A Survey of Seasonal Variations of Internal Tide in the Gulf of Maine through in-situ Observations

Both astronomical and internal tidal (IT) currents play significant roles in general circulation in the Gulf of Maine (GOM). While the former as a barotropic phenomenon has been well studied, the latter as a more variable baroclinic phenomenon is far less well understood. This study aims to investigate seasonal variations of the IT field and its spatial variability in GOM through analysis of 2-year (2011-2012) period time series of in-situ ocean current observations.

The observed data are acoustic Doppler current profiler (ADCP) measurements throughout the water column at seven buoy stations operated by the Northeastern Regional Association of Coastal Ocean Observing Systems (NERACOOS). The stations are positioned across the nearshore, the mid-Gulf, and close to the shelfbreak area. The region of coverage includes the eastern, central, and western Gulf, Massachusetts Bay, western Penobscot Bay, Jordan Basin, and the Northeast Channel. The unique positioning enables this study to reveal the spatial variability of IT dynamics throughout the Gulf.

To investigate the seasonal variability of the IT field, each 2-year time series was divided into 24 month long segments and was analyzed to derive tidal harmonic constants, as well as auto and rotary spectra. By comparing the results with observed river discharge data, the impact of river flow on the IT energy intensity was revealed. In addition, the study tested sensitivity of the derived harmonic constants to the length of time series under analysis. The results will help to guide future recommendations on suitable data lengths for IT harmonic analysis.