“Shamal” swells in the Arabian Sea and their influence along the west coast of India

Abstract

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.

Validation of Operational WAVEWATCH III Wave Model Against Satellite Altimetry Data Over South West Indian Ocean Off-Coast of Tanzania

“…Kurian and Baba (1987) showed the importance of shelf slope in controlling spatial contrasts in bottom frictional attenuation and consequently the coastal energy regime. Wave heights along the WCI are generally low during NE and pre-monsoon seasons, and higher during SW monsoon (Aboobacker et al, 2011;Vethamony et al, 2009;Kumar and Kumar, 2008). Waves along the WCI are generally multi-peaked (Kumar et al, 2003), which is due to co-existence of swell and wind sea (Vethamony et al, 2009).…”

“…Waves along the west coast of India (WCI) are dominated by swells during southwest (SW) and northeast (NE) monsoon seasons and by wind seas during pre-monsoon season (Vethamony et al, 2011;Kumar et al, 2000;Rao and Baba, 1996). Kurian and Baba (1987) showed the importance of shelf slope in controlling spatial contrasts in bottom frictional attenuation and consequently the coastal energy regime.…”

Observational Evidence of Summer Shamal Swells along the West Coast of India*

“…The waves along the west coast of the Indian subcontinent primarily depend on the wind conditions prevailing over the three different seasons: southwest (SW) monsoon (June-September), northeast (NE) monsoon (OctoberJanuary) and pre-monsoon (February-May). The general wave conditions in the Arabian Sea during the pre-monsoon period also depend on the swells coming from the far northwest (NW) Arabian Sea because of the northwesterly blowing Shamal winds (Aboobacker et al 2011). According to Aboobacker et al (2011), NW waves are observed along the west coast of India with mean periods ranging between 6 and 8 seconds.…”

“…The general wave conditions in the Arabian Sea during the pre-monsoon period also depend on the swells coming from the far northwest (NW) Arabian Sea because of the northwesterly blowing Shamal winds (Aboobacker et al 2011). According to Aboobacker et al (2011), NW waves are observed along the west coast of India with mean periods ranging between 6 and 8 seconds. These waves are due to the strong northwesterly winds blowing in the Arabian peninsula and the northwestern Arabian sea, and result in increase in wave height, decrease in swell period and common NW direction during the NE monsoon and early pre-monsoon season.…”

“…Rao et al (2001) reported that 51% of Shamal days occurred from May to July compared to the winter months of November to March. Aboobacker et al (2011) used the February data collected from the west coast of India to identify the winter Shamal events occurring at the Arabian Peninsula from November to March and the corresponding swells reaching the Indian west coast. The present study focuses on identifying the presence of summer Shamal winds and swells along the west coast of India.…”