A local stability analysis of idealized models of stratified eddies that appear in geophysical settings is presented. Specifically, detailed short-wavelength stability analysis is performed for an axisymmetric baroclinic vortex with background rotation and an out-of-plane stable stratification, and a radial stratification in thermal wind balance with the out-of-plane momentum gradient. The effects of curvature and Schmidt number are highlighted, and their potential relevance in the ocean (and ocean models) is discussed. Finally, we present results for the case of an elliptical barotropic vortex with an out-of-plane stable stratification.

In this talk I will discuss some statistical models of complex spatio-temporally varying datasets of the Indian summer monsoon rainfall. The main aim of such models is to uncover underlying spatial patterns as well as the temporal dynamics of these patterns. The techniques discussed vary from Markov Random Fields (MRF) to functional data analysis (FDA). I will emphasise the difficulties in obtaining results which are statistically robust in the sense of being similar even for different data-sets and / or different methods.

The variability of predicted variables at daily to seasonal scales in coupled models is primarily governed by surface boundary conditions between the ocean and atmosphere, namely, sea surface temperature (SST), turbulent heat, and momentum fluxes. Although efforts have been made to achieve good accuracy in surface fluxes and SST in observation and reanalysis products, less attention has been paid toward achieving improved accuracy in coupled model simulations. Improper diurnal phase and amplitude in intra-daily SST and precipitation are well-known problems in most global coupled general circulation models, including the Climate Forecast System v2 (CFSv2) model. The present study attempts to improve the representation of ocean-atmosphere surface boundary conditions in CFSv2, primarily used for India’s operational forecasts at different temporal/spatial scales. In this direction, the diurnal warm layer and cool skin temperature correction scheme are implemented along with the surface flux parameterization scheme following Coupled Ocean- Atmosphere Response Experiment (COARE) v 3.0. The coupled model re-forecasts with a revised flux scheme showed better characteristics in various ocean-atmosphere parameters and processes at diurnal and seasonal time scales. At the diurnal scale, the phase and amplitude of intra-daily SST and mixed layer depth variabilities are improved over most tropical oceans. Improved diurnal SSTs helped in enhancing the diurnal range of precipitation by triggering stronger intra-daily convection. The corrected diurnal ocean-atmospheric boundary state translated into a reduction in seasonal mean dry bias over Indian landmass and the wet bias over tropical oceans. Better simulation of non-linearity associated with El Niño–Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), ENSO-Indian Summer Monsoon Rainfall (ISMR), and IOD-ISMR relation is among the most critical improvements achieved by revising the turbulent flux parameterization. The revised flux scheme showed enhanced prediction skills for tropical SST indices and ISMR. Bay of Bengal is infested with lot of eddies in both SST and sea level. The air sea interaction over these eddies has been a topic of interest in the last few decades. The effect eddies have on atmospheric circulations has been explained by two mechanisms. When the wind blows over a warm eddy, the destabilisation of Atmospheric boundary layer causes the downward transfer of momentum, strengthening the surface winds (Zhang and McPhaden 1995). Also a secondary circulations arising from the pressure variations due to the atmospheric boundary temperature near the surface of thermal fronts were found to support the changes in wind speed (Lindzen and Nigam,1987). Thus, the SST gradients can create convergence and curl through changes in the surface wind speed. The surface wind stress curl and divergence fields created due to this change in the wind speed are related linearly to the crosswind and downwind SST gradients, respectively (Chelton et al. 2001,2004; O’Neill et al., 2003). Further these changes can induce convection on the leeward side of a warm water patch. Hence, it is expected that the BoB eddies impact significantly Indian summer monsoon rainfall significantly. In this presentation a possible pathways to impact ISMR are shown.

This talk highlights some of the important outcomes of the OMM-ASIRI campaign with a focus on salinity and near-surface stratification in the Bay of Bengal based on a combination of observational data from the moorings, satellite remote sensing, research ships and ocean reanalysis. The freshwater discharge from monsoonal rivers and rain into the bay results in about 5-10 m deep fresh layer which persists for nearly three seasons. The low-salinity surface layer leads to a very shallow density stratified mixed layer, profoundly influencing air-sea interaction during the summer monsoon on sub-seasonal to seasonal time scales. An overview of the talk includes: Sensitivity of stratification to vertical sampling resolution, dispersal mechanisms of freshwater, dominant space-time scales in surface salinity and the residence time of freshwater in the Bay of Bengal. These findings from observations indicate a real need for the ocean models to have a more realistic representation of the strong, salinity-dominated near-surface stratification, the shallow wind-driven currents, and the thin mixed layers in the Bay of Bengal.

