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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L16607, doi:10.1029/2007GL030634, 2007
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Intrinsic dynamics and long-term evolution of a convectively generated
oceanic vortex in the Greenland Sea
Angelo Rubino,1 Alexey Androssov,2 and Sergey Dotsenko3
Received 9 May 2007; revised 3 July 2007; accepted 31 July 2007; published 28 August 2007.
[1] Nonstationary dynamics, realistic long-term evolution,
and influence on local convection characterizing an oceanic
vortex similar to those observed in the central Greenland
Sea were investigated using in situ data, a hierarchy of
numerical models, and an analytical theory. A nonhydrostatic model for the simulation of convective plumes
was nested into a regional ocean-ice model forced and
initialized by a global, atmosphere/ocean/ice model. In the
central Greenland Sea, water masses similar to those
observed in a convectively-generated vortex and a
corresponding numerically reconstructed 3D velocity were
imposed in the non-hydrostatic model. Under atmospheric/
oceanic realistic conditions and forcing, the simulated
vortex evolved as an almost circular, nonstationary, longlived, predominantly anticyclonically rotating feature. As
time elapsed, it detached from the sea surface and became an
intermediate vortex, which influenced the local convective
preconditioning. Its inertial pulsations, shape, and velocity
structure closely resemble corresponding characteristics of
recently discovered theoretic nonstationary anticyclones.
Citation: Rubino, A., A. Androssov, and S. Dotsenko (2007),
Intrinsic dynamics and long-term evolution of a convectively
generated oceanic vortex in the Greenland Sea, Geophys. Res. Lett.,
34, L16607, doi:10.1029/2007GL030634.
1. Introduction
[2] Mesoscale as well as submesoscale vortices are
abundant in the World Ocean [see, e.g., McWilliams, 1985;
Olson, 1991]. In accordance to this relevance, the study of
origin, development, and decay of mesoscale as well as
submesoscale oceanic surface and intermediate vortices has
been the object of numerous theoretical, observational, and
experimental investigations [see, e.g., Csanady, 1979; Gill,
1981; Nof, 1983; McWilliams, 1985, 1988; Hedstrom and
Armi, 1988].
[3] During the last years a renewed attention has been
devoted to the study of mesoscale as well as submesoscale
oceanic surface and intermediate vortices [see, e.g., Lee and
Niiler, 1998; Rubino et al., 1998, 2002; Wadhams et al.,
2002; Gascard et al., 2002; Rubino and Brandt, 2003;
Wadhams et al., 2004; Budéus et al., 2004; Dotsenko and
Rubino, 2006; Zhai et al., 2007]. In fact, recent investigations have demonstrated the important role they exert in the
1
Dipartimento di Scienze Ambientali, Università Ca’Foscari, Venice,
Italy.
2
Alfred Wegener Institute für Polar- und Meeresforschung, Bremerhaven,
Germany.
3
Marine Hydrophysical Institute, National Academy of Science of
Ukraine, Sevastopol, Ukraine.
Copyright 2007 by the American Geophysical Union.
0094-8276/07/2007GL030634$05.00
formation and transformation of water masses [see, e.g.,
Gascard et al., 2002; Wadhams et al., 2002; Budéus et al.,
2004; Ronski and Budéus, 2005]. Moreover, they are able to
profoundly influence the downward propagation of windgenerated near-inertial waves [Lee and Niiler, 1998; Zhai et
al., 2007].
[4] In particular, convectively generated, long-lived
coherent vortices have been frequently observed in the
central part of the Greenland Sea, where they seem to
constitute ‘‘hot spots’’ for intermediate and deep water
formation [Gascard et al., 2002; Wadhams et al., 2002;
Budéus et al., 2004]. Their life span is supposed to be large,
mostly as a result of the capacity demonstrated by eddies
located in regions prone to open-ocean convection to ‘‘be
recharged’’ by surface interactions [Budéus et al., 2004].
Using the available in situ data, their evolution in the
Greenland Sea has been tentatively schematized as follows:
Open ocean convective activity is considered to be responsible for their formation, preferentially in the central part of
the Greenland gyre, where winter doming is strongest. As
time elapses, they tend to evolve into intermediate rotating
vortices, whose position and characteristics may be, at least
partly, preserved. These peculiarities render thus the local
recharge and hence the occurrence of similar features during
the next winter season not improbable [Wadhams et al.,
2004]. Their observed intrinsic dynamics seem to be characterized by several major characteristics: they rotate mostly
anticyclonically and show a core area characterized by high
negative vorticity, with swirl velocities linearly increasing
with their radius [Budéus et al., 2004].
