Graduate student Wenrui Jiang presented his doctoral thesis to the department of Earth & Planetary Sciences today. This is the final hurdle before conferral of the PhD degree. Wenrui’s thesis title is “Lagrangian budgets for ocean kinematics”. The abstract is below. Wenrui starts his postdoctoral training at the department of Earth, Atmospheric, & Planetary Sciences, MIT, in the New Year. Awesome work Wenrui!

Abstract: Understanding how oceanic tracers evolve in space and time is essential for diagnosing the physical processes that govern climate variability. Yet, traditional approaches—including Eulerian budget analyses and Lagrangian particle tracking—capture only part of the relevant dynamics. This thesis develops a new framework that closes tracer budgets along Lagrangian trajectories in a manner consistent with the Eulerian constraints of the finite-volume model. We apply this framework to explore the dynamical mechanisms shaping upper-ocean temperature and salinity variability.

The 2013–2016 Northeastern Pacific marine heatwave (MHW) is one of the most intense extratropical warming events on record. Particle trajectories separate the MHW into northern and southern components with distinct physical origins. The northern MHW is driven by weakened Ekman transport across the North Pacific Current and reduced wintertime surface heat loss, whereas weakened Ekman upwelling over the California Current plays the leading role in sustaining the southern MHW. Although the two components are kinematically independent, they are both dynamically driven by the same persistent low-pressure anomaly over the central Pacific via anomalous heat, moisture, and momentum fluxes. Elevated sea-surface temperatures, in turn, iireinforced this low-pressure anomaly, forming a positive feedback loop that prolonged and intensified the MHW.

We also use the method to quantify the origin and lifecycle of interannual salinity anomalies in the Eastern Subpolar North Atlantic (E-SPNA). Circulation changes, atmospheric forcing, and stratification variability are shown to be the main drivers of salinity fluctuations, with comparable contributions. Most anomalies originate within two years of arriving in the E-SPNA, forming near the Grand Banks and along the North Atlantic Current. Case studies reveal distinct mechanisms for driving extreme events, involving either one dominant driver, as in the 2016 fresh event, or multiple moderate contributors, as in the 2007 salty event.

By bridging Lagrangian and Eulerian perspectives, this thesis provides a robust method for diagnosing the physical controls on diverse forms of scalar variability. The framework is broadly applicable to other seawater properties, offering a pathway to deepen our understanding of ocean dynamics and their role in the climate system.