Stacking of EAC Eddies Observed From Argo.

Using data from Argo floats, satellite altimetry, and satellite sea surface temperature (SST), we investigate the merging of two anti‐cyclonic eddies in the Tasman Sea. The eddies are of different size and different density. Once the distance between the eddies falls below a critical separation distance, water from the smaller, denser eddy is observed to flow around the larger, lighter eddy, sink to greater than 400 m depth, and slowly spiral toward the eddy‐center. The merging event is characterized by filaments around the eddy perimeter, evident in high‐resolution, satellite SST data. The merged eddy develops multiple well‐mixed layers, each almost 400 m thick, with large anomalies in temperature and salinity that penetrate to about 1,000 m depth. After the merging is complete, water from the lighter eddy is stacked on top of water from the denser eddy. Using a simple conceptual model, we show that the merged profiles can be simply explained by subduction of surface waters from the denser eddy, and mixing with subsurface waters from the lighter eddy. The stacked eddy has a distinct signature in TS‐space. Using this TS‐signature, together with altimetry for context, we identify over 20 other examples when Argo floats sample stacked eddies in the Tasman Sea. We speculate that merged eddies may be more common than previously thought, with implications for ocean productivity, underwater acoustics, and the transfer of energy across scales. Plain Language Summary: Eddies are rotating features that exist everywhere in the ocean. They could be large or small and they often bump into each other. Eddies are easily seen by satellite measurements, but their properties below the surface aren't always known. Thousands of Argo floats drift around the ocean measuring ocean properties between the surface and 2,000 m depth. These Argo floats often sample eddies. In this study, we report observations from Argo floats that sample two nearby eddies as they join together. As they merge together, water from the smaller, denser eddy wraps around the larger, lighter eddy, and spirals downwards towards the eddy center. The resulting eddy has a distinct subsurface profile that resembles the light eddy stacked on top of the dense eddy. Having identified the key signature of these stacked eddies, we hunt for more, and find plenty. We think these stacked eddies are quite common, and may be important for many applications. Key Points: Observations from Argo measured the properties of two warm‐core eddies in the Tasman Sea that merge over a period of about a monthWater from smaller, denser eddy wraps around the larger, lighter eddy and sinks, with the merged eddy developing multiple well‐mixed layersAnalysis of other Argo profiles suggests that the stacked eddies, with similar features, may be more common than previously thought [ABSTRACT FROM AUTHOR]