The leading mode of Indian Summer Monsoon precipitation variability during the last millennium

The "internally" generated intraseasonal variability of the Indian Summer Monsoon is characterized by intermittent periods of enhanced ("active") and deficient ("break") precipitation, which produce a quasi east-west precipitation dipole over the Indian subcontinent. Here we present multicentennial-length and near annually-resolved reconstructions of monsoon precipitation, inferred from absolute-dated and instrumentally calibrated speleothem oxygen isotope records from regions (central and northeast India) that have diametric responses to active-break monsoon circulation patterns. On centennial timescales (AD 1400-2008), precipitation variability from these two regions exhibit opposing behavior, oscillating between periods with a persistently "active-dominated" (AD ∼1700 to 2007) and a "break-dominated" (AD 1400 to ∼1700) regime. The switch between these regimes occurs abruptly (within decades) at a time (AD ∼ 1650-1700) when a proxy record of upwelling intensity from the Arabian Sea suggest an abrupt increase in the monsoon winds. On the basis of these observations, we hypothesize that the frequency distribution of active-break periods varies on centennial timescales, implying a leading role of internal dynamics in governing the ISM response to slowly-evolving changes in the external boundary conditions.