Monsoon oscillations of the Findlater Jet and coastal winds of India

Abstract

Intraseasonal variability (ISV) of the Low Level Jet (LLJ) and its effects on coastal winds during the Indian summer monsoon are examined using National Centre for Environmental Prediction / National Centre for Atmospheric Research (NCEP) reanalyses winds and India Meteorological Department (IMD) wind observations along 11 coastal stations of India. The active-phase "peninsular- path" of the LLJ and its "branching- out" during beak-phase are well reproduced by NCEP 850 mb winds. Though the footprints of LLJ are visible in NCEP surface winds in the oceanic region, over land, it is not traceable. The ISV of the LLJ is found to have significant bearing on the coastal winds to affect the forcing on coastal circulation of India. During active phase, the south-westerly wind anomalies (approx. 1.5 m/s) in the south-east coast (Chennai/Tuticorin) favor upwelling in this region. During break phase, the ISV of the LLJ favors upwelling off Goa in the west and opposes upwelling off Tuticorin/Chennai in the east coast. Though NCEP surface winds do not carry the ISV of the LLJ over land, it captures the observed variability of coastal winds. Wavelet analysis on IMD coastal winds (10 m) for the 11 stations and the co-located 850 mb NCEP winds show 10-20 day and 30-60 day modes of ISV. While the amplitude of the 30-60 day mode, associated with the variability of the LLJ is 6 m/s, for IMD coastal winds its is approx. 2 m/s