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Ocean Turbulence and Mixing Around Sri Lanka and in Adjacent Waters of the Northern Bay of Bengal

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As a part of the US Air-Sea Interactions Regional Initiative, the first extensive set of turbulent kinetic energy dissipation rate (ε) measurements from microstructure profilers were obtained in the Bay of Bengal (BoB) and around Sri Lanka during 2013–2015. The observations span almost 1,200 km meridionally, and capture the dynamics associated with a variety of mesoscale and submesoscale features. High freshwater input in the northern part of the basin leads to regions of intense near-surface stratification, which become weaker moving south. The thin layers trap mechanical energy input from the atmosphere, often confining turbulence to the surface boundary layer. These thin layers can form shallow fronts, which at times resemble turbulent gravity currents (Sarkar et al., 2016, in this issue), and are associated with high levels of mixing. Away from the local frontal zones, turbulence in the surface low-salinity layer appears to be decoupled from the underlying pycnocline, where turbulence occurs only in rare and sporadic breaking events. A striking feature common to all of the data acquired is a dearth of turbulent mixing at depth, a condition that appears to be pervasive throughout the basin except during the passage of tropical storms. It is likely that the strong near-surface stratification effectively isolates the deeper water column from mechanical penetration of atmospheric energy.
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CITATION
Jinadasa, S.U.P., I. Lozovatsky, J. Planella-Morató, J.D. Nash, J.A. MacKinnon,
A.J. Lucas, H.W. Wijesekera, and H.J.S. Fernando. 2016. Ocean turbulence and
mixing around Sri Lanka and in adjacent waters of the northern Bay of Bengal.
Oceanography 29(2):170–179, http://dx.doi.org/10.5670/oceanog.2016.49.
DOI
http://dx.doi.org/10.5670/oceanog.2016.49
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Oceanography | Vol.29, No.2
170
By S.U.P. Jinadasa, Iossif Lozovatsky, Jesús Planella-Morató,
Jonathan D. Nash, Jennifer A. MacKinnon, Andrew J. Lucas,
Hemantha W. Wijesekera, and Harinda J.S. Fernando
Ocean Turbulence and Mixing
Around SriLanka and in Adjacent Waters
of the Northern Bay of Bengal
BAY OF BENGAL: FROM MONSOONS TO MIXING
Oceanography | Vol.29, No.2
170
Oceanography | June 2016 171
fresher surface water. ese eddies degen-
erate into smaller (submesoscale) fea-
tures, forming sharp salinity fronts and
laments (Sengupta etal., 2016). Lateral
mixing associated with submesoscale
dynamics aects sea surface temperature
(SST) and sea surface salinity (SSS), and
hence the overlying convection and prop-
agating disturbances in the atmosphere.
Small-scale mixing is central to turbu-
lence, which can be generated by wind
stress, shear, collapsing fronts, baroclinic
instabilities, convective overturning,
and internal wave breaking. Integrative
understanding of BoB dynamics from
large to dissipative scales is a challenging
puzzle. e smaller the scales of the pro-
cesses, the more dicult they are to sam-
ple, observe, and interpret. Until now, no
systematic measurements of turbulence
in the northern Indian Ocean existed, and
as a part of the US Air-Sea Interactions
Regional Initiative (ASIRI), we had an
opportunity to obtain turbulence mea-
surements in the BoB and in the coastal
waters of SriLanka. is paper describes
the ndings of this study.
During the boreal winter, the East
India Coastal Current (EICC; Figure1)
develops under the inuence of the north-
eastern monsoon and ows southward
along the eastern and southern coasts of
SriLanka (Shetye, 1993; Wijesekera etal.,
2015). During the summer monsoon,
southwestern winds drive the Summer
Monsoon Current (SMC; Figure 1),
which is directed along the southern coast
of SriLanka into the BoB. During transi-
tion periods (April–May and September–
October), the EICC reverses or weak-
ens along the Sri Lanka coast, and the
SMC gradually reverses westward,
becoming the winter monsoon current
(WMC; de Vos et al., 2014). ese cur-
rent systems, and their interactions with
nearby waters, initiate a rich variety of
turbulence- generating processes, which
cascade energy down to dissipation scales
(Lee etal., 2016, in this issue). e con-
vergence/divergence of currents, as well
as their reversals during monsoon transi-
tions, can be expected to produce remark-
able spatial variability of mesoscale and
small-scale phenomena near the eastern
and southern coasts of SriLanka (Shetye
et al., 1996; Mukherjee et al., 2014).
However, despite their importance, very
little information has been obtained about
small-scale processes in the BoB (Kunze
etal., 2006), and no direct measurements
of small-scale turbulence there and in
adjacent waters have been captured until
now. Dissipation rates and vertical dif-
fusivities inferred from Argo oat strain
proles in the 250–500 m depth range
indicate that the central and western parts
of the BoB have anomalously low turbu-
lence (Whalen etal., 2012).
e rst direct microstructure data
and associated hydrodynamic vari-
ables (stratication, currents) were col-
lected in 2013–2015 in the BoB and to
the south of SriLanka (Figure1) during
research cruises conducted onboard
R/V Roger Revelle and R/V Samuddrika
(a regional research vessel of the National
Aquatic Resources Research and Devel-
opment Agency of SriLanka). At many
of the sites in 2013–2014, the duration of
microstructure measurements was limited
to no more than several hours at a time,
so it was not possible to deduce details
of turbulence generation and dissipation
there; longer time series were obtained
during 2015. Here, we present snapshots
of the turbulent structure in a handful of
INTRODUCTION
e variability of key atmospheric and
oceanic processes in the Bay of Bengal
(BoB) over a range of scales—from mon-
soons to mixing—signicantly aects
regional (Indian Ocean) and global
weather and climate. As a result of heavy
rainfalls and enormous river discharge
into the northern BoB, mainly during the
summer monsoons, a very sharp den-
sity interface known as a barrier layer
is formed between low-salinity sur-
face waters and more saline deep-ocean
waters (Girishkumar et al., 2011). e
barrier layer is a very distinct feature of
BoB stratication, and small-scale mix-
ing across this layer is crucial for heat,
mass, momentum, and biogeochemical
uxes in the bay (Venyachandran etal.,
2013; Akhil etal., 2014).
