Short Communication

Spatial and temporal variation of surface waves in shallow watersalong the eastern Arabian Sea

T.R. Anoop, V. Sanil Kumar n, P.R. ShanasOcean Engineering, CSIR-National Institute of Oceanography, (Council of Scientific and Industrial Research), Dona Paula 403 004, Goa, India

a r t i c l e i n f o

Article history:Received 17 August 2013Accepted 17 February 2014Available online 6 March 2014

Keywords:Significant wave heightWave periodWave directionWave spectral energyWest coast of India

a b s t r a c t

We studied the spatial and temporal variation of surface waves along the eastern Arabian Sea during2011 and 2012. Measured directional wave data at two shallow water locations and re-analysis datasets(ERA-Interim) at 0.751 intervals at four locations were used for the study. The study region covers 270 kmalong the west coast of India and lies between Karwar and Ratnagiri. The temporal variations of waveparameters were less than 10% at both locations for temporal intervals up to 12 h except during themonsoon/storm period. During the storm period, the variation in significant wave height within 3 h wasaround 13% and was up to 26% inside 12 h. Comparatively low (o10%) spatial variation was found forwave height during the monsoon season and higher variation (420%) was observed during the non-monsoon season. The pattern of spatial variation of wave parameters was similar during both the yearsfor the measured and re-analysis datasets. The study shows that during the monsoon period, the wavecharacteristics were similar for the 270 km long stretch since the waves along this part were predomi-nantly (�72%) swells.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Engineers involved in the planning and design of marinestructures require information on waves at a specific location.To derive the wave climate at a location or region, time series wavedata covering many years are necessary. Wave data at a specificlocation is obtained through (i) wave measurements, (ii) numericalmodels, or (iii) satellites (Cavaleri and Sclavo, 2006). The wave dataobtained from numerical models and satellites need to be verifiedby the measured data. But due to the high expense towards theinstallation and maintenance of wave measuring instruments,in-situ measurements at many locations remain a challenge eventoday. Also, carrying out wave measurements in shallow waterlocations over a long-term basis is a difficult task due to theintense fishing activity in the near-shore area. Entanglement of thefishing vessels/nets with the moored buoy results in damage to thebuoy and the moorings. Similarly, the coastal domain represents achallenging target for exploiting satellite information, whereaccuracy is degraded due to a number of factors including issuesof land contamination in the altimeter and radiometer footprints(Andersen and Scharroo, 2011). In the absence of measured data

for a specific location, numerical models are calibrated with thedata available for the nearest location; hence it is important toknow the spatial variation of wave parameters. The re-analysisdatasets and the satellite data have limitations in temporal andspatial resolutions. Hence, it is necessary to understand thevariation of each parameter with respect to time and location.

The present study is focused on the eastern Arabian Sea, mainlyfrom Karwar to Ratnagiri (Fig. 1). The waters off the west coast ofIndia are exposed to seasonally reversing winds, with winds fromthe south-west (SW) during the SW/summer monsoon (hereaftermonsoon) period (June to September) and from the north-east(NE) during the post-monsoon period (October to January). Theperiod between the NE and SW monsoon is the pre-monsoonperiod or the fair weather (FW) period. The seasonal changes inwinds produce similar changes in the surface waves and hence it isimportant to know the variations in wave parameters seasonallyand inter-annually. Kumar et al. (2012) reported that the seasonalaverage wave height and period did not vary significantly betweenthree stations covering 200 km along the Karnataka coast, India.Glejin et al. (2012) compared the wave parameters at threelocations along the eastern Arabian Sea from June to August2010 covering the monsoon period and found that the waveheight increased from the south to the north. Since the study ofKumar et al. (2012) covered only the data for one month repre-senting each of the three seasons and that by Glejin et al. (2012)covered only the monsoon period, we have conducted a fresh

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Ocean Engineering

http://dx.doi.org/10.1016/j.oceaneng.2014.02.0100029-8018 & 2014 Elsevier Ltd. All rights reserved.

n Corresponding author. Tel: þ91 832 2450 327;fax: þ91 832 2450 602.E-mail address: sanil@nio.org (V. Sanil Kumar).URL: https://www.nio.org (V. Sanil Kumar).

