ICTS

V. Valsala, S. Singh and S. Balasubramanian

Variability of Bay of Bengal Mixing, Barrier Layer Formation and associated mixing energetics

A simulation with Coordinated Ocean‐Ice Reference Experiment (CORE) forcing for 15 years (from 1995 to 2009) is used as a test case to study the interannual variability (IAV) of the mixed layer depth and barrier layer thickness (BL) in BoB. The dominant modes of IAV in the BoB mixing are governed by the correspondingly varying surface momentum, heat, and fresh water fluxes with very little contribution from entrainment of heat and/or salt at the base of the mixed layer. Further, these fluxes are controlled by El Niño‐Southern Oscillation (ENSO) variability with very little influence from Indian Ocean Dipole‐Zonal Mode (IODZM). The BL IAV is predominantly controlled by precipitation forcing from ENSO. A stability analysis revealed that the turbulent kinetic energy (TKE) and the stability function (SH) are negatively correlated when ENSO is the dominant forcing. Such a correlation between TKE and SH is expected since unstable stratification conditions exist during positive ENSO, where the kinetic energy production is enhanced by the unstable buoyancy forcing leading to an increased TKE. During the negative phase of ENSO, stably stratified conditions exist, where the kinetic energy production is offset by the stable buoyancy force, reducing the TKE. This is indicative of high (low) turbulent kinetic energy production, low (high) flux Richardson number based stability function (SH), and low (high) dominance of buoyancy‐driven mixing during positive (negative) phases of ENSO. The results highlight that the counteracting influence of TKE and SH is a plausible reason for relatively weaker amplitude of IAV of BoB mixing compared to its normal seasonal cycle.

Reference:
Valsala, V., Singh, S., & Balasubramanian, S. (2018). A modeling study of interannual variability of Bay of Bengal mixing and barrier layer formation. Journal of Geophysical Research, 123, 3962–3981.

Eric D. Maloney

Propagation mechanisms of monsoon intraseasonal oscillations

The boreal summer intraseasonal oscillation (BSISO) has important impacts monsoon active and break periods, tropical cyclones, and precipitation in remote parts of the globe. Despite its importance and the increasing emphasis that the scientific community has placed on studying this phenomenon, the propagation mechanisms for the BSISO remain poorly understood. This talk will first provide a brief critical survey of the leading hypotheses for northward propagation of the BSISO. Then, recent results will be presented that suggest a role for air-sea interactions and horizontal moisture advection for northward propagation of the BSISO.
In particular, the role of SST anomalies in forcing boundary layer moisture convergence to the north of BSISO convection that fosters its northward propagation will be evaluated using a simplified model for the atmospheric boundary layer. Building on these findings, a local air-sea coupling model is then formulated that demonstrates that a 25 meter mixed layer depth is optimal for the Indian Ocean to support robust intraseasonal variability in the presence of air-sea coupling. Next, a moist static energy budget analysis is conducted indicating that horizontal moisture advection also plays an important role in northward BSISO propagation in the Indian Ocean, showing some similarities to the eastward propagation mechanism of the boreal winter Madden-Julian oscillation. Surface heat flux anomalies partially cancel the tendency for horizontal advection to move the BSISO northward. It is demonstrated the climate models with unrealistic mean meridional moisture gradients also produce worse simulations of northward propagation since horizontal advection is poorly simulated. Finally, the PISTON field program occurred in the west Pacific in the summer of 2018 with a goal to understand the northward propagation of the BSISO. An overview of PISTON and some initial findings of observational and modeling work will be presented.

