Transcript West Antarctic Peninsula circulation and implications for biological production

West Antarctic Peninsula circulation and implications for
biological production
Andrea Piñones, Eileen E. Hofmann, Michael S. Dinniman, John M. Klinck
Center for Coastal Physical Oceanography, Ocean Earth and Atmospheric Science Department, Old Dominion University, 4111 Monarch Way -3rd floor, Norfolk, VA23508, USA.
Contact: [email protected]
Research Objective
Introduction
The western Antarctic Peninsula (WAP, Fig. 1) is a biologically productive area that
supports large populations of upper trophic level predators. Field observations made
as part of the U.S. Southern Ocean Global Ocean Ecosystem Dynamics Program (SO
GLOBEC) showed that regions of enhanced biological production on the WAP
continental shelf tend to be associated with areas where onshelf intrusions of
Circumpolar Deep Water occur.
These regions also tend to have associated with them increased concentrations of top
predators. An implication of these observations is that the circulation on the WAP
continental shelf has a large role in structuring biological distributions of this region
through all components of the food web. This study was designed to address questions
related to the effect of circulation on biological distributions on the WAP continental
shelf.
Results
The objectives of this study are to:
•
Determine transport pathways and residence times on the WAP
continental shelf.
• Correlate patterns seen in transport pathways with those seen in
 The simulated particle trajectories showed preferred sites for cross-shelf
exchange and onshelf intrusions which are related to areas where higher
predator abundance was observed, such as inside and around Marguerite
Bay, and north of Adelaide Island (AI) (Fig. 2).
 The trajectories of floats released along the southwestern portion of the
WAP show inputs from the Bellingshausen Sea, particularly for the southern
portion of Marguerite Bay (Fig. 3).
biological distributions.
(b)
(a)
Crystal
Sound
A.I.
(a)
NW
Alexander Is.
Figure 1. (a) Map of the Antarctic continent
showing the location of the western Antarctic
Peninsula (WAP, red line). (b) WAP study area and
the bathymetry used in the model. Bottom depth in
meters is indicated by the color bar.
(c)
Laubeuf
Fjord
Figure
2.
Simulated
particle
trajectories for (a) Crystal Sound, (b)
Northwest of Alexander Island, and
(c) Laubeuf Fjord. These are areas
where increased abundance of
predators were observed during SO
GLOBEC. The numbers indicate the
float release locations. Areas where
Circumpolar Deep Water intrusions
preferentially occur are indicated by
purple arrows.
(b)
 Floats released below 350 m showed more cross-shelf exchange than
those released at shallower depths. Floats released at this depth are
associated with the core of Circumpolar Deep Water, which lies between 300500 m along the WAP continental shelf edge. These results show that this
water mass intrudes onto the shelf (Fig. 4).
Figure 3. Trajectories of floats
released in the southwestern
region of the WAP. The change
in color denotes the temporal
evolution of the floats in days.
The release site for individual
floats is indicated by the solid
circles.
days
 Residence times were calculated for floats released at the 3 sites shown in
Figure 2. Crystal Sound (CS) and Laubeuf Fjord (LF) have similar residence
times (2 months) which are greater than the residence time obtained for floats
released northwest of Alexander Island (Fig. 5). The CS and LF regions are
characterized by enclosed topography, which restricts exchanges and may
enhance or favor retention of nutrients and plankton at time scales of the
order of few months (Fig. 5).
Crystal Sound
The Rutgers/UCLA Regional Ocean Model System (ROMS) was used to simulate
the circulation along the WAP continental shelf. ROMS is a free-surface, terrainfollowing, primitive equations ocean circulation model, and was implemented for a
domain that extends along the western side of the Antarctic Peninsula to the tip of the
Peninsula and covers the entire continental shelf and about 500 km offshore of the
shelf break (Fig. 1b).
 Horizontal grid spacing is 4 km and 24 vertical levels. Details of implementation of ROMS for
the WAP are given in Dinniman and Klinck (2004, DSR II).
The isobaths of the last few grid-points were set normal to the boundary at every openboundary (Fig. 1b).
 Temperature and salinity along the model boundaries were relaxed to values obtained from the
Simple Ocean Data Assimilation package (SODA data climatology).
Prescribed sea ice concentrations were imposed using sea ice concentration climatologies
derived from the Special Sensor Microwave Imager (SSM/I).
Daily wind stress and wind speed were calculated from 6-hr winds from QSCAT data and NCEP
reanalyses.
Figure 5. Residence times
(in days) obtained from
floats released at the 3
sites shown in Figure 2.
The mean residence time
(in days) for each site is
given in the above table.
Days of simulation
Model configuration
Laubeuf Fjord
NW Alexander Is.
Floats released per site
Conclusion
Figure 4. Trajectories of floats released along the shelf break, at several depths below 250 m.
Bottom bathymetry (grey contours) is given in meters. The simulation was run for 1 year.
The relative contribution of circulation in producing regions of enhanced predator
abundance was investigated using Lagrangian particle tracking simulations. Neutrally
buoyant floats were released along the outer and mid- regions of the WAP continental
shelf at different seasons and depths to produce trajectories that covered most of the
model region.
Acknowledgments: This research is funded by National Science Foundation Grant ANT-0523172
and is part of the U.S. Southern Ocean GLOBEC Program synthesis and integration phase.
The float trajectories and residence times indicate that the
circulation is potentially important in developing localized areas of
high predator abundance perhaps through facilitating aggregation
of prey and/or providing areas of enhanced nutrient availability and
biological production.