Precipitation anomalies in the Brazilian southern coast related to the SAM and ENSO climate variability modes

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.

摘要 南里奥格兰德州（Rio Grande do Sul, RS）的降水格局正发生变化，整体呈增加趋势，但同时伴随长期干旱时段。多项研究指出，气候变率模态对该州降水存在影响。本研究针对南半球环形模（Southern Annular Mode, SAM）与厄尔尼诺-南方涛动（El Niño - Southern Oscillation, ENSO）对南里奥格兰德沿海平原（Rio Grande do Sul Coastal Plain, RGSCP）降水异常（Precipitation Anomalies, PP）的影响展开分析，并将该沿海平原划分为南部、中部与北部三个区域。研究采用列联表对各指数进行关联分析，将其分类为中性、低于或高于平均值三类；为检验统计显著性，针对每个区域采用学生t检验结合正确分类百分比进行评估。结果显示，南里奥格兰德沿海平原及南部沿海的降水异常与ENSO及SAM均存在关联，中部区域仅与SAM相关，而北部区域的降水异常则与二者均无关联。研究识别出降水异常、SAM及ENSO持续时长超过5个月的时段，其中低于平均值的事件占多数。结果表明，SAM正位相与拉尼娜事件的影响更为显著。结合当前SAM正位相的发展趋势以及ENSO活动的增强，可能会提升南里奥格兰德沿海平原的干旱发生频率。此外，本研究还证实，南里奥格兰德沿海平原的地理差异会对降水总量的分析结果产生影响。本研究有助于从区域与地理视角理解气候变率新趋势所带来的影响。