OSOcean ScienceOSOcean Sci.1812-0792Copernicus GmbHGöttingen, Germany10.5194/os-11-361-2015Eddy characteristics in the South Indian Ocean as inferred from surface
driftersZhengS.DuY.duyan@scsio.ac.cnLiJ.ChengX.State Key Laboratory of Tropical Oceanography, South
China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou
510301, ChinaNaval Institute of Hydrographic Surveying and Charting,
Tianjin 300061, Chinanow at: State Key Laboratory of Tropical Oceanography,
South China Sea Institute of Oceanology, Chinese Academy of Sciences, 164
West Xingang Road, Guangzhou 510301, ChinaY. Du (duyan@scsio.ac.cn)12May201511336137122October201411December201415April201524April2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://os.copernicus.org/articles/11/361/2015/os-11-361-2015.htmlThe full text article is available as a PDF file from https://os.copernicus.org/articles/11/361/2015/os-11-361-2015.pdf
Using a geometric eddy identification method, cyclonic and anticyclonic
eddies from submesoscale to mesoscale in the South Indian Ocean (SIO) have
been statistically investigated based on 2082 surface drifters from 1979 to
2013. A total of 19 252 eddies are identified, 60 % of them anticyclonic eddies. For the
submesoscale eddies (radius r < 10 km), the ratio of cyclonic eddies
(3183) to anticyclonic eddies (7182) is 1 to 2. In contrast, the number of
anticyclonic and cyclonic eddies with radius r≥ 10 km is almost equal.
Mesoscale and submesoscale eddies show different spatial distributions.
Eddies with radius r≥100 km mainly appear in the Leeuwin Current, a band
along 25∘ S, Mozambique Channel, and Agulhas Current, areas
characterized by large eddy kinetic energy. The submesoscale anticyclonic
eddies are densely distributed in the subtropical basin in the central SIO.
The number of mesoscale eddies shows statistically significant seasonal
variability, reaching a maximum in October and minimum in February.
Introduction
The South Indian Ocean (SIO) has unique current systems. A schematic general
circulation diagram is shown in Fig. 1. The South Equatorial Current (SEC)
in the SIO is in large part supplied by the Indonesian Throughflow (ITF).
The SEC splits into the Northeast Madagascar Current (NEMC) and Southeast
Madagascar Current (SEMC) when it reaches the east coast of Madagascar near
17∘ S (Schott et al., 2001, 2009). The NEMC flows
around the north tip of Madagascar at Cape Amber to the coast of Tanzania at
about 12∘ S and splits into northward and southward flows. The
northward branch of NEMC feeds into the East African Coast Current (EACC),
and the southward branch joins the Mozambique Channel throughflow with
anticyclonic eddies (de Ruijter et al., 2002; Schouten et al., 2003;
Ridderinkhof et al., 2010). The SEMC joins the Agulhas Current after passing
south of Madagascar and features with plentiful eddies and dipoles (de
Ruijter et al., 2004; Quartly et al., 2006). The Agulhas Current carries
warm and saline water from the Indian Ocean to the Atlantic Ocean through
the Agulhas Leakage (Gordon et al., 1992; Donners et al., 2004). The Agulhas
retroflection reenters the SIO as a broad northeastward flow and extends to
the west coast of Australia (Schott et al., 2009). The Leeuwin Current (LC)
off Western Australia is an anomalous poleward-flowing eastern boundary
current due to a large meridional pressure gradient (Godfrey and Ridgway,
1985; Feng et al., 2005). It carries warm, low-salinity water southward
along the coast of Western Australia (Cresswell and Golding, 1980; Pearce,
1991). The LC has seasonal variation: it flows strongly in the austral
winter with the weakest southerly winds (Cresswell, 1991; Fieux et al., 2005)
and strengthening alongshore pressure gradient (Godfrey and Ridgway, 1985;
Potemra, 2001).
A schematic diagram of identified current branches in the South
Indian Ocean, modified from Schott et al. (2009) and Beal et al. (2011).
Mean flow in the SIO is from the average of drifter-detected velocity in
bins with 1∘× 1∘ resolution, and velocity less
than 5 cm s-1 is omitted (for data details refer to Lumpkin and Johnson,
2013). Schematic current branches are the South Java Current (SJC),
Indonesian Throughflow (ITF), South Equatorial Current (SEC), Northeast and
Southeast Madagascar Current (NEMC and SEMC), East African Coast Current
(EACC), AC (Agulhas Current), and Leeuwin Current (LC). The subsurface
return flow is shown in magenta.
