Coastal Ocean Condition Forecasts during the Grand Lagrangian Deployment (GLAD) Experiment

GLAD is part of the Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) designed to understand the dispersion of surface materials under the action of ocean surface processes. As part of the experiment, ocean environment forecasts were used to aid in the initial deployment of 300 surface Lagrangian drifters in the northeastern coastal Gulf of Mexico during July 2012. The performance of the ocean predictions relative to the drifter observations indicates the present level of skill in predicting development of the physical processes controlling environmental conditions in such areas.

The predictions are based on numerical model forecasts, and in this case the Navy Coastal Ocean Model (NCOM) is used covering the Gulf of Mexico at 1km resolution nested into the global NCOM at 1/8 degree resolution. Comparisons to 3km resolution indicate the added energy in the submesoscale processes within the mixed layer that contribute to upwelling along the frontal edges of the mesoscale eddy field. The predictive models are forced by wind stress from COAMPS with forecasts to 72 hours every day. All publicly available satellite and in situ data were assimilated into the ocean model daily. Just prior to the experiment time period, the ocean community suffered loss of the satellite altimeter on ENVISAT, and the Jason-1 satellite was in the process of moving to a new orbit leaving only the Jason-2 data stream. Just prior to the experiment, the Jason-1 data stream was reestablished in its geodetic orbit, and the CryoSat2 data stream was added. These changes in the altimeter constellation presented an extreme challenge for the experiment, and did impact the forecast accuracies.

During the experiment time period, westerly winds dominated. This lead to an eastward flow along the south Louisiana coast, and the Mississippi River plume was flowing far to the east which is opposite to the expected direction of flow under no wind conditions. Salinity observations from the University of Miami ship Walton Smith used for the experiment provided confirmation of the exceptional eastward extent of the Mississippi River outflow fresh water that covered the coastal surface eastward to the DeSoto Canyon. East of the DeSoto Canyon, a cyclone along the shelf break contributed to a westward flow across the continental shelf. This cyclone was observed by satellite altimeter observations, and assimilation into the models provided proper positioning in the forecasts. The convergence of the continental shelf flows at the DeSoto Canyon resulted in general southwestward flow that flushed waters out of the canyon. This general circulation of southwestward flow out of the canyon was confirmed by drifters deployed extensively throughout the canyon, and the eastward flow near the Mississippi River Outflow was also confirmed by several drifters that recycled through this region.

A large anticyclonic Loop Current Eddy (LCE) had shed from the Loop Current just prior to the experiment. The LCE had several cyclones associated with it around its periphery as is typical of such processes, and these cyclones played important parts in the experiment and its results. The first cyclone mentioned above aided in the convergence at the DeSoto Canyon and also generated a large transport from the continental shelf into the deep waters. This flow connected to the LCE. The results of this are apparent in satellite MODIS imagery that show an intensive high chlorophyll area generated by the Mississippi River outflow east of the river mouth, and this chlorophyll was caught into the cyclone circulation and into the LCE. The stream of chlorophyll was observed by satellite all the way to the Florida Strait. The chlorophyll feature persisted for weeks during the experiment as the cyclone pulled the productive shelf waters into the deep ocean. The position of this cyclone was observed directly by the drifters and corresponded well with the model forecasts, which also predicted the surface current advection of shelf waters to the deep LCE by the cyclone.

A second small (50km diameter) cyclone was observed south of the Mississippi River mouth in satellite SST imagery on the periphery of the LCE. Initially the numerical models were not producing this cyclone, and examination of the satellite altimeter data indicated this area was a data void for quite some time due to the suboptimal sampling patterns induced by the satellite constellation changes prior to the experiment. Further into the experiment after the satellite altimeters had observed the cyclone, its placement in the ocean models was more accurate.

This experiment is the first time ocean forecasts played such an integral part in the placement of such a grand Lagrangian data set. It provides a good reference point for the systems' performance. In the future, assimilation of the drifter velocity observations and Lagrangian trajectories will be examined to demonstrate the impact of this data source that is not presently used.