The Arctic hydrologic cycle and its variability in a regional coupled climate model

186 Pages·2013·18.39 MB·English

The Arctic hydrologic cycle and its
variability in a regional coupled
climate model
Anne Laura Niederdrenk
138
Berichte zur Erdsystemforschung
2013
Reports on Earth System Science
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138
Berichte zur Erdsystemforschung
2013
The Arctic hydrologic cycle and its
variability in a regional coupled
climate model
Anne Laura Niederdrenk
aus Aachen
Hamburg 2013
138
Reports on Earth System Science
2013
ISSN 1614-1199
Anne Laura Niederdrenk
Max-Planck-Institut für Meteorologie
Bundesstrasse 53
20146 Hamburg
Als Dissertation angenommen
vom Department Geowissenschaften der Universität Hamburg
auf Grund der Gutachten von
Prof. Dr. Lars Kaleschke
und
Dr. Uwe Mikolajewicz
Hamburg, den 12. Juli 2013
Prof. Dr. Jürgen Oßenbrügge
Leiter des Departments für Geowissenschaften
ISSN 1614-1199
The Arctic hydrologic cycle and its
variability in a regional coupled
climate model
Anne Laura Niederdrenk
Hamburg 2013

Abstract
Modeling the Arctic hydrologic cycle implies big challenges because of the non-linear
behavior and interplay of its elements on different scales. Additionally, the Arctic envi-
ronment is rapidly changing due to global warming and so are the freshwater components.
Global general circulation models show remarkable differences in modeling the Arctic
freshwater cycle. While they agree on the general sinks and sources of the freshwater
budget, they differ largely in the magnitude of the mean values as well as in the variability
of the freshwater terms. Regional models can better resolve the complex topography and
small scale processes, but they are often uncoupled, thus missing the air-sea interaction.
Additionally, regional models mostly use some kind of salinity restoring or flux correction,
thus disturbing the freshwater budget.
Our approach to investigate the Arctic hydrologic cycle and its variability is a regional
atmosphere-ocean model setup, consisting of a global ocean model with high resolution
in the Arctic coupled to a regional atmosphere model. To account for all sinks and sources
of freshwater, we include a discharge model providing lateral terrestrial waterflows. In
contrast to most regional setups, we run the model without salinity restoring but with
freshwater correction. Additionally, this freshwater correction is set to zero in the Central
Arctic. To our knowledge, this enables for the first time the analysis of a closed freshwater
budget in the Arctic region with a fully coupled high resolution model.
In the framework of this study, we investigate how the dominant modes of large-scale
atmospheric variability impact the variability in the freshwater components. We focus on
thetwoleadingempiricalorthogonalfunctionsofwintermeansealevelpressure, aswellas
ontheNorthAtlanticOscillationandtheSiberianHigh. Thesemodeshavealargeimpact
on the Arctic Ocean circulation as well as on the solid and liquid export through Fram
StraitandthroughtheCanadianarchipelago. However,theycannotexplainthevariability
in river runoff. We find that not only winter conditions are responsible for increased river
runoff, but also an enhanced summer cyclone activity, especially over Eurasia.
To analyze how the freshwater components change in a global warming scenario, we
perform experiments for the first half of the 21st century and use data from an A1B
scenario simulation from the fourth IPCC Assessment Report as forcing. Consistent with
other model studies, we see an amplification of the Arctic hydrologic cycle. Our model
simulates strongly enhanced precipitation, especially over land, leading to an increase in
net freshwater input into the Arctic. The additional freshwater is partly stored within the
Arctic Ocean and accumulates north of the Canadian archipelago, but so far the export
of freshwater through the archipelago does not increase. The increased export through
Fram Strait and the warming of the surface temperature lead to a decrease in deep-water
formation in the Nordic Seas as well as in Labrador Sea.
Our approach to simulate the freshwater fluxes in the Arctic gives realistic results com-
pared with observations. This model is a useful tool for understanding the underlying
mechanisms causing the variability in the hydrologic cycle.

Contents
1 Introduction 1
1.1 The Arctic hydrologic cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Observations and model studies . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2.1 Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2.2 Model studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.3 Thesis Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.4 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2 Model setup and experimental design 11
2.1 The regional atmosphere model REMO . . . . . . . . . . . . . . . . . . . . 11
2.1.1 The hydrological discharge model . . . . . . . . . . . . . . . . . . . . 12
2.2 The global sea ice-ocean model MPIOM . . . . . . . . . . . . . . . . . . . . 12
2.3 OASIS coupling scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.4 Experimental design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3 Model validation 19
3.1 Comparison of model results with reanalysis and observational data . . . . 19
3.1.1 Mean Arctic climate in the model . . . . . . . . . . . . . . . . . . . 20
3.1.2 The Arctic freshwater components . . . . . . . . . . . . . . . . . . . 40
3.2 High resolution versus global model results . . . . . . . . . . . . . . . . . . 43
4 Arctic freshwater cycle in the 20th century 45
4.1 Atmospheric circulation and its impact on freshwater transports . . . . . . 46
4.1.1 Leading empirical orthogonal function of DJF mean sea level pressure 46
4.1.2 Second empirical orthogonal function of DJF mean sea level pressure 68
4.1.3 North Atlantic Oscillation . . . . . . . . . . . . . . . . . . . . . . . . 79
4.1.4 Siberian high . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
4.2 Arctic runoff variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
4.2.1 Eurasian river runoff . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
4.2.2 North American river runoff . . . . . . . . . . . . . . . . . . . . . . . 105
4.3 Summary and conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
4.3.1 Main findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
4.3.2 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
5 Arctic freshwater components in the 21st century 113
5.1 Changes in the atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
5.2 Changes in the ocean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
5.3 Influence of Arctic freshening and global warming on the global ocean cir-
culation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
5.4 Summary and conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
6 Conclusions and outlook 141
6.1 Answers to the research questions . . . . . . . . . . . . . . . . . . . . . . . . 141
6.2 Research perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
6.3 Concluding remark . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
A Methods 145
A.1 Correlation analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
A.2 Linear regression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
A.3 Composite analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
A.4 EOF analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
B Supplementary material for Chapter3 149
B.1 Seasonal cycle from climatologies . . . . . . . . . . . . . . . . . . . . . . . . 149
B.2 Stream function of the Atlantic ocean meridional overturning . . . . . . . . 153
C Supplementary material for Chapter4 155
C.1 Leading empirical orthogonal function . . . . . . . . . . . . . . . . . . . . . 155
C.2 Second leading empirical orthogonal function . . . . . . . . . . . . . . . . . 159
D Additional remarks on the forcing 161
Bibliography 163
Acknowledgements 173