Ocean Engineering 81 (2014) 150–157

study covering a larger spatial interval with continuous datacovering a 2 year period. The objective of the study was tounderstand the spatial and temporal variation in surface waveparameters between two locations spaced at 270 km along theeastern Arabian Sea. The variations in surface waves in the near-shore waters were studied with the data obtained from simulta-neous measurements carried out at two locations using mooreddirectional wave rider buoys and those for the offshore waterswere done with the ERA-Interim dataset.

The locations selected for the study were (i) off Karwar (south-ern location) at 15 m water depth (geographic position 14.82171N,74.05241E) and (ii) off Ratnagiri (northern location) at 13 m waterdepth (geographic position 16.98011N; 73.25841E). The distance ofthe locations from the west coast of the Indian mainland is 5 km atKarwar and 2 km at Ratnagiri. The distance between Karwar andRatnagiri is around 270 km and these locations are exposed todeep-water swell waves from the south Indian Ocean. Tides in thestudy region are mixed and are predominantly semi-diurnal andthe average tidal range at Karwar is 1.58 m during spring tide and0.72 m during neap tide. At Ratnagiri, the average spring tidalrange is about 1.8 m and the neap tidal range is 0.9 m (Glejin et al.,2013).

2. Data and methodology

Waves were measured using the Datawell directional waverider buoy (Barstow and Kollstad, 1991) for a period of 2 yearsfrom 1 January 2011 to 31 December 2012. Measurements weremade in Coordinated Universal Time (UTC) and the time referredin the paper is UTC. The data were recorded continuously at1.28 Hz and the data for every 30 min were processed as onerecord. The collected time series was subjected to standard errorchecks for spikes, steepness and constant signals (Haver, 1980) anda total 17,418 records measured simultaneously at both locationsduring 2011 and 17,416 records during 2012 were used for furtheranalysis. Zero-crossing analysis of the surface elevation time serieswas used to estimate maximum wave height (Hmax) and the waveperiod corresponding to maximum wave height (THmax). FastFourier Transform values of eight series, each consisting of 256measured vertical elevations of the buoy data, were added toobtain the wave spectrum. The high frequency cutoff is set at0.58 Hz and the resolution is 0.005 Hz. Significant wave height (Hs)which equals 4

ffiffiffiffiffiffiffimo

pand the mean wave period (Tz) which equals

2πffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffim0=m2

pwere obtained from the spectral moment. Wheremn is

the nth order spectral moment and is given by mn ¼R10 f nSðf Þdf ,

Annexure: Notations used

Bm directional spreadDp wave direction corresponding to spectral peakDps peak direction of swellDpw peak direction of wind-seaEmax maximum spectral energy densityECMWF European Centre for Medium-range Weather ForecastERA ECMWF Re-AnalysisHmax maximum wave height

Hs significant wave heightHss significant height of swellHsw significant height of wind-seaPD percentage differenceSwell (%) Swell percentageTHmax maximum wave periodTp Peak wave periodTz mean wave periodTzs mean period of swellTzw mean period of wind-sea

Fig. 1. Study locations in the eastern Arabian Sea. Red marks show the ERA-Interim locations. (For interpretation of the references to color in this figure caption, the reader isreferred to the web version of this paper.)

T.R. Anoop et al. / Ocean Engineering 81 (2014) 150–157 151

S(f) is the spectral energy density at frequency f. Peak wave period(Tp) was estimated at the spectral peak. Mean wave direction (Dp)and directional width (BM) at spectral peak were estimated as perKuik et al. (1988). Wind-sea and swell from the measured datawere separated using the method described by Portilla et al.(2009). The monthly mean of each parameter based on differenttemporal intervals (½, 1, 3, 6, 12 and 24 h) was estimated. Data athalf an hour intervals was used as reference data, and percentagedifference (PD) of each monthly mean data for different intervalsfrom its half hour interval was calculated as

PD¼((X�Y)/Y) 100, where X is the data at 1, 3, 6, 12 and 24 hinterval and Y is the data at ½ h interval.

For studying the spatial variation of wave parameters, the dataat the southern location (Karwar) was taken as the reference dataand the PD for the northern location (Ratnagiri) was calculated.Since the wave climate in the eastern AS shows seasonal varia-bility, while studying the spatial variation of wave parametersbetween Karwar and Ratnagiri, we used weekly average datainstead of the monthly/seasonal average.

