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Modeling slippery layers in the northern Bay of Bengal
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Modeling slippery layers in the northern Bay of Bengal Venkata Jampanaa, M. Ravichandranb, Lakshmi Kanthac,*, Hasibur Rahamana a b c
Indian Center for Ocean Information Services (INCOIS), Hyderabad, India National Centre for Polar and Ocean Research (NCPOR), Goa, India Department of Aerospace Engineering Sciences, University of Colorado, Boulder, CO, USA
ARTICLE INFO
ABSTRACT
Keywords: Bay of Bengal Mixing in the upper layers Second moment closure Turbulence OMNI mooring Solar insolation Precipitation Rainfall Slippery layers
In this study, we model a “slippery” layer observed during September 2011 at a National Institute of Ocean Technology (NIOT) mooring deployed at 18 oN, 89 oE in the northern Bay of Bengal (BoB). The mooring was located close to the mouths of the huge rivers draining the Indian subcontinent. The lateral advection of riverine water masses past the mooring resulted in a shallow brackish layer 10–15 m deep bounded by a strong halocline below, giving rise to the possibility of a slippery layer gliding past the layers below. The strong currents in this slippery layer were simulated by a simple slab-type dynamical model. In addition, a second moment turbulence closure-based model, driven by surface data from the buoy was also used to simulate the water mass structure and upper layer currents during the event. Both the slab and turbulence-closure models reproduce currents in the slippery layer reasonably well overall, although the currents are somewhat overestimated. This “haline” slippery layer observed in the BoB complements well the “thermal” slippery layers that have been observed under certain conditions during strong diurnal heating of the upper layers of the ocean.
1. Introduction Under a strong stabilizing buoyancy flux at the surface, as for example, under strong daytime solar heating or heavy rainfall during squalls, turbulence in the upper layers of the ocean tends to get damped, resulting often in the formation of a shallow surface layer just meters deep. Irrespective of the underlying cause (intense solar heating or heavy rainfall), the resulting shallow surface layer is bounded below by a strong density interface, which isolates and decouples it from the rest of the mixed layer. The strong pycnocline underneath inhibits transfer of momentum below the shallow but still perhaps turbulent mixed layer. Under these conditions, any momentum input by the wind at the surface to the ocean is confined to the shallow surface layer. If the wind stress is high enough, currents build up in the shallow layer, resulting in large near surface currents. It is as if the surface layers suddenly become “slippery” and glide past the water mass below. This phenomenon has been observed in diurnal mixed layers under strong solar insolation (e.g. Kudryavtsev and Soloviev, 1990). However, not much is known about a similar phenomenon under a heavy rainfall, although slippery layers have been reported in the Western Pacific Warm Pool (Anderson et al., 1996) and more recently in the Eastern Pacific Fresh Pool (Shcherbina et al., 2019). The large freshwater influx at the surface extinguishes turbulence in the prevailing seasonal
*
oceanic mixed layer (OML), and if the winds are strong enough, a slippery layer ensues. Price (1979) was the first one to report on a rainmixed layer in the upper ocean and Kantha and Clayson (1994) applied a second moment closure-based mixed layer model to simulate the temperature and salinity in a rain-mixed layer. However, shallow rainmixed layers are quite transient and subsequent mixing by wind or nocturnal convection distributes the rain water into the mixed layer. The Bay of Bengal (BoB) is a unique body of water in the global oceans in that it is strongly influenced by the huge freshwater influx from the major rivers draining the Indian subcontinent, as well as heavy rainfall during the Asiatic summer monsoon (Vinayachandran et al., 2002; Sengupta et al., 2006). The result is a shallow brackish layer about 10–15 m deep on top of the more saline oceanic water mass, in many regions of the BoB, especially the north. The strong pycnocline delineating the two water masses inhibits wind-driven mixing in the upper layers and can give rise to phenomena such as barrier layers. It also makes the northern BoB probably one of the best places to observe slippery layers. The characteristic feature of these “saline” slippery layers is a strong salinity increase at their base, whereas it is the strong temperature decrease at the base that characterizes the diurnal “thermal” slippery layers observed by Kudryavtsev and Soloviev (1990) and others. In this note, we describe a modeling effort to simulate a unique set of observations made in the northern BoB that constitutes a
Corresponding author. E-mail address: [email protected] (L. Kantha).
https://doi.org/10.1016/j.dsr2.2019.07.004 Received 22 February 2019; Received in revised form 8 July 2019; Accepted 8 July 2019 0967-0645/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Venkata Jampana, et al., Deep-Sea Research Part II, https://doi.org/10.1016/j.dsr2.2019.07.004
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after that period. Thus, the freshening of the upper layers is confined to the period between 15 September and 9 October. On the other hand, the temperature at 5 m depth dips below the temperatures at 10 and 15 m depths between the yellow-delineated focus period. Beyond this period (25 September), it rises above the temperatures at 10 and 15 m depths, with temperature at all three depths increasing slowly with time. The third panel shows the rainfall rate (in mm hr−1). Heavy rain occurred just before the salinity at 5 m depth began to decrease. Even heavier rain occurs between 19–21 September during the slippery layer focus period. The fourth panel shows the currents observed at 1 and 15 m depths. The near surface current reaches values as high as 0.8 m s−1 during the focus period, while the current at 15 m depth stays relatively unchanged at about 0.2 m s−1. This is what we call the “slippery layer.” Note that the wind stress (panel 5) increases to very high values starting from September and remains high until around 28 September. However, slippery layer phenomenon is not pronounced until 18 September. The oscillations in the near-surface currents are due to inertial oscillations initiated by winds (Fig. 2, panel 5) as expected. The inertial period at this latitude is 1.6 days. The near surface current decreases beyond 25 September and becomes roughly equal to the current at 15 m depth on the average after that time. Fig. 2, panel 6 shows the incoming solar insolation, which remains strong except on 19 and 20 September, when heavy clouds decrease it to very nearly zero. Clearly, this is responsible for the decrease of temperature at 5 m observed during this period, since the ocean loses heat mostly through latent heat flux but gains heat through penetrative solar insolation. Solar insolation recovers after 21 September, leading to increasing temperature in the upper layers beyond that time. Starting on 17, once again solar insolation decreases and the wind stress increases significantly until 20 October. Some rainfall occurs on 17 and 18 of October. Nevertheless, as the salinity plot shows, there is not much freshening and slippery layer phenomenon is not observed during this period. Fig. 3 shows time-depth plots of observed temperature and salinity in the upper 50 m from 10 September to 22 October from 6-hourly buoy data. The vertical lines delineate the focus period between 18–25 September. A sharp decrease in salinity in the upper layers can be seen during this period and beyond. Comparatively, the variability in the temperature of the upper layers is smaller. The water column below 15 m does not show much variability throughout the period 10 September to 22 October. Fig. 4 summarizes the findings in the form of temperature and salinity differences between 5 m and 15 m depths from 11 September to 22 October. The difference between surface currents (at 1 m) and currents at 15 m depth is also shown. Hourly values are shown by thin lines, along with 6-hourly running means.
