Wind Forcing, Stratification, and Along-channel Vertical Velocity Structure in Penobscot Bay, Maine, at Synoptic to Interannual Timescales

Abstract:

Long-term observations (10 years) of current velocity, hydrographic properties, and meteorological conditions collected at the University of Maine Ocean Observing System (UMOOS) buoy F in outer western Penobscot Bay, Maine, are used to characterize flow structure and its associated dynamics in this system. The mean vertical velocity structure is primarily density-driven, with characteristic outflow typically restricted to the upper layer and inflow at middle and deep depths. At the synoptic scale, weather-based events dominate current variability; along-channel velocity at the surface and at depth is highly correlated with along-channel wind. This is typical of two-layer competition between wind forcing and pressure gradient force: wind forcing drives the surface layer, and the deeper return flow is forced by an expected sea-level setup resulting from the wind forcing. This velocity structure is modulated seasonally; during fall-winter, the surface outflow layer is deeper, and the shear at depth stronger, than during spring-summer conditions. The seasonal change in the velocity profile appears related to the seasonal change in wind conditions rather than river discharge, although seasonal stratification is linked to the restriction of shear to shallower depths in spring-summer. At interannual timescales, the same link between wind forcing and along-channel velocity is observed. These findings have important implications for key processes in the bay, such as the transport of nutrients, planktonic larvae, and pollutants.