Mesoscale eddies are an important ocean dynamic phenomenon and provide
important material and dynamical fluxes for the equilibrium balances of the
general circulation and climate (McWilliams, 2013). Mesoscale eddies can
cause heat and salt transports (Qiu and Chen, 2005; Volkov et al., 2008;
Dong et al., 2014), and eddy-induced zonal mass transport is comparable in
magnitude with that of large-scale circulation (Zhang et al., 2014).
Mesoscale eddies modulate nutrient flux into the euphotic zone through
vertical and horizontal transport (Falkowski et al., 1991; Aristegui et al.,
1997; Crawford et al., 2005). Submesoscale eddies also play a key role in
biogeochemical budgets through intense upwelling of nutrients, subduction of
plankton and horizontal stirring (Ledwell et al., 1993; Abraham, 1998;
Abraham et al., 2000; Lévy et al., 2001; Lévy and Klein, 2004).
Mesoscale eddies in the SIO have been studied using satellite data (e.g.,
Palastanga et al., 2006; Chelton et al., 2011), ocean modeling (e.g.,
Backeberg et al., 2008) and in situ observations (e.g., de Ruijter et al.,
2004; Ridderinkhof et al., 2010). In the southeast Indian Ocean (IO),
altimetry measurements showed that anticyclonic eddies propagate westward
and equatorward, and cyclonic eddies propagate poleward (Morrow et al., 2004). The eddy kinetic energy (EKE) shows a seasonal cycle with a maximum
in austral summer and a minimum in austral winter (Jia et al., 2011). In
austral spring, enhanced heat flux forcing of combined meridional Ekman and
geostrophic convergence strengthens the upper-ocean meridional temperature
gradient and intensifies the vertical velocity shear. This modulation of
the vertical velocity shear changes the intensity of baroclinic instability
associated with the surface-intensified South Indian Countercurrent (SICC)
and underlying SEC system, leading to the seasonal variations of EKE (Jia et
al., 2011). In the Mozambique Channel, satellite SeaWiFs ocean color snapshots
showed large anticyclonic rings intermittently propagating poleward
along the western edge of the channel (Quartly and Srokosz, 2004). Analysis
of sea surface height (SSH) suggested a connection between mesoscale eddy
activity around Madagascar and large-scale interannual variability in the
SIO (e.g., Palastanga et al., 2006). Long-term moorings showed that four to
five anticyclonic eddies drifted southwards through the channel in each year
(Ridderinkhof et al., 2010). Southwest of Madagascar, cruise data showed
anticyclonic eddies propagated mostly westward, while cyclonic eddies
diverged more between west and southwest (de Ruijter et al., 2004). Altimetry
and sea surface temperature data showed that a number of
westward-propagating eddies exist along the zonal band near 25∘ S
(Quartly et al., 2006). In the Agulhas Current region, anticyclonic eddies
propagate northwestward and enter the southeast Atlantic Ocean (Backeberg et
al., 2008).
Though a few studies have worked on the mesoscale eddies in the SIO, the
investigation of eddy characteristics based on in situ observation is far
from enough. Compared with altimetry measurements, surface drifters give
fine spatial and temporal resolutions. Due to the long distance between
satellite tracks, altimetry can hardly detect eddies with radius r < 40 km (e.g., Chelton et al., 2011). Whereas surface drifters can detect
submesoscale eddies with radius r < 10 km (Berti et al., 2011; Li
et al., 2011; Schroeder et al., 2012). Surface drifters have a 6 h
temporal resolution, much more than altimetry data which, so far, usually
have 7- or 1-day intervals available to the public. Surface
drifters give reliable in situ observations and have been extensively used
in other ocean regimes (Deverdiere, 1983; Chaigneau and Pizarro, 2005;
Hamilton, 2007; Li et al., 2011; Zu et al., 2012).
(a) Release and (b) terminal locations of surface drifters.
Number of drifters passing through each 1∘× 1∘ bin in their trajectories.
(a) Meridional and (b) zonal accumulated number of drifters as a
function of longitude and latitude, respectively.
