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Variability of the ice-ocean system in the Pacific sector of the Southern Ocean [Elektronische Ressource] : numerical model studies / vorgelegt von Karen Aßmann

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Variability of the ice-oceansystem in the Pacific sector ofthe Southern OceanNumerical Model StudiesDissertationzurErlangung des Grades einesDoktors der Naturwissenschaften-Dr. rer. nat.-Dem Fachbereich Physik/ElektrotechnikderUniversit¨ at Bremenvorgelegt vonKaren AßmannDezember 2003Alfred-Wegener-Institut fur¨ Polar- und Meeresforschung, Bremerhaven1. Gutachter: Prof. Dr. Dirk Olbers2.hter: Prof. Dr. Peter LemkeIt was created in beauty. One [autumn] day the temperature dropsthirty degrees in four hours, and the sea grows as motionless as amirror. It’s waiting to reflect a wonder of creation. The clouds andthe sea now glide together in a curtain of heavy grey silk. The watergrows viscous and tinged with pink, like a liqueur of wild berries. Ablue fog of frost smoke detaches itself from the surface of the waterand drifts across the mirror. Then the water solidifies. Out of thedark sea the cold now pulls up a rose garden, a white blanket of iceblossoms formed from salt and frozen drops of water.(...)Then frazil ice is formed, grease ice, and pancake ice, whose platesfreeze together into floes. The ice separates out the salt, the seawaterfreezes from below. The ice breaks; packing, precipitation, and in-creased cold give it an undulating surface. Eventually the ice is forcedadrift.(...)You can try and live with the ice. You can’t fight it or change it orlive instead of it.

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Published 01 January 2004
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Variability of the ice-ocean
system in the Pacific sector of
the Southern Ocean
Numerical Model Studies
Dissertation
zur
Erlangung des Grades eines
Doktors der Naturwissenschaften
-Dr. rer. nat.-
Dem Fachbereich Physik/Elektrotechnik
der
Universit¨ at Bremen
vorgelegt von
Karen Aßmann
Dezember 2003
Alfred-Wegener-Institut fur¨ Polar- und Meeresforschung, Bremerhaven1. Gutachter: Prof. Dr. Dirk Olbers
2.hter: Prof. Dr. Peter LemkeIt was created in beauty. One [autumn] day the temperature drops
thirty degrees in four hours, and the sea grows as motionless as a
mirror. It’s waiting to reflect a wonder of creation. The clouds and
the sea now glide together in a curtain of heavy grey silk. The water
grows viscous and tinged with pink, like a liqueur of wild berries. A
blue fog of frost smoke detaches itself from the surface of the water
and drifts across the mirror. Then the water solidifies. Out of the
dark sea the cold now pulls up a rose garden, a white blanket of ice
blossoms formed from salt and frozen drops of water.
(...)
Then frazil ice is formed, grease ice, and pancake ice, whose plates
freeze together into floes. The ice separates out the salt, the seawater
freezes from below. The ice breaks; packing, precipitation, and in-
creased cold give it an undulating surface. Eventually the ice is forced
adrift.
(...)
You can try and live with the ice. You can’t fight it or change it or
live instead of it.
Miss Smilla’s Feeling For Snow
Peter HøegAbstract
In the framework of BRIOS (Bremerhaven Ice Ocean Simulations) a coupled ice-
ocean model of the Southern Ocean was adapted to investigate the variability of
the ice-ocean system in the Pacific sector of the Southern Ocean with a particular
focus on the factors affecting dense water formation on the Ross Sea continen-
tal shelf. The model’s ocean component is based on the S-coordinate Primitive
Equation Model (SPEM) which is coupled to a dynamic-thermodynamic sea ice
model. Also included is the ice-ocean interaction in major ice shelf cavities. Ex-
tensive validation with the available data base shows that the model BRIOS2.2
is able to provide a highly reasonable representation of ocean and ice conditions
in the Pacific sector of the Southern Ocean.
