Quantifying mesoscale eddies in the Lofoten Basin

Abstract

The Lofoten Basin is the most eddy rich region in the Norwegian Sea. In this paper, the characteristics of these eddies are investigated from a comprehensive database of nearly two decades of satellite altimeter data (1995–2013) together with Argo profiling floats and surface drifter data. An automated method identified 1695/1666 individual anticyclonic/cyclonic eddies in the Lofoten Basin from more than 10,000 altimeter‐based eddy observations. The eddies are found to be predominantly generated and residing locally. The spatial distributions of lifetime, occurrence, generation sites, size, intensity, and drift of the eddies are studied in detail. The anticyclonic eddies in the Lofoten Basin are the most long‐lived eddies (>60 days), especially in the western part of the basin. We reveal two hotspots of eddy occurrence on either side of the Lofoten Basin. Furthermore, we infer a cyclonic drift of eddies in the western Lofoten Basin. Barotropic energy conversion rates reveals energy transfer from the slope current to the eddies during winter. An automated colocation of surface drifters trapped inside the altimeter‐based eddies are used to corroborate the orbital speed of the anticyclonic and cyclonic eddies. Moreover, the vertical structure of the altimeter‐based eddies is examined using colocated Argo profiling float profiles. Combination of altimetry, Argo floats, and surface drifter data is therefore considered to be a promising observation‐based approach for further studies of the role of eddies in transport of heat and biomass from the slope current to the Lofoten Basin.

The Lofoten Basin eddy: Three years of evolution as observed by Seagliders

“…Irminger Current Anticyclones (or Irminger Rings) play an important role in the advection of heat into the central Labrador Sea due to their anomalous warm cores, and are argued to account for between 25 and 100% of the heat needed to balance the surface heat loss during wintertime [ Lilly et al ., ; Katsman et al ., ; Hátún et al ., ; Rykova et al ., ; Fan et al ., ]. Similar to Lofoten Eddies (see an example at 10°E, Figure ) shed from the slope current [ Köhl , ; Raj et al ., ; Søiland et al ., ], Irminger Rings are triggered from the west Greenland boundary current over steep topography [ Lilly et al ., ; Katsman et al ., ; Hátún et al ., ; Rykova et al ., ].…”

“…We call this anticyclonic eddy the Lofoten Basin Eddy (LBE). Since then, several studies addressed the mesoscale activity in the basin using observations of hydrography [ Ivanov and Korablev , ; Rossby et al ., ; Raj et al ., ], numerical models [ Köhl , ; Volkov et al ., ], satellite altimetry [ Raj et al ., ; Søiland et al ., ], and surface drifters and subsurface floats [ Rossby et al ., ; Voet et al ., ; Andersson et al ., ; Koszalka et al ., ; Søiland and Rossby , ]. Lagrangian studies described the pathways, eddy kinetic energy, lateral mixing rates in the basin, as well as position and radius of LBE.…”

“…Hydrographic and ocean current observations have been limited to cruises of opportunity and short duration surveys. The present consensus is that LBE is maintained by merging of anticyclonic eddies which drift westward after being shed off the eastern branch of the NwAC [ Köhl , ; Raj et al ., ; Volkov et al ., ; Raj et al ., ; Søiland et al ., ]. The geometry of the basin attracts the anticyclones to the bottom depression and enables dynamical stability for the vortex.…”

Monitoring the mesoscale eddies of the Lofoten Basin: importance, progress, and challenges

“…Eddy detection in the Lofoten Basin using this method has also been validated using Argo floats and surface drifters (Raj et al 2016). Raj et al found that the altimeter-derived geostrophic speeds of eddies are roughly 25-30% lower than the geostrophic speeds of surface-drifters co-located inside eddies.…”

“…However, the eddy activity of the Lofoten Basin in terms of its physical properties remained less explored. It is only recently that a study (Raj et al 2016) quantified the mesoscale eddies of the Lofoten Basin using a combination of gridded altimeter, Argo floats, and surface drifter data.…”