BoBBLE (Bay of Bengal Boundary Layer Experiment): Impact of the Bay of Bengal on Monsoon Processes

BoBBLE is a joint India-UK project to examine the impact of ocean processes in the Bay of Bengal (BoB) on the monsoon system. A field campaign was carried out in July 2016 with a research vessel, five ocean gliders, six profiling floats, and microstructure and subsurface radiation measurements, to measure key processes in the southern BoB. The saline Southwest Monsoon Current (SMC), a major control on the salt and SST distribution of the BoB, is shown to be controlled by both local (wind stress curl) and remote (equatorial wave propagation) factors, strongly linked to subseasonal variability over the wider Indian Ocean basin. The high salinity core (HSC) of the SMC is shown to have its origins in the western equatorial Indian Ocean, reaching the BoB via the Somali Current, the Equatorial Undercurrent and the SMC. Seasonal reversals that occur at the Somali Current and SMC junctions act as 'railroad switches' diverting water masses to different basins in the northern Indian Ocean. Barrier layer formation and erosion in the southern BoB were investigated using microstructure and conventional measurements, and were found to be largely controlled by differential advection and resulting mixing driven by shear stress. Chlorophyll in the southern BoB was strongly influenced by mixed layer processes and barrier layer strength. The effect of these oceanic processes in the BoB on the development of the South Asian monsoon were determined by a series of modelling experiments, using the MetUM-GOML model, which has a realistic representation of the monsoon in its default configuration. Perturbation experiments were carried out to show the impact on the monsoon of the north-south and east-west SST gradients in the BoB, the strength of barrier layers, and the effect of chlorophyll via changes to the scale depth of penetration of the blue light component of solar radiation.