Wave data collected off Ratnagiri, Goa and Dwarka along the west coast of India during winter season (NE monsoon and early pre‐monsoon) present distinct wave characteristics with periodicity ranging between 2 and 5 days associated with shamal events. The notable wave characteristics during these events are: an increase in wave height, decrease in swell period and a common propagation direction (northwest) for wind sea and swell. IFREMER/CERSAT blended winds clearly show the presence of strong northwesterly winds in the Arabian Peninsula and northwestern Arabian Sea, which are associated with the winter shamal events. The winds during such events generate large northwesterly swells (shamal swells) in the northwestern Arabian Sea and propagate towards the west coast of India in the NW direction with mean periods ranging between 6 and 8 s. Numerical simulations reproduce the shamal swells over the Arabian Sea, and they can be traced all along the west coast of India, however, with lesser order of magnitude from north to south. Generation and propagation of shamal swells and their influence along the west coast of India have been described.

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An analysis of altimeter significant wave height data of May 2007 revealed the occurrence of an extreme weather event off southern tip of South Africa in the Atlantic Ocean, and generation of a series of very high swells at 40° S. These swells propagated towards northeast and broke over La Réunion island in the Indian Ocean on 12 May 2007. The wave model WAVEWATCH III was used to study the propagation of these swells in the Indian Ocean. The model was validated for the Indian Ocean using moored buoy data at 12 locations and merged altimeter wave data. The wave model accurately reproduced the event of May 2007. Swell heights, of the order of 15.0 m, at the generation area reduced to 6.0 m near La Réunion island. This study shows that the swells generated in the Roaring Forties of the Atlantic Ocean (between 15° to 80° E longitude) propagate in the NE/NNE direction towards the north Indian Ocean, and wave characteristics of the Arabian Sea are least influenced compared to that of Bay of Bengal, when swells from the Atlantic Ocean enter the Indian Ocean. The double peak spectrum extracted for the Bay of Bengal indicates that one of the peaks is due to swells generated off southern tip of South Africa

Abstract. An attempt has been made to understand the coexistence of wind seas and swells along the west coast of India during non-monsoon season. Wave data were collected in different years during non-monsoon season (off Goa during May 2005, off Ratnagiri during January-February 2008 and off Dwarka during December 2007-January 2008), which is fairly a calm weather season along these regions. Diurnal variation in wave parameters is noticeable along the central west coast of India (off Goa and Ratnagiri), which is due to the interaction of multidirectional waves (both wind seas and swells) of varying magnitudes and frequencies. Swells are predominantly mature (91 %) and old (88 %) during late pre-monsoon and post-monsoon seasons, respectively. Sea Swell Energy Ratio quantifies wind sea, swell and mixed seas prevailing in these regions during non-monsoon season. Intermodal distance (ID) between the energy peaks is moderately separated during non-monsoon season, whereas, during the shamal events, energy peaks are very close 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 time during non-monsoon season. Wind sea growth has been found while the swell propagates opposite to the direction of the wind and wind sea. Wind seas have minimum angular spreads in multimodal state. Under low winds, the interaction between wind sea and swell dominates and thereby the multimodal state reduces to unimodal state. The fetch available for the evolution of the wind sea spectrum has been estimated, and it is found to be less than 150 km. For the fetch limited condition, a non-dimensional empirical relation has been derived relating the significant wind sea height in terms of wind speed and peak wind sea period, and this relation fits for the west coast of India.