Uncommon rainfall and circulation patterns sent some of last year's raging Mississippi River flood waters all the way to the Florida Keys and the east coast.
Nan D. Walker and Lawrence J. Rouse Jr., Coastal Studies Institute, Louisiana State University, Baton Rouge; Giulietta S. Fargion, Texas A&M University, Galveston; and Douglas C. Biggs, exas A&M University, College Station.
In the summer of 1993, the Mississippi River basin in the midwestern United States experienced anomalously high rainfall. Record flooding resulted from an abnormally persistent atmospheric weather pattern consisting of a quasi-stationary jet stream positioned over the central part of the nation; there, moist, unstable air flowing north from the Gulf of Mexico converged with unseasonably cool, dry air moving south from Canada.
In concert with the persistent weather pattern over the United States, highly anomalous circulation patterns were observed over much of the Northern Hemisphere. The heavy rainfall over the central United States produced abnormally high river discharge along the Louisiana coastline from the Mississippi and Atchafalaya Rivers during July and August, traditionally months of low river discharge. Some of the river water discharged into the northern Gulf of Mexico reached the Straits of Florida by September 1993.
The Mississippi River is the sixth largest river in the world in terms of discharge, with an annual average flow rate of 14,000 m/s and a freshwater discharge onto the continental shelf of 580 km/yr. River discharge into the Gulf of Mexico is distinctly seasonal, with highest flow occurring between March and May and lowest flow occurring between August and October. Approximately 70% of the Mississippi River flow enters the northern Gulf of Mexico through the bird-foot delta. The remaining 30% of the flow is discharged into the Gulf of Mexico by the Atchafalaya River through Atchafalaya Bay, further west.
In Figure 1, the monthly mean discharges of 1993 are compared with long-term climatological values of minimum, mean, and maximum discharges, compiled from data collected over the past 63 years. This data analysis reveals that monthly mean discharges were above normal during the first 9 months of 1993. Monthly mean discharges of April and May 1993 were approximately 50% higher than their long-term monthly mean values. August and September 1993 discharges exceeded those of the previous 63 years! Discharge in August 1993 exceeded that of an average month during the annual spring flood.
Fig. 1. Mississippi River discharge for January through September 1993 in comparison with long-term monthly means, maxima, and minima from 1930-1992.
During the 1993 summer, not only was river discharge anomalously high but the direction of flow of river water in the northern Gulf of Mexico was also unusual. Generally, much of the river water discharged into the northern Gulf of Mexico by the Mississippi and Atchafalaya Rivers moves westward along the Louisiana and Texas coasts, driven primarily by the prevailing easterly winds. Freshwater from these rivers has been observed as far as Port Aransas, Tex., 800 km from the Mississippi River delta.
However, during summer 1993, persistent eastward flow of river water was suggested by the orientation of sediment plumes as observed in several clear-sky satellite images of the NOAA advanced very high resolution radiometer (AVHRR). These data were obtained at the Earth Scan Laboratory, Coastal Studies Institute, Louisiana State University.
On August 10, an enormous plume of turbid river water was detected extending eastward from the Mississippi River delta region. River water discharged on the western side of the delta flowed eastward around the south side of the delta, joining additional river water discharged from passes on the south and east sides of the delta. The sediment plume covered 7000 km2, extending 270 km eastward onto the Mississippi, Alabama, and Florida continental shelves to 87°30'W (Figure 2).
Fig. 2. August 10, 1993, NOAA-11 satellite image processed to reveal surface suspended sediment distribution. Yellow and orange hues show highest suspended sediment concentrations, and blue depicts clearest waters. Salinity measurements were collected aboard the research vessel Gyre along lines 1, 2, and 3 from August 6 to 9, 1993. The lowest-salinity plume waters were encountered at A, B, C, and D.
The research vessel Gyre traversed portions of the plume from August 6-9 en route to Panama City, Fla., from Galveston, Tex. On this ship-of-opportunity transit, water was pumped from a depth of 3 m past a conductivity sensor to obtain underway salinity measurements. Salinity data were obtained along a line west of the delta (Figure 2, line 1) and along two lines east of the delta (Figure 2, lines 2 and 3). Several areas of low-salinity surface water were encountered east of the delta at B, C, and D. A minimum salinity value of 15 practical salinity units (psu) was encountered on the western end of line 3 at C, which confirmed the satellite-observed eastern limit of the turbid river plume. Between C and D salinity increased to as much as 30; however, at D, salinity dropped again to 22. Along line 2, surface salinity ranged from 25 near the 1000-m isobath - an isobath is a line on a chart or map that connects points of equal water depth - to 17 psu near the 200-m isobath. The cloud cover east of C prevented tracking of the sediment plume beyond 87°30'W, but the salinity measurements suggested the presence of Mississippi River water as far east as 86°40' W. The shipboard measurements also revealed an extensive area of low-salinity water west of the Mississippi River delta (Figure 2, line 1) with a minimum salinity at A. This water was probably discharged onto the shelf by the Atchafalaya River and driven eastward by the wind.
Wind measurements from Louisiana coastal stations suggest that the eastward flow of river water was at least partially wind-driven. From mid-July through the third week in August, winds along the Louisiana coast were predominantly westerly and southwesterly, which would have driven the buoyant plume eastward. Previous research has demonstrated that shelf currents and plume morphology respond rapidly to wind forcing in this area. Thus the persistence of westerly winds during July and August 1993 effectively reversed the westward flow of Mississippi River water and forced the buoyant plume to the east. Another important factor influencing circulation of Mississippi River water was anticyclonic flow along the northern margin of the Loop Current. The Loop Current, a branch of the Yucatan-Florida Current system, had recently intruded northward in close proximity to the Mississippi delta, introducing eastward flow over the continental slope and outer continental shelf, possibly enhancing the rapid eastward movement of river water.