Large-scale currents along the BoB
boundaries, as well as eddies propagating
toward the center of the bay, transport the
ABSTRACT. As a part of the US Air-Sea Interactions Regional Initiative, the rst
extensive set of turbulent kinetic energy dissipation rate (ε) measurements from
microstructure prolers were obtained in the Bay of Bengal (BoB) and around SriLanka
during 2013–2015. e observations span almost 1,200 km meridionally, and capture
the dynamics associated with a variety of mesoscale and submesoscale features. High
freshwater input in the northern part of the basin leads to regions of intense near-surface
stratication, which become weaker moving south. e thin layers trap mechanical
energy input from the atmosphere, oen conning turbulence to the surface boundary
layer. ese thin layers can form shallow fronts, which at times resemble turbulent
gravity currents (Sarkar etal., 2016, in this issue), and are associated with high levels of
mixing. Away from the local frontal zones, turbulence in the surface low-salinity layer
appears to be decoupled from the underlying pycnocline, where turbulence occurs
only in rare and sporadic breaking events. A striking feature common to all of the
data acquired is a dearth of turbulent mixing at depth, a condition that appears to be
pervasive throughout the basin except during the passage of tropical storms. It is likely
that the strong near-surface stratication eectively isolates the deeper water column
from mechanical penetration of atmospheric energy.
FACING PAGE. (inset) Oceanographers from
Sri Lanka’s National Aquatic Resources
Research and Development Agency work-
ing aboard R/V Samuddrika launch a ver-
tical microstructure profiler for turbulence
and stratification measurements in the Bay
of Bengal. Photo credit: B.M.D.H. Kumarasiri.
(background) A US scientist deploying a
smaller version of the same instrument o the
back of R/V Roger Revelle, using a fishing reel
and rod instead of the A-frame. Photo credit:
Gualtiero Spiro Jaeger
Oceanography | Vol.29, No.2
172
log10ε (W kg–1)
log10ε (W kg–1)
log10ε (W kg–1)
log10ε (W kg–1)
log10ε (W kg–1)
log10ε (W kg–1)
log10ε (W kg–1)
Depth (m)
Depth (m)
Depth (m)
Depth (m)
Depth (m)
Depth (m)
Depth (m)
18°N
16°N
Nov 19, 2013
16°N
Nov 23, 2013
8°N
12°N
16°N
20°N
4°N 81°E 84°E 87°E 90°E
T (°C)
regions, with a goal of providing a glimpse
of BoB turbulence spanning a broad range
of processes and locations at specic time
periods. Several noteworthy themes can
nevertheless be deduced, including the
wind-induced turbulence above and
below the shallow barrier layer; the turbu-
lence aected by narrow, sharp baroclinic
(salinity) fronts; and turbulence structure
across the EICC and WMC. Weak mix-
ing below the near-surface layer is also
a recurring theme.
OBSERVATIONS
Turbulence observations in the BoB were
acquired during four research cruises
spanning winter and summer monsoons
and the transition periods between them.
Turbulence data were collected using
loosely tethered Rockland Scientic
vertical microstructure prolers (VMPs)
deployed in two dierent modes. e
VMP-500 is a large proler that records
data internally but transmits real-time
pressure through its tether. is device
was deployed through the ships A-frame
using a large winch in 2013–2014 (see title
page photo), and it obtained data down
to 150 m depth. e smaller VMP-250
records data internally, and was deployed
from the ship’s starboard quarter and
recovered using a thin, nonconducting
tether and free-spooling electric shing
reel; its depth was limited to 100 m by
spool capacity. Both VMPs carried two
airfoil probes (to estimate the turbulent
kinetic energy [TKE] dissipation rate ε),
a three-component accelerometer, a pres-
sure sensor (to calculate depth), and accu-
rate temperature-conductivity sensors to
obtain precise estimates of temperature,
salinity, and potential density. e data
processing followed the methodology
of Roget et al. (2006); additional infor-
mation can be found in Liu etal. (2009)
and Lozovatsky etal. (2015). Data above
5–10 m have been discarded due to the
potential for ship-wake contamination.
e measurements discussed below
were conducted at the locations indicated
in Figure1. Also shown are proles of the
time-average TKE dissipation rate and
salinity at each location in order to give a
broader avor of the freshwater distribu-
tion across the BoB and its implications
for mixing. ese data are discussed in
more detail below. In November 2013,
55 VMP-500 proles were obtained along
two mini-sections (November 18–19),
across the head of a bore (two sections,
November 21), and over a short (~25km)
mesoscale transect (November 23) in
a region bounded by 15.95°N–16.25°N
and 86.7°E–87.0°E. Similar measure-
ments were taken by R/V Samuddrika
across Sri Lankan coastal waters south
of Weligama (Weligama section, WS,
80.4°E, 5.9°N–5.2°N; April 24–26, 2014)
and east of Trincomalee (Trincomalee
section, TS, 8.5°N–7.5°N, 81.5°E–83°E;
September 10–11, 2014). e merid-
ional WS section covered 11 stations
with 17 VMP-500 casts over a dis-
tance of ~125 km. In addition to this
FIGURE1. Summary of vertical micro-
structure profiler (VMP) measure-
ments in the northern Bay of Bengal,
with a sea surface temperature com-
posite from December 14, 2013,
shown in color. Counterclockwise
from bottom left are average verti-
cal profiles of dissipation (red shad-
ing) and salinity (solid line) from:
(a) Weligama section (WS) from
R/V Samuddrika (April 24, 2014),
(b–d) 16°N for November 23, 16°N
for November 19, and bore sections
from R/V Roger Revelle (November
21, 2013), and (e–g) 18°N, “front” and
“jet” sections from R/V Roger Revelle
(August–September 2015). Not shown
are the Trincomalee section (TS) data,
which were not averaged because of
substantial spatial variability.