[5] In this paper the nonstationary structure, the intrinsic
dynamics, as well as the long-term evolution of a convectively generated vortex observed in the Greenland Sea are
explored, under realistic conditions, using in situ data, a
hierarchy of nested numerical models and a recently
developed analytical theory. As a result, we recognize the
simulated features to belong to intermediate, stratified,
nonstationary pulson-like vortices of high order [see Rubino
et al., 1998; Dotsenko and Rubino, 2006]. By comparing
the simulated water mass squared Brunt-Väisälä frequency
in the central Greenland Sea with and without assuming the
presence of a convectively generated vortex in the area, the
influence exerted by such rotating features on the convective activity in the central Greenland Sea is discussed.
2. Numerical Experiments and Analytical
Modeling
[6] The configuration of the numerical models used in the
present investigation is schematized in Figure 1: The nonhydrostatic and the regional hydrostatic model are similar to
those used in previous studies [see, e.g., Romeiser et al.,
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Figure 1. Configuration of the numerical models used in the present investigation: (a) REMO/MPI-OM coupled model;
(b) regional hydrostatic and local nonhydrostatic model.
2004; Androssov et al., 2005], to which the reader is
referred for further details about the employed numerical
techniques. These models are coupled to an ice model
similar to that of Hibler [1979]. The regional model, which
describes the hydrodynamics of an oceanic area encompassing the Greenland Sea (see Figure 1) was initialized and
forced using the results of larger scale simulations
performed with the REMO/MPI-OM coupled model
[Maier-Reimer, 1996], which is an atmospheric/oceanic/
ice model. In particular, wind forcing consists of six-hourly
data simulated by the REMO model (its horizontal resolution is 1°) adequately temporally and spatially interpolated
to match the requirements of the higher resolution regional
model. In the central part of the model domain, which
corresponds to a mean position of the centre of the cyclonic
gyre of the Greenland Sea (supposed here to be located near
75°N 0°E/W, where a model focus was implemented), a
very high resolution, fully non-hydrostatic model (minimum
horizontal grid step = 125 m) was nested to the regional
model. The selected initial fields of the REMO/MPI-OM
model were chosen to represent, realistically, a winter
(January) scenario in the central Greenland Sea. In the
core-area of that region (centred at 75°N 0°E/W), in
correspondence with the centre of the simulated Greenland
Sea cyclonic gyre, temperature and salinity values similar to
those observed locally in a convectively generated vortex
during April 2001 [Wadhams et al., 2002] and a
corresponding simulated 3D velocity field were imposed
as a part of the model solution after three months simulation
time. Note that the vortex observed by Wadhams et al.
[2002] and, consequently, the vortex we imposed in the
central part of our model, are almost vertically homoge-
neous in salinity. Given the imposed vortex structure, a
diagnostic model run was thus used to simulate a 3D
associated velocity structure. As a result, an anticyclone
emerged, characterized by a high negative vorticity core,
in accordance with different theories and experiments
describing mesoscale and submesoscale coherent features
[see, e.g., Csanady, 1979; Gill, 1981; McWilliams, 1988;
Hedstrom and Armi, 1988; Rubino and Brandt, 2003;
Dotsenko and Rubino, 2006]. On such a basis, the numerical model was then allowed to further evolve, under
atmospheric and lateral forcing from the REMO/MPI-OM
coupled model, as long as 36 months. In Figure 2 zonal
vertical sections across the vortex centre depicting the
distribution of potential temperature one week (Figure 2a),
one month (Figure 2b), two months (Figure 2c), and three
months (Figure 2d) after the insertion of the vortex in the
model central area are delineated. Initially, the coldest
vortex isotherm largely outcrops (Figures 2a and 2b). This
stage of the vortex evolution corresponds to a possible stage
of its ‘‘recharge’’ by surface cooling. As time elapses, the
exposure of the vortex core to the atmosphere is substantially eroded (Figure 2c); but, rather than loosing its
coherence, it tends to transform into a noticeably stable,
intermediate vortex (Figure 2d). The results of our numerical simulations indicate that, in this stage of the vortex life,
its shape does not deviate substantially from circular symmetry. Note that, in a central region within the vortex body,
the shape of the isopycnals seems to be parabolic, whilst it
largely deviates from parabolicity at the vortex periphery.
