The monthly average spectrum of July 2005 at themoored buoy of National Institute of Ocean Technology at DS5 (83o 16′ 16″ E, 13o59′ 28″N; 3267 m depth) location in BOB has been obtained by averaging every threehourly spectra spanning the entiremonth.
It exhibited double-peaked spectrum withsea-dominated condition (Ep-sea >Ep-swell). Swells from distant storms in theSouthern Indian Ocean reach the Bay of Bengal and hence constituted approximately 11% (upto 0.5 Hz) of the total energy.
Co-existence of locally generated wind seas andpre-existing swells results in a diurnal pattern of wave parameters. The role ofSouthern Indian Ocean swells on the North Indian Ocean wave field has been detailed by13. In the present case, the measured spectrum is fitted with adouble-peaked(DP) JONSWAP spectrum (6 parameter) by curve fitting (MATLAB tool) using theappropriate separation frequency (0.082 Hz) and by adjusting other 5spectral parameters (Table 1 & 2) with 99.5% confidence and with RMSE of 0.
02788 and R as0.9955. This fitted DP spectrum is denoted as 1A. Torsethaugen,14 hasdeveloped a two peaked 1A type spectrum using curve fitting of experimental datafrom the North Sea in Norwegian waters by using a separation frequency. Thecomparison of measured spectrum with fitted (1A) is shown in Fig. 2.
The fitted 1Aspectrum slightly over-predicted (by 5%) the measured one. Further, swell and sea componentsof 1A (DP) are fitted 15 with separate single-peaked (SP) JONSWAP spectra (pureswell; 1B & pure sea, 1C) by defining their respective peak frequencies (fp)using curve fitting (MATLAB tool); (Table 1& 2). In this paper, twotypes of double peaked spectra 1A (sea-dominated; fitted to measured DS5location in BOB, July 2005) and 2A (swell-dominated;hypothetical spectrum withsame non-linear coupling status to that of 1A) arechosen and their Snlbehaviour iscompared.
1A and 2A are treated as wave conditions pertaining to different locationsand seasons with varying intensities. The total energies, separation.frequencies andcompositions are different for the two.
2A spectrum is obtained (from 1A) by magnifyingpeak swell energy(by 1.62 times) and curtailing sea energy (by 52%) with a nettotal energy reduction(by 34%), while the sum total of sea & swellpeak energies isincreased by 5% than that of 1A. fp-sea, fp-swell and f-ratio are kept constant to haveidentical nonlinear coupling status between the sea and swell systems for both 1A and2A; which is governed by f-ratio1. It is also associated with a reduction inseparation frequency (1A: 2A= 0.082 to 0.075 Hz). Thesestatistics apply only when the seaand swell systems are decoupled for DP spectra (fp-ratio(sw/sea)<0.
6).Here, 2Ais not an observed spectrum at the same location of 1A. The spectral energy balancebetween 1A & 2A is not resorted to.
If so, it would have changed the f-ratio and alteredthe nonlinear coupling status.