mid-to-late 2019. The +AMM climate can be interrupted by the MJO. When the MJO is in the longitude of the Americas the South America climate is biased wetter. The 14-day MJO forecast indicates the wetter support fades in late November. The concern is that the drier +AMM climate resumes as meteorological summer arrives and encourages dry (and anomalous hot) climate for December. Fig. 5: The GFS ENS indicates areas of beneficial rainfall over the next 15 days in both Brazil and Argentina. Discussion: ENSO is often regarded the lead climate influencer of South America climate. However, other oceanic indices can significantly affect the climate pattern, specifically rainfall, soil moisture and contribution to excessive heat risk (which can accelerate drought). The cross equatorial wind pattern known as the Atlantic Meridional Mode (AMM) identifies the moisture fetch into or away from northern South America which affects the ability of moisture entrainment into low pressure systems across the southern Brazil to central Argentina crop areas. The positive phase of the AMM is present when wind flow is from the southern hemisphere across the equator into the northern hemisphere. This pattern is most readily apparent by the warm SSTA regime in the North Atlantic tropics while the South Atlantic tropics are cooler than normal. This pattern is present now. The wind flow toward the northern hemisphere limits the ability for a moisture fetch southward into important crop areas increasing the risk of a dry climate. Presence of the +AMM regime the past 3-6 months is likely a contributor to widespread dry soils as of November 1, 2019. In November so far there have been areas of beneficial rainfall across South America more specifically parts of both the Brazil and Argentina crop areas. The 15-day forecast indicates additional beneficial rainfall is likely. The wetter regime is caused by presence of the convection phase of the Madden Julian oscillation (MJO) in the longitudes of the Americas. The MJO is a powerful intra-seasonal tropical oscillation and presence over-rules large-scale climate influences of the AMM (and ENSO) climate regimes. The 14-day MJO forecast indicates the presence in the longitudes of the Americas fades. There is concern that the drier biased +AMM climate pattern will resume in December as the onset of meteorological summer arrives. Given still-dry soil moisture conditions in key areas of Brazil and Argentina expectations of new drought concerns are likely during early meteorological summer.