Abstract
In this paper the relationship between the two dominant modes of the atmospheric low frequency oscillation viz. ENSO (El Nino Southern Oscillation) and NAO (North Atlantic Oscillation) is examined prior to the onset of the Indian summer monsoon season (June-September) and during the monsoon period. The analysis is carried out for 107 years of data (1881-1987). From the analysis it is observed that relationship between ENSO and NAO is an inverse before the monsoon season starts and during the monsoon season the relationship between these low frequency modes is direct. The relationship again becomes the inverse after the withdrawal of the monsoon. Behaviour of these two oscillations are monitored for the contrasting monsoon years (i.e. deficient and excess monsoon years). The study reveals that these two oscillations are improper phase in deficient monsoon years but in excess monsoon years these oscillations are in proper phase. Key words: North Atlantic Oscillation, Southern Oscillation and Indian summer monsoon
Highlights
This paper examines the relationship between the North Atlantic Oscillation (NAO) and Southern Oscillation (SO), two large scale alternations in atmospheric mass discovered by Sir Gilbert Walker in the 1920s
2) The relationship between NAO and SO is negative in the beginning of the monsoon season but during the monsoon season it changes towards positive relationship and it becomes reverse at the end of the season. 3)
During the deficient years these oscillations are not in proper phase while in excess monsoon years these two oscillations are in phase
Summary
Southern Oscillation (SO) indices for 107 years. Here the five month mean is represented as the central month of the respective five months. As preceded through the succeeding months, the relationship becomes negative in June which is significant at 5%. The monthly homogeneous updated data for 108 years (18811988) of the Southern Oscillation Index (SOI) is taken from the CAC (climate analysis centre) and up dated from monthly climatic diagnostic bulletin. The five monthly moving averages are considered for the analysis using simple correlation mean of NAO and SO for same 107 years. It is seen that only in June (average of April through August) an inverse relationship is significant at 5% level. This is the key point to be noticed about these two oscillations because the interaction between NAO and SO is significant during the months just before the beginning of monsoon and during monsoon season
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