Science Focus: The Orinoco River Plume as seen by MODIS

Andrey Savtchenko, NASA Goddard Earth Sciences Data and Information Services Center, Code 610.2,
L-3 Communications Government Service, Inc.

The Orinoco river plume and associated cyclonic eddies, reported in Eos, v85, 20, have indeed been quite evident during the week of August 12, 2003, as can be seen from these images of MODIS/Terra 8-day data products. What is worth noting though, is that the wind field likely played a major role in supplying additional instability and energy for the eddies.

Plate (a) shows a subset image of the MODIS/Terra SST parameter (11 µm retrieval), from the global 4.9-km resolution product, MO04MW, during the week of August 12, 2003. The vectors represent the 10-m winds acquired by SeaWinds instrument on board of QuickScat. The wind field is a three-day aggregate and manifests strong and persistent easterly flow. The plume is excellently revealed in the image of the normalized water-leaving radiance nLw_443, plate (b), by means of the strong absorption that occurs at the 443 nm band.

[Note: MODIS SST data is no longer available with the data filename shown above. MODIS SST data can now be acquired at the link below.]

A fascinating feature is that the winds drag the Essequibo river discharge, which consequently is fed into the Orinoco plume. With so much river discharge and help from the wind, this magnificent plume stretched for thousands of miles.

The nLw_443 image reveals two eddies, at about (14N, -64W) and (15N, -67W). For that period of 8 days, the (15N, -67W) eddy managed to close and entrap a "cleaner" core from ambient more saline water, plate (b). Cyclonic eddies in the northern hemisphere normally are associated with upwelling (Cushman, 1994), and the reported baroclinic profile (Corredor et al., 2004) does show shoaling. However, a close inspection of the SST image does not manifest distinct temperature gradients across the (15N, -67W) eddy, apparently indicating that the upwelling is not strong enough to bring colder waters to the surface. An example of such strong upwelling would be off the north coast of Venezuela. This upwelling, however, is driven by Ekman transport triggered by the strong along-shore easterly winds.

Ocean Color Web :
http://oceancolor.gsfc.nasa.gov

MODIS-Aqua SST data in Giovanni:
http://reason.gsfc.nasa.gov/OPS/Giovanni/ocean.aqua.shtml (allows ordering of MODIS-Aqua SST data from the Ocean Color Web with spatial and temporal criteria from Giovanni)

References:

Corredor, J., et al.: Cyclonic Eddy Entrains Orinoco River Plume in Eastern Caribbean, Eos, 85, 20, 2004, p.197

Cushman, B., Introduction to Geophysical Fluid Dynamics, Prentice Hall, 1994, p.66