Notes on Storm Tracks 

The storm tracks of interest for generation of rissaga are those propagating down out of the tropics. Therefore, we look for low pressures along the northern boundary of the domain (20 deg S). At this latitude, monthly minimum pressure varies a lot from summer to winter, as indicated in Figure 1. The figure shows, for example, that if pressures fall below 1008 hPa in September, there is a high likelihood of a storm, whereas if the same occurs in February, storm conditions are unlikely. So, in looking for low pressures at 20 deg S, we must take account of where we are in the year. The algorithm that seems to work best uses a threshold of 5 hPa below the average minimum monthly pressure. If pressures fall below that, a storm is signalled. Figure 1. Mean monthly minimum pressures at 20 deg S.

Once pressures along the northern boundary fall below the threshold, a storm state is initiated and the point of minimum pressure is tracked from forecast to forecast every 6 hours.

The weather system to the left is a typical rissaga-generating event. It shows atmospheric pressure at mean sea level (MSL), with high pressures being red and low pressures being blue, as indicated in the colour bar (in hPa).

At midnight on 10 Sept, a low-pressure system began to emerge from the tropics and propagate southwards. Over the next few days, it steadily tracked southwards, with the pressure at its centre dropping as it went, as indicated in the plot below which shows the trajectory plotted over the top of the bathymetry.