The temporal evolution of the simulated vortex tangential
velocity, normalized by a mean vortex velocity, about 20 km
from the vortex centre is depicted in Figure 3a. Indepen-
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Figure 2. Zonal vertical sections across the vortex centre depicting the distribution of potential temperature (a) one week,
(b) one month, (c) two months, and (d) three months after the insertion of the vortex in the model central area.
dently on depths, the vortex is characterized by exact
inertial pulsations which are in phase throughout its body.
Note, however, that the amplitude of the pulsations largely
varies with depth. During most of the inertial period, the
vortex rotates anticyclonically at all depths, its average
magnitude being largest around 1600 m depth and decreasing toward the bottom and toward the surface. Very noticeably, there are time intervals within the inertial period
during which the circulation of part of the vortex periphery
is reversed, as the rotation is cyclonic there (Figure 3a).
[7] In Figure 3b, the simulated vortex tangential velocity,
normalized by a mean vortex velocity, is depicted, as a
function of the vortex radius, for a selected time of the
inertial period, at three different depths. Independently on
depth, in a central region, extending about 12 km from the
vortex centre, the vortex tangential velocity can be
accurately described as a linear function of the radial
coordinate. Outside the core, the tangential velocity
substantially deviates from linearity: It decreases toward
the periphery and tends to connect to the ambient velocity
field. These characteristics of the simulated vortex, which
are consistent with observations [see, e.g., Budéus et al.,
2004] resemble characteristics of recently discovered theoretic oceanic nonstationary, stable pulsating vortices, which
Figure 3. (a) Temporal evolution of the simulated vortex tangential velocity, normalized by a mean vortex velocity
v* = 1,27 cm/s, about 20 km from the vortex centre, at three different depths. (b) Simulated vortex tangential
velocity, normalized by a mean vortex velocity v*, as a function of the vortex radius, for a selected time of the
inertial period, at three different depths. The time series (T = 0) begins circa 3 months after the insertion of the
chimney in the central Greenland Sea.
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Figure 4. (a) Shape of a stratified (seven layers) pulson of the 5th order and (b) structure of its tangential velocity
(normalized by a mean vortex tangential velocity v* = 1,27 cm/s), as a function of the vortex radius, for a selected time of
the inertial period. The theoretical vortex was obtained by considering stratification and density contrasts typical of the
vortex simulated numerically (see Figure 2).
are solutions of the nonlinear shallow water equations on an
f plane. Based on previous analytical theories [see, e.g., Gill,
1981; McWilliams, 1988; Rubino et al., 1998], Dotsenko
and Rubino [2006] found analytical solutions describing
nonstationary, nonlinear oceanic circular vortices. They are
inertially pulsating, stable, circular features characterized by
an arbitrary stable density distribution and by general
shapes and structures of their tangential velocity. In their
centre, the rotation is anticyclonic, but, at their periphery,
cyclonic circulation is possible during part of an inertial
cycle. In Figure 4 the shape of a stratified (seven layers)
pulson of the 5th order and the structure of its tangential
velocity (normalized by a mean vortex tangential velocity),
obtained by considering typical stratification and density
contrasts of the vortex simulated numerically, are depicted.
A large similarity between analytical and numerical results
can be evinced. In a central region within the vortex body,
the shape of the isopycnals resembles a parabola, whilst it
largely deviates from parabolicity at the vortex periphery. In
such a central region the vortex tangential velocity can be
accurately described as a linear function of the radial
coordinate. Outside this region, however, the tangential
velocity substantially deviates from linearity. Note that the
similarity between numerical and analytical solutions
encompasses different other properties, like, e.g., the presence of exact inertial pulsations, the vertical structure of the
tangential velocity, and the coexistence of time intervals
during which the vortices rotate cyclonically at its
periphery. These similarities account for the robustness of
pulson-like surface as well as intermediate vortices in the
World Ocean [see Rubino and Brandt, 2003]. As indicated
by different theoretical results [see, e.g., Lee and Niiler,
1998; Zhai et al., 2007], anticyclonic vortices are also able
to induce downward near-inertial wave propagation, thus
substantially contributing to drain wind-induced near-inertial energy toward the abyss. In order ascertain, at least
partly, the origin of the simulated inertial oscillations and to
test the capability of the simulated vortex to transport nearsurface, near-inertial energy induced by the wind toward the
abyss, we repeated our numerical simulations using a) a
much weaker wind forcing and b) monthly averaged wind
forcing. The results obtained in both simulations indicate
that, indeed, a noticeably lower near-inertial energy is
obtained within the vortex, when a lower wind activity is
imposed and/or typical temporal scales of synoptic disturbances are neglected. This demonstrates the influence
exerted by wind strength and synoptic variability in the
near-inertial wave transmission to the vortex interior.
