Detection and tracking of mesoscale eddies in the algerian basin from altimetric data and in-situ measurements
Author(s)
Pessini, Federica
Date Issued
June 15, 2017
Type
Doctoral Thesis
Abstract
The circulation of the Western Mediterranean Sea (WMED) is dominated by
highly variable and inhomogeneous mesoscale circulation. It is affected by the
formation of Algerian Eddies (AEs), mainly in the Algerian Basin. These strong
and large propagating structures can be both cyclonic and anticyclonic. Due
to the short lifespan of cyclonic eddies, only anticyclonic ones are considered
in this work.
In order to investigate the spatial and temporal distribution of eddy generation
and their respective paths in the WMED, we use an automated detection
and tracking method applied to 22 years of daily altimetric (SLA, Sea Level
Anomalies) data (AVISO merged, delayed-time, SLA data).
The tracking algorithm provides information about the lifetime of each detected
eddy, and permits the evaluation of their mean properties, such as radius, kinetic
energy and translational velocity.
We treat separately the eddies with lifetime respectively under and over 100
days. We find that short-life eddies mostly occur in the northern part of the
domain, above 39 N, along the North Balearic Front and we refer to them
as Frontal Anticyclonic Eddies (FAEs). Most are formed in fall and winter.
By contrast, longer-life eddies tend to arise in the southern part of the basin,
along the Algerian Current, mainly in spring and summer. A clear spatial and
seasonal complementarity of the two kinds of structures is evident.
We observe two preferred areas of formation along the Algerian slope, at 5 E
and at 7 E. The southern long-life AEs are more energetic and move eastward
along the coast to the Sardinia Channel. We suppose that high salinity on
the surface layer hinders the transit of the eddies through the channel, forcing
them to deviate northward with the cyclonic circulation. From the perspective of eddy kinetic energy (EKE), we find that the daily
MEKE in the south is higher than in the north of the domain, and has an
annual periodicity, due to the seasonality of heat fluxes, and a periodicity of
4:4 years. The latter should be more comprehensively examined with a larger
data set.
In order to investigate the vertical extension of the eddies and the water masses
involved, we use CTD data sampled in the course of three oceanographic cruises
in the WMED. We superimpose the CTD transects on the SLA maps in each
eddy location. Two transects intersect southern AEs and a third intersects a
FAE. In all cases, we measure a minimum level of salinity in the sub-surface
layer (50-70 m depth) co-located with the eddy core. It points to the presence
Atlantic Water trapped by the structure. The CTD data also provide the
physical characteristics (potential temperature, salinity and potential density
anomaly) of the water masses in the AEs. We found that southern AEs reach
1200 1500 m depth and the FAEs 600 700 m depth.
The detection and tracking method is a powerful instrument to study the eddies
and their variability, and the combination with in-situ data allows a more complete
characterization of these structures. Finally, this work helped to find the
preferred areas of eddy formation and their main pathways within the basin.
The interaction with the intermediate layer suggests that eddies are transported
by the Algerian Gyre in a cyclonic loop below 39 N.
Several studies have shown a strong correlation between the deflection of the
isopycnals, due to the eddies, and the distribution of the nutrients in the euphotic
layer. The study of the relationship between hydrodynamic processes
and the dynamics of the first trophic levels acquire importance in highly dynamic
seas such as the WMED. The complete description of mesoscale circulation
in the Algerian Basin provided by this dissertation is of fundamental
importance to examine the bio-ecological processes in the WMED.
Additional information
Dottorato di ricerca in Ecologia e gestione sostenibile delle risorse ambientali
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