Search

Argo float trajectory data has been used to estimate the velocities of mid-depth (1000 db) currents in the North Indian Ocean (NIO). The structure of the upper ocean absolute geostrophic currents can be derived more accurately from the Argo temperature and salinity profiles by referring to the estimated velocities rather than by assuming a level of "no motion". The derived flow field reveals that the eastward zonal velocities generate striation-like structures in the Arabian Sea, which is most prominent above the layer of 500 db layer, with a meridional scale of ~300 km; however, such structures are barely observed in the Bay of Bengal. This meridional scale as well as the distribution of the mid-depth striations are unique to the NIO, as compared with the other ocean basins. The nonlinear 1 1 2 -layer reduced-gravity model combined with the triad baroclinic Rossby wave interaction theory have been used to determine the essential factors controlling the characteristics of the quasi-zonal striation structure. Compared with the North Pacific Ocean, the meridional scale in the NIO is narrower, and is caused by the smaller basin size in the equatorial zone, rather than by the semiannual wind stress forcing period or the eastern boundary angle. Furthermore, the coastal trapped Kelvin waves significantly contribute to the generation of the zonal striations in the Arabian Sea. • Mid-depth striations in the North Indian Ocean have been observed by the Argo trajectory data and T/S profiles. • The striations are more prominent in the Arabian Sea while barely observed in the Bay of Bengal. • The meridional scale of those striations is 300 km, which is because of the smaller basin scale in the equatorial zone. • Coastal trapped Kelvin waves contribute significantly to the generation of the zonal striation in the Arabian Sea. [ABSTRACT FROM AUTHOR]

The Great Whirl (GW) is an anticyclonic mesoscale eddy, forming periodically in the tropical western Indian Ocean. The lifespan of the GW exhibits a marked interannual variability, which has a late generation time and short lifetime due to the abnormal local wind field in a strong positive IOD year. The anomalous northeasterly wind inputs positive wind stress curl into the ocean, which is not conducive to establishing a stable anticyclonic circulation at the early stage of GW formation. In addition, oceanic Rossby waves exert impacts on the GW generation. The abnormal Rossby waves account for the earlier generation time of the GW in 2019 despite the unfavorable local wind field. This study identifies the atypical lifespan of the GW associated with a positive IOD event and reveals the contribution of local Ekman pumping and Rossby wave propagation. • Great Whirl showed a late generation time in strong positive Indian Ocean Dipole (IOD) years. • The abnormal wind related to strong positive IOD events is key to the unstable gyre early in the Great Whirl formation. • The Great Whirl in 2019 (a prolonged IOD year) is an exception due to abnormal Rossby waves. [ABSTRACT FROM AUTHOR]