Assessing the Inter-annual variability of energy contained by wind waves in the tropical Indian Ocean

The study investigates the inter-annual variability of energy contained by wind waves over the tropical Indian Ocean using Empirical Orthogonal Function (EOF) analysis. Emphasizing the first two leading modes of variability, the regions with significant changes in wind wave power over the last four decades are identified utilizing reanalysis data sets spanning between 1979 to 2023. In the first two EOF modes, accounting for 36.29% and 14.56% of the total variance respectively, the variability exhibited is highest in the Tropical Southern Indian Ocean region.

The leading mode of variability (EOF 1) exhibits multiple distinct lobes of high variability, including the southern tropical Indian Ocean region (10°S–30°S, 70°E–100°E), the southwest Arabian Sea, the southeastern tip of the Indian Ocean, and the southeastern equatorial Indian Ocean, which shows contrasting trends. Notably, these regions of high variability align precisely with the zones of extreme values in the annual climatology of energy flux input into surface waves over the tropical Indian Ocean computed for the same study period. Although wind speed is often used as a general proxy to explain and reason wave power variability, the parameter ‘energy flux input into surface waves’ demonstrates the closest and precise resemblance to zones of spatial variability of wave power in the study region, as it directly measures the energy transfer from wind to waves, accounting for critical factors such as air-sea coupling, wave age, and sea state. Considering this, the study also examines the met-ocean parameters that influence the energy flux input into surface waves. The climatology and long-term variability of parameters such as wind stress, wave steepness, and wave age in the study region were analysed. Additionally, the relative contribution of each parameter to wave power variability in the region was assessed.

In EOF Mode 2, the entire study region, excluding the Arabian Sea and the Bay of Bengal, exhibits a clear contrasting pattern between the eastern and western sides, with a prominent dipole pattern observed in the tropical southern Indian Ocean, spanning 10°S to 25°S and 55°E to 110°E.

This study offers insights into the long term variabilities in the energy contained by wind waves and to identify and analyze the met-ocean drivers influencing these variations and to assess their contribution.

Figure: Spatial Distribution of Inter-Annual variability in power of wind waves based on

(a) EOF1 and (b) EOF 2

                                            (a)

                                        (b)