This work adopts a dynamical systems approach to evaluate the fidelity of various upper ocean turbulence parameterizations subject to realistic monsoon forcing, and presents a finite-time ensemble vector (EV) method to better manage the design and numerical principles of these parameterizations. This method resembles the bred vector approach, but emphasizes the dynamics of a turbulence closure multi-model ensemble. The dynamical systems framework focuses on identifying what constitutes the local behavior of the mixed layer system and isolates the forcing and ocean state conditions where turbulence parameterizations most disagree. Identifying disagreement provides the potential to evaluate SC models comparatively against the LES. A case study forced with observed surface fluxes during the 2018 Monsoon onset in the Bay of Bengal allows for model evaluation during variable forcing conditions. The case study results highlight two regimes where models disagree a) during wind driven deepening of the mixed layer and b) under strong diurnal forcing.

Year-long turbulence records from mixing meters called chi-pods in the WHOI 2015 mooring detail the seasonal nature of the Bay of Bengal turbulence modulated by the variability in the Indian monsoon. The seasonal changes in the monsoon winds control the upper ocean turbulence in a direct way. However, both winds and low-salinity water accompanying monsoonal precipitation and riverine discharge control sub-surface turbulence. Subtle diurnal variations of the winds during a few days of the summer monsoon drive diurnally-varying turbulence, most notably below the mixed layer. We hypothesise this diurnal turbulence to be driven by internal waves generated at the base of the mixed layer.

Dynamical systems in the form of nonlinear partial differential equations are widely employed in atmospheric science and oceanography. It's often the case that the variables that define the state are not the easiest to measure directly and hence a very natural problem arises of inferring the full state from suitable observations. In this talk, I'll focus on some recent results for state estimation in the context of the shallow-water equations covering theoretical and numerical aspects of the problem. Many of the ideas generalize to other models allowing for a simple and practical ways to incorporate measurements into PDE-based dynamical systems.

We describe the seasonal cycle of mixing in the top 30–100 m of the Bay of Bengal as observed by moored mixing meters (χpods) deployed along 88N between 85.58 and 88.58E in 2014 and 2015. All xpod observations were combined to form seasonal-mean vertical profiles of turbulence diffusivity KT in the top 100 m. The strongest turbulence is observed during the southwest and post-monsoon seasons, that is, between July and November. The northeast monsoon (December–February) is a period of similarly high mean KT but an order of magnitude lower median KT, a sign of energetic episodic mixing events forced by near-inertial shear events. The months of March and April, a period of weak wind forcing and low near-inertial shear amplitude, are characterized by near-molecular values of KT in the thermocline for weeks at a time. Strong mixing events coincide with the passage of surface-forced downward-propagating near-inertial waves and with the presence of enhanced low-frequency shear associated with the Summer Monsoon Current and other mesoscale features between July and October. This seasonal cycle of mixing is consequential. We find that monthly averaged turbulent transport of salt out of the salty Arabian Sea water between August and January is significant relative to local E-P. The magnitude of this salt flux is approximately that required to close model-based salt budgets for the upper Bay of Bengal.

During the southwest monsoon, seasonal storms bring torrential rainfall to the South Asian subcontinent and the northern Indian Ocean. Dense cloud-cover limits the amount of sunlight that reaches the ocean's surface, and sediment-laden river-runoff limits the depths to which light can penetrate. This variability may affect phytoplankton primary productivity and its dependent biogeochemical processes. Yet little is known about how monsoonal weather is linked to ecosystem processes below the ocean’s surface. Here, using novel measurements from an array of free-drifting, autonomous systems, we document the co-variability of subsurface irradiance and gross primary productivity (GPP) in the oligotrophic Bay of Bengal. These measurements show a factor-of-two change in subsurface carbon fixation rates between the 'active' and 'break' phases of the southwest monsoon. Bioacoustic measurements collected onboard the autonomous platforms suggest that this intra-seasonal variability in GPP generates a response in higher trophic levels. Long-term measurements from biogeochemical Argo floats provide context for our observations, and indicate that the magnitude of the intraseasonal variability reported here is similar to that of the annual cycle in the region. Our findings demonstrate that coupled air-sea intraseasonal variability in subtropical and tropical ocean systems modulates important regional biogeochemical ocean processes that ultimately impact the global climate system, and may help improve our ability to forecast the patterns and intensity of monsoon precipitation.