Why it happened

The monsoon story

Print edition : December 25, 2015

Figure 1. A schematic showing "normal" and "inverted" troughs. The north-east monsoon is characterised by inverted troughs.

Figure 2. Acummulated rainfall between November 28 and December 4 over Chennai and neighbourhood measured by NASA's GPM satellites. A large quantum of rain fell over the sea.

Figure 3. An infrared cloud image taken by the Indian Space Research Organisation's Kalpana-1 satellite on December 5. The rain systems have moved southwards towards Sri Lanka, causing heavy rains there.

Figure 4. A plot of the daily rainfall over Tamil Nadu and Puducherry between October 1 and December 4. The cumulative rainfall is also mentioned.

Figure 5. The sea surface temperature anomaly in the Nino 3.4 region; it touched 3 °C in November.

Figure 6. The lower topospheric sustained easterlies emanating from the Pacific El Nino region and stretching well across the Bay of Bengal and breeding storms over the bay.

Figure 7. The trajectory of the deep depression that formed over the Bay of Bengal on November 8, which traversed inland over the next two days and weakened first into a depression and then into a “pressure low” bringing heavy rain over interior Tamil Nadu and Karnataka.

Figure 8. Warm sea temperatures over the Bay of Bengal, which went up to 30 °C.

Figure 9. A panel of frame grabs from an animation video of rain-bearing systems moving across from the Pacific to the Bay of Bengal and over coastal south India between November 28 and December 4. Photo: NASA/TRMM

Figure 10. A graph showing the trend of increasing number of intense rainfall events (above 10 cm and 15 cm) over the past six decades.

This year, the effect of the north-east monsoon has been worsened by an intense El Nino and a quick succession of rain-causing weather systems.
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