Specifically, in our case, the vortex, initially located at the
sea surface, tends to preserve a large part of its initial nearinertial energy while intruding at depth. In the layers located
above its body, however, a complex, mostly cyclonic
circulation is present, which renders the near-inertial downward wave transmission intricate.
[8] It can be thus conjectured that characteristics of the
simulated vortex, together with the specific features of the
background circulation in the central Greenland Sea, be
crucial for a comprehension of the longevity attributed to
convectively generated vortices in the area. We performed,
to this purpose, different numerical simulations characterized by the fact that the vortex was posed outside the centre
of the Greenland Sea cyclonic gyre and we observed, in this
case, a much faster dissipation of the vortex coherence.
[9] In order to quantify the preconditioning exerted by
the simulated vortex to open-ocean convective events, we
repeated the numerical simulations described above without
inserting, as a part of the oceanic stratification, the structure
of the observed, convectively generated vortex in the central
Greenland Sea. We analyzed the differences in the water
mass squared Brunt-Väisälä frequency between the two
experiments: they can be then used as a measure of
the convective preconditioning induced by the simulated
vortex. In particular, we compared the temporal evolution of
the vertical distributions of the squared Brunt-Väisälä
frequency at 75N 0E/W for the two experiments (see
auxiliary material1). During the first winter, the differences,
which emerged, are obviously due to the insertion of the
observed vortex, which represents a noticeable input of
almost homogeneous water masses. Its presence, however,
leaves a long-lived trace, as, almost throughout the water
column, a smaller squared Brunt-Väisälä frequency was
simulated in the experiment with the convective vortex
during the successive winter. This trace decreased noticeably during the successive summer (which, from our model
results, can be interpreted as the joint result of a shift in the
position of the vortex core region and of a partial export of
1
Auxiliary materials are available in the HTML. doi:10.1029/
2007GL030634.
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the vortex water), but it was still evident during the
successive winter and, indeed, continued throughout the
whole simulation period (3 years).
3. Conclusions
[10] A hierarchy of nested numerical models was implemented in the Greenland Sea and surroundings aimed at
describing the nonstationary, convectively generated
dynamics in the central Greenland Sea and the processes
related to its small-scale and mesoscale manifestations by
retaining a large part of the complexity inherent in the larger
scale, oceanic and atmospheric variability in the area. As a
result, we were able to describe, under realistic atmospheric
and oceanic forcing, the intrinsic, nonstationary dynamics
of a convectively generated vortex: it is demonstrated
that the vortex can be accurately approximated by an
intermediate pulson of high order [Dotsenko and Rubino,
2006]. This findings account for the robustness of pulsonlike surface as well as intermediate vortices in the World
Ocean. Such a characteristic, together with the specific
features of the background circulation in the central
Greenland Sea, seem to be crucial to the comprehension
of the longevity attributed to convectively generated
vortices in the area: A comparison between two long-term
numerical experiments carried out with and without inserting the convectively generated vortex as a part of the
stratification in the central Greenland Sea reveals that such
vortices can contribute indeed to the destabilization of the
water column, their presence being noticeable even several
years after their insertion in the area. They seem moreover
to contribute, even when located in the interior ocean, to
drain wind-induced near-inertial energy toward the abyss.
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A. Androssov, Alfred Wegener Institute für Polar- und Meeresforschung,
Bussestrasse 24, Building F-210, D-27570 Bremerhaven, Germany.
S. Dotsenko, Marine Hydrophysical Institute, National Academy of
Science of Ukraine, Kapitanskaya st. 2, 99011 Sevastopol, Ukraine.
A. Rubino, Dipartimento di Scienze Ambientali, Università Ca’ Foscari,
Calle Larga Santa Marta, Dorsoduro 2137, I-30123 Venice, Italy.
([email protected])
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