Manikandan Mathur

In the Bay of Bengal, the warm, dry boreal spring concludes with the onset of the summer monsoon and accompanying southwesterly winds, heavy rains, and variable air-sea fluxes. Here, we summarize the 2018 monsoon onset using observations collected through the multinational Monsoon Intraseasonal Oscillations in the Bay of Bengal (MISO-BoB) program between the United States, India, and Sri Lanka. MISO-BoB aims to improve understanding of monsoon intraseasonal variability, and the 2018 field effort captured the coupled air-sea response during a transition from active-to-break conditions in the central BoB. The active phase of the ~20-day research cruise was characterized by warm sea surface temperature (SST > 30°C), cold atmospheric outflows with intermittent heavy rainfall, and increasing winds (from 2 to 15 m s−1). Accumulated rainfall exceeded 200 mm with 90% of precipitation occurring during the first week. The following break period was both dry and clear, with persistent 10-12 m s−1 wind and evaporation of 0.2 mm h−1. The evolving environmental state included a deepening ocean mixed layer (from ~20 to 50 m), cooling SST (by ~1°C), and warming/drying of the lower to midtroposphere. Local atmospheric development was consistent with phasing of the large-scale intraseasonal oscillation. The upper ocean stores significant heat in the BoB, enough to maintain SST above 29°C despite cooling by surface fluxes and ocean mixing. Comparison with reanalysis indicates biases in air-sea fluxes, which may be related to overly cool prescribed SST. Resolution of such biases offers a path toward improved forecasting of transition periods in the monsoon.

We perform a three-dimensional short-wavelength linear stability analysis of numerically simulated two-dimensional Kelvin-Helmholtz vortices in homogeneous and stratified environments at a fixed Reynolds number of. For the homogeneous case, the elliptic instability at the vortex core dominates at early times, before being taken over by the hyperbolic instability at the vortex edge. For the stratified case of Richardson number, the early-time instabilities comprise a dominant elliptic instability at the core and a hyperbolic instability influenced strongly by stratification at the vortex edge. At intermediate times, the local approach shows a new branch of (convective) instability that emerges at the vortex core and subsequently moves towards the vortex edge. A few more convective instability bands appear at the vortex core and move away, before coalescing to form the most unstable region inside the vortex periphery at large times. In addition, the stagnation point instability is also recovered outside the periphery of the vortex at intermediate times. The dominant instability characteristics from the local approach are shown to be in good qualitative agreement with the results based on global instability studies for both homogeneous and stratified cases. A systematic study of the dependence of the dominant instability characteristics on is then presented. While is identified as most unstable (with convective instability being dominant), another growth rate maximum at is not far behind (with the hyperbolic instability influenced by stratification being dominant). Finally, the local stability approach is shown to predict the potential orientation of the flow structures that would result from hyperbolic and convective instabilities, which is found to be consistent with three-dimensional numerical simulations reported previously.

Natural processes, ranging from blood transport to planetary formation, are strongly influenced by particle collisions induced by background turbulence. While inertial clustering and particle pair relative velocity are recognized as the main collision enhancement factors, their physical coupling is poorly understood. In this experimental study, we measure clustering and relative velocity in a polydisperse droplet field with background air turbulence, to directly demonstrate the physical coupling between these collision enhancement factors. This coupling is shown to cause an inverse relation between clustering and relative velocity in the mean-flow-dominated turbulent flow we study, thus suppressing the intuitive effect of an increase in droplet collision rate with background air turbulence. Turbulence modulation due to clustering, and the resultant reduction of caustic droplet pairs with large relative velocities, are found to be the key physical mechanisms, and should be a consideration in droplet collision rate estimates in warm rain initiation.

