
Invited talk - OS21   Marine Ecosystem Modeling on Regional to Global Scales 


Modeling Mesoscale Biogeochemical Processes
in a Topex/Poseidon Diamond
Surrounding the U.S. JGOFS Bermuda Atlantic Time-series Study

D.J. McGillicuddy, Jr., V.K. Kosnyrev, E.N. Sweeney and K.O. Buesseler

An interdisciplinary modeling system has been configured in
the Topex/Poseidon (T/P) "diamond" surrounding the Bermuda Atlantic
Time-series (BATS) site.  After extensive experimentation with
the treatment of the open boundary conditions, 
a realistic hindcast of sea level variations in the interior
of the domain has been achieved by prescribing information only along the
boundaries.
The time series of RMS difference between simulated and observed
SLA fields for the entire altimetric record available to date
shows hindcast skill that is in most cases the same order as
the altimetric measurement error (3-5cm).

The T/P diamond model is being used to diagnose mesoscale
biogeochemical processes in a retrospective analysis of BATS data.
This activity was begun with an attempt to
interpret a three-year time series record of particle flux based on thorium-234
measurements made by K.O. Buesseler.  During this time period,
there were three anomalously high flux events.  Analysis of 
contemporaneous results from the T/P diamond model reveals that
each of the three events took place when eddy features were present.  
The first two (June 1993 and August 1994) were
associated with cyclonic features (negative sea level anomalies),
while the last one (July 1995)
was associated with with a positive sea level anomaly.
Concurrent hydrographic measurements reveal the latter to be
associated with a so-called ``Mode water eddy,'' a thick bolus
of 18-degree water which depresses the main thermocline and lifts
the seasonal thermocline.  Previous work has shown that both
cyclones and Mode water eddies can inject nutrients into the
euphotic zone, causing the accumulation of phytoplankton biomass
in their interiors.  Thus the high particulate flux events
inferred from the thorium-234 flux measurements are consistent
with these eddy-driven mechanisms.