The re-analysis wave datasets (ERA-Interim) used in this studywere from the ECMWF (European Centre for Medium-rangeWeatherForecast) re-analysis models (see http://www.ecmwf.int/research/era/do/get/index). ERA-Interim is the most recent re-analysis pro-vided by the ECMWF (Simmons et al., 2007). ERA-Interim is the firstre-analysis using adaptive and fully automated bias corrections ofsatellite radiance observations (Dee and Uppala, 2008). Significantwave height (Hs) data is available at 0.751 � 0.751 grid points andthe data for the years 2011 and 2012 for four locations (14.251N,73.51E; 151N, 72.751E; 15.751N, 72.751E; and 16.51N, 72.751E) cover-ing Karwar to Ratnagiri were used in the study. These datasets have atemporal interval of 6 h. The southern location (14.251N, 73.51E) wastaken as the reference for studying the spatial variation of Hs.

3. Results and discussions

3.1. Temporal variation

The temporal variation of monthly average wave parameters(Hs, Hmax, Tz, Tp and Dp) shows that the PD of all parameters exceptTz was less than 10% (Figs. 2 and 3) for the temporal interval up to

24 h. For the temporal interval up to 12 h, the PD for all parametersexcept Tz was in the range of 0–3% during most of the time.At Ratnagiri, Tp shows a higher PD value (10–15%) during January2011 and November 2012 and except these months, the PD valueof Tp was less than 10% similar to other wave parameters. Thehigher PD value of Tp during these months was due to thepresence of swells coming from the northwest (NW) and SWdirections (Fig. 4a–d). The NW swells were more at Ratnagiri thanat Karwar and during the monsoon season the NW swells wereabsent at both the locations (Fig. 4e to 4f). Compared to otherparameters, the temporal variation of Dp was very small (o4%) forboth locations. The variation of PD of Hs at both locations duringboth years depends on the seasons since the wave climate ofthe eastern Arabian Sea is influenced by the monsoonal windsduring the SW monsoon resulting in high wave activity (Kumar,2006; Kumar et al., 2010; Glejin et al., 2013). The PD of Hs wascomparatively low during the SW monsoon period when thewave heights were high and the PD was high during the pre-monsoon period when the influence of sea breeze was high. Thesea breeze has a high impact along the west cost of India duringthe pre-monsoon season (Neetu et al., 2006; Glejin et al., 2013)and the horizontal extent of the sea/land breeze circulation overthe Arabian Sea was observed up to 80–100 km (Subrahamanyamet al., 2001). In the present case the measurement locations werewithin 5 km from the coast and hence the sea/land breezeinfluenced the wave parameters and resulted in comparativelyhigher PD of Hs during the pre-monsoon season. In the post-monsoon season, the impact of land/sea breeze was comparativelylower than that observed in the pre-monsoon season (Glejin et al.,2013). The influence of land/sea breeze on the waves was negli-gible during the monsoon seasons since the land/sea breezesystem was weak during the monsoon system and was mainlyactive during the break-in the monsoon (Mandal and Halder,1992). The study shows that the influence of sea/land breezewas higher on the temporal variation of wave parameters.

The present study indicates that the variation in monthlyaverage value of Hmax and Hs for data based on different recordingintervals was negligible (o10%). Glejin et al. (2012) found thatduring the monsoon season there was no change in the averagevalue of Hs and Hmax for the data based on different recordingintervals of 3, 6, 12 and 24 h.

Fig. 2. Percentage difference of monthly average wave parameters with different temporal intervals at Karwar during 2011 and 2012.

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The monthly maximum value of Hs for different temporalintervals is presented in Table 1. The variation in monthly max-imum value of Hs was less than 6% for the temporal interval up to3 h except during the storm period in November 2011 (deepdepression in the eastern Arabian Sea from 26 November to1 December 2011; IMD, 2012) at Karwar and the monsoon periodat Ratnagiri. During the storm/monsoon period, the variation in Hs

was around 15% within 3 h and 26% for 12 h. The variation inmaximum Hs for the temporal interval of 24 h was up to 40% inOctober at Ratnagiri (Table 1). The study indicates that data at atemporal interval of more than 6 h will miss the monthly max-imum Hs values during the SW monsoon and the storm period.

