Figure 1: IOOS Overview. I see a few new icons are being phased in. The Navy profilers now look real – like the vertical yellow cylinders with an antenna on the top. The Navy drifters are starting to look like the red drogues. The SIO/WHOI Spray 40 glider is heading into the center of Loop Current Eddy Franklin. it is surrounded by 2 Navy and 1 Horizon Marine drifter. Heading north, we encounter Navy Seaglider SG135 between the Loop Current Eddy and the slick. In the slick we see two Seagliders, the Navy’s SG137 and iRobot’s SG515. On the Florida shelf we see the Slocum gliders from USF and U.Delaware.
Figure 2: As usual, we’ll start today’s analysis in deepwater, looking at the general configuration of the Loop Current, the Loop Current Eddy, and its interactions with the shelf to the north and east. Then we’ll zoom into the shelf regions. Here we look at the Navy global HyCOM sea surface height (color fill) and the surface currents (white arrows). The meander crest in the Loop Current continues to develop in the HyCOM forecast, heading straight north from Cuba and making a sharp right hand turn to the east, flowing along the shelf break and out of the Gulf. The elliptical Loop Current Eddy (Franklin) has rotated so that its major axis is oriented east-west. The western side of the major axis has that lingering trail of an interaction with the Loop Current, and the eastern side has the evolving interaction with the Florida shelf. Interpretation of the link between the western side of Franklin and the Loop Current is aided by the Horizon Marine Drifter to the south of Franklin. The Slocums on the Florida shelf will help with the eastern side. In this orientation, the strong clockwise currents of the Loop Current Eddy are much farther south of the oil spill site than they were when the eddy orientation was north-south.
Figure 3: Here we switch to the nested higher-resolution SABGOM forecast from the ROMS model run at North Carolina State. Sea surface height is in color, and the surface currents vectors are white arrows so we can better see the heights. The main deepwater features are similar, and we commented yesterday how the small clockwise spinning eddy in the SABGOM forecast south of Franklin looks to be in even better agreement with the Horizon Marine drifter. In this image, we also see a band of light blue running north-south on the middle to outer Florida shelf. With the distance between the Loop Current Eddy Franklin and the spill site increasing, this ridge of slightly higher water on the Florida shelf is an important feature we will return to further down in today’s analysis.
Figure 4:But first lets continue moving north and zoom into the slick. First we turn off the SABGOM sea surface height so we can see the slick, but we leave the SABGOM surface current vectors turned on with a color change to black arrows. First we see that the NOAA oil slick forecast (light, medium & dark blue) has the same U-shape as yesterday with a similar area of uncertainty (thin black line). The base of the U-shaped slick is a bit wider than yesterday, and the tops of the U have been bent over a bit to the east. As we suspected from yesterday’s SABGOM forecast, the area of uncertainty along the southern coast of Louisiana has extended a bit farther to the west along the coast. Yesterday’s westward flowing coastal jet is much less prominent in today’s SABGOM forecast, so we expect less change in this portion of the area of uncertainty tomorrow. Looking north of the slick, we have the HF Radar surface currents as the patch of mostly green arrows. On the western side of the HF Radar coverage, we see a counterclockwise circulation pattern, with a strong jet towards the coast in the region of Dauphin Island. On the eastern side of the coverage we see the currents flowing to the northeast and turning to the east in the alongshore direction. Where they overlap, the black arrows of the SABGOM surface current forecast are in very good agreement with the HF Radar currents.
So lets use our combined observation and modeling database to look at transport pathways. I have drawn three paths as red lines on the image. We start with the most western red line that heads from the oil slick in towards Dauphin Island. Recall from yesterday that the CSTARS satellite imagery clearly identifies the oil slick at the base of this red arrow. From that location, the SABGOM currents flow north, turn northeast, than turn north in a jet that snakes its way in towards Dauphin Island. Along that transport pathway, we see a Horizon Marine Drifter, a small piece of the oil slick that has broken off, and the surface currents from the HF Radar array confirming the direction and strength of this shoreward transport pathway. As an old ocean forecaster, it is amazing for me to see how far our ocean forecast models have come. In oceanography, we witnessed the growth of Ocean Observatories over the last decade. This next decade we will see the explosion in the use of ocean forecast models.
Figure 5: So lets take that same SABGOM forecast model which just amazed us with its capabilities in Figure 4, and look for other transport pathways to the southeast. The most prominent feature is the alongshelf transport pathway that runs midshelf along Florida to the southeast. The tops of the U’shaped oil slick both join this midshelf jet near 86W. With the Loop Current Eddy pulling away from the spill site, this may be a transport pathway of growing importance for Mote’s upcoming glider deployments.
Figure 6: Turning the SABGOM sea surface height back on, we see that this mid shelf jet is associated with the eastern side of the ridge of slightly higher (light blue) water on the middle to outer Florida shelf. The USF and U.Delaware Slocums gliders are in excellent positions to investigate the northern and southern sections of this jet. Both are heading offshore into this feature. The Mote glider deployment planned for this coming week will be deployed in between the two Slocums, giving us an excellent assimilation and validation dataset for this critical feature.
Figure 7: Here we take Figure 6 with the SABGOM forecast, and add the HyCOM surface currents as the white arrows. Comparing the black arrows of the SABGOM forecast and the white arrows of HyCOM, we see many areas of agreement. This is important. A lot of time, effort and money is spent on the glider deployments, and we want as much knowledge of the environment as possible before we deploy. We can see the midshelf jet is present in both. We can also look at the outer shelf and see that many of the smaller scale interaction zones between deepwater and the shelf are also in good agreement. For example, the interaction between the eastern side of the Loop Current Eddy and the Florida shelf is observed in both models. The details vary, just as with an ensemble of weather forecast models, but the major features are reproduced in both.
Figure 8: So lets zoom in and produce one more image for the mission planners at Mote. The USF Slocum is heading southwest across the midshelf jet towards a small clockwise eddy that is interacting with the shelf break with significant cross-shelf break flows. The U.Delaware Slocum is heading northwest across the midshelf jet and towards another clockwise eddy interacting with the shelf break and causing the strong cross-shelf break flows. The Mote Slocum deployments are being planned for somewhere in the middle, where the flow at midshelf is alongshelf to the southeast in both models and in the HF Radar current fields (green arrows). In this central region, the flow along the outer shelf also is alongshelf to the northeast. The spacing of the three gliders is about as good as you can do with three.