Adjoint Modeling and Observing System Design in the Subpolar North Atlantic

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Nora Loose

Adjoint Modeling and Observing System Design in the Subpolar North Atlantic

2019

Thesis for the degree of Philosophiae Doctor (PhD) University of Bergen, Norway

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at the University of Bergen

Avhandling for graden philosophiae doctor (ph.d ) ved Universitetet i Bergen

.

2017

Dato for disputas: 1111

Nora Loose

Adjoint Modeling and Observing System Design in the Subpolar North Atlantic

Thesis for the degree of Philosophiae Doctor (PhD)

Date of defense: 30.08.2019

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The material in this publication is covered by the provisions of the Copyright Act.

Print: Skipnes Kommunikasjon / University of Bergen Name: Nora Loose

Title: Adjoint Modeling and Observing System Design in the Subpolar North Atlantic Year: 2019

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“It is absolutely necessary, for progress in science, to have uncertainty as a fundamental part of your inner nature. [...] And as you develop more information in the sciences, it is not that you are finding out the truth, but that you are finding out that this or that is more or less likely. That is, if we investigate further, we find that the statements of science are not of what is true and what is not true, but statements of what is known to different degrees of certainty. [...] Every one of the concepts of science is on a scale graduated somewhere between, but at neither end of, absolute falsity or absolute truth.”

— Richard Feynman,Science vs. Religion and Why Uncertainty Is Central to Morality

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Abstract

The near-surface ocean currents of the subpolar North Atlantic transport large amounts of heat from the subtropics to higher latitudes, affecting Arctic sea ice extent, the melting of the Greenland Ice Sheet, and the climate in western Europe and North America.

Moreover, deep water formation in the subpolar North Atlantic actively shapes the At- lantic meridional overturning circulation, which connects the surface with the deep ocean and the northern with the southern hemisphere. The recently acquired data from the OS- NAP (Overturning in the Subpolar North Atlantic Program) mooring array challenges our understanding of the processes that govern circulation and deep water formation in the subpolar North Atlantic. However, only long-term and sustained ocean observations can provide the much-needed benchmark to evaluate climate model simulations, to advance our understanding of key mechanisms, and to predict the role of the North At- lantic in future climate changes and anthropogenic carbon uptake. Unfortunately, most observational efforts rely on short-term funding periods.

Given the cost of deploying and maintaining ocean observing systems, these systems have to be designed carefully. Key questions are: What information is contained in already existing observation networks? What do existing networks, such as the OSNAP array, tell us about hydrographic and circulation quantities in remote oceanic regions with few observations? In this thesis, a novel approach to ocean observing system design is explored that is able to address these questions. The approach makes use of adjoint modeling and Hessian-based Uncertainty Quantification (UQ) within a global oceanographic inverse problem.

Adjoint-derived sensitivities reveal that the eastern boundary of the North Atlantic and the coasts of Iceland and Greenland are important pathways for communicating wind-driven pressure anomalies around the entire subpolar North Atlantic and the Nordic Seas. Consequently, the OSNAP observing array shares many dynamical pathways and mechanisms with oceanic quantities that are remote from the array. The OSNAP array has therefore potential to inform these unobserved - or unobservable - quantities: for instance, ocean heat content in the Nordic Seas or close to Greenland’s margins. In this thesis, this potential is quantified within the state-of-the-art ECCO (Estimating the Circulation and Climate of the Ocean) state estimation framework, by combining physical relationships in the model with prior information and data uncertainties.

The effectiveness of an observing system is determined by how well it captures climate-relevant signals and important dynamical adjustment mechanisms. A second important factor, however, is how strongly the monitored signals are masked by noise.

All factors combined, heat transport measurements across the OSNAP-West transect, extending from Labrador to South Greenland, impose an overall much stronger constraint on the ECCO state estimate than heat transport measurements across the OSNAP-East transect, extending from South Greenland to Scotland. This is largely explained by the

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fact that climate signals detected by OSNAP-West are less noisy compared to climate signals detected by OSNAP-East. As a result, transport and hydrographic quantities - even in the Nordic Seas - are constrained more efficiently by OSNAP-West than OSNAP-East observations, contrary to recent findings. This suggests that OSNAP-West is important for informing remote climate signals.

This thesis explores the physical mechanisms that link the subpolar North Atlantic and the Nordic Seas, translates the mathematical concepts that underlie Hessian-based UQ to dynamical concepts, and discusses benefits, shortcomings, and future challenges for designing an effective, long-term Atlantic observing system by means of UQ within ocean state estimation.

