Dynamical Interaction between
Atmosphere and Sea Ice
in Antarctica
Master’s Thesis
by
Alexander Haumann
Utrecht University
October 2011
Supervisor:
Prof. Dr. Michiel R. van den Broeke
E-mail: [email protected]
Co-Supervisors:
Jan H. van Angelen
E-mail: [email protected]
Jan T. M. Lenaerts
E-mail: [email protected]
Abstract
Sea ice that covers large parts of the polar oceans throughout most of the year responds to changes in
the atmosphere or the ocean within a short period of time. The rapid decrease of the Arctic sea ice
cover in the past decades has led to a fundamental discussion of the role of sea ice in the climate system.
Surprisingly, in contrast to the northern hemisphere, the sea ice in the Southern Ocean has been slightly
increasing over the last decades. This is owing to essentially different processes that take place around
Antarctica. There, the ice is not confined to a basin as in the Arctic Ocean but can move rather freely
around the Antarctic continent which results in a strong response to changes in the wind field.
In this Master’s thesis I examined the impact of the variations in the coastal Antarctic atmospheric
boundary layer on the sea ice. By studying wind driven sea ice transport in the Southern Ocean and
temporal and spatial variabilities in the period 1989 to 2006, I have revealed important characteristics of
the sea ice cover and processes that determine its growth and decay. The near surface wind field over the
coastal continent and ocean as well as its forcing mechanisms were described in detail by using output
from a regional atmospheric climate model. This showed strong relations to key parameters that I have
deduced from a satellite record of sea ice concentration and sea ice motion.
The regions of the largest sea ice extent, the Ross and Weddell Seas, are also those areas where most
of the sea ice transport takes place and where its variability is the largest. Interannual variations and
trends of transport are associated with varying sea ice concentration just north of these areas in the Ross
and Weddell Seas. Comparing the wind field and the sea ice motion, I found out that spatial patterns of
persistent southerly or south-easterly winds coincide with those of ice drift. The winds in these regions
result from combined effects of the large-scale pressure distribution, cold air that accumulates over the ice
shelves, and large topographic barriers that alter the flow. Adjacent to the large Ross and Ronne-Filchner
Ice Shelves constant outflow of cold air takes place almost year-round. Here, sea ice is constantly exported
from the coastal region, and large polynyas and leads form. As the cold winds not only lead to sea ice
transport but also support refreezing of the open water, these areas are associated with strong sea ice
formation. I have defined an index that captures the outflow of cold continental air from the ice shelves.
The long-term variations in outflow correlate well with variations of the sea ice cover and meridional sea
ice transport in the Ross and western Weddell Seas. Further, the results suggest that the positive trend
of sea ice cover in western Ross Sea and the negative trend in the western Weddell Sea are related to a
respective seasonal increase and decrease of cold air outflow. Overall, in my thesis, I showed that the
dynamical interaction between the atmospheric boundary layer and the sea ice is a regional key element
in the interannual variability and the long-term changes of the sea ice cover in the Southern Ocean.
i
ii
Contents
Abstract
i
Boxes
iv
Figures
v
Tables
v
1 Introduction
1
2 Theoretical Background
7
2.1
The Antarctic Atmospheric Boundary Layer over Snow, Ice and Ocean . . . . . . . . . . .
8
2.2
Forcing Mechanisms of the Antarctic Atmospheric Boundary Layer Dynamics . . . . . . .
16
2.3
Forcing Mechanisms of Sea Ice Formation and Transport . . . . . . . . . . . . . . . . . . .
21
3 Data & Model Description
27
3.1
The Regional Atmospheric Climate Model RACMO2.1/ANT . . . . . . . . . . . . . . . .
28
3.2
Sea Ice Concentrations from Passive Microwave Satellite Data . . . . . . . . . . . . . . . .
34
3.3
Sea Ice Drift Data from Satellites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
40
3.4
Data Assimilation and Grid Adaptation . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
4 Simulated Atmospheric Boundary Layer Processes
45
4.1
Cold Air Pooling and Near-Surface Temperature . . . . . . . . . . . . . . . . . . . . . . .
45
4.2
The Near-Surface Wind Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
56
4.3
The Forcing Terms of the Wind Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
62
4.4
Cold Air Pooling Along Topographic Barriers and Associated Flow . . . . . . . . . . . . .
68
5 Observed Sea Ice Cover & Dynamics
74
5.1
Sea Ice Cover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
74
5.2
Sea Ice Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
86
5.3
Open Water Formation and Sea Ice Production . . . . . . . . . . . . . . . . . . . . . . . .
99
6 Regional Assessment of Cold Air Outflow over Sea Ice
106
6.1
Ross Sea and Ice Shelf Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
6.2
Weddell Sea and Ronne-Filchner Ice Shelves . . . . . . . . . . . . . . . . . . . . . . . . . . 112
7 Conclusions & Discussion
118
Bibliography
123
Acknowledgment
131
iii