The present study investigates characteristics of eddy spatial and temporal
distribution in the SIO based on in situ surface drifters. We wish to
address the following questions: do eddies have uniform spatial
distribution? If not, what is different between cyclonic and anticyclonic
eddies with different radii. Does the number of eddies have seasonal variation? In
this paper, spatial distribution and temporal variation of cyclonic and
anticyclonic eddies with different radii are studied. The paper may extend
our knowledge of mesoscale and submesoscale eddies from in situ
observations and provide a background for biochemical study.
The rest of this paper is organized as follows. Data and methods are
introduced in Sect. 2. Eddy characteristics and eddy statistics in the SIO
are described in Sect. 3. Discussion and a summary are given in
Sect. 4.
Data and methods
The surface satellite-tracked drifter data used in this paper are archived
at the Atlantic Oceanographic and Meteorological Laboratory (AOML). The AOML
receives drifter positions from the Doppler shift of its transmission from
the American–French satellite-based system (Argos, Lumpkin and Pazos, 2007). The
Drifter Data Assembly Center (DAC) at AOML assembles these raw data, applies
quality control procedures, and interpolates them via the kriging method to
regular 6 h intervals (Hansen and Herman, 1989; Hansen and Poulain,
1996). The drifters have their drogues centered at 15 m depth to measure
surface currents, and a semi-rigid material throughout the drogue can
provide support for the drifter to maintain its shape in high-shear flows
(Lumpkin and Pazos, 2007). In this study, surface drifter data from
February 1979 to September 2013 are used. There were a total of 2226 drifters
released in the IO (Fig. 2a), and their distribution was not uniform.
Drifters covered most regions in the Arabian Sea and Bay of Bengal, but
drifters were sparse south of 20∘ S. A total of 2567 drifters
terminated (Fig. 2b) in the IO, a little more than those released in the IO;
some came from the adjacent oceans. Drifters were released in particular
areas, like along the commercial tracks, but their terminations were
uniform. Figure 3 shows drifter numbers passing through each 1∘× 1∘ bin in their trajectories. Despite release locations
mainly north of 20∘ S, drifters prefer to stay in the central SIO.
Therefore, we select all 2082 drifters captured in the region of 20–120∘ E, 5–60∘ S
(rectangle line in Fig. 3) to study the eddy characteristics. We also calculate drifter numbers as a
function of longitude and latitude. There are two maxima near longitudes
75 and 90∘ E (Fig. 4a), and two maxima near latitudes
41 and 33∘ S (Fig. 4b). These maxima are
corresponding to the NEMC, SEMC, Agulhas Current, and subtropical convergent
region, respectively. Figure 5a shows the number of drifter observations in
each year since 1985, and Fig. 5b shows the number of drifter observations
in each climatological calendar month. Although surface drifter plans began
in 1979, observations were sparse in the SIO before 1995 (Fig. 5a). The
number of observations increased dramatically since 1995 and reached
a maximum in 2010. Fortunately, it does not show significant seasonal
difference (Fig. 5b).
(a) Number of all drifter observations in each year since 1985,
each observation is one 6 h position from a drifter trajectory. (b) Number of all drifter observations at 6 h intervals in each
climatological calendar month.
A geometric eddy identification method based on surface drifters has been
successfully used in the northeastern Atlantic Ocean (Lankhorst, 2006;
Lankhorst and Zenk, 2006), Kuroshio Extension region (Dong et al., 2011) and
northern South China Sea (Li et al., 2011). In our study, we use the method
developed by Li et al. (2011) to identify eddies in the SIO. The method is
based on definition of a closed loop with the starting point overlapped by
the ending point. Eddies are identified through four steps (see more
detail in Li et al., 2011): first, find overlapping points along surface
drifter trajectories. Second, perform quality control on overlapping points to
eliminate false points and avoid losing internal overlapping points. Third,
determine polarity of loops (cyclonic or anticyclonic drifter track). In the
Southern Hemisphere, when a surface drifter is caught by a cyclonic
(anticyclonic) eddy it will make clockwise (counterclockwise) loops.
