To understand how our global climate will change in response to natural and anthropogenic forcing, it is essential to determine how quickly and by what pathways climate change signals are transported throughout the global ocean, a vast reservoir for heat and carbon dioxide. Labrador Sea Water (LSW), formed by open ocean convection in the subpolar North Atlantic, is a particularly sensitive indicator of climate change on interannual to decadal timescales. Hydrographic observations made anywhere along the western boundary of the North Atlantic reveal a core of LSW at intermediate depths advected southward within the Deep Western Boundary Current (DWBC). These observations have led to the widely held view that the DWBC is the dominant pathway for the export of LSW from its formation site in the northern North Atlantic towards the Equator. Here we show that most of the recently ventilated LSW entering the subtropics follows interior, not DWBC, pathways. The interior pathways are revealed by trajectories of subsurface RAFOS floats released during the period 2003-2005 that recorded once-daily temperature, pressure and acoustically determined position for two years, and by model-simulated 'e-floats' released in the subpolar DWBC. The evidence points to a few specific locations around the Grand Banks where LSW is most often injected into the interior. These results have implications for deep ocean ventilation and suggest that the interior subtropical gyre should not be ignored when considering the Atlantic meridional overturning circulation.

For decades oceanographers have understood the Atlantic meridional overturning circulation (AMOC) to be primarily driven by changes in the production of deep-water formation in the subpolar and subarctic North Atlantic. Indeed, current Intergovernmental Panel on Climate Change (IPCC) projections of an AMOC slowdown in the twenty-first century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep-water formation. The motivation for understanding this linkage is compelling, since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic Program (OSNAP), to provide a continuous record of the transbasin fluxes of heat, mass, and freshwater, and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014, and the first OSNAP data products are expected in the fall of 2017.

Recent observations of fluid parcel pathways in the Gulf Stream using isopycnal RAFOS floats revealed a striking pattern of cross-stream and vertical motion associated with meanders (Bower and Rossby 1989). In an attempt to explain the observed pattern, a two-dimensional kinematic model of a meandering jet has been developed which enables examination of the relationship between streamfunction patterns and fluid parcel trajectories. The streamfunction fields are displayed in a reference frame moving with the wave pattern so motions of fluid parcels relative to the jet can be seen more easily. The results suggest that the observed pattern of cross-stream motion results primarily from the downstream phase propagation of meanders. The model successfully reproduces several of the most distinctive features of the float observations: 1 ) entrainment of fluid into the Gulf Stream occurs at the leading edges of meander extrema while detrainment takes place at the trailing edges; 2) exchange between the Gu... Abstract Recent observations of fluid parcel pathways in the Gulf Stream using isopycnal RAFOS floats revealed a striking pattern of cross-stream and vertical motion associated with meanders (Bower and Rossby 1989). In an attempt to explain the observed pattern, a two-dimensional kinematic model of a meandering jet has been developed which enables examination of the relationship between streamfunction patterns and fluid parcel trajectories. The streamfunction fields are displayed in a reference frame moving with the wave pattern so motions of fluid parcels relative to the jet can be seen more easily. The results suggest that the observed pattern of cross-stream motion results primarily from the downstream phase propagation of meanders. The model successfully reproduces several of the most distinctive features of the float observations: 1 ) entrainment of fluid into the Gulf Stream occurs at the leading edges of meander extrema while detrainment takes place at the trailing edges; 2) exchange between the Gu...

A unique set of Lagrangian observations has recently been collected in the Gulf Stream using the newly developed isopycnal RAFOS float. Between January, 1984 and October, 1985, thirty-seven of these drifters were launched in the main thermocline of the current off Cape Hatteras and lacked acoustically downstream for 30 or 45 days. Temperature and pressure were also recorded along each float trajectory. The isopycnal capability of this drifter allows it to follow fluid parcel pathways quite accurately along the sloping density surfaces of the Gulf Stream. The RAFOS drifters revealed a striking pattern of vertical and cross-stream motion in Gulf Stream meanders. Floats were consistently observed to upwell (downwell) and move onshore (offshore) as they approached anticyclonic (cyclonic) meander crests (troughs). The rms vertical velocity in the center of the stream was observed to be 0.08 cm s−1 on the 12°C surface. No mean vertical motion was detected in the main thermocline of the Gulf Stream betw... Abstract A unique set of Lagrangian observations has recently been collected in the Gulf Stream using the newly developed isopycnal RAFOS float. Between January, 1984 and October, 1985, thirty-seven of these drifters were launched in the main thermocline of the current off Cape Hatteras and lacked acoustically downstream for 30 or 45 days. Temperature and pressure were also recorded along each float trajectory. The isopycnal capability of this drifter allows it to follow fluid parcel pathways quite accurately along the sloping density surfaces of the Gulf Stream. The RAFOS drifters revealed a striking pattern of vertical and cross-stream motion in Gulf Stream meanders. Floats were consistently observed to upwell (downwell) and move onshore (offshore) as they approached anticyclonic (cyclonic) meander crests (troughs). The rms vertical velocity in the center of the stream was observed to be 0.08 cm s−1 on the 12°C surface. No mean vertical motion was detected in the main thermocline of the Gulf Stream betw...