Authors: Sachiko Mohanty, D. Mitra, and R. P. Singh

Satellite altimetry sea surface height (SSH) data has been used to study the ocean circulation, mesoscale eddies, and internal tidal waves with a scale more than 100 km. However, sub-mesoscale ocean dynamics (~spatial scale less than 100 km) was unable to resolve using the conventional satellite data.  Surface Water and Ocean Topography (SWOT) satellite, launched on 22 December 2022 focused on the better understanding of the sub-mesoscale dynamics over the World Ocean and terrestrial surface water. SWOT is being jointly developed by NASA and CNES with contributions from the CSA and United Kingdom Space Agency. The radar system of SWOT named as Ka‐band radar interferometer (KaRIn) (Peral et al., 2024) which measures high-resolution SSH using synthetic aperture radar interferometric techniques, across two wide-swath of 120 km with a 20 km nadir gap. Jason-class Altimeter collects data in the gap between the KaRIn swaths. SWOT designed to capture the ocean mesoscale and sub-mesoscale features with spatial resolution of 15-km and greater (Fu et al., 2023). The Data Unification and Altimeter Combination System (DUACS) is the multi-mission altimeter product of SSH developed by CNES/CLS (Pujol et al., 2016). The DUACS L4 daily data with spatial resolution of 0.125^o is taken from the Copernicus Marine Service. In this study, the capability of SWOT in capturing the internal waves (IWs) and sub-mesoscale eddies over the Andaman Sea have been highlighted.

Figure 1: Spatial distribution of (a) DUACS SSH (b) SWOT SSH (c) Zoomed-in over the box marked in fig. (b) showing the IWs field on 28 February 2025

IWs are generated by the interaction of tide with bottom topography in a stratified ocean. They affect the long-term global processes such as the earth’s climate and abyssal circulation (Garrett, 2003) by transferring energy from tides and winds to mixing. IWs affect the acoustic transmission in the ocean by disturbing the density field which is important for submarine operations. Large-amplitude IWs also play an important role to support the biological activity, including locally enhanced growth of phytoplankton and distribution of fish larvae in the shelf slope region. IWs having very high amplitude of about 80 m was observed over the Andaman Sea (Perry and Schimke, 1965) and are mainly generated in the north of Sumatra coast, in the Sombrero channel, south of the Car Nicobar Island and north of the Andaman Island (Mohanty et al., 2018).

Eddies are ubiquitously present over various parts of the world’s oceans. They are mainly characterized by swirling motions with a horizontal scale of 10-100 kilometers and a temporal scale of a few days to months (Chelton et al., 2011). Sub-mesoscale eddies in the ocean play a significant role in maintaining the global ocean energy balance and thus significantly govern the weather and climate at the regional as well as global scales (Su et al., 2018). SWOT satellite products confirm the characteristics of sub-mesoscale eddies and waves and their impacts on ocean dynamics, the marine ecosystem, weather and Earth’s climate system (Archer et al., 2025).

Figure 2: Spatial distribution of SSH overlaid geostrophic current from (a) DUACS (b) SWOT on 31 March 2025. SWOT data is plotted over the box marked in fig. (a)

In the present study, the SSH is plotted over the Andaman Sea from DUACS and SWOT data for on February 2025 as shown in Figure 1. The IWS are clearly seen in SWOT SSH product over the north Sumatra region (Figure 1c). Similarly, the IWs signature is also clearly visible over the Andaman Sea in SWOT on 31 March 2025. SSH overlaid with geostrophic current is plotted using DUACS and SWOT data (Figure 2). Cyclonic and anticyclonic sub-mesoscale feature can be seen in SWOT data which is not visible in multi-mission DUACS product.

References:

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