WS
Oceanography | June 2016 173
27.0 27.2 27.4 27.6 27.8 28.0 28.2 28.4
30
25
20
15
10
5
0
z (m)
31.6 32.0 32.4 32.8 33.2 33.6 34.0
S (psu)
T (°C)
σθ
σθ
20.4 20.6 20.8 21.0 21.2 21.4 21.6
27.0 27.2 27.4 27.6 27.8 28.0 28.2 28.4
31.6 32.0 32.4 32.8 33.2 33.6 34.0
S (psu)
T (°C)
σθ
20.4 20.6 20.8 21.0 21.2 21.4 21.6
TS
(a) November 18, 2013
(c) BoB November18, 2013 (d) BoB November 19, 2013
(b) November 19, 2013
0
25
50
75
100
125
35.0
34.6
34.2
33.8
33.4
33.0
32.6
32.2
z (m)
S (psu)
–5
–6
–7
–8
–9
–10
–11
log
10
ε (W kg–1)
log10ε (W kg –1)
10–10 10 –8 10–6 10 –4
log10N2 (s–2)
10–7 10 –6 10–5 10 –4 10–3 10 –2
log10 ε (W kg–1)
10–10 10 –8 10–6 10 –4
log10 N2 (s–2)
10–7 10 –6 10–5 10 –4 10–3 10 –2
0
25
50
75
100
125
z (m)
Distance (m)
0 500 1,000 1,500 2,000
Distance (m)
0 500 1,000 1,500 2,000
N2εσθTSN2ε
transect, 14 VMP-500 proles were
obtained during a ~7 km long dri
near the Weligama shelf break. Another
nine VMP-500 casts were carried out
in deep waters near the shelf break on
February 3, 2014, allowing estimation
of the mixed layer depth (MLD) in the
WMC. A 130km long TS zonal transect
covered nine stations with 20 VMP-500
proles. In addition to these 2013–2014
data, a VMP-250 was used to acquire
turbulence proles to 80–100 m depth
from R/V Roger Revelle across a jet-
like feature at 13°N on August 26, 2015
(“jet”; 309 proles), across a submeso-
scale front near 17°N on September 9,
2015 (“front”; 123 proles), and near the
18°N moorings on September 12–14
(“18°N”; 699 proles).
e TKE dissipation rate ε was cal-
culated by tting a Nasmyth bench-
mark spectrum to the measured spec-
tra (e.g., Gregg, 1999) at consecutive
two- second segments (1,024 points).
As a result, vertical proles of ε(z)
were obtained with a vertical spac-
ing of ~1.4 m. e same spacing was
adopted for temperature T(z), salinity
S(z), potential density ρθ(z), and buoy-
ancy frequency N(z) proles, where
N2 = ( g/ρθ) × (
θ/dz). e squared shear
Sh2 = (Δu
z)2 + (Δv
z)2 and the gra-
dient Richardson numbers Ri = N 2/Sh 2
were calculated by matching the verti-
cal resolution of the calculations of tem-
perature/salinity and dissipation proles
measured by the VMP sensors. Here, u
and v
are the zonal and meridional com-
ponents of the mean currents, which
were measured by a 150 kHz acous-
tic Doppler current proler (ADCP)
with vertical resolution of 8 m and then
interpolated to a 5 m grid.
STRATIFICATION AND
TURBULENCE IN THE
NORTHERN BAY OF BENGAL
Surface Layer Turbulence and
a Weak Interfacial Mixing
In this section, we compare two series
of microstructure measurements col-
lected in the same location in the BoB on
November 18 and 19 around noon local
time. During the 23-hour time period of
VMP measurements, the wind speed peri-
odically changed from W
a ~ 10 m s–1 down
to ~4–8 m s–1 and up to ~14–16 m s–1.
Four periods of wind increase were reg-
istered, each lasting approximately two
hours. e air temperature uctuated
between 27.5°C and 24°C.
Under moderate winds (November 18),
a shallow (z <10–15 m) low-salinity
(32.2–32.6 psu) surface mixed layer
(Figure 2a) was eectively decou-
pled from the water below by a
sharp thermohalocline, where
N2 > (6–8) × 10–4 s–2 (Figure2c). Relatively
small but distinguishable horizontal vari-
ability of T/S and specic potential den-
sity σθ = (ρθ − 1,000) in the mixed sur-
face layer (Figure2a,c) indicate that the
measurements were in an area of a weak
local frontal zone. On November 19, aer
several relatively short periods of higher
winds, the mixed layer deepened only
slightly, still decoupled from the waters
below by an even stronger barrier layer
where N2 > 2 × 10–3 s–2 (Figure2b,d).
Wind mixing, however, erased the
FIGURE2. Salinity (upper) and the dissipation rate (lower) contour plots along two
mini-sections taken in the Bay of Bengal on November 18 (a) and 19 (b) and the cor-
responding vertical profiles of T(z), S(z), N2(z), σθ(z), and ε(z) in the upper 30 m
layer (c) and (d), respectively.
Oceanography | Vol.29, No.2
174
horizontal thermohaline and density gra-
dients in the surface layer almost com-
pletely, as Figure2c,d, shows in detailed
proles of T(z), S(z), and σθ(z) and ε for
the upper 30 m.
Under mild winds, the turbu-
lence intensity, characterized here
by ε, gradually decreased from
ε ~ (3 × 10–5−10–3) W kg–1 at z = 5 m to
ε ~ (10–6–10–8) W kg–1 between z = 10 m
and z = 15 m. ereaer, a sharp drop
to ε ~ 10–9 W kg–1 was followed by an
approximately constant value of ε(z) with
increasing depth. e horizontal dier-
ences of T, S, and σθ in the middle of the
surface layer (z = 7 m) were ΔxT ≈ 0.25°C,
ΔxS ≈ 0.4 psu, and Δxσθ ≈ 0.22, respec-
tively, over a ~2 km separation
(Figure 2a,c). An increase of the dissi-
pation rate to ε ≈ (10–6–10–7) W kg–1
across the entire mixing layer at the sec-
ond mini-transect most likely is asso-
ciated with periodic, but short-lived,
segments of wind stress intensication up
to ~0.5 N m–2. us, simple formulae for
mixed layer deepening based on a con-
stant friction velocity at the sea surface
(e.g.,Pollard etal., 1972; see the review
by Zilitinkevich etal., 2007) is expected
to fail in predicting the observed changes
of MLD from 10–15 m on November 18
to 22–25 m on November 19.
e enhanced turbulence in the sur-
face layer may also break down spa-
tial gradients by lateral stirring, as the
horizontal thermohaline dierences
along the second mini-section reduced
to ΔxT ≈ 0.017°C, ΔxS ≈ 0.02 psu, and
Δxσθ ≈ 0.008 over approximately the same
distance as they did in the rst section.
On the other hand, the observed increase
of the mixed layer depth to z ≈ 22 m
could be associated not only with local-
ized wind-induced mixing but also with
lateral advection of a deeper mixed layer
to the measurement site.
e observation that the surface layer
and pycnocline were eectively decou-
pled from each other suggests that inter-
nal sources of turbulence in the interior
(z ≈ 25–120 m) were weak (ε <10–8 W kg–1;
Figure 2a,b) under mild and even rela-
tively strong sporadic winds observed
before and on November 18 and 19.
Internal wave radiation and breaking of
the waves below the barrier layer appear
to be damped, possibly due to local wave
breaking in the pycnocline (where a slight
increase of ε could be seen).
Upper-Ocean Response
to a Moderate Storm
A 48-hour time series of almost 700 ver-
tical proles at 18°N, 89.5°E provides an
opportunity to assess the BoB’s upper-
ocean turbulent response to a moder-
ate wind event (Figure3). Initially, winds
were light (5 m s–1) and the near- surface
boundary layer was capped by a cooler,
low-salinity layer about 5 m thick and
delineated from the uid below by strati-
cation associated with both salinity and
temperature steps. As the winds picked
up to 10 m s–1, the surface boundary
layer gradually deepened, but remained
less than 10 m thick. Mixing at the base
of the surface boundary layer reached
10–6 W kg–1, but such directly forced tur-
bulence never penetrated deeper than
15 m, even during the peak of the wind
forcing. Below this depth, signicant mix-
ing events were weaker (10–8–10–7 W kg–1)
and relatively infrequent, presumably
because the strong near-surface strati-
cation limited downward energy transfer
(see also MacKinnon etal., 2016, in this
issue, for a discussion).
During this period, three notable
patches of enhanced turbulence can be
identied beneath the surface layer. One is
a region of elevated mixing that occurred
at 06:00 on September 13 at 20 m depth
as the surface mixed layer deepened but
was clearly distinct and separated from
the surface mixed layer by a layer of
weaker turbulence. e other two patches
occurred around 12:00 on September14
following an increase in wind speed at
FIGURE3. Upper-ocean response to a moderate strength wind event in the north-
ern Bay of Bengal as captured by 699 vertical profiles spanning a 48-hour period
near 18°N. (a) Wind stress from the Woods Hole Oceanographic Institution mooring
at 18°N, (b) turbulent kinetic energy (TKE) dissipation rate, (c) salinity, and (d) tem-
perature. Beneath the surface boundary layer, only a few patches of turbulence
have dissipation rates approaching 10–8 W kg–1 (circled in red); the time- averaged
dissipation below 20 m is less than 10–9 W kg–1, as summarized in Figure1e.
τ (N m–2)
Depth (m)Depth (m)Depth (m)
Temperature (°C) Salinity (psu) log10ε ( W kg–1)
0.15
0.10
0.05
0.00
0
20
40
60
0
20
40
60
0
20
40
60
29
27
25
33
31
29
–7
–8
–9
–10
12:00 12:0000:00 00:0018:00 18:0006:00 06:00 12:00
September 12–14, 2015
Weak
turbulent
response
Time series at 18°4'N, 89°27'E
Oceanography | June 2016 175
about 09:00; any connection between
this wind event and turbulence is specu-
lative. However, neither of these patches
signicantly exceed 10–8 W kg–1, such
that the time-averaged deep dissipation
is 10–9 W kg–1 (Figure1e), which is quite
weak for upper-ocean turbulence. In
addition to these localized events, there is
also a notable band of slightly enhanced
turbulence at 30 m depth that is persistent
over the entire record and appears in con-
junction with a thermal inversion there.
Surface-Layer Turbulence Aected
by Strong Salinity Fronts
Frontal zones between saltier oceanic
waters and lenses or laments of fresher
waters of riverine and/or rainfall ori-
gin strongly inuence turbulence and
mixing in the upper layer of the north-
ern BoB. Horizontal dimensions of the
low-saline features may vary from a few
to tens and hundreds of kilometers. Many
frontal zones were detected in the north-
ern BoB during R/V Roger Revelle ASIRI
cruises (Lucas et al., 2014; Wijesekera
et al., in press). Turbulence measure-
ments across one such front were made
on November 21; this is the same front
shown in FigureB1c in Box 1 in Sarkar
et al. (2016, in this issue). Two mini-
sections (550 m and 720 m long, each
with six and eight approximately equally
distributed casts) were obtained while
crossing approximately perpendicular
to the front from saltier warmer ambi-
ent waters to the fresher colder side. e
origin of this specic pool of low-salinity
water is most likely an eddy or lament
associated with southbound low-saline
ow to the east (within the SriLankan
Exclusive Economic Zone not sampled at
that time). A very sharp salinity change
xS ~ 0.71 psu) was observed at the
rst crossing over a distance of ~100 m
(Figure 4a); the temperature and den-
sity at the fresher side of the front (not
shown here) decreased by ΔxT ~ 0.52°C
and Δρx ~ 0.33 kg m–3, respectively. Over
the course of 10 hours, R/V Roger Revelle
crossed the front 10 times while tow-
ing a thermistor chain from its bow.
ese data permit us to determine that
the front was very sharp at times, drop-
ping by as much as 0.5 psu in less than
5 m horizontal distance. ese data also
permit us to determine that the fea-
ture’s propagation speed (0.15–0.2 m s–1
relative to the uid ahead of it) was close
to
g'H, consistent with internal grav-
ity current speed (Turner, 1973). Here,
g' = gΔρ/ρ = (3.2–3.5) × 10–3 m s–2 is
reduced gravity and H = 10 m is the
thickness of a colder, low-salinity layer.
From the VMP-500 data, the feature
appears turbulent, especially closer to the
sea surface (compare the regions delim-
ited by dashed lines in the upper and
lower panels of Figure 4). Detailed
inspection of T, S, and ρθ proles clos-
est to the front revealed many density
inversions in the upper 10 m layer (not
shown here), pointing to active turbu-
lent mixing by the frontal ow. Below the
low- salinity layer, turbulence was sharply
reduced, with ε < 10–8 W kg–1, but only
in a limited depth range (~25–35 m)
shown by blue-green regions in the ε(z,x)
contour plots in the two lower panels of
Figure4. Strong stratication in the lower
part of the frontal layer and immediately
below could have provided this isolation.
However, 20 m above the MLD, turbu-
lence is not signicantly inuenced by
the shielding eect of the front. In the
depth range ~40 < z < 60 m, the dissipa-
tion remained above (3–5) × 10–8 W kg–1
0
25
50
75
100
125
FIGURE4. Salinity (upper) and TKE dissipation rate (lower) contour plots along two frontal cross sections with six (left) and eight (right)
VMP profiles, respectively, taken on November 21, 2013. The heavy dashed lines are approximate lower boundaries of the low-salinity
lenses. The ε(z,x) plot on the left is overlaid by u(z) and v(z) current components measured in the front at the first (circles) and second
(diamonds) sections. Examples of the mean squared shear profiles Sh
2(z) are in the lower right panel.
Distance (m)
z (m)
z (m)
log10ε ( W kg–1)
log10ε ( W kg–1)
S (psu)
S (psu)
Distance (m)
0
–0.1 0.0 0.1 0.2 u and v (m s –1)Sh
2 (s–2)–0.2 10–6 10 –5 10 –4
0
25
50
75
100
125
z (m)
0
25
50
75
100
125
z (m)
0
25
50
75
100
125
–5
–6
–7
–8
–9
–10
–11
–5
–6
–7
–8
–9
–10
–11
34.8
34.2
33.6
33.0
32.6
32.0
35.0
34.4
33.8
33.2
32.6
32.0
0200 200500 500 600 700100 100400 400300 300
Oceanography | Vol.29, No.2
176
(yellow-green strips in the ε(z,x) plots)
due to persistently high vertical shear.
e squared shear was close or exceeded
Sh2 ~ 10–4 s–2, ensuring a Richardson
number of the order 0.1 for N2 ~ 10–5 s–2.
Below the MLD, stratication increased
in the pycnocline with N2 ~ 3 × 10–4 s–2
on average, while the mean shear sub-
stantially and continuously decreased to
less than Sh2 ~ 2 × 10–6 s–2, preventing
shear-induced instability. As a result, the
dissipation rate in the pycnocline drops
below 10–10 W kg–1 (Figure4).
In addition to these 2013 observa-
tions associated with the propagation of
a gravity current, regions of strong hor-
izontal gradients were also sampled at
two separate locations in 2015 and are
summarized in Figure1f,g. In both cases,
turbulence proling spanned a region
of strong horizontal velocity gradients
that were associated with ageostrophic
fronts, some of which were as sharp as
those observed in 2013. While the stron-
gest turbulence was conned to the sur-
face boundary layer, in both of these
cases, dissipation rates below the near
surface were also enhanced, but not to
the same extent as the 2013 event (com-
pare Figure1 panels f and g with d); dissi-
pation rates below 40 m depth remained
close to background levels.
STRATIFICATION, CURRENTS,
AND TURBULENCE TO THE
SOUTH AND EAST OF SRILANKA
IN THE SMC AND EICC
e VMP measurements to the south and
east of SriLanka along the WS and TS
lines (Figure1) revealed substantial dif-
ferences in stratication and turbulence
between the EICC and SMC branches
of the near-coastal currents that reect
seasonal and/or spatial variability. e
TKE dissipation rate proles along the
WS and TS (color strips of log10 ε) are
shown in Figure5a,b overlaying the con-
tour plots of potential density (the corre-
sponding temperature and salinity pan-
els are presented in Wijesekera etal., in
press). During the intermonsoon season
(April; Figure5a), the depth of the rela-
tively well-mixed surface layer in the blue
water (>22 km from the coast) appears to
be about 30–40 m across the entire south-
ern branch of the current, deepening
slightly toward the open sea. According
to the VMP measurements made in
February 2014 at two WS stations near
the shelf break (not shown), the depth
of the surface homogeneous layer was
~60 m, indicating the possibility of sub-
stantial convective cooling and/or strong
wind mixing in the upper layer south of
Sri Lanka during the winter monsoon,
and gradually relaxing forcing toward
the transition period. Lenses of slightly
fresher and cooler water near the sea sur-
face did not aect the near-surface den-
sity structure very much, as evidenced by
FIGURE5. The TKE dissipation rate (ε, colored strips) overlaid on a background
of specific potential density σθ along (a) the WS (80.4°E), and (b) the TS (8°N). The
enhanced vertical shear (c) centered along z = 75 m coincides with the layer of
high dissipation rate within the secondary pycnocline shown in (b).
