Prior to the incursion of “Superstorm Sandy” on the East Coast of the United States in October 2012, another hurricane – Isaac – captured headlines and posed a serious danger to the northern coast of the Gulf of Mexico in September. Isaac threatened the same region that had been devastated by Hurricane Katrina in 2005, exactly seven years earlier. Isaac made landfall as a Category 1 hurricane almost exactly seven years after Category 3 Katrina. Isaac actually made two official landfalls – first on Plaquemines Parish, the arm of land that extends southward from New Orleans as part of the Mississippi River delta, on August 28, and then another landfall on the southern Gulf Coast of Louisiana on the morning of August 29.
Part 2: The tale of Isaac’s tail in the Gulf of Mexico
All of the rainwater that Isaac deposited during its stormy journey over the Gulf Coast states and parts of the Midwest had to go somewhere – and most of that water entered the Mississippi River–Atchafalaya River drainage area of Louisiana, or drained back into the Gulf along the coast of Alabama and Mississippi. While the Mississippi River still carries the majority (~70%) of the water, the Atchafalaya River is the main ‘distributary’ of the river, separating from the Mississippi north of Baton Rouge and carrying a significant volume of water that enters the Gulf of Mexico west of the Mississippi River delta. In fact, the Atchafalaya is large enough to have its own delta region. The winds of Isaac stirred up shallow marine sediments along the Gulf coast, and the rivers delivered a massive flow of water, sediment, debris, and a large amount of organic matter into the Gulf.
In the Gulf itself, the circulation pattern of currents was somewhat unusual. Two rotating areas of water (called gyres) were located south of the Mississippi River delta. (The core of the Gulf of Mexico’s dominant current system, the Loop Current, was located much further south than normal, and was oriented southeastward). This situation enabled the formation of a fairly strong temporary surface current, flowing almost directly south, between the two gyres. (This same type of interaction in the atmosphere between weather systems can cause very windy days.) Maps of the circulation derived from measurements of the sea surface height (provided by the National Oceanic and Atmospheric Administration (NOAA) CoastWatch program), show this circulation pattern clearly. Figure 6 (a-d) shows the circulation pattern on September 13, 14, 15, and 16 (click on any of these images to see them larger). Figure 7 shows the circulation on September 14, with a large gray arrow indicating the direction of current flow between the two gyres. The currents shown, called geostrophic currents, are not actually measured or observed currents; they are calculated from the sea surface height data. Thus, the lighter current flow indicated is not necessarily real. However, a strong and persistent gyre will generally cause the establishment of a geostrophic current flow.
. Geostrophic current patterns in the Gulf of Mexico, September 13-16, 2012, derived from sea surface height measurements. Colored lines are tracks of drifting buoys. (a) September 14. (b) September 15. (c) September 16. (d) September 17. Maps were acquired from http://www.aoml.noaa.gov/phod/dhos/altimetry.php
. (Click on any image to view it larger.)
One question that arises from the images of the geostrophic currents is whether or not the water is actually moving in the direction indicated by the arrows, because winds – particularly strong winds from storms or weather fronts – will also cause the formation of surface currents. Figure 8 is a surface temperature and wind animation created using Modern Era Retrospective-analysis for Research and Applications (MERRA) data, showing how the winds varied with Isaac’s passage and subsequently. A weather front that influenced the area starting on September 9 actually reversed the predominant wind direction over the Gulf. So to determine the cumulative influence of wind and surface currents, the drifter movements can be examined.
Figure 9 shows an animation of the daily geostrophic current images. The drifting buoy tracks are shown with a length of 15 days, starting 15 days before the image (a yellow square marks the initial buoy position) and ending on the day of the image. Thus, the ‘free’ end of the track is the buoy position corresponding to the day of the image. The tracks for the buoys will change color at times. Watching the buoy movement shows that the current established between the two gyres was a strong influence, though the winds also affected the surface water movement, especially where the currents were weaker, as is the case near the Mississippi River delta. One of the buoys near the delta makes several tight loops, indicating the uncertainty of the current flow there. However, when the buoy in the central Gulf interacts with the current flow between the two gyres starting on September 8, it moves very quickly, indicating the strength of this current feature.
