The heat, freshwater and mass of the Arctic Ocean: a model overview

203 Pages·2015·4.38 MB·English

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UNIVERSITY OF SOUTHAMPTON

Faculty of Natural and Environmental Sciences

Department of Ocean and Earth Sciences

The Heat, Freshwater and Mass of the Arctic Ocean: A Model Overview

by

Stephen Andrew Fawcett

Thesis for the degree of Doctor of Philosophy

September 2015

UNIVERSITY OF SOUTHAMPTON
ABSTRACT
FACULTY OF NATURAL AND ENVIRONMENTAL SCIENCES
Ocean and Earth Sciences
Thesis for the degree of Doctor of Philosophy
THE HEAT, FRESHWATER AND MASS OF THE ARCTIC OCEAN: A MODEL
OVERVIEW
Stephen Andrew Fawcett
The Nucleus for European Modelling of the Ocean (NEMO) Ocean General
Circulation Model (OGCM) is shown for the first time to conserve Boussinesq
mass at the limit of uncertainty from computer precision when using output
data offline, which is sufficient for thermodynamic calculations to be reliable.
Long term variability and seasonal cycles of freshwater surface and boundary
fluxes show different response rates to perturbations of freshwater flux: a slow
baroclinic response leading to freshwater storage and a fast barotropic
response redistributing summer river inflow. Two thirds of Arctic Ocean
surface heat loss occurs in the Barents and Kara Seas, a region only 14% of its
total area. The seasonal thermodynamic storage cycle shows that the limited
observations of the Arctic Ocean happen to be made when minimum storage
occurs, implying that observations are unbiased by storage.
The Arctic Ocean has an important role in global ocean circulation, sea ice and
climate. It is poorly observed, so OGCMs complement observations but these
must conserve mass for heat and freshwater estimates to be reliable and
independent of any reference value. Atlantic Water is a layer of Atlantic Ocean
origin water that enters the Barents Sea, providing a major source of Arctic
Ocean heat. Its varying strength has implications for Arctic sea ice and
freshwater. The monthly, annual and seasonal patterns of heat and freshwater
storage in a modelled Arctic Ocean area are studied from 1981-2007, showing
that surface fluxes have a strong seasonal variability and boundary fluxes show
a diluted version of this variability. The water circulating through the Barents
Sea drastically decreases in temperature as heat is lost to the atmosphere from
cooling and formation of ice and denser water. A large surface heat loss also
occurs near the Arctic Ocean in the Nordic Seas, from recirculating water from
Fram Strait.

Table of Contents
ABSTRACT I
TABLE OF CONTENTS I
LIST OF TABLES V
LIST OF FIGURES VII
DECLARATION OF AUTHORSHIP XV
ACKNOWLEDGEMENTS XVI
SOME DEFINITIONS AND ABBREVIATIONS XVII
CHAPTER 1: INTRODUCTION 1
1.1 Modelling 1
1.2 Thermodynamic properties 4
CHAPTER 2: OVERVIEW 9
2.1 Geography of the Arctic Ocean 9
2.1.1 Definition of Arctic Ocean 9
2.1.2 Water masses 11
2.1.3 Circulation 12
2.1.4 Inflow/Outflow 14
2.2 Sea Ice 16
2.3 Figures 19
i
CHAPTER 3: MODELLING 25
3.1 NEMO OCGM 25
3.2 Sea Ice 32
3.3 Run configurations 34
3.3.1 Model outputs 36
3.4 Tables and figures 37
CHAPTER 4: MASS CONSERVATION IN NEMO 41
4.1 Mass conservation 41
4.2 Transport components 45
4.2.1 Advection 45
4.2.2 Surface fluxes 50
4.2.3 Content change 51
4.3 Single cell mass conservation 52
4.4 Simple region mass conservation 54
4.5 Results 56
4.6 Discussion 58
4.6.1 Single cell mass conservation 58
4.6.2 Single year regional mass conservation 59
4.6.3 50-year regional mass conservation 59
4.6.4 Sources of uncertainty 61
4.7 Tables and figures 65

ii
CHAPTER 5: ARCTIC OCEAN FLUXES 71
5.1 Region 72
5.2 Transports 72
5.2.1 Liquid heat component 72
5.2.2 Solid sensible heat component 73
5.2.3 Solid latent heat component 74
5.2.4 Freshwater 75
5.3 Surface Fluxes 76
5.3.1 Liquid heat 76
5.3.2 Latent heat 76
5.3.3 Freshwater 76
5.4 Content change of temperature and salinity 77
5.4.1 Heat content 78
5.4.2 Freshwater content 78
5.5 Total budget 79
5.5.1 Heat budget 79
5.5.2 Freshwater budget 80
5.6 Uncertainty from mass conservation 81
5.7 Results 81
5.8 Discussion 88
5.8.1 Mass 88
5.8.2 Heat 91
5.8.3 Freshwater 96
5.9 Tables and Figures 100
iii
CHAPTER 6: BARENTS AND KARA SEAS FLUXES 127
6.1 Region 127
6.2 Method 128
6.3 Results 129
6.4 Discussion 133
6.5 Tables and figures 137
CHAPTER 7: DISCUSSION AND FURTHER WORK 155
7.1 Mass Conservation 155
7.2 Arctic Ocean fluxes 156
7.3 Barents and Kara Seas fluxes 158
7.4 Future work 159
LIST OF REFERENCES 163

iv
List of tables
Table 3.1: Summary of model runs used. ......................................................37
Table 4.1: Single cell volume conservation for a single 5-day period. ............65
Table 4.2: Arctic Ocean mass conservation for a single year. .........................65
Table 5.1: Mean oceanic heat budget for the whole Arctic in different model
runs. Values are in TW, sign is positive into the region. Fluxes are monthly
averages and content change and imbalance are annual averages. ..............100
Table 5.2: Mean solid heat budget components for the whole Arctic in different
model runs. Values are in TW, sign is positive into the region. Advection and
flux are monthly averages and content change and imbalance are annual
averages. ....................................................................................................101
Table 5.3: Totals of monthly average surface and boundary flux components.
Values are in TW, sign is positive into the region. ........................................102
Table 5.4: Mean freshwater (FW) budget for the whole Arctic in different model
runs. Values are in mSv, sign is positive into the region. Fluxes are monthly
average and content change and imbalance are annual average. .................103
Table 5.5: Mean volume budget for the whole Arctic in different model runs.
Values are in mSv, sign is positive into the region. Fluxes are monthly
averages and content change and imbalance are annual averages. ..............104
Table 5.6: Mass equivalent volume budget for the whole Arctic in different
model runs. Values are in kT s-1, sign is positive into the region. Fluxes are
monthly averages and content change and imbalance are annual averages. 105
v