Fourth, cluster loops to oceanic eddies. Some drifters can be trapped in
eddies for a long time period (figure is omitted). The mean number of
(drifter-tracked) rotations for individual large cyclonic (anticyclonic)
eddies is 2.22 (2.23), while for submesoscale cyclonic (anticyclonic) eddies
it is 2.49 (3.29). Submesoscale eddies trend to rotate more times than large
mesoscale eddies. We cluster drifter loops with more than one rotation to
one eddy according to method of eddy identification (Li et al., 2011). The
loop center is estimated by geometrically averaging all the sample points in
the loop. The loop radius is defined by the mean distance between all loop
points to the loop center. The eddy radius is defined by mean radius of
clustered loops. To remove inertial oscillation, eddies with a period less
than double the local inertial oscillation period (T) are not included.
The inertial oscillation period is calculated as follows:
T=2πf,
where f=2Ωsin(φ), f is the Coriolis
parameter, Ω is the earth rotating frequency (Ω=2π/24h=7.27×10-5s-1), and φ is the latitude of the eddy
centers.
Histogram of eddy occurrence with a bin width of 5 km as eddy
radius. Blue and red bars indicate cyclonic and anticyclonic eddies,
respectively.
To compare with eddies detected by surface drifters, mesoscale eddies in
altimeter observation of SSH are also used. The mesoscale eddies were
detected from SSH-based automated eddy identification procedures distributed
by Chelton et al. (2011).
The gridded sea level anomaly (SLA) data product with 1/3∘ resolution
is used to calculate EKE: these SLA data are distributed by Archiving,
Validation, and Interpretation of Satellite Oceanographic data (AVISO). EKE
are calculated by geostrophic velocity anomalies (Pujol and Larnicol, 2005;
Jia et al., 2011) as follows:
EKE=12(Ug′2+Vg′2),Ug′=-gfΔη′Δy and Vg′=gfΔη′Δx,
where Ug′ and
Vg′ are the geostrophic velocity anomalies, and Δη′ is the
SLA.
(a) Number of eddies detected in 1∘× 1∘ bins. (b) Number of eddies with radii equal or greater than
10 km detected in 1∘× 1∘ bins. (c) Ratio of
number of eddies to number of drifters in 1∘× 1∘
bins. (d) Ratio of number of eddies with radii equal or greater than 10 km to
number of drifters in 1∘× 1∘ bins.
Eddy characteristics and statistics in the SIONumber and radius
On the basis of the surface satellite-tracked drifter data, a total of 57 228
loops were detected in the SIO. Among them 22 773 are cyclonic loops and
34 455 are anticyclonic loops. After clustering the loops, a total of 19 252
eddies are detected with 7657 cyclonic eddies (clockwise) and 11 595
anticyclonic eddies (counterclockwise). The number of anticyclonic eddies
account for 60 % of eddies. Figure 6 shows the histogram of eddy radii with
bin widths of 5 km, in which blue (red) bars depict cyclonic (anticyclonic)
eddies. Submesoscale eddies (radius r < 10 km) are identified
successfully, and its number is 10 365 accounting for 54 % of total
eddies. In terms of submesoscale eddies, the ratio of cyclonic eddies to
anticyclonic eddies is 1 to 2. With submesoscale eddies excluded, the mean
radii are 39, 37, and 41 km for all, cyclonic, and anticyclonic
eddies, respectively. As documented by Chaigneau and Pizarro (2005), if
drifters are on average statistically evenly distributed along the eddy
radius R, the probability density p(r,θ) of finding the drifter at
a radius r and direction θ relative to the eddy center is
constant. The formulate is as follows: p(r,θ)=1∫0R∫02πrdrdθ=1πR2. The mean
distance R1‾, or expectation E(r)=∫0R∫02πr2p(r,θ)drdθ of a drifter from the
eddy center is then given by R1‾=2R/3. The mean radius
R1‾ of all eddies is 19 km in the SIO, implying a
characteristic eddy diameter 2R of 57 km. This order of magnitude is
consistent with the mean Rossby deformation radius (20–100 km) in the region of
10–50∘ S in the SIO (Chelton et al., 1998).
