Using MODIS ocean color satellite images collected during March -- July 2004, an attempt was made to observe a series of eddies in the central Sargasso Sea before and during 2 first cruises of the EDDIES project. Five eddies of interest were identified prior to the cruises using satellite altimetry: 3 anticyclones and 2 cyclones. Using altimetry archive each of the eddies was  tracked  to determine its trajectory and amplitude. Then this data set together with cruise data was used to identify exact position and strength of a particular eddies in color images.

We have used SEADAS software to display MODIS color images processed to Level 2  chlorophyll-a maps using cylindrical map projection with the spatial resolution ~1 km. The chlorophyll levels in each of the eddies did not remain constant over the time; so, the features are not clearly evident in each image.

March-April 2004.

During this period, when the mixed layer is deep or just starts to shoal,  and the spring phytoplankton bloom is triggered, satellite images show increased concentrations of surface chlorophyll.

MODIS image acquired March 12, 2004 shows increased Chl-a concentrations tied with  a frontal zone located along 61W with a multiple small-scale swirls. Anticyclone eddy marked as "A1", in its central part  and north of it is obscured by clouds. But a broad belt of increased Chl-a concentration is clearly seen in the southern and eastern parts of the eddy. It is not clear if this increase is associated with  the local upwelling inside the eddy, or is due to horizontal advection  of phytoplankton from the adjacent cold eddy located NE of "A1". Another one  Chl-a patch located at 31N, 60W may be associated with  western periphery of cold eddy "C1". Altimetry maps show that center of this eddy is located at 31.5N, 59W.

Next image acquired 2 weeks later March 28,  shows  Chl-a maximum located SE and S of two patterns associated with 2 strong cold eddied (rings?). Anticyclonic eddy center marked as "A1", is located approximately 50 km SE of that feature.  At this time, increased Chl-a concentration is evident in center of cold eddy marked  "C1", and in center of another one cold eddy marked  "C2'. An interesting feature is a tongue of low chl-a water intruding through the  south periphery of C1 from SW. This tongue has  anticyclonic rotation and may be associated with the anticyclonic eddy A2.

Color image obtained on April 1, 2004, shows  Chl-a pattern consisting of two rings and A1 eddy. But in this case the area of increased chl-a is located right in the center of "A1" eddy. An interesting feature is a long "sleeve" connecting eastern parts of A1 and cold core eddy just to the north of A1. Taking into account direction of rotation in both eddies  we can conclude that A1 eddy advectively transports high chl-a waters and accumulates them in its central part. But these waters not originated from the adjacent cold eddy. Seem, both of them transport high chl-a waters which are resulted from the upwelling processes on the periphery of these eddies. In lower part of the image a low chl-a tongue has moved further to the east and the remnant of eddy "C1" is evident.

Image acquired April 4, 2004 shows that chl-a concentration in area occupied by A1 eddy is increased significantly. A tongue of low chl-a water which is associated with A2 eddy, almost completed anticyclonic rotation. Also, "C1" eddy is visible.

Two consequtive images obtained on April 30 and May 1, do not show any features of interest.

May-June 2004.

Next image was obtained on May 8, 2004. At this time of the year the spring bloom  rapidly utilizes the nutrient in the surface waters, and the subsurface deep chlorophyll maximum (DCM) begins to develop. Also, as water column stratification continues to intensify below the shallow mixed layer,  the seasonal thermocline inhibits further nutrient enrichment of the surface waters. Despite this reason color image shows concentration in the area associated with the position of A1 eddy is high enough to conclude that this increase is a result of vertical processes inside the  eddy but horizontal advection.  Position of eddy C2 is also indicated.

On images obtained on May 20, June 5, and June 6, 2004, eddy A1, migrating to the west, is still may be tracked due to chl-a pattern showing anticyclonic rotation. The higher chlorophyll values in A1 center appear to be advected from the northern, cyclonic eddy. The drawing of water from cyclonic eddy can explain why the anticyclonic eddy has high chl-a concentration and the cyclonic eddies C1 and C2 have low chl-a concentration. Examining these images showed that beginning from early May anticyclonic  eddy A1 draws chl-a rich chl-a waters from the northern eddy. Cyclonic eddies moving through the relatively clear water form DCM which may be not detected onboard the satellite. Note a large area of specific   (similar to triangle) form with very low chl-a concentration centered on 30N, 62W. It is interesting  that this low chl-a pattern may be tracked down in all images (until June 17) but it is unclear which mechanism forms this pattern.

A clear sky image acquired June 16 shows intense streamer drawing high chlorophyll waters from north in the southward direction. Approximately near 33N, this streamer changs its direction and high chl-a values appear to be advected  into the western, anticyclonic and cyclonic eddies. Comparison  to altimetry map shows that position and direction  of this streamer, originally  formed from Gulf stream, coinside with the eastern periphery of the long anticyclonic feature extending from northern boundary up to A1 eddy.

Unfortunately, we don't have a possibility to to identify this streamer in ADCP data: ship's track didn't cross it. The only small part of ADCP data revealed a southwest jet-like current near 32.5N, 62.5W. Position of C2 eddy is also indicated.

MODIS image acquired next day, shows a body of high chl-a water accumulated inside A1 eddy. A small (southern) part of the streamer is still presented on the image.

Althought the image obtained on June 28, 2004 is 70% clear sky, it is difficult to identify any chl-a patterns with  A1 eddy. The higher chl-a values near 30.75N, -60.75W appear to be cyclonic eddy C2. Location of another one cyclonic eddy C2 is evident due to cyclonic-like swirl at 30.25N, 65.0W.

On three consecutive images acquired  July 16, 17, and 18, the only  location of the cyclonic eddies C1 and C2 may be identified.