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Acknowledgements

First of all, I thank my supervisors, Kerim H. Nisancioglu and Patrick Heimbach, for all their support. I am grateful to Kerim for giving me the freedom to explore my own research interests, while providing useful mentorship, particularly in the writing and presentation of this work. Kerim’s everlasting enthusiasm and encouragement has helped me to never give up. I would also like to thank Kerim for field work opportunities and to support many fun summer schools, conferences, and workshops. I am grateful to Patrick for many inspiring discussions, for valuable input whenever needed, and for generously supporting me to finish this thesis from UT Austin. I also thank Patrick for hosting earlier stays at MIT and UT Austin and for opening the door to networking events.

I owe huge thanks to Helen Pillar and Tim Smith for tremendously helpful discussions and technical support. I am indebted to Helen for helping me to get started with the MITgcm and the adjoint and for giving me invaluable and thorough comments on most of this thesis. I would also like to thank Helen for excellent discussions and ideas on how to make computational tools more accessible and exciting to physical oceanographers. I hope we are on the right track! I am grateful to Tim for sharing his code and experience with the MITgcm and for all his technical assistance. I also thank Tim for giving me feedback on many chapters of this thesis, for in-depth discussions on computing and science, and for all his support outside of this thesis.

I am grateful to Marius Årthun for discussing and proofreading part of this thesis. I owe thanks to Dan Amrhein and Alex Kalmikov for sharing code and to Joel Pedro, An Nguyen, Chuncheng Guo and Mari F. Jensen for sharing data. Thanks are also due to Gael Forget, Ou Wang and Jean-Michel Campin for answering ECCO-related questions.

I would like to thank Dan Goldberg, Yavor Kostov, Jake Gebbie, Dan Jones, Helen Johnson, Peter Huybers, Carl Wunsch, and Bruce Cornuelle for inspiring discussions at various stages of my PhD.

I feel privileged to have been part of two excellent research groups in Bergen and Austin, as well as the Ice2Ice community. These groups have provided a wonderful research environment and introduced me to many exciting applications in climate, physical oceanography, paleoceanography, glaciology, and computational science. Finally, I would like to thank my PhD colleagues, office mates, friends and family. Special thanks are due to my friends in Bergen for many skiing, climbing, and running trips in the mountains that kept me sane during this PhD.

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Chapter 1 Introduction

This thesis is an effort to (i) contribute to our understanding of the physical mechanisms in the subpolar North Atlantic and Nordic Seas, and (ii) explore a novel approach to dynamics-based observing system design for the subpolar North Atlantic. In Section1.1, I provide a brief overview of the circulation and climate impacts of the subpolar North Atlantic and Nordic Seas, a recently launched subpolar North Atlantic observing system, and open science questions. The following two sections introduce the methods that are used in this thesis: In Section 1.2, I describe the concept and applications of adjoint models, and in Section 1.3, I review multiple techniques for evaluating and designing observing systems. Finally, I outline the main thesis objectives and goals in Section1.4.

1.1 The subpolar North Atlantic and Nordic Seas

1.1.1 Near-surface circulation

The subpolar North Atlantic comprises roughly the cyclonic ocean gyre north of about 50^◦N(Fig.1.1). The major current which brings warm and salty waters from the subtropics to the subpolar North Atlantic is the North Atlantic Current (NAC), which crosses the Atlantic near the surface as the north-eastward extension of the Gulf Stream.

The NAC then splits into several branches. The north-eastern branches flow through Rockall Trough and the Iceland Basin across the Iceland-Scotland ridge, transporting warm and salty waters into the Nordic Seas (the Greenland, Iceland, and Norwegian Seas) [Hansen and Østerhus,2000]. The two branches of the Norwegian Atlantic Cur- rent carry the warm and salty Atlantic waters further toward the Arctic Ocean [Orvik and Niiler,2002]. Another branch of the NAC recirculates in the Iceland Basin, and fol- lows the cyclonic boundary current of the subpolar gyre: The recirculated branch flows southward along the eastern flank of the Reykjanes Ridge, crosses the ridge, and con- tinues northward as the Irminger Current (IC) along the western flank of the ridge. A portion of the IC flows through Denmark Strait into the Nordic Seas, while another portion recirculates and flows around the southern tip of Greenland into the Labrador Sea [Holliday et al.,2009]. Here, the current circulates around the margins of the Labrador Sea, while another portion flows northward through Davis Strait [Cuny et al.,2002].

