This decline in LLC frequency was found to be modulated by the remote influence of the Pacific Decadal Oscillation (PDO, Mantua,1999) through inhibition of low-level cyclonic vorticity and increased vertical wind shear. The PDO is a long-term ocean fluctuation of the Pacific Ocean. The PDO waxes and wanes approximately every 20 to 30 years. The findings are important as the frequency of such cyclones is expected to increase in the favourable phase of PDO. We believe that this is the first study where such a remarkable decline in the frequency of NIO LLCs was detected and was attributed to local and remote forcing.

To put our study in a societal context, it may be noted that a recent LLC, Ockhi, in November 2017, travelled over 2000 km and devastated parts of Sri Lanka and India with extensive damage to properties worth US$920M and loss of lives of at least 844 people (Singh et al., 2020, https://en.wikipedia.org/wiki/Cyclone_Ockhi). The cyclone Ockhi was peculiar as it created havoc over the south-western coast of India (Kerala, Karnataka coast and Sri Lanka). The occurrence of tropical cyclones over these regions is rare as it is the first very severe cyclone to form over the Lakshadweep Sea since 1925 (Singh et al., 2020).  Normally, tropical cyclones do not form near the equator (6.5°N in the case of Ockhi) due to the lack of adequate Coriolis force[1]. This intrigued the team behind this study to look for occurrences of near-equatorial cyclones and their frequency.

  The study finds that there were more near-equatorial cyclones in the previous decades (1951-1980) compared to the recent (Fig. 1). The epochal change in two key circulation features—weakening low-level vorticity and increasing vertical wind shear—supports the decline in LLC frequency. The Pacific Decadal Oscillation (PDO, Fig. 1) modulates the epochal changes in LLC frequency through these circulation features. Further, we also show that this influence of PDO on LLC frequency is largely independent of El Niño Southern Oscillation (ENSO) influence. The results present an interesting situation where remote influence by natural climate variability (PDO) causes fewer cyclones, but favourable local thermodynamic conditions due to global warming make them slightly stronger. When this tug-of-war between the natural and anthropogenic forcing changes, and they begin to work synergistically, the risk of severe cyclones in the post-monsoon north Indian Ocean may be amplified.

[1] The invisible force that appears to deflect the wind is the Coriolis force. Tropical cyclones are difficult to form near the Equator because the Coriolis force is too small to generate a vortex.

References:

Mantua, N. J. ,199: The Pacific Decadal Oscillation:  A brief overview for non–specialists, to appear in the Encyclopedia of Environmental Change." Joint Institute for the Study of the Atmosphere and Oceans University of Washington, Seattle, Washington, USA.(http://jisao. washington.edu/pdo/).

Singh, V. K., M. K. Roxy, and M. Deshpande, 2020: The unusual long track and rapid intensification of very severe cyclone Ockhi. Curr. Sci., 771-779, Doi:10.18520/cs/v119/i5/771-779.