Moored Observations of Shelfbreak Processes at the Inflow to and Outflow from the Arctic Ocean ...

275 Pages·2013·30.76 MB·English

Moored Observations of Shelfbreak Processes at the
Inflow to and Outflow from the Arctic Ocean
by
Wilken-Jon von Appen
B.S., Jacobs University, Bremen, Germany, 2007
Submitted in partial fulfillment of the requirements for the degree of
Doctor of Philosophy in Physical Oceanography
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
and the
jARCHN~ES
WOODS HOLE OCEANOGRAPHIC INSTITUTION
"A3SACIH'J NTTUTE
February 2013
@ S i-
2013 Wilken-Jon von Appen LiBRR IE
All rights reserved.
The author hereby grants to MIT and to WHOI permission to
reproduce and to distribute publicly paper and electronic copies of this
thesis document in whole or in part.
A uthor .............. j. .V. ....................... .
Joint Program in Physical Oceanography
December 20, 2012
Certified by....................... ..... ................
Robert S. Pickart
Senior Scientist
Thesis Supervisor
A ccep ted by ...........................................................
Glenn Flierl
Chair, Joint Committee for Physical Oceanography
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Moored Observations of Shelfbreak Processes at the Inflow to
and Outflow from the Arctic Ocean
by
Wilken-Jon von Appen
Submitted to the Joint Program in Physical Oceanography
Massachusetts Institute of Technology and Woods Hole Oceanographic Institution
on December 20, 2012, in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy in Physical Oceanography
Abstract
Two high-resolution mooring arrays extending from the outer shelf to the mid conti-
nental slope are used to elucidate shelf-basin exchange at the inflow to and the outflow
from the Arctic Ocean. Pacific Water entering the Arctic Ocean forms the Western
Arctic shelfbreak current along the Beaufort Sea slope. Data from the mooring array
at 1520W-approximately 150 km east of Pt. Barrow, AK-reveals that this cur-
rent has two distinct states in summer depending on the water mass it transports.
When advecting Alaskan Coastal Water it is surface-intensified and both baroclini-
cally and barotropically unstable. This configuration lasts about a month with an
average transport of 0.7 Sv. When advecting Chukchi Summer Water the current
is bottom-intensified and is only baroclinically unstable. This state also exists for
approximately a month with an average transport of 0.6 Sv. The strong mean-to-
eddy energy conversion causes both configurations of the current to spin down over
a distance of a few hundred kilometers, suggesting that warm Pacific Water does not
enter the Canadian Arctic Archipelago via this route.
Dense water formed in the Nordic Seas overflows the Denmark Strait and un-
dergoes vortex stretching, forming intense cyclones that propagate along the East
Greenland slope. Data from the mooring array at 650N-roughly 300 km down-
stream of Denmark Strait-was used to determine the full water column structure of
the cyclones. On average a cyclone passes the array every other day in the vicinity of
the 900 m isobath, although the depth range of individual cyclones ranges between
the 500 m and 1600 m isobaths. The cyclones self-propagate at 0.45 m/s and are
also advected by the mean flow of 0.27 m/s, resulting in a total propagation speed of
0.72 m/s. They have a peak azimuthal speed of 0.22 m/s at a radius of 7.8 km and
contain overflow water in their core. In the absence of the cyclones, the background
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flow is dominated by the East Greenland Spill Jet. This is shown to be a year-round
feature transporting 2-4 Sv of dense water equatorward along the upper continental
slope.
Thesis Supervisor: Robert S. Pickart
Title: Senior Scientist
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Acknowledgments
There are many individuals who were instrumental in the making of this thesis either
by helping me with its content or by supporting me in my life. Unfortunately, this
list has to be finite, so I unfortunately cannot mention you all.
First and foremost I would like to thank my advisor Bob Pickart. He has taught
me a lot of science as well as how to think, do, and write science. He has always been
a great role model for what it means to be a scientist. I have gone to sea with him
multiple times which has always been a fun and instructional experience.
The members of my thesis committee Ken Brink, Tom Haine, and Paola Rizzoli
always had great ideas and comments and made sure that I would stay on track when
I was once again ready to go off in a different direction. They spent many hours
reviewing my work in great detail and finding the big and little things that improved
the argumentation.
This work would not have been possible without the diligent efforts of many people
in collecting and processing the data. We thank the crews of the R/V Arni Fri riksson
and R/V Knorr and the WHOI Mooring Operations, Engineering and Field Support
Group, in particular John Kemp and Jim Ryder. H66inn Valdimarsson assisted in
various aspects of the fieldwork. I am grateful to the following people for processing
various parts of the data: Paula Fratantoni, Frank Bahr, Dan Torres, Terry McKee,
Jeremy Kasper, Carolina Nobre, Leah Trafford, and Ben Harden. I have also asked
them many questions which were always happily answered. Fruitful science (and non-
science) discussions with Kjetil Vige, Donglai Gong, Lena Schulze, and Eric Brugler
gave me insights about this thesis and other more important things.
