Sketch explaining the ocean circulation features in this area. A SMOS 10-days averaged SSS map is shown in the background with vectors representing surface currents derived from altimer and surface wind products (OSCAR).

The Gulf stream is probably the most intensely studied ocean current. Over the past 70 years, many hydrographic sections have been taken accross the stream between Florida straits and the Grand Banks. These sections always reveal strong property gradients aligned with the current, indicating that the stream is a boundary between cold, fresher slope and shelf waters and warm, more saline central waters (Bower and Rossby, 1989). Biologists have also recognized the role of the Gulf stream as a boundray between two environments, each with its own unique set of flora and fauna.

The GulfStream is a turbulent jet that migrates into deeper waters after reaching Cape Hatteras, forming large-amplitude meanders downstream of Cape Hatteras from baroclinic and barotropic instability processes (see sketch above). Large enough individual meanders can separate from the main current, loop back on to themselves and form independent rings (Saunders, 1971; Csanady,1979). Rings that form from Gulf Stream meander crests engulf parcels of warm, salty Sargasso Seawater in their core and begin to interact with their surrounding waters through either mixing or stirring processes(Parker,1971; Churchilletal.,1993)

The common occurrence of Gulf Streal warm-core rings (WCRs) in the Slope & shelf seas, and their role in initiating cross-frontal events like shelf water entrainment in the western North Atlantic, have been well documented through satellite altimetry, sst and chlorophyll imagery, theoretical models and field observations. However, little is known about the salt transported by these Warm core rings and the new satellite microwave sea surface salinity products might bring  very interesting insight in that context.

During the first two years of mission operations, SMOS data in this area were howevever heavily contaminated by Radio Frequency Interferences (RFI) from the military radar arrays installed over North America. Since end 2011, a dramatic improvment was observed due to on-going action to refurbish L-band radar stations in Canada (see dedicated news).  In that frame, we analyzed the complete year 2012 of SMOS data to investigate if we can now tackle realistic SSS variability in the Gulf stream area with SMOS data. First results were highlighted during the 3-years SMOS press event organized by ESA in Madrid (see here).  Results from a more in depth analysis are presented herebelow.

 

 Figure 1: Climatologies of (a) the Mean Dynamic Topography (color) and geostrophic currents (arrows) (Rio, 2009); (b) World Ocean Atlas SSS; (c) Pathfinder SST (Casey) and (d) SeaWIFS 1997-2007 Chlorophyll. The black curve extending across the North Atlantic is the separating streamline estimated from the steady atltimer-based streamfunction.

   Annual climatologies of Sea surface Height (SSH), SSS, Sea Surface Temperature (SST) and Chlorophyll-a shown in Figure 1 reveal that in average , very large  SSH, SSS, SST and Chl-a concentration gradients are encountered along South-North sections trough the gulf stream. In particular the SSS exhibits zonal gradients greater than 5 unit/10° of  latitude and the SST can range from 24°C in the central waters down to less than 5°C in the shelf and slope waters. It therefore a good test-area for evaluating the ability of microwave salinity missions to retrieve SSS in moderate to cold seas.

 Specific scientific questions we are investigating are the following:

1) What accuracy of SMOS products at moderate to low SSTs ?

 

2) How SMOS data complement SST & SSH informations to better track meso-scale features

 

3) Can SMOS data be used to better monitor biological productivity in the separated Gulf stream area ?

 

4) Is SMOS able to track Near-Surface

Transport Pathways of salt in the North Atlantic Ocean ?

ÞLooking for Throughput from the Subtropical to the Subpolar Gyre

 

 

To answer the first question above, we conducted an ensemble of co-localizations between SMOS and in situ data collected in that area over year 2012.

    

 

 

 

Figure 2: SMOS SSS data (a) co-localized with in situ SSS data (b). In situ observations include VOS TSG data and ARGO float upper depth measurements in the Gulf stream area. The data are collected from 03/15/2012 to 10/31/2012. TSG data were spatially averaged at the SMOS product 0.25° x 0.25° spatial resolution. SMOS SSS data were temporally averaged over a 11 days-period centered on each ship or ARGO observation date. c) Comparison between co-localized SMOS and TSG SSS data as function of sea surface temperature (color). d) Comparison co-localized SMOS and ARGO SSS data as function of sea surface temperature (color); (e) Average differences ΔSSS between co-localized in situ and SMOS SSS observations per 0.2°C-width bins of Sea Surface Temperature. The black curve indicate the median  ΔSSS within each bin and the blue vertical bars indicate ±1 standard deviation. The number of samples per bin is indicated by the green curve.

 

 

 

 We considered Volountary Observing Ships Thermo-SalinoGraph data averaged at 0.25°, ARGO & SMOS data 10 days average at 0.25° (CEC-Ifremer v2 product).  In situ data include  ARGO (~2000 pts) and TSG (GOSUD (Matisse+Oleander)+ SAMOS data (Atlantis, Henri Biglow, Knorr, Nancy Foster, Pisces, Okeanos Explorer, RonBrown) ) with a spatial sampling distribution as shown in Figure 2. In general, we found an rms difference between SMOS and in situ observations of 0.7 and 0.5 for TSG and ARGO float data, respectively.  Given the large gradient and strong temporal variability encountered in this area, SMOS products data can therefore be used to perform some scientific analysis.  However, bining the in situ-minus-SMOS SSS differences as function of SST, it appears (Fig 2d above) that systematically, SMOS L3 CEC V2 products are too salty compared to in situ observations as SST decreases below about 13°C.  The colder the SST below that threshold, the more important the bias. The reasons for that are not yet fully clear but certainly involve a mixture of dielectric constant modelling issues (we rely on Klein adnd Swift's model), badly corrected roughness effects, rfi and land contaminations.

 

 

  How SMOS data complement SST & SSH informations to better track meso-scale features ?

 

To investigate that point, we combined SMOS observations with GHRSST SST products (ODYSSEA) and merged altimeter geostrophic+wind-induced ekman currents from OSCAR. The animation below is showing the co-varying SSS (color) and currents (arrows) in the Gulf stream area in 2012 (clik on the image to see a larger view) 

 

With an unprecedented space and time resolution, SMOS surface salinity data now bring additional information on the evolution of the meandering Gulf Stream. Off Cap Hatteras, strong lateral gradients are caused by the convergence of subtropical water carried northward by the Gulf Stream with sub-polar water carried southward along the east coast of North America by the Labrador current. Stirred by mesoscale (50-100 km) and larger scale processes, SMOS observations help delineate meanders pinching off from the current to form Gulf Stream rings. Saltier (fresher) water parcels from the southern (northern) water masses are then trapped in these eddies, to further propagate in distinct ocean salinity environment on both sides of the stream. 

Two examples of this capabilities of SMOs sensor to detect meso-scale features are given by the signal of warm-core and salty eddies detached from the Gulf stream in June and September around 65°W and 40°-42°N:

 

 

 

By chance the Matisse ship equipped with a TSG crossed the salty eddy dettached from the Gulf stream which was detected from mid-may to end July (top one in the figure above) with data acquired close in time to an ARGO float surfacing just in the eddy core: