Dissertation Defense: Tien-Yiao Hsu

Title: Understanding the Role of the Ocean in a Changing Climate through Model Hierarchies

Abstract:

Model hierarchies contribute to an improved understanding of the climate response to external forcing by isolating the influence of various physical processes. Simplified global ocean models that have been used to study the climate system consist of models with a temporally evolving mixed-layer acting independently in each water column and models with Ekman flows coupling the water columns. By superimposing the mixed layer induced entrainment and Ekman flow, I construct a model hierarchy of ocean models with four members, which can be individually used as the ocean component in the Community Earth System Model version 1 for perturbation climate studies. In particular, I apply two different forcings to the hierarchy, that of Arctic sea-ice loss and the abrupt quadrupling of CO2 concentration to understand the role and modulation of the ocean in the climate response.
 
In this dissertation defense, I will introduce the formulation of the Ekman mixed-layer ocean model (EMOM) that generates the hierarchy, the numerical methods to obtain the necessary flux correction to compensate for the missing ocean heat flux convergence in the intermediate ocean models, and the results of the unforced and forced simulations.
 
In the unforced simulations, I will demonstrate that including an interactive Ekman flow improves the representation of the Pacific Decadal Oscillation. I will also examine the outcome of two perturbation experiments. The Arctic sea-ice loss experiment demonstrates the importance of including the thermohaline circulation, which significantly modulates the Atlantic ocean heat uptake. The abrupt quadruple CO2 experiment demonstrates that if the modulation of the thermohaline circulation is sufficiently weak at or near equilibrium, the atmosphere model that couples to the Ekman included ocean model reproduces the enhanced tropical rainfall response simulated in the fully-coupled model, which is absent in more idealized settings.

In the final part of my dissertation, I will briefly introduce another hierarchy of simple meridional overturning circulation (MOC) models to understand the modulation of zonally asymmetric freshwater forcing on the existence of multiple equilibria of MOC. The first model is the zonally averaged two-slabs ocean model (ZATOM). ZATOM consists of two meridional-depth slabs to explicitly resolve the east-west buoyancy difference that supports the geostrophic MOC. The second model is the extended two-box model built on top of the Stommel two-box model to include the zonally asymmetric freshwater forcing. Through the bifurcation analysis, I found that the zonal asymmetry of freshwater forcing is a stability threshold of MOC that determines the existence of multiple equilibria. The asymmetry of freshwater forcing can suppress one of the two modes, i.e., thermal and haline modes, in the two-box model by changing the strength of salt-advection feedback. This result provides a potential explanation for the absence of multiple equilibria of Atlantic meridional overturning circulation in some of the atmosphere-ocean general circulation models.