Jennifer MacKinnon

Diagnosing the impact of sub-kilometer scale ocean boundary layer variability on air-sea interactions in the late Monsoon

Monsoon forecasting on both synoptic and intraseasonal timescales often lacks skill, in part due to inadequate representation of upper ocean processes. Here we use data from the 2015 ASIRI cruise on R/V Revelle to assess both the fidelity of one-dimensional representations of turbulent mixing and upper ocean heat distribution, and the particular ways in which three-dimensional structures come into play. In the late summer, intense freshwater input in the northern Bay of Bengal creates shallow mixed layers bounded by strong vertical stratification and strong lateral gradients in density that are challenging to both measure and model. During the cruise, a triangle-pattern survey, ~4 km across, was repeated for 2 days by R/V Revelle while acquiring profiles of CTD and turbulence data, horizontal velocities, and thermistor and CTD data from a towed bow-chain. During this survey, a robotic oceanographic surface sampler (ROSE) and a Wirewalker were deployed. Observations are compared with 1-D model run with the KPP mixing parameterization. The model does not reproduce the vertical offset of gradients in momentum and scalars seen in the observations, a limitation of KPP. A strong dependence on the initial stratification in the model runs indicates that the upper ocean response to wind bursts in the BoB varies on scales $<$ 1 km, related to sub-mesoscale density gradients. Implications of these finings for upcoming 2019 fieldwork will but put forward as discussion questions.

Authors & Affiliation:
Jennifer MacKinnon, Andrew Lucas and Kate Adams (Scripps Institution of Oceanography), Jonathan Nash and Emily Shroyer (Oregon State University)

Andrew Lucas

Diagnosing the impact of sub-kilometer scale ocean boundary layer variability on air-sea interactions in the late Monsoon

Monsoon forecasting on both synoptic and intraseasonal timescales often lacks skill, in part due to inadequate representation of upper ocean processes. Here we use data from the 2015 ASIRI cruise on R/V Revelle to assess both the fidelity of one-dimensional representations of turbulent mixing and upper ocean heat distribution, and the particular ways in which three-dimensional structures come into play. In the late summer, intense freshwater input in the northern Bay of Bengal creates shallow mixed layers bounded by strong vertical stratification and strong lateral gradients in density that are challenging to both measure and model. During the cruise, a triangle-pattern survey, ~4 km across, was repeated for 2 days by R/V Revelle while acquiring profiles of CTD and turbulence data, horizontal velocities, and thermistor and CTD data from a towed bow-chain. During this survey, a robotic oceanographic surface sampler (ROSE) and a Wirewalker were deployed. Observations are compared with 1-D model run with the KPP mixing parameterization. The model does not reproduce the vertical offset of gradients in momentum and scalars seen in the observations, a limitation of KPP. A strong dependence on the initial stratification in the model runs indicates that the upper ocean response to wind bursts in the BoB varies on scales $<$ 1 km, related to sub-mesoscale density gradients. Implications of these finings for upcoming 2019 fieldwork will but put forward as discussion questions.

Authors & Affiliation:
Jennifer MacKinnon, Andrew Lucas and Kate Adams (Scripps Institution of Oceanography), Jonathan Nash and Emily Shroyer (Oregon State University)

Simon de Szoeke

Observations of convection and atmosphere-ocean interaction

Intraseasonal to mesoscale variations in convection represent important departures from the radiative-convective and atmospheric boundary layer (BL) quasi-equilibria on which convective schemes are based.
We made observations of the simultaneous air-sea, turbulent, and convective processes in the central Indian and western Pacific Oceans from the Dynamics of the Madden-Julian Oscillation (DYNAMO) and Propagation of Intraseasonal Oscillations (PISTON) field experiments. The Madden-Julian Oscillation (MJO) in the central Indian Ocean, and modulation of the monsoon by typhoons and tropical depression-type disturbances in the western Pacific Ocean, lead to active and suppressed periods of convection on synoptic to intraseasonal time scales. Ample sunlight warms the ocean mixed layer and forms shallow diurnal warm layers of 1-5 °C under weak winds associated with convectively suppressed periods. Storms embedded within convectively active phases of intraseasonal oscillations reduce solar radiation and increase wind over the ocean surface, deepening the mixed layer and entraining salt from salinity-stratified barrier layers. The resulting cycle of sea surface temperature (SST) acts to reduce intraseasonal surface moist static energy (MSE) flux and to reduce the intraseasonal cycle of atmospheric MSE, but the spatial pattern of BL temperature related to SST generates low-level convergence and MSE import.
Downdrafts cooled by evaporation of hydrometeors enter the BL and reduce its MSE. Spreading cold pools increase winds and surface fluxes, and generate long-lasting convective squall lines when aligned with subsequent convective cells. Turbulent mixing vents BL MSE to the free troposphere and clouds, fueling moist convection. Intraseasonal convective variability alter the relative effects of downdrafts, turbulent mixing, and surface fluxes on the atmospheric BL.
Authors & Affiliation:
Simon P. de Szoeke and Eric D. Skyllingstad, Oregon State University

