Co-existence of wind seas and swells along the west coast of India during non-monsoon season

摘要

An attempt has been made to understand the co-existence of wind seas andswells along the west coast of India during non-monsoon season. Wave datawere collected in different years during non-monsoon season (off Goa duringMay 2005, off Ratnagiri during January–February 2008 and off Dwarka duringDecember 2007–January 2008), which is fairly a calm weather season alongthese regions. Diurnal variation in wave parameters is noticeable along thecentral west coast of India (off Goa and Ratnagiri), which is due to theinteraction of multidirectional waves (both wind seas and swells) of varyingmagnitudes and frequencies. Swells are predominantly mature (91%) andold (88%) during late pre-monsoon and post-monsoon seasons,respectively. Sea Swell Energy Ratio quantifies wind sea, swell and mixedseas prevailing in these regions during non-monsoon season. Intermodaldistance (ID) between the energy peaks is moderately separated duringnon-monsoon season, whereas, during the shamal events, energy peaks are veryclose to each other (ID ∼ 0). However, pure wind seas (ID ∼ 1)are weakly present and found to co-exist with the swells almost all the timeduring non-monsoon season. Wind sea growth has been found while the swellpropagates opposite to the direction of the wind and wind sea. Wind seas haveminimum angular spreads in multimodal state. Under low winds, the interactionbetween wind sea and swell dominates and thereby the multimodal state reducesto unimodal state. The fetch available for the evolution of the wind seaspectrum has been estimated, and it is found to be less than 150 km. For thefetch limited condition, a non-dimensional empirical relation has beenderived relating the significant wind sea height in terms of wind speed andpeak wind sea period, and this relation fits for the west coast of India.