West Antarctic Peninsula circulation and implications for biological production 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. 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 -3 rd floor, Norfolk, VA23508, USA. Contact: Results Conclusion 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.  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). 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. (a) (c) Crystal Sound Laubeuf Fjord (b) NW Alexander Is. A.I. days 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. 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. Research Objective 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 biological distributions. Model configuration The Rutgers/UCLA Regional Ocean Model System (ROMS) was used to simulate the circulation along the WAP continental shelf. ROMS is a free-surface, terrain- following, 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 open-boundary (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. 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 and is part of the U.S. Southern Ocean GLOBEC Program synthesis and integration phase. (b) 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. (a) 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. Crystal Sound NW Alexander Is. Laubeuf Fjord Days of simulation Floats released per site  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 m along the WAP continental shelf edge. These results show that this water mass intrudes onto the shelf (Fig. 4).  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).