Spatio-Temporal Distribution and Variability of High Threshold Wind Speed and Significant Wave Height for the Indian Oce
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Pure and Applied Geophysics
Spatio-Temporal Distribution and Variability of High Threshold Wind Speed and Significant Wave Height for the Indian Ocean S. SREELAKSHMI1 and PRASAD K. BHASKARAN1 Abstract—Space-borne satellites provide extensive coverage of measuring sea surface wind field and wave heights on a global scale. The present study provides a comprehensive assessment of distribution of extreme wind and wave fields over the Indian Ocean (IO) utilizing satellite-based scatterometer and altimeter datasets. Merged scatterometer data from ERS-1/2, QuikSCAT, and ASCAT for a period of 27 years produced continuous record of wind speed, whereas the calibrated altimeter data for different satellite missions provided information on the significant wave height. A comparison study between merged scatterometer and calibrated altimeter derived long-term maximum wind speed shows an overestimation of altimeter product by an average measure of 3 m/s and higher as compared to the scatterometer winds in 92.8% of the total observational points considered. A decadal variability was observed in the inter-annual spatial distribution of the 90th and 95th percentile winds for the study region. Interestingly, the study reveals a declining trend in the dispersion of higher waves (90th, 95th, and 99th percentiles) in the entire IO except the South China Sea. Further, 2011 onwards there is an increasing trend in the annual distribution of extreme winds and waves for the Extra-tropical South Indian Ocean having implications on the generation of swell wave fields. Keywords: Variability, extreme wind and waves, space borne observations, Indian Ocean.
1. Introduction The momentum transfer at the air-sea interface is responsible for the generation of surface gravity waves. The importance and role of ocean waves are very well recognized globally in the operational aspects of marine weather forecasting, climate-modelling studies, engineering activities related to the design and operation of offshore structures, optimal
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Department of Ocean Engineering and Naval Architecture, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721 302, India. E-mail: [email protected]; [email protected]
ship routing, coastal zone management activities, etc. Location specific, regional and basin scale wave related information have immense practical value in offshore industry and marine activities. In the past, the routine visually based wave data were reported by the voluntary observing ships (Gulev et al. 1998) which is spatially inhomogeneous and only confined to major shipping routes. A study by Hogben et al. (1986) reported on the initial efforts made to compile the global wave statistics data from voluntary observing ships. Numerical ocean wave models, on the other hand, are a cost-effective method for representing the global ocean wave data. The reliability of wave model products is in turn strongly dependent on inherent model physics, parameterization schemes and quality of forcing wind fields (Komen et al. 1994). A stud
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