Evidence That an Optical Tail in the Gulf of Mexico After Tropical Cyclone Isaac was the Result of Offshore Advection of Coastal Water

Abstract

A B S T R A C TThis study investigates the hypothesis of Acker's Web report in 2013 that an optical tail of high chlorophyll a, observed in the open Gulf of Mexico (GoM) approximately 2 weeks after tropical storm Isaac made landfall in coastal Louisiana, was due to advection of outflowing Mississippi River related with the mesoscale eddy field in the open GoM. By using available in situ data and data from multiple satellites, strong evidence was found to support Acker's hypothesis. Drifting buoy, remotely sensed sea surface salinity, and surface geostrophic current data were used to show that low-salinity water (LSW) was indeed associated with the optical tail. Remotely sensed colored dissolved organic matter indicated that the LSW was of coastal origin, and satellite-observed rain rate indicated that this LSW in the optical tail was not due to local precipitation. The path of freshwater from the Mississippi River Delta to the region offshore in the optical tail was shown to be similar to a simulated trajectory estimated by surface geostrophic currents; likewise, the drifting buoys deployed near the shelf break offshore of the Mississippi River Delta prior to the peak in discharge.

Biophysical Submesoscale Processes in the Wake of Hurricane Ivan: Simulations and Satellite Observations

“…An emerging paradigm for the oceanographic aftermath of such storm events in the GOM is that turbid waters overlying the continental shelf (overturned and well-mixed as a result of the storm) may be exported off of the shelf and into the pelagic GOM, along the peripheries of perturbed and intensified mesoscale circulation features. Whereas this offshore advection process may certainly occur following storm events [1,2] and at other times [3,4], the COAMPS-TC modeling results and satellite analysis presented herein demonstrate that the presumed horizontal advection that often appears in synoptic satellite images of the GOM (for example, the satellite ocean color chlorophyll-a product) may instead be due to submesoscale instabilities along the peripheries of larger circulation centers. Thus, the causative mechanism for the filament-like chlorophyll-a (Chla) COAMPS model simulations and satellite observational data in order to resolve and analyze this biophysical submesoscale process.…”

“…Thus, our simulations suggest that in addition to right sided asymmetry and CCE intensification/cooling, a separable and submesoscale biological-physical process may evolve in the aftermath of a TC passage. This also suggests that horizontally propagating "filaments" of discolored water (or high satellite-estimated chlorophyll) that appear in satellite ocean color images of the Gulf of Mexico may not always be the result of horizontal advection of shelf waters [2,59], but may instead be the result of frontogenetic instability along the periphery of mesoscale circulation features. These submesoscale filaments are brought about by a fundamentally different biophysical process, through which nitrogen-rich, colder waters can be brought to the ocean surface to promote enhanced primary productivity via ageostrophic vertical circulation.…”

“…A notable exception to this trend would be open ocean blooms of coccolithophorids [47]. It has also been previously argued that, in some instances, tropical cyclones or significant flooding/rainfall events may result in the advection of low-salinity coastal waters to several hundred kilometers away from the coast [2,48].…”

“…A notable exception to this trend would be open ocean blooms of coccolithophorids [47]. It has also been previously argued that, in some instances, tropical cyclones or significant flooding/rainfall events may result in the advection of low-salinity coastal waters to several hundred kilometers away from the coast [2,48]. The MODIS sea surface temperature (SST) product provides additional clarity on the underlying biophysical mechanisms resulting in the observed Csat distribution (Figure 10), particularly where it appears to match the COAMPS biophysical simulation.…”

Towards Constraining Sources of Lithogenic Metals in the Northern Gulf of Mexico

“…The deep Gulf of Mexico in summer is just as oligotrophic as the Sargasso Sea (Howe et al, 2020;Muller-Karger et al, 1991;Stukel et al, 2021), though the northern Gulf can sometimes be impacted by Mississippi River outlow (Kil et al, 2014;da Silva & Castelao, 2018) potentially leading to a higher annual average biological particle flux in the northern Gulf.…”