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The Chennai extreme rainfall event in 2015: The Bay of Bengal connection


Southeast India experienced a heavy rainfall during 30 Nov–2 Dec 2015. Particularly, the Chennai city, the fourth major metropolitan city in India with a population of 5 million, experienced extreme flooding and causalities. Using various observed/reanalysed datasets, we find that the concurrent southern Bay of Bengal (BoB) sea surface temperatures (SST) were anomalously warm. Our analysis shows that BoB sea surface temperature anomalies (SSTA) are indeed positively, and significantly, correlated with the northeastern Indian monsoonal rainfall during this season. Our sensitivity experiments carried out with the Weather Research and Forecasting (WRF) model at 25 km resolution suggest that, while the strong concurrent El Niño conditions contributed to about 21.5% of the intensity of the extreme Chennai rainfall through its signals in the local SST mentioned above, the warming trend in BoB SST also contributed equally to the extremity of the event. Further, the El Niño southern oscillation (ENSO) impacts on the intensity of the synoptic events in the BoB during the northeast monsoon are manifested largely through the local SST in the BoB as compared through its signature in the atmospheric circulations over the BoB.

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The authors acknowledge useful suggestions from the two anonymous reviewers on the earlier version of the manuscript. BA and GD acknowledge discussions with Dr. Ch. Nagaraju, UCLA, on the WRF model experiments. The WRF experiments have been carried using the MoES/IITM HPC facility, for which we are grateful to the MoES/IITM. The sources/publications for the various data used are listed in the references and supplements. The figures in this work have been generated using the COLA/GRADS software.

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Correspondence to Karumuri Ashok.

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See Figs. 8, 9, 10, 11 and 12.

Fig. 8

Geographical location of the State of Tamil Nadu of India

Fig. 9

Time series area-average of BoB SST OND season anomalies (85°E–95°E, 8°N–15°N) form the OISST dataset during the period from 1986 to 2015

Fig. 10

a Time series area-average of BoB SST OND season anomalies de-trend over a region bound by 85°E–95°E, 8°N–15°N from the HadISST dataset. bd Indicating the strong El Niño conditions during the OND season in 1982, 1997 and 2015 respectively from the top

Fig. 11

a Time series area-average of Arabian SST OND season anomalies (65°E–75°E, 8°N–15°N) form the HadISST dataset during the period from 1901 to 2015, b correlation between zonal wind and Niño3 index for the season OND (period: 1986–2015) and c correlation between TIO SST and Niño3 index for the season OND (1986–2015)

Fig. 12

Time series monthly anomalies of Nino3 index for 1982, 1997 and 2015

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Boyaj, A., Ashok, K., Ghosh, S. et al. The Chennai extreme rainfall event in 2015: The Bay of Bengal connection. Clim Dyn 50, 2867–2879 (2018).

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  • The Chennai extreme rainfall event
  • WRF model
  • Northeast monsoon
  • Bay of Bengal warming trend