Estimation of internal solitary wave propagation speed in the Andaman Sea using multi–satellite images

Abstract

Different image pairs of Synthetic Aperture Radar (SAR) from tandem satellites (i.e. ERS2 and EnviSat) and sunglint optical images of Moderate Resolution Imaging Spectroradiometer (MODIS), Visible Infrared Imaging Radiometer Suite (VIIRS) and Medium Resolution Imaging Spectrometer (MERIS) between December 2002 and May 2016 are investigated and used to estimate the propagation speed of Internal Solitary Waves (ISWs) in the Andaman Sea. 426 ISWs are identified during this period and are found to be concentrated in four regions of the Andaman Sea namely, (a) southeastern region near Sumatra, (b) central eastern region, (c) around Nicobar Islands, and (d) northern region. However, to estimate the phase speed of ISWs, 11 image pairs are found suitable, which captured 24 ISWs. Two methods, (i) Multiple Images Comparison (MIC), and (ii) Tidal Time Period (TTP) are used to estimate the phase speed of ISWs. The estimated phase speeds are well agreed with theoretical results derived from Sturm–Liouville equation using monthly climatology stratification data and local bathymetry. A maximum phase speed of 3.02 m/s was estimated at 3740 m depth in the western side of Nicobar Islands and minimum speed of 0.83 m/s at 100 m in the southeastern region of the Andaman Sea. It is found that the ISW phase speed decreases as they propagate from deep to shallow water. From the analysis of monthly variation of ISW phase speed, we found higher phase speed in December and January than March in the southeastern region of the Andaman Sea, which is attributed to monthly variation of environmental parameters such as mixed layer depth (MLD), thermocline depth, etc. over the region. It is found that water depth and monthly stratification play a vital role in controlling the phase speed of the ISWs in the Andaman Sea.