Fig. 1. The location of the BD08 OMNI mooring deployed by NIOT at 18 oN, 89oE in the BoB during 2011.
textbook example of slippery layers in the upper ocean. For articles on BoB dealing with related issues, see Buckley et al., 2019; Dandapat et al., 2019; Jampana et al., 2018; Sandeep and Pant, 2019; Sengupta et al., 2006; Shroyer et al., 2019. 2. Methods 2.1. Observations The National Institute of Ocean Technology (NIOT) of India has developed and routinely deployed instrumented OMNI (Ocean Moored Network of buoys for the northern Indian ocean) since 2011 (Venkatesan et al., 2013). One such moored buoy, BD8, was deployed at 18 oN, 89 oE in the northern BoB (Fig. 1). The buoy carried a suite of meteorological sensors at a nominal height of 3 m, as well as sensors in the water column below the buoy to measure subsurface temperature, salinity and currents. Temperature and salinity were measured in the water column at depths of 1, 10, 15, 20, 30, 50, 100, 200 and 500 m, whereas currents were measured at depths of 1.2 m by a current meter, and from 10 to 100 m depths at 5 m intervals by a downward looking ADCP located at 7.5 m depth. As such, time series of water column properties, several years long are now available at that location. However, this study focuses on September 11 to October 22, 2011, when the slippery layer phenomenon was observed and measured. These data are available at https://drive.google.com/open?id= 1egTwJKEnODdGgfmNaqF4k3Yqyj_xHF4U. Fig. 2 shows time series of various relevant parameters from buoy data. The yellow color highlighted region with two vertical lines delineate the 18–25 September 2011, when the slippery layer was observed. The top panel shows the temperature and the second panel shows the salinity, at depths of 5, 10 and 15 m in the water column (the temperature and salinity sensors at 1 m depth did not return any useful data). Prior to 19 September, the temperature at the three depths is roughly the same. However, salinity at the same three depths begins to diverge starting on 15 September. The depth of the well-mixed surface layer with uniform salinity (not shown) was at least 15 m prior to the 15th. A strong freshening begins to occur with salinity falling from 31 to about 21.5 (all salinities in psu) at 5 m, while the salinity at 15 m depth dips to 30 before increasing to about 32, remaining roughly the same throughout the rest of this period. Salinity at 5 m depth stays roughly at 21.5 until 18 September, before rising slowly back to values between 27 and 30 by 9 October and remaining roughly unchanged
2.2. A simple slab model Clearly, a strong decrease in salinity in a shallow surface layer, and concurrent high wind forcing are prerequisites for a slippery layer. The question is the relative role of lateral advection of freshwater and heavy precipitation. Both lead to rapid freshening of a shallow layer near the surface and a strong halocline/pycnocline below inhibiting mixing and confining wind-driven currents to the shallow layer. Heavy rainfall is known to produce a very shallow layer containing low-salinity water of only a meter or two deep, when winds are weak (e.g. Price, 1979). Even moderate winds, if present during heavy rainfall, tend to mix the rain water to larger depths. In the observations discussed above, the freshening occurs in the upper 15 m. Unfortunately, no salinity data were returned at 1 m depth and so we do not know if there exists a very shallow rain-mixed layer (less than a meter in depth) near the surface.
