Spectral wave climatology off Ratnagiri, northeast Arabian Sea

Wave spectra of the measured data at 13 m water depth off Ratnagiri using the Datawell Directional waverider buoy from March 2010 to November 2014 are used to study the daily, monthly, seasonal and annual variations in the wave spectral characteristics. Wave data are also analyzed to identify them as single-peaked or multi-peaked spectra. Single-peaked spectra are then identified as belonging to wind-seas or swells. The monthly averaged wave spectrum indicates that the wave spectra are mainly single-peaked during June to August with peak frequency distributed around 0.08 Hz (peak wave period ~12.5 s) and double-peaked during the remaining period, except in September. During January to May and from October to December, the swell peak is around 0.07 Hz. The wind-sea peak during the non-monsoon period is mainly at 0.2 Hz except during March and April. During March and April, due to the strong local winds and weak swells, the wind-sea peak shifted to higher frequencies (~0.3 Hz). The monthly averaged spectrum is wind-sea dominated during January to March and swell dominated during rest of the year. The inter-annual variations in the monthly average wave spectrum are high during February, May and September to November. The inter-annual variations in the direction of swell are marginal (<10°), whereas significant variations in wind-sea are observed during June to November. Parameters of the JONSWAP and Donelan model spectra are estimated for the corresponding sea-state by using a least square error method.