Observations of long-period waves in the nearshore waters of central west coast of India during the fall inter-monsoon period

The waves measured in the eastern Arabian Sea during the fall inter-monsoon period (October) are examined at 4 locations during 2010-2015. Multi-modal features have been observed in measured wave spectra due to independent wave systems corresponding to long-period swells generated by remote storms, swells generated by nearby storms and locally generated wind waves. In October, the long-period waves (peak wave period>18 s) appeared 5-7 times in different years and the significant wave height of these long-period waves are less than 1.4 m. The spatial distribution of peak wave period based on the numerical wave model shows that the presence of long-period swells along the 350 km stretch of the eastern Arabian Sea is almost in the same day. It is observed that the high waves (significant wave height >4 m) generated in an area bounded by 40-60°S and 20-40°E in the south Indian Ocean reached the measurement locations in 5-6 days and resulted in the long-period waves. Relatively high percentage of long-period waves are observed in 2011 and 2015 compared to other years due to the reduction in short-period waves at the study location due to the influence of Indian Ocean Dipole.