  The forecast of Indian monsoon droughts has been predicated on the notion of
a season-long rainfall deficit linked to warm anomalies in the equatorial
Pacific. Here, we show that in fact nearly half of all droughts over the past
century were sub-seasonal, and characterized by an abrupt decline in
late-season rainfall. Furthermore, the potential driver of this class of
droughts is a coherent cold anomaly in the North Atlantic Ocean. The vorticity
forcing associated with this oceanic marker extends through the depth of the
troposphere, and results in a wavetrain which curves towards the equator and
extends to East-Asia. This upper-level response triggers an anomalous low-level
anticyclonic circulation late in the season over India. This teleconnection
from the midlatitudes offers an avenue for improved predictability of monsoon
droughts.
