Using a three-dimensional coupled physical-biological model, this paper explores the creation of phytoplankton blooms around tropical islands in the presence of ambient currents and short-lived (~4 days) wind events. The ambient flow creates a retention zone of weak flows in the lee of the island, which is a typical feature of island wakes. Findings reveal that wind-induced upwelling effects are essential for the initial nutrient enrichment and phytoplankton growth that occur mainly in this retention zone. Oscillating flow, typical of island wakes, occasionally releases mesoscale patches of upwelled water and its phytoplankton load into the ambient ocean. The phytoplankton continues to grow within floating structures that are of up to 20 km in diameter. This mechanism complements the plankton growth associated with the formation of mesoscale eddies.

Using a reduced-gravity model, this study explores the ocean’s response to wind forcing in the South-East Tropical Indian Ocean (SETIO) during the development phase (July-September) of positive phases of the Indian Ocean Dipole (IOD), using the years of 2009 and 2019 for comparison. Findings show that IOD variability is exclusively controlled by zonal equatorial wind anomalies. In the average situation, represented by the 2009 season, westerly equatorial winds create downwelling Kelvin waves that operate to suppress the Sumatran upwelling. In contrast, positive phases of the IOD, represented by the 2019 season, are characterized by easterly equatorial winds that reverse the equatorial influence, which leads to strong coastal upwelling off Sumatra’s southeast coast. In addition, findings show that the Sumatran upwelling involves the formation of a strong northwestward surface flow, the South-East Sumatra Current, that transports colder seawater into the region, which contributes to the IOD anomaly.

This work explores the density-driven overturning circulation of the ocean using a process-oriented three-dimensional hydrodynamic model with a free sea surface. As expected, dense-water formation in polar regions creates a deep western boundary current (DWBC) spreading southward ward along the continental slope. Near the equator, the DWBC releases its water eastward into the ambient ocean to form a large upwelling zone. This upwelling is coupled with a slow westward surface recirculation feeding into a swift surface return flow along the western boundary that closes the mass budget. This recirculation pattern, which is fundamentally different to the Stommel-Arons model, is a consequence of geostrophic adjustment to anomalies of the surface pressure field that form under the influence of both coastal and equatorial Kelvin waves and Rossby waves. Based on the findings, the author presents a revised model of the ocean’s meridional overturning circulation to supersede earlier, incorrect suggestions.

Using the method of process-oriented hydrodynamic modelling, this work investigates the dispersal of particles in stratified fluids on continental margins. The focus is placed on steady-state density distributions that are governed by an advective-diffusive balance. In this case, particles can still be advected across isopycnal surfaces, given that turbulent fluctuations do generally not offset the advective displacement of a particle. The validity of this fundamental principle is demonstrated here with the diapycnal upslope sediment transport in a bottom Ekman layer that forms under a stratified geostrophic slope current. Similarly, this study demonstrates that interaction between slope currents with a submarine channel can facilitate a continuous diapycnal upslope flux of particles, confined to the lowermost 10-20 m of the water column. Velocity anomalies that facilitate this upslope sediment flux are the signature of standing topographic Rossby waves, that can only develop for slope currents that are left-bounded (right-bounded) by shallower water in the northern (southern) hemisphere. Findings of sensitivity studies confirm the existence of up-channel flows for a wide range of parameter values. Under the assumption that particles remain suspended in the water column, the inclusion of gravitational settling significantly increases the up-channel sediment flux. Sediment settling operates to trap particles close to the seafloor within the core of bottom-intensified up-channel flow. The author postulates that this mechanism plays an important role in biogeochemical cycles at continental margins.

Using the method of process-oriented hydrodynamic modelling, this study explores wind-driven upwelling features around atoll reefs. The three-dimensional physical model is coupled to a biological model to simulate phytoplankton blooms initiated by the upwelling. Findings demonstrate that wind forcing can create a zone of nutrient upwelling near atolls reefs fuelling significant subsurface phytoplankton blooms. Here we show that both lowering surface salinity due to enhanced precipitation, and submergence of the atoll reef due to sea level rise and enhanced erosion lead to a significant reduction of phytoplankton productivity. We expect that this climate-change related reduction in phytoplankton production around atoll reefs will cause severe disruptions to the functioning of marine ecosystems in tropical oceans.