Kerstin Cullen

Seasonality and Interannual Variability of the Sri Lanka Dome (Poster)

Located in the SW Bay of Bengal, the Sri Lanka Dome (SLD) is an upwelling tropical thermal dome that forms in a region of positive wind stress curl within the Southwest Monsoon Current (SMC) system. Here, we quantify variability in the SLD’s area, amplitude, sea surface slope, and position using a 23-year record of absolute dynamic topography. The SLD varies in size ranging from 10^4- 10^5 km^2 and in amplitude from 0.15 - 0.3 m. We observe a strong seasonal cycle in the SLD and its relationship to local wind stress curl. For the first two months after formation, the SLD’s sea surface slope correlates with the strength of the wind jet off the tip of Sri Lanka. Later in the southwest monsoon, the SLD’s position shifts northward outside the region of strong wind stress curl, resulting in decorrelation between the slope and wind stress curl. The Dipole Mode Index (DMI), which acts as an integrated measure of regional wind strength, is correlated with both the annual average wind stress curl and SLD slope. The amplitude and area of the SLD vary together, and both are correlated with the strength of the Southwest Monsoon Current. These results suggest alignment between the SMC and the region of wind stress curl may play a pivotal role in controlling the strength of the SLD on an interannual timescale.
Authors & Affiliation: Kerstin Cullen, Emily Shroyer, Oregon State Unviersity

Udeshika Wimalasiri

Distribution of plantonic bioluminescence in the off south coast of Sri Lanka

Bioluminescence is production and emission of light by living organisms and it is a common phenomenon in the marine ecosystem. Variations in bioluminescence intensity with respect to depth and vertical temperature profiles in the south coast of Sri Lanka were studied during 2016 using the Research Vessel Samuddrika under Air-sea interaction in the Northern Indian Ocean project. The intensity of light was measured using Recoverable Bathyphotometer (RBPM). Temperature, conductivity, chlorophyll and depth measurements were taken using a conductivity temperature depth (CTD) profiler. The vertical hauls of zooplankton samples were also collected from the 10 m depth to surface. Bioluminescence intensity decreased with deeper depths and the highest was recorded at the depths range from 20 - 80 m. This study proved that the bioluminescence intensity was comparatively higher above thermocline depth and it is decreasing below the thermocline. These results further confirm that the highest bioluminescence intensity was in the 26–29ºC temperature range. High ocean bioluminescent zooplankton species abundance was recorded in January to March and the October to December periods. Variations in bioluminescence profiles clearly reflect differences in bioluminescence distribution among hydrological areas. This observation can be applied to estimate planktonic distribution in a given area.
Authors & Affiliation: Wimalasiri H.B.U.G.M (National Aquatic Resources Research and Development Agency, Colombo 15, Sri Lanka), Jinadasa S.U.P (National Aquatic Resources Research and Development Agency, Colombo 15, Sri Lanka), Dissanayake D.C.T (University of Sri Jayewardenepura, Nugegoda, Sri Lanka) Authors & Affiliation: Kerstin Cullen, Emily Shroyer, Oregon State Unviersity

