The South Asian summer monsoon (June-September) provides 80% of the annual rainfall for over one billion people, many of whom depend on monsoon rains for subsistence agriculture and freshwater. It is critical to forecast accurately not only the seasonal rainfall, but also rainfall variations within the summer. Sub-seasonal active and break phases can last weeks, resulting in floods and droughts across broad areas of South Asia.

Air-sea interactions are key to understanding and predicting monsoon behaviour. Ocean surface temperatures in the Bay of Bengal, east of India, remain very warm (above 28 Celsius) throughout the summer. Evaporation from the Bay provides moisture and energy to monsoon depressions that form over the Bay and bring substantial rain to India. It is not understood how the Bay remains warm despite losing energy to these systems. Ocean temperature and salinity variations across the Bay are known to drive changes in rainfall over the Bay and surrounding land, but it is not clear how these arise or how they are maintained. This is particularly true for east-west variations in the southern Bay, a focus of this project. Although air-sea interactions are important to the monsoon, weather predictions are made with models of only the atmosphere. There is potential to improve monsoon forecasts by including well-represented air-sea interactions in models.

The Bay of Bengal Boundary Layer Experiment (BoBBLE) comprises of an observational campaign for the southern Bay, during the established monsoon (mid-June to mid-July). BoBBLE will deploy two ships, six ocean gliders and eight floats to collect an unprecedented range of oceanic and air-sea flux observations. The ships will occupy locations in the southwest and southeast Bay, as well as tracing east-west and north-south paths between those locations, measuring ocean temperature, salinity and currents. Two gliders (automated underwater vehicles) will accompany each ship, with two others between the ships, diving to 500 metres every 2 hours to measure temperature, salinity and currents. Diurnal variations in air-sea fluxes and ocean temperatures may affect the development of weather systems. A novel configuration of the gliders will allow computations of horizontal transports of heat and salt. The floats (automated submersibles) will be deployed in the Bay to measure the ocean to 2000 metres every 5 days. They will remain in the Bay after BoBBLE, enhancing the observing network. Ships and gliders will also measure ocean chlorophyll, which absorb sunlight and alter near-surface ocean temperature, influencing air-sea interactions.

BoBBLE scientists will analyse these observations, along with routine datasets, to understand the evolution of conditions in the Bay and how they influence the atmosphere. Particular emphasis will be placed on estimating the uncertainty in existing datasets of air-sea fluxes by validating them against available observations. The best-performing datasets will be used to improve estimates of air-sea exchanges and their variability on daily to decadal timescales, to calculate budgets of heat and freshwater fluxes in the Indian Ocean and the Bay, and to validate model simulations.

A hierarchy of model simulations will reveal how conditions in the Bay are maintained and how air-sea interactions influence the monsoon. Simulations with an ocean model, forced by and validated against BoBBLE observations, will isolate the roles of air-sea fluxes (including the diurnal cycle), chlorophyll and horizontal transports in maintaining and recharging ocean structure after each weather system passage. Retrospective forecasts of the BoBBLE period with atmosphere-only and atmosphere-ocean coupled models will demonstrate how air-sea interactions influence monsoon rainfall predictions. Multi-decadal simulations will evaluate how air-sea interactions and coupled-model systematic errors influ

The overall aim of this project is to determine, quantify and model ocean-atmosphere interactions that drive variability in the South Asian monsoon. The Bay of Bengal (BoB) is identified as a key region for these processes, where oceanic and ocean-atmosphere processes have a major impact on monsoon precipitation and circulation, but are inadequately measured and understood. The objectives are to answer the following questions:

How does ocean-atmosphere interaction in the BoB impact monsoon precipitation?

How do ocean mixed layer and dynamical processes across the BoB affect sea surface temperature (SST) and subsequently monsoon precipitation?

What causes the large-scale SST gradients in the BoB, and how do they impact the monsoon?

How do these processes vary on a range of spatio-temporal scales, specifically related to the diurnal cycle, individual weather systems, and intraseasonal oscillations?

What are the best estimates of the surface fluxes that drive ocean-atmosphere interaction over the Indian Ocean, with uncertainties?

How well do models, particularly the UK Met Office Unified Model, simulate these processes and how does this impact prediction of monsoon variability?

What is the heat and freshwater budget of the BoB, and how does this constrain model simulations?

What is the role of ocean biogeochemistry in the BoB and its impact on the monsoon?

Data from this project will be stored at the British Oceanographic Data Centre (BODC)