The revisiting rate of polar orbiting satellites is 1 or more days;hence these satellites do not adequately resolve the short-termevent and process characteristics of the coastal environment(Brown et al., 2005). To determine the chance of missing an Hs

event between the revisiting intervals of these satellites here wecompared the maximum value of Hs during the half, 24 and 48 htemporal intervals (Fig. 5). The maximum PD between the 24 hand 48 h interval datasets was up to 40%. During the monsoonseason, the maximum PD for the 48 h and 24 h interval data waswithin the range of 16–26%. Also, during the storm event atKarwar, a high PD (�26%) was observed. Compared to the post-monsoon season, the pre-monsoon season shows a high PD in Hs

due to the effect of the land/sea breeze system. From Fig. 5, we cansee that during the monsoon season, the missing of higher waveheight was more than that during the pre- and post-monsoonseasons. The average of the difference between maximum Hs forthe half and 12 h intervals was 0.17 m and that between half and24 h was 0.25 m. The maximum difference in Hs for the ½ h and24 h interval data was 0.8 m and that for 48 h was 1.4 m. From thiswe can see that the measurement of wave parameters in onelocation using one satellite with a temporal interval of 2 days isinsufficient. Multiple observations are needed for better observa-tion of wave parameters, if not it may cause us to miss high andshort-term wave events generated by oceanographic/meteorolo-gical factors. The correction factor was estimated to minimize theerror between the data with different temporal intervals. Thecorrection factor for Karwar was 19% and 23.5% for the 24 h and48 h data and that for Ratnagiri was 14% and 29% respectively.Goda (1988) observed that wave data with recording intervals of6 or 12 h tend to miss the peaks of some storm events and can leadto underestimation of severe wave conditions. In order to com-pensate for this effect, Allen and Callaghan (1999) used a simpleblock adjustment factor of 7% for the 6 h recording interval dataand 10% for the 12 h recording intervals for each peak Hs value.

Yearly maximum PD value for other wave parameters is shownin Table 2. The PD variation of these parameters also depends upon

Fig. 3. Percentage difference of monthly average wave parameters with different temporal intervals at Ratnagiri during 2011 and 2012.

Fig. 4. Time series plot of peak wave period and mean wave direction during(a) January 2011 at Ratnagiri (b) January 2011 at Karwar, (c) November 2012 atRatnagiri, (d) November 2012 at Karwar, (e) August 2012 at Ratnagiri and (f) June2011 at Karwar.

T.R. Anoop et al. / Ocean Engineering 81 (2014) 150–157 153

the seasons. The variation of directional width with time washigher (10–12%) at Ratnagiri and lower (5–8%) at Karwar. Theparameters Emax, Swell (%), Hsw, THmax, and Tzw generally show ahigher PD (4 10%) than other parameters. The PD of swellpercentage shows a comparatively low value during the monsoonseason and that of Emax shows this trend in both the monsoon and

post-monsoon seasons since the waves were swell dominated inthe monsoon and post-monsoon seasons (Kumar et al., 2012; Sajivet al., 2012).

3.2. Spatial variation

Spatial variation of wave parameters was studied using mea-sured data and the significant wave height (Hs) from ERA-Interim.A comparison based on buoy data (Fig. 6) indicates that the spatialvariation of most of the parameters during the pre-monsoonseason was comparatively higher than that observed during thepost-monsoon season. Variation of Hs observed betweenthe locations was equal to or slightly greater than 20% duringthe pre- and post-monsoon seasons, whereas it was less than 10%during the monsoon season. A similar trend was observed in thecase of Hmax as well. The decreasing of PD of all wave parametersduring the monsoon season was due to similar wave conditions inthe eastern Arabian Sea due to the influence of the SW monsoon.A large variation in the wave climate was unlikely along theeastern Arabian Sea since a substantial part of the wave energyalong this was due to the swells (Kumar et al., 2000; Glejin et al.,2012; Sajiv et al., 2012; Glejin et al., 2013). However, as the wavespropagate from deep to shallow water they undergo transforma-tion through their interaction with the seabed and the wavecharacteristics will vary with change in water depth.

The PD of Emax was higher among all other parameters duringthe post- and pre-monsoon seasons since single-peaked spectrawere large (63%) in the southern location resulting in high Emaxvalues compared to the northern location (46%) because of varia-tion in local winds (Kumar et al., 2014). Also the maximumspectral energy density during the monsoon was much higher(up to 35 m2/Hz) than that (o5 m2/Hz) during the remainingperiods (Kumar et al., 2012). The PD of THmax has a higher value inthe pre-monsoon season than that in the post-monsoon season.For Dp, a higher PD was observed in April and May for both yearssince during these months the influence of the land/sea breezesystem was active and at Ratnagiri the NW waves were observed

Table 1Monthly maximum values of significant wave height at different temporal intervals.

