Blue-shifted deep ocean currents in the equatorial Indian Ocean

Abstract

Spectra from two decades of zonal current data at ∼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 • E , the 60-120-day period at 83 • E, and the 30-90-day period at 93 • 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.

Energetic Bottom Current at the Equatorial Gap of the Ninety East Ridge in the Indian Ocean Based on Mooring Data

“…Thus, besides Kelvin wave revealed by Amol et al. (2022), the deep ISV in this region can also be partly attributed to the westward propagating Rossby wave.…”

“…Previously, Amol et al (2022) pointed out that equatorial Kelvin wave could propagate eastward and downward to get to the bottom near this region and may contribute to the deep ISV in this region. As shown in Figure 8b, the eastward propagating signal of Kelvin wave is consistent with results of Amol et al (2022). On the other hand, besides the eastward propagating signals of Kelvin wave, notable westward propagating signals of Rossby wave were also detected with a phase speed of ∼10.7 cm s −1 (Figure 8b), and these westward propagating signals coincided with the enhancement of the ISV of the bottom current at NR (Figures 6a and 6b).…”

“…Hovmöller diagram of the MO zonal velocity corresponding to the depth of the mooring at NR along 0.1°N shows that eastward and westward propagating signals coexist in this region, both of which probably contribute to the ISV of the bottom current at NR (Figure 8b). Previously, Amol et al (2022) pointed out that equatorial Kelvin wave could propagate eastward and downward to get to the bottom near this region and may contribute to the deep ISV in this region. As shown in Figure 8b, the eastward propagating signal of Kelvin wave is consistent with results of Amol et al (2022).…”

“…Previously, Amol et al. (2022) pointed out that equatorial Kelvin wave could propagate eastward and downward to get to the bottom near this region and may contribute to the deep ISV in this region. As shown in Figure 8b, the eastward propagating signal of Kelvin wave is consistent with results of Amol et al.…”

“…Further, to identify the potential origin of the ISV of the bottom current at NR, we first compared the geostrophic current of the surface ocean to the bottom current at NR to evaluate the correlation between the motions of the upper ocean and the bottom layer; and we found no obvious correlation (∼0.01) between them, indicating that the variability of the bottom current at NR could not be directly induced by local mesoscale or large‐scale motions in the upper ocean. In fact, previous studies revealed the ubiquity of ISV in the upper ocean of the equatorial Indian Ocean, and wind‐forced oscillations in the upper ocean of the equatorial Indian Ocean, that is, remote wind‐forced Yanai wave (Miyama et al., 2006; Nagura & McPhaden, 2012; Ogata et al., 2008; Sengupta et al., 2004), the equatorial Kelvin waves or reflected Rossby waves (Amol et al., 2022), which could propagate vertically into the subsurface ocean; and some of them could penetrate the pycnocline and leak into the deep layer (Kessler & McCreary, 1993; McCreary, 1984), causing the ISV in the deep layer (Amol et al., 2022; David et al., 2011; Greatbatch et al., 2018; Jain et al., 2021; Ménesguen et al., 2019; Murty et al., 2006; Rothstein et al., 1985). In light of these previous studies, we speculate that the ISV of the bottom current at NR be associated with some remote forcing in the equatorial Indian Ocean.…”