We study the nucleation dynamics and microstructure selection in a model two-dimensional solid undergoing a square to rhombic transformation, using coarse-grained molecular dynamics (MD) simulations. We find a range of microstructures depending on the depth of quench. The transformations are accompanied by the creation of transient and localized non-affine zones (NAZ), which evolve with the rapidly moving parent-product interface. These plastic regions are created beyond a threshold stress, at a rate proportional to the local stress. We show that the dynamics of NAZs determines the selection of microstructure, including the ferrite and martensite.

Assimilation methods, meant to constrain divergence of model trajectory from reality using observations, do not exactly satisfy the physical laws governing the model state variables. This allows mismatches in the analysis in the vicinity of observation locations where the effect of assimilation is most prominent. These mismatches are usually mitigated either by the model dynamics in between the analysis cycles and/or by assimilation at the next analysis cycle. However, if the observations coverage is limited in space, as it was in the ocean before the Argo era, these mechanisms may be insufficient to dampen the mismatches, which we call shocks, and they may remain and grow. Here we show through controlled experiments, using real and simulated observations in two different ocean models and assimilation systems, that such shocks are generated in the ocean at the lateral boundaries of the moored buoy network. They thrive and propagate westward as Rossby waves along these boundaries. However, these shocks are essentially eliminated by the assimilation of near-homogenous global Argo distribution. These findings question the fidelity of ocean reanalysis products in the pre-Argo era. For example, a reanalysis that ignores Argo floats and assimilates only moored buoys, wrongly represents 2008 as a negative Indian Ocean Dipole year.

We study the nucleation dynamics of a model solid state transformation and the criterion for microstructure selection. Using a molecular dynamics (MD) simulation, we had shown that the dynamics of the solid is accompanied by the creation of transient non-affine zones (NAZ), which evolve with the rapidly moving transformation front. Guided by our MD results, we formulate a dynamical continuum theory of solid state transformation, which couples the elastic strain to the non-affine deformation. We demonstrate that our elastoplastic description recovers all qualitative features of the MD simulation. We construct a dynamical phase diagram for microstructure selection, including regimes where martensite or ferrite obtains, in addition to making several testable predictions.

Changes in sea level may be attributed either to barotropic (involving the entire water column) or baroclinic processes (governed by stratification). It has been widely accepted that barotropic sea level changes in the tropics are insignificant at intraseasonal time scales (periods of 30–80 days). Based on bottom pressure records, we present evidence for significant basin-wide barotropic sea level variability in the tropical Indian Ocean during December–April with standard deviations amounting to ∼30–60% of the standard deviation in total intraseasonal sea level variability. The origin of this variability is linked to a small patch of wind over the Eastern Indian Ocean, associated with boreal winter Madden–Julian Oscillations (MJO). These large fluctuations are likely to play a prominent role in the intraseasonal sea level and mass budgets. Because of their much faster propagation than baroclinic processes, they allow the basin to adjust to climatic perturbations much more rapidly than was previously thought.