Study of the Mechanisms of Vortex Variability in the Lofoten Basin Based on Energy Analysis

Abstract

Purpose. The Lofoten Basin is one of the most energetic zones of the World Ocean characterized by high activity of mesoscale eddies. The study is aimed at analyzing different components of general energy in the basin, namely the mean kinetic and vortex kinetic energy calculated using the integral of the volume of available potential and kinetic energy of the Lofoten Vortex, as well as variability of these characteristics. Methods and Results. GLORYS12V1 reanalysis data for the period 2010–2018 were used. The mean kinetic energy and the eddy kinetic one were analyzed; and as for the Lofoten Vortex, its volume available potential and kinetic energy was studied. Mesoscale activity of eddies in winter is higher than in summer. Evolution of the available potential energy and kinetic energy of the Lofoten Vortex up to the 1000 m horizon was studied. It is shown that the vortex available potential energy exceeds the kinetic one by an order of magnitude, and there is a positive trend with the coefficient 0,23·1015 J/year. It was found that in the Lofoten Basin, the intermediate layer from 600 to 900 m made the largest contribution to the potential energy, whereas the 0–400 m layer – to kinetic energy. The conversion rates of the mean kinetic energy into the vortex kinetic one, and the mean available potential energy into the vortex available potential one (baroclinic and barotropic instability) were analyzed. It is shown that the first type of transformation dominates in summer, while the second one is characterized by its increase in winter. Conclusions. The vertical profile shows that kinetic energy of eddies in winter is higher than in summer. The available potential energy of a vortex is by an order of magnitude greater than the kinetic energy. Increase in the available potential energy is confirmed by a significant positive trend and by decrease of the vortex Burger number. The graphs of the barotropic instability conversion rate demonstrate the multidirectional flows in the vortex zone with the dipole structure observed in a winter period, and the tripole one – in summer. The barotropic instability highest intensity is observed in summer. The baroclinic instability is characterized by intensification of the regime in winter that is associated with weakening of stratification in this period owing to winter convection.

Vertical variability of the intensity of the frontal zones of the Norwegian Sea

“…The interannual variability of the number of eddies has a significant positive linear trend, the strongest for anticyclones. A similar feature is also noted in [Travkin and Belonenko, 2021], where the presence of a positive significant linear trend in available potential energy associated with climate change in the North Atlantic is noted. There is also a similarity between the number of long-lived eddies in the FZs and the average annual AO and NAO indices from [Akhtyamova and Travkin, 2023].…”

Changes in the Available Potential and Kinetic Energy of Mesoscale Vortices When They Are Stretched into Filaments

“…Next, we analyze separately the estimates of kinetic and available potential energy terms in Equation ( 1). The horizontal integration area is bound by the vortex area (see Figure 1), and the vertical integral is taken from 0 to 1000 m depth [5,38,39]. Analysis (Figure 4) quantifies that the available potential energy of the vortex is 1.5 times higher than its kinetic energy.…”

“…Therefore, there are no corresponding estimates in Table 1. The bottom row of Table 1 shows the results of theoretical calculations using Equations ( 2)- (5). When the vortex is stretched, the core energy is found to decrease faster than the total energy of the vortex, which includes the energy of the external rotating water.…”

“…This is primarily due to the presence of baroclinic and barotropic instabilities of the current, which is the main reason for the generation of vortices. Nowadays, convincing evidence has been obtained that mesoscale vortices make a great contribution to the low-frequency variability in the sea level [1][2][3][4][5].…”