Modulation of low frequency intraseasonal oscillations of northern summer monsoon by El Nino and southern oscillation (ENSO)

In order to improve our understanding of the interannual variability of the 30-50 day oscillations of the northern summer monsoon, we have performed numerical experiments using a 5-level global spectral model (GSM). By intercomparing the GSM simulations of a control summer experiment (E1) and a warm ENSO experiment (E2) we have examined the sensitivity of the low frequency intraseasonal monsoonal modes to changes in the planetary scale component of the monsoon induced by anomalous heating in the equatorial eastern Pacific during a warm ENSO phase. It is found that the anomalous heating in the equatorial eastern Pacific induces circulation changes which correspond to weakening of the time-mean divergent planetary scale circulation in the equatorial western Pacific, weakening of the east-west Walker cell over the western Pacific ocean, weakening of the time-mean Reverse Hadley circulation (RHC) over the summer monsoon region and strengthening of the time-mean divergent circulation and the subtropical jet stream over the eastern Pacific and Atlantic oceans. These changes in the large scale basic flow induced by the anomalous heat source are found to significantly affect the propagation characteristics of the 30-50 day oscillations. It is noticed that the reduction (increase) in the intensity of the time-mean divergent circulation in the equatorial western (eastern) Pacific sectors produces weaker (stronger) low-level convergence as a result of which the amplitude of the eastward propagating 30-50 day divergent wave decreases (increases) in the western (eastern) Pacific sectors in E2. One of the striking aspects is that the eastward propagating equatorial wave arrives over the Indian longitudes more regularly in the warm ENSO experiment (E2). The GSM simulations reveal several small scale east-west cells in the longitudinal belt between 0-130 degrees E in the E1 experiment. On the other hand the intraseasonal oscillations in E2 show fewer east-west cells having longer zonal scales. The stronger suppression of small scale east-west cells in E2 probably accounts for the greater regularity of the 30-50 day oscillations over the Indian longitudes in this case. The interaction between the monsoon RHC and the equatorial 30-50 day waves leads to excitation of northward propagating modes over the Indian subcontinent in both cases. It is found that the zonal wind perturbations migrate northward at a rate of about 0.8 degrees latitude per day in E1 while they have a slightly faster propagation speed of about 1 degrees latitude per day in E2. The low frequency monsoonal modes have smaller amplitude but possess greater regularity in E2 relative to E1. As the wavelet trains of low latitude anomalies progress northward it is found that the giant meridional monsoonal circulation (RHC) undergoes well-defined intraseasonal oscillations. The amplitude of the monsoon RHC oscillations are significantly weaker in E2 as compared to E1. But what is more important is that the RHC is found to oscillate rapidly with a period of 40 days in E1 while it executes slower oscillations of 55 days period in E2. These results support the observational findings of Yasunari (1980) who showed that the cloudiness fluctuations on the 30-60 day time scale over the Indian summer monsoon region are associated with longer periods during El Nino years. The oscillations of the monsoon RHC show an enhancement of the larger scale meridional cells and also a stronger suppression of the smaller scale cells in E2 relative to E1 which seems to account for the slower fluctuations of the monsoon RHC in the warm ENSO experiment. It is also proposed that the periodic arrival of the eastward propagating equatorial wave over the Indian longitudes followed by a stronger inhibition of the smaller meridional scales happen to be the two primary mechanisms that favour steady and regular northward propagation of intraseasonal transients over the Indian subcontinent in the warm ENSO experiment (E2). This study clearly demonstrates that the presence of El Nino related summertime SST anomalies and associated convection anomalies in the tropical central and eastern Pacific are favourable criteria for the detection and prediction of low frequency monsoonal modes over India.