Abstract This study investigated the intensity and spatial distribution of precipitation anomalies in Rio Grande do Sul state (RS) during the 20th century by analyzing the influence of El Niño - Southern Oscillation (ENSO) in its three phases. Reanalyzes from the University of Delaware (V5) were used. Precipitation anomalies were divided into three zones (Campaign, Plateau, Coast) to investigate possible differences in precipitation under ENSO influence. Alexandersson's Standard Normal Homogeneity Test was applied to verify possible structural breaks. Wavelets were used to rate the periodicity of precipitation anomalies into three phases. Anomalies that occurred in El Niño and La Niña phases were classified in contingency tables as weak, average, and strong to evaluate the qualitative behavior of these precipitation anomalies. It was found that precipitation anomalies presented a structural break in 1955 when the tendency to positive anomalies increased. From 1955, precipitation anomalies increased at least 0.5 standard deviation while the frequency of these anomalies decreased from 85 to 60 months. Peaks of positive anomalies in El Niño were higher than 200 mm. In neutral cases, anomalies intensified in 0.5 standard deviation since 1970. Negative anomalies did not show specific behavior in any ENSO phase. Contingency tables indicated that La Niña events did not present any visible influence pattern. It could intensify the episodes of positive and negative precipitation anomalies in at least 0.5 (-0.5) standard deviation. Therefore, La Niña events could increase or decrease the monthly anomaly but do not show any tendency to negative anomalies. Weak El Niños tends to contribute to negative precipitation anomalies while strong cases are associated with an average increase of 2 standard deviations in positive anomalies. It was concluded that the influence of ENSO in RS is stronger in El Niño than in La Niña. Still, both may induce negative and positive anomalies, depending on the intensity of each case. Wavelet analysis revealed that cycles that did not coincide with El Niño/La Niña showed an increase (decrease) of 0.5 (-0.5) standard deviation is positive (negative) anomalies. The increase of anomalies in neutral phases indicated that other climatic variability modes and the intensity of meteorological events decreased the 25 and 43 months cycles and increased the precipitation in RS.

RESUMO O padrão de precipitação do estado do Rio Grande do Sul (RS) esta mudando, indicando aumento, ainda que se observe longos períodos de seca. Diversos estudos apontam influencia de modos de variabilidade climática na precipitação do RS. Este trabalho analisa a influência do Modo Anular do Hemisfério Sul (SAM) e do El Niño – Oscilação Sul (ENOS), nas anomalias de precipitação (PP) da Planície Costeira do Rio Grande do Sul (PCRGS), a dividindo em três regiões: sul, central e norte. Tabelas de contingência foram utilizadas para correlacionar os índices, os classificando como neutros, abaixo ou acima da média. Para significância estatística utilizamos o percentual corretamente classificado, com o qual se aplicou o teste t de Student para cada região. As PP da PCRGS e da costa sul possuem correlação com o ENOS e SAM; a central apenas com o SAM. As PP do norte não são correlacionadas há nenhum dos dois indices. Períodos com mais de 5 meses de PP, SAM e ENOS anômalos foram identificados. Eventos abaixo da média foram maioria. Os resultados indicam maior influência SAM+ e La Niña. A tendência ao SAM+ e intensificação do ENOS, poderá aumentar a frequência de secas na PCRGS. Além disso, foi possível interpretar que as diferenças geográficas da PCRGS podem influenciar nos resultados dos totais de precipitação. Este trabalho contribui para a compreensão dos efeitos das novas tendências da variabilidade climática sob aspecto regional e geográfico.

ABSTRACT The precipitation pattern of the State of Rio Grande do Sul (RS) is changing, indicating an increase, although there are long periods of drought. Several studies indicate the influence of climate variability modes on RS precipitation. This work analyzes the influence of the Southern Annular Mode (SAM) and the El Niño - Southern Oscillation (ENSO) on precipitation anomalies (PP) of the Rio Grande do Sul Coastal Plain (RGSCP), dividing it into three regions: south, central and north. Contingency tables were used to correlate the indices, classifying them as neutral, below ou above the mean. To statistical significance we used percentage correctly classified with which the Student’s t was aplied for each region. The PP of the RGSCP and the south coast have correlation with the ENSO and SAM; the central only with SAM. The PP of the north is not correlated to either index. Periods with more than 5 months of PP, SAM and ENSO anomalies were identified. Below average events were majority. The results indicate greater influence of SAM + and La Niña. The trend towards SAM+ and intensification of ENSO, could increase the frequency of droughts in RGSCP. In addition, it was possible to interpret that the geographical differences of the RGSCP can influence the results of precipitation totals. This work contributes to the understanding of the effects of the new trends of climatic variability under regional and geographical aspects.

