Mechanisms of variability in Atlantic ocean heat transport and meridional overturning in global climate models

Abstract

Meridional ocean heat transport (OHT) plays a major role in global climate. The Atlantic Ocean is particularly relevant to the global climate because its OHT is northward in both hemispheres due to the existence of the strong Atlantic Meridional Overturning Circulation (AMOC). This thesis gives new insights into what mechanisms drive changes in North Atlantic OHT. The first chapter focuses on the mechanisms that drive changes in OHT into the Arctic from the North Atlantic under both internal variability and CO2 forced climate change, which are important to understand because the Arctic is experiencing particularly rapid climate change. Our results indicate that the mechanisms differ depending on whether the OHT changes occur under CO2 forcing or internal variability. We also find that an increase in OHT into the Arctic can occur despite a decrease in the strength of AMOC un- der global warming. Chapter 2 considers the entire North Atlantic, and aims to determine the mechanisms driving low-frequency OHT variability using a novel method that isolates a mode of low-frequency variability without any explicit low-pass filtering of the data. Here our results suggest that in global climate models, North Atlantic OHT and AMOC are driven primarily by changes in water-mass transformation in the Labrador Sea regardless of which deepwater formation regions dominate the climatological water-mass transformation and AMOC. In Chapter 3, we investigate how these mechanisms differ in higher resolution models. Chapter 3 Part I focuses on determining how well the time-mean AMOC and high-latitude water-mass transformation are represented in a high-resolution coupled model compared to an equivalent low-resolution version. We find that a high-resolution coupled model reproduces the water-mass transformation found in an atmospheric reanalysis-forced ocean simulation fairly effectively, especially compared to a low-resolution version. Chapter 3 Part II applies a similar analysis to what is used in Chapter 2 to a high-resolution model to see whether the same mechanisms of OHT variability found in low-resolution models hold true. Here we find that, although the Labrador Sea plays a much smaller role in climatological water-mass transformation and AMOC in the high-resolution model compared to the low-resolution version, it still appears to play a major role in the WMT and AMOC variability at decadal and longer timescales. This work as a whole contributes to providing a better understanding of North Atlantic ocean variability, how it differs between models with differing climatologies and resolutions, and how it is linked to Arctic changes.