An extreme aggregation of precipitation events on seasonal timescales, leading to a so-called extreme wet season, can have substantial environmental and socio-economic impacts. Nevertheless, the analysis of the conditions leading to extreme wet seasons has not gained high visibility in climate research so far. This study addresses this research gap by shedding some light onto the complex relationship between synoptic scale weather patterns and seasonal precipitation.

Due to the high variability of wet seasons around the globe, in terms of both duration and intensity, a new method has been developed to identify extreme wet seasons objectively. This method first applies 90-day running accumulations of precipitation to every grid point in order to identify the central date of extreme wet seasons. Then, it concatenated grid points to patches to determine the spatial extent affected by the same extreme wet season. Finally, the we use objectively identified weather systems to quantify the contribution of these weather systems to the formation of extreme wet seasons.

As an example, this figure shows the complexity of weather systems contributing to the common extreme season (27 November 2010 to 3 April 2011) that is shared by the locations within the hatched area. The accumulated precipitation is shown in shadings of blue colour, the frequency of cyclones occurrence during this extreme season is shown in red contours for values of 20, 40 and 60%, while the brown contour shows the areas affected by Rossby wave breaking with a frequency of occurrence of 60%. The red dashed line indicates the track of tropical cyclone Yasi, while its position on 5 February 2011, 18 UTC, is indicated by the red dot.

In contrast to extreme precipitation events, typically caused by single weather systems, an extreme accumulation of precipitation on seasonal scales may be attributed to a combination of different and/or recurring weather systems. Indeed, the precipitation within the hatched area of the figure has been formed through the contribution of climatological features such as the ITCZ, enhanced precipitation by Rossby wave breaking, but also due to the frequent occurrence of cyclones, including the single, prominent system of tropical cyclone Yasi.