Understanding sea-level variations in the Bay of Bengal

Author: J. de Juan, Rui M. Ponte and Nadya Vinogradova
December 5, 2012
Poster presentation
AGU Fall Meeting 2012

Julia de Juan; James L. Davis; Emma M. Hill; Mark E. Tamisiea; Rui M. Ponte; Nadya T. Vinogradova (2012) Understanding sea-level variations in the Bay of Bengal. AGU Fall Meeting, San Francisco, CA.

Sea level is affected by a wide range of processes, resulting in a response that varies on seasonal, interannual, and decadal time scales, and that has clear regional variations. Understanding such variability is important in order to quantify and interpret global trends in long-term sea level. One cause of this variability, of many, is the seasonal exchange of water between the continents and the ocean, which induces changes in the shape and gravity field of the Earth. This so-called ‘self-attraction and loading’ (SAL) causes a spatial and temporal variation of sea level, with an annual amplitude that ranges from ~2 mm to >18 mm. Previous studies show that the effect of SAL on the annual cycle of sea level is larger in the Bay of Bengal than anywhere else on Earth. In addition, tide-gauge measurements of the annual cycle in sea level show among the largest disagreements with ocean model predictions and near-coastal altimeter measurements in this region. The study of sea level in the Bay of Bengal is important, both socially and scientifically. Three rivers converge in Bangladesh, with one of the world’s highest annual discharge, of ~1300 GT/yr. The large delta covers the highly populated regions of southern Bangladesh and West Bengal. River flow is highly seasonal, with almost all discharge taking place during the summer monsoon. These conditions result in widespread flooding over Bangladesh every summer, with ~100 GT of water stored within Bangladesh during these events, as observed with GRACE and in-situ measurements. This large hydrological load is the cause for the observed large annual SAL effect in this region, and may account for at least part of the discrepancy between tide-gauge measurements and ocean-model predictions. Furthermore, comparison with measurements suggests that the hydrology models used to estimate the global SAL effect on the annual sea-level cycle may be underestimating the water load over this region. The problem is compounded by the fact that this river system has also the largest rates of sediment discharge, of ~1 GT/yr, resulting in a highly dynamic delta. Additionally, due to the low elevation of the coastal areas, changes in shoreline location may be occurring. Careful inspection of tide-gauge RSL time series from this area reveals large temporal and regional variations in the annual cycle and at longer periods. These factors suggest that the variations in sea level may also be strongly impacted by local effects.