In this paper, we use asymptotic theory and numerical methods to study resonant triad interactions among discrete internal wave modes at a fixed frequency in a two-dimensional, uniformly stratified shear flow. Motivated by linear internal wave generation mechanisms in the ocean, we assume the primary wave field as a linear superposition of various horizontally propagating vertical modes at a fixed frequency. The weakly nonlinear solution associated with the primary wave field is shown to comprise superharmonic (frequency) and zero frequency wave fields, with the focus of this study being on the former. When two interacting primary modes and are in triadic resonance with a superharmonic mode, it results in the divergence of the corresponding superharmonic secondary wave amplitude. For a given modal interaction, the superharmonic wave amplitude is plotted on the plane of primary wave frequency and Richardson number, and the locus of divergence locations shows how the resonance locations are influenced by background shear. In the limit of weak background shear , using an asymptotic theory, we show that the horizontal wavenumber condition is sufficient for triadic resonance, in contrast to the requirement of an additional vertical mode number condition in the case of no shear. As a result, the number of resonances for an arbitrarily weak shear is significantly larger than that for no shear. The new resonances that occur in the presence of shear include the possibilities of resonance due to self-interaction and resonances that occur at the seemingly trivial limit of, both of which are not possible in the no shear limit. Our weak shear limit conclusions are relevant for other inhomogeneities such as non-uniformity in stratification as well, thus providing an understanding of several recent studies that have highlighted superharmonic generation in non-uniform stratifications. Extending our study to finite shear (finite) in an ocean-like exponential shear flow profile, we show that for cograde-cograde interactions, a significant number of divergence curves that start at will not extend below a cutoff. In contrast, for retrograde-retrograde interactions, the divergence curves extend all the way from to. For mixed interactions, new divergence curves appear at for and extend to other primary wave frequencies for smaller. Consequently, the total number of resonant triads is of the same order for small as in the limit of weak shear , although it attains a maximum at.

We present a simple, novel kinematic criterion - that uses only the horizontal velocity fields and is free of arbitrary thresholds - to separate line plumes from local boundary layers in a plane close to the hot plate in turbulent convection. We first show that the horizontal divergence of the horizontal velocity field has negative and positive values in two-dimensional (2D), laminar similarity solutions of plumes and boundary layers, respectively. Following this observation, based on the understanding that fluid elements predominantly undergo horizontal shear in the boundary layers and vertical shear in the plumes, we propose that the dominant eigenvalue of the 2D strain rate tensor is negative inside the plumes and positive inside the boundary layers. Using velocity fields from our experiments, we then show that plumes can indeed be extracted as regions of negative, which are identical to the regions with negative. Exploring the connection of these plume structures to Lagrangian coherent structures (LCS) in the instantaneous limit, we show that the centrelines of such plume regions are captured by attracting LCS that do not have dominant repelling LCS in their vicinity. Classifying the flow near the hot plate based on the distribution of eigenvalues of the 2D strain rate tensor, we then show that the effect of shear due to the large-scale flow is felt more in regions close to where the local boundary layers turn into plumes. The lengths and areas of the plume regions, detected by the criterion applied to our experimental and computational velocity fields, are then shown to agree with our theoretical estimates from scaling arguments. Using velocity fields from numerical simulations, we then show that the criterion detects all the upwellings, while the available criteria based on temperature and flux thresholds miss some of these upwellings. The plumes detected by the criterion are also shown to be thicker at Prandtl numbers greater than one, expectedly so, due to the thicker velocity boundary layers of the plumes at 1$]]>.

We present an experimental study of resonant generation of superharmonic internal waves as a result of interaction between horizontally propagating vertical internal wave modes m and n at frequency ω0 in a uniformly stratified finite-depth fluid. Thorpe [J. Fluid Mech. 24, 737 (1966)JFLSA70022-112010.1017/S002211206600096X] has shown theoretically that modes m and n at frequency ω0 and mode p=|m-n| at frequency 2ω0 are in triadic resonance at specific values of ω0. We demonstrate the occurrence of this triadic resonance by forcing a primary wave field of modes m and n at various ω0 using a novel internal wave generator, and observing the spontaneous growth (or lack thereof) of the superharmonic mode p=|m-n| at frequency 2ω0. A superharmonic wave field with a predominantly mode-p=|m-n| structure is observed over a finite range of frequency (Δω0≃0.03N) around the resonant value, where N is the uniform buoyancy frequency. The spatial growth of the superharmonic wave field is then quantitatively measured, to subsequently compare with the predictions from amplitude evolution equations at resonance at various forcing amplitudes, thereby validating this model. It is furthermore shown that a large-scale spatial evolution of the wave field is more suited to describe our experiments than the slow temporal evolution approach. The paper concludes with a brief discussion of viscous effects

