Puthenveettil Narayana Vinayachandran

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Extreme rainfall events (EREs) over India are strongly influenced by the Boreal Summer Intraseasonal Oscillation (BSISO), yet how this relationshiphas evolved in recent decades remains poorly understood. Using observational datasets and reanalysis from the past four decades, we examine the changes in BSISO characteristics in the recent past and their role in modulating EREs over the Indian monsoon region. We find a marked strengthening of BSISO-associated rainfall over central India (15N–25N), along with a spatially coherent increase in rainfall accumulation from EREs as well as in seasonal mean monsoon rainfall.Our results suggest that these trends mainly stem from an increase in the number of active BSISO days. Increased BSISO activity creates a more favourable environment, which supports the occurrence and persistence of extreme rainfall. A dynamic-thermodynamic decomposition of the BSISO precipitation shows that the dynamic component, associated with the BSISO circulation, dominates the changes in precipitation. However, increased vertical velocity is limited to areas with increased background moisture, indicating a strong connection between dynamic forcing and thermodynamic conditions.In summary, our findings highlight a linkage between BSISO variability and extreme rainfall in India over recent decades. The mechanisms we identified provide a physical framework for understanding observed changes in monsoon rainfall and offer insights into how intraseasonal variability might impact future monsoon extremes in the warming climate.

Rainfall plays an important role in shaping the dynamics of the ocean surface layer. However, the impact of prolonged rainfall, lasting a few days, on the Bay of Bengal's surface layer remains poorly understood. This study investigates the formation of a rain plume (low‐salinity plume) associated with a rainfall event that occurred from 22 to 31 July 2015, using a very high‐resolution (1 km) ROMS model simulation. The rain plume exhibits distinct impacts across its lifecycle, modifying upper‐ocean salinity, temperature, stratification, and the mixed layer (ML), as well as intensifying submesoscale activity. The rainfall event was associated with a cyclonic storm KOMEN. Strong wind‐driven surface currents pushed the plume southwestward from the northern bay. Afterward, offshore movement of the plume was dominated by geostrophic currents. By August 2, it reached maximum intensity, forming a mushroom‐like shape approximately 140 km in length, 37 km in width, and a depth of 20 m. Within the rain plume, the salinity was fresher by 4 psu, the temperature cooler by C, and the ML vanished. The temporal evolution of the ML within the rain plume led to three distinct phases: deepening, steady, and shallowing. Our findings also highlight enhanced submesoscale activity along the edges of the rain plume. The life cycle of the rain plume lasted 13 days from formation, intensification, to dissipation. These findings enhance the understanding of upper‐ocean responses to freshwater plumes and highlight the role of high‐resolution models in capturing them, with implications to intense weather systems and marine ecosystem.

Mode water acts as a barrier layer controlling surface-to-interior fluxes of key climatic properties. In the Arabian Sea, mode water stores heat and provides an oxygen-rich layer for rapid remineralization, and its subduction is a direct pathway for oxygen into the upper oxygen minimum zone. We use float observations to characterize the properties of the Arabian Sea mode water layer (MWL). The MWL forms when springtime warming stratifies the surface layer and caps the deep surface mixed layer formed during the winter monsoon. During the summer monsoon, a second MWL is formed south of 20° N following the cessation of wind-driven mixing. We use 1D and 3D models to disentangle the contributions of atmospheric and oceanic forcing to this water mass. The 1D model accurately represents the mode water's formation and erosion, showing that atmospheric forcing is the first-order driver, in agreement with observations. However, there are regions where advective processes, eddy mixing, or biological heating are essential for the formation and/or erosion of the MWL. For instance, in the eastern Arabian Sea, freshwater-driven stratification advected via the West Indian Coastal Current reduces the potential for deep mixed layers via convective mixing, resulting in a thinner MWL. The 3D model shows that the MW contributes 5 ± 1 % to the oxygen content of the upper ocean, with its maximum during spring in the northern Arabian Sea (40 ± 17 %), thus highlighting the key role of the water mass in storing and transporting heat and oxygen to the interior.

Antarctic-origin abyssal waters occupy a significant volume and play a critical role in ocean circulation and ventilation. The abyssal water volume undergoes a major reduction in the recent warming climate. Despite the Indian Ocean (IO) housing a significant fraction of abyssal waters, it remains understudied, with no direct measurements of abyssal upwelling. Previous observation campaigns have mapped the pathways of these waters, however, substantial uncertainties remain due to limited spatiotemporal resolutions. Here, we investigate the abyssal water transports using an eddy-resolving regional ocean model and observational datasets. Our analysis reveals a net northward abyssal transport of 12.5 Sv along 32oS. We identified regional “hot-spots” of abyssal upwelling, primarily concentrated along mid-ocean ridges and continental boundaries, collectively contributing about 10 Sv of vertical transport. These findings provide new insights into the lower limb of the IO circulation by quantifying the abyssal upwelling and its spatial structure for the first time.

