Doctoral Proposal Defense – Parth Sastry

Date:  Thursday, November 6, 2025

Time: 1:00pm

Topic:  Mesoscale Eddies and Tracer Transports Into the Eastern Tropical Pacific Oxygen Deficient Zone

Zoom:  https://umassd.zoom.us/j/95944167939

                           Meeting ID: 959 4416 7939

Passcode: 222984

Oxygen deficient zones (ODZs) are biogeochemically relevant regions of interest, characterized by very low to undetectable oxygen concentrations within the water column. They are found in all the world’s oceans, with the largest being in the eastern tropical Pacific (ETP). In conjunction, the region is home to a rich mesoscale eddy field, assumed to be associated with frequent wind jet events over the Gulf of Tehuantepec along with instabilities in the larger circulation. These wind events drive upwelling over the ocean, causing cold, dense, nutrient-rich waters to be oxygenated, potentially

being a significant source of oxygen into the ODZ.

A landmark study in 2015 by Cole et al. utilized information from Argo profiles – analyzing salinity anomalies – to obtain estimates of horizontal eddy di!usivities throughout the global ocean. These estimates help us account for smaller scale mixing processes a!ecting tracers in models, including oxygen. In the first part of the thesis, we improve upon this previous study by deriving a method to obtain a lower bound for horizontal eddy di!usivity via analyzing profiles within mesoscale eddies utilizing our knowledge of the eddy tracks to constrain observed salinity anomalies to a purely local-stirring induced anomaly. Hence, our analysis accounts for advective contributions to these anomalies and shows a reduction compared to di!usivities in the previous study. We still observe an elevated band of di!usivities along the “eddy alley” in the ETP – of the order of 5 → 103 m2s→1 – showcasing agreement with estimates from other studies that parameterize di!usivities. However, this estimate is still higher than what is used in common global climate models – leading them to potentially underestimate the oxygen flux into the ODZ.

Coastal waters in the region are more oxygenated than waters further out in the open ocean. Mixing of local climatological oxygen gradients by mesoscale eddies is one component of oxygen transport into the ODZ, which we quantify in the second chapter via our obtained di!usivity estimates. The aforementioned upwelling events are another source of oxygen. They oxygenate cold, dense water which then subducts into the ODZ. We propose to examine the statistics and nature of upwelling events via sea surface temperature maps from reanalysis products to quantify the amount of oxygen that gets transported cross-shelf as a consequence of these events. This analysis could examine whether wind-driven upwelling is a significant source of oxygen for the deeper waters of the ODZ. We also examine and diagnose particular upwelling events with models like HYCOM, to quantify the fraction of oxygenated water that gets advected to the ODZ, compared to along the coast – giving us better estimates of net oxygen transport. We seek to answer three key questions in this chapter – 1) How significant are upwelling events compared to local stirring in cross-shelf oxygen transport into the ODZ?, 2) Do high-resolution ocean models capture upwelling events as an oxygen source in the region? and 3) How do model computations of net oxygen transport compare with our estimates?

The mesoscale eddies observed in this region have a statistically significant relationship with the Tehuantepecer, a mountain-gap wind travelling through the Chivela pass and passing over the Gulf of Tehuantepec, that can often reach storm intensities. Our aim for the final part of the thesis is to investigate, via a hierarchy of increasingly complex ocean models in the Julia software package ClimaOcean, the generation and evolution mechanism of these eddies. A previous study utilizing the Regional Ocean Modeling System (ROMS) to model the region, showed that the predominant source of eddy kinetic energy (EKE) in the region are instabilities of the mean seasonal circulations and not transient wind work. This conclusion was drawn purely from a EKE budget analysis and did not analyze the generation mechanism of these eddies. We wish to examine, via process studies, what local processes drive eddy generation and whether they can explain observed asymmetries among anticyclones/cyclones in stability and size.

ADVISOR(s):  Dr. Amit Tandon, Department of Estuarine and Ocean Sciences, SMAST

                                                          (atandon@umassd.edu)

                                                           Dr. Geoffrey Cowles, Dept. of Fisheries Oceanography, SMAST

                                                           Dr. Eric D’Asaro, Applied Physics Laboratory, University of Washington

NOTE: All EAS Students are ENCOURAGED to attend.