within the isobath of 3000 m. Note that the western and northern parts of this vortex
are formed by the western NwAC jet. In the northern part, there are two currents: a
jet of the Atlantic waters directed northward and confined to the Mona Ridge, as
well as a jet along the edge of the LB, which weakens when moving eastward. The
structure of the AV in the LB is asymmetric: the velocities at its northern periphery
at the 30 m level are 6–12 cm/s, 4–7 cm/s at 400 m, and 1–3 cm/s at 1000 m; the
velocities at its southern periphery are 3–6 cm/s, 3–5 cm/s, and 0–1 cm/s,
respectively, and in the center they range from 1–2 cm/s to zero. This means that a
circular current exists in the 0–1500 m layer along the perimeter of the isobath
3000 m, which is better pronounced in its northern part. The velocities are weak
(0–1 cm/s) in its central part. This current is located within the boundaries of the
isobath 3200 m. Therefore, after analyzing these materials, it is possible to assume
that an intrathermocline lens exists at the depths of 250–700 m within the LB with
an average long-term position of its center at 69.5° N, 3.5° E. The lens forced by the
topographic beta-effect drifts along a cyclonic trajectory. A similar result, according
to the hydrological observations, was obtained by Alekseev et al. [1], in which the
authors proposed to consider the scales of 60 and 130 km for describing the AV
and LB.
Although the Argo observations give us a clear pattern of currents in the LB as
part of the general circulation in the PS, they cannot serve as a basis for explaining
the mechanisms of formation and displacement of vortices. In this relation, it is
extremely important to conduct model studies within the framework of an adequate
mathematical model, using estimates of the natural space and time scales. Below,
we investigate the evolution of an intrathermocline lens in the central part of LB for
different types of current fields using the three-layer version of the Contour
Dynamics Method.
Numerical Model and Simulation
Let us consider a model basin to explain the mechanism of the impact of the
underwater depression on the behavior of the anticyclonic intrathermocline lens.
We model the motion in the form of a set of two circular cylinders with displaced
centers and with vertical walls which coincide with the 3000 and 3200 m isobaths.
Let the outer circle radius be equal to 132 km and the internal to 60 km; the
height difference on each circle is 200 m, and their centers are in coordinates
X 1 ; Y 1
ð
Þand X 2 ; Y 2
ð
Þ, respectively. We will use a quasi-geostrophic three-layer
ocean model [20] with a piecewise-constant densities of ρ 1 , ρ 2 , ρ 3 between
0–250 m, 250–655 m, and 655–3000 m, respectively, and Δρ 1 = ρ 2 − ρ 1 =
0.00018 g cm
− 3 , Δρ 2 = ρ 3 − ρ 2 = 0.00015 g cm
− 3 . These values are typical for this
water basin [4]. We assume that the vertical and horizontal scales are H = 3 km
and L = 40 km, respectively; for the dimensionless layer thicknesses and radii of
cylinders we get h 1 = 0.0833, h 2 = 0.1383, h 3 = 0.7784, and R 1 = 3.3, R 2 = 1.5.
Evolution of an Intrathermocline Lens …
337
are formed by the western NwAC jet. In the northern part, there are two currents: a
jet of the Atlantic waters directed northward and confined to the Mona Ridge, as
well as a jet along the edge of the LB, which weakens when moving eastward. The
structure of the AV in the LB is asymmetric: the velocities at its northern periphery
at the 30 m level are 6–12 cm/s, 4–7 cm/s at 400 m, and 1–3 cm/s at 1000 m; the
velocities at its southern periphery are 3–6 cm/s, 3–5 cm/s, and 0–1 cm/s,
respectively, and in the center they range from 1–2 cm/s to zero. This means that a
circular current exists in the 0–1500 m layer along the perimeter of the isobath
3000 m, which is better pronounced in its northern part. The velocities are weak
(0–1 cm/s) in its central part. This current is located within the boundaries of the
isobath 3200 m. Therefore, after analyzing these materials, it is possible to assume
that an intrathermocline lens exists at the depths of 250–700 m within the LB with
an average long-term position of its center at 69.5° N, 3.5° E. The lens forced by the
topographic beta-effect drifts along a cyclonic trajectory. A similar result, according
to the hydrological observations, was obtained by Alekseev et al. [1], in which the
authors proposed to consider the scales of 60 and 130 km for describing the AV
and LB.
Although the Argo observations give us a clear pattern of currents in the LB as
part of the general circulation in the PS, they cannot serve as a basis for explaining
the mechanisms of formation and displacement of vortices. In this relation, it is
extremely important to conduct model studies within the framework of an adequate
mathematical model, using estimates of the natural space and time scales. Below,
we investigate the evolution of an intrathermocline lens in the central part of LB for
different types of current fields using the three-layer version of the Contour
Dynamics Method.
Numerical Model and Simulation
Let us consider a model basin to explain the mechanism of the impact of the
underwater depression on the behavior of the anticyclonic intrathermocline lens.
We model the motion in the form of a set of two circular cylinders with displaced
centers and with vertical walls which coincide with the 3000 and 3200 m isobaths.
Let the outer circle radius be equal to 132 km and the internal to 60 km; the
height difference on each circle is 200 m, and their centers are in coordinates
X 1 ; Y 1
ð
Þand X 2 ; Y 2
ð
Þ, respectively. We will use a quasi-geostrophic three-layer
ocean model [20] with a piecewise-constant densities of ρ 1 , ρ 2 , ρ 3 between
0–250 m, 250–655 m, and 655–3000 m, respectively, and Δρ 1 = ρ 2 − ρ 1 =
0.00018 g cm
− 3 , Δρ 2 = ρ 3 − ρ 2 = 0.00015 g cm
− 3 . These values are typical for this
water basin [4]. We assume that the vertical and horizontal scales are H = 3 km
and L = 40 km, respectively; for the dimensionless layer thicknesses and radii of
cylinders we get h 1 = 0.0833, h 2 = 0.1383, h 3 = 0.7784, and R 1 = 3.3, R 2 = 1.5.
Evolution of an Intrathermocline Lens …
337