The sea ice regimes in the western and eastern Ross Sea were shown to be de-
coupled from each other with sea ice characteristics in the western Ross Sea
determined predominantly by the local atmospheric conditions and those in the
eastern Ross Sea by ice import from the Amundsen Sea. The resulting strong
gradient in sea ice formation provides the thermohaline driving force for the shelf
circulation. The cooling, salt input and subsequent deep convection during the
seasonal cycle modifies the waters of the shelf inflow so that their density is
sufficient participate in Antarctic Bottom Water formation.
Model results show that the region around Ross Island and McMurdo Sound has
a key role in controlling the exchange between the ice shelf cavity and the open
ocean in the Ross Sea. Freshwater outflow from McMurdo Sound shapes the
dome structure in the salinity distribution along the ice shelf edge. Drainage
of High Salinity Shelf Water through McMurdo Sound into the cavity in winter
prevents brine accumulation and thus lowers High Salinity Shelf Water salinities
to the range observed. This also affects the balance of Ice Shelf Water and High
Salinity Shelf Water, the two parent water masses for the formation of Antarctic
Bottom Water in the Ross Sea.
The interannual variability of dense water characteristics is, however, predomi-
nantly controlled by variations in the shelf inflow through a sub-surface salinity
and a deep temperature signal whose origin can be traced into the Amundsen
and Bellingshausen Seas. The temperature anomalies are induced in the western Sea where the meridional transport of Circumpolar Deep Water
causes temperature anomalies at the continental shelf break.Zusammenfassung
Im Rahmen von BRIOS (Bremerhaven Ice Ocean Simulations) wurde ein gekop-
peltes Meereis-Ozean Modell des Sudp¨ olarmeeres modifiziert, um das Eis-Ozean-
Systems im Pazifischen Sektor und die Faktoren, die die Bildung dichter Wasser-
massen auf dem Rossmeer-Kontinentalschelf beeinflussen, zu untersuchen. Das
Modell setzt sich aus dem S-coordinate Primitive Equation Model (SPEM) und
einem dynamisch-thermodynamischen Meereismodell zusammen. Es beinhal-
tet ausserdem die Eis-Ozean Wechselwirkung in den grossen Schelfeiskavernen.
Grundlic¨ he Validierung mit den vorhandenen Beobachtungen zeigt, dass BRIOS2.2
eine sehr gute Darstellung der Ozean- und Eisbedingungen im Pazifischen Sektor
des Sudp¨ olarmeeres liefert.
Es konnte gezeigt werden, dass die Meereisregime im westlichen und ostlic¨ hen
Rossmeer voneinander unabh¨ angig sind, wobei sie im westlichen Rossmeer haupt-
s¨achlich von den lokalen atmosph¨ arischen Bedingungen abh¨ angen und im ostlic¨ hen
vom Meereisimport aus dem Amundsenmeer. Der daraus entstehende starke Gra-
dient im Meereiswachstum stellt den themohalinen Antrieb fur¨ die Schelfzirkula-
tion dar. Die saisonale Abkuhlung,¨ der Salzeintrag und die darauf folgende tiefe
Konvektion modifizieren die Wassermassen des Schelfeinstroms derart, daß ihre
Dichte ausreicht um an der Bildung von Antarktischem Bodenwasser teilzunehmen.
Modellergebnisse zeigen, dass die Region um Ross Island und McMurdo Sound
eine Schlusselrolle¨ fur¨ den Austausch zwischen Schelfeiskaverne und offenem Ozean
spielen. Ein Sußw¨ asserausstrom durch McMurdo Sound formt die Domstruk-
tur in der Salzgehaltsverteilung entlang der Schelfeiskante. Der Abfluss von
hochsalinem Schelfwasser in die Kaverne verhindert eine starke Salzanreicherung
und h¨ alt die Salzgehalte so bei den beobachteten Werten. Dieses beeinflusst auch
die Balance zwischen Schelfeiswasser und hochsalinem Schelfwasser, die beide
Quellwassermassen des Antarktischen Bodenwassers im Rossmeer sind.
