Rapid vertical exchange at fronts in the Northern Gulf of Mexico.

Over the Texas-Louisiana Shelf in the Northern Gulf of Mexico, the eutrophic, fresh Mississippi/Atchafalaya river plume isolates saltier waters below, supporting the formation of bottom hypoxia in summer. The plume also generates strong density fronts, features of the circulation that are known pathways for the exchange of water between the ocean surface and the deep. Using high-resolution ocean observations and numerical simulations, we demonstrate how the summer land-sea breeze generates rapid vertical exchange at the plume fronts. We show that the interaction between the land-sea breeze and the fronts leads to convergence/divergence in the surface mixed layer, which further facilitates a slantwise circulation that subducts surface water along isopycnals into the interior and upwells bottom waters to the surface. This process causes significant vertical displacements of water parcels and creates a ventilation pathway for the bottom water in the northern Gulf. The ventilation of bottom water can bypass the stratification barrier associated with the Mississippi/Atchafalaya river plume and might impact the dynamics of the region's dead zone.

Enhanced mixing across the gyre boundary at the Gulf Stream front.

The Gulf Stream front separates the North Atlantic subtropical and subpolar ocean gyres, water masses with distinct physical and biogeochemical properties. Exchange across the front is believed to be necessary to balance the freshwater budget of the subtropical gyre and to support the biological productivity of the region; however, the physical mechanisms responsible have been the subject of long-standing debate. Here, the evolution of a passive dye released within the north wall of the Gulf Stream provides direct observational evidence of enhanced mixing across the Gulf Stream front. Numerical simulations indicate that the observed rapid cross-frontal mixing occurs via shear dispersion, generated by frontal instabilities and episodic vertical mixing. This provides unique direct evidence for the role of submesoscale fronts in generating lateral mixing, a mechanism which has been hypothesized to be of general importance for setting the horizontal structure of the ocean mixed layer. Along the Gulf Stream front in the North Atlantic, these observations further suggest that shear dispersion at sharp fronts may provide a source of freshwater flux large enough to explain much of the freshwater deficit in the subtropical-mode water budget and a flux of nutrients comparable to other mechanisms believed to control primary productivity in the subtropical gyre.

Anomalous Near-Surface Low-Salinity Pulses off the Central Oregon Coast.

From mid-May to August 2011, extreme runoff in the Columbia River ranged from 14,000 to over 17,000 m(3)/s, more than two standard deviations above the mean for this period. The extreme runoff was the direct result of both melting of anomalously high snowpack and rainfall associated with the 2010-2011 La Niña. The effects of this increased freshwater discharge were observed off Newport, Oregon, 180 km south of the Columbia River mouth. Salinity values as low as 22, nine standard deviations below the climatological value for this period, were registered at the mid-shelf. Using a network of ocean observing sensors and platforms, it was possible to capture the onshore advection of the Columbia River plume from the mid-shelf, 20 km offshore, to the coast and eventually into Yaquina Bay (Newport) during a sustained wind reversal event. Increased freshwater delivery can influence coastal ocean ecosystems and delivery of offshore, river-influenced water may influence estuarine biogeochemistry.