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Fig. 2. Time series from 10 September to 22 October 2011. of, from the top: Panel 1, Temperature at 5 m (blue), 10 m (magenta) and 15 m (red) depths; Panel 2, Salinity at 5 m (blue), 10 m (magenta) and 15 m (red) depths (Note that the sensor at 1 m depth did not return any data); Panel 3. Thin blue line shows precipitation in mm hr−1 and the thick red line shows cumulative rainfall in meters multiplied by a factor of 20; Panel 4, Currents at 1 m (red) and 15 m (blue) depths; Panel 5, Wind stress magnitude (black), eastward (red) and northward (blue) components; Panel 6, Incoming solar insolation qsol (blue) and net outgoing heat flux at the surface qsur (red) In Panels 1, 2 and 4, the thin lines are hourly observations and the thick lines are 6-h running means. The yellow shaded region shows the focus period containing slippery layers. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
However, this is not as important as the observations showing that the large currents extend much deeper. Therefore, it is clearly lateral advection that is responsible for the slippery layer and not the rainfall. The slippery layers are simulated in Section 2.3, using second moment closure models of the OML (e.g. Kantha and Clayson, 1994, 2004). However, a simple slab model is useful to understand the basic dynamics and is therefore applied first. Let D be the depth of the shallow mixed layer that results from a shallow pycnocline, whether due to strong stabilizing buoyancy flux (heavy rain, strong solar insolation) or due to lateral advection of riverine water mass. If it is the former, D is proportional to the Monin-Obukhov length scale LMO =
u3 *, qb
u t
fv =
1
v t
+ fu =
1
x
z y
(1)
z
where t is the time, z is the vertical coordinate, u and v are the eastward and northward velocity components, x and y are the eastward and northward wind stress components, f is the Coriolis parameter, and is the density. Integrating in the vertical from z = 0 to z = -D, we get
where qb is
the buoyancy flux at the surface (e.g. Kantha, 1980). However, in the northern BoB, the shallow pycnocline during summer/early fall is invariably due to advection of freshwater from rivers draining the continent. We assume conditions are horizontally homogeneous. The governing equations are then
d dt
(uav D)
f (vav D) =
wx
d dt
(vav D ) + f (uav D) =
wy
(2)
where uav and vav are average velocities in the shallow mixed layer. Note that we ignore the stress at the bottom of the mixed layer because we assume the layer is slippery. Eq. (2) can be written as:
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Fig. 3. Observed temperature (top panel) and salinity (bottom panel) in the upper layers from 10 September to 22 October 2011. Note the significant freshening between 16 September and 2 October. The black vertical lines delineate the focus period containing slippery layers.
Fig. 4. Temperature difference (top panel) and salinity difference (middle panel) between 5 m and 15 m from 10 September to 22 October 2011. Note the freshening of the layers above 15 m. The bottom panel shows difference in currents between 1 m and 15 m. The thin lines are hourly observations and the thick lines are 6-h running means. The yellow shaded region shows the focus period containing slippery layers. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 5. Currents in the upper layer from the slab model (blue) compared to observed (red) currents for 10 September to 22 October 2011, at 1 m depth. From the top, Panel 1, Wind stress components to the east (blue) and north (red) and total wind stress (black); Panel 2, Current magnitude; Panel 3, Eastward component of the current, U; Panel 4. Northward component of the current, V. The yellow shaded region shows the focus period containing slippery layers. In the bottom three panels, blue lines show modeled currents and the red line shows observed currents at 1 m depth. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
dM + ifM = dt
w
where M = D (uav + ivav );
study. There is no need to go into the details of the model, such as the governing equations and solution techniques, which can be found in those papers (See also Kantha, 2011 for a concise review of second moment closure). It is important to point out that the model includes Stokes production of turbulence kinetic energy (TKE), which is the energy extracted from wave motions by the action of turbulent Reynolds stresses working on the shear of the Stokes drift induced by surface waves (see Kantha et al., 2010 for the importance of Stokes production in mixing of the upper layers of the ocean).
(3) w
=
wx
+i
wy
(4)
These are classic Ekman equations and were integrated from 10 September to 22 October 2011. The model was initialized with observed currents on 10 September, and driven by wind stresses for the period derived from BD08 met data using TOGA/COARE algorithm. ML depth D was taken as equal to 15 m consistent with observations described above in Section 2.1.
3. Model results
2.3. One-dimensional second moment turbulence closure model
3.1. The slab model
A one-dimensional turbulent mixing model based on second-moment closure of turbulence is well suited to exploring the slippery layer in the Bay of Bengal. There has been extensive work on second moment closure of turbulence over the past several decades, and Mellor-Yamada type closure models are now routinely used in 3-D models of both the atmosphere and the global ocean, for both research and operational forecasts (e.g. Mellor and Yamada, 1982, Galperin et al., 1988, Kantha and Clayson, 1994, 2004, Kantha and Mellor, 1989, Kantha, 2003, Kantha and Carniel, 2009, Zhou et al., 2018, Kantha et al., 2019 and references therein). Of these, Kantha and Clayson (1994, 2004) second moment closure model, often used in ocean models, is used in this
Fig. 5 shows the slab model results. The top panel shows the wind stress forcing used. The modeled currents (blue) are compared with observed surface (1-m depth) currents (red) in the next three panels. The model produces the currents in the slippery layer quite well. The model overestimates currents before 18 September, when the wind stress reached levels as high as during the focus period. This is likely because of the absence of a shallow layer freshened and bound by a strong halocline. Both modeled and observed currents exhibit strong inertial oscillations throughout the observation period. However, note that the slab model addresses the issue of momentum transfer from wind to the upper layers and not issues related to the heat and salt
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Fig. 6. T-S profiles used for initialization of the 1-D mixed layer model, corresponding to 10 September 2011. Red lines show the observed data and the blue lines, data interpolated to model grid. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
fluxes, since T/S structure is prescribed and not allowed to evolve under the influence of heat and salt fluxes. For this, we turn to the Kantha and Clayson (KC) mixed layer model described in Section 2.3.
