Kerkar, J.P.; Seelam, J.K., and Jena, B.K., 2020. Wave height trends off central west coast of India. In: Sheela Nair, L.; Prakash, T.N.; Padmalal, D., and Kumar Seelam, J. (eds.), Oceanic and Coastal Processes of the Indian Seas. Journal of Coastal Research, Special Issue No. 89, pp. 97-104. Coconut Creek (Florida), ISSN 0749-0208.
Wave height trends off three sites along 100 m and 20 m depth contours off the central west coast of India have been estimated using 46 years hindcast-wind-wave data simulated for the Indian Ocean region. The wind waves were simulated with NCEP/NCAR winds as input to a third generation spectral wind wave model. This study showed an increasing trend for the annual mean significant wave heights from south to north ranging between 0.25 and 0.36 cm/year. The wave heights during the southwest monsoon and fair weather periods were observed to have similar trends. The study also showed increasing trends in water depths of 20 m compared to that of 100 m water depth, wherein a maximum increase of 0.22 cm/year is observed.
Understanding long-term variations in the wind and wave parameters is a critical element in studying the coastal environment. Long-term wind and wave records based on in situ measurements are still sparse in the Indian seas, and only short-term records are available (Kumar, 2006; Premkumar et al., 2000). Given the sparse measured wave data, the numerical model derived wave data gains vital importance and many studies on numerical wave prediction have been carried out for the Indian Ocean region using scatterometer winds (Muraleedharan et al., 2009; Vethamony et al., 2006). For coastal regions of India, nearshore wave parameters have been estimated by using spectral wind wave model (Aboobacker et al., 2009). There is no long-term measured wave available for the north Indian Ocean. Wind-waves are crucial for coastal activities, and the prediction of waves through numerical models has been the primary source of information in the entire ocean. Owing to the sparse availability of wave measurements, a numerical model derived wave data gains vital importance, and few studies have been carried out for the Indian Ocean region using different wind inputs. Regional studies on the changes in wave climate, especially in the North Pacific and North Atlantic regions, have been conducted. Very few studies on wave climate trend variations in the north Indian Ocean exist based on the long-term wave modelling results.
This paper presents the trends of wind waves in the central west coast of India. The north Indian Ocean is divided by the Indian subcontinent into two semi-enclosed seas viz., the Arabian Sea and the Bay of Bengal, both of these seas encroach onto the equatorial regime of the basin (Shankar and Shetye, 2001). It is observed that the Arabian Sea shows high wind speed and wave conditions due to the monsoons (Kumar and Anand, 2004; Kumar, Philip, and Nair, 2010; Young, 1999).
There have been many studies on the change in wave climate especially in North Pacific and North Atlantic regions (Allan and Komar, 2000; Caires and Swail, 2004; Carter and Draper, 1988; Gulev and Gringorieva, 2004; Soomere and Raamet, 2011; Vanem and Walker, 2013). In the Indian Ocean, positive trends in wave heights were observed in the south Indian Ocean (Bhaskaran, Gupta, and Dash, 2014). Gupta, Bhaskaran, and Das (2015) which then studied the trends in the wind wave climate in the Indian Ocean using altimeter data. Anoop et al. (2015) showed that there is an increasing trend of greater than 1 cm per year in the northern north Indian Ocean based on the 34 years of data. Hithin, Sanil Kumar, and Shanas (2015) studied the variability of annual mean and maximum SWH at a selected deep water location in the central Arabian Sea using along-track satellite altimeter data and showed a positive trend of 0.63 cm per year in the mean SWH and negative trend of 2.66 cm per year in the maximum SWH.
The study area lies along the central west coast of India. The northeastern boundary of the study area is covered by the Arabian Peninsula and the Persian Gulf. The waves along the central west coast of India depend mainly on the wind conditions prevailing over three different seasons; viz. Southwest monsoon (June-September), Northeast monsoon (October-January) and the fair weather season (February-May).