Este estudo investiga a ocorrência de Complexos Convectivos de Mesoescala (CCM) e a precipitação associada aos eventos no Estado do Rio Grande do Sul (RS), entre outubro e dezembro de 2003. A identificação dos CCM foi feita através do aplicativo ForTraCC e, para quantificação dos totais de precipitação oriundos de CCM, foram utilizados dados diários de 31 estações meteorológicas. Durante o período de estudo, foram identificados 22 eventos de CCM sobre o RS. Deste total, sete ocorreram em outubro, sete em novembro e oito em dezembro. Os dados analisados apontam para um volume médio de precipitação de CCM no RS de 119 mm em outubro, 90 mm em novembro e 152 mm em dezembro. Do total de precipitação registrado para o período, o percentual oriundo de CCM ficou em 64% no mês de outubro, 65% em novembro e 61% em dezembro. O volume médio de precipitação de CCM acumulado no trimestre ficou em 361 mm, uma participação percentual de 63%, confirmando que os CCM contribuíram significativamente para a precipitação no Estado e foram responsáveis por quase dois terços da chuva observada no RS no período.

This study investigates the occurrences of Mesoscale Convective Complexes (MCC) and the associated precipitation in the southernmost Brazilian state, Rio Grande do Sul (RS), from October to December of 2003. The identification of MCC events was done using ForTraCC. The MCC precipitation was based on the daily data of 31 local weather stations. Twenty-two MCC events were identified over RS during the three month study period, and this total, seven events occurred in October, seven in November and eight in December. Results point to an MCC associated precipitation average of 119 mm in October, 90 mm in November and 152 mm in December. The percentages for MCC precipitation averages, for the period, were 64% in October, 65% in November and 61% in December. The average accumulated volume of MCC precipitation for the three months was 361 mm (63%). This confirms that MCC events contributed significantly to the precipitation totals in RS, and were responsible for almost two thirds of the precipitation in RS, for the period.

Os Complexos Convectivos de Mesoescala (CCM) são aglomerados organizados de nuvens convectivas, que atingem com freqüência a Região Sul do Brasil e produzem uma grande variedade de desastres. Este trabalho identificou os CCMs ocorridos entre outubro e dezembro de 2003 no Rio Grande do Sul (RS) e avaliou os desastres associados aos eventos. A identificação dos CCMs foi feita através do aplicativo ForTraCC - DSA/INPE. O levantamento das ocorrências de desastres foi realizado a partir do cruzamento dos dias de eventos de CCM com as informações da Defesa Civil do RS (DCRS) e do Jornal Correio do Povo. Foram identificados 22 eventos de CCM no período, responsáveis por 42 episódios de vendaval, 21 de enxurrada, 14 de enchente, cinco de granizo associado à vendaval, três de granizo, dois de alagamento, dois de inundação e um deslizamento, totalizando 90 ocorrências. Houve registro de seis mortes no RS e cinco em Santo Tomé na Argentina, fronteira com o Estado. Mais de 16.500 pessoas foram atingidas e cerca de 60% dos municípios que comunicaram desastres à DCRS decretaram situação de emergência. Em média, a cada quatro eventos de CCM, três produziram algum tipo de desastre no RS.

Mesoscale Convective Complexes (CCM) are organized clusters of convective clouds, which often arrive at Southern Region of Brazil and produce a variety of hazards. This study evaluated hazards associated with CCM events in the Rio Grande do Sul (RS) State, the southernmost of Brazil, between October and December of 2003. The detection of CCM was done using ForTraCC application - DSA/INPE. Data coming from Defesa Civil do RS (DCRS), the RS civil defense organization, and from the archives of Jornal Correio do Povo, a local newspaper, served as source material to evaluate the hazards on the CCM event occurring days. During the period, 22 CCM events were identified over the RS. Based on DCRS reports, 90 incidents were associated with CCM events: strong gales and storms (42), flash floods (21), stream overflows (14), hail and strong gales (5), hail storms (3), swamping (2), floods (2) and earth flows (1). These CCM events caused the death of eleven people, of which six were in RS and the five in San Tomé, on the Argentinean side of the boarder. More than 16,500 people were affected by hazards caused by CCM in the RS. Approximately 60% of the municipal districts affected requested emergency situation. On average, three to four events produce some type of hazard.