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Keywords:

  • extratropical tropospheric temperature pattern;
  • Indian Summer Monsoon Rainfall;
  • standing wave

ABSTRACT

In this article, role of extratropical circulation features in modulating Indian Summer Monsoon Rainfall (ISMR) is examined. Principal component (PC) analysis of extratropical tropospheric temperature data of the monsoon season for 1951–1980 and 1981–2010 years was performed separately. Analysis revealed that a PC (PC 3) of the first period and two PCs (PC 2 and PC 6) of the second period had significant correlation with the ISMR. Composite geopotential height anomaly of middle and upper troposphere, for the dates when extratropical tropospheric temperature anomaly pattern was similar to that of Eigen Vector of the PC 3 of the first period and PC 6 of the second period, highlighted presence of standing waves in the extratropics. These waves had trough over northwestern/northern parts of the Indian subcontinent and adjoining extratropics and ridge eastwards. Such circulation anomaly pattern of the extratropics apparently reduced the north-south tropospheric temperature gradient required for good monsoon flow. This led to weak monsoon currents, resulting subdued ISMR activity during the period. This pathway of interaction of the extratropical circulation anomalies and the ISMR was reported by earlier studies also. Composite geopotential height anomaly of the middle and upper troposphere, for the dates when extratropical tropospheric temperature anomaly pattern was similar to that of Eigen Vector of the PC 2 (explaining larger variance of the temperature data) of the second period also depicted presence of standing waves. These waves were of relatively large amplitude and wavelength and were located at more northern latitudes. The configurations had warm high/ridge with centre over higher latitudes to the north of India and cold low/trough eastwards. Further analysis highlighted a new pathway of modulation of the ISMR by the extratropical circulation anomalies with above features.