Eastern Caribbean Circulation and Island Mass Effect on St. Croix, US Virgin Islands: A Mechanism for Relatively Consistent Recruitment Patterns.

The northeastern Caribbean Sea is under the seasonal influence of the Trade Winds but also of the Orinoco/Amazon freshwater plume. The latter is responsible for intensification of the Caribbean Current in general and of its eddy activity in the northern part of the Caribbean Sea. More importantly, we show in this study that the front of the freshwater plume drives a northward flow that impinges directly on the island of St. Croix in the United States Virgin Islands. The angle of incidence of the incoming flow controls the nature of the wake on both sides and ends of the island, which changes from cyclonic to anticylonic wake flow, with either attached or shed eddies. Using an off-line bio-physical model, we simulated the dispersal and recruitment of an abundant Caribbean coral reef fish, the bluehead wrasse (Thalassoma bifasciatum) in the context of the wake flow variability around St. Croix. Our results revealed the role played by the consistent seasonal forcing of the wake flow on the recruitment patterns around the island at the interannual scale. The interannual variability of the timing of arrival and northward penetration of the plume instead controls the nature of the wake, hence the regional spatial recruitment patterns

Potential vorticity anomaly (PVA) (s^-1) maps overlaid with current vector (m.s^-1) of the wake south of St. Croix in model year 2008 during the month of August for days 2, 8, 9 and 12.

<p>Left (right) column shows the vorticity at 30 (100)-m. Pink (white) arrows show cyclones (anticyclones). Red lines show the cross section locations used in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0150409#pone.0150409.g012" target="_blank">Fig 12</a>.</p

Time-series of 8-day average SeaWiFS chlorophyll a concentration (mg. m^-3) images from 05 September to 06 October 2000 showing the northward advection of the Chl-a plume.

<p>Time-series of 8-day average SeaWiFS chlorophyll a concentration (mg. m^-3) images from 05 September to 06 October 2000 showing the northward advection of the Chl-a plume.</p

Potential vorticity anomaly (PVA) (s^-1) time series at 30-m (left column) and 100-m (right column) during the eastern cyclones shedding wake-flow period in model month August 2007.

<p>From top to bottom, model day 21(a, b), 22(c, d), 23(e, f), and 25(g, h). Yellow (orange) arrows show the negative (positive) vorticity anomaly generation regions. White (pink) arrows show the anticyclone (cyclone).</p

Percentage of the larvae recruiting on the northern and southern shore for the ROMS model configurations used (2007, 2008, 2009, and climatological).

<p>“clim” = climatological model.</p

Proportion of larvae settled in (a) the northwest site (NW), and in (b) the southeast site (SE) in June, July, August, and September of model year 2007, 2008, 2009, and the climatological model.

<p>“clim” = climatological.</p

Surface flow field snapshots from the model first child.

<p>(a) Snapshot obtained on 26 August of model year 2007. (b) Flow field snapshot on 7 September of the climatological year. (c) Same as (b) on 29 September.</p

Potential vorticity anomaly (PVA) (s^-1) snapshot at 30-m (left column) and meridional cross-section (right column) during the anticyclonic wake flow in August of model year 2007.

<p>(a) PVA on 4 August. (b) PVA meridional section in the cyclonic eddy at 64.58°W. (c) PVA on 6 August. (d) PVA meridional section in the cyclonic vorticity core at 64.47°W. (e) PVA on 7 August. (b) PVA meridional section in the cyclonic vorticity core at 64.32°W. Yellow arrows show the anticyclone and the red line shows the cross-section location across the cyclone. The orange arrow in the right panels shows the cyclonic vorticity.</p

Potential vorticity anomaly (PVA) (s^-1) meridional cross-sections during the cyclonic wake-flow period in model month September of the climatological simulation.

<p>(a) In the anticyclonic eddy at 65.00°W on model day 4 September. (b) In the wake of the anticyclonic eddy at 65.25°W on model day 4 September. (c) In the wake at 65.25°W on model day 5 September. (d) In the wake at 65.25°W on model day 6 September. White (orange) arrows indicate negative (positive) vorticity anomalies.</p

Chlorophyll a concentration in model parent grid in mg.m^-3.

<p>(a) Model initial field obtained from SeaWiFS chlorophyll a concentration 8-day average using the time period 29 September 2000–06 October 2000. Black square shows the boundary of the first child grid. (b) Model field twenty-two days after initialization. (c) same as (a) for the time period 14–21 September 1998.</p