My fellow SSFs in 2006 Nate, Jeff, Carly, Sophie, and Maya as well as my SSF
advisor Amy Bower gave me a great introduction to Woods Hole that made me want
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to come back for much more. I would also like to thank Amy for supporting me again
after so many years as the chair of my thesis defense.
The WHOI Academic Programs Office always kept any sort of administrative
issues off my back and special thanks go to Marsha Gomes, Julia Westwater, and
Tricia Gebbie.
My fellow students in physical oceanography Julian, Nick, Ru, Sophie, Malte,
Daniela, Rachel, Elise, Ping, and Marilena helped me learn a huge amount of oceanog-
raphy, kept me sane to go through the General Exam and have been a great resource
ever since. Thanks for the many lunch time (and other) conversations.
Woods Hole would not be the same without my friends (many have already been
mentioned above). Julian was the best housemate and friend to have and I happily
look back at our countless adventures. I would also like to thank Dan 0, Nate, Kate,
Elise, Leah, and Marilena for a great time.
Susan and Bill are my Massachusetts family while Christin, Laura, and Lennart
helped me keep in touch with "back home".
Didi and Christa deserve a special thanks for never giving up the hope that some-
thing good would come of that little boy and for always quietly working toward that
end.
My mom and my dad have always supported me and I can't thank them enough
for raising me to be who I am. Without them I would not have been the confident
running maniac that is so obsessed with the natural wonders of the ocean to spend
five years to write 250 pages about the physics of the water.
Finally I would like to thank Liz for being the luckiest thing that has happened
to me in Woods Hole.
Financial support for this work was provided by National Science Foundation
grants OCE-0726640 and OCE-0612143, by the Arctic Research Initiative at WHOI,
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by the Y-S Anonymous Fellowship from the Office of the Dean of Graduate Education
at MIT, and by WHOI Academic Programs Office funds.
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Contents
1 Introduction 13
1.1 The Importance of Shelfbreak Processes at High Latitudes . . . . . . 14
1.2 Pacific Water Circulation in the Arctic Ocean . . . . . . . . . . . . . 17
1.3 Atlantic Water Circulation in the Arctic Ocean and the Nordic Seas . 18
1.4 Export of Arctic and Nordic Seas Waters to the North Atlantic . . . 20
1.5 Motivation and Outline of the Thesis . . . . . . . . . . . . . . . . . . 22
2 Two Configurations of the Western Arctic Shelfbreak Current in
Summer 25
2.1 A bstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.3 Data and M ethods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.3.1 Mooring Array at 152'W . . . . . . . . . . . . . . . . . . . . . 33
2.3.2 Shipboard Sections . . . . . . . . . . . . . . . . . . . . . . . . 35
2.3.3 Two Year Mean State . . . . . . . . . . . . . . . . . . . . . . . 36
2.4 Characteristics and Timing of Summer Water Masses . . . . . . . . . 38
2.4.1 Definition of Summer Water Types . . . . . . . . . . . . . . . 38
2.4.2 Temporal Patterns of Summer Water . . . . . . . . . . . . . . 42
2.4.3 Seasonal Variability . . . . . . . . . . . . . . . . . . . . . . . . 44
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2.4.4 Interannual Variability . . . . . . . . . . . . . . . . . . . . . . 44
2.5 Summer Water Current Configurations and their Transports . . . . . 45
2.5.1 Alaskan Coastal Water Configuration . . . . . . . . . . . . . . 45
2.5.2 Chukchi Summer Water Configuration . . . . . . . . . . . . . 49
2.5.3 Transports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
2.6 Energetics and Downstream Fate of the Shelfbreak Current . . . . . . 56
2.6.1 Energetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
2.6.2 Decay Distances and Downstream Fate . . . . . . . . . . . . . 65
2.6.3 Limitations of Distance Estimates . . . . . . . . . . . . . . . . 69
2.7 D iscussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
2.8 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
3 A Case Study of Denmark Strait Overflow Water Cyclon es from a
Cross-stream Mooring Array 77
3.1 Abstract ..... ............................. 77
3.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
3.2.1 Overview of Region . . . . . . . . . . . . . . . . . . . 78
3.2.2 Denm ark Strait . . . . . . . . . . . . . . . . . . . . . 79
3.2.3 Steady Models of the Overflow South of the Strait . . . . 83
3.2.4 DSOW Eddies and Cyclogenesis Mechanisms . . . . . . . 84
3.2.5 The Region Between Denmark Strait and Cape Farewell 89
3.2.6 Cape Farewell Region . . . . . . . . . . . . . . . . . . . . 93
3.2.7 Motivation and Outline of the Chapter . . . . . . . . . . 95
3.3 Data 96
Morn ra................................
3.3.1 M ooring Array . . . . . . . . . . . . . . . . . . . . . . . 96
3.3.2 Synoptic CTD Data . ....... .... .. ... .. . 100
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