Figure 7. Map of surface currents derived from sea surface height measurements on September 14, 2012. The direction of current flow between the western gyre (orange) and the eastern gyre (blue-green) is shown with the gray arrow. The blue, red, magenta, and black colored lines are the tracks of drifting buoys used to monitor surface ocean currents. Note in this figure that the warm Loop Current (orange) which flows from the Caribbean Sea into the Gulf of Mexico between the Yucatan Peninsula and Cuba was in a far southern position and oriented to the east, rather than extending into the central Gulf. Thus, the circulation influenced by the two gyre systems dominated the movement of surface water. (Click the image to view it larger.)
Figure 8. Animation of surface temperature and wind data from MERRA. The wind circulation of Isaac is clearly seen at the beginning of the animation. A reversal of wind direction over the Gulf of Mexico due to a weather front can be observed near the end of the animation. (Click the image to view it larger.) Alternate versions of this animation are provided to allow pausing so that individual frames of the animation can be examined:
Figure 9. Animation of daily geostrophic current, altimeter data, and drifter buoy tracks for the Gulf of Mexico, August 30 – September 14, 2012. Note in particular the magenta track and the brown (changing to black on September 6) track south of the Mississippi River delta, and the red (changing to blue on September 6) track in the central Gulf. Alternate versions of this animation are provided to allow pausing so that individual frames of the animation can be examined:
Now that the current systems in the Gulf at the time of Isaac’s passage have been examined, the next topic is the influence of Isaac on the “color” of the Gulf waters. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite provides many different data products for Earth scientists, including data products for the atmosphere, the land, and the ocean. The MODIS-Aqua ocean data products are sea surface temperature (SST) and many different variables classified as “ocean color radiometry” data products, or more simply, “ocean color”. The Giovanni data tool allows exploration and simple analysis of these data products, which are produced by the Ocean Biology Processing Group (OBPG) at NASA Goddard Space Flight Center. For events such as Isaac, ocean color data products can sometimes detect the movement of flood waters in the ocean based on their unique optical characteristics.
Thus, as the storm surge waters and flooding river waters caused by Isaac drained into the Gulf of Mexico, some of this runoff was captured by this unusual circulation pattern and transported rapidly south, towards Mexico’s Yucatan Peninsula. The most commonly used data product, chlorophyll a concentration (chl a) indicates the movement of the waters stirred and muddied by Isaac. The images shown here are 8-day averages, meaning that all the data collected for an 8-day period is collected and averaged to produce the image. This process helps improve coverage of a region by reducing the impact of clouds, while providing improved resolution of rapidly-changing ocean features. For the period August 28-September 4, immediately following the passage of Isaac, an area of higher chl a is observed just south of Louisiana (Figure 10a). The next 8-day interval, September 5-12, captures the movement of the disturbed water southward (Figure 10b). The following period, September 13-20, shows a larger area of elevated chl a south of Louisiana (Figure 10c). Circles indicate each of the areas of interest in the figures.
Figure 10. 8-day average images of chl a in the Gulf of Mexico. Features related to Hurricane Isaac which are discussed in the text are indicated with black circles. (a) August 28 – September 4, 2012. (b) September 4-12, 2012. (c) September 13-20, 2012. (Click on any image to view it larger.)
Chl a is a basic data product used by oceanographers, but in turbid waters such as those created by Isaac, it can inaccurately indicate higher chlorophyll concentrations from phytoplankton, because colored dissolved organic matter (CDOM) also absorbs some of the light that is absorbed by chlorophyll. The algorithms used to calculate chl a depend on precise measurements by MODIS of the intensity of light radiating from the ocean surface, which has been modified by absorption, scattering, or reflection off substances and particles in the water. So chl a does not provide all the information necessary to determine the actual cause of the altered optical properties of the water in this case. In clear waters where phytoplankton are the primary factor affecting the optical properties of seawater, chl a generally provides accurate estimates of phytoplankton chlorophyll concentration.
There are other data products allow an even more refined characterization of this event in the Gulf of Mexico. Based on the wavelengths where substances absorb light most efficiently, they are the absorption coefficient of dissolved and detrital matter (adg), the absorption coefficient of phytoplankton (aph), and the backscatter coefficient (bbp). These data products are calculated from the same observations of light intensity that MODIS acquired for the calculation of chl a and Rrs. Essentially, adg indicates light absorption by non-living organic matter, aph indicates light absorption by phytoplankton, and bbp indicates reflection from suspended particles, usually sediments.