The various NAC branches transport large amounts of heat from the subtropics to higher latitudes. While being carried cyclonically around the subpolar North Atlantic, a

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NAC IC

NwA C

NwCC

LS IS IB RT

DS ISR DvS

LS RR

IB RT AO

IS

NS

SPNA

STNA

Figure 1.1: Schematic of the major near-surface currents in the subpolar North Atlantic (SPNA) and Nordic Seas (NS). Warm Atlantic-origin water pathways are shown as red to yellow arrows, cold Arctic-origin water pathways as blue arrows. The major basins of the SPNA carry light blue labels. The thin contour lines mark the isobath drawn at1500 m.

NAC = North Atlantic Current; IC = Irminger Current; NwAC = Norwegian Atlantic Current; NwCC = Norwegian Coastal Current; SPNA = subpolar North Atlantic; STNA

= subtropical North Atlantic; RT = Rockall Trough; IB = Iceland Basin; IS = Irminger Sea; LS = Labrador Sea; NS = Nordic Seas; AO = Arctic Ocean; ISR = Iceland-Scotland Ridge; RR = Reykjanes Ridge; DS = Denmark Strait; DvS = Davis Strait.

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1.1 The subpolar North Atlantic and Nordic Seas 3 fmars-06-00138 March 28, 2019 Time: 18:10 # 5

Straneo et al. GrIOOS

FIGURE 2 |Schematic of a Greenland glacier/fjord system showing relevant physical processes that govern circulation in the fjord and at the glacier-fjord boundary, typical stratification and water masses, and sources of freshwater to the fjord.

ongoing East GRIP Ice-Core Project focusing on the Northeast Greenland Ice Stream).

Building a Case for GrIOOS: The Last Decade

The rapid increase in mass loss from the GrIS began in the early 2000s (Krabill et al., 2004;Rignot et al., 2008;Murray et al., 2015;

Catania et al., 2018;Wood et al., 2018) and it was only a decade later that the importance of processes at the ice sheet-ocean margins became apparent, making ice sheet-ocean interactions in Greenland and globally a novel and rapidly growing area of research. Key to community progress have been a series of workshops, and related follow-up documents, that sought for the first time to bring together the diverse disciplines needed to advance the science.

The first of these workshops, a multi-disciplinary International Workshop on “Understanding the Response of Greenland’s Marine-Terminating Glaciers to Oceanic and Atmospheric Forcing,” was organized by the US Climate and Ocean Variability, Predictability, and Change (US CLIVAR) Working Group on Greenland Ice Sheet-Ocean interactions in June 2013. It brought together over 100 international scientists and program managers with the goals to summarize the current state of knowledge and questions (Straneo et al., 2013) and to develop several key recommendations to make progress (Heimbach et al., 2014). One major recommendation was the collection of long-term time series (bothin situ and remotely sensed) of critical glaciological, oceanographic, and atmospheric variables at key locations in and around Greenland through the establishment of GrIOOS. The research community recognized that such measurements are needed to provide information on the time-evolving relationships between climate forcing, ice sheet dynamics, and ocean characteristics. The lack of such data has hindered our ability to explain and model the complex interactions among ice-ocean-climate, leaving major gaps in our ability to project future changes. The community noted that GrIOOS data would be critical, not only to validate hypotheses, but also to provide boundary conditions, forcings, and a point of comparison for both ocean and ice sheet model simulations.

Following the recommendations made in the 2014 report, the Study of Environmental Arctic Change Land Ice Action Team, in collaboration with the Greenland Ice Sheet Ocean Interaction Science Network (GRISO), and the Climate and Cryosphere Project (CliC) of the World Climate Research Program, organized a workshop to make progress on the design and implementation of GrIOOS. The resulting 2015 workshop was attended by 47 participants from seven countries, including U.S.

agency program managers (National Science Foundation, NSF, and National Aeronautics and Space Administration, NASA) and a representative of the Greenland government. Participant expertise included oceanography, glaciology, climate and ice sheet modeling, marine ecosystems, and paleoclimatology.

Together, this group examined questions such as: (i) What are the essential ice sheet and ocean variables? What measurements and observing systems already exist? (ii) What should be the structure of the GrIOOS system regarding target observing sites and optimal instrumentation? (iii) How could data be collected, quality controlled, and distributed? Tentative answers to these questions are summarized in Straneo et al. (2018) and have informed the GrIOOS system design discussed here.

Societal Benefits From GrIOOS

Our inability to quantify GrIS-ocean exchanges, and their climate forcing, is a major scientific obstacle to understanding causal origins of past variability and to predicting the future of GrIS and its impact on the neighboring ocean regions, including the marine ecosystems. Connecting science across disciplines and countries, which is necessary to address key climate-related questions, has proven challenging because there is no integrating framework for a comprehensive ice sheet and ocean observing system, including needed structures for data management and dissemination, translating observations to usable model data and parameterizations, and overall cyberinfrastructure to support FAIR (Findable, Accessible, Interoperable, Re-usable) data.