Die zwischenj¨ ahrliche Variabilit¨ at der Eigenschaften des dichten Wassers wird
allerdings haupts¨ achlich durch Ver¨ anderungen des Schelfeinstroms bestimmt, ins-
besondere durch ein ober߬ achennahes Salzgehaltssignal und ein tiefes Temper-
atursignal, deren Ursprung ins Amundsen- und Bellingshausenmeer zuruc¨ kverfolgt
werden kann. Die Temperaturanomalien entstehen im westlichen Bellingshausen-
meer, wo der meridionalen Transport von Zirkumpolarem Tiefenwasser Temper-
aturanomalien nahe der kontinentalen Schelfkante verursacht.Contents
1 Introduction 5
2 Model description 14
2.1 Ocean model ............................. 14
2.1.1 Model basics ......................... 14
2.1.2 Parametrization of subscale processes ............ 16
2.2 Seaicemodel 19
2.2.1 Thermodynamics ....................... 20
2.2.2 Momentum balance...................... 23
2.2.3 Snow.............................. 24
2.3 Sea ice-ocean coupling ........................ 25
2.3.1 Heat flux ........................... 25
2.3.2 Salt flux ............................ 26
2.3.3 Momentum flux 26
2.4 Ice shelf-ocean interaction ...................... 27
2.5 Model configuration and forcing ................... 28
2.5.1 Model area, grid, and bathymetry .............. 28
2.5.2 Model initialisation and boundary conditions........ 30
2.5.3 Atmospheric forcing ..................... 32
3 Sea ice conditions in the Pacific sector of the Southern Ocean 33
3.1 Sea ice concentration and extent................... 3
3.2 Sea ice thickness ........................... 36
3.3 Sea ice drift - large scale pattern 39
3.4 Seaicegrowth............................. 42
3.5 Summary ............................... 4
12 CONTENTS
4 Ocean properties and circulation 46
4.1 Water mass characteristics and distribution............. 46
4.2 Circulation pattern .......................... 51
4.2.1 The Ross Gyre ........................ 51
4.2.2 Ross Sea continental shelf .................. 5
4.3 Seasonal cycle on the Ross Sea continental shelf .......... 57
4.3.1 Water mass distribution and horizontal circulation..... 59
4.3.2 Melting and freezing at the Ross Ice Shelf base....... 61
4.3.3 Regional fresh-water budget ................. 64
4.4 The effect of McMurdo Sound topography on water mass exchange
across the Ross Ice Shelf front .................... 65
4.5 Summary ............................... 71
5 Sea ice variability in the Pacific sector of the Southern Ocean 73
5.1 Sea ice drift in the Amundsen Sea - validation and implications . 73
5.1.1 Drift data ........................... 73
5.1.2 Comparison of observed and modelled buoy tracks and drift
velocities............................ 75
5.1.3 Influence of wind and ocean currents ............ 85
5.2 Sea ice transports 86
5.3 Sea ice retreat in the Bellingshausen Sea .............. 8
5.4 Sea ice export from the Amundsen Sea ............... 94
5.5 Interaction with the Ross Sea ice cover 97
5.6 Summary ...............................10
6 Variability of dense water formation in the Ross Sea 102
6.1 Variability of Ross Sea shelf water properties............102
6.2 Local atmospheric variability and its effect .............108
6.3 Variability of the continental shelf inflow ..............18
6.4 Origin and propagation of the inflow signal in the Amundsen &
Bellinghausen Seas ..........................12
6.5 Summary132
7 Conclusions & Outlook 134
Acknowledgements 141CONTENTS 3
A Horizontal Grid Structure 143
B Validation of NCEP winds for the Amundsen Sea 145
Bibliography 147