Observations and Modelling of the Boreal Summer Intraseasonal Oscillation during the Bay of Bengal Boundary-Layer Experiment (BoBBLE)

Synoptic disturbances that form and intensify over the Bay of Bengal during the summer monsoon season often bring heavy precipitation to South Asia. In some seasons, these monsoon depressions may contribute up to 50% of the seasonal total precipitation to central and northern India. The frequency and intensity of these systems is modulated by large-scale sub-seasonal variability, known as monsoon active and break phases, which are associated with the Boreal Summer Intraseasonal Oscillation across the Indo-Pacific warm pool. These active and break phases often develop in the equatorial Indian Ocean, then propagate north through the Bay of Bengal, where air-sea interactions over the relatively warm sea-surface temperatures (SSTs) in the Bay are believed to sustain them and encourage further northward propagation to India. The precise effect of air-sea interactions on sub-seasonal variability and synoptic monsoon depressions remains uncertain, however, as are the mechanisms by which propagating atmospheric convection responds to variations in SST as these systems move across the Bay.

The southern Bay of Bengal is a relatively under-explored region, characterised by sharp zonal and meridional gradients in SST and sea-surface salinity (SSS) with the waters in the northern Bay and the open Indian Ocean to the south. The southern Bay features an intense northeastward current that diverts around Sri Lanka and advects high SSS water from the Arabian Sea, providing a contrast with the fresher water from river outflow into the northern Bay. The Sri Lanka "cold dome" also presents a mystery: a local SST minimum in a relatively dry region with high net surface flux into the ocean. The interactions of these features with developing and propagating weather systems, as well as the effects of these systems on the oceanic mixed layer below, are unknown.

The Bay of Bengal Boundary-Layer Experiment (BoBBLE) aims to improve understanding of air-sea interactions in the Bay of Bengal and their effects on developing weather systems and ultimately South Asian monsoon rainfall. It comprises a field campaign in the southern Bay during the summer monsoon in 2016, as well as related modelling experiments with forced ocean models and coupled atmosphere-ocean weather and climate models. The BoBBLE campaign was conducted on RV Sindhu Sadhana in July 2016 and obtained detailed oceanographic and atmospheric measurements with radiosondes, underwater autonomous vehicles, Argo floats and shipboard acoustic depth current profilers (ADCP), among other instruments. The ship conducted two transects across the southern Bay, as well as a two-week timeseries at 89E, 8N. The campaign observed a prolonged break phase of the monsoon and a transition to the next active phase, which included SST warming and strong salinity advection by the monsoon current.

This presentation will review the BoBBLE campaign, including observations and related modelling experiments. For the latter, the presentation will show experiments with the Met Office Unified Model Global Ocean Mixed Layer model (MetUM-GOML) that demonstrate how an improved representation of local air-sea interactions in the Indian Ocean improves the structure and northward propagation of the boreal summer intraseasonal oscillation to India. Initial results from coupled numerical weather prediction of the 2016 monsoon season will also be shown, including a comparison against operational atmosphere-only forecasts to demonstrate the impact of air-sea coupling on sub-seasonal monsoon prediction.