Intrusions of Amazon River Waters in the Virgin Islands Basin During 2007–2017.

The Virgin Islands basin (VIB) includes several Marine Protected Areas (MPAs) of interest as biologically unique spawning aggregation sites. The ecological structure in and around these MPAs is regulated by several factors, including changes in near‐surface water properties. Anomalously low near‐surface salinity is observed in the VIB during April 2009/2011, and March 2010, with a salinity signature consistent with Amazon plume waters. Other low salinity events in the region are found during 2007–2017 using output from an ocean reanalysis. The reanalysis shows that horizontal salinity advection explains near‐surface salinity variability in the VIB to a high degree, including events observed in the in situ measurements. We use a Lagrangian Particle tracking model to track particles over the 2007–2017 period and identify the source and pathways of water imports to the VIB. We describe three pathways. The northernmost one is often associated with advection of salty Atlantic waters. The two southernmost paths are associated with advection of low salinity waters from the Amazon into the VIB. The latter two pathways arrive to the Caribbean Sea as described in previous studies on low salinity advection to the wider Caribbean from the Amazon River; we find that once in the Caribbean Sea, the low salinity water makes its way into the VIB when steered northward by mesoscale features. This results in Amazon River waters regulating salinity variability in the VIB during April–November. During December–March, when mesoscale activity is at its minimum, the Atlantic inflow regulates the salinity variability within the VIB instead. Plain Language Summary: The Virgin Islands basin (VIB) is an ecologically important region of the northeastern Caribbean Sea. Changes in upper‐ocean properties affect the regional biology. Measurements of water properties in the VIB during April 2009/2011 and March 2010 show low salinity waters near the surface consistent with salinity from the Amazon River plume. Similar events are found during the 2007–2017 period using outputs from an ocean model. We show that salinity in the region is driven by the horizontal transport of salt via ocean currents. We then track the origin of the waters arriving to the VIB. A path from the east/north‐east transports salty ocean waters into the VIB during December–March. Two other paths from the south/south‐east are instead associated with the transport of low salinity waters into the VIB from the Amazon River area and occur frequently during April–November. Key Points: Salinity variability in the Virgin Islands basin (VIB) is primarily driven by horizontal salinity advectionThree paths dominate the import of waters into the VIB: two paths linked with Amazon plume waters and the third with Atlantic inflowMesoscale features steer low salinity waters from the southeastern Caribbean toward the VIB [ABSTRACT FROM AUTHOR]