Chesapeake Bay outflow plume and coastal upwelling events: Physical and optical properties

Abstract

One of a series of Chesapeake Outflow Plume Experiments, COPE‐2, was conducted in May 1997 along the coast of Virginia/North Carolina. The objective of this experiment was to describe the coastal buoyancy jet formed by the outflow of water from Chesapeake Bay, its dispersion into midshelf, and the optical property changes that accompany these events. Wind forcing scales of 3–5 days were responsible for formation of the jet (downwelling favorable winds) and its dispersion (upwelling favorable winds). During downwelling favorable events the jet established itself along the coast with speeds exceeding 0.5 m s−1, slowing and becoming more stable as it progressed. Downwelling circulation helped maintain the jet against the coast, and variations in wind strength produced wavelike variations in jet width. During upwelling favorable events, water from the jet was dispersed into midshelf in a thin near‐surface layer. Optical surrogates for dissolved organic matter (DOM), chlorophyll, and suspended particles were formed using data from a pumped optical absorption/attenuation meter (ac‐9). High loads of these materials were associated with the buoyancy jet and were subsequently dispersed into midshelf during upwelling events. During mature upwelling states, nearshore increases in chlorophyll and suspended particles mimicked the plume load, although relatively low levels of DOM made it separable from plume water by analysis of the optical signature. Upwelling relaxation fronts in these optical conditions could be seen.