lateral advection of less saline water mass is the principal reason, although the rainfall may also contribute. Since rivers draining the Indian subcontinent are in flood stage during this period and the mooring is close to the mouth of the rivers, it is likely that water masses of riverine origin were advected past the mooring between 15 September and 9 October. The 1-D model was run with a layer of depth 200 m, enough to encompass processes in the upper layers. The vertical resolution is 1 m and the time step is 15 min. The model was run for 42 days for the period 10 September to 22 October and initialized with T-S profiles from mooring data on 10 September (Fig. 6), It was forced with observed wind stress, heat fluxes and precipitation and evaporation based on observed latent heat flux (shown in Fig. 1), however, a 1-D model cannot simulate lateral advection. To overcome this problem, the model was reinitialized by observed profiles on 16 September and 9 October 9. This is somewhat crude and cannot fully simulate advection effects (Fig. 1). However, it simulates the approximate water mass structure in the water column. This is the best that can be done without deploying a full 3-D model of the BoB. Fig. 7 shows the model results. The simulated advection by reinitialization can be seen very clearly in salinity profiles. Panels 3–5 show turbulence properties (TKE, dissipation rate of TKE and dissipation rate of temperature variance) in the upper 60 m of the water column. These clearly show the extent of the turbulent region in the water column. Marked diurnal variability can be seen in all three turbulence parameters, with the minimum TKE occurring near the surface at around local noon. There is also an interesting downward propagation of turbulence thereafter. Deepening of the ML starting on 13 September is seen in temperature and turbulence parameters, but the reinitialization by less saline water on 16 September makes the ML
3.2. The 1-D model Heavy rainfall precedes the slippery layers (Panel 3, Fig. 2) and occurs also during the focus period containing slippery layers. Naturally, the question is: Do these heavy rainfall events cause slippery layers or is it the advection of less saline riverine water masses near the surface starting around 15 September 2011, as indicated by the salinity at 5m depth (blue line, Panel 2, Fig- 2)? The cumulative rainfall is about 1.7 cm prior to the focus period and an additional 5.2 cm of rainfall occurs during the focus period. Note that heavy rainfall occurs also during 17–18 October, accompanied by strong wind forcing but a slippery layer does not occur then. Assume an upper layer of depth D (in meters) and salinity S0. If precipitation of P (in meters) occurs, then the resulting salinity S is given by
S = S0
D D+P
(5)
Using observed values of S0 ~ 31, S ~ 21 and P ~ 0.05 m, we obtain D ~ 0.1 m. In other words, the rainfall must be confined to a layer of approximately 10 cm for the salinity to be depressed to 21 from the initial 31 value. This is unlikely. On the other hand, if the rainfall is spread over a layer of 15 m depth as observations indicate, then the salinity will be only 30.9 psμ. Observations indicate that the salinity is about 21 psμ. Consequently, heavy rainfall could not have depressed the salinity in the upper layers to the extent observed, which means that
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Fig. 7. Time-depth plots of model results (temperature, salinity, turbulence kinetic energy, TKE dissipation rate, and the dissipation rate of temperature variance) from 10 September to 22 October 2011, the simulation period. Note the simulated intrusion of less saline water mass on 16 September and back to a more saline water mass on 9 October. The black vertical lines delineate the focus period containing slippery layers. Reinitializations of the model can be clearly seen in the salinity plot on 16 September and 9 October as a sharp change in the salinity profile.
shallow, which then deepens slowly under the strong wind forcing. However, the strong halocline confines the momentum input by the wind to the shallow ML resulting in strong currents in the layer, as seen in Fig. 8, which shows the time series of currents. The slippery layer phenomenon can be clearly observed from 16 September to 25 September as a significant difference between currents at 2 m and 20 m depths. Beyond 16 October, the ML deepens significantly, and turbulence penetrates to around 50 m depth (bottom 3 panels of Fig. 7). For the surface layer to be slippery, the momentum input from the wind must be confined to the surface layer by the strong pycnocline underneath, which must not be eroded by turbulent entrainment. While the model simulates this phenomenon, it clearly overestimates the resulting currents. Further fine-tuning of the model parameters can result in better agreement, but we chose not to do so, since the objective is to demonstrate the model can simulate the phenomenon, not necessarily reproduce currents with great fidelity. On the other hand, without reinitialization, strong wind forcing deepened the ML to around 40 m depth (not shown) and the slippery layer phenomenon was not observed (Fig. 9), because the wind-driven momentum was distributed over the deep mixed layer.
4. Discussion and concluding remarks We have presented observations on a slippery layer in the northern Bay of Bengal during September–October 2011 from a NIOT mooring deployed at 89 oE and 18 oN in the northern Bay of Bengal close to the mouths of huge rivers draining the Indian subcontinent. Due to the heavy rainfall during the summer monsoon, these rivers are in flood stage during the observation period. We showed that the significant freshening of the upper 15 m cannot be the result of heavy rainfall observed during the phenomenon, although rainfall can contribute to the process. The occurrence of rainfall is a mere coincidence and the freshening is due to lateral advection past the mooring of riverine water masses in the upper layers. We showed that a simple slab-type dynamical model can explain the observed phenomenon of slippery layers. We also showed that the 1-D version of the KC second moment turbulence-closure model, properly modified to approximately simulate the lateral advection of less saline water past the mooring by reinitialization, can also simulate the phenomenon. The dynamical model can simulate only the currents, whereas the turbulence closure model can simulate the T/S structure as well as turbulence properties in the
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Fig. 8. Modeled currents from the 1-D model for 10 September-22 October 2011 simulation period, showing slippery layers during the focus period highlighted in yellow. In the top panel, the blue line shows the current at 2 m depth and the black line, at 20 m. In the bottom panel, the blue line shows the U component of velocity and the red line the V component at 2 m depth. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 9. As in Fig. 8, but with no reinitialization.
slippery layer. NIOT moorings were deployed continuously at the site since 2011 and it would be interesting to see if slippery layers occurred at other times in the northern Bay of Bengal.
Appendix A. Supplementary data
Acknowledgements
References
The authors wish to thank Dr. Satheesh Shenoi, the Director of INCOIS for the encouragement and support to carry out this study. They also acknowledge E. Pattabhi Rama Rao, R.Venkat Shesu, Suprit Kumar and N. Kameshwari for useful discussions related to data processing and analysis. This work was supported by the Ministry of Earth Sciences (MoES), Govt. of India. This is also INCOIS Contribution number 343 and NCPOR Contribution number J-14/2019-20. LK was supported by the U.S. Office of Naval Research (ONR) MISO/BoB DRI under grant number N00014-17-1-2716.