The trends in significant wave height (SWH) along the central west coast of India is the primary focus of this paper. There are few studies in this region on the long-term trends of SWH. The aim of this paper is to fill the gap using numerical wave modelling data. Wind-wave modelling has been carried out to hindcast the waves in the Indian Ocean during 1970-2015. The model which has been validated with the available in situ measurements. MIKE by DHI's third generation spectral wind wave model (MIKE-SW) is used in this paper. MIKE-SW uses flexible mesh, which allows for coarse spatial resolution in the offshore area and high resolution in the shallow coastal waters. MIKE-SW model includes two different formulations: a directional decoupled parametric formulation and a fully spectral formulation of the wave action balance equation. The first formulation is suitable only for nearshore conditions whereas the second one is applicable in both nearshore and offshore regions, and therefore, the second formulation has been used in this paper to account for both the shallow and offshore regions. MIKE-SW includes the following physical phenomena: (i) wave growth by action of wind, (ii) non-linear wave-wave interaction, (iii) dissipation due to white capping, (iv) dissipation due to bottom friction, (v) dissipation due to depth-induced wave breaking, (vi) refraction and shoaling due to depth variations (vii) wave-current interaction, (viii) effect of time-varying water depth and flooding and drying. The discretisation of the governing equations in geographical and spectral space is performed using a cell-centered finite volume method.
An unstructured mesh technique is used in the geographical domain and the time integration is carried out using a fractional step approach, where a multi-sequence-explicit method is adopted for the propagation of wave action. The model domain considered in this study covers the Indian Ocean region from 60°S-30°N and 20°E - 115°E (Figure 2). The consideration of such a large domain was necessary as the swells from the south west or south directions strongly influence the wave climate along the west coast of India. The model uses a flexible mesh wherein the grid resolution varies from 0.25° near the coast to 1° as we moved from coast to offshore. The bathymetry data was obtained from the Earth Topography and Bathymetry version 2 (ETOPO2) from the National Geophysical Data Centre USA for deep water and DHI coastal mapping data (MIKE CMAP) for the coastal region. The accuracy of the wave hindcast is dependent mainly on the accuracy of the input winds (Holthuijsen, Booji, and Bertotti, 1996). NCEP/NCAR reanalysis wind data from 1970 to 2015 were used as input to generate the waves. These data are available in the form of daily 6 hourly mean zonal (east-west), and meridional (north-south) surface wind components, with a resolution of 2.5°×2.5° (http://www.esrl.noaa/gov/psd/data/reanalysis/reanalysis.shtml).
The NCEP/NCAR reanalysis wind represents the output of a high-resolution atmospheric model, that is run using observations collected from surface and upper air stations and satellite observation platforms, which are routinely used for climate change simulations when measured wind records are not available (Mujumdar and Ghosh, 2008).
Six locations have been considered to study the annual and seasonal trends, three at 100 m water depth named as W1, W2, and W3; and three locations named as P1, P2 and P3, parallel to W1, W2 and W3 respectively, at 20 m along the coast (Figure 1). The linear trend analysis is carried out following the method as in Shanas and Kumar (2014), wherein the seasonal and annual significant wave heights at each of the location are subjected to statistical analysis to obtain maxima, mean and 90th percentile for each year or season in each year. For each of the location, 46 data points are obtained and are used further to obtain the linear trend.
RESULTS AND DISCUSSION
The MIKE-SW model results have been compared with measured data available in the Indian Ocean (Figure 3). Three locations in the Arabian Sea, viz., AD02, AD06, and AD09, wherein the measured data was available are used to compare with the MIKE-SW model results. The statistical parameters used for comparison are Bias, Root Mean Squared Error (RMSE), Scattering Index (SI) and the Correlation Coefficient. The measured data showed an excellent comparison with the modelled data. The model results at locations AD02 and AD06 showed correlation coefficients of 0.95 and 0.94 respectively with a bias of about 0.5 for both locations. However, at location AD09 the correlation coefficient was relatively less about 0.64 (Table 1).
