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Keywords:

  • India;
  • boreal summer monsoon;
  • local-scale onset;
  • sea surface temperature

Abstract

Variability of Indian summer monsoon local-scale onset dates is investigated at 1° spatial resolution by applying an agronomic definition (i.e. the first significant rains without a potentially crop-threatening dry spell thereafter) to gridded observed daily rainfall data (1901–2004). Median onset dates are compared well with previous estimates. The inter-quartile range varies from less than 2 weeks over the monsoon zone and Western Ghats, to about a month over the northwestern desert. A relationship between local-scale rainfall onset date and regional-scale indices of onset is demonstrated over the eastern part of the monsoon zone (i.e. approximately from Orissa and eastern Maharashtra to Western Bengal) and along the coast of the Arabian Sea; these regions are also found to exhibit significant correlations with Niño 3.4 sea surface temperature index in May. However, these relationships are found to be rather weak and geographically confined, which is attributed to low values of spatial coherence of local-scale onset date variability. A weak relationship is also found between local-scale onset date and the June–September seasonal rainfall total, while the latter is shown to be weakly more spatially coherent than local-scale onset date over the monsoon zone, and thus slightly more potentially predictable.