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Contents lists available at ScienceDirect
Deep-Sea Research Part II journal homepage: www.elsevier.com/locate/dsr2
Modeling slippery layers in the northern Bay of Bengal Venkata Jampanaa, M. Ravichandranb, Lakshmi Kanthac,*, Hasibur Rahamana a b c
Indian Center for Ocean Information Services (INCOIS), Hyderabad, India National Centre for Polar and Ocean Research (NCPOR), Goa, India Department of Aerospace Engineering Sciences, University of Colorado, Boulder, CO, USA
ARTICLE INFO
ABSTRACT
Keywords: Bay of Bengal Mixing in the upper layers Second moment closure Turbulence OMNI mooring Solar insolation Precipitation Rainfall Slippery layers
In this study, we model a “slippery” layer observed during September 2011 at a National Institute of Ocean Technology (NIOT) mooring deployed at 18 oN, 89 oE in the northern Bay of Bengal (BoB). The mooring was located close to the mouths of the huge rivers draining the Indian subcontinent. The lateral advection of riverine water masses past the mooring resulted in a shallow brackish layer 10–15 m deep bounded by a strong halocline below, giving rise to the possibility of a slippery layer gliding past the layers below. The strong currents in this slippery layer were simulated by a simple slab-type dynamical model. In addition, a second moment turbulence closure-based model, driven by surface data from the buoy was also used to simulate the water mass structure and upper layer currents during the event. Both the slab and turbulence-closure models reproduce currents in the slippery layer reasonably well overall, although the currents are somewhat overestimated. This “haline” slippery layer observed in the BoB complements well the “thermal” slippery layers that have been observed under certain conditions during strong diurnal heating of the upper layers of the ocean.
1. Introduction Under a strong stabilizing buoyancy flux at the surface, as for example, under strong daytime solar heating or heavy rainfall during squalls, turbulence in the upper layers of the ocean tends to get damped, resulting often in the formation of a shallow surface layer just meters deep. Irrespective of the underlying cause (intense solar heating or heavy rainfall), the resulting shallow surface layer is bounded below by a strong density interface, which isolates and decouples it from the rest of the mixed layer. The strong pycnocline underneath inhibits transfer of momentum below the shallow but still perhaps turbulent mixed layer. Under these conditions, any momentum input by the wind at the surface to the ocean is confined to the shallow surface layer. If the wind stress is high enough, currents build up in the shallow layer, resulting in large near surface currents. It is as if the surface layers suddenly become “slippery” and glide past the water mass below. This phenomenon has been observed in diurnal mixed layers under strong solar insolation (e.g. Kudryavtsev and Soloviev, 1990). However, not much is known about a similar phenomenon under a heavy rainfall, although slippery layers have been reported in the Western Pacific Warm Pool (Anderson et al., 1996) and more recently in the Eastern Pacific Fresh Pool (Shcherbina et al., 2019). The large freshwater influx at the surface extinguishes turbulence in the prevailing seasonal
*
oceanic mixed layer (OML), and if the winds are strong enough, a slippery layer ensues. Price (1979) was the first one to report on a rainmixed layer in the upper ocean and Kantha and Clayson (1994) applied a second moment closure-based mixed layer model to simulate the temperature and salinity in a rain-mixed layer. However, shallow rainmixed layers are quite transient and subsequent mixing by wind or nocturnal convection distributes the rain water into the mixed layer. The Bay of Bengal (BoB) is a unique body of water in the global oceans in that it is strongly influenced by the huge freshwater influx from the major rivers draining the Indian subcontinent, as well as heavy rainfall during the Asiatic summer monsoon (Vinayachandran et al., 2002; Sengupta et al., 2006). The result is a shallow brackish layer about 10–15 m deep on top of the more saline oceanic water mass, in many regions of the BoB, especially the north. The strong pycnocline delineating the two water masses inhibits wind-driven mixing in the upper layers and can give rise to phenomena such as barrier layers. It also makes the northern BoB probably one of the best places to observe slippery layers. The characteristic feature of these “saline” slippery layers is a strong salinity increase at their base, whereas it is the strong temperature decrease at the base that characterizes the diurnal “thermal” slippery layers observed by Kudryavtsev and Soloviev (1990) and others. In this note, we describe a modeling effort to simulate a unique set of observations made in the northern BoB that constitutes a
Corresponding author. E-mail address: [email protected] (L. Kantha).
https://doi.org/10.1016/j.dsr2.2019.07.004 Received 22 February 2019; Received in revised form 8 July 2019; Accepted 8 July 2019 0967-0645/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Venkata Jampana, et al., Deep-Sea Research Part II, https://doi.org/10.1016/j.dsr2.2019.07.004
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after that period. Thus, the freshening of the upper layers is confined to the period between 15 September and 9 October. On the other hand, the temperature at 5 m depth dips below the temperatures at 10 and 15 m depths between the yellow-delineated focus period. Beyond this period (25 September), it rises above the temperatures at 10 and 15 m depths, with temperature at all three depths increasing slowly with time. The third panel shows the rainfall rate (in mm hr−1). Heavy rain occurred just before the salinity at 5 m depth began to decrease. Even heavier rain occurs between 19–21 September during the slippery layer focus period. The fourth panel shows the currents observed at 1 and 15 m depths. The near surface current reaches values as high as 0.8 m s−1 during the focus period, while the current at 15 m depth stays relatively unchanged at about 0.2 m s−1. This is what we call the “slippery layer.” Note that the wind stress (panel 5) increases to very high values starting from September and remains high until around 28 September. However, slippery layer phenomenon is not pronounced until 18 September. The oscillations in the near-surface currents are due to inertial oscillations initiated by winds (Fig. 2, panel 5) as expected. The inertial period at this latitude is 1.6 days. The near surface current decreases beyond 25 September and becomes roughly equal to the current at 15 m depth on the average after that time. Fig. 2, panel 6 shows the incoming solar insolation, which remains strong except on 19 and 20 September, when heavy clouds decrease it to very nearly zero. Clearly, this is responsible for the decrease of temperature at 5 m observed during this period, since the ocean loses heat mostly through latent heat flux but gains heat through penetrative solar insolation. Solar insolation recovers after 21 September, leading to increasing temperature in the upper layers beyond that time. Starting on 17, once again solar insolation decreases and the wind stress increases significantly until 20 October. Some rainfall occurs on 17 and 18 of October. Nevertheless, as the salinity plot shows, there is not much freshening and slippery layer phenomenon is not observed during this period. Fig. 3 shows time-depth plots of observed temperature and salinity in the upper 50 m from 10 September to 22 October from 6-hourly buoy data. The vertical lines delineate the focus period between 18–25 September. A sharp decrease in salinity in the upper layers can be seen during this period and beyond. Comparatively, the variability in the temperature of the upper layers is smaller. The water column below 15 m does not show much variability throughout the period 10 September to 22 October. Fig. 4 summarizes the findings in the form of temperature and salinity differences between 5 m and 15 m depths from 11 September to 22 October. The difference between surface currents (at 1 m) and currents at 15 m depth is also shown. Hourly values are shown by thin lines, along with 6-hourly running means.