Location Interval (h) Month

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Karwar (2011) ½ 0.9 1.3 1.2 1.1 1.4 3.2 4.1 3.2 3.6 1.2 2.3 0.91 0.9 1.3 1.2 1.1 1.4 3.1 4.0 3.2 3.6 1.2 2.2 0.83 0.9 1.2 1.2 1.1 1.4 3.1 4.0 3.2 3.4 1.2 2.0 0.86 0.9 1.2 1.2 1.0 1.4 3.1 3.8 3.0 3.4 1.2 2.0 0.8

12 0.9 1.2 1.2 1.0 1.3 3.0 3.6 3.0 3.4 1.0 1.7 0.824 0.8 1.2 1.2 1.0 1.3 3.0 3.3 3.0 3.4 1.0 1.7 0.7

Ratnagiri (2011) ½ 1.0 1.8 1.2 1.2 1.6 3.2 3.9 3.1 4.2 0.9 1.2 0.91 1.0 1.7 1.2 1.2 1.6 3.2 3.8 3.0 4.2 0.9 1.2 0.93 1.0 1.5 1.1 1.1 1.5 3.1 3.8 2.9 3.6 0.9 1.2 0.86 1.0 1.5 1.1 1.1 1.5 3.1 3.8 2.9 3.6 0.9 1.2 0.8

12 0.9 1.5 1.1 1.1 1.4 3.1 3.4 2.9 3.6 0.9 1.2 0.824 0.8 1.5 1.1 1.1 1.4 3.0 3.4 2.9 3.2 0.9 1.2 0.8

Karwar (2012) ½ 1.2 1.6 1.3 1.5 1.3 3.3 3.6 3.0 2.7 1.3 0.8 0.91 1.2 1.6 1.3 1.5 1.3 3.3 3.6 3.0 2.7 1.3 0.8 0.93 1.2 1.5 1.3 1.5 1.3 3.3 3.6 3.0 2.7 1.3 0.8 0.96 1.1 1.5 1.3 1.4 1.3 3.3 3.3 2.8 2.5 1.3 0.8 0.9

12 1.0 1.5 1.3 1.4 1.3 3.0 3.3 2.8 2.5 1.3 0.8 0.924 1.0 1.5 1.1 1.4 1.2 3.0 3.3 2.5 2.5 1.2 0.7 0.8

Ratnagiri (2012) ½ 1.5 2.1 1.3 1.6 1.3 3.4 3.5 2.8 2.5 1.5 0.9 0.91 1.5 2.0 1.3 1.6 1.3 3.4 3.4 2.8 2.5 1.5 0.9 0.93 1.5 2.0 1.3 1.5 1.3 2.9 3.4 2.8 2.5 1.4 0.9 0.96 1.4 1.8 1.2 1.4 1.3 2.9 3.4 2.8 2.5 1.4 0.9 0.9

12 1.4 1.6 1.2 1.4 1.3 2.8 3.3 2.8 2.2 1.4 0.9 0.924 1.4 1.6 1.2 1.4 1.3 2.7 3.3 2.8 2.2 0.9 0.9 0.9

Fig. 5. Maximum value of significant wave height observed at temporal intervals ofhalf, 24 and 48 h.

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whereas at Honnavar (80 km south of Karwar), swells from the SWwere predominant (Kumar et al., 2014).

The presence of a strong seasonal variation in the wave heightsin the eastern Arabian Sea is a well-known feature due to thesimilar variation in the wind speed (Kumar et al., 2012; Glejinet al., 2013). The monthly mean Hs varies at Karwar from about0.56 m during December to about 2.30 m in July (Fig. 7). Thevariation of monthly mean Hs at the northern location, 0.54 m inDecember to 2.36 m in July, almost follows with that of thesouthern location. In the post-monsoon and pre-monsoon seasons,the value of Hs was lower at Karwar than that observed atRatnagiri. The seasonal variation in Hmax was also similar to thatof Hs at both locations.

In the case of Tp, there was an abnormally higher PD observedin the pre-monsoon season. Kumar et al. (2014) observed that thedouble-peaked spectra were mostly swell dominated in the south-ern location of the eastern Arabian Sea and at the northernlocation during December to May the spectra were wind-seadominated due to the strong local winds blowing from northwest.

