Yann Krien (LEGOS, Toulouse, France); Fabien Durand (LEGOS, Toulouse, France); Laurent Testut (LEGOS, Toulouse, France); AKM Saiful Islam (Institute of Water and Flood Management, Bangladesh); Fabrice Papa (LEGOS, IFCWS, Bangalore, India)

The Northern Bay of Bengal with its adjoining Ganges-Brahmaputra-Meghna delta (GBM) forms the largest deltaic region in the world. It is surrounded by a wide area of low-lying land (less than a few meters above mean sea level), very densely populated. It is home to a strong variability of sea level, with ample tides and frequent storm surges. It is also subject to extended river flooding during the monsoon season, with frequent overflows of two of the world's largest rivers (Brahmaputra and Ganges). There is thus a need to understand and predict the various mechanisms responsible for coastal sea level variability in this area. In this study, we address one of the least understood facets of this variability: the low-frequency modulation of tides. We concentrate on the seasonal changes of amplitude of the dominant tidal component: the semi-diurnal lunar tide, M2. It is found that M2 amplitude shows marked changes between winter and summer seasons, incommensurate with most of the world's coastal ocean. We observe contrasted patterns from the open areas of the coastal ocean to the inner part of the GBM estuary. In the coastal ocean and southern part of the GBM delta, M2 amplitude is stronger during summer and decreases until winter. Conversely, in the northern part of GBM estuary, M2 amplitude is stronger during winter and weaker during summer. We make use of a hydrodynamic tidal model to decipher the processes responsible for this contrasted evolution. It is found that the southern part of the domain is dominated by the influence of frictional effect (bottom dissipation of tidal energy) whereas the northern part of the domain suggests a simple convergence/divergence of tidal energy over a time-varying thickness of the water column. Our study demonstrates the need to carefully account for these processes for a proper representation of the tidal dynamics as well as of the flooding hazard of the Bay of Bengal.This peculiar modulation of tidal range also appears highly relevant for a successful de-aliasing of the future swath altimetry products expected from SWOT mission in the near-shore ocean.