BoBBLE: Bay of Bengal Boundary Layer Experiment

Lead Research Organisation: University of East Anglia
Department Name: Environmental Sciences


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) proposes 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 influence daily-to-seasonal monsoon variability

Planned Impact

Here we summarise who is likely to benefit from the BoBBLE research and how they will benefit. The Pathways to Impact attachment describes what will be done during BoBBLE to maximize the likelihood of the BoBBLE research reaching those beneficiaries.

Academic researchers in the fields of climate science, meteorology and oceanography will benefit from the advances made and also the new techniques (Seagliders, Argo floats, numerical modelling schemes) that BoBBLE will develop and apply. Some of these techniques will be specific to the local challenges of monsoon system in the Bay of Bengal, but others will be globally applicable. The focus of BoBBLE is on the physics of air-sea interaction, but both the observations and the modelling will include elements of biogeochemistry, so our results will benefit those working on fishery management, carbon sequestration and ecosystem modelling. Ocean scientists in the Bay of Bengal region will benefit by learning how to deploy, operate and pilot Seagliders and how to best design cost-effective surveys to obtain near-surface multidisciplinary data.

Weather forecasters both in government agencies and commercial companies around the world will benefit from BoBBLE, directly through provision of novel data sets in a rarely-observed region, and indirectly through the improvements in climate and weather forecasting techniques and understanding. Fundamentally, BoBBLE is designed to deliver improvements to the forecasting of the monsoon rainfall, and thus will benefit those in Southeast Asian countries tasked with management of water resources. The Indian NCMRWF will benefit from improved modelling parameterisations, and the UK Met Office will benefit from improved understanding of monsoon triggers contributing to improving the HadGEM and UM family of models. This contributes directly to increasing the value of weather/climate forecasting products that they can sell.

The PDRAs working on the project will gain valuable skills in numerical modelling, seagoing oceanography and meteorology, data processing and analysis, Seaglider operation and piloting, and science communication. They will also gain transferable skills including time management, networking, project management, leadership, communication to different audiences, and computer programming. These will be valuable skills should they move into business or industrial careers.

In the longer term, we will make a contribution to resolving the skill shortage in UK scientists and technologists, by attracting further young people into science and into tertiary education through our outreach and media efforts. Norfolk in particular is a region where take-up of tertiary education is low, so we will focus our efforts to raise ambitions and confidence in East Anglia.


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Sheehan P (2020) Injection of Oxygenated Persian Gulf Water Into the Southern Bay of Bengal in Geophysical Research Letters

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Vinayachandran P (2018) BoBBLE: Ocean-Atmosphere Interaction and Its Impact on the South Asian Monsoon in Bulletin of the American Meteorological Society

Description Horizontal advection of temperature has been directly measured in the ocean for the first time. This was enabled through a unique configuration of ocean research ship and ocean gliders during the BoBBLE cruise of summer 2016. The mechanisms that determine the strength and location of the Southwest Monsoon Current in the Bay of Bengal have been identified. This is important as the Southwest Monsoon Current exerts a control on the temperature and salinity of the Indian Ocean in the Bay of Bengal, which then impacts on weather systems and rainfall over Indian and South Asia. A high impact paper on this work has been published in Nature Scientific Reports.

A new pathway for ocean circulation has been discovered in the Bay of Bengal. This pathway brings high salinity water from the Arabian Sea by a deep ocean current into the Bay of Bengal. This water is also highly oxygenated, and contributes to maintaining oxygen levels in the Bay of Bengal, which is critical for ecosystems and fisheries.

The role of absorption of solar radiation by chlorophyll (from marine plankton) in the Bay of Bengal, has been quantified in a series of climate model experiments. This has shown that the changes in absorption by chlorophyll through the seasonal cycle of plankton blooms (which is not usually taken into account in climate models) has a significant impact on the monsoon circulation and ultimately changes rainfall patterns around the Bay of Bengal and over India.

The origin of waters in the "high salinity core" in the Bay of Bengal has been determined through modelling the trajectories of water particles back in time. This has been traced back to the western equatorial Indian Ocean. Interannual variability in the strength of the currents that move this high saline water and critical timings at junctions of these currents have been shown to be crucial in determining the salinity of waters entering the Bay of Bengal. This then drives the distribution of salt content in the ocean across the Bay of Bengal, which partially determines the structure of ocean currents in the Bay, with impacts on the Asian monsoon.

Mechanisms of how "barrier layers" form and are destroyed in the Bay of Bengal were determined from fieldwork data. Barrier layers are key components of the climate system as they insulate the deep ocean from the atmosphere above. The existence or not of a barrier layer then has impacts on the interactions between the ocean and atmosphere and can lead to changes in monsoon circulation and precipitation.
Exploitation Route The technique can be used generally to measure ocean advection where it is an important component of the local dynamics.
Sectors Aerospace, Defence and Marine,Environment

Title Ocean glider data from five Seagliders deployed in the Bay of Bengal during the BoBBLE (Bay of Bengal Boundary Layer Experiment) project in July 2016. 
Description As part of the Bay of Bengal Boundary Layer Experiment (BoBBLE) project, five Seagliders funded through NERC grant reference NE/L013827/1, were deployed in the southern Bay of Bengal from ORV Sindhu Sadhana during the BoBBLE cruise SSD-024. The first glider was deployed on 28 June 2016, and the final glider was recovered on 20 July 2016, giving a total deployment time of 22 days. The gliders were deployed along the west-east section at 8 degrees N, from 85 degrees 18 minutes E to 89 degrees E. During each deployment these autonomous vehicles fly in a continuous repeating saw-tooth pattern from the surface down to a maximum depth of 1000 m. They are all equipped with conductivity-temperature-depth (CTD) sensors. Additional sensors include dissolved oxygen, chlorophyll fluorescence, backscatter and photosynthetically active radiation (PAR). Three Seagliders (including one microstructure enabled glider) are from the University of East Anglia (UEA), UK glider facility. The remaining two Seagliders are from the Marine Autonomous Robotics Systems (MARS) national UK facility. All five Seagliders were deployed and piloted by UEA and associated personnel. The major objective of the BoBBLE project was to understand the east-west contrast in the upper layer characteristics of the southern Bay of Bengal and its interaction with the summer monsoon. The BoBBLE project is funded jointly by Ministry of Earth Sciences (MoES), Government of India and Natural Environmental Research Council, UK, through the "Drivers of Variability in the South Asian Monsoon" programme. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
Impact Data source was used in production of BoBBLE papers, uploaded as publications. Now of general use and and open access to community. 
Description Collaboration with PN Vinaychandran 
Organisation Indian Institute of Science Bangalore
Department Centre for Atmospheric and Oceanic Sciences
Country India 
Sector Academic/University 
PI Contribution Providing hardware and expertise in ocean glider and Argo float deployment in Bay of Bengal for fieldwork in summer 2016.
Collaborator Contribution Providing shiptime in Bay of Bengal for fieldwork in summer 2016.
Impact Planning for 2016 field campaign. Execution of 2016 field campaign
Start Year 2015
Description Seaglider training (India) 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact UEA and NOC personnel ran two training events for Indian scientists in the use of Seagliders for oceanographic research. The first was at National Institute for Ocean Technology (NIOT), Chennai, and the second was on board R/V Sindhu Sadhana in the Bay of Bengal during the BoBBLE cruise.
Year(s) Of Engagement Activity 2016