Severe cyclonic storm Dana which made landfall in Odisha is the second cyclonic disturbance originating in the Bay of Bengal (BoB) this October. The first such disturbance, which was of lower intensity, had caused the floods in the more southern parts of the eastern. Are two such back-to-back storms unusual in the BoB in October? While two storms might appear unusual because this has not happened in the past few years, it is not unusual if one is looking at long-term trends. On the other hand, Dana’s intensification into a severe cyclone storm will strengthen a long-term trend of post-monsoon cyclonic disturbances in the BoB intensifying long-term.

Severe cyclonic storm Dana which made landfall in Odisha is the second cyclonic disturbance originating in the Bay of Bengal (BoB) this October. The first such disturbance, which was of lower intensity, had caused the floods in the more southern parts of the eastern. Are two such back-to-back storms unusual in the BoB in October? While two storms might appear unusual because this has not happened in the past few years, it is not unusual if one is looking at long-term trends. On the other hand, Dana’s intensification into a severe cyclone storm will strengthen a long-term trend of post-monsoon cyclonic disturbances in the BoB intensifying long-term.

The storm trends for BoB summarised above will make sense only if one understands the classification of storms. The term cyclonic disturbance, for example, encompasses all low-pressure areas with anti-clockwise wind movement that have a maximum sustained wind speed of at least 31 kmph, according to the India Meteorological Department (IMD). These cyclonic disturbances are then further classified according to their intensity, which is measured by their maximum sustained wind speed: depression (31-49 kmph), deep depression (50-61 kmph), cyclonic storm (62-88 kmph), and severe cyclonic storm (89 kmph or higher). To be sure, IMD also further classifies severe cyclonic storms by their degree of intensity. That classification is excluded here for the sake of brevity.

So how common are cyclonic disturbances in the BoB in October? Quite common is the short answer. At least one cyclonic disturbance occurred in October in BoB in 117 of 133 years from 1891 (the earliest year for which this data is available) to 2023, which gives an 88% probability of such a storm occurring in October. This means that a cyclonic disturbance in itself is not surprising in October. In the full October-December season, such a storm has occurred in 131 years (98% probability).

Even two cyclonic disturbances are not rare in the BoB either in the October-December season (117 out of 133 years) or just the month of October (61 of 133 years). This means that even in October, two cyclonic disturbances occurring and not occurring in the BoB are roughly equally probable events. However, there is a reason this might appear unusual. One reason is that the last time two cyclonic disturbances occurred in the BoB in October was in 2020. Moreover, the 30-year average of such storms in October peaked at two in 1971 and has since decreased to 1.13 in the 30-year period ending in 2023. Clearly, the complete historical trend of such storms does not capture the relatively recent decline.

To be sure, the recent decline notwithstanding, there is no long-term trend (over the entire 1891-2023 period) in the occurrence of cyclonic disturbances in the BoB in the October-December season. This can be seen in the trendline drawn over the 30-year averages in the charts above. However, one aspect of the two storms this year – that of Dana turning into a severe cyclonic storm -- is part of a long-term trend in the BoB. Severe cyclonic storms show a rising trend for the 1891-2023 period for both the October-December season as a whole and for the month of October. This can be seen in the trendline drawn over the 30-year averages of severe cyclonic storm frequency below.

To be sure, even severe cyclonic storms might appear unusual in this season in the BoB. As the charts above show, severe cyclones have not increased in every 30-year period. Their current frequency is also lower than the peak. This explains why a severe storm in this season might appear unusual although intensification has become more common in the longer-term picture.

Abhishek Jha, HT’s senior data journalist, analyses one big weather trend in the context of the ongoing climate crisis every week, using weather data from ground and satellite observations spanning decades.