Hence, a higher PD was observed for Tp in the pre-monsoon season(Fig. 6). Also, due to the variation in the swell/sea domination atthe locations studied, a large variation was found in the monthlyaverage value of Tp at both locations during December to May(Fig. 7). At Ratnagiri, the percentage of swells was 26, 71 and 40during the pre-monsoon, monsoon and post-monsoon periodsrespectively. At Karwar, the percentage of swells was 41, 73 and 53during the pre-monsoon, monsoon and post-monsoon periodsrespectively. The variation in swell percentage between the twolocations was high during the pre- and post-monsoon periods(Figs. 6 and 7) due to the larger presence of swells at Karwar thanat Ratnagiri. During the monsoon period, the average value of Tp(�11 s) and the percentage of swells were the same at bothlocations.

It was found that along the study area, the PD of Hs based onthe ERA-Interim dataset increased with an increase in latitude(Fig. 8). For the location at 0.751 from the reference location, thePD was less than 10% for all seasons. In the case of the location at1.51 away from the reference location, the PD was less than 20% for

Table 2Maximum value of percentage difference of monthly average wave parameters during temporal intervals from half to 24 h at Karwar and Ratnagiri in 2011 and 2012.

Parameter 2011 2012

Karwar Ratnagiri Karwar Ratnagiri

Significant wave height of swell (Hss) 8.2 6.9 3.3 8.3Significant wave height of wind sea (Hsw) 10.9 13.2 8.2 10.6Wave period associated with maximum wave height (THmax) 10.9 9.9 8.9 10.0Mean wave period of swell (Tzs) 6.3 5.7 1.9 7.4Mean wave period of wind sea (Tzw) 11.1 13.9 8.9 10.4Swell direction (Dps) 1.4 2.8 1.6 3.8Wind sea direction (Dpw) 11.6 4.0 6.5 2.3Directional width (BM) 5.7 10.7 8.4 12.6Percentage of swell 9.2 15.0 7.5 11.8Maximum spectral energy density (Emax) 11.9 17.2 11.9 17.0

Fig. 6. Spatial variation of weekly average wave parameters at Ratnagiri with respect to Karwar in 2011 and 2012 based on measured data. Week 5–21 is pre-monsoon, 22–40 is monsoon and 41–52 and 1–4 is post-monsoon period.

T.R. Anoop et al. / Ocean Engineering 81 (2014) 150–157 155

most of the time during the post- and pre-monsoon seasons. Forthis location during the monsoon season, the PD was less than10%. The PD was greater than 20% during the pre-monsoon andpost-monsoon seasons at the location 2.251 away from the

reference location and was less than 20% during the monsoonseason.

4. Conclusion

The study results indicate that all parameters which describethe wave characteristics are highly depend upon the seasons. Thevariation in monthly average values of Hmax and Hs for data basedon different recording intervals was less than 10%. In October, thevariation in maximum Hs for the temporal interval of 24 hcompared to the ½ h interval was up to 40%. In the case of there-analysis data, the local influence on significant wave height wasnegligible compared to that in the case of the buoy data. Hence,the re-analysis data show the latitudinal variation of wave para-meters more clearly. For the study locations, high Hs and waveenergy were observed during the monsoon season. Most of thewave parameters show low spatial and temporal variation duringthe monsoon season and the same trend was observed in the caseof the buoy data and the re-analysis data. During the non-monsoon period, a higher difference in swell percentage wasobserved between the locations. The spatial variation of waveparameters between Karwar and Ratnagiri, on the west coast ofIndia during the monsoon season, was very low (o10% for Hs andHmax).

Acknowledgments

The authors acknowledge the financial support given by theEarth System Science Organization (ESSO)-Indian National Centrefor Ocean Information Services (INCOIS), Ministry of EarthSciences, Government of India, to conduct this research. Dr. S.W.A.Naqvi, Director, CSIR-NIO, Goa, provided encouragement to carryout the study. We thank Dr. T.M. Balakrishnan Nair, Head, OSISG,and Mr. Arun Nherakkol, Scientist, INCOIS, Hyderabad, and Mr. JaiSingh, Technical Assistant, CSIR-NIO, for help during the datacollection. Logistics during the data collection were provided bythe Centre for Coastal and Marine Biodiversity (Dr. BabasahebAmbedkar Marathwada University) at Ratnagiri and the PostGraduate Department of Marine Biology (Karnatak University) atKarwar. This is NIO contribution No. 5533.

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