Real-time analysis of altimetric observations will provide maps of the eddy field prior to and during our sampling operations (Figure 1).

Figure 1: Example objective analysis of sea level anomaly (SLA) for 10 July 2003.  Near real-time data feeds from Jason, Topex, Geosat Follow On, and ERS were obtained through the Colorado Center for Astrodynamics Research Real-Time Altimetry Project; see http://www-ccar.colorado.edu/~realtime/welcome/.

As far as we know, there are three different types of mid-ocean eddies in the Sargasso Sea (Figure 2): cyclones, anticyclones, and mode-water eddies (MWEs).  Cyclones and MWEs are of interest to this project, as both tend to displace upper ocean isopycnals toward the surface, causing nutrient input into the euphotic zone.  Whereas cyclones are identifiable in satellite altimetry by virtue of their negative sea level anomaly (SLA), MWEs are not distinguishable from anticyclones on the basis of altimetry alone becauseboth result in positive SLA.  In principle, satellite-based SST could distinguish these two, as anticyclone and MWEs would be characterized by warm and cold SST anomalies, respectively.  However, given the paucity of reliable SST imagery in the Sargasso Sea during summer, we will likely have to rely on in situ measurements to unequivocally distinguish MWEs from anticyclones.

Eddy age is another key issue.  Whereas an intensifying cyclone will have upwelling in its center, the isopycnals in the interior of a decaying cyclone will be downwelling.  The earlier phase of the eddy’s lifetime will be when nutrient injection and the associated biological response occur.

Summary of desirable characteristics for the target eddy:
1. Young
2. Strong imprint on upper ocean physics, biology, and chemistry (not necessarily a large amplitude SLA in the case of a MWE).
3. Intensifying
4. Chemical impact discernible in real time (optical NO₃ sensor)
5. Biological impact discernible in real time (fluorometry, microscope counts, VPR?)
6. Cyclone versus MWE?
   a. unequivocal satellite determination favors cyclones
   b. trapping of near-inertial motions and possible enhanced mixing favors MWEs
   c. some of the big events at BATS have been MWEs
     Jenkins (1988) Summer 1986 event
     McNeil et al. (1999) July 1995 eddy.
7. Proximity to BBSR: must be within 1 day’s steam for Weatherbird II

Clearly, it will behoove us to sample several eddies during the first survey cruise prior to making a decision about which eddy we wish to spend the rest of the summer in.

See also:
Project Summary and Cruise Logistics:

        HTML document updated April 7, 2004
          Word file (.doc) of same
          Powerpoint file from project meeting July 23, 2003