Meteorological and Internal Wave Forcing of Seiches along the Sri Lanka Coast

Regularly observed seiches from tide gauge records around the Sri Lanka coast have been analysed to determine the forcing mechanisms. The seiche periods range from a few tens of minutes to 2 hr. Fortnightly and seasonal variations of seiche amplitudes are clearly visible on the east coast. These seiche amplitudes are particularly large approximately 6-8 days after spring tides, with maximum seiche amplitudes observed during March-April and October-November, suggesting that the seiche variation could be related to stratification and internal wave activity. There is no visible fortnightly and seasonal variation of seiche amplitudes in west coast records. Instead, they show a daily pattern with relatively larger seiches around 10:00 hrs local time, suggesting that the seiches on the west coast could result from diurnal atmospheric forcing. Barotropic and 2-layer models have been developed to investigate the influence of atmospheric forcing and internal wave activity on the seiches. A barotropic model applied to the west coast shows that the daily seiche amplitude variation could be simulated with cyclic diurnal meteorological forcing. Two-layer model runs for Trincomalee Bay and the adjacent east coast shelf suggest that seiche amplitudes are proportional to vertical stratification and mixed layer depths. Therefore, the observed seiches there could be excited by internal waves which originate as far away as the Andaman Sea during spring tides and have a travel time of 6-8 days to the Sri Lankan east coast. However, further studies, including direct measurements of internal wave activity within the region are required to confirm this hypothesis