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Multisensor observations of the Amazon-Orinoco river plume interactions with hurricanes

posted Dec 2, 2014, 7:00 AM by Salinity CERSAT

Figure 1:  The number of 1950 through 2010 “best track” TC per one degree square (smoothed by a 3° x 3° block average) (a) that evolve as category 4-5 somewhere along their path and (b) that intensified locally to category 4-5. The black curve is showing the historical extent of the Amazon-Orinoco river plume during the hurricane peak season (August to October).

An analysis has been presented in Reul et al. (2014) for the spatial and intensity distributions of North Atlantic extreme atmospheric events crossing the buoyant Amazon-Orinoco freshwater plume.

1)- we show that the cat 4-5 hurricanes locally intensify more systematically at the northern reach of the Amazon-Orinoco plume (see Figure 1 above) during their westward transport within the North Atlantic Hurricane main development region.

Figure 2: Mean sea surface cooling amplitude in the wake of North-Atlantic Hurricane as function of the Saffir-Simpson Wind scale with error bars showing the 90% and 95% significance levels for errors in the mean. Responses estimated over  open water waters (red dots) are distinguished from those evaluated within the historical plume region (blue dots). (a) Slow moving (or low latitude) tropical cyclones with V/f <1 (b) fast moving (or high-latitude) storms with V/f >1.

2)- for the first time, we evidence from observations (satellite sst & in situ SSS) that there is a systematic SST cooling reduction in the wake of hurricanes traveling across the Amazon-Orinoco plume compared to hurricanes of identical intensity and, similar effective translation speed, but traveling in surrounding open-ocean waters (see Figure 2). These results are of interest for better forecasting hurricane intensification (Emmanuel, 1995) in the MDR region. Similar results were evidenced in Balaguru et al. 2012 but relied mainly on oceanic model simulations.

3)-that the ocean subsurface properties, deduced from historical observations, reveal a maximum in the subsurface salinity-driven density stratification at the base of the northern tip of the river plume, around 18°N. We also show that this longitude below the plume  is coinciding with the location where the most intense Cape-Verde hurricane are most likely to travel when they develop toward the west.

3)-As already demonstrated in Grodsky et al. 2012 for the case of hurricane Katia in 2011, we further confirm on another hurricane case (Igor) that satellite SSS measurements provide a reliable measure of the ocean surface salinity response to hurricanes in such strong SSS frontal zone, with high spatio-temporal variability.

Figure 3: Surface wakes of Hurricane Igor.  Post minus Pre-hurricane (a) Sea Surface Temperature (ΔSST ) (b) Sea surface Salinity (ΔSSS).The thick and thin curves are showing the hurricane eye track and the locii of maximum winds, respectively.  The dotted lines is showing the pre-hurricane plume extent. ΔSST and ΔSSS wakes were only evaluated at spatial locations around the eye track for which the wind exceeded 34 knots during the passing of the hurricane.  

We evidenced the detection of a 89000 km2 upwelling of cooler and saltier subsurface water inducing a 1-1.5 psu SSS change after IGOR passage, very coherent with in situ Argo observations. The satellite SSS observation over the plume therefore present a clear potential to complement SST and ocean color observations of the upper ocean response to TCs in that region.

4) -from synergistic use of SMOS, SST and Argo float observations, we show that very small SSS & SST changes are found in the wake of a Cat 4 hurricane traveling over the plume when a thick Bariier Layer (BL) and strong vertical stratification is observed at its base, even if the atmospheric forcing is locally very intense.

5) - for the first time we show from historical in situ observation that the Sea surface salinity in the plume waters is an excellent proxy of the stratification strength at depth below the plume waters -and we provide an empirical law relating SSS to the brunt-vaissala frequency maximum over depth for the plume waters, itself a proxy for the upper ocean stratification strength at the base of the plume.  This simple observation relates an horizontal plume property (its salinity) at the surface to the vertical structure of the plume density, a key variable for better monitoring the local ocean-atmosphere interaction processes (e.g., through the Richardson number).

6) -the results of our statistical analysis conducted over 2008-2010 and demonstrating the dependencies of the SST changes in the wake of storms passing, or not, over the river plume, and first conducted using an in situ-based SSS climatology to determine the location of the latter, is also confirmed when using satellite SSS observations from SMOS over the period 2010-2012. Indeed, Reul et al. 2010 & 2014, Salisbury 2011, Grodsky et al. 2012,  Coles et al. 2013, Tzortzi et al. 2013, Grodski et al. 2014 already clearly demonstrated that satellite SSS is certainly more accurate to determine the actual, very variable, spatial extent of the Amazon river plume relative to an SSS climatology (e.g WOA or Levitus). Despite a little amount of data yet collected to derive very robust statistics (our SMOS statistics based on 40 storms collected over 2010-2012 can not be fully robust given the low number of samples), the results shown by the use of limited SMOS data tend however to give confidence to the historical results deduced from an SSS climatology but also for a much larger ensemble of TCs.

SMOS SSS data can thus be used to consistently anticipate the cooling inhibition in the wake of TCs traveling over the Amazon-Orinoco plume region.

Reul, N., Y. Quilfen, B. Chapron, S. Fournier, V. Kudryavtsev, and R. Sabia (2014), Multisensor observations of the Amazon-Orinoco river plume interactions with hurricanes, J. Geophys. Res. Oceans, 119, doi:10.1002/2014JC010107.