Figure 1: Cut and Paste from the NOAA National Hurricane Center early morning update on Bonnie:
THE REMNANT LOW OF BONNIE…CENTERED ABOUT 25 MILES EAST-SOUTHEAST
OF GRAND ISLE LOUISIANA…CONTINUES TO PRODUCE DISORGANIZED SHOWERS AND A FEW THUNDERSTORMS. RE-DEVELOPMENT OF THE SYSTEM IS NOT EXPECTED BEFORE THE LOW REACHES THE SOUTH CENTRAL LOUISIANA COAST EARLY THIS MORNING. THERE IS A LOW CHANCE…NEAR 0 PERCENT…OF THIS SYSTEM BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HOURS.
Figure 2: Oceanweather’s compilation of ship and offshore platform weather report’s has strong winds towards the coast of Mississippi and Alabama on the back side of Bonnie’s Low.
Figure 3: Wave forecast from Oceanweather has the largest waves on the right side of the Bonnie’s track, just off the coast of Mississippi & Alabama. Just like the textbook says.
Figure 4: IOOS Overview showing the official NOAA track (in red) for Bonnie and this morning’s location for the Low (the red L) just at the edge of the Louisiana coast. The track passed very close the drill site, within 5 nautical miles by my google earth measurement. The current cloud coverage shows how Bonnie has dissipated. Probably the most striking observation are the strong currents in the HF radar (normally green arrows all red) off the coast of Mississippi and Alabama where we saw the strong winds and waves in Figures 2 & 3. To better compare these currents with the SABGOM forecast later in this report, we switch to the hourly currents in this image. This is much easier to do when the wind driven currents are strong and are much larger than the tides. Under normal conditions when the wind driven currents are similar in size to the tides, we almost always have to look at the 25 hour average currents to remove the tides for comparisons to the model.
Figure 5: First we turn on the HyCOM model to look at the overall view of the Gulf, the Loop Current and the Loop Current Eddy. Here we plot the sea surface height (high is red, low is blue) and the surface currents (white arrows). The red colors indicate the areas where the warm surface layer of the upper ocean is thickest. The ocean’s thick warm layers of the fuel for the tropical storms. Bonnie’s track is northeast of the thick layer of warm water associated with the Loop Current Eddy.
Figure 6: First we zoom into the Florida shelf and switch to the nested high resolution SABGOM model. We see a collection of assets, the Spray 40, Seaglider 135, a Horizon Marine drifter and Navy drifter heading from deep water straight at the Florida shelf. The USF Slocum has turned south along the outer shelf towards this area.
Figure 7: Moving north to the coast of Louisiana, Mississippi & Alabama, and again switching to the high-resolution SABGOM model to compare the forecast coastal currents with the hourly HF Radar observed currents. North of the drill site and on the continental shelf, we see the strongest HF radar currents in red. Both the observed HF Radar currents and the SABGOM forecast currents are heading north across the entire shelf towards the coast of Mississippi and Alabama. In the deeper water of the canyon to the northeast of the drill site, both the HF Radar and the forecast have the currents heading to the northeast, up the canyon. On the shelf to the east of the drill site, and east of the canyon, we see a clockwise circulating eddy on the outer shelf, with the HF radar seeing the northern half of the clockwise circulation. Moving nearshore, to the far northeast corner of the HF Radar coverage, we see a current reversal, with currents heading due east, opposite most of the other currents in the field. An initial thought may be bad data. But when you look at the forecast model, we see a strong coastal current running north and east along the Florida coast, and the far western edge of this coastal current is just inside the HF Radar coverage. This is an interesting point, since as an oceanographer, when you get a current observation, you first try to explain it in terms of the forcing to determine if it is believable. If we only look at the windfield, the first guess would be that the currents cannot possibly be in the direction opposite to the wind. Perhaps they are wrong. But then you look at the SABGOM model, and there are more forcing functions than just the winds. SABGOM says a coastal current has developed in this region with flow to the east, we see flow to the east in the observations, and suddenly we are very happy with our sensors.
I know I’ve said this many times in this blog, but once again, I am amazed by our community’s growing ability to forecast the ocean. Perhaps it comes from being in the forecasting business for 30 years and seeing the progression. It may also come from the perspective of the ensemble forecast. These are common in weather forecasting, like the many tracks of Tropical Storm Bonnie in yesterday’s forecast, but much less common in ocean forecasting. Often we get one ocean forecast, we compare it with one current meter at a fixed location, we see they don’t agree but don’t know why, and we conclude the models don’t work. But one of the most important things to forecast in the ocean is where is the water going. We do this with the U.S. Coast Guard to compare various methods of forecasting the dispersive cloud of an ensemble of drifters. The drifter cloud dispersion does not depend on a single forecast current value being exactly correct. It integrates the effects over many current values along a drift path. It tells you whether the general circulation pattern is correct or not, and if something that is in the water, be it oil or people, are where you expect them to be.