Post-doc Dr. Roshin P. Raj is first author of two publications that provide new knowledge on the mesoscale ocean variability in the Lofoten basin, published in cooperation with colleagues at the Nansen Center, Geophysical Institute at University of Bergen and Cape Peninsula University of Technology in South Africa. The papers in respectively Journal of Geophysical Research and International journal of Remote Sensing provide new quantitative knowledge on the eddy generation, development and variability. Further the importance of eddies in transport of heat and nutrients in the Lofoten Basin are quantified. Methodologically the two papers demonstrate the potential of combined use of satellite radar altimeter data, profiling Argo floats and surface drifters for locating and studies of 3-dimensional variability mesoscale eddies. The studies recommend generation of a new gridded regional radar altimeter data set at better temporal and spatial resolutions to preform studies of mesoscale ocean variabity in the Lofoten Basin.

The circulation in the Nordic Seas.: The Nordic Seas with schematic pathways indicating the overturning circulation from warm inflowing Atlantic Water in the sur- face (red) to cold and dense overflows to the deep North Atlantic (blue). The Norwegian Atlantic slope current (NwASC), here termed as the slope current, and the Norwegian Atlantic front current (NwAFC) are represented by red arrows. The fresh Norwegian Coastal Current (NCC) is indicated in green.Mesoscale eddies are vortices or flows with scales ranging from the baroclinic Rossby radius of deformation to several hundreds of kilometers. The Lofoten Basin situated in the Norwegian Sea is the most eddy rich region in the entire Nordic Seas. The mesoscale eddies of the Lofoten Basin can be coupled to the heat transport, local climate, and fisheries of the region. The paper published in the Journal of Geophysical Research-Oceans provides a comprehensive observation-based quantitative analysis of eddies in the Lofoten Basin from nearly two decades of altimeter data, complemented with Argo profiling floats and surface drifters. The eddies in the Lofoten Basin are found to be both generated and residing locally. The high nonlinearity of Lofoten Basin eddies further reaffirms their importance for the heat and nutrient transport in the Lofoten Basin. The study also confirms the transfer of energy from the Norwegian Atlantic slope current to the Lofoten Basin eddies, especially during winter. The vertical structure of the altimeter-based eddies is examined using colocated Argo profiling float profiles. The findings and results offer encouraging evidence that altimeter data combined with Argo floats and surface drifters can successfully be used to locate and study the three-dimensional thermodynamic properties of mesoscale eddies in the Lofoten Basin.

The paper published in the International Journal of Remote Sensing mainly highlights the progress and challenges in monitoring the eddies of the Lofoten Basin. The study assesses the performance of the new daily gridded satellite data compared to the previous weekly data in terms of eddy detection and eddy statistics in the Lofoten Basin. The three main challenges identified are: (1) the higher rate of disappearance of eddy signature in daily data, (2) anisotropy in the grid spacing of the daily data, and (3) the inability to resolve smaller-scale eddies in both data sets. Analysis found that the anisotropy in the grid spacing of the daily data is results in the overestimation of eddy radius, while the eddy intensities are underestimated. However, the mean eddy drift in the Lofoten Basin is better represented in the daily data. The study thus supports the need for a regional gridded data set with high temporal and spatial resolution (equal-spaced grid) in order to optimize the use of satellite altimeter data in the Lofoten Basin.

Citations to the published papers:

Raj, R. P., J. A. Johannessen, T. Eldevik, J. E. Ø. Nilsen, and I. Halo (2016), Quantifying mesoscale eddies in the Lofoten Basin, J. Geophys. Res. Oceans, 121, doi:10.1002/2016JC011637.

Roshin P. Raj & Issufo Halo (2016) Monitoring the mesoscale eddies of the Lofoten Basin: importance, progress, and challenges, International Journal of Remote Sensing, 37:16, 3712-3728