Abstract
Spectra from two decades of zonal current data at $$\sim $$ 4000 m in the central and western equatorial Indian Ocean show a shift in the dominant frequencies from the west to the east. The 120–180-day period is stronger at 77 $$^\circ $$ E, the 60–120-day period at 83 $$^\circ $$ E, and the 30–90-day period at 93 $$^\circ $$ E. The weakening of lower frequencies near the eastern boundary can be explained using theoretical ray paths of Kelvin waves and reflected Rossby waves. The equatorial Kelvin wave forced by winds reflects from the eastern boundary as Rossby waves with different meridional modes. After reflection, the low (high) frequency Rossby beams travel a larger (shorter) distance before reaching the bottom, thereby creating a shadow zone, a region with low wave energy, between the ray path and the eastern boundary. The shift in frequency with longitude is not evident in the top 1000 m, where the current is dominated by the semi-annual cycle.
Highlights
Observations from the equatorial Indian Ocean show energetic current fluctuations in the deeper waters (Luyten and Swallow 1976; Eriksen 1980; O’Neill and Luyten1984; Ponte and Luyten 1990; Dengler and Quadfasel 2002)
The Kelvin beam reflects from the eastern boundary as a Rossby beam, which has a steeper ray path that can reach the bottom of the ocean at seasonal time scales
The surface circulation in the equatorial Indian Ocean is dominated by the semiannual cycle
Summary
Observations from the equatorial Indian Ocean show energetic current fluctuations in the deeper waters Luyten and Roemmich (1982) showed that the mid-depth zonal currents in the western equatorial Indian ocean were dominated by the semi-annual cycle. They suggested that these fluctuations were caused by equatorially trapped Kelvin waves and first meridional mode Rossby waves. — the 120–180-day period is stronger at 77◦ E , the 60–120-day period at 83◦ E, and the 30–90-day period at 93◦ E — and this shift in frequency can be explained using theoretical ray paths of Kelvin and Rossby waves The signatures of these waves propagating downwards as equatorial beams are evident in the current observations from the near-surface and intermediate depths. We compare the theoretical ray paths with observed spectra and highlight the role of beams in causing a shift in dominant frequencies along the longitude (Section 4.2)
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