Variability in Lagrangian-derived throughput from the subtropical to the subpolar gyres in the North Atlantic and its impact on inter-gyre heat transport

Variability in the strength of the inter-gyre throughput of water from the subtropical to the subpolar gyres in the North Atlantic as part of the upper limb of the Atlantic Meridional Overturning Circulation has been hypothesized to control the variability in the inter-gyre heat transport. Here, we first quantify the variability in the inter-gyre throughput by tracking backwards-run Lagrangian trajectories in a high-resolution ocean circulation model and secondly determine the controlling mechanisms of this variability by analyzing the strength and spatial extent of the subtropical and subpolar gyres as measured by satellite altimetry. Backwards-run Lagrangian trajectories provide an accurate measure of the origin of water masses without the subjective designations of water mass classes and assumptions on the mixing along pathways. Similarly, by explicitly tracking the strength (sea-surface height of gyre center – sea-surface height of gyre boundary) and spatial extent (area enclosed by the largest closed sea-surface height contour), no assumptions are necessary on the role of statistical modes that change through time. Results from these analyses will be discussed as they pertain to the observed warming of the subpolar gyre during the past two decades.