Recent Change in the Connection from the Asian Monsoon to ENSO

Abstract

The Asian monsoon and El Niño?Southern Oscillation (ENSO) are known to interact with each other. In this paper, four primary indices (the Indian monsoon rainfall index, the Webster and Yang monsoon index, the tropical-wide oscillation index, and the Southern Oscillation index) that characterize the temporal variation of these complex, chaotic and quasi-oscillatory phenomena are used to assess the action from the Asian monsoon to ENSO, that is, the linkage between the strong/weak monsoon and La Niña/El Niño. The evolution of the four previously documented indices and other auxiliary data over a 43-yr period is examined using the observed database and the reanalysis of the National Centers for Environmental Prediction. The Asian monsoon and ENSO intersect in a common area, namely, the warm pool in the western tropical Pacific. This region (e.g., 10°S?5°N, 110°?170°E) is located at the longitudinally central portion of the Walker circulation and also the equatorial end of the Indo-Pacific meridional overturning cell that is part of the zonal mean Hadley circulation. In recent decades, the connection between the monsoon and ENSO has changed considerably. This change is related to the atmospheric circulation over the entire North Pacific Ocean, which entered a new regime in about 1976. Before 1976, the correlations among the four primary indices, and those between the indices and the Niño-3 index of sea surface temperature, were strong. In recent decades, the ocean temperature in the entire North Pacific became considerably colder. The lower-tropospheric winds became simultaneously more cyclonic over the North Pacific. ENSO is now related to atmospheric fluctuations both in the Indian sector and in northeastern China. The western North Pacific monsoon in the vicinity of the Philippine Islands (9°?19°N, 139°?141°E) may play an important role together with the off-equatorial ocean heat content in a larger region (5°?15°N, 135°?170°E) in maintaining or even increasing ENSO activities.