ICTS

TBA

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.

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

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.

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)

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"

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)

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

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