z (m)
0
25
50
75
100
125
150
Distance (km)
Distance (km)
Distance (km)
(a) Distance from the Coast Along 80.4°E
(b) Distance from the Coast Along 8°N
(c) Distance from the Coast Along 8°N
0 40
40
40
110
110
110
20
20
20
90
90
90
60
60
60
10
10
80
80
80
50
50
50
120
120
120
30
30
30
100
100
100
70
70
70
log10 Sh (s–1)
–1.4 –2.2–1.8 –2.6
–7.0
–7.5
–8.0
–8.5
–9.0
–9.5
–10.0
–10.5
log10ε ( W kg–1)
z (m)z (m)
0
25
50
75
100
125
150
25
50
75
100
Oceanography | June 2016 177
the almost uniformly mixed upper layer
and the sharp, but not very narrow, pyc-
nocline. In general, the pycnocline deep-
ens toward the south, suggesting predom-
inantly eastward geostrophic transport at
the end of April. is is well supported
by the contour plot of geostrophic veloc-
ity (Figure 6a) calculated using deep-
water Sea-Bird CTD proles with a zero
velocity reference level at 550 db. An
interesting dynamical feature appears in
Figure6a closer to the SriLankan coast,
where the eastward-directed along-slope
current ows next to the westward-
directed oshore current (dark blue con-
tours). Although it is possible that the
observed westward current is a remnant
of the seasonal WMC during the transi-
tion season (April) between winter and
summer monsoons, it is even more likely
that we observed a well-developed (down
to ~320 m) clockwise-rotating anti-
cyclonic mesoscale eddy approximately
50 km in diameter with a core (~zero
velocity) located about 45–48 km from
the coast. Note that the regular SMC usu-
ally does not extend below ~200 m depth
(Jensen, 2001). Sea surface elevation
maps retrieved from the Aviso archive
(http://eddy.colorado.edu/ccar/ssh/nrt_
global_grid_viewer) show a mesoscale
feature to the south of Sri Lanka, with a
positive height (up to 6 cm max), which
could be associated with an anticyclonic
eddy that separated from westward ow
near the SriLanka shelf break.
In September, toward the end of the
summer monsoon, the thermohaline
structure along the TS to the east of the
SriLanka coast (Figure5b) is much more
complex than that of the WS. It exhib-
its a sharp thermohalocline in the depth
range z = 30–40 m near the shelf break.
However, approximately 80–90 km o-
shore, the pycnocline bends toward the
sea surface, forming a striking baroclinic
front that separates the fresher BoB sur-
face water (S <33.8 psu, σθ <21.4) moving
southward along the SriLanka coast from
the saltier water of Arabian Sea origin
(S > ~35.2–35.4 psu at z ≈ 40–50 m, cor-
responding to the specic density range
σθ ~ 22.4–22.5 in Figure5b) that is mov-
ing north-northeastward at the east-
ern end of the section. e calculation
of geostrophic velocities across this sec-
tion (Figure 6b) and ADCP measure-
ments (Figure 6c) support this notion.
Interestingly, in September, the south-
ward-owing branch of the EICC is
present (green strip in the lower panel
of Figure 6c), although it is quite nar-
row, extending from the coast only
~40–45 km oshore. It has two cores, at
~30 m and 90 m depths, with maximum
velocity of about 0.4 m s–1. Simulations by
Jensen (2003) indicate that the southward
branch is perennial in the northeastern
part of Sri Lankan waters. On the con-
trary, the northward branch of the SMC is
still a powerful current, with a maximum
longitudinal velocity component vmax of
~ 1.5 m s–1 in the mixed layer at a distance
of 90–110 km from the coast (intense red
FIGURE6. Geostrophic velocities (the reference level is 550 db)
across the (a) WS and (b) TS. Positive velocities are to the (a) east
and (b) north. (c) ADCP velocity components u and v along the TS.
Distance (km)
Distance (km)
Stations Stations
Distance (km)
(a) Distance from the Coast
Along 80.4°E
(b) Distance from the Coast
Along 8°N
Distance from the Coast Along 8°N
Zonal Velocity u (m s–1)
Distance from the Coast Along 8°N
Meridional Velocity v (m s–1)
30
20 10060 14040 12080
3 3 4 5 7 9 11 134 5 6 7 8 9 10 11
3070 7050 5090 90110 110
z (m)
z (m)z (m)
0
100
200
300
400
500
25
50
75
100
25
50
75
100
1.5
1.0
0.5
0.0
–0.5
0.5
0.0
–0.5
1.2
0.8
0.4
0.0
–0.4
–0.8
–1.2
Geostrophic velocity (m s–1)
(c)
Oceanography | Vol.29, No.2
178
area in Figure6c). e zonal component
u is much weaker (umax ~ 0.2 m s–1) com-
pared to vmax ; it is always directed east-
ward in the upper 75 m (pink segments
in the upper panel of Figure6c), show-
ing oscillations with approximate wave-
lengths of ~35–40 km. Below 75–80 m
depth, the u component, which is still
associated with the SMC because of
the strong northward ow in the same
depth range (v ~ 0.5–0.7 m s–1), is, how-
ever, directed westward (blue segments
in the upper panel of Figure6c), having
approximately the same maximum value
as its eastward counterpart. (Comparing
Figure6c and b, it is clear that the core
of the SMC is composed of very salty
Arabian Sea water [ = 35.3–35.4 psu]
and that the EICC water owing along its
east side originates in the BoB and exhib-
its salinity as low as 33.7–33.8 psu.)
