Gulf Stream Transport and Mixing Processes via Coherent Structure Dynamics

The Gulf Stream has been characterized as either a barrier or blender to fluid transfer, a duality relevant to gyre-scale climate adjustment. However, previous characterization depended on relatively sparse, Lagrangian in situ observations. The finite-time Lyapunov exponent (FTLE) is calculated from satellite altimetry to identify Lagrangian coherent structures (LCS) in the Gulf Stream region. These LCS provide dense sampling of flow and capture distinct regions associated with mixing. Independent observations of ocean color contain similar flow-dependent structures, providing verification of the method and highlighting transport and mixing processes that influence sea surface temperature and chlorophyll, among other water properties. Diagnosed LCS support the existing Bower kinematic model of the Gulf Stream, but also highlight novel behavior of comparable importance. These include vortex pinch-off and formation of spiral eddies, clearly identified by LCS and which may be explained by considering changes to flow topology and the dynamics of shear-flow instability at both small and large Rossby number. Such processes, seen though LCS, may further enable validation of climate models. The spatial distribution of these intermittent processes is characterized in terms of the criticality of jet dynamics with respect to Rossby wave propagation, and whether the jet is in an unstable or wave-maker regime. The generation and connectivity of hyperbolic trajectories in the flow appears to play an important role in governing large-scale transport and mixing across the Gulf Stream. Plain Language Summary Earth is heated by the sun more strongly near the equator than near the poles. The atmosphere and ocean dynamics redistribute this heating imbalance via fluid transfer to achieve a climate that is more moderate and less extreme. At midlatitudes, the ocean transports more heat poleward than the atmosphere, and the physical processes in the Gulf Stream affect the rate of heat transport for the North Atlantic. Changes in heat transport set ocean surface temperature patterns, which affect atmospheric weather and also govern climate variability. These crucial transport processes involve the interaction of the Gulf Stream with ocean eddies, typically tens of kilometers in diameter, the ocean's equivalent of atmospheric weather systems. This study uses the latest satellite observations and a new method for identifying structures in fluid flow to reexamine a picture of the transport processes, first developed 30 years ago. In order to accurately predict future global climate, computer simulations need to represent these key transport processes at ocean eddy scales. We show that the traditional view is still relevant for part of the Gulf Stream, but that fascinating new behavior is also seen from space, showing the generation and evolution of flow structures central to fluid transport.