Academic literature on the topic 'Diagnosing stirring in the Bay of Bengal'

AbstractThe scale-dependent variance of tracer properties in the ocean bears the imprint of the oceanic eddy field. Anomalies in spice (which combines anomalies in temperature T and salinity S on isopycnal surfaces) act as passive tracers beneath the surface mixed layer (ML). We present an analysis of spice distributions along isopycnals in the upper 200 m of the ocean, calculated with over 9000 vertical profiles of T and S measured along ~4800 km of ship tracks in the Bay of Bengal. The data are from three separate research cruises—in the winter monsoon season of 2013 and in the late and early summer monsoon seasons of 2015 and 2018. We present a spectral analysis of horizontal tracer variance statistics on scales ranging from the submesoscale (~1 km) to the mesoscale (~100 km). Isopycnal layers that are closer to the ML-base exhibit redder spectra of tracer variance at scales km than is predicted by theories of quasigeostrophic turbulence or frontogenesis. Two plausible explanations are postulated. The first is that stirring by submesoscale motions and shear dispersion by near-inertial waves enhance effective horizontal mixing and deplete tracer variance at horizontal scales km in this region. The second is that the spice anomalies are coherent with dynamical properties such as potential vorticity, and not interpretable as passively stirred.

Abstract A simple coupled model is used in a zonally symmetric aquaplanet configuration to investigate the effect of ocean–atmosphere coupling on the Asian monsoon intraseasonal oscillation. The model consists of a linear atmospheric model of intermediate complexity based on quasi-equilibrium theory coupled to a simple, linear model of the upper ocean. This model has one unstable eigenmode with a period in the 30–60-day range and a structure similar to the observed northward-propagating intraseasonal oscillation in the Bay of Bengal/west Pacific sector. The ocean–atmosphere coupling is shown to have little impact on either the growth rate or latitudinal structure of the atmospheric oscillation, but it reduces the oscillation’s period by a quarter. At latitudes corresponding to the north of the Indian Ocean, the sea surface temperature (SST) anomalies lead the precipitation anomalies by a quarter of a period, similarly to what has been observed in the Bay of Bengal. The mixed layer depth is in phase opposition to the SST: a monsoon break corresponds to both a warming and a shoaling of the mixed layer. This behavior results from the similarity between the patterns of the predominant processes: wind-induced surface heat flux and wind stirring. The instability of the seasonal monsoon flow is sensitive to the seasonal mixed layer depth: the oscillation is damped when the oceanic mixed layer is thin (about 10 m deep or thinner), as in previous experiments with several models aimed at addressing the boreal winter Madden–Julian oscillation. This suggests that the weak thermal inertia of land might explain the minima of intraseasonal variance observed over the Asian continent.

The stirring of passive tracers driven by altimetry-derived daily surface geostrophic currents is studied on subseasonal timescales in the Bay of Bengal. Advection of latitudinal and longitudinal bands highlights the chaotic nature of stirring in the Bay via repeated straining and filamentation of the tracer field. An immediate finding is that stirring is local, i.e., of the scale of the eddies, and does not span the entire basin. Further, stirring rates are enhanced along the coast of the Bay and are relatively higher in the pre-and post-monsoonal seasons. The spatially non-uniform stirring at the surface of the Bay is reflected in long-tailed probability density functions of Finite-Time Lyapunov Exponents (FTLEs), which become more stretched for longer time intervals. Quantitatively, advection for a week shows that mean FTLEs lie between 0.13$\pm$0.07 day$^{-1}$, while extremes reach almost 0.6 day$^{-1}$. Averaged over the Bay, relative dispersion initially grows exponentially, followed by a power-law at scales between approximately 100 and 250 km, which finally transitions to an eddy-diffusive regime. Quantitatively, below 250 km, a scale-dependent diffusion coefficient is extracted that behaves as a power-law with cluster size, while above 250 km, eddy-diffusivities range from $6 \!\times \!10^3$ $\!-\!$ $1.6\times 10^4$ m$^2$s$^{-1}$ in different regions of the Bay. These estimates provide a useful guide for resolution-dependent diffusivities in numerical models that hope to properly represent surface stirring in the Bay.\\ A particularly important tracer field in the Bay is the sea surface salinity; indeed, freshwater from rivers influences Indian summer monsoon rainfall and tropical cyclones by stratifying the upper layer and warming the subsurface ocean in the Bay of Bengal. We use {\it in situ} and satellite data with reanalysis to showcase how river water experiences a significant increase in salinity on sub-seasonal timescales. This involves the trapping and homogenization of freshwater by a cyclonic eddy in the Bay. Using a specific example from 2015, river water is shown to enter an eddy along its attracting manifolds within a period of two weeks. This leads to the formation of a highly stratified subsurface layer within the eddy. When freshest, the eddy has the largest sea-level anomaly, spins fastest, and supports strong lateral gradients in salinity. Subsequently, observations reveal a progressive increase in salinity inside the eddy within a month. In particular, salty water spirals in, and freshwater is pulled out across the eddy boundary. Lagrangian experiments elucidate this process, whereby horizontal chaotic mixing provides a mechanism for the rapid increase in surface salinity.\\ The eddy-freshwater interaction, or adjustment, is then studied using a high-resolution Regional Ocean Modeling System. Apart from lateral advection, a mixed layer salinity budget shows the importance of ageostrophic vertical advection during the evolution of salinity within the eddy. An analysis of the depth-integrated eddy kinetic energy indicates the development of both barotropic and baroclinic instabilities. The vertical profile associated with these conversion terms reveals that the surface freshwater was likely involved in developing baroclinic terms in the mixed layer. In addition, an eddy available potential energy (EPE) budget suggests that the entrainment of the river water raises the EPE, which is reflected in the development of gradients in salinity within the eddy. The EPE is lowered with homogenization, signifying irreversible mixing. Further, EPE rates are modulated by the correlation of buoyancy fluxes with density anomalies, which involves lateral advection of freshwater associated with surface cooling and local, regional rainfall. Finally, the adjustment of this freshwater eddy triggers submesoscale dynamics that appear to be an integral part of salinity homogenization. The observation and reanalysis data also showcase the presence of these events across different years, thus bringing out the broader impact of mixing freshwater into high salinity ambient water by eddies in the Bay. This pathway is distinct from vertical diffusive mixing and is likely to be important for the evolution of salinity in the Bay of Bengal.
Ministry of Education (MoE), Indian Institute of Science, Bengaluru; University Grants Commission; National Monsoon Mission, Indian Institute of Tropical Meteorology, Pune; Divecha Centre for Climate Change, Indian Institute of Science, Bengaluru