Modulation of the inflow into the Caribbean Sea by North Brazil Current rings

A two-year long record from a triangular mooring array between the Lesser Antilles islands Tobago, Barbados, and St. Lucia is used to investigate the inflow into the Caribbean Sea, the amount of South Atlantic Water (SAW) carried with the inflow, and the role of North Brazil Current (NBC) rings in the observed variability. The data set consists of time series from temperature/conductivity recorders and current meters in the moorings, bottom-mounted inverted echo sounders at the Tobago and St. Lucia mooring positions, and supplementary shipboard measurements. The acoustic travel time measurements of the inverted echo sounders and the conductivity/temperature time series are used for continuous estimation of dynamic height profiles and geostrophic currents between the surface and 1000 dbar as well as the amount of SAW found at the mooring positions.The observations show a domination of intraseasonal variability between 0 and 15 Sv, superimposed on the long-term fluctuations. With time scales of one to three months, these represent the signature of the NBC rings. The baroclinic transport time series shows nine periods of increased variability, indicative of the rings interacting with the Lesser Antilles island arc; with the exception of one, these periods were associated with corresponding sea surface height anomalies. No marked seasonality was observed in the transport variability or the ring frequency.The arrival of individual rings leads to a weakening of the inflow into the Caribbean. Nevertheless, the rings carry large amounts of SAW into the area, and the immediate increase of the transport towards the end of a ring event suggests a subsequent flow of this SAW-rich water into the Caribbean. At St. Lucia, rings sometimes cause a short-term decrease of SAW content, indicative of an influx of northern hemispheric water and a blocking situation. The average transport of SAW into the Caribbean south of St. Lucia during the observations amounted to 5.5 Sv, with no significant seasonal cycle, but a small positive trend in SAW fraction as well as in transport of about 15% and 1 Sv, respectively; a corresponding trend in the baroclinic volume transport was not observed.