e main features of turbulence in the
SMC somewhat resemble the observa-
tions in the northern BoB (Figures 2–4),
where turbulence was mostly conned to
the surface mixed layer, which is detached
from interior water by a strong barrier
layer. In the BoB pycnocline, however,
only a small number of patches with ε
exceeding 10–8 W kg–1 were observed, but
there was patchy intermittent turbulence
in the pycnocline along the entire WS sec-
tion above z ~ 75 m (Figure5a). At dis-
tances of 20–40 km from the SriLankan
coast, the dissipation patches were pres-
ent at all depths where turbulence is likely
to be sustained by the instability of radia-
tive internal waves generated at the shelf
break. In situ measurements of internal
waves in SriLankan waters are currently
underway (Wijesekera etal., in press). A
more speculative possibility is that advec-
tion and regeneration of shear-induced
turbulence in the interior water could
also be in play. is is similar to the shelf-
generated patches of turbulence as exem-
plied by Phillips etal. (1986) using labo-
ratory experiments. Note that Lozovatsky
et al. (2012) reported topographically
induced turbulence at a distance greater
than 15 km from the source; this tur-
bulence was advected downstream by
strong currents and sustained along the
way by shear instability (ow behind
Baker Island in the western Pacic).
e spatial structure of the dissipa-
tion rate appears to be quite dierent
along the TS (Figure5b). e turbulent
patches with ε >10–8 W kg–1 occupy the
EICC at all depths (up to ~40 km from
the coast). East of the EICC, the high
turbulence is mostly conned within a
very narrow and sharply sloping upper
pycnocline, which starts 16 km from
the coast at z = 35–40 m and crops out
at the sea surface about 80 km oshore
(Figure5b), and in the lower secondary
pycnocline at depths of z = 70–80 m. e
most probable source of this turbulence
is strong shear instability at narrow inter-
faces (e.g., Strang and Fernando, 2001).
Indeed, we detected a layer of strong
shear (Figure5c), Sh >(2–3) × 10–2 s–1,
that coincided with the secondary pycno-
cline centered at z = 75 m, ensuring spo-
radic generation of the high-level of tur-
bulence shown in Figure5b.
CONCLUSIONS: SPATIAL
PATTERNS OF MIXING IN THE
BAY OF BENGAL
e rst measurements of TKE dissi-
pation rate ε in the northern BoB and
adjacent waters around Sri Lanka pro-
vide an initial glimpse into mixing in the
BoB. From these data, we gather a con-
sistent picture of the patterns of mixing,
as summarized by the proles in Figure1.
Most striking is the general nding that
the dissipation rate is extremely weak
(~10–9 W kg–1) below ~20–30 m depths,
except during rare storm events. We also
observe a general trend that the deep dis-
sipation rates are weaker to the north,
where surface stratication is strongest,
and increases southward, where near-
surface stratication is reduced. ese
observations lead to the general conclu-
sion that turbulence in the northern BoB
is largely inuenced by details of the very
thin, near-surface layer that is controlled
by monsoon rainfall and river inow and
the complex meso- and submesoscale
motions that advect it (MacKinnon etal.,
2016, in this issue). More specic conclu-
sions of this study are:
1. Very strong stratication in the sharp
BoB pycnocline can damp wind-
induced mixing almost completely,
preventing penetration of turbulence
below a thin, lower-salinity mixed sur-
face layer (MLD <15–20 m). Under
moderate winds (Wa ~ 11–12 m s–1),
the surface layer is eectively decou-
pled from the thermohalocline, but
horizontal/temporal gradients of T
and S still exist above the MLD. Under
stronger winds (Wa ~ 16–18 m s–1),
the homogeneous mixed layer deep-
ens only slightly (Figure1c), but is still
largely decoupled from the pycnocline.
e horizontal/temporal gradients of
T, S, and specic potential density in
the surface layer almost completely
vanish, possibly due to enhanced
wind stirring.
2. e northern BoB is characterized by
shallow and very sharp density fronts,
which at times resemble thin gravity
currents with strongly turbulent heads.
Turbulence observations across fronts
made in both 2013 and 2015 indicate
enhanced mixing associated directly
with gravity current shear. Below the
low-salinity near-surface frontal fea-
tures, turbulence is enhanced, possi-
bly as a result of strain induced by the
overlying dynamics, leading to dissipa-
tion rates as high as 5 × 10–8 W kg–1
down to depths of almost 50 m in
2013. In the pycnocline, turbulence is
sharply reduced, with ε reduced to less
than 10 –9 W kg–1.
3. Substantial deepening of the surface
layer south of Sri Lanka during the
winter monsoon was detected (the
MLD deepens to 60 m in February
compared to ~30–40 m in April). e
forcing gradually relaxes toward the
transition period.
4. e spatial structure of the dissipa-
tion rate is quite dierent along merid-
ional and zonal transects made to the
south (WS) and to the east (TS) of
SriLanka that cross the SMC and the
EICC. e main features of turbulence
Oceanography | June 2016 179
in the SMC resemble the observations
in the northern BoB, where turbu-
lence is mostly conned to the surface
mixed layer, which is detached from
waters below it by a strong pycnocline.
However, coastal bathymetry inu-
ences pycnocline turbulence in the
northern part of the WS, where turbu-
lent patches may be related to internal
waves generated at the shelf break.
5. In contrast to turbulence in the SMC,
high-level dissipation that occurs
along the TS (east of the EICC) is
mostly conned within a very narrow
and sharply sloping upper pycnocline,
which starts at z = 35–40 m near the
coast and crops out at the sea surface
about 80 km oshore. e most proba-
ble source of such turbulence is strong
shear instability at narrow interfaces,
which is documented by ADCP data
in the lower secondary pycnocline at
the depths z = 70–80 m.
