DeepWater Horizon Oil Spill

Gliders continue their surveys.  The four gliders off Louisiana and Mississippi continue their surveys off the shelf.  Meanwhile the four gliders offshore Florida continue to their coordinated continental shelf survey.

The Spray glider from Scripps/WHOI continues to make good progress.  It has reached further offshore which continues to reveal a consistent story.  The ~ 100 m water depth, reveals the subsurface salinity maximum and that combined with the temperature stratification have resulted in a strongly stratified system.  The low salinity buoyant plumes on the shelf are particularly dramatic.

The NAVO gliders make good progress.  The NAVO glider 135 has swung inshore and has reached the edge of the shelf.  The 135 glider has once again encountered the large optical backscatter feature at depth as in early last weekend.  The feature, the identity of it remains an open question, spans 200 meters at depth.  Of all the glider data so far during this response, this remains the largest scientific mystery.  The backscatter complexity does not mirror the CDOM variability which shows a general increase with depth or with the phytoplankton which shows a subsurface maximum at ~100m.  The NAVO glider 137 shows similar patterns in CDOM and chlorophyll however does not show the bright subsurface optical backscatter maximum.  This glider is a little further offshore and the most recent vertical casts suggest the we are reaching edge of the backscatter feature. For both gliders there does appear to be an increase depth of the subsurface chlorophyll maximum at depth.

The iRobot/APL Seaglider is making good progress. the strongly stratified system is still characterized by the highly variable CDOM fluorescence at depth.  The CDOM variability is below the subsurface chlorophyll maximum which at the start of the weekend showed a discontinuity with a displacement of the chlorophyll maximum to the surface.  Over the weekend this layer has deepened to be consistent with the earlier measurements.

The Webb gliders along Florida continue “their dance” as quoted by Prof. Oliver on NPR on Friday.  The USF BASS glider is making very good progress in the northern Gulf. The optical backscatter and the chlorophyll fluorescence show a detachment bottom boundary of the particle/phytoplankton maximum at the shelf edge. The detachment appears to be related to the presence of the high salinity water at depth. While the CDOM variability is present, it is not nearly as dramatic as observed in the optical backscatter and fluorescence data.

Rutgers RU21 glider after the great Mote repair last week is flying like a champ.  Given the improved flying skill it has been directed towards the offshore and so it is now moving the continental slope. particle and chlorophyll fluorescence is highest at depth.  This mirrors the CDOM variability.  Recent profiles in the last day, suggest the Rutgers glider has transitioned to the offshore condition where now the particle maximum is at ~50-60 m water depth and not associated with the bottom boundary layer.  We will have to check whether this layer deepens as the glider heads further offshore to be more consistent with the other glider in deepwaters in the Gulf.

RU23 is making good progress as it heads inshore.  It has re-entered the shelf system characterized with a two layer system with enhanced CDOM, backscatter, and chlorophyll fluorescence associated with deepwater at depth.  With luck this glider will be recovered later this week as the batteries are running low.

UDel is making great speed, and is banging through way-points extremely quickly.  The University of Delaware’s mascot is the Blue Hen, which are known for striking speed in the farmyard, and this glider is living up to its namesake.  Features generally are similar to BASS, RU21, and RU23 with enhanced particles/CDOM/chlorophyll in the bottom waters.  One extremely interesting feature that has begun to emerge in the UDel glider’s data is the regular variability in CDOM fluorescence in the surface waters.  In the surface waters, the CDOM fluorescence shows a repeated pattern with what appears to be a diel variability (outlined in the red box).  Whether this CDOM diel variability reflects local production due to phytoplankton metabolism, a bias to an increase in ambient sunlight, or if it reflects enhanced degradation during the daylight hours when radiant energy is high remains a feature which we will have to be examine over the next week with the team.

Figure 1. The big change we see here is in the NOAA oil spill forecast.  The lower concentration slick that was being wrapped around the eastern side of the Loop Current Eddy has followed the path of the many drifters and has also completed the loop around the eddy. Isolated slicks are now found all the way around the large Loop Current Eddy identified here by the Navy and the Horizon Marine drifter tracks. The higher concentration slick in the northern Gulf continues to move toward shore and spread to the east.

Figure 2: Here is the same view with the HyCOM model forecast for today showing the sea surface height in color fill and the surface currents as white arrows.  The red high of the separated Loop Current Eddy is the dominant feature, well defined by the drifters and the altimetry.

Figure 3.  Zooming into the interaction zone between the Loop Current and the Loop Current Eddy, we see that some of the black line indicating the outer edge of the oil slick forecast is crossing out of the Loop Current Eddy and starting to approach the edge of the Loop Current itself.  This will be an area to watch.  If the slick keeps moving into the eastward flows of the Loop Current, it heads back towards the Florida shelf.

Figure 4.  Three gliders are deployed on the Florida Shelf in this region in case that occurs.  RU23 and UDel134 are heading out, and RU21 is head in right in between the other two.

Figure 5. Continuing the lap around the Gulf, we zoom into the high concentration slick in the north.  The HF Radar 25-hour average currents (green arrows) over the oil slick are trending shoreward and east, just as the oil slick in the forecast is moving shoreward and spreading eastward.  Outside the oil slick on the eastern side of the HF Radar coverage, the 25-hour average currents are trending westward.  The black line indicating the edge of the oil slick in this region is right in the convergence zone between the eastward flowing and westward flowing nearshore surface currents.