资源环境科技发展态势分析平台

The impacts of the Madden-Julian oscillation (MJO) on precipitation over Northeast Brazil (NEB, also known as Nordeste) are evaluated based on daily raingauge data from 492 stations over 30-year period (1981-2010). Composites of precipitation, outgoing longwave radiation and moisture-flux anomalies are performed for each phase of the MJO, and over all four seasons, based on the Jones-Carvalho MJO index. To distinguish the MJO signal from other patterns of climate variability, daily data are filtered using a 20-90 day band-pass filter; only days classified as MJO events are considered in the composites. The results show strong seasonality of the MJO's impact on precipitation. The most spatially coherent signals of precipitation anomalies occur in the austral summer, when about 80% of the raingauge stations showed increased precipitation in phases 1-2 and suppressed precipitation in phases 5-6 of the oscillation. Although the MJO impacts precipitation on intraseasonal timescales in all seasons in most locations, these impacts vary in magnitude and depend on the phase of the oscillation. Precipitation anomalies over NEB are explained by the interaction of convectively coupled Kelvin-Rossby waves with the dominant climatic features in each season. During the austral summer and spring, westerly regimes increase precipitation over most NEB. In the austral winter and fall, precipitation anomalies exhibit more complex spatial variability. In these seasons, precipitation anomalies in coastal areas depend on the strength of the South Atlantic anticyclone, which is largely modulated by Rossby waves. The strengthening of the anticyclone intensifies the convergence of the trade winds in coastal areas and precipitation windward of the coastal range. Conversely, the intensification of the subsidence is responsible for precipitation deficits in the lee side of the range. These conditions are typically observed when easterly regimes dominate over tropical South America and NEB decreasing moisture flow from the Amazon.