Horizontal and vertical spreading of dye in the coastal ocean of the northern Mid-Atlantic bight

Abstract The paper investigates the horizontal and vertical spreading of a neutrally-buoyant chemical tracer (rhodamine) that was released into the surface mixed layer in the coastal ocean. Rhodamine concentrations from the dye release experiment were combined with observations of water column properties and velocity measurements from the high-frequency radar (HFR) and ship-based ADCP in order to investigate and interpret the downward penetration of dye from the mixed layer into the stratified part of the water column and to quantify the horizontal diffusivity associated with the lateral dye spreading. Of the total amount of dye encountered during the 33-h long survey, 99% was observed within the mixed layer, suggesting that the downward penetration of dye from the mixed layer into the pycnocline is a slow process that occurs on the time scale of at least multiple days. The observed elevated dye concentrations below the mixed layer were always co-located with the elevated concentrations within the mixed layer. However, the distribution of dye below the mixed layer was patchy, and the below-mixed-layer dye concentrations were poorly correlated with those within the mixed layer. Combined with the absence of strong vertical shear of horizontal velocity across the base of the mixed layer, this suggests that the observed dye could have been ejected from the mixed layer earlier and at a different location, and then advected to the observed location by the horizontal currents below the mixed layer. The vertical diffusivity in the mixed layer is estimated to be approximately 7x10−4 m 2 s , about two orders of magnitude larger than the diffusivity across the base of the mixed layer, which is 5x10−6 m 2 s . Lateral spread of dye was found to be well represented by the HFR velocities (with roughly 2-km resolution) with the added small-scale turbulent diffusivity of 5 m 2 s . Plain Language Summary. The top part of the water column, called the surface mixed layer, is well mixed to a nearly uniform density. Below lays the pycnocline where density changes rapidly with depth. The exchange of water and properties between the mixed layer and pycnocline is an important but poorly understood oceanographic process and is the main subject of the current observational study. Specifically, this paper describes and interprets the initial 36 h of the vertical and horizontal spreading of a dye plume, released into the surface mixed layer of the coastal ocean south of Martha's Vineyard, MA. Our results indicated that the ejection and penetration of dye down from the mixed layer is a slow process that occurs over a time scales longer than multiple days. The episodic nature of the downward dye ejection from the mixed layer and the resulting patchy distribution of the subsurface dye is another important finding from this work, which has far-reaching implications for the parameterization of these small-scale vertical exchange processes in numerical models. The vertical diffusivity within the mixed layer (7x10−4 m 2 s ) is estimated to be about two orders of magnitude larger than the diffusivity across the base of the mixed layer (5x10−6 m 2 s ). Finally, the lateral spreading of the dye plume was found to be well represented by advection by the high-frequency-radar-based velocities with the added small-scale turbulent diffusivity of 5 m 2 s – the value that is in line with our prior estimates for this coastal region, but is larger than many prior estimates from different regions of the ocean.