Salinity Intrusions in the Bay of Bengal from a highly resolved Argo float array

by A. Sanchez-Franks1, B.A. King1, A.J. Matthews2,3, P.N.Vinayachandran4, and B.G.M.Webber2

1 National Centre of Oceanography, Southampton, UK

2 Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, Norwich, UK

3 Centre for Ocean and Atmospheric Sciences, School of Mathematics, University of East Anglia, Norwich, UK

4 Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India

corresponding author: Alejandra Sanchez-Franks
Reference: Sanchez-Franks A., B. A. King, A. J. Matthews, P. N. Vinayachandran, and B. G. M. Webber. Variability of the Southwest Monsoon Current and the High Salinity Core in the Bay of Bengal (to be submitted).
Context

A joint India-UK field experiment called BoBBLE (“The Bay of Bengal Boundary Layer Experiment”) took place in summer 2016 in the Bay of Bengal during the South Asian summer Monsoon. The purpose of the experiment was to understand the evolution of the near-surface stratification and air-sea exchange, in order to understand and improve monsoon predictability.

Data and method

Five Seagliders, seven Argo floats and multiple drifters were deployed during the campaign. Salinity measurements from CTD, gliders and Argo floats are shown in Figure 2. India provided the research vessel Sindhu Sadhana and gathered shipboard CTD measurements, while the UK provided profile data from gliders and seven Argo floats.

Four of the Argo floats were equipped with SeaBird near surface temperature and salinity sensors (STS) and three carried radiometers to show the depth at which incoming solar radiation is being absorbed. The floats cycled daily while the research vessel was nearby, and reverted to a standard Argo 10-day mission after an initial intensive observing period ended.

Data from the BoBBLE floats have been combined using Optimal Interpolation with data from other Argo floats in the region, plus gliders and shipboard CTDs, to produce maps of the distribution of temperature and salinity.

Results

The OI gridded time series (Figure 3) enables analysis of the evolution of the salinity and temperature variability in the surface (top 500 m) layer of the southern Bay of Bengal. Salinity profiles indicate the observed salinity maxima are likely salinity intrusions advected from the Arabian Sea via the Southwest Monsoon Current.

These results suggest that during the summer monsoon, the heat and freshwater budgets are not locally driven. Elevated salinity is also observed in the uppermost (top 50 m) surface layer, generally corresponding in time to the salinity intrusions at depth. The elevated surface salinity events have a duration of 2-3 days and occur every few weeks (and do not appear to be restricted to the southwest monsoon). The combination of datasets from the BoBBLE field campaign has provided an important ocean observing system that will be will be critical in determining seasonal and interannual heat and freshwater variability in the Bay of Bengal. 

References
  • Sanchez-Franks A, Kent E.C., Matthews A.J., Webber BGM, Peatman SC, Vinayachandran PN. (In review) : Intraseasonal Variability of Air-Sea Fluxes over the Bay of Bengal during the Southwest Monsoon, Journal of Climate. 
  • Vinayachandran, P., A. Matthews, K. Vijay Kumar, A. Sanchez-Franks, V. Thushara, J. George, V. Vijith, B. Webber, B. Queste, R. Roy, A. Sarkar, D. Baranowski, G. Bhat, N. Klingaman, C. Parida, K. Heywood, R. Hall, B. King, E. Kent, A. Nayak, C. Neema, P. Amol, A. Lotliker, A. Kankonkar, D. Gracias, S. Vernekar, A. D.Souza, G. Valluvan, S. Pargaonkar, K. Dinesh, J. Giddings, and M. Joshi, 2018: BoBBLE (Bay of Bengal Boundary Layer Experiment): Ocean– atmosphere interaction and its impact on the South Asian monsoon. Bull. Amer. Meteor. Soc. doi:10.1175/BAMS-D-16-0230.1, in press.

 

More information on the BoBBLE Experiment : http://www.bobble.ac.uk