Spatial distribution
Figure 7a shows eddies are densely distributed in the subtropical region in
the central SIO. As drifters are also densely distributed in the central SIO
(Fig. 3), a question is whether the spatial distribution of eddies depends
on that of drifter number or not. The ratio of the number of eddies to the number of
drifters is relatively high in the band of 25–40∘ S (Fig. 7c), which suggests that more drifters (Fig. 3) would give more detected
eddies. When the submesoscale eddies (radii < 10 km) are excluded,
the spatial distribution of the number of eddies is almost uniform (Fig. 7b), and
the relatively large ratio is mainly in the Mozambique Channel and Agulhas
Current (Fig. 7d), which is not fully corresponding to the drifter
distribution (Fig. 3). The method of eddy detection still works well in the
sparse drifter region. We separate cyclonic eddies (Fig. 8a) from
anticyclonic eddies (Fig. 8c) to study their spatial distribution in detail.
The cyclonic eddies have a relatively uniform spatial distribution and in
most regions are fewer than 15 in 1∘× 1∘ bins.
In contrast, the anticyclonic eddies are densely distributed in the central
SIO with more than 15 eddies in 1∘× 1∘ bins.
When submesoscale eddies are excluded, cyclonic (Fig. 8b) and anticyclonic
eddies (Fig. 8d) show similar spatial distributions. Therefore, different
spatial distributions according to eddy size imply that eddy properties also
affect the results irrespective of drifter density.
(a) Number of cyclonic eddies detected in 1∘× 1∘ bins. (b) Number of cyclonic eddies with radii equal or greater
than 10 km detected in 1∘× 1∘ bins. (c) Same as
(a) but for anticyclonic eddies. (d) Same as (b) but for anticyclonic
eddies.
The distribution of cyclonic and anticyclonic eddies detected from
drifters in the SIO.
Climatological mean of eddy kinetic energy in the SIO derived
from altimetry data over 1993–2012. Blue rectangles represent the Leeuwin
Current region (106–120∘ E, 18–39∘ S), a band along 25∘ S (57–106∘ E, 20–30∘ S), Mozambique Channel
(29–57∘ E, 10–34∘ S) and
Agulhas Current (20–70∘ E, 34–46.5∘ S). Black rectangle represents the interior ocean (70–106∘ E, 30–55∘ S) in the SIO.
Spatial distribution of eddies varies with the size. All eddies are
categorized into four groups according to their size: large eddies (r≥ 100 km), medium eddies (100 km > r > 60 km), small
eddies (60 km ≥r≥ 10 km), and submesoscale eddies (r < 10 km). Figure 9 shows the trajectories of drifters in cyclonic eddies (blue
lines) and anticyclonic eddies (red lines) for the four groups, and the
numbers of cyclonic (anticyclonic) eddies are 203 (340), 565 (619), 3706
(3454), and 3183 (7182), respectively. We also used two loops as a criterion
for eddy detection (figure is omitted), and the result was similar to
Fig. 9. Actually, eddies with periods less than double the local inertial
oscillation period are excluded to remove inertial oscillation, and loops
are clustered to one eddy in detection algorithm. The radii of eddies are
smaller than the basin scale (Fig. 6) in our study. There are more large anticyclonic
eddies than cyclonic ones. Large eddies are numerous in the Leeuwin Current, a
band along 25∘ S, Mozambique Channel, and Agulhas Current (Fig. 9a). The climatological mean EKE in Fig. 10 is mean EKE derived from AVISO
SLA data from 1993 to 2012. When we regard this climatological mean EKE as
background EKE, the mean EKE at the eddies' centers (along the eddies' trajectories)
could be calculated through linear interpolation in the SIO for large
eddies, medium eddies, small eddies and submesoscale eddies, giving 357
(346) cm2 s-2, 301 (298) cm2 s-2, 243 (243) cm2 s-2, and 167 (168) cm2 s-2, respectively. The mean EKE for
large eddies is larger than for other groups of eddies. Five regions are
selected in the SIO – including the Leeuwin Current at the eastern boundary, a
band along 25∘ S, Mozambique Channel and Agulhas Current at the
western boundary, and the interior Ocean (rectangles in Fig. 10) – to study
relationships between different kinds of eddies and mean EKE. The results
show that the mean EKE of large eddies at the eastern boundary, in a band along
25∘ S and at the western boundary, is larger than for other groups,
but this comparison is reversed in the interior ocean. The number of large
eddies in large EKE regions is 457 accounting for 84 % of all large
eddies in the SIO. Therefore, large eddies tend to populate regions with
large EKE. We also calculate the ratio of number of eddies where mean EKE is
equal to or greater than 200 cm2 s-2 to the number of all eddies, which is
similar for criterion 250 cm2 s-2. This ratio is larger (for large
eddies) than for other eddy groups in large EKE regions, except in the
interior ocean. Hence, large eddies populate a band along 25∘ S
and the eastern and western boundaries with large EKE. Whereas in the interior
ocean with small EKE the number of large eddies is small. Although many
drifters can be found northeast of Madagascar (Fig. 3), no large eddies are
detected there. There are few large eddies south of 43∘ S,
probably due to the Rossby deformation radius of less than 30 km (Chelton et
al., 1998). The spatial distribution of large cyclonic eddies and
anticyclonic eddies are different. Anticyclonic eddies are rich in the
Mozambique Channel and Agulhas Current, while many cyclonic eddies appear
south of Madagascar. In the Mozambique Channel, the shape of large eddies is
restrained by coastline in a northeast–southwest direction, and large eddies
show similar character with eddies detected by in situ observations (de
Ruijter et al., 2002; Swart et al., 2010) and altimetry
(Schouten et al., 2003), which also validated our method
of eddy detection. At same time, large eddies appear mainly along the center
line of the Mozambique Channel (Fig. 9a), and the western boundary current does not
show strong influence on eddies in this region. Medium eddies have a similar
spatial distribution to that of large eddies but occur in a wider area.
