Monday, November 11, 2013

The Mississippi River is the largest river in North America, draining ~41% of the contiguous United States. More than half of the freshwater input into the Gulf of Mexico (GoM) comes from the Mississippi River. River discharge in the northern GoM shows significant seasonal variability, with maximum flow around spring (March-June) and minimum flow in fall (September-November) (Figure 1). Mississippi River discharge has implications to the general ocean circulation and ecology of the GoM.

  Figure 1.  Time series of USGS daily mean discharge (x10^3 m^3/s) from May – November 2011 for the Mississippi River at the Baton Rouge, Louisiana gauging station (7374000 located at 30°26'44.4"N, 91°11'29.6"W).  The red line represents the annual mean discharge of ~13,000 m^3/s.

May 2011 was a record-breaking flood year in the central United States, comparable to major flood events in 1927 and 1993. Widespread flooding was attributed to excessive precipitation from major storm systems in combination with extensive snowmelt. The combination of the two caused the Mississippi River and many of its tributaries to swell to record levels, devastating the states of Illinois, Missouri, Tennessee, Arkansas, Mississippi, and Louisiana and their inhabitants. At Baton Rouge, Louisiana, the USGS gauging station (7374000 at 30°26'44.4"N, 91°11'29.6"W) observed maximum flooding of the Mississippi River on 18-19 May 2011, with an approximate peak discharge of 40,000 m^3/s into the GoM (Figure 1). Here we demonstrate the applicability of Aquarius data to observe SSS freshening associated with the extreme Mississippi River flooding event in 2011. This is the first successful demonstration of the utility of Aquarius data to study the salinity variation in marginal seas, regions that are challenging for the application of satellite-derived SSS measurements, in peer-reviewed literature.
Aquarius data began in late August 2011 and captured part of this discharge event in the GoM. Aquarius observed low SSS values (< 34 psu) in the northeastern GoM from August-September 2011 (3-4 months after peak discharge), with peak freshening and maximum intrusion in the open GoM in August 2011 (Figures 2a-b). By October 2011 (5 months after peak discharge), the effect of the discharge was no longer visible in the Aquarius data (Figure 2c).


  Figure 2.  7-day running averaged Aquarius SSS (psu) (a) 28 August 2011, (b) 26 September 2011, and (c) 26 October 2011 in the GoM. The time mean spatial average of Aquarius SSS was adjusted in (a-c) for improved visualization of the spreading of the freshwater plume. Black vectors superimposed on (a-c) are 7-day running averaged OSCAR ocean surface currents. Red lines indicate the propagation of the river plume from the mouth of the Mississippi River.
Seasonal winds and general circulation variability in the GoM impacted the observed salinity response to the record 2011 Mississippi River flooding event. Southerly/southeasterly winds were prevalent, as is typical during spring and summer months when Mississippi River discharge is maximal, driving an eastward current in the northern GoM that transported freshwater over the narrow Missisippi-Alabama-Florida shelf from August-September 2011. The narrower shelf and deeper waters near the shelf edge permitted the coastally confined river plume to interact with the general circulation of the GoM. In the GoM, the ocean circulation is dominated by the Loop Current (LC) and its associated warm-core (anticyclonic or clockwise-rotating) and cold-core (cyclonic or counterclockwise-rotating) eddies (vectors in Figure 2). The position of the LC fluctuates within the GoM, but its most northward position occurs prior to shedding a large warm-core eddy, which happens every 3 to 17 months.  From August-September 2011 (3-4 months after peak discharge), the LC and its associated warm-core eddy caused freshwater to spread into the GoM. By October 2011, the low SSS values (< 34 psu) observed in the open GoM had been transported out of the GoM by the LC, and likely continued via the Florida Straits or possibly even as far as the Gulf Stream. At this time, changes to SSS values along the coasts were likely due to a shift in the prevailing wind pattern from southerly/southeasterly during spring/summer to northerly/northeasterly during fall/winter. The response could additionally be associated with wind-driven mixing that weakened the low salinity signal at the ocean surface.





PO.DAAC Science Team,
Jet Propulsion Laboratory, Pasadena, Calif.