Deepak Cherian

The wind-forced seasonal cycle of mixing in the Bay of Bengal

We describe the seasonal cycle of turbulent mixing as observed from moored turbulence sensors (χpods) deployed on moorings distributed throughout the Bay of Bengal in the north Indian Ocean during 2014 and 2015.
We combine all χpod observations to form seasonal-mean vertical profiles of diffusivity in the top 100m and find that the seasonal cycle of near-surface turbulent diffusivity (top 45m) in the Bay appears to follow the seasonal cycle in wind forcing.
In the thermocline between 50- and 100-m, we repeatedly find that high mixing events coincide with the passage of surface-forced downward propagating near-inertial waves and occasionally with the presence of enhanced low-frequency shear associated with the Summer Monsoon Current.
The months of March and April, a period of weak wind forcing and low near-inertial shear amplitude, are characterized by near-laminar flow and near-molecular temperature diffusivities for weeks at a time. Both observations lead us to link the seasonal cycle of thermocline turbulence to the seasonal cycle of near-inertial energy flux in the Bay.
We also find that the enhanced thermocline mixing during the southwest monsoon in the southwestern Bay results in significant export of salt out of the warm salty Arabian Sea water mass.
Authors & Affiliation: "Deepak Cherian, Emily Shroyer, Jim Moum Oregon State University"

Amala Mahadevan

Portrayal of an ISO event over the Bay of Bengal

The spatiotemporal development of ISO events is described by compositing several years of rainfall, wind and sea-surface-temperature data from satellites. Such a generalized analysis reveals the northward propagation and intensification of the events, and phase relationships between variables. This is then compared with observations from a single ISO event sampled during the 2018 summer monsoon MISO-BOB cruise in the Bay of Bengal using radiosonde and ship-based measurements.
Authors & Affiliation: Gualtiero Spiro Jaeger (MIT/WHOI Joint Program), Amala Mahadevan (WHOI)

Simon de Szoeke

Atmosphere-ocean fluxes and atmospheric turbulence observations over warm tropical oceans

TBA

Hyodae Seo

Coupled effects of ocean current on wind stress in the Bay of Bengal: Energetics of mesoscale circulation and upper ocean stratification

This study examines the effect of including surface current in the bulk formula for the wind stress, referred to as the relative wind (RW) effect, on the energetics of the circulation and the upper ocean stratification in the Bay of Bengal (BoB), based on the two SCOAR (WRF- ROMS) coupled model experiments. With the RW effect, the energetics of geostrophic mean circulation and eddy activity are greatly reduced, with the most significant reduction found along the path of the northward East India Coastal Current (EICC) and to the south of the separated latitude. The damping rate of the time-mean kinetic energy, both in the mean (MKE) and eddy (EKE), by the RW effects exceeds 100%, which is much higher than the previous studies in other ocean basins. The energetics calculations reveal that the damping of EKE is primarily due to the reduced eddy wind work, which, according to the spectral analysis, shows the most significant reduction at wavelength close to the first baroclinic Rossby deformation radius. Further examination reveals that the MLD is reduced where the EKE is most strongly weakened. This is attributed to the doming of the isopycnals above the thermocline within the anticyclonic eddies and the resulting increase in the near-surface stratification. Overall, that the geostrophic circulation and ML energetics along the EICC path and south of its separated latitude exhibit the most significant responses to the RW effect implies that it is the area of the hot spot for the momentum coupling between the surface circulation and the monsoonal winds, thus a potential region for focused field measurements.
Authors & Affiliaton: Hyodae Seo (hseo@whoi.edu)1 , Aneesh Subramanian2 , and Hajoon Song3 1Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA University of Colorado, Boulder, Colorado, USA3 Yonsei University, Seoul, South Korea

