Intraseasonal and low frequency processes contributing to the December 2013 heat wave in Southern South America

In the second fortnight of December 2013, the longest heat wave registered until then occurred in Buenos Aires city and over a large region of Argentina, with large socio-economic impacts. Excess heat indexes were used to characterize this heat wave, which occurred within the warm season with largest number of heat wave days. This extreme event resulted in the longest consecutive period of heat wave conditions between 1979 and 2014. This event was the result of the combined activity of short intraseasonal (10–30 days), long intraseasonal (30–90 days) and larger (more than 90 days) time scales. Accounting for the lower frequency, dry soil moisture anomalies were observed in Argentina during autumn, winter and spring of 2013 probably favoring more extreme values in the temperature anomalies. Weekly geopotential heights anomalies computed during the event showed that the combination of the negative phase of the Southern Annular Mode and a wave-4 pattern at midlatitudes favored the development of positive geopotential height anomalies over southern South America which promoted subsidence motions there. Intraseasonal variability played a key role in the persistence of the heat wave. The development of an MJO event over the Indian Ocean (RMM phase 3) 12 days prior to the beginning of the heat wave may have contributed to organize the extratropical wave train which in turn favored the anticyclonic upper-level anomaly development over southern South America. Furthermore, an intense SACZ event during the first 12 days of the heat wave favored clear-sky conditions and diabatic heating as well as subsidence, while during the last days it was the northerly advection of warmer air in the 10–30-day time scale that maintained the intensity of the temperature anomalies.