The Lofoten Basin in the Norwegian Sea is an area where the warm Atlantic Water is subject to the greatest heat losses anywhere in the Nordic Seas. A long‐lived, deep, anticyclonic eddy is located in the central part of the basin (the Lofoten Basin Eddy, LBE). Here we use observations from Seagliders, collected between July 2012 and July 2015, to describe LBE in unprecedented detail. The missions were designed to sample LBE repeatedly, allowing for multiple realizations of radial sections across the eddy. LBE has a mean radius of 18 ± 4 km and propagates cyclonically with a mean speed of approximately 3–4 cm s−1. The anticyclonic azimuthal peak velocity varies between 0.5 and 0.7 m s−1, located between 700 and 900 m depth. The average contribution of geostrophy in the cyclogeostrophic balance is 44%. The relative vorticity of the core is close to the local Coriolis parameter. The evolution of core water properties shows substantial interannual variability, influenced by surface buoyancy flux and advection of anomalous low‐salinity near‐surface waters that may affect the vertical extent of winter convection. A comparison of the eddy properties to those inferred from automated tracking of satellite altimeter observations shows that the location of eddy center is successfully detected to within one half eddy radius, but vorticity is underestimated and the radius overestimated, each approximately by a factor of 2, because of excessive smoothing relative to the small eddy radius.

A differentiable function whose contours are orthogonal to potential density (σ) contours does not exist. However, such a function, called potential spicity (π), can be defined in the least square sense; these two functions form a practically orthogonal coordinate system in potential temperature-salinity (θ-S) space. Thus, in addition to the classical potential temperature-salinity (θ-S) diagram, seawater properties can be studied in the potential density-potential spicity (σ À π) diagram. Plain Language Summary Potential spicity is defined in the least square sense, forming a practically orthogonal coordinate system with the potential density in potential temperature-salinity (θ-S) space, which can be used to study seawater properties.