The objectives of GrIOOS are to address all of these challenges by determining essential observations (‘Essential Variables’) for the ice sheet-ocean-atmosphere system, establishing guidelines for instrumentation that can be used across institutions and

Frontiers in Marine Science | www.frontiersin.org 5 March 2019 | Volume 6 | Article 138

Figure 1.2: Schematic of a glacial fjord in Greenland. Warm Atlantic waters (purple shading) that cross the continental shelf and submarine sill (brown rise in bottom to- pography) reach the glacier at depth and drive glacier submarine melting (red swirls).

Figure from Straneo et al.[2019].

portion of this heat is released back to the atmosphere, thereby having an effect on many climate and weather phenomena in western Europe and North America [e.g.,Sutton and Hodson,2005;Sutton and Dong,2012]. Moreover, variability in the poleward progression of ocean heat from the subpolar North Atlantic towards the Arctic Ocean has been linked to Arctic sea-ice extent [Carmack et al.,2015;Zhang,2015;Polyakov et al.,2017].

1.1.2 Shaping the AMOC

The northward flow of warm, salty waters in the near-surface layers of the Atlantic is often described as the upper limb of the Atlantic merdional overturning circulation (AMOC) [e.g.,Buckley and Marshall,2016]. As the warm waters are carried from the Gulf Stream cyclonically around the subpolar North Atlantic by the various NAC branches, they gradually cool along their path [McCartney and Talley, 1982; Brambilla and Talley, 2008]. The gradual transformation causes these surface waters to become denser - and eventually sink to great depth in the Nordic Seas and Labrador Sea, where heat loss to the atmosphere destabilizes the water column in winter [e.g.,Marshall and Schott,1999].

These cold, dense waters that are formed in the Nordic Seas and Labrador Sea constitute the components of North Atlantic Deep Water, which are exported southward at depth and feed the lower limb of the AMOC. The subpolar North Atlantic is therefore a region where the strength and structure of the AMOC is actively shaped. The AMOC connects the Northern with the Southern hemisphere, as well as the surface with the deep ocean.

It is therefore a key component of the global climate system, for instance, through its cross-equatorial ocean heat and freshwater transport and its role in the global carbon cycle [Buckley and Marshall,2016;Khatiwala et al.,2013].

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0 10 20 30 40 50 60 70 80 90 10 110 120

−45

−40

−35 −50

−40

−30

Time (kyr ago)

δ18O[‰] Temperature[◦C]

Figure 1.3: Oxygen isotope measurements (black, [North Greenland Ice Core Project members, 2004]) and temperature reconstruction (orange, [Kindler et al., 2014]) from the Greenland ice core NGRIP.

1.1.3 Interaction with Greenland’s marine-terminating glaciers

The cyclonic boundary currents of the subpolar North Atlantic bring warm and salty Atlantic waters around the continental slopes of Greenland. Closer to the coast, cold and fresh waters of Arctic origin flow around Greenland’s shallow (200-300 m deep) continental shelves, partially buffering Greenland’s coast from the warm, Atlantic waters (Figs.1.1 and1.2). In the mid-1990s, Greenland’s marine-terminating glaciers started to retreat and accelerate, at a time when the subpolar North Atlantic experienced a rapid warming [Straneo and Heimbach,2013]. Observational studies show evidence that, concurrent with the warming of the subpolar North Atlantic, the layer of Atlantic water thickened and the shelf waters warmed [e.g.,Holland et al., 2008]. Coming in contact with the glaciers, warm ocean waters trigger increased submarine melting (see Fig.1.2).

It is therefore suggested that the subpolar North Atlantic has an important impact on the melting and acceleration of Greenland’s glaciers [Holland et al., 2008; Straneo et al., 2010, 2012; Vieli and Nick, 2011; Joughin et al., 2012; Straneo and Heimbach, 2013;Rainsley et al.,2018]. There are two main implications of increased ice loss from Greenland for the climate system. First, sea-level change [e.g., Stammer, 2008]; and second, ocean surface freshening, which may impact the AMOC, when reaching the deep water formation sites [e.g.,Böning et al.,2016;Luo et al.,2016;Yang et al.,2016].

