Explaining Unexpected Heavy Rains Across Northeast Brazil

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Discussion: During November the wind component of the Atlantic meridional mode (AMM) became very negative (Fig. 1) meaning wind is blowing across the equator from the northern to southern hemisphere (from the northeast). Typical of –AMM regimes the surface water warms south of the equator while comparatively cool north waters are north of the equator (Fig. 2). The northeast wind into warmer waters along and south of the equator is the perfect set-up to produce a wet pattern across northeast Brazil. MUCH ABOVE normal rainfall is in the forecast by operational models over the next 15 days for northeast Brazil (Fig. 3).

The wet pattern in this region is a surprise. A developing El Nino in the equatorial East Pacific correlates to a dry summertime climate across northeast Brazil (Fig. 4). However, the atmosphere is struggling to couple with the warming in the eastern equatorial Pacific to produce an El Nino climate.  The lack of coupling of the atmosphere to the eastern equatorial Pacific is best diagnosed by the monthly southern oscillation index (SOI) which is a measure of the sea level pressure anomaly (SLPA) patterns across the tropical Pacific which should be consistently negative if El Nino is present (Fig. 5). –SOI indicates lower pressure in the tropical eastern Pacific related to increased convection (which has also minimal).

So…the lack of coupling of the atmosphere to the ocean warming in the eastern equatorial Pacific is allowing other regional climate factors to produce non-El Nino climate. Why is the atmosphere unable to link with the warming of the eastern equatorial Pacific? One reason may be the general warmer than normal Pacific Ocean related to climate change (less interaction with constricted polar ice cap and oceanic currents). Note the global sea surface temperature anomalies (SSTA) now (Fig. 6) versus a moderately strong El Nino at this time of the year in 1994 (Fig. 7).

Fig. 1: The 2018 monthly Atlantic meridional mode wind index.

Fig. 2: Conditions created by –AMM in the equatorial Atlantic Ocean region.

Fig. 3: Today’s 12Z GFS OP percent of normal rainfall forecast for days 1-15 across South America.

Fig. 4: El Nino DEC/JAN/FEB global climate anomalies.

Fig. 5: Monthly southern oscillation index for 2018. Several consecutive months of –SOI are required for El Nino climate development.

Fig. 6: November 2018 global SSTA and near-record warmth indicated.

Fig. 7: November 1994 global SSTA and the more typical focus of most ocean warming within the El Nino system.