The East India Coastal Current (EICC) flows poleward along the east coast of India during spring and equatorward during winter. In the summer, it flows equatorward in the northern bay and poleward in the south, converging in the central part of the coast. Data from three pairs of HF radars located at three stations, Odisha (ORCO), Andhra Pradesh (APCO), and Tamil Nadu (TNCO), along the east coast of India are used in this study to present the characteristics and dynamics of eddies for a decade, from 2010 to 2020. The seasonal reversal of the EICC occurs through a gentle transition in the north, marked by strong offshore flows in the middle part of the coast, and eddies of different signs in the south. Eddy formation peaks in winter and most frequent off TNCO, followed by APCO and ORCO. Cyclonic eddies have longer lifespans and larger sizes in winter, while anticyclonic eddies show similar characteristics in spring. Most eddies have a radius between 10–30 km and life of 5 days. Seasonal mean Eddy Kinetic Energy (EKE) is higher in spring and winter, when EICC is well organized. Barotropic (BT) energy conversion takes place for all three stations for all seasons. Baroclinic (BC) energy transfer occurs in winter for all three stations and additionally off ORCO during summer. Spatial patterns of energy conversion occur in both mean‐to‐eddy and eddy‐to‐mean directions; they are spatially non‐uniform and show marked differences between coastal and offshore regions.

Mode water acts as a barrier layer controlling surface-to-interior fluxes of key climatic properties. In the Arabian Sea, mode water stores heat and provides an oxygen-rich layer for rapid remineralization, and its subduction is a direct pathway for oxygen into the upper Oxygen Minimum Zone. We use float observations to characterize the properties of the Arabian Sea mode water layer (MWL). The MWL forms when springtime warming stratifies the surface layer and caps the deep surface mixed layer formed during the winter monsoon. During the summer monsoon, a second MWL is formed south of 20° N following the cessation of wind-driven mixing. We use a 1D and 3D model to disentangle the contributions from atmospheric and oceanic forcing on this water mass. The 1D model accurately represents the mode water's formation and erosion, showing that atmospheric forcing is the first-order driver, in agreement with observations. However, there are regions where advective processes, eddy mixing or biological heating are essential for the formation and/or erosion of the MWL. For instance, in the eastern Arabian Sea, freshwater-driven stratification advected via the West Indian Coastal Current reduces the potential for deep mixed layers via convective mixing, resulting in a thinner MWL. The 3D model shows that the MW contributes 4±1 % to the oxygen content of the upper ocean, with its maximum during spring in the northern Arabian Sea (30±6 %), thus highlighting the key role of the water mass in storing and transporting heat and oxygen to the interior.

Mode water acts as a barrier layer controlling surface-to-interior fluxes of key climatic properties. In the Arabian Sea, mode water stores heat and provides an oxygen-rich layer for rapid remineralization, and its subduction is a direct pathway for oxygen into the upper Oxygen Minimum Zone. We use float observations to characterize the properties of the Arabian Sea mode water layer (MWL). The MWL forms when springtime warming stratifies the surface layer and caps the deep surface mixed layer formed during the winter monsoon. During the summer monsoon, a second MWL is formed south of 20° N following the cessation of wind-driven mixing. We use a 1D and 3D model to disentangle the contributions from atmospheric and oceanic forcing on this water mass. The 1D model accurately represents the mode water's formation and erosion, showing that atmospheric forcing is the first-order driver, in agreement with observations. However, there are regions where advective processes, eddy mixing or biological heating are essential for the formation and/or erosion of the MWL. For instance, in the eastern Arabian Sea, freshwater-driven stratification advected via the West Indian Coastal Current reduces the potential for deep mixed layers via convective mixing, resulting in a thinner MWL. The 3D model shows that the MW contributes 4±1 % to the oxygen content of the upper ocean, with its maximum during spring in the northern Arabian Sea (30±6 %), thus highlighting the key role of the water mass in storing and transporting heat and oxygen to the interior.

Mode water acts as a barrier layer controlling surface-to-interior fluxes of key climatic properties. In the Arabian Sea, mode water stores heat and provides an oxygen-rich layer for rapid remineralization, and its subduction is a direct pathway for oxygen into the upper Oxygen Minimum Zone. We use float observations to characterize the properties of the Arabian Sea mode water layer (MWL). The MWL forms when springtime warming stratifies the surface layer and caps the deep surface mixed layer formed during the winter monsoon. During the summer monsoon, a second MWL is formed south of 20° N following the cessation of wind-driven mixing. We use a 1D and 3D model to disentangle the contributions from atmospheric and oceanic forcing on this water mass. The 1D model accurately represents the mode water's formation and erosion, showing that atmospheric forcing is the first-order driver, in agreement with observations. However, there are regions where advective processes, eddy mixing or biological heating are essential for the formation and/or erosion of the MWL. For instance, in the eastern Arabian Sea, freshwater-driven stratification advected via the West Indian Coastal Current reduces the potential for deep mixed layers via convective mixing, resulting in a thinner MWL. The 3D model shows that the MW contributes 4±1 % to the oxygen content of the upper ocean, with its maximum during spring in the northern Arabian Sea (30±6 %), thus highlighting the key role of the water mass in storing and transporting heat and oxygen to the interior.