Upwelling signature around small oceanic islands and the case of the Maldives Archipelago

Around the Maldives, it was found that the interaction between currents and the steep bathymetry is responsible for a local cooling of about 0.2°C in the Archipelago during the warmest season, with respect to the surrounding waters. The reduced SST is probably linked to the Island Mass Effect: the enhanced productivity around small islands discovered in the sixties and documented worldwide. Despite its effects on marine productivity, the exact description of the physical processes behind the local cooling and nutrient input that enhances productivity is still unclear. From the analysis of SST variations and net primary productivity (NPP) around small islands and archipelagos, two kinds of signals can be identified, depending on the altitude and dimension of the islands. Around islands with considerable elevation and greatest diameters, cold/warm anomalies, most likely corresponding to upwelling/downwelling zones, emerge. Warmer areas don’t appear around smaller islands that usually display only a local cooling. Several oceanic and atmospheric processes might be involved. The case of the Maldives has been analyzed in detail using CROCO and a particle tracking model: Ariane. More than one process might coexist in generating the described patterns, the prevailing one varying along the year and depending on the strength and direction of the incoming flow. Near the Maldives, the frictional break of the currents in the presence of shallow bathymetry produces a strong vertical shear in the flow that favors vertical mixing and produces a nearly symmetric cooling around the islands. A different mechanism dominates the cooling pattern when the currents are particularly intense, such as during the monsoons: intense zonal currents cross the Archipelago and give rise to intense wakes with large horizontal shear; strong upwelling originates in the lees, creating an asymmetric temperature signal (larger cooling in the lee of the islands) and obfuscating the effects of the enhanced vertical mixing.