Hydrographic measurements, obtained from the research vessel Pelican during a Gulf of Mexico survey southeast of the delta in late August, confirmed the presence of anticyclonic transport along the northern margin of the Loop Current, from 90° to 88°30'W between the 100- and 2000-m isobaths. Eastward transports of 4-6 x 106 m3s were estimated from geopotential heights calculated from cruise measurements (Figure 3b) along lines southeast and southwest of the Mississippi River delta. Salinity measurements obtained along the 100-m isobath revealed the river discharge plume to be a thin lens (<5 m thick) of fresher water overlying relatively saline water at 30 m depth. The fraction of freshwater within the top 3 m was computed at each cruise station, revealing a maximum value of 56% east of the delta (see Figure 3b).
Fig. 3. a) Geopotential heights in centimeters and b) surface salinity units (psu) superimposed on the satellite image of August 10, 1993. In a) the direction of transport is indicated. In b) the red dot indicates a freshwater fraction in excess of 50%; the black dot indicates 30-40%, and the yellow dots depict fractions between 20 and 30%. Values less than 20% are not shown.
Moving eastward, the freshwater fraction remained above 20% to 87°30'W and seaward to the 2000-m contour. The values east of the delta were approximately double those obtained in August 1992 in the same region. The freshwater fraction southwest of the delta in 1993 was 35%, considerably lower than the freshwater contribution east of the delta in 1993, but slightly higher than the 29% encountered in August 1992.
Drifting buoys were deployed west of the delta in late July. The drift tracks confirmed the prevalence of eastward surface flow along the continental shelf around the Mississippi River delta from late July through late August. Two of the drifters moved around the south side of the delta and were caught up in the turbid, low-salinity plume extending eastward from the bird-foot delta (as observed on August 10). Both drifters were observed to jog northeastward east of the delta, either due to a wind-induced current or a small anticyclonic eddy impinging on the continental shelf southeast of the delta. (See Figure 4.)
Fig. 4. Tracks of LATEX-A drifting buoy 6935 from July 29 to October 1, 1993. Buoy positions are indicated by black dots.
From August 13 to 19, one drifter moved eastward between the 200- and 1000-m isobaths at a velocity of 50-60 cm/s, then continued southward along the west Florida shelf, reaching the Florida Straits by September 20 (see map). This drifter moved at an average velocity of 39 cm/s from August 13 to October 1, with a velocity range of 15-110 cm/s. Another drifter initially followed a similar path, but it never reached southern Florida. Instead, it moved back toward the Mississippi delta after reaching 26°N around October 1. Another buoy, which was also near the Mississippi delta in early August, reached the Florida Keys in early September, as it traveled in the Loop Current rather than in the plume and along the continental shelf edge.
The drifter tracks help explain observations of unusually low-salinity water in the Florida Key region and on the in-shore edge of the Florida current from September 9 to 13. Low salinity water was also encountered along the U.S. east coast off North Carolina on September 22. The available data suggest that currents favorable for eastward and southward flow of river water in the northeastern Gulf of Mexico were present for at least 6 weeks from mid-July through late August.
The satellite imagery, hydrographic measurements, and drifter data show that the low-salinity water encountered in the northeastern Gulf of Mexico, south Florida region, and along the U.S. east coast during the 1993 summer was of Mississippi River origin. In 1973, another high river discharge year, low-salinity water was also reported along the U.S. east coast and hypothesized to be of Mississippi River origin. Its movement from the northern Gulf of Mexico through the Florida Straits was linked to a northward incursion of the Loop Current.
Northward intrusions of the Loop Current, as occurred in summer 1993, can occur several times each year and thus are not, in and of themselves, unusual occurrences. What was anomalous in 1993 was the simultaneous occurrence of high river discharge onto the Louisiana continental shelf, the abnormal persistence of westerly winds along the Louisiana coastline, and the prevalence of anticyclonic loop currrent transport in close proximity to the Mississippi River delta. These three factors ensured that a detectable amount of river water reached the Florida Keys and the U.S. east coast, entrained on the in-shore margin of the Loop and Florida Currents. This circulation process probably occurs on a smaller scale more often than has been noted. As with many phenomena of nature, much is learned by the investigation of extreme events.
Some of the consequences of the Mississippi River flooding during the 1993 summer were documented. In terms of the coastal ocean, hypoxic conditions - oxygen concentrations below 2 mg/L - were elevated and more widespread than usual along the Louisiana coastline west of the delta. This can be attributed to two main causes. First, the high river discharges introduced abnormal amounts of nutrients during the summer months, fueling phytoplankton growth. Second, the widespread low-salinity plume of river water was rapidly warmed by solar heating, resulting in a very stable water mass on the continental shelf.
The increased phytoplankton biomass and the highly stratified water mass exacerbated hypoxic conditions, which covered approximately double the area that would be expected in summer. The effects of hypoxic conditions on the productive Louisiana fishery has yet to be determined; however, the widespread hypoxia in 1993 did have a negative impact on the benthic community west of the delta. The effects of increased river discharges east of the Mississippi delta are not known; however, hypoxia has not previously been found to be a problem in this area.
Source:Eos, Vol. 75, p. 409, September 6, 1994.
Return to Science and
Society 