The number of eddies is about twice the number of large eddies (Fig. 9b).
Unlike other kinds of eddies, the number of small cyclonic eddies is greater
than small anticyclonic eddies (Fig. 9c). The small eddies are in the region
ranging from 20 to 44∘ S, and the distribution of
cyclonic eddies appear in the northeast–southwest direction west of Australia. Here
the distribution of cyclonic eddies is consistent with poleward propagation
at the eastern boundary, which can be explained by theories for vortex
propagation on a β-plane (Morrow et al., 2004). The submesoscale
eddies are densely distributed over the entire SIO (Fig. 9d), with a similar
pattern to that of total eddies (Fig. 7a). The submesoscale anticyclonic
eddies are densely distributed in the subtropical basin in the central SIO. The
area with a high number of submesoscale anticyclonic eddies agrees well with
the location of garbage patches (Maximenko et al., 2012; Van Sebille et al.,
2012). The appearance of more submesoscale anticyclonic eddies maybe come
from drifter aggregation, maintained by converging Ekman currents (Maximenko
et al., 2012), and Fig. 3 also reveal that drifters aggregated in the central
SIO. At the same time, the surface current in the region of drifter aggregation
is counterclockwise. Hence, there are more anticyclonic eddies than cyclonic
eddies. However, it is still unclear why such a mechanism only affects
submesoscale eddies. Nakamura et al. (2012) found that counterclockwise
submesoscale eddies are generally larger in number than cyclonic
submesoscale eddies near the Kuril Islands. The possible causes of such
asymmetry are planetary-vorticity tube stretching and asymmetric advection
by the rotating tidal flow. The mechanism of submesoscale eddy generation
needs study in the future.
(a) The number of eddies as a function of the calendar months for
cyclonic (blue), anticyclonic (red), and both types of eddies (green). The
bars indicate the standard error of eddy number estimates. (b) as in (a),
but for mesoscale eddies with radius larger than 10 km.
Temporal variations
The occurrence of eddies shows significant seasonal variations (Fig. 11a),
with more eddies in austral autumn and winter and fewer eddies in austral
spring and summer. The number of cyclonic eddies reaches a maximum in
August, whereas the number of anticyclonic eddies reaches a minimum in
September. If the submesoscale eddies are not included, temporal variations
of mesoscale eddies (Fig. 11b) are different from that of all eddies (Fig. 11a). The number of mesoscale eddies reaches a maximum in October and a
minimum in February, and the cyclonic and anticyclonic mesoscale eddies show
similar seasonal variations.
To compare with seasonal variability of eddies detected from satellite
observations, we use eddy data provided by Chelton et al. (2011). Because
the number of drifter observations has increased dramatically since 1995
(Fig. 5a), we check eddy fields derived from altimetry SSH from 1995 to
2012. The result represents a similar temporal distribution as in Fig. 11b.
There are more eddies generated in austral spring (Fig. 12), and fewer
eddies in austral summer.
