Diagnosing Low Frequency Hydrologic Variability in the Monsoon-ENSO teleconnection

The correlation between ENSO and Indian monsoon rainfall has recently experienced significant interdecadal variability, reaching a peak in magnitude of about -0.8 in the 1960’s and falling to near zero during the late 1990’s. Though fluctuations in the relationship have been well documented, their causes are poorly understood. Since it is known that moisture availability is key to the existence of the monsoon and aspects of its interannual variability, the hydrologic cycle is investigated for processes that may be relevant to the evolving ENSO-monsoon relationship. It is found that many of the large-scale features of the ENSO-monsoon hydrologic coupling are evident in recent decades despite weakness in the observed correlation. For example, easterly moisture transport anomalies across the Pacific due altered trade winds associated with ENSO are as strong in recent decades as in the decades when the ENSO-monsoon correlation was strong. Our findings show that many important aspects of the teleconnection’s modulation over recent decades reveal small-scale structure in the Indian Ocean region rather a planetary-scale signature involving the Pacific. For example, the reduced correlation of ENSO with rainfall over India since 1980 is not evident in other monsoon regions such as in the Bay of Bengal or the western Pacific Ocean. Moreover, anomalies in easterly moisture advection from the Pacific Ocean associated with ENSO during recent decades is not significantly different than during the 1960’s and 1970’s. Rather, a local redistribution of correlation and rainfall anomalies within the northern Indian Ocean and southeast Asia are associated with the decadal modulation of the ENSO-monsoon relationship.