The linear trend analysis of SWH for three different locations along the 100 m contour viz., W1, W2, and W3, along the central west coast of India, was carried out. Trends of SWH for annual data as well as seasonal data, considering three seasons, viz., (1) Fair-weather (February-May), (2) NE monsoon (October-January) and (3) Southwest monsoon (June-September) are presented.
During the fair weather season, SWH at W1 showed a boost of 0.42 and 0.55 cm/year in mean and the 90th percentile of SWH, whereas, a negligible decreasing trend of 0.05 cm/year in maximum SWH (Figure 4a). Also at W2 (Figure 5a) and at W3 (Figure 6a) the maximum SWH showed a subsiding trend of the order of 0.19 and 0.18 cm/year respectively, but the mean and the 90th percentile SWH showed an increasing trend. During the northeast monsoon, SWH at all the three locations showed a positive trend for the maximum, mean and the 90th percentile. The maxima and the 90th percentile was almost similar for W1 and W3. However, for the mean SWH, a considerable increase of 0.51 and 0.46 cm/year was seen at W1 and W3 respectively (Figures 4b and 6b), whereas, the SWH at W2 showed an increasing trend of about 0.39, 0.45 and 0.51 cm/year for the maxima, mean and the 90th percentile of SWH (Figure 5b).
During the south-west monsoon, at W1, the maximum SWH showed a negligible increase, whereas, the mean and the 90th percentile SWH showed a step up of the order of 0.2 cm/year (Figure 4c), however, decreasing trends were observed at W2 and W3 in the maximum and the 90th percentile of SWH. The mean SWH at W2 and W3 showed a negligible positive trend, whereas, the maximum SWH at W2 showed a negative trend of about 0.27 cm/year (Figure 5c). Also, W3 showed a negative trend of 0.53 cm/year in maximum SWH (Figure 6c). The 90th percentile of SWH at W2 and W3 showed a similar negative trend.
The annual trends showed a progressive trend in mean SWH with values of 0.36, 0.25 and 0.25 cm/year for W1, W2 and W3 respectively. W1 also showed a positive trend for maxima and 90th percentile of SWH (Figure 7a). However, at location W2, the maximum SWH showed a negative trend of 0.27 cm/year and a negligible increasing trend of 0.06 cm/year for 90th percentile SWH (Figure 7b). Similarly, at location W3 the maximum SWH showed a negative trend of about 0.54 cm/year and 0.03 cm/year in the 90th percentile of SWH (Figure 7c).
In addition to the SWH trends in water depths of 100 m, annual trends were studied at a shallow water depth of 20 m at locations parallel to W1, W2, and W3, which are referred to as P1, P2 and P3 respectively.
During the fair weather season, a diminishing trend in the maximum SWH was observed at P1, P2 and P3 of the order of 0.13, 0.24 and 0.37 cm /year respectively. However, the mean and the 90th percentile of SWH showed a rising trend. At P1, the mean and the 90th percentile of SWH showed a positive trend of about 0.42 and 0.55 cm/year respectively (Figure 8a). The mean SWH at P2 and P3 showed similar values whereas the 90th percentile of SWH at P2 showed a developing trend of 0.55 cm/year. Also, P3 showed an increasing trend of the order of 0.26 cm/year of SWH (Figures 9a and 10a).
During the northeast monsoon, all the locations showed a positive trend for the maxima, mean and the 90th of SWH. At P1, the maxima, mean and the 90th percentile of SWH showed an effective trend of the order of 0.4, 0.38 and 0.36 cm/year respectively (Figure 8b). Also, P2 and P3 showed almost similar values of increasing trend in the maxima, mean and the 90th percentile of SWH (Figures 9b and 9c).
