William E. Johns | Searcharxiv

arXiv2026

Density and sea level changes weaken the Atlantic Meridional Overturning Circulation through ocean bottom pressure

Observations from the RAPID array at 26.5$^\circ$N indicate a linear decline in the Atlantic Meridional Overturning Circulation (AMOC) over the past two decades, linked to contrasting boundary changes: a weakening western-boundary contribution that is partly compensated by strengthening at the eastern boundary. Yet it remains unclear whether this partial compensation reflects a basin-wide adjustment or a regional feature, and what processes drive it. Here we use a high-resolution ocean model to investigate the spatial structure and underlying mechanisms of the AMOC change across the mid-latitude North Atlantic. The model reproduces a meridionally coherent decline in western-boundary deep overturning transport together with a partially compensating strengthening at the eastern boundary, consistent with observations at 26.5$^\circ$N. These opposing trends arise from a vertically coherent ocean bottom pressure trend shaped by two competing drivers: rising coastal sea level and decreasing interior density. Through geostrophic balance, this mechanism produces partial boundary compensation across latitudes, yielding a basin-wide AMOC decline throughout the mid-latitude North Atlantic.

physics.ao-ph↗

arXiv2022

Lagrangian coherence and source of water of Loop Current Frontal Eddies in the Gulf of Mexico

Loop Current Frontal Eddies (LCFEs) are known to intensify and assist in the Loop Current (LC) eddy shedding. These eddies can also modify the circulation in the eastern Gulf of Mexico (GoM) by attracting water and passive tracers such as chlorophyll and pollutants to the LC-LCFE front. During the 2010 Deepwater Horizon oil spill, part of the oil was entrained not only in the LC-LCFE front but also inside an LCFE, where it remained for weeks. This study assesses the ability of the LCFEs to transport water and passive tracers without exchange with the exterior (i.e., Lagrangian coherence) using altimetry and a high-resolution model. The following open questions are answered: (1) How long can the LCFEs remain Lagrangian coherent at and below the surface? (2) What is the source of water for the formation of LCFEs? (3) Can the formation of LCFEs attract shelf water? The results show that LCFEs are composed of waters originating from the outer band of the LC front, the region north of the LC, and the shelf, and potentially drive cross-shelf exchange of particles, water properties, and nutrients. At depth (~180 m), most LCFE water comes from the outer band of the LC front in the form of smaller frontal eddies. Once formed, LCFEs can transport water and passive tracers in their interior without exchange with the exterior for weeks: these eddies remained Lagrangian coherent for up to 25 days in the altimetry dataset and 18 days at the surface and 29 days at depth (~180 m) in the simulation. LCFE can remain Lagrangian coherent up to a depth of ~560 m. Additional analyses confirm that the LCFE involved in the oil spill formed from water near the oil rig location. Temperature-salinity diagrams show that LCFEs are composed of GoM water as opposed to LC water. Thus, LCFE formation modify the surrounding circulation and the transport of oil and other passive tracers in the eastern GoM.

physics.ao-ph↗