Figure 1: (a) AQUARIUS and (d) SMOS SSS before hurricane Katia. Crosses are the hurricane daily position at 00:00UTC with size scaled between 20kt and 120kt of maximum sustained winds (from NHC analysis). (b, e) SSS differences after (5–10 SEP/2011) minus before (25AUG–1SEP/2011) the hurricane passage. 35 psu contour before the passage of Katia is overlain. The differences are color-scaled between −2 and 2. Land-contaminated coastal data are blanked.
At its seasonal peak the Amazon/Orinoco plume covers a region of 106 km2 in the western tropical Atlantic with more than 1 m of extra freshwater, creating a near-surface barrier layer (BL) that inhibits mixing and warms the sea surface temperature (SST) to >29°C. Here new sea surface salinity (SSS) observations from the Aquarius/SACD and SMOS satellites help elucidate the ocean response to hurricane Katia, which crossed the plume in early fall, 2011. As illustrated in the above figures, its passage left a 1.5 psu high haline wake covering >105 km2 (in its impact on density, the equivalent of a 3.5°C cooling) due to mixing of the shallow BL. Destruction of this BL apparently decreased SST cooling in the plume, and thus preserved higher SST and evaporation than outside. Combined with SST, the new satellite SSS data provide a new and better tool to monitor the plume extent and quantify tropical cyclone upper ocean responses with important implications for forecasting.
The strong SSS increase in hurricane wake within the plume is explained by an erosion of the BL. This is evident by Argo profiles collected within the plume (see Figs below) that indicate the presence of shallow, about 15 m deep mixed layer overlying the halocline. Mixed layer salinity is lower by 2 to 4 psu than the water beneath. This shallow haline stratification is destroyed by hurricane-forced entrainment (mixed layer deepening and upwelling), which is stronger on the right side of hurricane eye. Although the hurricane strengthened further along the trajectory, the SSS change is much weaker there corresponding to weak vertical salinity stratification outside the plume.
On the left side of the trajectory there is an area of SSS decrease (Figure 4) sampled by Argo #4 and #5. In contrast to the increase in SSS within the plume where the BL is eroded, the surface (down to 30 m) adjacent to the northwestern corner of the plume is 1 psu fresher after the passage of hurricane. The decrease in salinity implies an addition of 1 m of freshwater, much larger than could have come from direct rainfall. The most likely explanation is freshwater advection from the plume with some additional contribution due to direct rainfall.
Magnitude of satellite SSS increase in the haline wake (about 1.5 -2 psu) agrees well with the Argo observations. SSS changes observed by Aquarius and SMOS qualitatively well agree as well suggesting that satellite sensing of SSS is a mature technique for strong signals >1 psu.
More detail can be found in our recent published paper here:
Grodsky, S. A., N. Reul, G. Lagerloef, G. Reverdin, J. A. Carton, B. Chapron, Y. Quilfen, V. N. Kudryavtsev, and H.-Y. Kao (2012), Haline hurricane wake in the Amazon/Orinoco plume: AQUARIUS/SACD and SMOS observations, Geophys. Res. Lett., 39, L20603, doi:10.1029/2012GL053335. http://www.agu.org/pubs/crossref/2012/2012GL053335.shtml
Note as well our dedicated web site related to the ESA/STSE SMOS+Stotm fstudy ocussing on the use of SMOS data for Storm monitoring: