ENSO signal on the distribution of precipitation, improving seasonal information for stakeholders

PANTANO V.C., PENALBA O.C.

Guayaquil

Congreso; IV International Conference on El Niño Southern Oscillation: ENSO in a warmer Climate; 2018

El Niño- Southern Oscillation (ENSO) is one of the most important modes of inter-annual climate variability taken into account for predicting seasonal rainfall, useful information for decision makers. In the monthly meetings of the Argentine National Weather Service (scientists, technicians, decision makers, users, etc), this information is supplied as the probability of seasonal accumulated rainfall, separated in terciles. However, for certain users, it is also necessary to know how rainfall is distributed through the season. For agriculture planning, moderate rainfall at first stages of summer crops and during vegetative growing, represent benefit for the yields. The objective of this study is to analyze the probability of occurrence of seasonal rainfall under different ENSO phases and to explore whether the distribution through the season supplies water demand from different phenological stages of summer crops.Daily rainfall was used from 30 meteorological stations in Southeastern South America for the period 1970-2010. Three ENSO phases were discriminated based on Oceanic Niño Index. Firstly, from the terciles of rainfall, it was calculated their probability of occurrence under ENSO phases. Secondly, since seasonal rainfall is not necessarily equally distributed through the season, it was calculated if monthly rainfall was higher than the third part of its seasonal total. This study was performed during summer when high evapotranspiration can reduce or reinforce the impact of high and low rainfall, respectively. The probability of reaching the 3rd tercile is between 40% and 60% in most of the stations under El Niño. It is noticeable that some stations near the southern border of the study region present lower probability, between 20% and 40%. The probability is lower under La Niña, between 20% and 30% in the center and 30% to 40% in the rest of the study region. For the 1st tercile, under La Niña, the probability is 30% to 50% in most of the stations whereas it is below 30% in most of them under El Niño. Additionally, the probability of exceeding a threshold -third of seasonal rainfall accumulation- was evaluated in December (maize and soybean sowing stage), and January (vegetative growing stage). In December, the probability of exceeding the threshold is between 50% and 70% to the east and northeast and 70% to 80% in the rest of the region, under El Niño. Lower probabilities with higher spatial variability are presented under La Niña. When December is rainy, the probability of exceeding a threshold in January is above 50% in almost all the stations under the three phases. This result is relevant for short-term planning in agricultural systems.Finally, it is worth to mention the two most relevant ENSO events in the recent years. During 2011´ La Niña event, 13 out of 30 stations reached the third tercile but only 3 of them presented high rainfall during December. More recently, during 2015´El Niño event, 17 stations reached the third tercile and 10 of them presented high rainfall during December.