Roshin P. Raj

Quantifying mesoscale eddies in the Lofoten Basin

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.

Surface velocity estimates of the North Indian Ocean from satellite gravity and altimeter missions

ABSTRACT The circulation in the North Indian Ocean (NIO) is one of the most complex systems compared with other regions of global oceans, mostly due to its interactions with the monsoon winds. In recent years, our ability to measure the ocean’s mean dynamic topography (MDT) from space has improved immensely with the availability of satellite gravity measurements from Gravity Recovery and Climate Experiment (GRACE) and Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) missions. The present study uses data from GOCE and GRACE satellite gravity missions together with altimeter data in retrieving the geoid, satellite-only MDT, and surface velocities in the NIO. The study estimates geoid heights of the NIO from all five releases of the direct approach and the time-wise GOCE gravity data. The formal error associated with geoid heights at different resolutions is found to be the lowest for the latest release of direct approach GOCE data. In addition, a new satellite-only MDT is estimated from the direct approach GOCE geoid and the CNES_CLS11 mean sea surface. This MDT corrected to a 20-year time reference is used together with the newly reprocessed sea level anomaly data to estimate absolute dynamic topography and surface geostrophic velocities in the NIO. The total surface velocities computed from the Ekman and geostrophic velocity fields reproduce all major surface currents in the NIO, along with their seasonality. Furthermore, total surface velocity estimates computed here are validated using surface drifters and are found to be highly comparable (difference within ± 10 cm s–1) with more than 170,000 individual surface drifter observations. Finally, the total velocities estimated here are used to examine the variability of the East India Coastal Current.

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

ABSTRACT The Lofoten Basin is a ‘hot spot’ of intense mesoscale eddy activity in the Nordic Seas. The mesoscale eddies of the Lofoten Basin can be coupled to the heat transport, local climate, and fisheries of the region. During the past two decades, the European satellite missions European Remote Sensing (ERS) satellite and Environmental Satellite (ENVISAT) have played a major part in delivering continuous altimeter measurements over the Lofoten Basin. In recent years, automated eddy detection and tracking methods have revolutionized the long-term monitoring of eddies in the Lofoten Basin from the gridded altimeter data. Another major recent development in this field is the possibility of monitoring eddies on a daily basis from the globally available daily gridded altimeter data. This study assesses the performance of the new daily data compared to the previous weekly data in terms of eddy detection and eddy statistics in the Lofoten Basin. Analysis found that the long-lived eddies detected using the weekly data are counted as separate short-lived eddies when using daily data. In addition, the daily data returns lower eddy intensities (22–40%) with higher eddy radius estimates, which is linked to the anisotropy in the grid spacing of the data. Interestingly, the mean eddy drift in the Lofoten Basin is better represented in the daily data. The daily data shows the westward movement of eddies from the slope current and their subsequent northwestward movement following the topography, before joining the cyclonic drift in the western Lofoten Basin. The study supports the need for a regional gridded data set with high temporal and spatial resolution (in both zonal and meridional scales), in order to optimize the use of satellite altimeter data in the Lofoten Basin.

Northern North Atlantic Sea Level in CMIP5 Climate Models: Evaluation of Mean State, Variability, and Trends against Altimetric Observations

AbstractThe northern North Atlantic comprises a dynamically complex area with distinct topographic features, making it challenging to model oceanic features with global climate models. As climate models form the basis for assessment reports of future regional sea level rise, model evaluation is important. In this study, the representation of regional sea level in this area is evaluated in 18 climate models that contributed to phase 5 of the Coupled Model Intercomparison Project. Modeled regional dynamic height is compared to observations from an altimetry-based record over the period 1993–2012 in terms of mean dynamic topography, interannual variability, and linear trend patterns. As models are expected to reproduce the location and magnitude but not the timing of internal variability, the observations are compared to the full 150-yr historical simulations using 20-yr time slices. This approach allows one to examine modeled natural variability versus observed changes and to assess whether a forced signal ...