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Wijesekera, H.W., T.G. Jensen, E. Jarosz,
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S.U.P. Jinadasa, K. Arulananthan, L. Centurioni, and
H.J.S. Fernando. 2015. Southern Bay of Bengal
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Wijesekera, H.W., E. Shroyer, A. Tandon,
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H.J.S. Fernando, N. Agrawal, K. Arulanathan,
G.S. Bhat, and others. In press. ASIRI: An ocean-
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ACKNOWLEDGMENTS
We are grateful to the crew of R/V Roger Revelle
and R/V Samuddrika for assistance with the VMP
measurements. Support for ship operation was pro-
vided by the US Oce of Naval Research and Naval
Research Laboratory. We acknowledge funding
through ONR grants N00014-13-1-0199 and N00014-
14-1-0279 (SUPJ, IL, HJSF, and JPM), N00014-14-1-
0455 (JM and JN), and N00014-13-1-0489 (AL).
AUTHORS
S.U.P. Jinadasa is Head of Division and Principal
Scientist, National Aquatic Resources Research
and Development Agency, Crow Island, Colombo,
SriLanka. Iossif Lozovatsky (i.lozovatsky@nd.edu)
is Research Professor, Department of Civil &
Environmental Engineering and Earth Sciences,
University of Notre Dame, Notre Dame, IN, USA.
Jesús Planella-Morató is Visiting Researcher,
University of Notre Dame, Notre Dame, IN, USA, and
Adjunct Professor, Department of Physics, University
of Girona, Girona, Spain. Jonathan D. Nash is
Professor, College of Earth, Ocean, and Atmospheric
Sciences, Oregon State University, Corvallis, OR,
USA. Jennifer A. MacKinnon is Professor, Scripps
Institution of Oceanography, University of California,
San Diego, La Jolla, CA, USA. Andrew J. Lucas
is Assistant Research Oceanographer, Scripps
Institution of Oceanography, University of
California, San Diego, La Jolla, CA, USA.
Hemantha W. Wijesekera is Oceanographer,
US Naval Research Laboratory, Stennis Space
Center, MS, USA. Harinda J.S. Fernando is the
Wayne and Diana Murdy Endowed Professor,
Department of Civil & Environmental Engineering &
Earth Sciences, and Department of Aerospace and
Mechanical Engineering, University of Notre Dame,
Notre Dame, IN, USA.
ARTICLE CITATION
Jinadasa, S.U.P., I. Lozovatsky, J. Planella-
Morató, J.D. Nash, J.A. MacKinnon, A.J. Lucas,
H.W. Wijesekera, and H.J.S. Fernando. 2016.
Ocean turbulence and mixing around SriLanka
and in adjacent waters of the northern Bay
of Bengal. Oceanography 29(2):170–179,
http://dx.doi.org/10.5670/oceanog.2016.49.
... In the coastal upwelling system, FP is an effective indicator of hydrodynamics 448 [39,41,65]. When the along-shore wind forces off-shore Ekman transport, cold water from 449 the subsurface is forced to upwell to the surface and fronts subsequently develop around 450 the boundary of water masses, associating with a strong SST gradient [17,66]. The cold 451 water near the coast further reduces the wind aloft and produces a positive wind stress 452 curl [67][68][69]. ...
... The wind stress curl, however, does 455 not have the winter maximum (Figure 3b). FP also reveals the submesoscale process, 456 which produces strong vertical mixing/convection and provides nutrients to the surface 457 layer [66,70,71]. In the SLD box, the maxima of FP occur in August, the same month as the 458 occurrence of chlorophyll maximum. ...
... In the coastal upwelling system, FP is an effective indicator of hydrodynamics [39,41,65]. When the along-shore wind forces off-shore Ekman transport, cold water from the subsurface is forced to upwell to the surface and fronts subsequently develop around the boundary of water masses, associating with a strong SST gradient [17,66]. The cold water near the coast further reduces the wind aloft and produces a positive wind stress curl [67][68][69]. ...
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... These 2019 BoB diffusivity estimates were subjected to a comprehensive statistical analysis and compared with K N from our previous turbulence measurements in the region. In 2013, the dissipation measurements were taken in the northern BoB [23]. In 2014, the data were collected along two sections to the south and to the east of Sri Lanka [11,23]. ...
... In 2013, the dissipation measurements were taken in the northern BoB [23]. In 2014, the data were collected along two sections to the south and to the east of Sri Lanka [11,23]. In 2018, a field campaign was carried out in the western BoB [16] during the presence of a cyclonic mesoscale eddy, commonly known as the Sri Lanka Dome. ...
... Note that MacKinnon and Gregg [1] found 0 = 6.9 × 10 −10 W/kg for the dynamically active New England shelf, while Sun et al. [28] reported 0 = 8.6 × 10 −10 W/kg for sufficiently strong turbulence in the northern South China Sea. It is noted that in June 2019, stratification in the southeastern BoB at 8°N, 89°E (8Ne station) did not exhibit a sharp density jump below the surface mixed layer (SML), which is a common observation in the northern BoB (e.g., [23]). As such, the relatively uniform pycnocline was not decoupled from the SML (~ 50 m deep), allowing an energy flux from SML to penetrate downwards more easily, thus enhancing background mixing in stratified water interior. ...
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... Until recently, only a few direct measurements of small-scale turbulence and related mixing in BoB have been conducted [11,15]. As a continuation of this works, in April and September of 2014, Sri Lankan oceanographers launched first ever measurements of microstructure and associated hydrodynamic variables (temperature, salinity, stratification, currents) to the south and to the east from the Sri Lanka coast (Figure 1). ...
... Jinadasa et al. [11] suggested that shear-induced turbulence is a major possible source of diapycnal mixing in monsoon driven currents around Sri Lanka, and if so, the gradient Richardson number is a salient parameter that determines the eddy diffusivity. To verify this hypothesis, we analyzed the dependence of turbulent diffusivity, on the gradient Richardson number as in many previous publications [26][27][28][29][30]. ...
... MacKinnon and Gregg (2005) also attributed the elevation of turbulent mixing to the increment of shear, which is caused by the reinforcement of stratification. Strong near-surface stratification can apparently influence mixing by isolating the deeper water from mechanical penetration of atmospheric energy (Jinadasa et al., 2016). ...
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