Sources of Variability in the Timing of the Rainy Season over Monsoonal Regions

Monsoonal regions across the world are similar in that they exhibit well-defined wet and dry seasons. The interannual variability of the timing of the rainy season over monsoonal regions is dependent the interactions among the seasonal variation of insolation, the three-dimensional atmospheric flow, and ocean-land atmospheric fluxes. However, it is possible that a significant portion of the variance of the timing of the rainy season can be explained by global patterns such as the El Nino Southern Oscillation (ENSO) or by simple metrics of atmospheric instability. Therefore, we investigate the relationship between ENSO and the rainy season using lagged correlations between the Niño 3.4 Index and global onset and end date anomalies. We found that ENSO is positively correlated with onset dates while negatively correlated with end dates in the western Pacific, implying an El Nino (La Nina) pattern is associated with shorter (longer) rainy seasons in southeast Asia and northern Australia. We also investigated local thermodynamic conditions associated with the variability of the Indian and North American monsoons We found that early onset years exhibited locally high moisture content and low temperatures throughout the boundary layer in the two weeks before and after the onset of the rainy season. We also observed a weaker gradient of boundary layer moist static energy in the Indian monsoonal region.