Fig. 1. The location of the BD08 OMNI mooring deployed by NIOT at 18 oN, 89oE in the BoB during 2011.
textbook example of slippery layers in the upper ocean. For articles on BoB dealing with related issues, see Buckley et al., 2019; Dandapat et al., 2019; Jampana et al., 2018; Sandeep and Pant, 2019; Sengupta et al., 2006; Shroyer et al., 2019. 2. Methods 2.1. Observations The National Institute of Ocean Technology (NIOT) of India has developed and routinely deployed instrumented OMNI (Ocean Moored Network of buoys for the northern Indian ocean) since 2011 (Venkatesan et al., 2013). One such moored buoy, BD8, was deployed at 18 oN, 89 oE in the northern BoB (Fig. 1). The buoy carried a suite of meteorological sensors at a nominal height of 3 m, as well as sensors in the water column below the buoy to measure subsurface temperature, salinity and currents. Temperature and salinity were measured in the water column at depths of 1, 10, 15, 20, 30, 50, 100, 200 and 500 m, whereas currents were measured at depths of 1.2 m by a current meter, and from 10 to 100 m depths at 5 m intervals by a downward looking ADCP located at 7.5 m depth. As such, time series of water column properties, several years long are now available at that location. However, this study focuses on September 11 to October 22, 2011, when the slippery layer phenomenon was observed and measured. These data are available at https://drive.google.com/open?id= 1egTwJKEnODdGgfmNaqF4k3Yqyj_xHF4U. Fig. 2 shows time series of various relevant parameters from buoy data. The yellow color highlighted region with two vertical lines delineate the 18–25 September 2011, when the slippery layer was observed. The top panel shows the temperature and the second panel shows the salinity, at depths of 5, 10 and 15 m in the water column (the temperature and salinity sensors at 1 m depth did not return any useful data). Prior to 19 September, the temperature at the three depths is roughly the same. However, salinity at the same three depths begins to diverge starting on 15 September. The depth of the well-mixed surface layer with uniform salinity (not shown) was at least 15 m prior to the 15th. A strong freshening begins to occur with salinity falling from 31 to about 21.5 (all salinities in psu) at 5 m, while the salinity at 15 m depth dips to 30 before increasing to about 32, remaining roughly the same throughout the rest of this period. Salinity at 5 m depth stays roughly at 21.5 until 18 September, before rising slowly back to values between 27 and 30 by 9 October and remaining roughly unchanged
2.2. A simple slab model Clearly, a strong decrease in salinity in a shallow surface layer, and concurrent high wind forcing are prerequisites for a slippery layer. The question is the relative role of lateral advection of freshwater and heavy precipitation. Both lead to rapid freshening of a shallow layer near the surface and a strong halocline/pycnocline below inhibiting mixing and confining wind-driven currents to the shallow layer. Heavy rainfall is known to produce a very shallow layer containing low-salinity water of only a meter or two deep, when winds are weak (e.g. Price, 1979). Even moderate winds, if present during heavy rainfall, tend to mix the rain water to larger depths. In the observations discussed above, the freshening occurs in the upper 15 m. Unfortunately, no salinity data were returned at 1 m depth and so we do not know if there exists a very shallow rain-mixed layer (less than a meter in depth) near the surface.