During the southwest monsoon, a mixed trend was observed in the maxima, minima and the 90th percentile of SWH. At P1, a negligible increasing trend in the maximum, mean and the 90th percentile of SWH was observed (Figure 8c). However, at P2, a negative trend of about 0.27 cm/year was observed in the maximum, and the 90th percentile of SWH whereas a slightly mounting trend in the mean SWH was observed (Figure 9c). At location P3, a negative trend of about 0.49 and 0.30 cm/year was observed in the maxima and the 90th percentile of SWH (Figure 10c). The trend in the mean SWH at P3 was negligible.
The annual trends at P1 showed a sharpening trend in the maxima, mean and the 90th percentile of SWH (Figure 11a). At P2 the maximum SWH showed a negative trend of 0.27 cm/year whereas the mean, and the 90th percentile showed a constructive trend of the order of 0.19 and 0.05 cm/year respectively (Figure 11b). A negative trend was observed at P3 in the maximum and the 90th percentile of SWH, but the mean showed a positive trend of 0.14 cm/year of SWH (Figure 11c).
A few researchers tried to study the variations in wave climate through ship observations, buoy measurements, satellite altimetry, and numerical modelling. Hemer, Church, and Hunter (2010) showed that SWH revealed an increasing trend in the Indian Ocean sector. Alves (2006) showed that the swells generated in the southern ocean propagate into the tropical and extratropical latitudes of the Indian Ocean and the Pacific Ocean through numerical modelling. Hence, the annual mean SWH showed an increasing trend due to a build up in the wave intensity. The annual mean SWH in the central Arabian Sea is increased by 0-25% per year from 1985-2008 as studied by Young, Zieger, and Babanin (2011). The increase as per Young, Zieger, and Babanin (2011) is 0-0.28 cm /year. On the regional scale Hithin, Sanil Kumar, and Shanas (2015) showed a confident trend in mean SWH of 0.63 cm/year and a counter active trend of 2.66 cm/year in maximum SWH using altimeter data at a deep water location at around 3500 m depth in the central Arabian Sea. This study also showed an increasing trend in mean SWH of the order of 0.3 cm/year at a 100 m water depth.
The maximum SWH showed a negative trend of about 0.54 cm/year in the deep waters. The shallow waters at 20 m also showed similar trends. The maximum SWH showed a declining trend of 0.5 cm/year, and the mean SWH showed increasing trends with values varying from 0.14-0.22 cm/year.
This study showed a rising trend in the mean of SWH at both shallow and deep water locations. However, the maxima and the 90th percentile of SWH showed mixed trends. Fair-weather season showed contrary trends in maximum SWH but a positive trend for mean and the 90th percentile of SWH. However, the southwest monsoon showed mixed trends in the maximum and the 90th percentile of SWH but a positive trend in the mean SWH. During the nort-east monsoon, the variations in the trends are insignificant. Similar results have been reported by Gupta, Bhaskaran, and Das (2015), wherein, a seasonally positive trends in mean SWH were observed along a transect in the deep waters of the Arabian Sea.
Based on the study carried out, it has been observed that there is an upsurge in annual maximum SWH trends at 100 m water depth as we go from south to north. A similar trend is observed in the shallow waters. Also, the wave heights during the southwest monsoon and fair weather season showed an increasing trend as we move from south to north. During the northeast monsoon, there is not much variation in the maximum, mean and the 90th percentile of SWH trends. There is no linear relationship between the trends of shallow and deep water. SWH trends are mostly less than 1 cm/year for the central west coast of India.
Authors thank their respective Institutes for the support. Dept. of Science and Technology (DST), Govt. of India is acknowledged for supporting the research work of the first author through Women scientist scheme (WOS-A). We also thank Director, CSIR-National Institute of Oceanography, Goa for encouraging for carrying out the study. The buoy data used for validation was provided by the Indian National Centre for Ocean Information Services (INCOIS), Ministry of Earth Sciences, India. NIO contribution number is 6398.