Jayesh Phadtare

Blocking of Bay of Bengal Storms by the Eastern Ghats

The southern Bay of Bengal during the northeast monsoon season (Oct-Dec), is a fertile ground for the genesis of cyclonic vortices. Subsequently, these vortices propagate westward; they grow in strength with the time they spend over the warm bay. These systems dump heavy rainfall at their landfall over the east coast of India. Some of the rainfall episodes over the coast can last for three consecutive days, causing severe local flooding (for e.g., Chennai floods during 30 November-2 December 2015). Infrared images from the INSAT-3D satellite show that such events are associated with quasi-stationary storms. These storms are blocked by the Eastern Ghats from moving further westward, hence they become stationary over the coast. With four case-studies of cyclonic vortices, it is proposed that the blocking of storm is governed by a nondimensional parameter known as the vortex Froude number, Frv = Vmax/(NH), where, Vmax is the maximum tangential velocity of winds in the vortex, H is the height of the orography, and N is the Brunt-Vaisala frequency of atmosphere along the orography. Strong vortices with Frv > 1 overcome the orographic barrier and propagate further westward. While those with Frv < 1 get blocked by the orography and become stationary. Therefore, the seemingly innocuous weak cyclonic vortices born over the southern bay can also have devastating consequences, given their propensity to hover over the coast for long durations.

Leah Johnson

Are we using the correct boundary layer mixing parameterizations in the Bay of Bengal?

Current boundary layer (BL) mixing schemes tend to disagree on the length of the ocean surface boundary layer (OSBL). This implies significant uncertainty in air-sea dynamics that depend on the depth of the OSBL. Constraining the appropriate BL parameterization is one actionable step towards model improvement of the monsoonal intra-seasonal oscillation (MISO). Many studies that examine variance in OSBL parameterizations do so under idealized scenarios or inspired by ocean regimes very different from the Bay of Bengal. Here the consistency of BL mixing schemes is tested using realistic forcing and initial conditions drawn from the 2018 MISO-BoB pilot cruise observations. Results show the evolution of predicted OSBLs between different parameterizations disagree during the transition from active to break by more than 20m. This ultimately generates spread in the modeled SST with an implied heat flux back into the atmosphere that varies on the order of 20 W m^-2. This highlights the importance of constraining the correct BL parameterizations to better predict the upper ocean buoyancy budget and feedback to the atmosphere.

Amit Tandon and Sanjiv Ramachandran

Air-sea coupling at O(1-100 km) scales: results from submesoscale-permitting simulations coupled thermodynamically to a simplified atmosphere.

Air-sea interactions are crucial to the generation and propagation of monsoon intra-seasonal oscillations (MISOs) in the Bay of Bengal. But we still lack a clear picture of the scale-dependent nature of these interactions. In particular, recent observations (ASIRI/MISOBOB) suggest the range of scales spanning the submesoscale to the mesoscale could have an important role in modulating air-sea coupling during the summer monsoon. We perform idealized simulations using a three-dimensional ocean model (MITgcm) coupled thermodynamically to the atmosphere, allowing for dynamical estimation of the air-sea fluxes forcing the ocean. The atmospheric boundary layer is represented by a single point at a fixed reference height above the ocean surface (CheapAML). We prescribe the winds and the solar insolation, while 'CheapAML' estimates various components of the heat flux using a bulk algorithm at each model iteration. This configuration allows for thermodynamical feedbacks between the ocean and the atmosphere but does not capture oceanic effects on the wind field.
We present preliminary results from simulations of an idealized shallow mixed-layer front. The simulations permit submesoscales and are capable of containing O(100 km) eddies. After allowing the unforced front to develop meanders, we perform two sets of simulations, one with idealized forcing and the other forced by a six-hourly reanalysis product. We present horizontal maps of various components of the heat flux across scales O(1-100 km) and explore oceanic signatures at these scales within the heat fluxes. We also estimate spectra of the heat fluxes to assess the relative contribution from different scales to the spatial variability in the fluxes.