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Fig. 2. Time series from 10 September to 22 October 2011. of, from the top: Panel 1, Temperature at 5 m (blue), 10 m (magenta) and 15 m (red) depths; Panel 2, Salinity at 5 m (blue), 10 m (magenta) and 15 m (red) depths (Note that the sensor at 1 m depth did not return any data); Panel 3. Thin blue line shows precipitation in mm hr−1 and the thick red line shows cumulative rainfall in meters multiplied by a factor of 20; Panel 4, Currents at 1 m (red) and 15 m (blue) depths; Panel 5, Wind stress magnitude (black), eastward (red) and northward (blue) components; Panel 6, Incoming solar insolation qsol (blue) and net outgoing heat flux at the surface qsur (red) In Panels 1, 2 and 4, the thin lines are hourly observations and the thick lines are 6-h running means. The yellow shaded region shows the focus period containing slippery layers. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
However, this is not as important as the observations showing that the large currents extend much deeper. Therefore, it is clearly lateral advection that is responsible for the slippery layer and not the rainfall. The slippery layers are simulated in Section 2.3, using second moment closure models of the OML (e.g. Kantha and Clayson, 1994, 2004). However, a simple slab model is useful to understand the basic dynamics and is therefore applied first. Let D be the depth of the shallow mixed layer that results from a shallow pycnocline, whether due to strong stabilizing buoyancy flux (heavy rain, strong solar insolation) or due to lateral advection of riverine water mass. If it is the former, D is proportional to the Monin-Obukhov length scale LMO =
u3 *, qb
u t
fv =
1
v t
+ fu =
1
x
z y
(1)
z
where t is the time, z is the vertical coordinate, u and v are the eastward and northward velocity components, x and y are the eastward and northward wind stress components, f is the Coriolis parameter, and is the density. Integrating in the vertical from z = 0 to z = -D, we get
where qb is
the buoyancy flux at the surface (e.g. Kantha, 1980). However, in the northern BoB, the shallow pycnocline during summer/early fall is invariably due to advection of freshwater from rivers draining the continent. We assume conditions are horizontally homogeneous. The governing equations are then
d dt
(uav D)
f (vav D) =
wx
d dt
(vav D ) + f (uav D) =
wy
(2)
where uav and vav are average velocities in the shallow mixed layer. Note that we ignore the stress at the bottom of the mixed layer because we assume the layer is slippery. Eq. (2) can be written as:
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Fig. 3. Observed temperature (top panel) and salinity (bottom panel) in the upper layers from 10 September to 22 October 2011. Note the significant freshening between 16 September and 2 October. The black vertical lines delineate the focus period containing slippery layers.
Fig. 4. Temperature difference (top panel) and salinity difference (middle panel) between 5 m and 15 m from 10 September to 22 October 2011. Note the freshening of the layers above 15 m. The bottom panel shows difference in currents between 1 m and 15 m. The thin lines are hourly observations and the thick lines are 6-h running means. The yellow shaded region shows the focus period containing slippery layers. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 5. Currents in the upper layer from the slab model (blue) compared to observed (red) currents for 10 September to 22 October 2011, at 1 m depth. From the top, Panel 1, Wind stress components to the east (blue) and north (red) and total wind stress (black); Panel 2, Current magnitude; Panel 3, Eastward component of the current, U; Panel 4. Northward component of the current, V. The yellow shaded region shows the focus period containing slippery layers. In the bottom three panels, blue lines show modeled currents and the red line shows observed currents at 1 m depth. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
dM + ifM = dt
w
where M = D (uav + ivav );
study. There is no need to go into the details of the model, such as the governing equations and solution techniques, which can be found in those papers (See also Kantha, 2011 for a concise review of second moment closure). It is important to point out that the model includes Stokes production of turbulence kinetic energy (TKE), which is the energy extracted from wave motions by the action of turbulent Reynolds stresses working on the shear of the Stokes drift induced by surface waves (see Kantha et al., 2010 for the importance of Stokes production in mixing of the upper layers of the ocean).
(3) w
=
wx
+i
wy
(4)
These are classic Ekman equations and were integrated from 10 September to 22 October 2011. The model was initialized with observed currents on 10 September, and driven by wind stresses for the period derived from BD08 met data using TOGA/COARE algorithm. ML depth D was taken as equal to 15 m consistent with observations described above in Section 2.1.
3. Model results
2.3. One-dimensional second moment turbulence closure model
3.1. The slab model
A one-dimensional turbulent mixing model based on second-moment closure of turbulence is well suited to exploring the slippery layer in the Bay of Bengal. There has been extensive work on second moment closure of turbulence over the past several decades, and Mellor-Yamada type closure models are now routinely used in 3-D models of both the atmosphere and the global ocean, for both research and operational forecasts (e.g. Mellor and Yamada, 1982, Galperin et al., 1988, Kantha and Clayson, 1994, 2004, Kantha and Mellor, 1989, Kantha, 2003, Kantha and Carniel, 2009, Zhou et al., 2018, Kantha et al., 2019 and references therein). Of these, Kantha and Clayson (1994, 2004) second moment closure model, often used in ocean models, is used in this
Fig. 5 shows the slab model results. The top panel shows the wind stress forcing used. The modeled currents (blue) are compared with observed surface (1-m depth) currents (red) in the next three panels. The model produces the currents in the slippery layer quite well. The model overestimates currents before 18 September, when the wind stress reached levels as high as during the focus period. This is likely because of the absence of a shallow layer freshened and bound by a strong halocline. Both modeled and observed currents exhibit strong inertial oscillations throughout the observation period. However, note that the slab model addresses the issue of momentum transfer from wind to the upper layers and not issues related to the heat and salt
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Fig. 6. T-S profiles used for initialization of the 1-D mixed layer model, corresponding to 10 September 2011. Red lines show the observed data and the blue lines, data interpolated to model grid. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
fluxes, since T/S structure is prescribed and not allowed to evolve under the influence of heat and salt fluxes. For this, we turn to the Kantha and Clayson (KC) mixed layer model described in Section 2.3.
