A Comparison of Profiling Float and XBT Representations of Upper Layer Temperature Structure of the Northwestern Subtropical North Atlantic Robert L. Molinari National Oceanic and Atmospheric Administration Atlantic Oceanographic and Meteorological Laboratory Miami, Florida United States of America

Outline Objectives Data Sources: MBT’s, XBT’s, WOCE profiling floats A Gulf Stream recirculation gyre including definition of an index to represent gyre size Characterization of the annual cycle of the gyre and properties of the Gulf Stream: Temporal and Spatial Characterization of the decadal cycle of the gyre and properties of the Gulf Stream: Temporal and Spatial Dynamics of the annual and decadal signals

Objectives Describe the temporal and spatial characteristics of a recirculation gyre located south of the Gulf Stream with emphasis on annual and decadal time-scale variability. Demonstrate the ability of a profiling float array to capture the properties of the recirculation gyre. Using model results, provide possible mechanisms for forcing the decadal signal of the recirculation gyre.

Forcing Mechanisms for Decadal Signals Groetzner et al. (1998): ‘Gyre adjustment’, decadal variability in subtropical gyre structure, including SST, and NAO is a coupled mode of air-sea interaction. Time scale is set by the ocean through eastern-basin generation and westward propagation of planetary Rossby waves. Hakkinen (2000): Surface heat flux forcing rather than wind forcing is the atmospheric component of a coupled air-sea decadal mode of variability. Largest oceanic response occurs in the area of the Wang and Koblinsky (1996) recirculation gyre. Planetary waves establish the time-scale of the variability. Spall (1996a, 1996b): Feedbacks between the Gulf Stream, Deep Western Boundary Current and recirculation gyres can result in self-sustaining oscillations with a decadal period. The signal is independent of time-dependency in the NAO or planetary wave propagation in the subtropics.

Summary On annual time-scales, the size of a shallow recirculation gyre located south of the Gulf Stream varies in phase with the transport and position of the Gulf Stream. On decadal time-scales, the same correlations are observed, but the variability in gyre structure extends considerably deeper into the water column. Because the gyre variability is associated with changes in Gulf Stream position it becomes an agent for climate change through the effect of the Stream’s position on SST. Profiling float data reproduce the signals in gyre and Gulf Stream properties estimated from the longer XBT record. Since altimetric data only capture the near surface variability in Gulf Stream characteristics, the float and XBT data are needed to characterize deeper changes in the Atlantic boundary currents and gyres.