Luc Rainville and Craig Lee

Circulation in the Southern Bay of Bengal and Arabian Sea: intra-seasonal to interannual variability from direct observations

Monsoon-driven circulation in the Bay of Bengal and Arabian Sea plays a critical role in governing heat and freshwater transport in the northern Indian Ocean. Net evaporation and inflow of high-salinity water masses from the Red Sea and Persian Gulf produce an annual net salt surplus in the Arabian Sea, while large riverine discharge and excess precipitation produce an annual net freshwater surplus in the Bay of Bengal. Seasonal exchanges between the two basins, around the southern tip of the Indian subcontinent, act to maintain the salinity balance in the northern Indian Ocean. A series of autonomous glider and surface drifter deployments have captured watermass variability and circulation in the region around Sri Lanka. Observations near Sri Lanka span multiple years (2013 to present) and cover the complete annual cycles, including the monsoon transitions. Persistent sampling by long-endurance Seagliders characterize surface and sub-surface transport of freshwater out of the Bay of Bengal, and of high-salinity Arabian Sea water into the Bay of Bengal, in the context of monsoon circulation. Eighteen realizations of a section east of Sri Lanka were occupied in 2013-2015, and 30 sections from the shelf break to 2​​N (400 km) were obtained more recently south of Sri Lanka (ongoing since early 2016). These direct measurements provide an unprecedented view into the circulation across this important gateway, and allow us to estimate volume, heat, and freshwater transports of the water masses that set the upper ocean properties.
L. Rainville, C. Lee, S.U.P. Jinadasa, H. J. S. Fernando, H. W. Wijesekera

Amala Mahadevan

Portrayal of an ISO event over the Bay of Bengal

The spatiotemporal development of ISO events is described by compositing several years of rainfall, wind and sea-surface-temperature data from satellites. Such a generalized analysis reveals the northward propagation and intensification of the events, and phase relationships between variables. This is then compared with observations from a single ISO event sampled during the 2018 summer monsoon MISO-BOB cruise in the Bay of Bengal using radiosonde and ship-based measurements

Ritabrata Thakur

Seasonality and Buoyancy Suppression of Turbulence in the Bay of Bengal as measured by Chipods

We show how the upper ocean turbulence in the Bay of Bengal varies by winds most of the times until a shallow layer of low-salinity water arrives. We show the effect of this low-salinity water on the suppression of turbulence below the mixed layer.

Sutanu Sarkar

LES of a wind-driven boundary layer with vertical and horizontal gradients of buoyancy

We will present results from high-resolution LES of upper-ocean turbulence in two process studies whose environmental parameters (stratification, wind stress, etc.) are guided by recent observational campaigns in the Bay of Bengal. The first problem is the response of a thin filament of fresh water to wind that is perpendicular to the direction of buoyancy contrast. We will discuss the frontal instabilities and turbulence that emerge and lead to qualitative differences in the vertical structure between the two sides of the front. The numerical study is coordinated with a complementary analysis of observations by other members of the MisoBob team. The second problem is a stratified boundary layer that undergoes a diurnal cycle. The temporal modulation of turbulent fluxes and surface temperature in response to the diurnal cycle of heat flux is quantified. Other motivations of this problem are related to collaborative efforts, first, to provide a benchmark solution to evaluate the accuracy of 1-D mixing parameterizations and, second, to provide upscaled fluxes in regional-model computations of the Bay of Bengal.
Authors:Sutanu Sarkar & Hieu Pham, University of California at San Diego

Jared Buckley

The Impact of Lateral Advection on SST and SSS in the Northern Bay of Bengal during 2015

The Bay of Bengal is subject to strong lateral advection of low-salinity water from the Ganga-Brahmaputra-Meghna river system. This advection leads to the development of strong upper-ocean stratification (salinity stratification), which can have a significant impact on the evolution of SST and SSS by modifying mixing near the surface. In this study, we use a 1-dimensional ocean mixing model (PWP) forced with in-situ air-sea fluxes from a surface mooring, along with estimates of horizontal surface salinity and temperature advection from a satellite ocean current product. We develop an ensemble of 1-dimensional simulations by varying estimates of the advection used to force the model and analyze how the advection of temperature and salinity influences the evolution of SST and SSS. The use of an ensemble of solutions, rather than any single solution, compensates for uncertainty in our satellite estimates and significantly improves confidence in our results. Our results strongly indicate that air-sea fluxes are not sufficient to properly simulate the evolution of SST in the northern Bay of Bengal, and that accounting for the advection of salinity is required in order to reduce error in SST.