lateral advection of less saline water mass is the principal reason, although the rainfall may also contribute. Since rivers draining the Indian subcontinent are in flood stage during this period and the mooring is close to the mouth of the rivers, it is likely that water masses of riverine origin were advected past the mooring between 15 September and 9 October. The 1-D model was run with a layer of depth 200 m, enough to encompass processes in the upper layers. The vertical resolution is 1 m and the time step is 15 min. The model was run for 42 days for the period 10 September to 22 October and initialized with T-S profiles from mooring data on 10 September (Fig. 6), It was forced with observed wind stress, heat fluxes and precipitation and evaporation based on observed latent heat flux (shown in Fig. 1), however, a 1-D model cannot simulate lateral advection. To overcome this problem, the model was reinitialized by observed profiles on 16 September and 9 October 9. This is somewhat crude and cannot fully simulate advection effects (Fig. 1). However, it simulates the approximate water mass structure in the water column. This is the best that can be done without deploying a full 3-D model of the BoB. Fig. 7 shows the model results. The simulated advection by reinitialization can be seen very clearly in salinity profiles. Panels 3–5 show turbulence properties (TKE, dissipation rate of TKE and dissipation rate of temperature variance) in the upper 60 m of the water column. These clearly show the extent of the turbulent region in the water column. Marked diurnal variability can be seen in all three turbulence parameters, with the minimum TKE occurring near the surface at around local noon. There is also an interesting downward propagation of turbulence thereafter. Deepening of the ML starting on 13 September is seen in temperature and turbulence parameters, but the reinitialization by less saline water on 16 September makes the ML
3.2. The 1-D model Heavy rainfall precedes the slippery layers (Panel 3, Fig. 2) and occurs also during the focus period containing slippery layers. Naturally, the question is: Do these heavy rainfall events cause slippery layers or is it the advection of less saline riverine water masses near the surface starting around 15 September 2011, as indicated by the salinity at 5m depth (blue line, Panel 2, Fig- 2)? The cumulative rainfall is about 1.7 cm prior to the focus period and an additional 5.2 cm of rainfall occurs during the focus period. Note that heavy rainfall occurs also during 17–18 October, accompanied by strong wind forcing but a slippery layer does not occur then. Assume an upper layer of depth D (in meters) and salinity S0. If precipitation of P (in meters) occurs, then the resulting salinity S is given by
S = S0
D D+P
(5)
Using observed values of S0 ~ 31, S ~ 21 and P ~ 0.05 m, we obtain D ~ 0.1 m. In other words, the rainfall must be confined to a layer of approximately 10 cm for the salinity to be depressed to 21 from the initial 31 value. This is unlikely. On the other hand, if the rainfall is spread over a layer of 15 m depth as observations indicate, then the salinity will be only 30.9 psμ. Observations indicate that the salinity is about 21 psμ. Consequently, heavy rainfall could not have depressed the salinity in the upper layers to the extent observed, which means that
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Fig. 7. Time-depth plots of model results (temperature, salinity, turbulence kinetic energy, TKE dissipation rate, and the dissipation rate of temperature variance) from 10 September to 22 October 2011, the simulation period. Note the simulated intrusion of less saline water mass on 16 September and back to a more saline water mass on 9 October. The black vertical lines delineate the focus period containing slippery layers. Reinitializations of the model can be clearly seen in the salinity plot on 16 September and 9 October as a sharp change in the salinity profile.
shallow, which then deepens slowly under the strong wind forcing. However, the strong halocline confines the momentum input by the wind to the shallow ML resulting in strong currents in the layer, as seen in Fig. 8, which shows the time series of currents. The slippery layer phenomenon can be clearly observed from 16 September to 25 September as a significant difference between currents at 2 m and 20 m depths. Beyond 16 October, the ML deepens significantly, and turbulence penetrates to around 50 m depth (bottom 3 panels of Fig. 7). For the surface layer to be slippery, the momentum input from the wind must be confined to the surface layer by the strong pycnocline underneath, which must not be eroded by turbulent entrainment. While the model simulates this phenomenon, it clearly overestimates the resulting currents. Further fine-tuning of the model parameters can result in better agreement, but we chose not to do so, since the objective is to demonstrate the model can simulate the phenomenon, not necessarily reproduce currents with great fidelity. On the other hand, without reinitialization, strong wind forcing deepened the ML to around 40 m depth (not shown) and the slippery layer phenomenon was not observed (Fig. 9), because the wind-driven momentum was distributed over the deep mixed layer.
4. Discussion and concluding remarks We have presented observations on a slippery layer in the northern Bay of Bengal during September–October 2011 from a NIOT mooring deployed at 89 oE and 18 oN in the northern Bay of Bengal close to the mouths of huge rivers draining the Indian subcontinent. Due to the heavy rainfall during the summer monsoon, these rivers are in flood stage during the observation period. We showed that the significant freshening of the upper 15 m cannot be the result of heavy rainfall observed during the phenomenon, although rainfall can contribute to the process. The occurrence of rainfall is a mere coincidence and the freshening is due to lateral advection past the mooring of riverine water masses in the upper layers. We showed that a simple slab-type dynamical model can explain the observed phenomenon of slippery layers. We also showed that the 1-D version of the KC second moment turbulence-closure model, properly modified to approximately simulate the lateral advection of less saline water past the mooring by reinitialization, can also simulate the phenomenon. The dynamical model can simulate only the currents, whereas the turbulence closure model can simulate the T/S structure as well as turbulence properties in the
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Fig. 8. Modeled currents from the 1-D model for 10 September-22 October 2011 simulation period, showing slippery layers during the focus period highlighted in yellow. In the top panel, the blue line shows the current at 2 m depth and the black line, at 20 m. In the bottom panel, the blue line shows the U component of velocity and the red line the V component at 2 m depth. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 9. As in Fig. 8, but with no reinitialization.
slippery layer. NIOT moorings were deployed continuously at the site since 2011 and it would be interesting to see if slippery layers occurred at other times in the northern Bay of Bengal.
Appendix A. Supplementary data
Acknowledgements
References
The authors wish to thank Dr. Satheesh Shenoi, the Director of INCOIS for the encouragement and support to carry out this study. They also acknowledge E. Pattabhi Rama Rao, R.Venkat Shesu, Suprit Kumar and N. Kameshwari for useful discussions related to data processing and analysis. This work was supported by the Ministry of Earth Sciences (MoES), Govt. of India. This is also INCOIS Contribution number 343 and NCPOR Contribution number J-14/2019-20. LK was supported by the U.S. Office of Naval Research (ONR) MISO/BoB DRI under grant number N00014-17-1-2716.
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