Figure 11 (a-c) displays images of adg for the three 8-day periods shown previously. However, the images of aph (Figure 12 a-c) do not show the same features. The difference between the images is an indication that the main substance in the water affecting the optical properties was CDOM, not phytoplankton. Examination of bbp images (not shown here) indicated little evidence that suspended sediments were carried very far from the Gulf coast.
Figure 11. 8-day average images of adg in the Gulf of Mexico. (a) August 28 – September 4, 2012. (b) September 4-12, 2012. (c) September 13-20, 2012. (Click on any image to view it larger.)
Figure 12. 8-day average images of aph in the Gulf of Mexico. (a) August 28 – September 4, 2012. (b) September 4-12, 2012. (c) September 13-20, 2012. Note the absence of the features which can be seen in the adg images in Figure 11. (Click on any image to view it larger.)
These images provide one more interesting facet of Isaac’s impact on the Gulf coast and the Gulf of Mexico, beyond identifying the main factor causing the optical “tail” of Isaac in the normally clear waters of the Gulf. In the first two 8-day periods following the passage of Isaac, coastal water with entrained organic matter from disturbed coastal sediments appears to have been immediately captured by the southward-flowing current, which carried this discolored water toward the Yucatan Peninsula. But in the third period (which began approximately two weeks after Isaac made landfall) the large area of discolored water south of Louisiana is more likely due the increased volume of flow in the Mississippi and Atchafalaya rivers reaching the Gulf of Mexico, as the storm-stirred coastal waters would have dispersed by this time.
The ocean color data acquired by MODIS, processed by the OBPG, and visualized with Giovanni can thus provide important information on processes affecting the optical properties of surface ocean waters, the changing populations of phytoplankton, and the influence of storms like Isaac. These factors are important to a basic understanding of the physical and biological dynamics of the ocean, as well as factors important to humans, such as water quality and seafood safety (as fish and shellfish can be contaminated by toxic phytoplankton blooms).
Because of the unusual formation of the currents in the Gulf of Mexico when this event occurred – especially with the core of the Loop Current located much more eastward than normal – river water potentially laden with excess nutrient concentrations and pollutants was carried near the Mexican coastal zone.
Furthermore, the northern Gulf waters adjacent to the Mississippi River delta also harbor one of the largest ‘creeping dead zones’ in the world ocean, an area of the seafloor almost devoid of oxygen due the digestive action of bacteria on organic matter sinking to the ocean bottom. Efforts to reduce the size and spread of this area have influenced agricultural practices in the Midwest. The ongoing multi-year drought, however – which has caused the Mississippi River to become shallower and shallower (affecting commercial barge traffic upriver) – means that one of the factors contributing to the persistence of the seafloor dead zone has been diminishing for several years.
NASA satellite and model data on rainfall, hydrology, and the optical properties of the ocean surface thus contribute to a better understanding of the interaction of land and ocean, and the influence of and effects on humans, in the Earth system. As the environment of the Earth changes, it is important to maintain these data records so that natural and altered ecosystems can be studied in detail. Maintaining such data, and providing it for use in research, is the mission of the NASA GES DISC and the other Earth science data centers managed by NASA.
Article by James Acker (Adnet Inc. / GES DISC). Figure 8 created by Andrey Savtchenko. Technical review comments gratefully received from Eurico D'Sa (Louisiana State University), Mitchell Roffer (Roffer's Ocean Fish Forecasting Service), David Mocko (SAIC/GSFC), and Andrey Savtchenko (Adnet Inc. / GES DISC).
Questions or comments? Email the NASA GES DISC Help Desk: firstname.lastname@example.org
Funding for support of ocean color radiometry data in Giovanni is provided through the "Water Quality for Coastal and Inland Waters Project," NNX09AV57G, from the National Aeronautics and Space Administration (NASA). Zhongping Lee of the University of Massachusetts – Boston is the Principal Investigator on this project.
Go to Part 1: The tale of Isaac's tail in the Midwest >