Indian summer monsoon intra-seasonal oscillation associated with the developing and decaying phase of El Niño

Abstract

The present study investigates the characteristics of active and break cycles associated with the Indian summer monsoon (ISM) during developing and decaying phase of El Nino and in the years in which Indian Ocean Dipole (IOD) co-occurred with El Nino (co-occurred years). Observations show that break days are more in number (∼2–3 times) and long lasting (∼15–20 days) than active days in El Nino developing summers and vice versa for decay years. During El Nino developing years, northward propagation is well organized with significant anomalies in both active and break phases. The increased convection associated with active phase persists longer over Indian Ocean, than over the monsoon region, while the reduced convection in break phase propagates faster from ocean to land and persists there for a longer time. Compared to break events, active events have slower (faster) propagation over the monsoon (oceanic) region during El Nino decay years. The present study put forward the argument that the contrasting persistent circulations over Indo-western Pacific regions favour particular phase of intra-seasonal oscillation (ISO) in developing and decay phase of El Nino. These long-lasting circulations advect anomalous dry (moist) air to ISM region for longer period, resulting in long-lasting break (active) events in El Nino developing (decay) years. During co-occurrence years, the number of break (active) days is reduced by two to three times compared to the developing (decaying) phase of El Nino. It is found that 30- to 60-day scale ISO is strongly modulated, than 10 - to 20-day scale, by the changes in seasonal mean state associated with El Nino. Thus, this study demonstrates that the ISO characteristics such as its variance, northward propagation, spatial distribution and duration of active and break days are strongly modulated by seasonal background anomalies over the Indo-Pacific region.