George Mason University
The South-Asian (Indian) monsoon and ENSO (El Niņo/ Southern Oscillation) are known to interact with each other. In this paper, several indices are used to assess the action from the SA-monsoon to ENSO, i.e., the sequential linkage between the strong/weak monsoon and La Niņa /El Niņo. The evolution of six indices and other auxiliary data over a 43-year period are examined, using the observed database and the reanalysis of National Centers for Environmental Prediction.
The domains of these two phenomena intersect in a common area, namely, the warm pool in the western tropical Pacific. This domain (100S-50N, 1100-1700E) is located at the western edge of the Walker circulation and also the equatorial end of the local portion of the meridional overturning or Hadley circulation. In recent decades, the connection of the monsoon-ENSO has considerably changed (for example, Kumar et al. 1999). The atmospheric circulation over the entire North Pacific entered into a new regime in about 1976.
Before 1976, the correlation coefficients among the six indices are very high, and those between the index and NINO 3 SST (sea surface temperature) are also high. In other words, the SA-monsoon is a good precursor for ENSO. In recent decades, the ocean temperature over the entire North Pacific became considerably colder. As a result, the winds at 850 hPa level, for example, became more cyclonic over the North Pacific. ENSO is now related to the Indian sector as well as the northeastern China. It appears that the Western North Pacific monsoon near Philippine Islands (90-190N, 1390-1410E) plays an important role together with the ocean heat content (50-150N, 1350-1700E).
The relationship among the SA-monsoon, the Walker circulation, and the wind system over the warm pool region has changed in the recent period.. The intensification/weakening of the Walker circulation is not synchronized with the so-called Indian-monsoon index any more, but it is well synchronized with, for example, the TOI (Tropical-wide Oscillation index) before and after 1976.