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The basin-scale wave propagation in the Southern Indian Ocean (SIO) is inferred to affect the low-frequency variability of eddy kinetic energy (EKE) within the Agulhas current system (ACS). Observations and numerical simulations indicate that westward-propagating waves, in the form of sea level variability from the east boundary, may cause "non-linear" oceanic responses (e.g., sea level and EKE) in the ACS. This is likely due to two factors: (i) the movement of wave crests and troughs continuously alters the stratification of the upper ocean, generating a positive baroclinic instability and prompting conversions of EKE from mean flow in the source regions, and (ii) wave-current interactions induce a rapid intension of an anticyclone gyre in the interior SIO and the corresponding EKE in the downstream regions. These basin-scale wave modulations may provide an additional perspective to the traditional wind driving view based on the barotropic Sverdrup relation. The local winds over the SIO play an important role in maintaining basin-scale Rossby waves in the southwest Indian Ocean and further modulating oceanic low-frequency variability in a "quasi-linear" way. • The basin-scale wave propagation in the Southern Indian Ocean (SIO) have an influence on the dynamics of the Agulhas current system. (ACS). • The waves can alter the stratification of the upper ocean, as well as strengthen the anticyclone gyre in the interior SIO and the corresponding EKE in the downstream regions. • These basin-scale wave modulations may provide an additional perspective to the traditional wind driving view based on the barotropic Sverdrup relation. [ABSTRACT FROM AUTHOR]

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 circulation structure in the western tropical Indian Ocean (WTIO) was investigated based on surface drifters and Simple Ocean Data Assimilation (SODA), with particular attention to the characteristics of the circulation structure associated with seasonal monsoon transitions. Surface drifter trajectories manifest three distinct seasonally dependent patterns of western boundary currents in the WTIO. The northward Somali Current (SC) flows across the equator to ∼10 °N in the boreal summer (June to August; JJA). The northward SC turns offshore near the equator during monsoon transitions. The southward SC feeds the South Equatorial Countercurrent together with the northward East African Coast Current in the boreal winter (December to February; DJF). While during the monsoon transition (MAM and SON), drifters prefer to turn eastward and feed the strong equatorial currents. Based on satellite observations, the results of a balanced diagnostic model demonstrate that the geostrophic component U ζ and the wind-driven component U τ predominate the mean state and the seasonal variations of circulation in the WTIO. The vertical motion shows that upwelling primarily occurs along the coasts of Somali and Oman and is induced by offshore Ekman transport. Ekman transport and associated vertical motions constitute meridional overturning circulation. The SODA results confirm the existence of an equatorial roll in the boreal summer and winter. The results elucidate the dynamic relationships between 3-D circulation and monsoon transitions in the WTIO. • Drifter trajectories describe three seasonally dependent circulation patterns in the western tropical Indian Ocean. • The geostrophic component U ζ and wind-driven component U τ predominate the mean state and the soasonal variability of circulation. • The 3-D structures, show seasonal variability associated with monsoon transitions. [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]