1.1.4 A player in paleoclimate shifts

During the last glacial period, North Atlantic climate was characterized by large millenial-scale variability and a number of abrupt climate shifts. Fig.1.3 shows oxygen isotope measurements and temperature reconstructions from the Greenland ice core NGRIP. The record shows about 25 large positive spikes, reflecting sudden warmings by 5−10^◦C within at most a few decades. These sudden changes from cold Green- land stadial conditions to warmer Greenland interstadial conditions are referred to as the Dansgaard-Oeschger (D-O) events [Dansgaard et al.,1993;North Greenland Ice Core Project members,2004]. The schematics in Fig.1.4show land and sea-ice conditions in

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1.1 The subpolar North Atlantic and Nordic Seas 5

6 Introduction

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Figure 1.3: The hypothesized configuration of the Nordic Seas during a) Stadial and b) Interstadial conditions on Greenland. Figure adapted from Dokken et al. (2013).

als than warm interstadials. Enhanced ice rafted debris and low seawater oxygen isotope values suggest a fresher surface layer during the stadial periods (e.g., Dokken and Jansen, 1999), but also toward the end of the interstadials (Dokken et al., 2013). Using planktic foraminifera assemblages, Dokken et al. (2013) show a gradual warming of the subsurface during stadials and a warm overshoots at the start of each interstadials.

This is further described in Sec. 1.2.2.

Circulation changes during DO-events are also thought to occur in the Nordic Seas.

The evidence for deep-water convection in the Nordic Seas is generally elusive and indirect. Open ocean convection probably occurred during interstadials (Rasmussen et al., 1996; Kissel et al., 1999; Dokken and Jansen, 1999; Rasmussen et al., 2014b).

Deep-water formation through open ocean convection is suggested to stop during stadials due to an insulating sea-ice cover and/or fresh surface layer as indicated by lowered δ¹³C (stable carbon isotope) signals of planktic foraminifera (Dokken et al., 2013).

Ezat et al. (2014) reconstructed bottom water temperatures (here: at 1179 m) from Mg/Ca measurements, showing an increase of 2-5^◦C during stadials as opposed to the colder values during interstadials. This is interpreted as a pause in deep-water production as the marine sediment core is located in an overflow area. Lowerδ¹⁸O-values on benthic (deep-dwelling) foraminiferas are also interpreted as showing a warming of the deep ocean during stadials. In contrast, Dokken and Jansen (1999) and Dokken et al.

(2013) interpret the values as a brine signal from enhanced sea-ice production during stadials. Brine is released when sea ice freezes and the salty water may penetrate to depths depending on its density. Dokken and Jansen (1999) suggest that brine production from surface waters with lowδ¹⁸O-values sinks to the deep ocean and contributes to the low benthicδ¹⁸O-signal.

Sea ice has been hypothesized to be present in the eastern Nordic Seas during stadials based on the Arctic-like stratification (Dokken et al., 2013) and fore-mentioned benthic δ¹⁸O-signal. More direct evidence is emerging as new records of IP25 (a biomarker for sea ice) are presented (Hoff et al., 2016, H. Sadatzki et al. 2017; un- der review for Nat. Geosc.). On the other hand, studies based on dinoflagellate cyst assemblages (Eynaud et al., 2002; Wary et al., 2016) suggest a warmer surface in the Nordic Seas during stadials and more sea ice during interstadials. However, the more extensive sea ice during interstadials is not consistent with the majority of other proxy

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Figure 1.4: Schematics showing land and sea-ice conditions in the North Atlantic and the Nordic Seas during (a) cold Greenland stadials and (b) warmer Greenland interstadials, as suggested byDokken et al.[2013]. Figure fromDokken et al.[2013].

180ôW 120ôW 60ôW 0ô 60ôE 120ôE 180ôW 80ôS

40ôS 0ô 40ôN 80ôN

Cores with coverage of GI−7/GI−8 and at least 500−yr resolution

ACER database extra subsurface T

Figure 1.5: Globally available SST proxy data from marine sediment cores that cover GI-7/GI-8 and have at least 500-yr resolution. Figure courtesy Joel Pedro. Cf. with data coverage of modern observations, e.g., Argo floats in Fig. 1.10.

Adjoint Modeling and Observing System Design in the Subpolar North Atlantic

Nora Loose

Adjoint Modeling and Observing System Design in the Subpolar North Atlantic

at the University of Bergen

Avhandling for graden philosophiae doctor (ph.d ) ved Universitetet i Bergen

.

2017

Nora Loose

Adjoint Modeling and Observing System Design in the Subpolar North Atlantic

Thesis for the degree of Philosophiae Doctor (PhD)

Date of defense: 30.08.2019

Abstract

Acknowledgements

Contents

Chapter 1 Introduction

1.1 The subpolar North Atlantic and Nordic Seas

Building a Case for GrIOOS: The Last Decade

Societal Benefits From GrIOOS