The number of mesoscale eddies generated in the SIO (1995–2012)
as a function of the calendar months for cyclonic (blue), anticyclonic
(red), and both types of eddies (green). The bars indicate the standard
error of number of eddies. Mesoscale eddy data come from Chelton et al. (2011) based on altimetry observations of SSH. Chelton and Schlax have
updated and extended the eddy data set to April 2012 in the new third version
(http://cioss.coas.oregonstate.edu/eddies/).
The vertical velocity shear associated with the SICC and SEC system
intensifies due to enhanced heat flux forcing of combined meridional Ekman
and geostrophic convergence in austral spring (Jia et al., 2011). The
seasonal change of baroclinic instability and EKE variations, induced by
modulation of vertical velocity shear in the southeast Indian Ocean, favor
the generation of eddies. In addition, other nonlocal processes including
the Leeuwin Current (Feng et al., 2007; Rennie et al., 2007) and Agulhas Current
systems (Backeberg et al., 2008; Beal et al., 2011) may also modulate the
activity of mesoscale eddies. The mechanism of temporal variations of eddies
needs further study.
Discussion and conclusions
The dynamics of submesoscale eddies are distinct from the traditional
mesoscale quasi-geostrophic theory (Thomas et al., 2008), and in situ
submesoscale observations are still relatively scarce. From model studies in
the Southern California Bight, most eddies are in geostrophic balance, but
some submesoscale eddies are ageostrophic with a finite value of the local
Rossby number (Dong et al., 2012). The spatial distributions of submesoscale
eddies are different from those of large eddies and may be controlled by
ageostrophic balance, including pressure gradient, Coriolis acceleration and
nonlinear effect. The submesoscale eddies are densely distributed over
the entire SIO, with many submesoscale anticyclonic eddies in the subtropical
basin in the central SIO (Fig. 9d), possibly attributable to submesoscale
coherent vortices (SCVs). The SCVs have scales not exceeding the first
baroclinic radius of deformation and are usually anticyclonic (Thompson and
Young, 1989). McWilliams (1985) proposed the mechanism of anticyclonic SCVs,
produced by geostrophic adjustment when a volume of water with weak
stratification created by diapycnal mixing is injected into a more
stratified fluid. This mechanism has been demonstrated in laboratory models
(e.g., Hedstrom and Armi, 1988). Nencioli et al. (2013) estimated in situ
submesoscale horizontal eddy diffusivity across an ocean front in the
western Gulf of Lion in the Mediterranean Sea, which may extend our
understanding about submesoscale process with more in situ observations in
the future.
Eddy characteristics in the SIO were investigated on the basis of in situ
satellite-tracked drifter data from 1979 to 2013. In total 19 252 eddies were
detected. Among them 7657 (11 595) are cyclonic (anticyclonic) eddies. For
the submesoscale eddies, the ratio of cyclonic eddies (3183) to anticyclonic
eddies (7182) is 1 to 2. Large eddies (r≥100 km) populate the Leeuwin
Current, a band along 25∘ S, Mozambique Channel, and Agulhas
Current. The spatial distribution of large eddies corresponds to the large
EKE region (Figs. 9a, 10). In the Mozambique Channel, the shape of
large eddies is restrained by coastline in the northeast–southwest direction.
The number of mesoscale eddies shows significant seasonal variations, consistent
with eddies in the SIO detected from altimetry observations.
Acknowledgements
We thank Wei Zhuang for useful discussions and comments. The
satellite-tracked drifter data were provided by the Drifter Data Assembly
Center (DAC) at NOAA's Atlantic Oceanographic and Meteorological Laboratory
(http://www.aoml.noaa.gov/envids/gld/index.php). The
drifter-derived mean flow data were also provided by DAC (http://www.aoml.noaa.gov/phod/dac/dac_ meanvel.php). The
mesoscale eddies data detected by altimeter observation were obtained from
mesoscale eddies in altimeter observation of SSH (http://cioss.coas.oregonstate.edu/eddies/). The SLA data were provided by
AVISO (http://www.aviso.altimetry.fr/en/home.html). This work
was supported by the Strategic Priority Research Program of the Chinese
Academy of Sciences (XDA11010103), the Natural Science Foundation of China
(41306018), the National Basic Research Program of China (2010CB950302,
2012CB955603), and the Knowledge Innovation Program of the Chinese Academy of
Sciences (SQ201108).
Edited by: J. M. Huthnance
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