Monsoon Events that Trigger Deadly Heat Waves May be Predictable

Six years ago a brutal heat wave prior to the summer monsoon took hundreds of lives across the Indian subcontinent. Temperatures averaged in excess of 100 degrees Fahrenheit throughout much of central and northern India, marking the second consecutive year that high temperatures took lives across the region.

In a new study, researchers at Scripps Institution of Oceanography at the University of California, San Diego, and the Naval Research Laboratory show that a traceable pattern of circumstances is responsible for such events. They argue that the sequence leading up to it is in fact predictable, opening the door to better preparation for similar heat episodes in the future.

In the November issue of the Journal of Climate, Maria Flatau, Piotr Flatau, and Daniel Rudnick published a study that analyzes the climatology of monsoon onsets related to heat waves in India. Their research revealed that in certain years the region’s annual monsoon season is preceded by a deceptive "bogus onset."

These bogus onsets, the study found, depend on the timing of an intraseasonal oscillation in the Indian Ocean and its link to the western Pacific Ocean. Certain sea surface temperature developments in the Bay of Bengal and western Pacific also play a role in this phenomenon.

While the authors say intraseasonal oscillations propagate randomly, they believe that once the sequence leading to the "double monsoon onset" is in place, they can predict hot and dry conditions in India before the real monsoon hits.

"This information can be used as a tool," said Maria Flatau, first author of the study. "You won’t be able to see this coming six months in advance, but once you have certain events in motion you can see it coming. This is an agricultural region so timing is important."

The group looked at 32 years of climatological data and identified six cases of the double monsoon onset.

Their analysis began in 1994-1995, when Rudnick deployed instruments analyzing a variety of ocean properties in the Arabian Sea. His data on the physical and oceanographic aspects of the region were cross-examined with an eye for atmospheric and climatological science by Maria and Piotr Flatau.

They began to closely analyze the events and discovered that the double monsoon onset develops when a strong convection in the Bay of Bengal is accompanied by a monsoon-like circulation. This appears in the Indian Ocean in approximately early May, roughly three weeks earlier than the date of the true onset in June.

They found that the initial bogus onset is followed by clear skies and dry conditions over the monsoon region. As in 1995, the monsoon-like conditions that appear in mid-May disappear by the end of the month and are followed by the deadly heat wave that delays the onset of the real monsoon.

"Monsoons affect millions of people," said Piotr Flatau. "This research may help people differentiate between a real, sustained monsoon and a bogus one."

Maria Flatau was a physical oceanography project scientist working on atmospheric science and oceanography topics at Scripps Institution. She is now with the Naval Research Laboratory in Monterey, Calif. Piotr J. Flatau is an atmospheric scientist at Scripps Institution currently visiting the Naval Research Laboratory. Dan Rudnick is a professor in the Physical Oceanography Research Division at Scripps Institution.

The research was funded by a grant from the National Science Foundation Climate Dynamics Program; with additional funding from the Office of Naval Research.