Internal gravity waves are propagating disturbances in stably stratified fluids, and can transport momentum and energy over large spatial extents. From a fundamental viewpoint, internal waves are interesting due to the nature of their dispersion relation, and their linear dynamics are reasonably well-understood. From an oceanographic viewpoint, a qualitative and quantitative understanding of significant internal wave generation in the ocean is emerging, while their dissipation mechanisms are being debated. This paper reviews the current knowledge on instabilities in internal gravity waves, primarily focusing on the growth of small-amplitude disturbances. Historically, wave-wave interactions based on weakly nonlinear expansions have driven progress in this field, to investigate spontaneous energy transfer to various temporal and spatial scales. Recent advances in numerical/experimental modeling and field observations have further revealed noticeable differences between various internal wave spatial forms in terms of their instability characteristics; this in turn has motivated theoretical calculations on appropriately chosen internal wave fields in various settings. After a brief introduction, we present a pedagogical discussion on linear internal waves and their different two-dimensional spatial forms. The general ideas concerning triadic resonance in internal waves are then introduced, before proceeding towards instability characteristics of plane waves, wave beams and modes. Results from various theoretical, experimental and numerical studies are summarized to provide an overall picture of the gaps in our understanding. An ocean perspective is then given, both in terms of the relevant outstanding questions and the various additional factors at play. While the applications in this review are focused on the ocean, several ideas are relevant to atmospheric and astrophysical systems too.

Internal waves in the ocean are well-recognized to play an important role in the global energy budget. Triadic resonance is one mechanism via which these internal waves transfer their energy to other spatial and temporal scales before dissipation, at locations blue both near and away from their generation sites. In this paper, we perform a combined theoretical and numerical study of triadic resonance in internal wave modes in a finite-depth ocean with an arbitrary stratification profile. Considering internal waves generated at spatially localized regions, the spatial evolution of the modal amplitudes within a resonant triad are derived based on the method of multiple scales. Two representative cases are considered: (i) modes 1 and 2 at a specific frequency ω0 in triadic resonance with the mode-1 superharmonic wave (frequency 2ω0) in a uniform stratification, and (ii) a self-interacting mode-3 at a specific frequency ω0 in triadic resonance with the mode-2 at frequency 2ω0 in an ocean-like nonuniform stratification. In case (ii), any initial energy in mode-3 at frequency ω0 gets permanently transferred to mode-2 at frequency 2ω0. Numerical simulations are performed to show the spontaneous excitation of superharmonic internal waves resulting from modal interactions in the aforementioned cases, and quantitatively validate the initial spatial evolution of the wave field predicted by the amplitude evolution equations. Furthermore, numerical simulations at off-resonant frequencies are used to identify the range of primary wave frequencies (around the resonant frequency) over which spontaneous superharmonic wave excitation occurs. Quantitative estimates of energy transfer rates within the resonant triads considered show that superharmonic wave excitation resulting from modal interactions should be an important consideration alongside other triadic resonances like parametric subharmonic instability (PSI). We conclude by giving estimates of the relative importance of superharmonic wave excitation in the ocean, and provide motivation for future studies.

In this work, we present a study of the horizontal mixing properties in the North Indian Ocean using finite size Lyapunov exponents (FSLE) analysis generated using numerical ocean model simulated surface velocity fields. The period of analysis in this study is from December 2016 to 2020. Sensitivity of FSLE fields with respect to the final relative dispersion distance suggest that the FSLE features are optimally resolved when the distance is set at 110 km. Based on mixing activities inferred from the annual mean of backward and forward FSLE (bFSLE and fFSLE) fields, few sub-domains are selected in the western, southeastern Arabian Sea and in the Bay of Bengal. An investigation of links between satellite derived chlorophyll concentration and biological activity is done in the selected subregions and relation between surface horizontal stirring and chlorophyll standing stocks is also studied. Seasonal and interannual variations of bFSLEs field are analyzed to study the mixing characteristics of ocean on these time scales. Further, co-variability between FSLE field structure and chlorophyll-field structure is also studied. The current study is useful to understand the relationship between the horizontal mixing and chlorophyll concentration.